Techniques for achieving >20% conversion efficiency Si-based solar cells Presenter: TSUI, Kwong Hoi

 Abundant materials (~25% of Earth’s crust)  Non-toxic to human (Green materials)  High energy conversion efficiency (>20%)  Long-term stable in outdoor performance (>20 years) Reason for crystalline Si-based solar cells

Working principle of Single c-Si solar Band gap for crystalline Si: 1.1eV (~1100nm)

Solar Spectrum Spectral losses in a solar cell. The figure shows the maximum achievable energy of a silicon solar cell in relation to the sun spectrum (AM1.5) Theoretical Maximum Achievable efficiency of c-Si:~29%

1)the loss occurs since the photon does not enter the solar cell, this might be due to reflection from the metalized areas of the active surface of the cell. 2)the photon enters the cell but leaves it again without absorption within the cell. This is mainly controlled by the internal reflectance at front and rear and takes place for near-band-gap photons. Two ways leading to optical loss

1) Texturization of surface 2) Deposition of SiNx 3) Deposition of Al back reflector Three technical approaches to enhance the optical absorption

Schematic of Structure of Solar cell Structure of Al-BSF solar cell

Fabrication process

Fabrication Process

SEM of c-Si surface 10μm a b

Optical improvement

Conclusion

1.S.W. Glunz, R. Preu, D. Biro, Crystalline silicon solar cells: State-of- the-art and future developments, in: W. van Sark (Ed.), Comprehensive Renewable Energy, Elsevier, Oxford, 2012, pp. 353e E. Yablonovitch, “Statistical ray optics,” J. Opt. Soc. Am. A 72(7), 899–907 (1982). 3.Zhao, J, Wang, A, Green, MA, Ferrazza, F. Novel 19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells. Appl Phys Lett 1998, 73:1991– K. H. Tsui, Q. Lin, H. Chou, Q. Zhang, H. Fu, P. Qi and Z. Fan, Adv. Mater., 2014, 26, 2805– P. Gao, H. Wang, Z. Sun, W. Han, J. Li, and J. Ye, Appl. Phys. Lett. 103, (2013). Reference

Thank you!

 Selection of Si wafer: Boron doped c-Si wafer (p-type substrate) ( main functions for a p-type substrate are 1) to absorb incoming photons on a large surface efficiently, 2) to enable diffusion of minority carriers (electrons), and a good conductor to enable efficient majority carrier (holes) transport to contacts.  Texturization of front and back surface (increase the optical path length of the incoming photon inside the absorber by multiple internal reflection and gradual change of refractive index.)  Deposition of amorphous hydrogenated SiNx (incorporation of hydrogen concentration 10%at. 1) Formation of anti-reflection coating which has refractice inde ) Passivation of n-type surface and hydrogen  Deposition of Silver electrode at the front surface The main function of the H-pattern is efficient carrier transport and transparency for the incoming light, i.e. low shading.  Metaliization of rear side by Al Collection of current from the metallized area and reflection of light from rear side of solar cell Fabrication process