Solar Cells: An Overview Onkar S. Game Senior Research Fellow, National Chemical Laboratory, Pune.
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Transcript of Solar Cells: An Overview Onkar S. Game Senior Research Fellow, National Chemical Laboratory, Pune.
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!!!