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

2. 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

3. Sun: An ultimate source of energy
* Present : 12.8 TW
2050 : 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!!!

7. 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)

8. 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.
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.

9. Silicon Solar Cell Schematic
Why thickness of p type and n type semiconductor layers are different?

10. 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.

11. Equivalent Circuit
Rseries
Junction IL
Rshunt
External Load

12. 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

13. 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.

14. Solar Cell IV Measurement in Lab
Solar Simulator

15. Quantum Efficiency Set up

16. Current Status of Si Solar Cells
Factors Limiting Efficiencies:

17. 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

18. Cost Comparison of Various Photovoltaics

19. Nanotechnology: Towards low cost solar cells

20. Pre-requisite concepts
Transparent Conducting Oxide: Eg ≥ 3 eV e.g. ZnO, TiO2, SnO2 etc.
Molecular Levels:
HOMO: Highest Occupied Molecular Orbital
LUMO: Lowest Unoccupied Molecular Orbital

21. Dye Sensitized Solar Cells (DSSC)
Iodide/tri-iodide electrolyte e -
LOAD
Dye/QD
TiO2 (~ 20 nm)
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

22. 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

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

24. Iodide/tri-iodide electrolyte
LOAD
Dye/QD
TiO2 (~ 20 nm)
….continued
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 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.

27. 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

28. Sensitizers Dyes: Quantum Dots: 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.

29. DSSC Fabrication protocol
Name
Voc (V)
Jsc (mA/cm2)
FF (%)
η (%)
Sol-Gel TiO2
0.76
12.5 60
5.7

30. Transparent coatings for DSSC
Transparency a critical issue to avoid loss of incident radiation due to reflection at nanoparticle/TCO interface.
Without Dye
With Dye

31. Carbon based Nano-Materials for DSSCs
ZnO CNT composite
TiO2-MWCNT
TiO2-Graphene
Eff. 7.4%
Eff. 6%

32. Some Results: Efficiency Over 7% Name Voc (V) Isc (A) FF (%) η (%) 1st
η (%)
1st
0.76
0.0044
54.51
7.26
2nd
0.74
0.0043
56.51
7.23

33. 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

34. 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.

35. Organic Solar Cells

36. New Types of Solar Cells
LUMO
LUMO e– e– e–
LUMO
LUMO
ECB h+ h+
HOMO h+
HOMO
EVB
HOMO h+
HOMO
n-type semiconductor
p-type semiconductor
Anode
n-type semiconductor
P-type materials
Cathode
Anode
Electron acceptor
Hole acceptor
Cathode
Inorganic cells
Hybrid solar cells
Organic cells
Fast carriers mobility
Long life time
High production cost
Brittle
Low Production Cost
Flexible
Tunable color
Light weight
Slow carrier mobility
Short life time
Inorganic n + Organic p
ETA Cell
Dye-sensitized Solar Cells

37. Example of a organic-inorganic hybrid solar cell

38. Nano p-n junction solar cells

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

40. Thank You!!!