Improved fill factor and open circuit voltage by crystalline selenium at the Cu(In,Ga)Se₂/buffer layer interface in thin film solar cells

A surface treatment by evaporated selenium on Cu(In,Ga)Se 2 (CIGS) is shown to improve open circuit voltage, V oc , and in some cases fill factor, FF, in solar cells with CdS, (Zn,Mg)O or Zn(O,S) buffer layers. V oc increases with increasing amount of crystalline Se, while FF improves only for small amounts. The improvements are counteracted by a decreasing short circuit current assigned to absorption in hexagonal Se. Improved efficiency is shown for device structures with (Zn,Mg)O and Zn(O,S) buffer layers by atomic layer deposition. Analysis by grazing incidence X‐ray diffraction and photoelectron spectroscopy show partial coverage of the CIGS surface by hexagonal selenium. The effects on device performance from replacing part of the CIGS/buffer interface area by a Se/buffer junction are discussed.