In this contribution, we have discussed current models of the chalcopyrite-based heterojunction solar cell and their implications for achieving reliable and efficient devices. Numerical simulation of the device is an important tool for assessing the influence of various parameters. It has been used here to provide examples for the topics discussed. The key points for the minimisation of interface recombination are: doping of absorber and window electrical charge and band line-up at the interface. and interface recombination velocity.