The Use of Amorphous Silicon in Fabricating a Photovoltaic-Thermal ...

The Use of Amorphous Silicon in
Fabricating a Photovoltaic-Thermal
System
Pratish Mahtani, Davit Yeghikyan, Nazir P. Kherani, and
Stefan Zukotynski
Advanced Photovoltaics and Devices Group
Department of Electrical and Computer Engineering
University of Toronto
June 11 th , 2007

Outline
I. Photovoltaic-Thermal (PV/T)
Systems
II.
Amorphous Silicon in PV/T Systems
III. Proposed Research
University of Toronto

Thermal Generation in PV
• Convert 60-70% of incident radiation to thermal energy
• Can reach 35ºC above ambient [van Helden et al, 2004]
• With increasing temperature:
• I sat increases -> V oc decreases:
• Bandgap decreases -> I sc increases
• Net negative effect on efficiency
• Temperature Coefficients:
• c-Si: -0.4%/ºC [Emery et al, 1996]
• a-Si:H: -0.1%/ºC [Emery et al, 1996]
University of Toronto

PV/T System Overview
• Co-generate electricity and heat
• Heat generated from PV extracted by heat exchanger
• Photons transmitted through PV panel (Eph < Eg) absorbed by heat
exchanger
• System configuration:
• covered/uncovered
• air/water
• fluid flow rate
PV panel
heat exchanger
laminate
adhesive
insulating frame
Figure 1: Basic covered PV/T system configuration
water / air
flow
University of Toronto

Current Status in PV/T
• Practical conversion efficiency PV/T (electrical + thermal): 60-
80% [Bergene et al, 1995]
• Research focus on balancing electrical and thermal efficiencies
• Design tradeoffs between electrical and thermal efficiency
Table 1
Trade-offs between thermal & electrical efficiency of c-Si PV/T [Zondag et al, 2003]
Type
Thermal
Efficiency
Electrical
Efficiency
Stand-alone
PV
9.7%
Stand-alone
STC
83%
Covered
PV/T
58%
8.9%
Uncovered
PV/T
52%
9.7%
University of Toronto

PV/T Issues
• Reduced thermal efficiency:
• PV/T collected 33% less thermal energy than STC [Zondag et al, 2002]
• High Emissivity of PV Module:
• PV laminates ~ 90% emissive in IR [Zondag et al, 2004]
• Low-E coatings low transmission in visible spectrum (60-70%) [van Helden et
al, 2004]
• Lower incident radiation on STC:
• PV conversion
• Reflective back rear foil
• AR coating not optimized for IR radiation
• Reduced electrical efficiency:
• PV temperature covered PV/T 20ºC higher than stand-alone PV
[Sandnes et al, 2002]
• Higher Cost:
• 1m 2 PV/T costs $(CAN)29 MORE than 1m 2 PV + 1m 2 STC
[Bakker et al, 2005]
University of Toronto

a-Si:H PV/T
• Lower cost
• Improved high-temperature performance
• Staebler-Wronksi Effect
• Positive Temperature Coefficient
• Improved thermal efficiency
University of Toronto

Lower Cost
c-Si PV:
• Cylindrical ingots from molten Si
• New c-Si “thin-film” deposition temperature: 800ºC-1400ºC [B.P. Nelson et
al, 2003]
• Metals have high thermal exp. and low melting point
a-Si:H thin-film PV:
• Can be deposited over large areas at low temperatures (

Staebler-Wronski Effect
• a-Si:H PV suffers from light-induced degradation - Staebler-Wronksi
Effect (SWE)
• Exposure to light increases defect density
• Increase defect density leads to increase in recombination current and
decrease in efficiency
• Effect saturates after 1000h of light exposure with efficiency leveling off
at 30% lower than initial efficiency [Luque et al, 2003]
• SWE can be reversed by annealing at T ~ 150°C for few minutes [Luque
et al, 2003]
University of Toronto

Temperature Coefficient
• a-Si:H PV temperature coefficient:
• short-term: -0.1%/ºC [Emery et al, 1996]
• if kept at high temperature: +0.5%/ºC [Luque et al, 2003]
• a-Si:H PV module case study:
• module kept at 60ºC during daylight hours generated 20% more
energy than module kept at 35ºC [Hof et al, 1996]
• Attributed to slow, low-temperature anneal of SWE [Luque et
al, 2003]
• Alleviates thermal-electrical tradeoff
University of Toronto

Improved Thermal Efficiency
Increased Incident Radiation:
• Direct deposition onto STC increases incident radiation onto STC
Lower Emissivity:
• Using low emissivity PV cells will reduce significance of highly emissive laminate
• PV emissivity:
• c-Si: 40% [Platz et al, 1997]
• a-Si:H: 30% [Affolter et al, 2000]
• Commercial low-E coatings used in STC:

Proposed Research
• Developing and modeling a-Si:H PV
directly deposited on STC
• Research Focus:
• Lower cell emissivity
• Cell interconnection
University of Toronto

Lower Cell Emissivity
• Replace p-type a-Si:H layer with p-type a-C:H
• a-C:H/Ti films used as low-E coating for STC
• IR Emittance (100ºC): 6% [Schuler et al, 2000]
• a-C:H acts as window layer and helps increase PV efficiency
• Eg of p-type a-C:H: 2.0eV [Han et al, 2007]
• Eg of p-type a-Si:H: 1.7eV [Han et al, 2007]
• Efficiency increased from 5.1% to 5.6% [Han et al, 2007]
p-type a-Si:H
i-type a-Si:H
n-type a-Si:H
h+
e-
E
p-type a-C:H
i-type a-Si:H
n-type a-Si:H
(a)
(b)
Figure 4: (a) conventional a-Si:H p-i-n design (b) a-C:H window layer design
h+
e-
h+
e-
E
University of Toronto

Cell Interconnection
• Thermally conductive, electrically insulative materials (TCEI):
• Al 2
0 3
: ρ = 10 16 Ωm [Dickey et al, 1989] κ= 30Wm -1 k -1 [Ogden et al, 1987]
• Al: ρ = 2.82X10 -8 Ωm [Serway et al, 1998] κ= 205 Wm -1 k -1 [Sears et al, 1987]
• Glass: ρ= 10 14 Ωm [Serway et al, 1998], κ= 0.8 Wm -1 k -1 [Sears et al, 1987]
• Monolithic Design:
• Entire module fabricated directly on STC
• Interconnection made by laser welding [Vijh et al, 2006]
• Slab Design:
• Cut large area cell into smaller cells
• Cells adhered to STC
front contacts
laser
weld
a-Si:H
STC
(a)
front contacts
TCO
back contacts
TCEI
TCO
a-Si:H
TCEI
STC
(b)
TCO
a-Si:H
metallic
foil
adhesive
Figure 5: (a) monolithic design (b) slab design
University of Toronto

Summary
• PV/T utilizes intrinsic losses in PV devices
• PV/T Issues:
• manufacturing cost
• PV efficiency at high temperatures
• lower absorption and higher emittance than stand-alone STC
• a-Si:H for PV/T:
• thin-film direct deposition
• strong high temperature performance
• lower emittance than c-Si cells
• Research at UofT:
• cell Interconnection
• decrease PV emittance
University of Toronto

Acknowledgements
• Dr. Steve Harrison
• Jarrett Carriere
• Advanced Photovoltaics and Devices Group
• SBRN
University of Toronto

References
Affolter, P., Ruoss, D., Toggweiler, P., Haller, A., 2000. “New generation of Hybrid Solar PV/T
collectors”, EPFL.
Bakker, M., Zondag, H.A., Elsijk M.J., Strootman, K.J., Jong M.J.M., 2005. “Performance and
costs of a roof-sized PV/thermal array combined with a ground couple heat pump”, Solar
Energy, vol. 78, pp. 331.
Bergene, T., Lovvik, O.M., 1995. “Model calculations on a flat-plate solar heat collector with
integrated solar cells”, Solar Energy, vol. 55., pp. 453.
Dickey, J.R., Davidson, J.L., and Tzeng, Y., 1989., “Improved dielectric properties of anodic
aluminum oxide films by soft/hard two-step electrolytic anodization”, J. Electrochem. Soc.,
vol. 136. no. 6.
Emery, K., Burdick, J. Calyem, Y., Dunlavy, D., Field, H., Kroposki, B., Moriary, T., Ottoson, L.,
Rummel, S., Strand T., Wanlass M., 1996. “Temperature dependence of photovoltaic cells,
modules and systems”, Proceedings of 25th Photovoltaic Specialists Conference, pp. 1275.
University of Toronto

References
Gellin, K., Roos, A., Geotti-Bianchini, F., van Nijnatten, P., 2005. “Thermal emissivity of coated
glazing – simulation versus measurements”. Optical Materials, vol 27, pp. 705.
Han, J., Tan, M., Zhu, J., Meng, S., Wang, B., Mu, S., Cao, D., 2007. “Photovoltaic
characteristics of amorphous silicon solar cells using boron doped tetrahedral amorphous
carbon films as p-type window materials”, Applied Physics Letters, vol. 90.
Hof, C., Ludii, M., Goetz, M., Fischer, D., Shah, A., 1996. “Long term behaviour of passively
heated or cooled a-Si:H modules”, Proceedings of 25th Photovoltaic Specialists
Conference, pp. 1057.
Luque, A., Hegedus, S., 2003. Handbook of Photovoltaic Science and Engineering, West
Sussex: Wiley.
Nelson, B.P, Atwater, H.A., von Roedern, B., Yang, J., Sims, P. Deng, X., Dalal, V., Carlson, D.,
Wang, T., 2003. “Amorphous and Thin-Film Silicon”, NREL.
University of Toronto

References
Ogden, T.R., Rathsam, A.D., Gilchrist, J.T., “Thermal conductivity of thick anodic oxide coatings
on aluminum”, 1987. Materials Letters. vol. 5, no. 3.
Platz, R., Fischer, D., Zufferey, M.A., Selven, J.A., Haller, A., Shah, A., 1997. “Hybrid collectors
using thin-film technology”, Proceedings of 26th Photovoltaics Specialists Conference, pp.
1293.
Sandnes, B., Rekstad, J., 2002. “A photovoltaic/thermal (PV/T) collector with a polymer
absorber plate. Experimental study and analytical model”, Solar Energy, vol. 72. iss. 1, pp.
63.
Schuler, A., Geng, J., Oelhafen, P., Brunold S., Gantenbein, P., Frei, U., 2000. “Application of
titanium containing amorphous hydrogenated carbon films (a-C:H/Ti) as optical selective
solar absorber coatings”, Solar Energy Materials, vol. 60, pp. 295.
Sears, F.W., Zemansky, M.W., Young, H.D., 1987: University Physics, 7th Ed. Reading, MA:
Addison-Wesley Publishing Company
Serway, Raymond A., 1998. Principles of Physics, 2nd ed, Fort Worth, Texas; London:
Saunders College Pub
University of Toronto

References
van Helden, W.G.J., van Zolingen, R.J.C., Zondag, H.A., 2004. “PV Thermal Systems: PV
Panels Supplying Renewable Electricity and Heat”, Prog. Photovolt: Res. and Appl., Vol. 12,
pp.415-426.
Vijh, A., Yang, X., Du, W., Deng, X., 2006. “Triple-junction amorphous silicon-based flexible
solar minimodule with integrated interconnects”, Solar Energy Materials and Solar Cells, vol.
90, pp. 2657.
Zondag, H.A., De Vries, D.W., van Helden, W.G.J., van Zolingen, R.J.C., van Steenhoven, A.A.,
2002. “The thermal and electrical yield of a PV-Thermal collector”, Solar Energy, Vol. 72,
No. 2.
Zondag, H.A., De Vries, D.W., van Helden, W.G.J., van Zolingen, R.J.C., van Steenhoven, A.A.,
2003. “The yield of different combined PV-thermal collector designs”, Solar Energy, Vol. 74,
No. 3.
Zondag, H.A., van Helden, W.G.J., Elswijk, M.J., Bakker, M., 2004. “PV-Thermal collector
development – an overview of the lessons learnt”, Proceedings of the 19th European PV
Solar Energy Conference and Exhibition”, pp. 7.
University of Toronto

Thank you for your attention!
Any Questions or Comments?
University of Toronto