Nanoscale measurements of the surface photovoltage in Cu(In, Ga)Se2, Cu2ZnSnS4, and Cu2ZnSnSe4 thin films: The role of the surface electronics on the efficiency of solar cells

We report on recent advances in the development of nanoscale measurements of the surface photovoltage (SPV) based on scanning tunneling microscopy (STM) and its application to the kesterites Cu₂ZnSnS₄ (CZTS) and Cu₂ZnSnSe₄ (CZTSe). One critical aspect of the electronic structure of Cu(In, Ga)Se₂ (CIGS) that has yet to be determined in their related kesterite compounds is the character of the surface electronics. In CIGS, spontaneous deviations in the stoichiometry of the surface cause a depletion (or even a type inversion) region that reinforces the CIGS homojunction. First-principle calculations predict that this inversion region will be more difficult to form in CZTS. In this contribution, the characteristics of the surface space charge region for both CIGS and CZTS(e) are investigated by STM. The implications of the results of these measurements on the future development of CZTS solar cells will be discussed.