Absorption enhancement by transition metal doping in ZnS

Transition metal doping is an effective tool for controlling optical absorption in ZnS and hence the number of photons absorbed by photovoltaic devices. By using first principle density functional calculations, we compute the change in number of photons absorbed upon doping with a selected transition metal and found that Ni offers the best chance to improve the performance. This is attributed to the formation of defect states in the band gap of the host ZnS which give rise to additional dipole-allowed optical transition pathways between the conduction and valence band. Analysis of the defect level in the band gap shows that TM dopants do not pin Fermi levels in ZnS and hence the host can be made n- or p- type with other suitable dopants. The measured optical spectra from the doped solution processed ZnS nanocrystal supports our theoretical finding that Ni doping enhances optical absorption the most compared to Co and Mn doping.