Thin-film Solar Cells
Friday 07:00-09:00 pm
Textbook: Solar Cells
edited by T. Markvart and L. Castaner
Lecturer: Prof. Yeong-Cheol Kim
Chap. Iic-1 Amorphous silicon solar cells
1.
2.
3.
4.
5.
Introduction
Amorphous silicon alloys
1) Deposition conditions and microstructure
2) Optoelectronic properties
3) Doping
4) Light-induced degradation
Amorphous silicon solar cells
1) Physics of operation
2) Device structures
3) Performance and stability
4) Reliability
Production of amorphous silicon solar cells
1) Manufacturing process
2) Manufacturing costs
3) Environmental issues
Future trends
1. Introduction
- large-area deposition techniques for mass production
- 40 organizations
- first investigation, silane discharge, Chittik, 1969.
- lower density of defects than evaporated or sputtered a-Si.
- doping by Carlson, Spear&LeComber
- Carlson&Wronski, efficiency 2% in 1976.
- 5%
- H, alloy of H and Si, hydrogenated a-Si (a-Si:H).
- Staebler-Wronski effect (SWE): large change in photoconductivity, reversible at
150C for a few hrs.
- how to minimize the effect.
- PECVD, DC, RF, VHF (30~110 MHz), microwave (~2.45 GHz)
- substrate T: ~200-250C
- dilution of silane with hydrogen, protocrystalline a-Si:H
- bandgap change by H.
- C or Ge
- wide bandgap p-type a-Si:C:H alloys: little absorption, high built-in potentials.
- narrow bandgap a-Si:Ge:H: tandem and triple junction cells.
- C or Ge alloying: additional defect states
- ~1.3 eV (~75 at.% Ge) to ~2.1 eV (~15 at.% C)
- multijunction allows thinner component cells, reduce SWE.
- decrease in absorption of sunlight, increase in shorts and shunts.
- optical inhancement by textured optical reflectors.
- PECVD, excellent uniformity over 1 m2 area.
- RCA lab.: curing of shorts and shunts by reverse bias, laser scribing process, largearea, monolithic PV module of a-Si SCs connected in series.
2. Amorphous silicon alloys
2.1 Deposition conditions and microstructure
- decomposition of feedstock gases by plasmas, DC, RF (13.56 MHz), VHF (30-110
MHz), microwave (2.45 GHz).
- SiH4 decomposed by e impact into mixture of radical and ionic species.
- variables: sub T, P, flow rate, plasma power, frequency, electrode spacing, source
gases.
- impurities, O, C, N
- T, reactions on surface  material quality
- P, mean free path for collision  reaction locations
- flow rate, residence time  growth kinetics
- power, rate of dissociation  film growth rate
- frequency, nature of plasma  ion bombardment, less significant at VHF and micro.
- GeH4, CH4, dopant gases.
- H, key element
- variables, interdependent  complex PECVD process
2. Amorphous silicon alloys
2.1 Deposition conditions and microstructure
- deposition process
dissociation of molecules
transport to surface by diffusion
reaction
desorption of byproducts and unreacted silane radicals.
optimum condition: pure silane or H-diluted silane, low RF power, 200-300C
substrate T
- H in plasma, passivate dangling bonds, reconstruction of network.
- H dilution, R=[H2]/[SiH4]=10, thickness dependent microstructure.
- initially amorphous, eventually microcrystalline
2. Amorphous silicon alloys
2.1 Deposition conditions and microstructure
- rapid decrease of proto regime at high H
dilutions  limit i-layer thickness
- H increases bandgap.
Figure 1. Film thickness at which the
different phase transitions occur during
Si:H film growth plotted as a function of
the hydrogen dilution ratio R.
2. Amorphous silicon alloys
2.2 Optoelectronic properties
- absence of long-range order  bandtails, localized states due to disorder, broken
bonds.
- carrier trapping, recombination centers.
- high absorption coefficient due to random nature of atomic ordering.
- H incorporation, reduce defects, widen gap.
- densities of dangling bonds < 1016 cm-3 in PECVD a-Si:H with H~10 at.%.
2. Amorphous silicon alloys
2.3 Doping
- undoped, slightly n-type
- dopant introduction, move EF towards conduction and valence band.
- high density dopants, defect states at midgap  limit doping efficiency, lifetime.
- doped materials are not appropriate for absorber layers.
- thin films of p-type a-SiC:H or protocrystalline Si:H for p/i heterojunctions.
- n-type a-SiC:H and uc-Si:H as ohmic contacts.
- p-i-n or n-i-p cells
2. Amorphous silicon alloys
2.4 Light-induced degradation
- SWE
- no general consensus on exact nature of light-induced defects.
- because of no unique a-Si:H material.
- neutral dangling bonds, charged defects
3. Amorphous silicon solar cells
3.1 Physics of operation
- p-i-n or n-i-p heterojunction cell structure
3. Amorphous silicon solar cells
3.1 Physics of operation
- choice of TCO materials (window layer), optical transmission, conductivities, good
contact to p-layers.
- p-type: ~10 nm
- carriers collected by inter electric field, drift current.
- adverse effect of defects: recombination centers, reduce E field.
- buffer layer in p/i later
3. Amorphous silicon solar cells
3.2 Device structures
- multijunction structures, higher stabilized efficiencies
- substrates: float glass, BP Solar, Energy Photovoltaics, Intersolar, Kaneka,
Phototronics, Sanyo, Sharp
SS, Canon, United Solar Systems
plastic substrates, Fuji Electric, Iowa Thin Films, Sanyo
- glass/textured tin oxide/p-i1-n/p-i2-n/zinc oxide/Al/EVA/glass, BP Solar
i1, A-Si:H, i2, a-SiGe:H
- SS foil/textured silver/zinc oxide/n-i3-p/n-i2-p/n-i1-p/ITO/EVA/fluoropolymer
i2, i3, a-SiGe:H, i1, a-Si:H
fluoropolymer, Tefzel by DuPont
roll-to-roll, multi-chamber PECVD system
3. Amorphous silicon solar cells
3.3 Performance and stability
- light-induced degradation, triple-junction, 10-15%
3. Amorphous silicon solar cells
3.4 Reliability
- electrical performance, electrical isolation (dry and wet), visual inspection, thermal
cycling between -40 and +85 C, light soaking, ultraviolet light exposure, humidity
freeze cycling between -40 and +85 C (85% humidity above room T.), static and
dynamic mechanical loading, hail impact tests, surface cut susceptibility, hot-spot
endurance and outdoor exposure.
- moisture ingression, delamination for thin-film SC.
4. Production of amorphous silicon solar cells
BP Solar, tandem modules on glass
United Solar, triple-junction modules on SS foil
Iowa Thin Films, single-junction modules on plastic
4.1 Manufacturing process
- a-Si/a-SiGe tandem modules
- ~9 mm wide scribing by Nd:YAG laser
- ~15 um in diameter
from GETWATT
from GETWATT
Plasma frequency
 N ee
p  
 m e
2
1/2




Reflectivity
1
0
wp
Angular Frequency
ω
→
(C. M. Dunsky, Industrial Laser Solutions, 2008)
(C. M. Dunsky, Industrial Laser Solutions, 2008)
4. Production of amorphous silicon solar cells
4.2 Manufacturing costs
- BIPV: roofing shingles, PV laminates by Uni-Solar, PowerView by BP Solar
4. Production of amorphous silicon solar cells
4.3 Environmental issues
- mining and refining raw materials, manufacturing PV modules, disposing obsolete
modules.
- trimethylboron (~1-5% in silane), less toxic than diborane.
- silane, pyrophoric.
- no wet chemicals in BP
- no harmful materials in PV modules, breakage, fires, or long-term disposal in
landfills.
5. Future trends
- Sanyo in 1980 for calculators, 3.5 MWp by 1985
- ~40 MWp in 2001.
- key drivers for terrestrial PV market: efficiency, price, reliability.
- balance of system, area-related costs.
- stabilized efficiency, lower manufacturing cost, long term reliability
- microcrystalline Si
- BIPV
Industry status
Oerlikon solar
: better (and cheaper) reflective backsheet, thinner silicon layer
Pramac: 9.2%
Sharp, IBC solar: 10%
Schott ASI TM 103: 7.1%
Kaneka GSA-60: 6.3%
Inventux X 140: 9.8%
Uni-Solar PVL-144: 6.7%
Oerlikon Solar
ThinFab production line
Micromorph tandem
1.4m2 solar module
10%
143Wp
EUR 0.5/Wp
Solar cell panel price ($1/Wp) vs. 1 kWh price
System price: $2.5/Wp
Electric power of 1W cell:
1Wx3.5hrx365daysx20year=25.5kWh
Therefore, $2.5/25.5kWh=$0.1/kWh