Transcript Onkar Game on solar cells

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
Sun: An ultimate source of energy
* 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
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.
Equivalent Circuit
Rseries
Junction
IL
Rshunt
External Load
Parameters that characterize solar cell IV
curve
0.0025
• Isc : Short Circuit Current
0.0020
• Pmax: Maximum Power Delivered
• Vm: Voltage corresponding to Pmax
• Im: Current corresponding to Pmax
Current (A)
• Voc: Open Circuit Voltage
Isc
Im
0.0015
0.0010
Pmax
0.0005
• FF (Fill Factor):
• Efficiency =
FF 
Vm  Im
 100 %
Voc  Isc
P max Voc  Jsc

 FF %
Pin
Pin
• Series Resistance: (dI/dv)-1 at Voc
• Shunt Resistance: (dI/dv)-1 at Isc
0.0000
0.0
Vm
0.2
0.4
Voltage (V)
0.6
Voc
0.8
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 Cell IV Measurement in Lab
Solar Simulator
Current (A)
0.0025
0.0020
Isc
Im
0.0015
0.0010
Pmax
0.0005
0.0000
0.0
Vm
0.2
0.4
Voltage (V)
0.6
Voc
0.8
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 Orbital
b) LUMO: Lowest Unoccupied Molecular Orbital
Dye Sensitized Solar Cells (DSSC)
Dye/QD
Dye/QD
TiO2
nm)
TiO (~(~2020
nm)
2
e
-
Iodide/tri-iodide
electrolyte
Prof. Michael Gratzel
LOAD
LOAD
 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)
 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
Development of
Dyes with broad
visible light
absorption is
current area of
research !!!
….continued
Dye/QD
Dye/QD
TiO2
nm)
TiO (~(~2020
nm)
2
e
-
Iodide/tri-iodide
electrolyte
LOAD
LOAD
Why Nanoparticles?: Higher Surface area than what is projected. Higher dye
adsorption leads to higher photocurrent
Why 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 film
Why Platinum nanodot coated Fluorine doped Tin Oxide: To catalyze the I3reduction 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
ZnO Flowers
ZnO Nanorods
Rutile TiO2 Needles
TiO2-Nanotubes
Cu2O Nanoneedles
TiO2-Nanoleaves
TiO2-Nanofibers
Cu2O nano Spheres
Cu2O nano Cubes
TiO2 Spheres
TiO2-Nanowires
ZnO CNT composite
Sensitizers
• Dyes:
• Quantum Dots:
Ruthenium based synthetic dyes
Inorganic Quantum Dots viz. CdS,
CdSe, PbS, PbSe etc.
Dyes extracted from natural
resources: (e.g. Anthocyanidins
extracted from grapes)
DSSC Fabrication protocol
2
Current Density (mA/cm )
14
12
10
8
6
4
TiCl4 Treated Film
2
Area 0.25cm
0
0.0
0.2
2
0.4
0.6
0.8
Voltage(V)
50
QE ~ 43%
40
Name
Sol-Gel TiO2
Voc
(V)
Jsc
(mA/cm2)
FF
(%)
η
(%)
0.76
12.5
60
5.7
QE (%)
30
20
10
0
400
500
600
Wavelength (nm)
700
800
Transparent coatings for DSSC
 Transparency a critical issue to avoid loss of incident radiation
due to reflection at nanoparticle/TCO interface.
70
60
%R
50
40
Opaque Film
Transparent Film
30
20
10
0
200
Without Dye
300
400 500 600 700
Wavelength (nm)
With Dye
800
Carbon based Nano-Materials for DSSCs
TiO2-MWCNT
ZnO CNT composite
100nm
Eff. 7.4%
TiO2-Graphene
Eff. 6%
Some Results:
0.005
Efficiency Over 7%
Current (A)
0.004
0.003
0.002
Transparent TiO2 20nm + HS
0.001
1st Film
2nd Film
0.000
0.0
0.2
0.4
0.6
0.8
Voltage (V)
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
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
e–
e–
LUMO
LUMO
e–
e– LUMO
e– LUMO
ECB
HOMO
h+
h+
HOMO
h+
EVB
HOMO
n-type
semiconductor
p-type
semiconductor
Inorganic cells
Fast carriers mobility
Long life time
High production cost
Brittle
Anode
HOMO
h+
n-type
P-type
Cathode
semicond materials
uctor
Hybrid solar cells
Inorganic n + Organic p
ETA Cell
Dye-sensitized Solar Cells
Anode
Electron
acceptor
Hole
Cathode
acceptor
Organic cells
Low Production Cost
Flexible
Tunable color
Light weight
Slow carrier mobility
Short life time
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!!!