A computer analysis of double junction solar cells with a-Si : H absorber layers

Abstract

An integrated electrical-optical model has been used to examine the design of double junction solar cells, where the component cells have a-Si : H absorber layers of identical material quality in the initial state. The model takes into account both specular interference effects; and diffused reflectances and transmittances due to interface roughness. The carrier transport at the junction between the two p–i–n subcells is simulated with the help of a thin heavily defective “recombination” layer with a reduced mobility gap. Analysis of the transport properties as a function of position in the device indicates that for the highest double junction cell efficiency the thicknesses of the absorber layers of the component subcells are such that the electric fields over these absorber layers are high simultaneously. Our results also show that whereas in the initial state, the open-circuit voltage and the fill factor of the double junction cell are heavily dependent on the electric field in the thicker bottom subcell; in the light-stabilised state, the more degraded top subcell plays an important role in limiting double junction cell performance. The quantum efficiency under AM 1.5 bias light has been shown to be very sensitive to thickness variations of the component subcells. Using this tool we have arrived at a simplified procedure for designing the double junction structure likely to exhibit the highest efficiency in the stabilised state.