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

Speaker： Hsiao-Wei Liu08/18/2010 Wed

Outline

Introduction

Parameters to describe a rough interface

Optical modeling and data fitting

Conclusion

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.

Introduction

For light incident the rough interface

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

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.

Asahi U-type TCO

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 diffuse transmittance to obtain the haze parameter

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

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 σr by Bennett and Porteus’s theory

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.

Optical modeling

Assumed the diffuse transmittance is similar to the diffuse reflection :

For the normal incident and introduce correction factors C1.C2:

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.

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.

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

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.

Thanks for your kind attention!