Optical Modeling of a- Si:H Thin Film Solar Cells with Rough Interfaces

1. Optical Modeling of a-Si:H Thin Film Solar Cellswith Rough Interfaces Speaker：Hsiao-Wei Liu 08/18/2010 Wed

2. Outline • Introduction • Parameters to describe a rough interface • Optical modeling and data fitting • Conclusion

3. Introduction • Light trapping has become a standard to increase absorption of the incident light in the active layer. • In highly a-Si:H solar cells is mainly on use of texture substrates and highly reflective back contact. • The light path in the layer is increased and the light absorption enhanced.

4. Introduction • For light incident the rough interface

5. Introduction • For light incident a rough interface，the relation between the diffuse part and the total light is as follow • C: haze parameter • F angle dependence • The rough interface would be describe by rms roughness and scattering data. scattering data

6. Rms roughness • The parameter “σr” is the root-mean- square value of the thickness variation on the interface. • In this paper the rms roughness is the average form 20 measurements carried out on 3*3 um^2 scanned area.

7. Scattering parameter • Haze parameter • defined as the ratio between the diffuse part of transmitted light to the total transmitted light. • Measure total and specular transmittance and calculated the diffusetransmittance to obtain the haze parameter Asahi U-type TCO

8. Scattering parameter • Angular dependence • angular distribution function of the transmitted light this article is obtained from the ARS measurements • the angular distribution function of the transmitted diffuse part of light could be fitted by a Gaussian distribution function as follow Asahi U-type TCO

9. Optical modeling • Relation between surface roughness and scattering is important since we usually cannot measure the haze parameter of each layer in the solar cell structure. • For σr comparable or small than λ of the incident medium Rs is related to σrby Bennett and Porteus’s theory

10. Optical modeling • The ratio of Rd to R0 as a function of wavelength for several values of the rms roughness of the rough interface: (a)Asahi U-type TCO material and (b) n-type a-Si:H.

11. Optical modeling • Assumed the diffuse transmittance is similar to the diffuse reflection : • For the normal incident and introduce correction factors C1.C2:

12. Optical modeling • To fit the Asahi a-Si:H TCO , extracted the following formula • The ratio of Td to T0 as function of wavelength for several values of σr for: (a)the TCO/p interface and (b)the p/i interface.

13. Optical modeling • The haze parameter HT would be a formula as follow: • In this formula C is a factor that depends on the two media.

14. Assuming C =1 the simulated HT as a function of wavelength for different σr and measured HT for Asahi U-type TCO with σr = 40 nm • for simulation of HT of Asahi U-type TCO/air interface C= 0.5 is a better match

15. Conclusion • The angular distribution function for the transmitted light can be fitted by a Gaussian distribution function • Using optical modeling we obtain the relationship between surface roughness and haze parameter. But need to modify if not render a good agreement with experimental results • The measured haze parameter and angular distribution function will be incorporated in the optical model for a-Si:H solar cells that we are developing, in order to get accurate simulation results for a-Si:H solar cells.

16. Thanks for your kind attention!