Wet-milled anatase titanium oxide nanoparticles as a ...

RESEARCH ARTICLE

Wet-milled anatase titanium oxide nanoparticles as abuffer layer for air-stable bulk heterojunction solar cellsJen-Hsien Huang1, Mohammed Aziz Ibrahem1,2,3 and Chih-Wei Chu1,4*1 Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan 1152 Department of Physics, National Taiwan University, Taipei, Taiwan 106173 Nanoscience and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei, Taiwan 1154 Department of Photonics, National Chiao-Tung University, Hsinchu, Taiwan 300

ABSTRACT

In this study, an air-stable bulk heterojunction organic solar cell demonstrated by utilization of titanium oxide (TiO2)nanoparticles as a hole blocking layer was prepared through high-energy grinding method. The large clumps of the anataseTiO2 underwent deaggregation to form a stable dispersed solution during the grinding process. The resultant suspensioncan form a uniform and smooth TiO2 film through spin coating on various substrates. Because of substantial oxygen andwater protection effect of TiO2 thin film, the bulk heterojunction solar cells exhibit a significant long-term stability. It isalso found that the cell performance can be promoted dramatically after ultraviolet activation. The mechanism responsiblefor the enhanced cell efficiency was also investigated. This solution-based method does not require surfactants, thus pre-serving the intrinsic electronic and optical properties of TiO2 that makes these proposed buffer layers quite attractive fornext-generation flexible devices appealing high conductivity and transparency. Copyright © 2014 John Wiley & Sons, Ltd.

KEYWORDS

solar cells; solution process; TiO2; electron; buffer layer; stability

*Correspondence

Chih-Wei Chu, Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan 115.E-mail: [email protected]

Received 5 January 2012; Revised 3 July 2013; Accepted 1 May 2014

1. INTRODUCTION

The development of organic solar cells, with their flexibility,ready availability of raw materials, and easy processing, is apromising technology for the future provision of cost-effective energy [1–3]. At present, organic solar cells facetwo major limitations affecting their commercialization:low conversion efficiencies and poor long-term environmen-tal stability. Several solutions have been proposed to addressthese issues, including the use of low band gap organic andpolymeric compounds as the active layer (to improve theconversion efficiency) [4–6] and the fabrication of invertedsolar cells and metal/organic interface engineering (to ensureimproved stability) [7–12]. Recently, power conversionefficiencies (PCEs) of over 9% have been reported [13],close to the benchmark PCEs of 10% or higher predicted ifsuitable low band gap donor materials could be designedand implemented [14]. On the basis of these significantadvances, it is likely that the 10% hurdle will be overcomesoon. In the meantime, the issues of production cost and sta-bility must also be solved.

A good interface between the active layer and the elec-trode requires the establishment of ohmic contacts for effi-cient charge extraction. An interface with a barrier heightof a few tens of millivolts may result in significant chargeaccumulation and, therefore, significant recombination lossand inferior photovoltaic performance. Transition metaloxides, such as titanium oxide (TiOx) [7,15–19] and zincoxide (ZnO) [20,21], and alkali carbonates [22,23] are effi-cient materials for use as electron extraction and holeblocking layers in the fabrication of solar cells withinverted architecture. Among these candidates, TiOx is aparticularly promising material because of its high electronmobility, environmental stability, high transparency acrossthe visible spectrum, and ease of preparation. Indeed,inserting a TiOx interfacial layer can extend the lifetimeof a bulk heterojunction (BHJ) solar cell; furthermore, thethin layer of TiOx can also act as an optical spacer to redis-tribute the light intensity within the BHJ active layer,thereby enhancing the harvesting of photons [24]. TiOx

is, however, usually prepared through sol–gel processing

PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONSProg. Photovolt: Res. Appl. (2014)

Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2522

Copyright © 2014 John Wiley & Sons, Ltd.

involving a complex mixture of chemically bound alkoxidegroups. Under illumination, these bound organic moietieswould undergo photooxidation, because of the TiOx

functioning as a photocatalyst. These reactions lead to theproduction of CO2 and H2O, which can damage theinterface between the active layer and the TiOx layer[25]. Although these organic moieties can be removeddirectly through annealing, the high temperatures requiredcan lead to the diffusion of metal atoms (Ti, In, and Sn)between the TiOx and indium tin oxide (ITO) thin films,resulting in altered optical and electronic properties [26].Moreover, the need for an annealing procedure increasesthe production cost and disqualifies the materials frombeing used in plastic electronics.

To overcome these problems, in this study, the wetgrinding method is used to disperse pure TiO2 nano-particles (NPs), having the anatase phase, and prior to theformation of TiOx films. During the grinding process, largeclumps of the TiO2 (titanium dioxide) NPs underwentdeaggregation to form a stable dispersed solution. Usingspin coating, the anatase TiO2 dispersions were success-fully deposited onto a variety of substrates. Because thissolution-based method does not require any surfactants, itpreserves the intrinsic electronic properties of the TiOx

phase. This clean, solution-processable, low-temperaturemethod provided high-quality TiO2 films without the needfor any organic compounds. Applying these pure TiO2 thinfilms as electron extraction and hole blocking layers signif-icantly enhances the long-term stability of BHJ solar cells.

2. EXPERIMENTAL

2.1. Materials

High-purity anatase TiO2 powder (99.7%, Nanostructuredand Amorphous Materials, Inc., Los Alamos, New Mexico,USA) and isopropyl alcohol were used as the starting mate-rial and solvent, respectively. High-energy ball milling wasperformed at a speed of 2000 rpm at room temperature usinga batch-type grinder (JBM-B035, Just Nanotech Co., Ltd,Taiwan). The milling duration was typically between 30and 360min; the concentration of TiO2 was 0.1wt%. Afterhigh-energy grinding, the TiO2 suspensions can be dilutedto any concentration without precipitation.

2.2. Device fabrication

The fabrication of the devices was initiated by cleaning theITO-coated glass and then exposing the substrates to ultravio-let (UV) ozone for 15min. For the preparation of invertedcells, a TiO2 solution having a concentration of 0.01wt%was spun onto the ITO substrates, which were subsequentlydried at 120 °C for 1 h. Next, a “slow-grown” layer of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acidmethyl ester (PCBM) (1 : 1, w/w; 2% in dichlorobenzene)was cast upon the TiO2 buffer layer [27]. Finally, thermalevaporation of Al and V2O5 provided the reflective anodes.

For the preparation of conventional cells, the device structureincluded a thin PEDOT:PSS buffer layer, P3HT:PCBM, anda layer of TiO2. Prior to coating with TiO2, the active layerswere treated with oxygen plasma for 5 s to form a temporarilyhydrophilic surface. Finally, thermal evaporation of Alprovided the reflective cathodes.

2.3. Characterization

Particle sizes and zeta potentials were measured using aparticle size analyzer (Brookhaven 90Plus Sn11408, LongIsland, NY, USA). X-ray diffraction (XRD) studies wereperformed using a Philips X’Pert/MPD (Amsterdam, Dutch)apparatus. The cell performance was tested under simulatedAM1.5G irradiation at 100mWcm–2 using a Xe lamp-basedsolar simulator (Thermal Oriel 1000W, Newport, Wales,UK). The light intensity was calibrated using a monosiliconphotodiode equipped with a Hamamatsu KG-5 (Bunkoh-keikiCo Ltd, Japan) color filter. The whole measurement processwas performed at room temperature in a N2-filled glove box.The surface morphologies of the polymer films were investi-gated using atomic force microscopy (AFM) (Digital Instru-ment NS 3a controller equipped with a D3100 stage,Novascan Technologies, Inc., Ames, USA States) and scan-ning electron microscopy (Hitachi S-4700, Japan).

3. RESULTS AND DISCUSSION

Figure 1(A) displays the average particle sizes of the TiO2

powder as a function of the grinding time. The averageparticle size of TiO2 decreased rapidly upon grinding forup to 180min. The TiO2 particles deaggregated and brokeinto smaller particles as a result of plastic deformation,leading consequently to smaller particles. The inset toFigure 1(A) provides images of the TiO2 solutions aftervarious grinding times. These images were recorded fromthe as-prepared solutions after standing for 72 h. Aftergrinding for 240min, the TiO2 was fully dispersed,suggesting that the smaller particles were more highlycharged and, therefore, possessed enhanced dispersibility.To investigate the correlation between the surface chargeof the NPs and the grinding time, zeta potential measuredwith respect to the pH of the TiO2 solution and thegrinding time (Figure 1(B)). The isoelectric point of theTiO2 solution was a function of the particle size (grindingtime). When grinding time increased from 30 to 360min,the isoelectric point decreased from 8.2 to 7.0. Moreover,the surface charge of the TiO2 NPs also increased signifi-cantly after longer grinding times, indicating that repulsiveforces increased, leading to more dispersed TiO2 solutions.

Figure 2 presents representative transmission electronmicroscopy images of the TiO2 NPs before and aftergrinding for 6 h. Notably, the average size of the TiO2

NPs decreased significantly, from 30 to 8 nm, and theTiO2 NPs were highly dispersed, without aggregation,after grinding. The crystallinity of the TiO2 powdersexamined, by using XRD (Figure 2(C)). The intensities

Titanium oxide nanoparticles as a buffer layer for organic solar cells J.-H. Huang, M. A. Ibrahem and C.-W. Chu

Prog. Photovolt: Res. Appl. (2014) © 2014 John Wiley & Sons, Ltd.DOI: 10.1002/pip

of the XRD peaks corresponding to the initial powderdecreased, and peak broadening occurred, after grinding.These features are consistent with decreases in the grainsize and lattice strain induced by high-energy grinding.

The morphologies of TiO2 thin films cast onto ITO andP3HT:PCBM surfaces characterized by using AFM. Thefilm treated with UV ozone and O2 plasma for 15minand 5 s, respectively, prior to spin coating a layer ofTiO2. The bare ITO (Figure 3(A)) and P3HT:PCBM film(Figure 3(B)) featured many grain boundaries and chain-like features running through their surfaces. After the spincoating of TiO2, the surface coverage of TiO2 was veryhigh, as evidenced by the even surface morphologies with

nanoscale roughness. Moreover, the average grain size inthe TiO2 films was approximately 20 nm, suggesting thatthe TiO2 films lacked severe aggregation and were formedfrom isolated TiO2 NPs. In the fabrication of an optoelec-tronic device, excessive surface roughness would beproblematic because the rough features might protrudethrough the active layers and, thereby, result in shorting.As a result, uniform and smooth TiO2 films are preferredfor integration in BHJ solar cells. Scanning electronmicroscopy and large-scale AFM images are provided inthe Supporting Information.

Next, the stable TiO2 suspensions applied to thefabricate P3HT:PCBM-based solar cells with inverted

Figure 2. (A) and (B) Transmission electron microscopy images of the TiO2 powder (A) before and (B) after grinding for 360min. Aftergrinding, the particle size decreased significantly, allowing the TiO2 to disperse well without aggregation. (C) X-ray diffraction patternsof the TiO2 powder before and after grinding. The lower crystallinity of the TiO2 powder after grinding was caused by the decrease in

particle size and the lattice strain.

Figure 1. Plots of the (A) average particle size in the TiO2 powder with respect to the grinding time (inset: photograph of the TiO2

solutions obtained after different grinding times) and (B) zeta potentials of the TiO2 solutions with respect to pH.

Titanium oxide nanoparticles as a buffer layer for organic solar cellsJ.-H. Huang, M. A. Ibrahem and C.-W. Chu


[ITO/TiO2/P3HT:PCBM/V2O5/Al] and conventional[ITO/PEDOT:PSS/P3HT:PCBM/TiO2/Al] architectures;Figure 4(A) and (B), respectively, displays their perfor-mance. The as-prepared devices exhibited poor perfor-mance, with S-shaped J–V curves. Commonly, S-kinksare attributed to barriers at the contacts. Some simulationdata in the literature indicate that low charge carrier mobil-ities could also cause S-kinks. For very low mobilities, aspace charge is built up in the complete device, becauseelectrons and holes cannot be extracted sufficiently fast.Their concentration at the D/A interface is increased andhence recombination probability as well, which leads to alower fill factor (FF) [28]. Interestingly, the performancesof both the inverted and conventional devices wereenhanced significantly through UV illumination. Uponincreasing the irradiation time, a gradual enhancement in theshort-circuit current (JSC), open-circuit voltage (VOC), andFF can be monitored. After UV illumination for 150min, thePCEs of the inverted and conventional devices—initially(without UV activation) 0.11% and 0.13%, respectively—increased to 3.07% and 3.7%, respectively. Because thepresence of a metal oxide as the interfacial layer can enhancethe long-term stability of BHJ solar cells, we measured(Figure 4(C)) the change in solar cell efficiency with respect

to the exposure time for devices possessing various cellstructures. Figure 4(D) presents the corresponding J–V curvesfor the devices before and after performing durabilitytests. To investigate the durability of the BHJ solarcells, four kinds of devices were fabricated: [ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al], [ITO/PEDOT:PSS/P3HT:PCBM/TiO2/Al], [ITO/Cs2CO3/P3HT:PCBM/V2O5/Al], and [ITO/TiO2/P3HT:PCBM/V2O5/Al], designated as devices I–IV,respectively. The data in Figure 4(C) and (D) clearly revealthat both the inverted and conventional solar cells exhibitedenhanced durability relative to that of their Ca and Cs2CO3

counterparts, which may be attributed to TiO2 film substantialoxygen andwater protection effect because of the combinationof photocatalysis and inherent oxygen deficiency. In addition,the rapid photoresponse of the treated cell was attributed to theformation of hydrogen bonds between adsorbed watermolecules and carbonyl oxygen atoms in PCBM close to theTiOx surface. When the TiOx surface was positively chargedby UV-induced holes, the carbonyl oxygen in PCBM closeto the TiOx surface can quickly join to the TiOx surface,rapidly transporting photogenerated electrons from PCBM toTiOx in competition with the photocatalyzed degradation [29].

To better understand the mechanism responsible for theenhanced cell efficiency after activation with UV light, a

Figure 3. Surface morphologies of (A) the bare ITO substrate, (B) TiO2 cast on the ITO substrate, (C) the P3HT:PCBM surface, and (D)TiO2 cast on the P3HT:PCBM surface. Scale bar: 400 nm. The P3HT:PCBM surface had been treated with oxygen plasma for 5 s, lead-ing to a temporarily hydrophilic surface, prior to spin coating of the TiO2 solution. The surface coverage of TiO2 was very high and free

of severe aggregation.



systematic analysis of the devices and TiO2 layers beforeand after UV treatment was performed. Figure 5(A) dis-plays the dark current of the inverted solar cell [ITO/TiO2/P3HT:PCBM/V2O5/Al]. In the absence of UVillumination, a very weak current response was obtained,indicating that the device was insulated after insertion ofthe TiO2 layer. After light activation of the TiO2 film,however, a typical diode behavior in the dark wasobserved, revealing a change in the electronic propertiesof the TiO2 film. A similar result was reported previouslyby Yang et al. [30,31] Ultraviolet photoelectron spec-troscopy was used to determine the influence of lightactivation on the energy band of TiO2. The upper andlower emission onset energies for the TiO2 film werealmost identical, regardless of whether the TiO2 layer hadbeen illuminated with UV light or not. This phenomenonsuggests that the valence band of TiO2 was unchangedand that the enhanced cell performance was not a resultof better interfacial energy alignment between the TiO2

film and the active layer. To further scrutinize theelectronic properties of the TiO2 film after light activation,another device having the structure [ITO/TiO2/Al] wasconstructed. To avoid device shorting, in this case, amultilayered TiO2 film having a thickness of approxi-mately 100 nm has been used. The dark current of the

device increased slightly after AM1.5 illumination and sig-nificantly after UV illumination, relative to that obtainedwithout illumination. This behavior, which is commonfor TiO2 structures, is usually attributed to photodopingof the TiO2 [32]. UV light with energy greater than theTiO2 band gap (~3.2 eV) can be absorbed, leading tophotogenerated charge carriers, thereby increasing theTiO2 conductivity. Of the total AM1.5 irradiation, approx-imately 5% is within the UV range, explaining why thecurrent response increased slightly under AM1.5 illumina-tion. To calculate the electron mobility in the TiO2 film, thespace-charge-limited current of the device under variousconditions was measured. As indicated in Figure 5(D),the calculated electron mobilities of TiO2 under darkconditions and after AM1.5 and UV illumination were2.4 × 10–5, 3.1 × 10–5, and 1.8 × 10–4 cm2V–1 s–1, respec-tively. Thus, although the interfacial energy alignmentbetween the TiO2 film and the active layer remained almostunchanged, the electron mobility in the TiO2 film wassignificantly enhanced after UV activation. Therefore, wepropose that the enhanced cell performance observed inFigure 4 resulted entirely from the higher charge carrierdensity induced by photodoping, leading to improvedcharge transport within the TiO2 layer. Previous work hasshown that the UV–ozone treatment can tune the oxygen

Figure 4. (A) and (B) Cell performances of the (A) inverted [ITO/TiO2/P3HT:PCBM/V2O5/Al] and (B) conventional [ITO/PEDOT:PSS/P3HT:PCBM/TiO2/Al] devices, recorded under various ultraviolet (UV) illumination times. (C) Durability tests of bulk heterojunction solarcells possessing various structures. (D) J–V curves for the inverted device [ITO/TiO2/P3HT:PCBM/V2O5/Al], recorded before and after

the durability test.



concentration but not alter the TiO2 NP size, shape, ordistribution of the nanoclusters in the films [33]. TiO2 iswell known as a nonstoichiometric compound, which hasbeen generally considered as an oxygen-deficient com-pound, TiO2�x. Atomic concentrations of oxygen andtitanium also play an important role in the photovoltaicperformance of the TiO2-based inverted PSCs because ofthe inherent oxygen deficiency in which the dominantdefects in TiO2 are oxygen vacancies, which will signifi-cantly affect its conductivity. Trap filling in the TiO2 filmby photogenerated charges leads to increased photocon-ductivity [31,34].

Furthermore, utilization of TiO2 NPs thin film as holeblocking layer investigated with other type of polymer(thiophene (ADS2008P)(F8T2)) : fullerene combinationin conventional device structure [ITO/PEDOT:PSS/F8T2:PCBM/TiO2/Al] (PCE = 2.71%, JSC = 5.73mA/cm2, VOC = 0.91V, and FF = 51.7) (Figure S3) structurein order to show the versatility of our finding. Thedevices performance with this polymer is comparablewith devices incorporating PC70BM in the bilayer struc-ture and higher than the devices with PC60BM [35–38].However, device structure without buffer layers (control

device) shows very low cell performance parameterscompared with devices featuring buffer layers, which ismay be correlated to the suppression of energy losses,such as low contact resistance, electron doping, andreduction of exciton quenching at the metal/buffer inter-faces [18,21,23].

4. CONCLUSIONS

In conclusion, a high-energy wet grinding method wassuccessfully employed to prepare stable TiO2 suspensionsfor use in the fabrication of BHJ solar cells. This tech-nology is facile, inexpensive, scalable, clean, and compatiblewith flexible substrates. It requires no vacuum processing, arelatively low annealing temperature, and removes the needfor PEDOT:PSS Upon increasing the grinding time, theTiO2 powder was transformed into smaller particles, andthe zeta potential increased, leading to improved disper-sibility. Corresponding solution-processed TiO2 interlayerseffectively block holes and collect electrons from PCBM,in addition to significant enhancement in the durability ofP3HT:PCBM-based solar cells. The dispersed TiO2

Figure 5. (A) Plots of the dark current of the inverted cell [ITO/TiO2/P3HT:PCBM/V2O5/Al] with and without ultraviolet (UV) activation.In the absence of UV activation, the inserted layer of TiO2 was an insulator that blocked charge transport. (B) Ultraviolet photoelectronspectroscopy spectra of the TiO2 films before and after UV activation. The upper and lower emission onset energies for the TiO2 filmswere almost identical, suggesting the valence band of TiO2 was unchanged. (C) J–V curves of TiO2 films under various conditions. Thehigher current density under UV illumination resulted from the photodoping effect. (D) Space-charge-limited currents of TiO2 films

recorded under various conditions.



solution was readily deposited onto various substratesand, thereafter, integrated into organic optoelectronics.Such TiO2 suspension would also provide appropriateinterconnection layers for organic tandem cells.

ACKNOWLEDGEMENTS

We thank the National Science Council (NSC), Taiwan,(NSC 100-2221-E-001-009-) and the Thematic Projectof Academia Sinica, Taiwan, (AS-100-TP-A05) forfinancial support.

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SUPPORTING INFORMATION

Additional supporting information may be found in theonline version of this article at the publisher's web-site.



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