Optical modeling of a-Si:H solar cells with rough interfaces: Effect of back contact and interface roughness

An approach to study the optical behavior of hydrogenated amorphous silicon solar cells with rough interfaces using computer modeling is presented. In this approach the descriptive haze parameters of a light scattering interface are related to the root mean square roughness of the interface. Using this approach we investigated the effect of front window contact roughness and back contact material on the optical properties of a single junction $\mathrm{a-}\mathrm{Si:H}$ superstrate solar cell. The simulation results for $\mathrm{a-}\mathrm{Si:H}$ solar cells with ${\mathrm{SnO}}_{\mathrm{2}}\mathrm{:F}$ as a front contact and ideal Ag, ZnO/Ag, and Al/Ag as a back contact are shown. For cells with an absorber layer thickness of 150–600 nm the simulations demonstrate that the gain in photogenerated current density due to the use of a textured superstrate is around $\text{2.3\hspace{0.05em}}{\mathrm{mA cm}}^{\mathrm{-2}}$ in comparison to solar cells with flat interfaces. The effect of the front and back contact roughness on the external quantum efficiency (QE) of the solar cell for different parts of the light spectrum was determined. The choice of the back contact strongly influences the QE and the absorption in the nonactive layers for the wavelengths above 650 nm. A practical Ag back contact can be successfully simulated by introducing a thin buffer layer between the n-type $\mathrm{a-}\mathrm{Si:H}$ and Ag back contact, which has optical properties similar to Al, indicating that the actual reflection at the n-type $\mathrm{a-}\mathrm{Si:H}/\mathrm{Ag}$ interface is smaller than what is expected from the respective bulk optical parameters. In comparison to the practical Ag contact the QE of the cell can be strongly improved by using a ZnO layer at the Ag back contact or an ideal Ag contact. The photogenerated current densities for a solar cell with a 450 nm thick intrinsic $\mathrm{a-}\mathrm{Si:H}$ layer with ZnO/Ag and ideal Ag are 16.7 and $\text{17.3\hspace{0.05em}}{\mathrm{mA cm}}^{\mathrm{-2}},$ respectively, compared to $\text{14.4\hspace{0.05em}}{\mathrm{mA cm}}^{\mathrm{-2}}$ for the practical Ag back contact. The effect of increasing the roughness of the contact interfaces was investigated for both superstrate and substrate types of solar cells. Increasing the roughness of the carrier electrode, i.e., the rough electrode on which the silicon cell structure is deposited, up to 35 nm leads to a strong increase in the photogenerated current density; for higher values of the interface roughness the photogenerated current density tends to saturate.