1. Techniques for achieving >20% conversion efficiency Si-based solar cells
Qingkai QIAN
Department of Electronic and Computer Engineering
The Hong Kong University of Science and Technology
December 3, 2014

2. Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency
Maximize light absorption
Minimize recombination
Reduce resistance
To transcend the classical limit

3. Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency
Maximize light absorption
Minimize recombination
Reduce resistance
To transcend the classical limit

4. p-n junction: soul of solar cell
Built in electric field separates the photon-generated electron-hole pair.

5. I-V curve of solar cell IL : light generated current
Light generated current basically equals to absorbed photon number

6. Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency
Maximize light absorption
Minimize recombination
Reduce resistance
To transcend the classical limit

7. Response to different wavelength
Blue light absorbed at the front surface,
Red light absorbed at the back surface,
Different absorption site, different efficiency.

8. Artificial sun standard
International Electrotechnical Commission (IEC)
IEC 60904–3: 2008
AM1.5 standard
1000 W/m2 at 25 ℃
= 1 cos⁡(48°) =1.5

9. Theoretical output limit
Under AM1.5 solar standard
Maximum ISC 46mA/cm2.
Every photon >1.12eV corresponds to an e-h pair
Maximum VOC
Depend on dark current
Thermal equilibrium limit 0.85V
Auger recombination limit 0.72V
Shockley-Queisser efficiency limit 29.8%
Efficiency mainly limited by voltage loss.
For example, 3eV photon produces 0.7V voltage.

10. Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency
Maximize light absorption
Minimize recombination
Reduce resistance
To transcend the classical limit

11. Techniques to improve the efficiency
Maximize light absorption
Antireflection layer
Surface texturing
Reduce front metal contact
Minimize recombination
Front and back passivation
Heavily doped metal contact
Hetero-junction with amorphous silicon
Reduce resistance

12. Light trap: antireflection layer
Bare silicon has a high surface reflection of over 30%.
This wavelength is chosen since it is close to the peak power of the solar spectrum.
Minimize reflection for a wavelength of 0.6 µm
Double layer anti-reflection coating (DLARC)
ZnS+MgF2
Further reduce reflection
too expensive

13. Light trap: surface texturing
“Roughening" reduces reflection by bouncing back onto the surface.
Anisotropic etching of (100) in KOH
random pyramid texture
Rear reflector:
total internal reflection
random inverted-pyramid texture

14. Light trap: reduce metal shadowing
Possible to move to backside?
EWT: Emitter wrap through solar cell
MWT: Metal wrap through solar cell
Tandem package advantage

15. Reduce recombination
N-type silicon has a higher surface quality, near p-n junction
SiO2 surface passivation
Heavily contact region doping
Recombination will cause Current Losses Voltage Losses Lower efficiency Recombination Within a diffusion length of the junction, electron-hole pair will split, otherwise it will recombine. Unpassivated surface as a localized high recombination site Metal contact region

16. Reduce recombination HIT: hetero-junction with intrinsic thin layer
a-Si /Si/ a-Si junction as passivation

17. Reduce series resistance
Resistance of bulk silicon
Trade off with carrier diffusion length, recombination.
higher levels of doping result in damage to the crystal to the extent that carriers recombine before reaching the junction.
Doping of Base (1 Ω·cm)
Doping Level of Emitter(100 Ω/☐)

18. Reduce series resistance
Resistance of busbar metal
Trade-off with light shadowing.
The resistivity of silicon is too high. Lower resistivity metal grid is placed on the surface to conduct away the current.

19. Effect of these techniques

20. Records of efficiency
2014 Panasonic HIT® Solar Cell: heterojunction with intrinsic thin layer
Highest efficiency of silicon solar cell: 25.6%

21. Outline I-V curve of solar cell Theoretical limit
Keys to improve the efficiency
Maximize light absorption
Minimize recombination
Reduce resistance
To transcend the classical limit

22. To transcend the classical limit
Make the full use of sunlight
Tandem solar cell
One photontwo e-h pairs
two photonone e-h pair

23. Reference
[1] D. M. Chapin, C. S. Fuller, and G. L. Pearson, “A New Silicon p-n Junction Photocell for Converting Solar Radiation into Electrical Power,” J. Appl. Phys., vol. 25, no. 5, p. 676, 1954.
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[4] M. A. Green, “Limits on the Open-circuit Voltage and Efficiency of Silicon Solar Cells Imposed by Intrinsic Auger Processes.pdf,” IEEE Trans. Electron Devices, vol. 31, no. 5, pp. 671–678, 1984.
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[7] P. Campbell and M. A. Green, “Light trapping properties of pyramidally textured surfaces,” J. Appl. Phys., vol. 62, no. 1, p. 243, 1987.
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[9] F. Kiefer, C. Ulzhöfer, T. Brendemühl, N.-P. Harder, R. Brendel, V. Mertens, S. Bordihn, C. Peters, and J. W. Müller, “High Efficiency N-Type Emitter-Wrap-Through Silicon Solar Cells,” IEEE J. Photovolt., vol. 1, no. 1, pp. 49–53, Jul
[10] A. Wang, J. Zhao, and M. A. Green, “24% efficient silicon solar cells,” Appl. Phys. Lett., vol. 57, no. 6, p. 602, 1990.
[11] M. Tanaka, M. Taguchi, T. Matsuyama, T. Sawada, S. Tsuda, S. Nakano, H. Hanafusa, and Y. Kuwano, “Development of New a-Si/c-Si Heterojunction Solar cells: ACJ-HIT (Artificially Consructed Juncion-Heterojunction with Intrinsic Thin-Layer),” Jpn J Appl Phys, vol. 31, pp. 3518–3522, 1992.

24. Thank you!