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Published online by Cambridge University Press: 15 February 2011
An integrated electrical-optical model has been used to simulate and examine ways of optimising the performance of double junction solar cells, where both the component cells have a-Si:H absorber layers of identical material quality. In the optical modelling part we take into account both specular interference effects; and diffused reflectances and transmittances due to interface roughness. The model simulates carrier transport in the junction between the two p-i-n subcells with the help of a thin heavily defective recombination layer (RL) having a reduced band gap.
Our results reveal that in order to simulate the current-voltage and the quantum efficiency (QE) characteristics of these cells, window losses and light-trapping effects need to be properly accounted for. Results indicate that the highest open-circuit voltage is attained when the majority carrier quasi-Fermi levels on either side of the RL coincide. Also for the highest multijunction cell efficiency the thicknesses of the component subcells are such that the electric field in both are fairly close to one another. Finally, the QE under AM 1.5 bias light at the maximum power point has been shown to be extremely sensitive to thickness variations of the component subcells and hence an useful tool for multijunction cell optimisation.