Tunnelling enhanced recombination in polycrystalline CdS/CdTe and CdS/Cu(In,Ga)Se2 heterojunction solar cells

This article investigates the results of current-voltage measurements made at different temperatures on vacuum deposited ZnO/CdS/Cu(In,Ga)Se2 and CdS/CdTe heterojunction solar cells. We propose that the current-voltage data of a typical CdS/CdTe solar cell can be analysed by tunnelling enhanced bulk and interface recombination which gives a quantitative description of the electronic loss mechanisms in the chalcopyrite based heterojunction solar cells. We show that the temperature dependence of the saturation current and the diode ideality factor of the CdS/CdTe device are well described by this model. Below 240 K, tunnelling enhanced recombination at junction interface is found to play a major role for recombination with activation and tunnelling energies as about 0.77 eV and E00 = 36 meV, respectively. The transport mechanism in ZnO/CdS/Cu(In,Ga)Se2 device appeared to be also dominated by interface recombination. This unexpected behaviour is attributed to the presence of Cu-rich and indium depleted thin layer which might possibly be formed on the absorber surface.