Influence of Processing Conditions on thePerformance of Perovskite Solar Cells

Master’s Thesis

Matti UlkuniemiUniversity of Oulu

Faculty of Science/Physics22.11.2017

Abstract

Direct conversion of solar radiation into electricity is a renewable, abun-dant and clean method for producing energy. Organic-inorganic metal halideperovskites have attained great attention in developing efficient and cost-effective solar cells. They show many ideal properties such as high carriermobility, large absorption coefficient, an appropriate and tunable band gapand they can be manufactured at low costs. Moreover, contrary to organicmaterials they have shown non-excitonic behavior. Development of perovskitesolar cells has been fastest among all cell types exceeding 20% efficiency infirst 6 years. However, there are several factors complicating their commer-cialization including device stability, hysteresis and environmental impactsfrom lead.

In this thesis, solar cells and cell parameters are introduced. Commonsurface preparation and thin-film deposition techniques are also described.Since deposition conditions essentially influence the perovskite film prop-erties and device efficiency, they are discussed more extendedly. Finally,experiments concerning the effect of solution based processing method onan ITO/PEDOT:PSS/perovskite/PCBM/Ca/Al device performance are re-ported. Perovskite films were prepared mainly via two-step spin coating fromPbI2 and CH3NH3I precursors and cells reached power conversion efficiencyof 3.81%. In addition to perovskite solar cells, the content of this thesis canbe applied also for other perovskite based applications such as transistors,photodetectors, light emitting diodes and lasers.

Keywords: methylammonium lead halide, perovskite, solar cell, solution pro-cessing, thin-film

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Acknowledgments

The experiments presented in this thesis were done in Optoelectronics and Measure-ments Techniques (OPEM) Unit in the University of Oulu. The project was fundedby PrintoCent and supervised by D.Sc. Rafal Sliz, who advised in the experimentalpart of the project, and Prof. Tapio Fabritius, who guided the writing process.

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List of terms, symbols and abbreviations

a AbsorbanceAC Alternating currentAl AluminumAl2O3 Aluminum(III) oxideAM1.5 Air Mass 1.5, reference solar spectrumAr ArgonAu GoldBr Brominebs Spectral photon flux density normal to surfacec CompactCa CalciumCaTiO3 Calcium titanium oxideCB ChlorobenzeneCH3(CH2)n Aliphatic chainCH3NH2 MethylamineCH3NH3Cl Methylammonium chlorideCH3NH3I Methylammonium iodideCH3NH3PbI3 Methylammonium lead iodideCH3NH3PbI3−xClx Methylammonium lead mixed iodide-chlorideCl ChlorineCu CopperCuSCN Copper(I) thiocyanateCVD Chemical vapor depositionDC Direct currentDCB 1,2-DichlorobenzeneDMF N,N-DimethylformamideDMSO Dimethyl sulfoxideE EnergyEcb Energy of conduction band edgeEg Band gap energye.m.f Electromotive forceETM Electron transporting materialFDC Fast deposition-crystallizationFF Fill factorFTO Fluorine doped tin oxideGBL γ-ButyrolactoneGS Specific surface free energyH2O WaterHCl Hydrochloric acidHI Hydrogen iodideHOMO Highest occupied molecular orbitalHTM Hole transporting materialI CurrentI Iodine

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i IntrinsicIdark(V ) Dark currentIPA Isopropyl alcoholITO Indium tin oxideJ0 Saturation current densityJdark(V ) Dark current densityJe Electron fluxJh Hole fluxJm Maximum current densityJsc Short circuit current densityJ(V ) Current densitykb Bolzmann’s constantMAI Methylammonium iodideMAPbI3 Methylammonium lead iodideMAPbX3 Methylammonium lead halidemin Minutemp MesoporousNiO Nickel(II) oxideNREL National Renewable Energy LaboratoryO2 DioxygenP Power densityPb LeadPbCl2 Lead(II) chloridePbI2 Lead(II) iodidePbI2 · HCl Complex of HCl with PbI2PbX2 Lead halidePC61BM [6,6]-Phenyl-C61-butyric acid methyl esterPC71BM [6,6]-Phenyl-C71-butyric acid methyl esterPCE Power conversion efficiencyPEDOT:PSS Poly(3,4-ethylenedioxythiophene) polystyrene sulfonatePH 500 Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate

product with PEDOT to PSS weight ratio of 1:2.5Ps Incident light power densityq Electronic chargeQE Quantum efficiencyRL Load resistanceRs Series resistanceRsh Shunt resistanceSb2S3 Antimony(III) sulfidespiro-MeOTAD 2,2’,7,7’-Tetrakis(N,N-di-p-methoxyphenylamine)-

9,9’-spirobifluoreneSS-DSSC Solid-state dye-sensitized solar cellSSE Solvent-solvent extractionSTC Standard Test ConditionT TemperatureTCO Transparent conducting oxide

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Ti TitaniumTiN Titanium nitrideTiO2 Titanium dioxideUV UltravioletV VoltageVm Maximum voltageVoc Open circuit voltageXPS X-ray photoelectron spectroscopyε Emissivityγ Surface tensionγlv Liquid-vapor interfacial tensionγsl Solid-liquid interfacial tensionγsv Solid-vapor interfacial tensionτ Surface stretching tensionθa Advancing contact angleθr Receding contact angleθY Contact angleµm MicrometerC Degree Celsiuscm2 Square centimetermA/cm2 Milliampere per square centimetermJ/m2 Millijoule per square meterW/m2 Watt per square meterΩ/sq Ohm per square, unit of sheet resistancecm CentimetereV Electron volth HourkHz KilohertzmA Milliamperemg/ml Milligram per millilitermol/l Mole per litermol% Molar percentagenm Nanometerppm Parts per millionrpm Revolutions per minuterpm/s Revolutions per minute per seconds Secondv% Percentage by volumewt% Percentage by weightA/s Angstrom per second

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Contents

Abstract 2

Acknowledgments 3

List of terms, symbols and abbreviations 4

1 Introduction 9

2 Solar cells overview 11

3 Theoretical considerations for solar cells 123.1 A single p-n junction solar cell . . . . . . . . . . . . . . . . . . . . . . 123.2 Tandem solar cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.3 Spectral converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4 Parameters for evaluating solar cells 154.1 Current-voltage (IV) curve . . . . . . . . . . . . . . . . . . . . . . . . 154.2 Photocurrent and quantum efficiency . . . . . . . . . . . . . . . . . . 154.3 Dark current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.4 Power conversion efficiency . . . . . . . . . . . . . . . . . . . . . . . . 164.5 Parasitic resistances . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.6 Hysteresis and charge recombinations . . . . . . . . . . . . . . . . . . 18

5 Surface wetting and surface preparation 195.1 Surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.2 Contact angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205.3 Surface cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215.4 Surface modification with plasma . . . . . . . . . . . . . . . . . . . . 21

6 Main thin-film deposition technologies 236.1 Spin coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236.2 Sputtering and evaporation . . . . . . . . . . . . . . . . . . . . . . . 246.3 Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . 25

7 Perovskite solar cells 267.1 Perovskite solar cell architectures . . . . . . . . . . . . . . . . . . . . 26

7.1.1 Mesoporous structure . . . . . . . . . . . . . . . . . . . . . . . 267.1.2 Planar structure . . . . . . . . . . . . . . . . . . . . . . . . . . 277.1.3 Charge transporting materials and electrodes . . . . . . . . . . 27

7.2 Stability of perovskite solar cells . . . . . . . . . . . . . . . . . . . . . 287.2.1 Moisture and oxygen . . . . . . . . . . . . . . . . . . . . . . . 287.2.2 UV light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297.2.3 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

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8 Perovskite formation methods 308.1 Spin coating methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 318.2 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328.3 Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338.4 Solvent engineering and antisolvent treatment . . . . . . . . . . . . . 33

9 Experiments 349.1 Single-step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369.2 HCl-assisted single-step method . . . . . . . . . . . . . . . . . . . . . 369.3 HCl-assisted two-step sequential method . . . . . . . . . . . . . . . . 379.4 HCl-assisted two-step method . . . . . . . . . . . . . . . . . . . . . . 37

9.4.1 First fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 379.4.2 MAI annealing variation . . . . . . . . . . . . . . . . . . . . . 379.4.3 Open air environment . . . . . . . . . . . . . . . . . . . . . . 38

9.5 Two-step mixed-halides . . . . . . . . . . . . . . . . . . . . . . . . . . 389.6 HCl-assisted two-step mixed-halides . . . . . . . . . . . . . . . . . . . 399.7 Two-step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

10 Results 4010.1 Single-step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4010.2 HCl-assisted single-step method . . . . . . . . . . . . . . . . . . . . . 4010.3 HCl-assisted two-step sequential method . . . . . . . . . . . . . . . . 4010.4 HCl-assisted two-step method . . . . . . . . . . . . . . . . . . . . . . 41

10.4.1 First fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 4110.4.2 MAI annealing variation . . . . . . . . . . . . . . . . . . . . . 4210.4.3 Open air environment . . . . . . . . . . . . . . . . . . . . . . 43

10.5 Two-step mixed-halides . . . . . . . . . . . . . . . . . . . . . . . . . . 4310.6 HCl-assisted two-step mixed-halides . . . . . . . . . . . . . . . . . . . 4310.7 Two-step method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

11 Discussion 45

12 Conclusions 46

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1 Introduction

Continuous growth of population sets challenges for energy production throughoutthe world. Additional energy should be harvested mostly from renewable energysources and solar cells offer one of the best solutions [1]. The current market ofphotovoltaics are dominated by inorganic solar cells such as silicon solar cells [2].Silicon is considerably used in bulk (1st generation), thin-film (2nd generation), andsome of the nano-structured (3rd generation) solar cells [1]. However, the efficienciesof silicon cells are difficult to further improve and their manufacturing is expensive[1].

Solar cells based on organic semiconductors have been developed as a low-costalternative to silicon cells. Fabrication of such solar cells is based on solution pro-cessing, which allows fast production using affordable organic solutions [3]. However,organic solar cells are excitonic and strongly bound excitons have to migrate to adonor-acceptor interface and dissociate into free electrons and holes, resulting inenergy losses [3][4]. New materials that are abundant, less-toxic, stable and easilydeposited are still needed to fabricate low-price and effective solar cells [1]. In Figure1, the efficiencies of different solar cells have been plotted from 1976 onward showingperovskite based solar cells to be very promising.

Figure 1: Development of the efficiencies of various solar cell types. [5]

Perovskites are compounds characterized by a general formula ABX3, where Aand B denote an organic and metallic cation and X indicates a halide anion [6][7].Perovskites have a similar crystalline structure to calcium titanium oxide (CaTiO3)shown in Figure 2 [6]. Many properties including long charge diffusion length, highabsorption coefficient and great defect tolerance make methylammonium lead halides(CH3NH3PbX3, MAPbX3, X=I, Cl, Br) potential photoharvesting material [6][7].Moreover, varying the composition allows their band gaps to be tunable between the

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ultraviolet and infrared regions [7]. In contrast to organic materials, organolead tri-halide perovskites have revealed a non-excitonic nature and thus offer a very promis-ing thin-film technology for producing efficient and low-cost solar cells [3][4][8].

Figure 2: Crystal structure of organic-inorganic metal halide perovskite ABX3. Thered, blue and green spheres represent X, B and A atoms, respectively. [9]

The aim of this thesis is to investigate the influence of fabrication method andthin-film processing parameters on the ITO/PEDOT:PSS/perovskite/PCBM/Ca/Aldevice efficiency. Perovskite layers were prepared via single- and two-step spin coat-ing using mainly PbI2 and CH3NH3I precursors.

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2 Solar cells overview

In solar cells the energy of sunlight is converted to electrical energy via a conversionprocess, where light-induced charges are transported and collected into electrodes[4]. In classical p-n junction cells made of inorganic semiconductors, absorption ofphotons higher in energy than the band gap excites some electrons from the valenceband to the conduction band, generating electron-hole pairs with small exciton bind-ing energy. For example, even thermal energy is sufficient to dissociate excitons intofree carriers in a silicon cell and thus most excitons will instantaneously become freecharges. Drive of electrons into n-type and holes into p-type semiconductor side isachieved by the electric field of a depletion region, as illustrated in Figure 3 [10].Formed electromotive force (e.m.f) gets the electrons that reach the contact to flowthrough a connected load to do electrical work [2]. Although the general idea issame among all types of solar cells, their precise operation mechanisms are different[11].

Load

n p

Current

Sunlight+

+

Depletion region

+

+

+

+

++

+

++

+ +

++ +

+

Figure 3: Electric field at the junction drives collection of photogenerated electronsand holes into n-type and p-type semiconductor sides, respectively.

Current density generated by a single cell is typically some tens of mA/cm2 butthe DC voltage often lies between only 0.5 to 1 volts and is insufficient for mostapplications. Connecting cells in series into modules raises voltage and connectingthe modules in series and parallel into an array (a photovoltaic generator) furtherincreases voltage and current depending on the application. Cells within a moduleand modules within arrays are protected from the complete loss of power due to afailure in some cell by bypass and blocking diodes. Components for a charge storagesystem and power regulation are almost always required to compensate variation ofillumination and an inverter is used to convert DC current to AC, if necessary. [2]

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3 Theoretical considerations for solar cells

Thermal photons are emitted by all objects on account of their finite temperatures.According to the principle of detailed balance, the cell must emit thermal photonsat the same rate as it absorbs thermal photons from the surroundings. Duringthis spontaneous emission an excited electron relaxes to its ground state, which isneeded to maintain the electron concentration in the cell constant in steady state.Stimulated emission can be neglected because the excited state is almost empty.The principle of detailed balance leads to the result that for photon energy E theemissivity ε(E) equals to absorbance a(E). Under solar illumination, the rate ofspontaneous emission is increased as solar photons are absorbed and more electronswill exist at raised energy. This radiative recombination unavoidably limits theefficiency of a photovoltaic cell and current is generated by the net absorbed solarflux. [2]

3.1 A single p-n junction solar cell

A classical structure of a solar cell is a p-n junction, where a p-type semiconductorhas greater work function and thus greater electrostatic potential, which generatesan electric field at the junction region for driving photogenerated electrons intothe n-type semiconductor and holes into the p-side. Leaving an undoped, intrinsic(i) semiconductor layer between p-side and n-side forms the p-i-n junction, wherethe electric field is expanded while retaining the built-in bias of the correspondingp-n junction. Such a junction is appropriate for materials, where carriers formedoutside the depletion region in p- or n-layers probably can not contribute to the pho-tocurrent. Carriers formed in the i-layer have often greater lifetimes than in dopedmaterials and are transferred over greater distances towards the contacts. However,in the i-region, smaller conductivities can cause series resistance, recombination islikely at similar electron and hole populations and charged impurities can dimin-ish the electric field. In p-n and p-i-n heterojunctions, utilization of two dissimilarsemiconductor materials with distinct band gaps can enhance carrier collection orbe an only option for materials with limited doping properties. Potential step inconduction and valence band edges generates different effective fields for electronsand holes and the alignment of the energy bands critically determines the propertiesof the junction [2].

A perfectly absorbing and non-reflecting material would absorb all incident pho-tons of energy greater than the band gap energy Eg and only one electron perphoton is promoted to the upper band (without multiple carrier generation). Incase of perfect charge separation all electrons survived from radiative recombinationare aggregated to the negative terminal of the solar cell and reach the external cir-cuit. These assumptions give the maximum photocurrent, which is dependent onlyon the band gap and the incident spectrum. Very low Eg will lead to an insufficientworking value of voltage V while very high Eg will result in too low photocurrent.For the standard AM1.5 solar spectrum theoretical efficiency has a maximum ofabout 33% at a band gap of around 1.4 eV. [2]

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In a real solar cell all incident photons are not absorbed, reducing the generatedphotocurrent. Reflections from the front surface or from the contacts exist and somephotons pass through the cell without absorption. In addition to radiative recom-bination, carriers can be trapped at defect sites and non-radiative recombinationoccurs before collection. The phenomenon is likely at the surfaces, near interfacesand near the junction due to higher defect densities and increased dark current leadsto reduced photocurrent and voltage. Effect of series and parallel resistances in thecircuit will be discussed later. [2]

3.2 Tandem solar cell

In a single junction cell only a limited part of the solar radiation spectrum is har-vested. Photons at lower energy than the band gap are not absorbed and an excessenergy of higher energy photons is lost by thermalization losses. For a silicon basedcell these losses can include approximately 50% of the available solar energy [12][13].Using multiple band gap junctions enables incident light to be harvested more ef-ficiently. Incident light enters the higher band gap material first, and low-energyphotons not absorbed enter the next layer at a lower band gap to be absorbed. Inde-pendent electrical contacts between the cells are difficult to attain in separately con-nected stacked arrangement with several terminals, and the most practical methodis to connect the cells directly in series, where generated voltage is a sum of the indi-vidual cells and a single current passes the device with two terminals. Their differentp-n junctions are connected by heavily doped n-p junctions, which are called tunneljunctions and supposed to generate an Ohmic contact between p-type and n-typeterminals of the superimposed cells [2]. The tunneling layer must be appropriatefor optical transparency, and electrical conductivity to be matched to both the sur-rounding cells [14]. This arrangement in series has additional losses since currentsfrom each cells cannot be matched under all illumination conditions. Tandem cellsare expensive to produce and they have been predominantly used in space applica-tions [2]. Perovskites are potential candidates in future tandem constructions withe.g. crystal silicon cells, as they have a tunable band gap between 1.5-2.3 eV andhigh open circuit voltage Voc [14].

3.3 Spectral converters

Another methods for harvesting incident light more efficiently are spectral conversionlayers, which are down- and up-converters and down-shifters. These modify theincident light spectrum and they can be developed independently of the cell. Theincident spectrum and intensity affect gained benefits and combined use of bothconverters could compensate regionally varying efficiencies. [13]

Down-conversion and downshifting layers are placed on top of a solar cell [13],but nanoparticles can be incorporated in an appropriate absorber medium to actas down-converters and photon scatterers [12]. In the down-conversion process onehigh-energy photon is converted into two or more lower energy photons, which aremore efficiently utilized [15]. In downshifting one high-energy photon is convertedinto one lower energy photon via a luminescence phenomenon [13]. Improved amount

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of absorbed photons increases the photocurrent while retaining the cell’s properties[12]. Moreover, avoiding exposure of e.g. perovskite cells with TiO2 to UV-lightreduces UV-induced degradation [16]. Non-radiative relaxation pathways complicatethe development of down-conversion layers and impeding normally absorbed photonsreduces power conversion efficiencies (PCE) [13].

An up-conversion layer applied on bottom of a solar cell converts two or more low-energy photons into one high-energy photon [15]. This utilizes passed high-energyphotons and it could improve especially cells with higher band gap than silicon[13]. Contrary to down-converters, reflections from the up-conversion layer does notcause additional losses in PCEs [15]. Non-linear response to intensity reduces theperformance but even at low up-conversion efficiency, up-converters improve PCEs.In up- and down-conversion processes, some converted photons are emitted awaydue to isotropic emission and reflection layers can be used for directing them intothe cell [13].

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4 Parameters for evaluating solar cells

Various parameters are utilized for describing the performance of a cell. Intensityand energy spectrum of the incident radiation intrinsically affect the amount ofgenerated photocurrent within a solar cell. Thus parameters strongly dependent onthe light source are measured under standard test conditions. The Standard TestCondition (STC) for solar cells consists of the Air Mass 1.5 spectrum, an incidentpower density of 1000 W/m2 and a temperature of 25C. [2]

4.1 Current-voltage (IV) curve

Output current is independent of load for all but the largest loads and a solarcell is modeled as a current generator. The current-voltage (IV) curve presents thecurrent as a function of voltage and is used to determine basic parameters and modeldevice’s behavior within an electrical circuit [2]. The curves are generated startingthe photocurrent sweep from forward bias (V > 0) or reverse bias (V < 0) [17].

4.2 Photocurrent and quantum efficiency

Under illumination and terminals isolated, open circuit voltage Voc occurs. Theshort circuit current density Jsc develops when the terminals are connected together.These are the maximum values of a cell, and for any intermediate load resistance RL

the output voltage V lies between 0 and Voc and for the output current I = V/RL.The cell’s quantum efficiency QE is the probability that an incident photon ofenergy E will deliver one electron to the external circuit, and it depends upon theabsorption coefficient and the efficiencies of charge separation and collection butnot upon the incident illumination. Thus it is used to estimate cell properties undervarious conditions and it connects to Jsc as

Jsc = q

∫bs(E)QE(E)dE (1)

where q is the electronic charge and bs(E) is incident spectral photon flux density,the amount of photons of energy in the range E to E + dE emerging on unit areain unit time. QEs should be high at wavelengths corresponding to high solar fluxdensity. [2]

4.3 Dark current

A potential difference between the terminals of the cell occurs when a load is con-nected. A current drawn in the opposite direction to the photocurrent is called adark current Idark(V ), and it reduces the net current from its short circuit value Jsc.Dark current occurs under an applied voltage or bias in the dark. Due to an asym-metric junction most solar cells act as a diode in the dark and admit significantlygreater current under forward bias V > 0 than under reverse bias V < 0. Darkcurrent density Jdark(V ) for an ideal diode has a form of

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Jdark(V ) = J0(eqV/kbT − 1) (2)

where kb is Bolzmann’s constant, T is temperature in degrees Kelvin [2] and J0 issaturation current density [18].

According to the superposition approximation the net current density can bedesignated as the sum of the short circuit photocurrent and the dark current den-sities. The dark currents under or in the absence of illumination are not formallyequal but the approximation holds reasonably for many photovoltaic materials. Thephotocurrent is defined to be positive and the current density J(V ) in the cell is

J(V ) = Jsc − Jdark(V ) . (3)

At the open circuit voltage Voc the dark current and the short circuit photocurrentexactly cancel out and Voc for an ideal diode is presented by the following Equation(4). [2]

Voc =kbT

qln(

JscJo

+ 1) (4)

4.4 Power conversion efficiency

A cell generates power at voltages between 0 and Voc. The cell’s power densityP = JV has the optimum value at the maximum power voltage Vm with the cor-responding current density Jm. A higher load resistance leads to a higher potentialdifference and a lower net current. Thus, the load resistance should correspond tothe maximum power point of the photovoltaic generator. The power conversion ef-ficiency PCE of a cell is defined as a quotient of maximum power density JmVm andincident light power density Ps as

PCE =JmVmPs

. (5)

Fill factor FF expresses the ’squareness’ of the JV curve and is defined as

FF =JmVmJscVoc

. (6)

In Figure 4 the maximum power point for an ideal cell has been marked to the JVcurve and the power density curve is also shown. [2]

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Cur

rent

den

sity

, J

Bias voltage, V

Powerdensity

Maximumpower pointCurrent density

Vm Voc

Jm

Jsc

Figure 4: The JV curve and PV curve of an ideal cell. Power density has a maximumat a bias Vm. The area of the inner rectangle gives the maximum power density JmVm

and the area of the outer rectangle is JscVoc. At the fill factor value of 1 the JVcurve would follow the outer rectangle.

4.5 Parasitic resistances

Resistance of the contacts and leakage currents around the sides of a device reducethe generated power by lowering net current density. An equivalent circuit formodeling such a solar cell is presented in Figure 5, where two parasitic resistanceshave been marked in series Rs and in parallel Rsh. [2]

Rs

Rsh V

JscJdark

+

-

Figure 5: An equivalent circuit to model a solar cell with series and shunt resistances.

Series resistance originates from the resistance of a cell material to current flowand from resistive contacts, and is remarkable at high current densities. Parallelresistance is caused by leakage currents around the edges, through the cell andbetween contacts of different polarity. The effect of resistances within a device isseen in IV curves as a reduction of the maximum power density, as can be seen inFigure 6. It is desirable to get Rs as low and Rsh as high as possible. [2]

17

Cur

rent

Bias

Rs increasing

a)

Cur

rent

Bias

Rsh decreasing

b)

Figure 6: The effect of parasitic resistances. Increasing series a) and decreasingparallel b) resistances reduces the value of JmVm compared to JscVoc and the fillfactor falls. In both cases for the outer curve Rs = 0 and Rsh = ∞.

Presented model does not take into account capacitance elements such as chargetraps [17], where charge trapping and detrapping processes cause delayed photocur-rent response seen as hysteresis in JV curves [19]. Thus, more complex circuitswith capacitors are used to model such cells for identifying charge recombinationpathways [17].

4.6 Hysteresis and charge recombinations

Differences in measured JV curves depending on the scan direction and rate indicatecurrent hysteresis, and it impedes the precise determination of solar cell’s PCE[20]. Perovskite cells often show significant hysteresis [17][19], and its extent isstrongly dependent on cell architecture and processing conditions [20]. Hysteresishas required intensive research and many causes have been proposed to explain itsorigin, such as ion migration within the crystal [21] and recently mentioned chargeaccumulation to traps [17][19].

Detrimental charge recombination in polycrystalline solar cells occurs mainly atsurfaces, electrode interfaces and grain boundaries. Recombination within grains isinsignificant when carrier diffusion length exceeds crystal size, which is a very desiredand reasonable property in perovskites [17][22]. By contrast, traps at the absorberinterfaces severely impede the hole and electron extractions [22]. Photoluminescencemeasurements generally show red-shifted emission peaks due to the spontaneousradiative recombination between trap states [17][23]. Contrary to that, blue-shiftingof photoluminescence peaks indicates passivated states [17] or lowered trap densityby the improved crystallinity [22][23] or larger grains [22].

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5 Surface wetting and surface preparation

Prerequisite for high-quality thin-films along with a clean substrate is sufficientsurface wettability to a fluid to be deposited [24]. Good wettability is required forstrong adhesive bonding [25], but it is not a guarantee for that [26]. Besides in liquidcoating, wetting is taken into account in oil recovery, lubrication, spray quenchingand investigation of superhydrophobic surfaces. Surface wetting is strongly relatedto solid surface tension which is most simply measured indirectly from contact anglevalues. Direct measurements are often based on surface deformation, being trouble-some for solids [24].

5.1 Surface tension

Surface tension γ is the work needed to increase the surface area isothermally andreversibly by unit amount [27]. It can be identified as surface free energy per unitarea or force per unit length. In the latter case surface tension has directionalfeatures whereas specific surface free energy GS is a scalar attribute of the surfacearea. For liquids in equilibrium at constant conditions, specific surface free energyand surface tension equal contrary to solids. Increasing the surface area can bedivided into two steps. In the beginning, material is split and atoms stay in theiroriginal places. After that, atoms in the surface region move to equilibrium positions.For liquids these happen simultaneously but for solids the rearrangement can beslow, and therefore a solid surface can be stretched keeping the amount of atomsin it constant. The surface stretching tension τ describes the force per unit lengthneeded to keep species in their initial equilibrium position [28].

Solid surfaces with a surface energy GS ≈ 200-5000 mJ/m2 are classified as high-energy surfaces constructing of strong chemical bonds such as ionic, metallic andcovalent [28]. Metals and inorganic compounds such as oxides, silicates, silica, dia-mond and nitrides are examples of this group of materials [27], whereas solid organiccompounds belong to low-energy surfaces with surface energies of 10-50 mJ/m2.These materials are based on weak van der Waals chemical bonds which includeshort range attractive interactions between permanent dipolar molecules (Keesomforce), between a permanent dipolar molecule and a corresponding induced dipolarmolecule (Debye force) and between temporary dipoles (London dispersion force).The London dispersion force dominates for most organic liquids and solids [28].High-energy surfaces absorb low surface energy materials and thus a cleaned surfacewill be soon recontaminated in ambient atmosphere by water and organic particles[27]. This adsorption lowers the surface free energy and hence wettability [26], whichrelates to spreading of liquid on a solid surface [28].

Surface tensions of liquids can be altered chemically by surfactants which aremolecules having hydrophobic and hydrophilic groups. Molecules will orientate atsurfaces or interfaces according to greatest affinities. Often the hydrophobic partis constructed by aliphatic chains CH3(CH2)n and the hydrophilic part is an ionhaving an affinity to liquids with a high dielectric constant, e.g. water. Surfactantsare used in many industrial applications for altering wetting properties. [28]

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5.2 Contact angle

Molecules in pure bulk liquid are surrounded completely by others but near the liquidsurface neighboring molecules do not exist in all directions, and hence molecules havedifferent energy states. Liquids minimize its surface area by decreasing the amountof interface molecules leading to minimum surface free energy, and liquid dropletsresting on flat horizontal surfaces become spherical [28]. The surface tension andexternal forces such as gravity determine the shape of such a liquid droplet [24].

Solid surface tensions are evaluated by investigating contact angles of varioustesting fluids. Contact angle is the angle formed between the solid-liquid interfaceand a tangent drawn in the intersection along the droplet profile as represented inFigure 7 [24]. This wetting situation holds for a droplet inserted on a dry surfaceand it should be distinguished from a surface with a thin layer of absorbed liquidmolecules surrounding the deposited droplet [28]. Wetting is propitious at contactangles less than 90while greater angles indicate unfavorable wetting and compactdroplet formation. Young’s contact angle θY for a drop on an ideally smooth, chemi-cally homogeneous, isotropic, non-reactive and non-deformed rigid surface is definedas the mechanical equilibrium by Young’s Equation (7)

γlv cos θY = γsv − γsl (7)

where γlv, γsv, and γsl are the liquid-vapor, solid-vapor and solid-liquid interfacialtensions, respectively [24].

θ = 90°θ < 90° θ > 90°γlvγsl γsvθ

Figure 7: Contact angles of different sessile drops on a smooth and homogeneoussurface.

Measuring a contact angle of a static droplet simply inserted on the surface actu-ally does not give reliable values since various metastable states exist. For example,inflating and deflating a droplet with a syringe leads to advancing and recedingcontact angle θa and θr, respectively. They are threshold values at which the triple(three-phase) line starts to move. A subtraction of these maximum and minimumangles describes hysteresis and it complicates evaluation of the surface energy ofsolids. The phenomenon occurs even on ideal surfaces due to intermolecular in-teraction between molecules of solid and liquid, which pins the triple line to thesubstrate. Contact angle hysteresis is influenced also by many other factors such asroughness, chemical heterogeneity and deformation of the surface, and liquid pen-etration. Rough and/or chemically heterogeneous surfaces require different modelsfor describing the wetting, such as the Wenzel and Cassie-Baxter models. [28]

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5.3 Surface cleaning

Contaminants can be removed by suitable solvents. Ionic polar contaminants aremiscible into polar solvents such as water and water-alcohol mixtures and non-polar contaminants such as grease can be removed by non-polar solvents. Mixturesof solvents are often utilized for removing both polar and non-polar species anddetergents containing wetting agents and surfactants are used for easing the particleremoval from the substrate. Wetting agents lower the surface energy of liquids andsurfactants reduce the interfacial energy between different materials, such as oil andwater. [29]

An addition of cleaning fluids can be performed by various methods, and me-chanical disturbance is often incorporated for removing particulate contaminants.Some common methods include fluid spraying, ultrasonic and megasonic cleaning.Liquid spray systems use a high-pressure stream and it removes large particles oversubmicrometer scale well. Resonant vibrations induced by spraying may deterioratecomponents. In ultrasonic cleaners operating at 18-120 kHz, a surface is immersedin a fluid and adjacent collapsing cavitation bubbles produced by transducers gen-erate a high-pressure jet which removes dust from the surface. Many variables suchas properties of transducers and fluids affect cleaning. High-pressure jetting andresonance effects can damage sensitive surfaces and such brittle surfaces can becleaned by megasonic cleaners, where higher frequency waves (>400kHz) are usedfor displacing dust particles more gently without surface damage. [29]

Any wet cleaning processes should be followed by rinsing residues off with an ultrapure liquid, typically deionized water. Subsequent drying removes absorbed fluidsfrom substrates and it should be done as fast as possible to prevent particles stickingfirmly on wet surfaces during drying. Drying can be done by heating, spinning thesubstrate, blow-off with a gas or displacing water with fluids, e.g. anhydrous alcohol.Drying water by vaporization can leave concentrated residues on the surface. [29]

Volatile constituents trapped in bulk material are removed by out-gassing, wherethey diffuse to the surface and evaporate. For that reason, materials are heated ine.g. an oven and the effect can be improved by heat treating in vacuum. The progressof the process can be monitored by measuring decreased mass of the material as afunction of time. In out-diffusion, the diffused material does not evaporate from thesurface and surface cleaning procedures have to be repeated until criteria for purityis reached. [29]

5.4 Surface modification with plasma

Clean polymer surfaces have lower surface energies than metals and weak adhesionbonds are formed in coating processes without surface treatment [27]. For enhancingadhesive bonding to another polymer or metal, polymer surfaces are treated forincreasing their surface energy or modifying their functional groups [29]. Formedreactive functional groups on a surface can enable covalent bonding via chemicalreaction to metal [25]. Various treatments for improving adhesion exist. Somemethods utilizing plasma are described next, as they are very common treatmentsfor making surfaces harder, rougher, more wettable and more adhesive [27]. Plasmaalso removes contaminates from surfaces [29].

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Plasma consists of positive and negative ions and it can contain also other frag-ments such as free radicals, atoms and molecules [27]. In surface treatments withplasma, charged particles formed by exciting gas atoms are accelerated towardsa surface and ionization or excitation energies will be released during collisions[29]. Plasma treatment of polymers consist of simultaneous processes of ablation,cross-linking and activation, whose extent depends on the nature of the process.During ablation, smaller polymer chains are formed when covalent bonds of thepolymer backbone are broken by radiation and bombarding reactive species andelectrons. Volatile oligomers and monomers evaporate and are removed from thesurface. Cross-linking occurs in an inert plasma gas such as Ar, where bond break-ing in the absence of free-radical scavengers enables new bond formations betweennearby free radical chains. In activation, free radicals formed on a substrate reactwith the polymer backbone itself or with reactive species present in the plasmaresulting in replacement of the functional groups [25].

Treatments based on plasma are categorized according to energies of chargedparticles and gases used in a plasma. Plasma (glow discharge) treatment is done atreduced pressure and a gas can be inert or reactive [27]. Sputter cleaning occurs bybombardment of high-energy particles, which results in a collision cascade near thesurface. A lot of energy of the bombarding particles is released as heat, but partof the energy is transferred to surface atoms which will eject with contaminants.Sputter cleaning in a plasma region can result in a reaction between the surface andactivated contaminants such as oxygen. For that reason, plasma can be formed ina distinct region using inert or reactive gases, from where ions are extracted andaccelerated into the substrate at good vacuum conditions. Ion scrubbing occurs bybombardment of low-energy inert gas ions and it promotes desorption of surfacecontaminants usually without sputtering or damaging the surface. Ion scrubbingwith a reactive gas, such as oxygen or hydrogen, in the plasma is called reactiveplasma cleaning as reactive species can react with contaminants and form volatileproducts to leave the surface [29]. Treatment with atmospheric pressure plasma iscalled corona discharge [27].

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6 Main thin-film deposition technologies

Thin-film coatings offer a way to tailor the physical and chemical properties andmorphology of a surface. By surface engineering improved performance, functional-ity, material’s usage efficiency and cost reduction can be achieved along with newproperties not possible with bulk material. Thin-films can be utilized e.g. for wearprotection and improving lubricity and optical, electrical and thermal properties.Numerous thin-film preparation methods are available, such as sputtering, ther-mal deposition, chemical vapor deposition (CVD) and spin coating [30]. Mentionedcommon technologies are described next.

6.1 Spin coating

Spin coating is very common and relatively simple method for thin-film prepara-tions. In spin coating, fluid is added onto a substrate and during spinning the fluidspreads and excess solution is removed by centrifugal force. Properties of a residualthin-film depend on the solution and spinning parameters. Compared to other com-mon deposition techniques, spin coating is low-cost, low-temperature, fast and wellcontrollable without complex coupled variables. Disadvantages arise when substratesize increases, and ineffectively utilized fluid is costly or very hazardous. In staticdispensing the solution is spread over a stationary surface before spinning and itusually requires large amounts of material. In dynamic dispensing the substraterotates during addition of fluid and usually needs less solution to wet the surfacecompletely, being beneficial for expensive coatings or for materials with poor wettingproperties. [31]

During acceleration, excess fluid is removed fast and slight vorticity occurs in theinitial thick fluid layer, aiding the solution to spread over the surface optimally with-out shadowed areas. Finally the thin-film completely co-rotates with the substrateand the layer is uniform in thickness. Acceleration has a great influence on coatings,and generally greater spinning speeds and longer spinning times lead to thinnerfilms. Viscous flow and solvent evaporation take place constantly, but initially fluidviscous forces dominate layer thinning effect when the substrate is rotating at a con-stant speed. Thinning is fair homogeneous but at the edges droplets are generatedand difficultly thrown away, possibly leaving a bit thicker area of a coating especiallyon the corners of the square substrate. Eventually a fluid flow rate diminishes andvaporization of volatile solvents will have main influence on the deposition process.Viscosity of the remaining fluid increases rapidly and the coating is attached on thesubstrate while prolonged spinning times will not decrease thickness significantlyany more. Depending on the process a subsequent annealing is often required. [31]

Rotating disk interacts with surrounding air resulting in a radially directed shear-ing stress to the interface, and under vacuum conditions the layer thickness is ob-served to diminish at much lower rate. The ambient air along with fluid propertieshas a considerable influence on the drying rate and thus coating conditions have tobe taken into consideration. For example, humidity can lead to roughness, microc-racking during drying and some defects. Spin coaters are constructed to overshadowthe effects of slight variation of surroundings by allowing minimal vapor exhaust.

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It also slows down the drying process leading to more uniform films because theviscosity will stay more constant between the center and the edges of the substrate.[31]

Working in a clean environment and filtering a solution reduces contaminantparticles which can stick on a surface and impede a radial fluid flow, leading to sur-face defects called comets [32]. Radially oriented striations of thickness variation areother common defects, which are thought to result from surface tension effects drivenby evaporation. Faster evaporating solvent species can increase the concentration ofless-volatile material. If that uppermost part of a layer has a greater surface tensionthan the initial film still existing at deeper levels, it draws material in at regular in-tervals, and the areas between them evaporate faster leading to lines directed alongmajor liquid flow. This phenomenon is called the Marangoni effect. The thermalconductivity of the substrate also affects film formation and marks can occur froma metallic sample holder. These chuck marks exist due to thermal driving forcesand temperature differences [31]. Cooling of the surface by solvent evaporation isbetter compensated at areas with better back-side contact, and slightly higher sol-vent temperature allows solvent vapor pressure to rise exponentially leading to fasterevaporation and thicker and darker film areas. A less-volatile solution, solvent-richenvironment and chucks with uniform contact can reduce these defects [32].

6.2 Sputtering and evaporation

Sputtering is a common method for fabricating thin-films and in addition to thedeposition of metals, it can be exploited also for insulators. In sputter depositionprocess, ionized atoms are bombarded into surface of a target material and duringcollisions atoms eject from the surface. Those ejected atoms are condensed onto asubstrate to form a coating. In thermal evaporation method, material evaporatesfrom e.g. a resistively heated source and condenses on a substrate. As opposedto evaporation process, sputtering yields energetic flux with high surface mobilityleading more easily to condensation into smooth, continuous and dense coatings.Moreover, the target source retains its stoichiometry, in contrast to evaporationor chemical vapor deposition. Sputtering is broadly superseded in semiconductormanufacturing. [33]

Argon ions Ar+ are typically used as bombardment species for sputtering inindustry. Argon is inert and low-cost and has an atomic mass near to that ofcommonly used coating materials, such as Ti, Al and Cu. For sputtering metallic orinsulating compounds such as TiN and Al2O3, sputtering of a pure metallic target inthe presence of a compound gas (typically oxygen or nitrogen) is preferred as reactivesputtering. This allows to control the film’s stoichiometry more, but formation ofan insulating layer on the target surface due to the reaction with the compoundgas sets requirements for operation and the design of the device. Ions are typicallyaccelerated using the potential difference between ionized plasma vapor and targetcathode. The target can be powered by a DC power supply or an alternating currentRF source depending on deposition requirements. [33]

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6.3 Chemical vapor deposition

In chemical vapor deposition processes, a main gas flow consisting of gaseous re-actants undergoes a reaction forming solid material to be deposited and gaseousproducts. The reaction occurs on a heated substrate or near it. Also photons, elec-trons and ions can be used to maintain reactions. CVP yields to high throwing powerfor depositing uniform and low-porous coatings even on complex shapes, and alsopatterned coatings via selective deposition can be made. CVD processes are sen-sitive to contamination originating from the reactants, various unwanted chemicalreactions and air leakage. [34]

Reactions utilized in CVD processes are often complex but can be grouped asthermal decomposition, reduction, exchange, disproportionation and coupled reac-tions. The process is chosen according to the substrate and requirements for e.g.temperature, total pressure, gas mixture, costs and toxicity. CVD phase diagramsare calculated typically using free energy minimization and they are used to estimatesuitable processing conditions for a desired coating material. [34]

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7 Perovskite solar cells

A great absorption coefficient across a wide wavelength range allows relatively thinperovskite absorber layers, and the rate of non-radiative recombination is relativelylow leading to open circuit voltages Voc > 1V, higher than for efficient polymersolar cells. This may be explained by low density of deep-lying defect states [21].Perovskite films can be deposited via many methods compatible with low-cost andlarge-scale production techniques [14]. While energy losses from exciton migrationand dissociation impede in organic cells [3][4], perovskites with small exciton bindingenergies are very promising for developing efficient and low-cost photovoltaic devices[3][4][8].

7.1 Perovskite solar cell architectures

Perovskites are used as absorber films usually between layers of electron and holetransporting materials (ETM and HTM) [35] and extraction of photogeneratedcharges occurs near the interfaces [11]. Different device architectures exist depend-ing on porosity of the films and positioning of ETM and HTM layers, as shown inFigure 8. The construction is called n-i-p or p-i-n when the incident light entersthe ETM or HTM layer first, respectively. Appropriate materials and preparationmethod for electrodes and charge transporting layers are determined by the devicearchitecture [14]. Since perovskites are able to transport electrons and holes, ETM-free and HTM-free constructions are also possible with high-quality perovskite layersand controlled interfaces [35]. They are developed for more simple manufacturingwith decreased costs and for overcoming device degradation mechanisms inducedby poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as HTM[35] and metal oxides as ETM [36]. Different interfaces are formed depending on thecell architecture [4] and despite similarities to traditional junctions of classical cells,precise operation mechanism of perovskite cells requires different modeling [37].

AnodeHTM

ETMTCO

Perovskite

AnodeHTM

ETMTCO

Perovskite

Cathode

HTM

ETM

TCO

Perovskite

HTM

ETM

TCO

Perovskite

Cathode

a) Mesoporous n-i-p b) Planar n-i-p c) Planar p-i-n d) Mesoporous p-i-n

Figure 8: Typical structures of perovskite cells. a) mesoporous n-i-p, b) regularplanar, c) inverted planar and d) mesoporous p-i-n. Mesoporous layer could denoteTiO2 or Al2O3 in the mesoporous n-i-p and NiO in the mesoporous p-i-n cell.

7.1.1 Mesoporous structure

At first perovskite absorber materials were successfully used infiltrated in a meso-porous (mp) TiO2 layer having a n-i-p structure similar to a solid-state dye-sensitizedsolar cell (SS-DSSC) [38], as presented in Figure 8a. Mesoporous constitution in-creases the contact area for electron injection at the common TiO2/MAPbI3 interface

26

[39], decreases the carrier transport distance and enhances absorption due to lightscattering [14]. Moreover, total filling of pores prevents a direct contact betweenHTM and ETM [6][14][40]. Controlling the interface area by varying mp-TiO2 layerthickness enables the electron flux to be equalized to the hole flux [39]. The mostefficient results are attained by controlling the thickness and porosity of the TiO2

for improving pore filling and formation of an upper perovskite capping layer [6].Effective charge generation and transport occurs particularly in that layer of largerperovskite crystals [6][40] where crystal growth is not confined by pores, which leadto disordered and amorphous phases [14] and limited grain size [40]. Formation ofthe high-quality capping layer seems to be another main function of the mesoporouslayer [41].

Despite the advantages of a mesoporous layer, it is inappropriate for flexiblecells due to high-temperature processing, which can also impede cost-effective man-ufacturing [7]. Photocatalytic TiO2 is also problematic for device stability [42].Inorganic oxide HTMs have provided alternatives for selective contacts and enabledan inverted mesoporous cell construction shown in Figure 8d. Examples of such cellsinclude NiO/mp-Al2O3 and c-NiO/mp-NiO HTMs [14].

7.1.2 Planar structure

Resulting from long charge diffusion lengths and ability to transport both holesand electrons, perovskites were later reported to allow sufficient charge transportand collection also in planar type structures [7][23]. This type includes regular n-i-p and inverted p-i-n constructions, as shown in Figure 8. Many charge transportlayers used in organic solar cells are appropriate also in perovskite cells [7][14], andoriginally a planar device consists of a perovskite layer between two opposite organicHTM and ETM [14].

In n-i-p planar cells, the interface contact between compact metal oxide ETM andperovskite is often limited and it makes electron extraction ineffective compared toextraction of holes at the HTM interface [14]. Unbalanced electron flux Je and holeflux Jh result in charge accumulation into traps and retarded recombination. Chargetraps have been reported as the major factor for hysteresis in planar perovskite solarcells. The high density of trap states near perovskite top surface can be explainedby defects formed in thermal decomposition of surfaces and grain boundaries [17],while traps at bottom surface can result from the low-quality interface with smallgrains and poor crystallinity [22]. Fullerene ETMs are often reported to drasticallyreduce hysteresis in inverted planar construction and besides from passivation effect[7][17][22], it results presumably from balanced charge collection [14].

7.1.3 Charge transporting materials and electrodes

Fluorine doped tin oxide (FTO) and indium tin oxide (ITO) are common transparentconducting oxides (TCO) used as front electrodes, while back contact materials areusually metals such as Au and Al [14]. Hole and electron transporting materialsimprove the extraction of charges from the absorber to the electrodes and theyalso prevent oppositely signed charges to reach the inappropriate electrodes and

27

recombine [43]. Additional buffer layers can be utilized e.g. for improving ohmiccontact between fullerene ETM and metal cathode in the inverted structure [7].

Common HTMs include small molecules, organic polymers and inorganic com-pounds. Very common small molecular HTM is 2,2’,7,7’-tetrakis(N,N-di-p-methoxy-phenylamine)-9,9’-spirobifluorene (spiro-MeOTAD) [14], but it is expensive to syn-thesize and it has low hole mobility [44]. Traditional organic PEDOT:PSS has greathole transporting properties [45]. However, the effect of its acidic and water absorb-ing nature on stability is a matter of concern. Inorganic HTMs such as NiO andCuSCN are developed for more affordable and stable cells [7].

Metal oxides, such as TiO2, are traditional ETMs especially in mesoporous cells[14]. Very promising ETMs in planar structures include fullerenes and its derivatives,such as n-type [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM). Fullerenes dif-fuse into perovskite along pinholes and grain boundaries during deposition and an-nealing [7]. They effectively passivate charge traps near the top perovskite surfaceand/or along grain boundaries and successfully reduce interface charge recombina-tion and device hysteresis [17]. Vertical and straight grain boundaries might improvethe diffusion of fullerenes deeper to passivate some traps also near the bottom per-ovskite surface [22]. Thickness of the PCBM layer has critical influence on PCE,as an insufficiently thick layer results in leakage current [46][47] and conductivityin the PCBM layer is decreased by increasing its thickness. The increased travel-ing distance of charge carriers leads to increased recombination and device’s seriesresistance. Best performances are obtained at as thin PCBM layers as possible tostill ensure full coverage, for which a flat perovskite top surface is crucial [47].

7.2 Stability of perovskite solar cells

Solar cells must retain sufficiently their performance during long term usage [14][21].Current products on the market are guaranteed to work with system loss of <1%/year during a warranty period of 20-25 years. Intrinsic instabilities and extrinsicfactors cause problems with stability of perovskite solar cells. Oxygen and moisture,UV light and thermal effects are key factors of a degradation process [21].

7.2.1 Moisture and oxygen

Perovskites are sensitive to water [21] and under ambient conditions the presenceof moisture is often assumed to be the main cause for degradation of MAPbI3 [48].Moreover, decomposing by-products such as PbI2 create environmental problems[21]. However, further research has shown oxygen-induced degradation during illu-mination to be the dominant decomposition process of MAPbI3. Electron extractionlayers reduce this effect by extracting electrons before they react with diffused oxygenfor generating reactive superoxide O−

2 , which causes deprotonation of methylammo-nium cation [48].

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7.2.2 UV light

Many solar cell technologies are sensitive to UV radiation and it has been shownto induce degradation of perovskite on a photocatalytic TiO2 ETM layer, as pre-viously mentioned. UV-induced degradation can be reduced by using a blockinglayer, by reducing incident UV radiation or by exploiting another ETM [21]. Forexample, depositing a Sb2S3 blocking layer at the TiO2/CH3NH3PbI3 interface hasbeen shown to enhance device stability. Reduced degradation indicated that decom-position of the perovskite under UV radiation may originate from a reaction at theTiO2/CH3NH3PbI3 interface. The Sb2S3 layer was supposed to block UV-inducedphotocatalysis in TiO2 [49].

7.2.3 Temperature

Perovskites have much lower thermal stability than traditional inorganic semicon-ductors, causing degradation under elevated temperatures [17]. Reducing this effectis essential, because temperature will increase during operation and production offilms usually requires an annealing step. The following Equation (8) shows a pro-posed reaction for thermal decomposition of CH3NH3PbI3 films [21]. Their surfacesand grain boundaries are deduced to have the lowest thermal stability and as pre-viously stated, defects formed by their decomposition can result in charge traps[17]. Material of the film under a perovskite layer can have an influence on thermalstability [21].

CH3NH3PbI3 → PbI2 + CH3NH2 ↑ +HI ↑ (8)

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8 Perovskite formation methods

Dense and uniform perovskite layer with suitable thickness, high crystallinity andlarge grains typically gives high Jsc values, while high-quality interfaces and reduceddefect densities in and within grains enable high Voc values [14]. These propertiesare sensitive to deposition conditions and hence crystal growth is investigated thor-oughly and attempted to be controlled [14][50][51]. Pinhole formation results inpartial coverage and areas without absorber material will not harvest light. In addi-tion, in produced low-resistance shunting paths ETM and HTM layers have a directcontact, which acts like a parallel diode in the circuit equivalent to a solar cell andlowers Voc, FF and PCE [41].

At a thicker absorber layer more absorption and larger Jsc values are attained[22][50][52][53] particularly at wavelengths between the red and infrared range [22][52].Around a 300-600 nm thick MAPbX3 layer is sufficient to effectively absorb sunlight[22]. Moreover, decreased risk for short circuiting paths for thicker films is favorablefor large scale production via e.g. printing [52]. However, film thickness must beappropriate for a charge diffusion length not to limit charge transport efficiencies[7][46][54], and high coverage and crystallinity should still be ensured [50].

High crystallinity is also crucial for minimizing defects, which can provide short-ing and trapping sites and alter charge transport, dissociation and diffusion length[50]. Larger grains reduce total grain boundary area and thus charge recombinationdue to reduced charge trap density [22][42][52][55], which can be indicated by thecell’s smaller series resistance Rs and thus higher FF. Larger crystals can also im-prove the quality of interfaces [22]. Optimally, the grain size is larger or comparableto the film thickness in planar construction [22][42][52] to simulate single-crystal cells[22]. Generally, a grain size in solution-processed MAPbI3 perovskite films is just afew hundred nanometers originating from the fast crystallization reaction betweenPbI2 and methylammonium iodide (MAI) [52].

Crystal growth is strongly affected by the bottom material. More crystallinelayer appears to improve perovskite crystallization [7] and reduce trap density ata bottom absorber surface [22]. Wettability to the precursor solvent is anotherfactor. Although it was previously stated that appropriate surface wettability to adeposited fluid is needed for strong adhesion [25] and high-quality coatings [24], non-wetting condition is favorable for growing large crystals with enhanced crystallinity.High wettability facilitates deposition of continuous film, but surface tension ofthe wetting surface causes forces which decrease the mobility of grain boundariesand thus impedes formation of larger grains. A non-wetting and smooth surfaceenables drastically reduced interaction and the growth of larger crystals from fewernucleation sites. Smooth surface has also less small pores for smaller grains to beformed [22].

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8.1 Spin coating methods

In a one-step process, precursors of MAI and PbX2 (X=I or Cl) are dissolved in asuitable solvent [6][51]. They must dissolve precursors for depositing films at uniformthicknesses well and have e.g. suitable boiling points for desired crystallizationrate [53]. During deposition the solvent evaporates and precursors convert intoperovskite crystals [6]. Preparing a homogeneous and pinhole-free film is difficultwithout additives [51]. For example, conventional MAPbI3/DMF precursor solutionscan leave PbI2 impurities [19] and produce low coverage [56]. Reason could bepractical difficulty to mix equimolar amounts of MAI and PbI2 or decomposition orsublimation of MAI during MAPbI3 formation [19]. Compared to a stoichiometricmix, large excess amount of MAI in a 1:3 molar ratio PbI2/MAI solution has beenreported to lead to larger grains and smoother films [57]. A homogeneous crystallinelayer of quickly crystallizing ionic materials such as perovskites is challenging tosolution deposit, especially on smooth organic surfaces such as PEDOT:PSS due tothe poor affinity [45][51].

In a two-step sequential process, spin-coated lead halide layer is immersed intoMAI solution for conversion to occur. It is usually used for mesoporous structuresbecause of controllable crystallization in pores [6]. In a two-step method, spincoating of lead halide film is done before spin coating the second precursor [7], whichcan be added on the stationary or spinning film [51]. Morphology of a first precursorfilm is critical for high-quality perovskite layers [22][51][58]. Two-step processes offera way to solution deposit high-quality ionic perovskite layers on PEDOT:PSS, asa continuous and smooth PbI2 film is easier to be spin coated because it is lesspolar than perovskites [45]. Moreover, perovskite formation is better controlled byvarying the concentration of a MAI/IPA solution [51], which has been shown tostrongly affect the size of formed crystals and device performance [40].

Complete conversion of lead halides into high-quality perovskite is difficult dueto large crystal size of PbI2 and its rapid reaction with MAI [51]. An outer formedperovskite layer impedes MAI to diffuse deeper at the PbI2 crystal centers throughthe whole dense layer, and residual content reduces light absorption [6]. The dis-tribution and content of PbI2 define its final effect on PCE. In inverted structures,a thick remained PbI2 layer between HTM and perovskite blocks the hole trans-fer due to its low conductivity and valence band lower than in perovskite [51]. Ina conventional construction, an appropriate thick remnant PbI2 layer sandwichedbetween TiO2 ETM and MAPbI3 perovskite has been shown to act as a barrierlayer which inhibits the electron recombination and interception. In recombination,electrons transfer back from ETM to holes in the perovskite material’s valence bandand in interception electrons return from ETM to the holes in the HOMO of HTM,as depicted in Figure 9. Energy of a conduction band edge Ecb must be higher in thebarrier layer than in ETM. The barrier effect can be another reason for generallyhigher PCEs of two-step processed cells versus single-step ones [59].

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HTMPbI2 PerovskiteETM

h+

e-

1

2

E

Figure 9: Detrimental electron back-transfer processes. 1. Charge recombination.2. Charge interception.

8.2 Annealing

Annealing removes solvent residues and promotes perovskite crystallization [60].Temperature, solvents, rates of mass evaporation and transport, and layer thicknessessentially affect perovskite film morphology. For example, annealing temperaturehas an influence on the wetting of the film [41]. Raising the value has been shown toproduce less pores in perovskite films prepared from a PbCl2/CH3NH3I/DMF (1:3molar ratio) solution, but with increased size leading to lowered coverage. This hasbeen observed on PEDOT:PSS [50], on compact TiO2 [41] and on mesoporous TiO2

[60]. At lower temperatures slower crystallization from many nucleation sites intosmaller islands was possible [60].

Despite the detrimental effect of moisture on perovskites, annealing under con-trolled humidity has been reported to reduce hysteresis and increase grain size, cov-erage and photoluminescence lifetime in CH3NH3PbI3−xClx films on PEDOT:PSS.The reason is attributed to the strongly hygroscopic nature of MAI. Moisture cancombine adjacent grains and improve the precursor ion diffusion length while toohigh humidity level of over 80% began to induce decomposition [54]. However,preparing CH3NH3PbI3 films from a similar precursor solution via vacuum annealingwas reported to improve removal of the CH3NH3Cl by-product, whose accumulationwas found to induce pores, hysteresis and decomposition of perovskite probably byabsorbing moisture [61].

Annealing in ambient air can be considered as a solvent annealing process, whereH2O plays a key role. Gaseous species alter perovskite film growth and varioussolvents have been investigated for improving crystallization during annealing pro-cess. Common polar solvents for precursor solution include GBL, DMF and DMSO,whose vapors can erode film surfaces during annealing. During solvent annealing,substrates are annealed in a closed space containing a solvent, whose molecules con-dense on the film and concurrently re-evaporate forming a liquid or quasi-liquidphase. In the liquid phase grains bond together and many grain boundaries remelt.Liquid precursors move across the liquid-solid interface and recrystallize in areaswith less grains, optimally leading to improved coverage. [23]

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8.3 Additives

Properties of perovskite films can be effectively improved by introduction of appro-priate additives [14][51]. For example, chloride content is known to drastically im-prove diffusion lengths of charges in CH3NH3PbI3−xClx mixed-halide perovskite, en-abling effective charge extraction in thicker active layers. Reported diffusion lengthsover 1 µm for electrons and holes are multiple times greater than its absorptiondepth of 100-200 nm, contrary to the diffusion lengths of ∼100 nm on the order of orslightly shorter than the absorption depth in CH3NH3PbI3 [62]. Various compoundscontaining chloride can be added into precursor solutions [58]. One promising wayis to add HCl in a PbI2/DMF precursor solution [56][63] to produce compact anduniform novel PbI2 · HCl precursor film of small unique crystal grains, which istotally and rapidly converted into a high-quality MAPbI3 film. Addition of HClinto a single-step (PbI2/MAI/DMF) precursor solution has also been reported withpromising results [56]. There was no traceable amount of Cl detected in these films[56][63]. That is consistent with the reported amount of Cl in CH3NH3PbI3−xClxfilms, which is very small or even under the detection limit of a X-ray photoelectronspectroscopy (XPS) instrument [46].

8.4 Solvent engineering and antisolvent treatment

In solvent engineering various solvents with different properties are utilized for im-proving film morphology. For example, crystallization can be optimized by mixingsolvents, and antisolvents are used for solvent extraction induced crystal growth [58].In a single-step process, an antisolvent is highly miscible in the previous solvent buta poor solvent for perovskites. Exposing a wet and spinning perovskite film to theantisolvent rapidly reduces solubility of perovskite in the mixed solvent, leading toits fast crystallization [64] as a result from decreased Gibbs energy. Antisolventtreatment can also remove residue halides and ions [65]. The remnant solventsare removed and crystallization further promoted by subsequent thermal annealing.The process is called a fast deposition-crystallization (FDC) process, which has beenreported to result in full coverages with micron-sized grains [64]. Similar solvent-extraction-induced crystallization has also been used in a solvent-solvent extractionprocess (SSE), where a wet polar single-step solution coated film is immersed in asecond non-polar solvent bath [53].

33

9 Experiments

The inverted cell structure of ITO/PEDOT:PSS/perovskite/PCBM/Ca/Al present-ed in Figure 10 was used as in [45][51] and in one experiment Ca layers were notdeposited as in [46][50]. The devices consisted of 8 cells with an active area of0.15 cm2 for each, defined as the geometric overlap of the electrodes. They wereprepared on 6.25 cm2 square glass substrates with 1.3 cm wide and 150 nm thickITO stripe in the middle and 20 Ω/sq sheet resistance (Thin Film Devices Inc.). Allused substances are shown in Table 1.

AlCaPCBM

Perovskite

PEDOT:PSSITO

Figure 10: The cell structure used in the experiments.

Table 1. Materials used in the experiments

Material Purity Producer

PEDOT:PSS (PH 500) 1.0 - 1.3 % (solid content) HeraeusPbI2 99.999 % Sigma-AldrichPbCl2 98 % Sigma-AldrichCH3NH3I (MAI) 98 % Sigma-AldrichPC61BM >99 % OssilaPC71BM >99.5 % Nano CDMF 99.8 % Sigma-AldrichIPA ≥99.9 % Sigma-AldrichCB 99.9 % Sigma-AldrichDCB 99.9 % Sigma-AldrichDMSO ≥99.9 % Sigma-AldrichHCl 37 wt% Sigma-AldrichAcetone ≥99.9 % Sigma-AldrichMethanol ≥99.9 % Sigma-Aldrich

In substrate cleaning, substrates arranged in a stack were nitrogen blown andultrasonicated in acetone, IPA and methanol for 30 minutes for each solution. Be-fore immersion into the next solvent the substrates were rinsed with that solvent.After final drying by nitrogen blow the stack with the substrates were stored packedin foil. Before spin coating PEDOT:PSS layers, the substrates were plasma treatedfor 5 minutes in argon using a Plasma PREEN II-862 asher and nitrogen blown be-fore placing on the chuck. Cleaning effect was investigated and microscope imagesof the untreated, ultrasonicated and both ultra- and plasma-treated substrate arepresented in Figure 11. Some larger particles remained still on the surface.

34

(a) (b) (c)

Figure 11: A microscope image of the a) untreated, b) ultrasonicated and c) ultra-and plasma-treated surface on the corner of the glass/ITO interface. The scale bardenotes 100 µm.

Aqueous dispersion of PEDOT:PSS was used as HTM material in every experi-ment. It was mixed by ultrasonication for ca. 15 min before spin coating through a0.8 µm filter on room temperature substrates at 4000 rpm for 60 s with an acceler-ation value of 1000 rpm/s to produce 45 nm thick layers. Deposition and annealingin a Petri dish on a heating plate at 70-125 C for ca. 15 min starting from thelastly coated film was performed in room atmosphere, and usually the substrateswere further annealed in a glove box located vacuum oven at 120 C.

Precursor and PCBM solutions prepared in a nitrogen filled glove box were al-ways kept at 70 C with magnetic stirring overnight before depositions. PCBMlayers were prepared in a glove box and unless otherwise stated, perovskite layerswere spin coated with static dispensing in a nitrogen filled glove box with levels ofO2 and H2O of 0.1 ppm. An acceleration of ≈1000 rpm/s holds for every spin coat-ing process. Used solution of 40 mg/ml MAI in IPA for spin coating was adaptedfrom [42][57][66].

Thermal depositions of 5 nm thick Ca (∼0.8 A/s) and 100-150 nm thick Al (∼3.3 A/s) layers were done using Angstrom Among deposition system. Device per-formances were evaluated using class AAA solar simulator (Oriel Inc.) and BotestLIV organic electronics tester system located in the glove box. Scanning was donefrom negative to positive bias using 1000 W/m2 (1 Sun) radiation conditions cali-brated with NREL calibrated crystalline silicon reference cell. Eclipse LV100DA-U(Nikon, Japan) light microscope was used for surface imaging and Veeco Dektak 8Surface Profiler for film thickness measurements. The most important experimentssummarized in Table 2 are reported next. Additional experiments mean methodstried for single device along with the main experiments.

35

Table 2. Experiments, their corresponding references and results

Experiment Precursors Highest PCE Adapted referencesSingle-step MAI/PbCl2/DMF 0.71% [62]Single-stepHCl-assisted

MAI/PbI2/DMF + 2.5v% HCl - [56][63]

Two-step sequentialHCl-assisted

PbI2/DMF + 2.5 v% HClMAI/IPA

- [63]

Two-stepHCl-assisted

PbI2/DMF + 2.5 v% HClMAI/IPA

3.81% [56][63]

Two-stepmixed-halides

PbI2/PbCl2/DMFMAI/IPA

- [67]

Two-step mixed-halidesHCl-assisted

PbI2/PbCl2/DMF + 1.7 v% HClMAI/IPA

- [56][63][67]

Two stepPbI2/DMFMAI/IPA

- [66]

Additional experiment

Vacuum-annealedPbI2/DMF + 2.5 v% HClMAI/IPA

0.29% [61]

Non-annealed PbI2PbI2/DMF + 2.5 v% HClMAI/IPA

-

CB solvent-annealedETM

PbI2/DMF + 2.5 v% HClMAI/IPA

- [51]

DMSO solvent-annealedPbI2

PbI2/DMFMAI/IPA

- [55]

9.1 Single-step method

Spin-coated PEDOT:PSS layers were annealed at 70 C in room air during deposi-tions and at 120 C in a vacuum oven for 1 h, after which heating was turned offand the substrates were left in vacuum overnight. A 70 C MAI/PbCl2 3:1 molarratio (2.64 mol/l MAI and 0.88 mol/l PbCl2) solution was spin coated with a 0.45µm filter on 90 C substrates at 3000-6000 rpm for 45 s, adapting the reference [62].After 30 min of slow drying in a Petri dish, they were annealed at 90 C for 1.5 hand then at 100 C for 40 min. A syringe was always filled with a hot solution tobe deposited before each coating. A 70 C PC71BM/CB (50 mg/ml) solution wasspin coated with a 0.2 µm filter on the room temperature substrates at 1000 rpmfor 60 s and annealed at 100 C for 5 min. Finally 5 nm thick Ca and 120 nm thickAl layers were thermally deposited.

9.2 HCl-assisted single-step method

Vacuum annealing of PEDOT:PSS layers was done at 120 C overnight and aMAI:PbI2/DMF 1:1 molar ratio (45 wt%) solution containing 2.5 v% HCl (37 wt%)was used for perovskite formation. The experiment was based on [56], which do notreveal the amount of used acid. The concentration of 2.5 v% was taken from [63]and the precursor concentration of 45 wt% was adapted from [64], which is higherthan reported in [56] being appropriate for high rotation speeds up to 6000 rpm.Precursor solution temperature was 70 C and it was spread through a 0.2 µm filteron 70 C PEDOT:PSS coated substrates before spinning at 4000-6500 for 30-40 sand annealing at 100 C for 5-30 min. ETM layers from a 70 C PC71BM/CB (50mg/ml) solution were spin coated with a 0.2 µm filter on a 70 C substrates at 1000rpm for 60 s and annealed at 100 C for 15 min. The substrates were kept in vacuumovernight before thermal deposition of 5 nm thick Ca and 110 nm thick Al layers.

36

9.3 HCl-assisted two-step sequential method

Room temperature PEDOT:PSS coated substrates prepared as previously were spincoated with a 70 C PbI2/DMF (461 mg/ml) precursor solution containing 2.5 v%HCl (37 wt%) without a filter at 1250 rpm for 30 s and annealed at 70 C for 30 min.After cooling to room temperature and immersion in a room temperature MAI/IPA(30 mg/ml) solution for 90 s, the substrates were rinsed with IPA for removingexcess MAI and annealed at 90 C for 60 min. This perovskite formation methodwas taken from [63]. Cooled substrates were spin coated with a 70 C PC71BM/DCB(20 mg/ml) solution through a 0.2 µm filter at 1000-1750 rpm for 35 s. Annealingat 100 C for 15 min was preceded by thermal deposition of a 150 nm thick Al layer.

9.4 HCl-assisted two-step method

9.4.1 First fabrication

A 70 C PbI2/DMF (450 mg/ml) precursor solution containing 2.5 v% HCl (37 wt%)was spin coated on 70 C PEDOT:PSS layers prepared as previously at 6000 rpmfor 40 s with a 0.2 µm filter before annealing at 70 C for 30 min. The experimentwas based on [56][63], but the MAI layers were prepared via spin coating instead ofimmersion. Precursor concentration and spin coating speed were taken from [17].Substrates were transferred rapidly from a heating plate on the coater chuck andspin coated with a 70 C MAI solution (40 mg/ml) with a 0.45 µm filter at 6000 rpmfor 40 s. Syringes in this experiment were always filled with hot precursor solutionsright before each coating. A coated substrate was transferred on the heating plateat 70 C and after transferring each deposited substrate, temperature was raised to100 C for annealing for 2 h. Temperature was adjusted back to 70 C and ETMlayers were spin coated on 70 C substrates with a 70 C PC71BM/CB (50 mg/ml)solution and 0.2 µm filter at 1000-2000 rpm for 35-60 s following annealing at 100 Cfor 15 min. The substrates were kept in vacuum overnight before thermal depositionof 5 nm thick Ca and 110 nm thick Al layers.

9.4.2 MAI annealing variation

In the another experiment annealing conditions of MAI were varied. Concentrationfor PbI2/DMF solution with 2.5 v% HCl was increased into 461 mg/ml as in [56][63]and syringes were filled with 70 C precursor solutions before each coating. Anneal-ing of PEDOT:PSS films was done in room air at 125 C for 15 min starting fromthe last deposited substrate, after which they were rapidly transferred in a glovebox. One substrate was left without PEDOT:PSS and PbI2 · HCl layers were spincoated on room temperature substrates using a 0.2 µm filter at 6000 rpm for 60 sand annealed at 70 C for 30 min, expect one substrate, which was annealed at 100C for 30 min. After cooling to room temperature, the substrates were spin coatedwith a 50 C MAI solution using a 0.2 µm filter at 6000 rpm for 60 s and let try ina glove box for 30 min, which allowed slow solvent evaporation as in [41]. Loadingtime of 60 s for MAI was left for one substrate. The substrates were annealed at 70C for 30 min and one substrate was rinsed with IPA and rotated before annealing.Adapting the reference [61], one substrate was vacuum annealed with heating from

37

room temperature to 70 C and after reaching that value, heating was turned off.Temperature dropped to ca. 58 C after 30 min annealing. ETM layers from a 70C PC61BM/CB (20 mg/ml) solution were spin coated on the room temperaturesubstrates at 1500 rpm for 60 s. Used concentration was taken from [23][57]. Thesubstrates were annealed at 80 C for 5 min, except one, which was solvent annealedwith 3 µl CB in a covered Petri dish at 80 C for 6 min, based on the reference [51].All substrates were kept in vacuum overnight and coated via thermal depositionwith 5 nm thick Ca and 120 nm thick Al.

9.4.3 Open air environment

Acid-assisted two-step process was tried in room air. PEDOT:PSS coated substrateswere annealed at 120 C for 20 min starting from the last deposited film and thenkept at 70 C to prevent absorption of moisture during spin coating layers from a70 C PbI2/DMF/HCl solution. Spin coating was performed to 70 C substrateswith a 0.2 µm filter at 6000 rpm for 60 s following annealing at 70 C for 30 minunder a Petri dish, except for one substrate, which was kept in a Petri dish withoutannealing. A syringe was filled with a hot solution before each coating. MAI wasspin coated from a 70 C solution on 70 C substrates (room temperature for non-annealed) using a 0.2 µm filter at 6000 rpm for 60 s, except one, which was spincoated at 1500 rpm for 60 s. A syringe was kept filled on a heating plate. The filmswere annealed at 100 C for 60 min under a Petri dish and transferred in a glovebox, where PCBM layers were spin coated from a 70 C PC61BM/CB (20 mg/ml)solution on room temperature substrates with a 0.2 µm filter at 1500 rpm for 60 sbefore annealing at 100 C for 5 min. The substrates were kept in vacuum overnightbefore thermal deposition of 5 nm thick Ca and 120 nm thick Al layers.

9.5 Two-step mixed-halides

Chlorine was utilized by adding 33 mol% PbCl2 in PbI2/DMF solution as in [67].Annealing of PEDOT:PSS layers was done at 70 C in room air during the spincoating process and at 120 C in a vacuum oven for 1 h, after which heating wasturned off and the substrates were left in vacuum overnight. A 70 C mixed-halidesolution containing 306 mg/ml PbI2 and 92 mg/ml PbCl2 in DMF was spin coatedon 70 C PEDOT:PSS layers with a 0.45 µm filter at 2000 rpm for 40 s and annealedat 100 C for 60 min. A syringe was filled with a hot solution before each coating. A50 C MAI solution was spin coated through a 0.45 µm filter on 50 C substrates at2000 rpm for 40 s and annealed at 100 C for 60 min to produce CH3NH3PbI3−xClxfilms. A filled syringe was kept on a heater. A 70 C PC61BM/CB (20 mg/ml)solution was spin coated on the room temperature substrates with a 0.2 µm filterat 750-1250 rpm for 60 s and annealed at 100 C for 5 min. Finally 5 nm thick Caand 120 nm thick Al layers were thermally deposited.

38

9.6 HCl-assisted two-step mixed-halides

Annealing of PEDOT:PSS layers was done at 100 C in room air during spin coatingprocess and at 120 C in vacuum oven for 1 h. A similar 70 C PbCl2/PbI2/DMFsolution with 1.7 v% HCl (37 wt%) was spin coated with a 0.2 µm filter on roomtemperature PEDOT:PSS layers at 6000 rpm for 40 s and annealed at 100 C for60 min. A MAI solution at 50 C was spin coated through a 0.2 µm filter onroom temperature substrates at 6000 rpm for 40 s and annealed at 100 C for60 min. Syringes were always filled with hot precursor solutions for each coating.After cooling to room temperature, the substrates were spin coated with 70 CPC61BM/CB (20 mg/ml) solution using a 0.2 µm filter at 750-1250 rpm and annealedat 100 C for 5 min. Finally 5 nm thick Ca and 100 nm thick Al layers were thermallydeposited.

9.7 Two-step method

PEDOT:PSS layers were annealed at 125 C for 15 min starting from the last spin-coated substrate in room air and transferred rapidly in a glove box. Room temper-ature substrates were spin coated with a 70 C PbI2/DMF 461 mg/ml solution at6000 rpm for 60 s and annealed at 70 C for 30 min, except one, which was annealedunder a Petri dish with 50 µl DMSO at 45 C for 5 min following thermal annealingat 100 C for 5 min as in [55]. A 70 C MAI/IPA solution was spin coated on roomtemperature substrates via dynamic dispensing at 1500-3000 rpm for 60 s. Amountof deposited solution was 0.1-0.2 ml dispensed via a pipette or a syringe with a 0.2µm filter. Annealing was done in two steps at 70 C for 10 min and at 100 Cfor 60 min, as in [66]. PCBM layers were prepared from a 70 C PC61BM/CB (20mg/ml) solution via dynamic dispensing at 1500 rpm for 60 s on room temperaturesubstrates and annealed at 100 C for 45 min. Thermal deposition of 5 nm thick Caand 120 nm thick Al layers was done before leaving the cells in vacuum overnight.

39

10 Results

10.1 Single-step method

Perovskite films were prepared using a mixture of CH3NH3I and PbCl2, which is themost commonly used in one-step planar cells [61]. Despite prolonged annealing, thelayers were greyish and contained unconverted yellowish areas. The highest PCEwas 0.71% for the cell spin coated at 6000 rpm. Slow growth rate could be explainedby lattice distortion induced by Cl− during crystallization [62]. Longer annealingat higher temperature is often required in a two-step process for a reaction betweenPbCl2 and MAI compared with that of PbI2 [67]. Better performances could havebeen achieved by preparation in room air as moisture involvement is known to alterthe reaction kinetics for favorable crystal growth in such films [68].

10.2 HCl-assisted single-step method

The single-step processed perovskite layers from a MAI:PbI2/DMF 1:1 molar ratio(45 wt%) solution containing 2.5 v% HCl (37 wt%) were very hazy and the cells didnot work. One device is presented in Figure 12. It should be noted that according tothe reference, annealing for 5 min was done for water removal due to acid additionrather than for perovskite formation [56]. Here, annealing was prolonged to 30 minafter the first substrate, because film darkening was still significant after 5 min.Two-step spin coating on PEDOT:PSS has many previously discussed advantagesand it was utilized in the following experiments.

Figure 12: An ITO/PEDOT:PSS/perovskite/PC71BM/Ca/Al device evice preparedvia an HCl-assisted one-step process. The precursor solution was spin coated at 6000rpm for 40 s.

10.3 HCl-assisted two-step sequential method

A PbI2/DMF (461 mg/ml) precursor solution with 2.5 v% HCl (37 wt%) was triedin a two-step sequential process as in [63]. The prepared devices were light browncolored, non-uniform and non-working. However, prepared PbI2 · HCl layers fromthe first precursor solution seemed very smooth and homogeneous. MAI was decidedto be spin coated in the following experiments without rinsing with IPA. Also, muchhigher spin coating speed for PbI2 layers was chosen.

40

10.4 HCl-assisted two-step method

10.4.1 First fabrication

The PbI2 · HCl layers were smooth and clear as shown in Figure 13a. Formedperovskite layers seemed dark brown and smooth, but prepared cells revealed somecomet streaks. One device is presented in Figure 13b. The highest PCE of 3.81%was attained for the device whose PCBM layer was spin coated at 1000 rpm for60 s. Variations in PCEs of its cells was quite large and although another deviceduring the same batch was similarly prepared, it achieved only 0.33%. Other devicesdid not work. The JV curve of the champion cell is presented in Figure 14, whichindicates relatively large series resistance Rs probably as a result of charge traps inmany grain boundaries between small perovskite grains [22][40]. Low Voc = 0.74 Vis consistent with suspected low-quality interfaces and large defect density in andwithin grains, while low Jsc = 8.97 mA/cm2 can be attributed to poor crystallinityand small grain size [14].

(a) (b)

Figure 13: a) A spin-coated PbI2 ·HCl layer from a PbI2/DMF solution containing2.5 v% HCl (37 wt%). b) An ITO/PEDOT:PSS/perovskite/PC71BM/Ca/Al deviceprepared via an HCl-assisted two-step process. The PCBM layer was spin coated at2000 rpm for 35 s.

-15

0

15

30

45

-0,5 -0,25 0 0,25 0,5 0,75 1

Cu

rren

t d

ensi

ty (

mA

/cm

2)

Voltage (V)

Figure 14: The JV curve of an ITO/PEDOT:PSS/perovskite/PC71BM/Ca/Al cellprepared via an HCl-assisted two-step process. It was generated under forward scandirection. PCE=3.81 %, FF=0.57, Jsc = 8.97 mA/cm2, Voc = 0.74 V.

41

The experiment was repeated using a 0.45 µm filter for both precursor solu-tions and older PC71BM powder as the main differences, but only 0.37% PCE wasachieved. Similar experiment was also done with a 0.2 µm filter for all solutions,new PC61BM powder for PCBM/CB solution and annealing of MAI layers directlyat 100 C for 2 h. Nothing was improved and reason for early efficiency of 3.81%remained unclear. All prepared devices revealed some comet streaks, and despitetheir shiny and dark appearance the cells seemed to have a whitish thin layer ofunknown material on top of annealed perovskite.

10.4.2 MAI annealing variation

Annealing of PbI2 · HCl at 100 C for 30 min was tried for turning the complexinto PbI2 as in [56], but nothing was improved and the highest PCE was 0.71%.Suspecting that whitish layer could have been excess MAI, IPA rinsing was donefor removing such residuals as in [56][63]. Probably this was done too copiouslyremoving too much MAI leading to incomplete conversion into perovskite and thedevice did not work. Lower MAI solution temperature of 50 C was adapted from[67] as a purpose to lower the evaporation rate of IPA for trying to reduce proposedremnant MAI formed by precipitation. Lower annealing temperature of MAI wastried based on the reference [41], which suggested the lowest possible value forcomplete crystallization to still occur as an optimal annealing temperature. Diffusionof MAI was tried to improve by loading time as in [40]. No effect was achieved onPCEs by these treatments. The vacuum-annealed device reached 0.29% efficiencyand the device with solvent annealed PC61BM was very hazy and non-working. Thedevice without PEDOT:PSS also failed to work. The devices are shown in Figure15. Two first upper row devices again show comet streaks which occurred afterspin coating PC61BM. Also, the second device in the bottom row reveals a circularpattern resulting from first contact with MAI.

Figure 15: ITO/PEDOT:PSS/perovskite/PC61BM/Ca/Al devices prepared via anHCl-assisted two-step process. Upper row: Without PEDOT:PSS, PbI2 · HCl an-nealed at 100 C and MAI spin coated with loading time. Bottom row: solvent-annealed PC61BM, vacuum-annealed MAI and IPA-rinsed.

42

10.4.3 Open air environment

Followed HCl-assisted method has been reported to give successful results in am-bient air environment with controlled humidity of ∼60% [56] and in a nitrogenfilled glove box [63]. However, the devices prepared under a fume hood remainedlight colored indicating poor perovskite conversion and only 0.17% efficiency wasachieved. Leaving one device without annealing of the lead halide layer was triedbefore examining the reported two-step sequential experiment [6]. In order to achieveamorphous/polycrystalline domains for improved penetration of MAI, the MAI so-lution should be spin coated right after spin coating a PbI2/DMF solution. Thisway IPA should extract DMF in a wet PbI2 film and a reaction between MAI andPbI2 occurs preferably compared to PbI2 formation, crystallization and aggregation,leaving remarkably reduced PbI2 residues. Here, keeping the deposited PbI2 · HClfilm in a Petri dish for over 30 min probably allowed DMF to dry and there did notremain wet environment for similar improved conversion.

10.5 Two-step mixed-halides

The CH3NH3PbI3−xClx devices prepared using a PbI2/DMF solution with 33 mol%PbCl2 were non-working being hazy with a lot of comet streaks. Estimated layerthicknesses were around 340 nm for perovskite and 150-200 nm for PC61BM. Amicroscope image of comet streaks is shown in Figure 16. The mixed-halide precur-sor solution contained undissolved species, although common ion effect drasticallyimproves solubility of PbCl2 in DMF [69].

Figure 16: Comet streaks on an ITO/PEDOT:PSS/perovskite/PC61BM/Ca/Al cell.Light and dark areas denote aluminum and PC61BM layers, respectively.

10.6 HCl-assisted two-step mixed-halides

For preparing a homogeneous solution trying to diminish comet streaks and enhanceefficiencies, 17 µl/ml (37 wt%) HCl was added in a PbCl2/PbI2/DMF solution.Despite a fully dissolved clear solution, smaller filters and higher spin coating speeds,the cells were non-working containing a lot of comet streaks as in the previousexperiment without HCl. Thickness of the perovskite layers was ca. 360 nm andPC61BM layers were 150-200 nm thick. Since an excessively thick PCBM layerdecreases device performance and 110-140 nm should be sufficient even for relativelyrough perovskite layers to prevent leakage current [47], higher spin coating speed of1500 rpm turned out to be more appropriate for a PC61BM solution (20 mg/ml) forslightly reducing the thickness.

43

10.7 Two-step method

The prepared devices via a conventional two-step process are presented in Figure17. The PbI2/DMF solution contained undissolved material, but with a 0.2 µmfilter similar shiny layers than with acid were achieved. Applying MAI precursor ona rotating substrate eliminated circular patterns that often existed in the previousexperiments, originating from first contact with the solution. IPA washing wasdone for an extra substrate for trying to remove whitish layer on top of annealedperovskite, in contrast to previously tried common rinsing method. Now washingafter crystallization reaction did not wash MAI off detrimentally, but the whitishlayer was not removed indicating it might not be MAI. Annealing of PC61BM at100 C for 45 min was taken from [17] for trying to improve permeation of PC61BMand passivation of charge traps. All devices had less comet streaks but they did notwork.

DMSO solvent annealing for one PbI2 layer was done for preparing a porousPbI2 layer for improving diffusion of MAI. In the process, complexes of DMSO-PbI2exist during low-temperature DMSO annealing, and in the second step thermalannealing pores are generated during the release of DMSO. Porous structure wasreported to result in almost complete conversion into MAPbI3 with larger, cuboid-like grains grown evenly in every directions on mp-TiO2 [55]. Solvent annealingseemed to turn the shiny layer into such a film and it darkened instantly after MAIinsertion. However, the device was non-working and the reason is unclear. Perhapsthe penetration of MAI was not improved sufficiently via dynamic dispensing of aMAI/IPA solution (40 mg/ml), contrary to reported spin coating on mesoporoussurface with loading time using a lower concentrated solution of 8 mg/ml.

Figure 17: ITO/PEDOT:PSS/perovskite/PC61BM/Ca/Al devices prepared via anHCl-assisted two-step process. The DMSO solvent-treated device lies on left on thebottom row.

44

11 Discussion

An unreacted PbI2 hole injection blocking layer might have remained to inhibitoperation in the two step processed inverted cells. It can result from a too slowreaction with MAI as a consequence of used additives [51] or from too rapid crystalgrowth [6][51]. The excessively fast reaction also induces uncontrollable morphologyand is a common challenge especially on organic layers as PEDOT:PSS [51]. Thissituation is assumed to hold also here, as the films were usually relatively dark browncolor right after spin coating. The PbI2 layers might have been also very dense withlarge aggregates, which is the another key factor for poor penetration of MAI [6].As previously stated, MAI diffusion could have been improved by spin coating aMAI solution immediately on wet non-annealed PbI2 films in a glove box. This wayPbI2 layers are not densely crystallized and IPA extracts DMF. Perovskite formationreaction is energetically favorable and almost totally eliminated PbI2 residues havebeen reported in MAPbI3 films prepared via MAI immersion on mp-TiO2 [6]. Here,the prepared PbI2 layers were also relatively thick leading to perovskite thicknessof around 300 nm, which was conservatively estimated from the values reported in[17], and from the thicknesses attained using a mixed-halide solution containing HClwith similar spin coating parameters and lower total concentration. Thinner PbI2layers could be supposed to result in reduced thickness of the possible remnant layer.Too rapid crystallization and dense PbI2 films probably explain poor performanceof the cells prepared with and without HCl.

Another probable explanation for poor performance in both single- and two-step processed cells is the detrimental effect of grain boundaries [40][67], whichcontributes to high series resistances and thus low fill factors [40]. The perovskitelayers were relatively thick and at small grain size there must have been a lot ofgrain boundaries. In HCl-assisted two-step experiments, that is in agreement withreported crystal size to 1 nm and less [63]. The impact of such boundaries couldhave been decreased by reducing the film thickness using lower precursor concentra-tions. Considerable charge recombination could have occurred particularly in thickCH3NH3PbI3 films without improved charge transporting properties by doped Cl[46][62]. Also, crystal size could have been controlled by varying the concentrationof a MAI solution [40]. Moreover, too long annealing of perovskite layer in totalmay have induced defects at surfaces and grain boundaries, which are inferred asmost sensitive to thermal decomposition [17].

This study broadened the understanding about preparing perovskite cells in theOPEM laboratory, although prepared devices did not reach similar performances asin adapted references. The unit did not have earlier studies focusing on perovskitecells, and afterward more efficient devices with improved conversion into perovskitehave been prepared.

45

12 Conclusions

Most probable explanations for poor efficiencies despite dark and shiny appearanceof the absorber layers are existence of PbI2 residues and many grain boundaries.Appropriate thin-film characterization methods could have given information aboutthe quality of perovskite material especially for evaluating charge traps and remnantnon-conductive lead halides, whose excess amount can efficiently destroy the cellperformance. This study offers and example about difficulty of perovskite solarcells’ fabrication and it extended the knowledge about perovskite formation in theOPEM unit in the University of Oulu.

46

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