The role of NaF post-deposition treatment on the photovoltaic characteristics of semitransparent ultrathin Cu(In,Ga)Se2 solar cells prepared on indium-tin-oxide back contacts: a comparative study

Cu(In,Ga)Se2 (CIGSe) solar cells with absorber thicknesses of <500 nm are important for lowering the cost of photovoltaic (PV)-generated electricity. Moreover, ultrathin bifacial CIGSe solar cells can be prepared on indium-tin-oxide (ITO) back contacts (BCs). In contrast to Mo BCs, ITO BCs suppress the diffusion of Na from soda-lime glass (SLG) to the CIGSe absorber. Na present in the absorber is supposed to ameliorate the PV properties of CIGSe solar cells, but in the absence of Na or when the Na concentration is extremely low, the PV performance is expected to be poor. In this study, a NaF post-deposition treatment (PDT) was applied to a <500 nm thick semitrasparent CIGS absorber prepared by a 1-stage co-evaporation process. A detailed comparison is made between the CIGSe solar cell that underwent the NaF PDT (C–Na) and a reference CIGSe solar cell in which no Na was supplied from an external source (C0). All the PV parameters (i.e., the open-circuit voltage, short-circuit current density, fill factor, and efficiency) of C–Na considerably improved compared with those of C0. To understand the factors that led to this improvement, the solar cells are analyzed by various characterization techniques, including JV measurements, external quantum efficiency measurements, temperature-dependent measurements of the open-circuit voltage, capacitance–voltage measurements, drive level capacitance profilometry, and admittance spectroscopy. Furthermore, the reaction occurring at the CIGSe/ITO interface is investigated with transmission electron microscopy, and the implications of this reaction on the device performance are discussed.