A global electrical-optical model of thin film solar cells on textured substrates

In order to simulate the performance of the present day state-of-the-art multijunction solar cells in its entirety, an integrated electrical-optical model has been developed. The one-dimensional ab initio electrical model for the analysis of the transport properties of such devices can handle a very general semiconductor device structure where the material properties vary with position and the gap state properties with position and energy. The original semi-empirical optical model used takes into account both specular interference effects, and diffused reflectances and transmittances due to interface roughness. The latter are derived from angular-resolved photometric measurements and used as input parameters to the numerical programme. Comparison of the illuminated current density-voltage (J-V) characteristics, calculated on the basis of(a) a simple exponential absorption law and (b) the optical model, reveals an increase of similar to 1 mA cm(-2) in the short-circuit current and similar to 8% in the cell conversion efficiency for case (b). Also the long wavelength quantum efficiency (QE) shows a marked improvement, while the blue QE decreases since proper account is taken of the absorption in the transparent conducting oxide and reflection from the device. The combined model is being applied to simulate the characteristics of wideband-gap-emitter-layer solar cells deposited in a three chamber conventional glow discharge reactor onto (i) highly textured SnO2 and (ii) weakly textured indium tin oxide substrates. The cells have been characterised experimentally by J-V and QE measurements. Preliminary results indicate that the integrated model matches the experimental J-V and QE data with a more realistic set of material parameters as compared to case (a).