A quantitative investigation into the trapping mechanisms responsible for the improvement of CdTe solar cell performance upon progressive treatment with MgCl_2 is presented. It was found for the first time that the improvement, maximisation and decline in performance (due to overtreatment) all correlated well with the systematic variation of the trap properties and carrier concentrations with the activation treatment. In particular the trap energies in the CdTe were not related to specific point defects but were instead continuously variable in energy as the MgCl_2 treatment progressed. It was found that the maximum of the solar cell efficiency correlated well with a) a minimum in the trap energy, its capture cross section and density and b) a corresponding maximum in the acceptor carrier density. The improvement in solar cell performance arising from chloride treatment is therefore considered to be due to either to a reduction in the recombination loss from deep levels and/or a corresponding increase in the acceptor density, itself due to the electrical passivation of states that would otherwise act to compensate doping.
展开▼
