1. Amorphous silicon based solar cell technology
Prof. Partha Chaudhuri
Indian Association for the Cultivation of Science
Kolkata
Talk delivered at the NCPRE school at IIT Bombay
On 16th September, 2011

2. Structure of the talk Crystalline silicon (c-Si)
Amorphous silicon (a-Si) and hydrogenated amorphous silicon (a-Si:H)
Solar cell Basics
Amorphous silicon thin film solar cell
Various Solar cell Structures with a-Si
Future Technologies based on thin film silicon

3. Crystalline silicon solar cell
bond length = 2.33Å
bond angle = 109 28’

4. Crystalline silicon technology
Starting material: near Semiconductor grade p-type Si produced by Siemens process
Melt at about 1600C (m.p. of Si =1415C)
For monocrystalline Si - Czochralski process
For multicrystalline Si – the molten Si is cast in a graphite crucible
Blocks of multicrystalline Si is cut into equal size (15cm15cm  15cm) by band saw
Thin wafers of equal size (15cm15cm 250m) of multicrystalline silicon using wire saw
n-type doping by diffusion and subsequent annealing
Module Efficiency 15-17%
Energy pay back time 4.5 – 5 years
Life time 25 years
Slow
pulling
Monocrystalline
Silicon ingot
Molten silicon
Monocrystalline
Silicon wafers
Czochralski process
Polycrystalline
Silicon wafers
Crystal growing and casting are metallurgical processes which are relatively energy intensive since they are processing molten silicon at around 1600°C.  They consist of a large number of units of process equipment operating in parallel. This modular nature makes for relatively easy expansion of plant throughput.
The starting material is lumps of chemically pure polycrystalline silicon, of a quality close to semiconductor-grade, produced by the Siemens process.  The solar industry has historically taken off-specification material that is rejected by the semiconductor industry.  However, as growth of the PV industry is overtaking that for semiconductors, this scrap is in short supply and an increasing proportion of more expensive prime-grade Si is having to be used as meltstock. 
A small number of companies, either integrated PV companies or independent wafer production operations, use one of two main methods of manufacture.  The traditional route for monocrystalline wafers is the Czochralski process in which a single crystal of up to about 150mm diameter is pulled from molten Si held in a large heated quartz crucible.  In the more recently developed method, Si is cast in a re-useable graphite mould to produce blocks of multicrystalline silicon (cubes of over 0.5m dimensions).  When sawn into bars and then wafers (just bigger than a compact disc) using a wire saw, the cleaned product is ready for cell manufacturing.  Multicrystalline wafers from the latter process are cheaper than monocrystalline wafers, but make slightly less efficient solar cells.  Click here for more details on crystalline Si solar cell technology.
Crystal growing and casting plants are best sited where there is an abundant source of reliable, cheap energy to power the high temperature operations.  They do not need to be sited close to solar cell plants because wafer transportation is cheap, but most are because the investment has been by PV manufacturers to secure wafer supply to their cell plants.
Thin film plants do not utilize crystalline Si wafers, so this whole piece of the manufacturing chain is avoided.  Instead, as a starting point for manufacture, they generally use large area glass sheet coated with a transparent conducting oxide layer (of the type used for special low emissivity glass products).  This is manufactured either on-line in a float glass factory, or off-line in large scale chemical vapor deposition (CVD) plants.  Some thin film plants are based around the use of roll-to-roll stainless steel sheet as the substrate for the cell, rather than glass. 
Polycrystalline
Silicon ingot

5. The energy band structure for silicon

6. Amorphous silicon

7. Electronic band structure of amorphous silicon (unhydrogenated)
Conduction band
(Electrons are mobile)
Extended states
EC (Conduction band
mobility edge) E
Localised states
(elctrons and holes are trapped)
EF (Fermi
Energy)
EV (Valence band
mobility edge)
Valence band
(Holes are mobile)
Extended states
N(E)

8. Discovery of photosensitive hydrogenated amorphous silcion
R.C. Chittick, J.H. Alexander and H.F. Sterling. J. Electrochem. Soc. 116 (1969), p. 77
A radio-frequency glow discharge is used to deposit films of amorphous Si from silane gas on substrates at °.  These films have resistivities at 21° of up to 1014 Scm-1 and have large temp. coeffs. of resistivity.  A photoconductive effect is observed which reaches a max. for films deposited at 300°, and a sample is compared with a CdS cell.  The effects of heat treatment, aging, and doping on the properties of amorphous Si are reported.  The variation of properties with deposition temp. is related to the structural changes with temp. that have been observed for this material.

9. Different methods of deposition of hydrogenated amorphous silicon (a-Si:H)
PECVD - Plasma Enhanced Chemical Vapour Deposition [DC, RF(13.56 MHz), VHF( MHz), Microwave(GHz)]
HWCVD - Hot Wire Chemical Vapour Deposition
Photo-CVD - Photo Chemical Vapour Deposition
Reactive Sputtering of Silicon Target in Hydrogen Environment

10. PECVD system
PH3

11. a-Si:H deposition mechanism by radio frequency (13.56 MHz) PECVD
Amorphous silicon solar cell is deposited from decomposition of silane (SiH4) gas by radio frequency (13.56 MHz) electrical discharge
A fast electron breaks SiH4 into successively lower hydride radicals
SiH4 + e- = SiH3 + H + e-
= SiH2 + 2H + e-
= SiH + H2 + H + e-
= Si + 2H2 + e-
SiH3 radical has been found to deposit device
quality amorphous silicon
Dilution of silane with argon gas has also been found to give device quality amorphous silicon under certain deposition conditions

12. Electronic band structure of hydrogenated amorphous silicon
Conduction band EC
Energy (E) EF
Dangling
bonds states Eg
Tail states
Band gap, Eg=1.75 eV EV
Valence band
Density of states N(E)

13. Absorption coefficient for c-Si and a-Si:H

14. First report on amorphous silicon solar cell
Solar cells using Schottky barriers on amorphous silicon
By Carlson, D. E.; Wronski, C. R.; Triano, A. R.; Daniel, R. E.
Conference Record of the IEEE Photovoltaic Specialists Conference (1976), 12, 893-5. 
Thin (≤1μ thick) film solar cells were fabricated by using metal Schottky barriers on discharge-produced amorphous Si.  Power-conversion efficiencies of ≤5.5% were obtained by using Pt Schottky barriers and ZrO2 antireflection coatings.  These cells have the potential of producing low-cost power since inexpensive materials such as steel and glass were used as substrates.

15. Light induced degradation of photoconductivity (S-W effect)
Reversible conductivity changes in discharge-produced amorphous silicon
By Staebler, D. L.; Wronski, C. R.
Applied Physics Letters (1977), 31(4), 292
A new, reversible photoelectronic effect is reported for amorphous Si produced by glow discharge of SiH4.  Long exposure to light decreases both the photocond. and the dark cond., the latter by nearly 4 orders of magnitude.  Annealing at >150° reverse the process.  A model involving optically induced changes in gap states is proposed.  The results have strong implications for both the phys. nature of the material and for its application in thin-film solar cells, as well as the reproducibility of measurements on discharge-produced Si.

16. Light induced degradation
LS time dependence of the density of DBs measured by ESR.
Decrease in σd and σp by light soaking

17. Light induced effect on the density of states
Conduction band EC
Energy (E)
Dangling
bonds states
after light soaking
Dangling
bonds states
before light soaking EV
Valence band
Density of states N(E)

18. Solar cell basics: Conversion of light to electricity
Absorption of light
Generation of charge carriers (electrons, holes, excitons)
Separation of charge carriers
Collection of charge carriers at the electrodes
Electrical power in the external circuit

19. Solar cell basics (Contd.) Current flow in a p-n junction diodeV
Drift Current :
Ln= Diffusion length for electrons
Lp= Diffusion length for holes
gTh= Thermeal generation rate + - - n
Majority carrier e-
Minority carrier h+ + p
Majority carrier h+
Minority carrier e- - + E - + -
gop= Optical generation rate + -
Sunlight
Depletion layer
Diffusion Current :

20. Short circuit current:
Solar cell basics (Contd.) Diode characteristics in dark and with light
Open circuit voltage:
Short circuit current:
P +ve
Voc
Power, P = Voltage x Curent
P +ve
P -ve
Jsc
In Fourth quadrant:
P is -ve
SOLAR CELL

21. Fill factor and efficiency
Solar cell basics (Contd.)
Fill factor and efficiency
Maximum Output Power;
Pmax=Vmp.Imp
Pmax
Input power (optical);
Pin=Popt(1-R)
Fill Factor = Pmax/(Voc.Isc)
ConversionEfficiency is the percentage of power converted (from absorbed light to electrical energy) and collected, when a solar cell is connected to an electrical circuit.
h=Pmax/Pin

22. Solar cell basics (Contd.) Maximum Current
Conduction Band
Eg is the Band gap
of the Semiconductor Eg
dnPh/d(h) h
Valence Band
h(eV)

23. Solar cell basics (Contd.)
Optimal Band Gap
Em=Average Electrical Energy per photon  Eg
Selection of optimum band gap material for solar cell:
With low Eg, absorption is high i.e. greater portion of solar spectrum is utilized for EHP generation. But, Em is low i.e. energy converted to electrical power per absorbed photon is low.
With high Eg, Em is high but total absorption is low

24. Solar cell basics (Contd.)
Optimal bandgap for single junction solar cell
Shockley-Queisser limit
a-Si:H
a-Si:H
a-Si:H

25. Solar cell basics (Contd.) Quantum Efficiency
Quantum efficiency (QE) refers to the percentage of absorbed photons that produce electron-hole pairs (or charge carriers).
Let, f0 be the flux of incident photons with energy hn
Flux entering the device is f0(1-R), where R(n) is the reflectance of the front surface
Let, J = Current obtained due to absorption of this photon flux
QE = J/f0(1-R)

26. Solar cell basics (Contd.) Quantum efficiency
QE falls both at low energy and high energy
Lower enrgy limit is determined by the band gap Eg
In the high energy limit QE falls due to surface recombination of electrons and holes
Maximum value of QE depends on Lp and Ln values as also Sp and Sn
High energy QE may be increased by use of heterojunction
Amorphous silicon solar cells

27. Various designs of solar cells incorporating amorphous silicon
Industrially produced
Single junction p-i-n or n-i-p
Double junction p-i-n/p-i-n
Triple tandem a-Si/a-SiGe/a-SiGe
Micromorph double tandem a-Si/µc-Si
Heterojunction intrinsic thin layer (HIT)
Future Technologies
Si quantum dot in dielectrics such as a-SiO, a-SiN, a-SiC
Plasmonic solar cell

28. Fabrication of a single junction (p-i-n) a-Si:H solar cell
By sputtering or
vacuum evaporation
Silver
By sputtering
Transparent conducting oxide, ZnO, 180 Å
Load
n-a-Si:H, 200 Å, Eg=1.75 eV
i-a-Si:H, 4000 Å, Eg=1.75 eV
By PECVD
p-a-SiC:H, 100 Å, Eg=1.9 eV
By spray pyrolysis
Transparent conducting oxide, SnO2, 500 Å
Glass (3 mm)
Direction of Sun light

29. Band diagrams of the p-i-n single junction and pin/pin double junction a-Si:H solar cells

30. Features of amorphous silicon solar cell
No Silicon wafer required
Deposition process: Plasma Enhanced Chemical Vapour Deposition Process (PECVD)
Source materials: Silane (SiH4), Phosphine (PH3), Diborane (B2H6) etc. gas
Alloy of silicon with carbon or germanium can be easily formed by mixing SiH4 with CH4 or GeH4 respectively for band gap engineering
Solar cell is deposited on low cost substrates like glass, stainless steel, plastic etc. Hence cost is lower than c-Si solar cells
Stabilised module Efficiency 6-8%
Energy pay back time years
Life time 20 years

31. Radio frequency (13.56 MHz) plasma enhanced chemical vapour deposition (rf-PECVD)

32. Deposition system of a-Si:H double junction modules at IACS
Typical 1ft x 1ft
module produced
Module efficiency = 6.5%

33. Conversion Efficiency and Quantum Efficiency measurement set up

34. Current voltage characteristic (Single junction a-Si)

35. Quantum eficiency (Single junction a-Si)

36. Current voltage characteristics of double junction a-Si solar cell

37. Quantum efficiency of double junction a-Si/a-Si solar cell

38. Triple junction a-Si based solar cell
Middle cell – a-SiGe:H, 1.6 eV
Back cell – a-SiGe:H, 1.4eV
Aluminium
Front cell – a-Si:H, 1.75 eV
TCO
Glass

39. Flexible Triple Junction solar cell on Stainless Steel (United Solar, USA)

40. Flexible amorphous silicon solar modules

41. Amorphous / micro-crystalline silicon “micromorph” tandems
Such cells have higher current density, efficiency and stability, and being in thin film form, may be deposited over wide area.
Challenges
Right current matching, development of junction between subcells required.
Maximum efficiency of a solar cell (small area) 14.7%

42. Efficiency Losses in Solar Cell
1 = Thermalization loss
2 and 3 = Junction and contact voltage loss
4 = Recombination loss

43. Next generation technology
Silicon nanostructures Bandgap engineering of silicon.
Applications could be tandem solar cells and energy selective contacts for hot carrier solar cells.
Fabrication of silicon nanostructures consisting of quantum well and quantum dot super lattices to achieve band gap control

44. Next generation technology (cont.)
Up/Down converters Luminescent materials that:
EITHER absorb one high energy photon and emit more than one low energy photon just above the bad gap of the solar cell (down-conversion)
OR that absorb more than one low energy photon below the band gap of the cell and emit one photon just above the band gap (up-conversion).

45. Expected Efficiencies > 40% (UNSW, Australia)
Si quantum dot solar cells : Dense array of Si nano-crystals
Dense ordered array of nano-Si crystals for satisfactory quantum confinement effect
By varying size of dots (PECVD deposition conditions), band gap can be varied, to absorb light of different wavelengths.
Expected Efficiencies > 40% (UNSW, Australia)

46. Nanocrystalline silicon
High resolution
Transmission Electron
micrograph
X-ray diffraction spectrum
Raman spectra for nano-crystalline Si

47. 0.7

48. Si quantum dot super lattice structure
C.B.
Si q-dot Mini bands
Wider band gap
V.B.
Wider band gap layer (a-SiO, a-SiN, a-SiC)
Si dot embedded in Wider band gap layer

49. Present status of amorphous and nano- crystalline silicon based solar cells
Efficiency (%)
Area (cm2)
Voc (V)
Jsc (mA/cm2)
FF (%)
a-Si (single)
9.5
1.07
0.859
17.5
63.0
a-Si (nano-crystalline)
10.1
1.199
0.539
24.4
76.6
a-Si/µc-Si (sub-module)
11.7
14.23
5.462
2.99
71.3
a-Si/a-Si/a-SiGe
12.1
.27
2.297
7.56
69.7
a-Si/a-SiGe/a-SiGe
10.4
905
4.353
3.285
66.0

50. Present status of a-Si based modules
Company
Stabilized efficiency (Aperture area)
Device configuration
BP Solar
8.1% (0.36 m2)
a-Si/a-SiGe tandem on glass
7.6% (0.74 m2)
Fuji Electric
9.0% (0.32 m2)
Intersolar
~ 4.5% - 5% (0.30 m2)
Single junction on glass
Iowa Thin Films
Same gap tandem on plastic
Kaneka
8.1 % (0.41 m2)
~10% (0.37 m2)
a-Si/c-Si tandem on glass
Phototronics
~ 6% - 6.5% (0.55 m2)
Sanyo
9.3% (0.51 m2)
United Solar
10.1% (0.09 m2)
Triple junction on steel foil
7.9% (0.45 m2)

51. Thank you for your attention