Post-deposition annealing and interfacial ALD buffer layers of Sb$_2$Se$_3$/CdS stacks for reduced interface recombination and increased open-circuit voltages
Currently, Sb$_2$Se$_3$ thin films receive considerable research interest as a solar cell absorber material. When completed into a device stack, the major bottleneck for further device improvement is the open circuit voltage, which is the focus of the work presented here. Polycrystalline thin film Sb$_2$Se$_3$ absorbers and solar cells are prepared in substrate configuration and the dominant recombination path is studied using photoluminescence spectroscopy and temperature dependent current-voltage characteristics. It is found that a post-deposition annealing after the CdS buffer layer deposition can effectively remove interface recombination since the activation energy of the dominant recombination path becomes equal to the bandgap of the Sb$_2$Se$_3$ absorber. The increased activation energy is accompanied by an increased photoluminescence yield, i.e. reduced non-radiative recombination. Finished Sb$_2$Se$_3$ solar cell devices reach open circuit voltages as high as 485 mV. Contrarily, the short-circuit current density of these devices is limiting the efficiency after the post-deposition annealing. It is shown that atomic layer deposited intermediate buffer layers such as TiO$_2$ or Sb$_2$Se$_3$ can pave the way for overcoming this limitation.