Optical and recombination losses in thin-film Cu(In,Ga)Se2 solar cells

Abstract

Optical and recombination losses in thin-film solar cells based on CuInxGa1-xSe2 with the bandgaps 1.14–1.16 and 1.36–1.38eV have been evaluated. Parameters used for the analysis and calculations were verified by comparing the measured quantum efficiency spectra with the results of calculations. Optical losses due to reflections from the interfaces and absorption in the ZnO and CdS layers are found using the optical constants of materials taking into account anti-reflection coating on the ZnO surface. It is shown that for typical parameters of solar cells studied the optical losses amount to 16–18%. Losses due to the recombination of charge carriers at the front and back surfaces of the absorbing layer and in the space charge region (SCR) are calculated based on their dependences on the carrier lifetime, the concentration of uncompensated acceptors (determined the width of the SCR) and other parameters of the absorber. Total recombination losses in solar cells with the bandgaps of CuIn(Ga,In)Se2 1.14 and 1.36eV are equal to 7.0% and 4.5%, respectively. It is also shown that for solar cell with the bandgap of absorber 1.14eV, an incomplete charge collection is caused also due to non-optimal width of the SCR. An improvement of charge collection in this cell can be achieved by varying the concentration of uncompensated acceptors in the absorber. However, it is impossible to achieve such improvement for solar cells with the bandgap of 1.36eV due to shorter carrier lifetime in the material.