Solar Cells: An Overview

Onkar S. Game Senior Research Fellow,

National Chemical Laboratory, Pune.

Outline

• Introduction: Need for harnessing solar energy

• Historical development of modern photovoltaic effect: Example of p-n junction

• Thin Film Solar Cells: Examples • Modern Solar Cells: Nanotechnology and

Polymers• Current Status and Future Prospective

* Present : 12.8 TW 2050 : 28-35 TW

* Needs at least 16 TW Bio : 2 TW Wind : 2 TW Atomic : 8 TW (8000 power plant) Fossil : 2 TW

* Solar: 160,000 TW

Sun: An ultimate source of energy

If you want money and fame (and if you are not excellent at acting or sports) develop an efficient Solar Cell!!!

Task: Creating free electrons using photons

Semiconductors offer solution: Converting incoming photons into electron-hole pairs but creation of electron hole pair competes with electron-hole recombination!!! (which takes place within microseconds)

Modern Solar Cell Technology: 1954• In the early 1950s R.S. Ohl

discovered that sunlight striking a wafer of silicon would produce unexpectedly large numbers of free electrons.

• 1954 - The multidisciplinary research team at Bell Labs of Gerald Pearson, Calvin Fuller and Daryl Chapin, physicist, chemist and electrical engineer, respectively, announce the creation of the first practical solar cell made of silicon, known as the Bell Solar Battery. These cells had about 6% efficiency.

This revolution may mark the beginning of a new era, leading eventually to the realization of one of mankind’s most cherished dreams—the harnessing of the almost limitless energy of the sun for the uses of civilization.- New York Times 1954.

Silicon Solar Cell Schematic

Why thickness of p type and n type semiconductor layers are different?

Working of Si p-n junction solar cell

Processes:• Absorption of incoming photons (Ephoton ≥ Band Gap) and creation of free electron-hole pair. (Note: The absorption process has to dominant near junction)• Separation of electron hole pairs in presence of internal potential (junction potential). • Vectorial transport of electrons and holes in opposite direction.

IL

JunctionRshunt

Rseries

External Load

Equivalent Circuit

Parameters that characterize solar cell IV curve

• Voc: Open Circuit Voltage

• Isc : Short Circuit Current

• Pmax: Maximum Power Delivered

• Vm: Voltage corresponding to Pmax

• Im: Current corresponding to Pmax

• FF (Fill Factor):

• Efficiency =

• Series Resistance: (dI/dv)-1 at Voc

• Shunt Resistance: (dI/dv)-1 at Isc

%100Im

scoc

m

IV

VFF

%max

FFP

JV

P

P

in

scoc

in

0.0 0.2 0.4 0.6 0.80.0000

0.0005

0.0010

0.0015

0.0020

0.0025

Im

Vm

Pmax

Voc

Isc

Cu

rren

t (A

)

Voltage (V)

Factors Affecting Various Parameters in Solar Cell IV curve

• Voc: Depends on difference between the fermi energy of p and n type

semiconductor or semiconductor band gap. Ideal limit = Egap/q

• Jsc or Isc : Absorption properties of semiconductor i.e. band gap and

recombination rate of electron-hole pairs.

• Series Resistance: Depends on ohmic losses at front contact (n type semiconductor and metal). Ideally = 0

• Shunt Resistance: Depends on leakage current within solar cell. Ideally = ∞

• FF (Fill Factor): Depends on values of series and shunt resistance. Ideally = 100. i.e. The IV loop should look as ‘rectangular’ as possible.

• Efficiency: Depends on Voc, Isc and Fill Factor.

Solar Simulator

Solar Cell IV Measurement in Lab

0.0 0.2 0.4 0.6 0.80.0000

0.0005

0.0010

0.0015

0.0020

0.0025

Im

Vm

Pmax

Voc

Isc

Cu

rren

t (A

)

Voltage (V)

Quantum Efficiency Set up

Current Status of Si Solar Cells

Factors Limiting Efficiencies:

Alternative Thin Film Technologies

Disadvantages of Thin Film Solar Cell Technology:• Large scale production is difficult because of sophisticated fabrication techniques. Hence Expensive• Presence of rare elements viz. Indium, Gallium further adds to cost.•Presence of some toxic elements viz. Cadmium may create environmental hazards

Cost Comparison of Various Photovoltaics

Nanotechnology: Towards low cost solar cells

Pre-requisite concepts

• Transparent Conducting Oxide: Eg ≥ 3 eV e.g. ZnO, TiO2, SnO2 etc.

• Molecular Levels: a) HOMO: Highest Occupied Molecular Orbitalb) LUMO: Lowest Unoccupied Molecular Orbital

Dye Sensitized Solar Cells (DSSC)

Iodide/tri-iodide electrolyte

e-

LOAD

Dye/QD

TiO2 (~ 20 nm)

e-

LOAD

Dye/QD

TiO2 (~ 20 nm)

Excitation of dye molecule or Quantum Dot (QD) by incident sunlight

Transfer of electron from dye/QD to TiO2

Regeneration of oxidized dye/QD using a hole carrying electrolyte

Transport of electron through TiO2 and external load

Regeneration of electrolyte at counter electrode

Prof. Michael Gratzel

Excitation of dye molecule or Quantum Dot (QD) by incident sunlight

Transfer of electron from dye/QD to TiO2

Regeneration of oxidized dye/QD using a hole carrying electrolyte

Transport of electron through TiO2 and external load

Regeneration of electrolyte at counter electrode

Cross-sectional SEM of DSSC(counter-electrode and electrolyte missing)

Development of Dyes with broad visible light absorption is current area of research !!!

….continued

Iodide/tri-iodide electrolyte

e-

LOAD

Dye/QD

TiO2 (~ 20 nm)

e-

LOAD

Dye/QD

TiO2 (~ 20 nm)

Why Nanoparticles?: Higher Surface area than what is projected. Higher dye adsorption leads to higher photocurrentWhy ZnO or TiO2?: Light absorption and electron transport are separated. Why liquid electrolyte: Porous nature of TiO2 Film needs better percolation of hole conducting species throughout the filmWhy Platinum nanodot coated Fluorine doped Tin Oxide: To catalyze the I3

-

reduction at counter electrode.Why Fluorine doped Tin Oxide as Bottom electrode? FTO is a transparent conducting oxide hence it allows light to pass through it and it is conducting.

Nanostructured Metal Oxides For DSSC

Cu2O Nanoneedles

ZnO Flowers ZnO Nanorods Rutile TiO2 Needles TiO2-Nanotubes

TiO2-Nanoleaves TiO2-Nanofibers Cu2O nano Spheres

Cu2O nano Cubes TiO2 Spheres TiO2-Nanowires ZnO CNT composite

Sensitizers• Dyes: Ruthenium based synthetic dyes

Dyes extracted from natural resources: (e.g. Anthocyanidins extracted from grapes)

• Quantum Dots:Inorganic Quantum Dots viz. CdS, CdSe, PbS, PbSe etc.

DSSC Fabrication protocol

0.0 0.2 0.4 0.6 0.80

2

4

6

8

10

12

14

Cu

rren

t D

en

sit

y (m

A/c

m2 )

Voltage(V)

TiCl4 Treated Film

Area 0.25cm2

400 500 600 700 800

0

10

20

30

40

50

QE ~ 43%

QE

(%

)

Wavelength (nm)

Name Voc (V)

Jsc (mA/cm2)

FF (%)

η (%)

Sol-Gel TiO2 0.76 12.5 60 5.7

Transparent coatings for DSSC Transparency a critical issue to avoid loss of incident radiation

due to reflection at nanoparticle/TCO interface.

200 300 400 500 600 700 8000

10

20

30

40

50

60

70

%R

Wavelength (nm)

Opaque Film Transparent Film

Without Dye With Dye

Carbon based Nano-Materials for DSSCs

ZnO CNT composite

100nm100nm

TiO2-MWCNT TiO2-Graphene

Eff. 7.4% Eff. 6%

Some Results:

0.0 0.2 0.4 0.6 0.80.000

0.001

0.002

0.003

0.004

0.005

Cu

rren

t (A

)

Voltage (V)

Transparent TiO2 20nm + HS

1st Film 2nd Film

Name Voc (V)

Isc (A) FF (%) η (%)

1st 0.76 0.0044

54.51 7.26

2nd 0.74 0.0043

56.51 7.23

Efficiency Over 7%

Various Experimental Techniques Used to Characterize DSSC

• IV measurement under Solar Simulator• Wavelength Dependant IV measurement: IPCE

Setup or Quantum Efficiency Setup• Electrochemical Impedance Spectroscopy: To

determine time dynamics in DSSC upto microsecond scale

• Transient pump-probe measurement setup: To determine time dynamics in DSSC on nanosecond and picosencond time scale

Current Status of DSSC

• Highest Efficiency on small area test cells: 11.3%. Further increase is a challenge.

• Highest efficiency on modules: 9.2%• Issues related to use of liquid electrolyte and

its evaporation. Development of solid state electrolytes.

• Development of dyes with enhanced visible light absorption.

Organic Solar Cells

New Types of Solar Cells

n-type semiconductor

p-type semiconductor

h+

e–

ECB

EVB

Inorganic cells Hybrid solar cells

Electron acceptor

Hole acceptor

CathodeAnode

HOMO

HOMO

LUMO

LUMO

h+

e–

e–

Organic cells

n-type semiconductor

P-type materials

CathodeAnode

e–

HOMO

HOMO

LUMO

LUMO

h+

e–

Fast carriers mobilityLong life timeHigh production costBrittle

Low Production CostFlexibleTunable colorLight weightSlow carrier mobilityShort life time

h+

ETA CellDye-sensitized Solar Cells

Inorganic n + Organic p

Example of a organic-inorganic hybrid solar cell

Nano p-n junction solar cells

Coaxial silicon nanowires as solar cells and nanoelectronic power sources NATURE, 449, 885, 2007

Thank You!!!