Open Access Version

Abstract:
In recent years, post deposition treatment (PDT) of the absorber with RbF has led to a significant improvement in the efficiency of Cu(In,Ga)Se2 (CIGSe) thin-film solar cells. In this work, the influence of RbF by PDT on the recombination and current transport mechanisms was studied. Temperature-dependent current-voltage (JVT) measurements were performed with a set of CIGSe samples prepared under various preparation conditions at Helmholtz-Zentrum Berlin (HZB) and in other laboratories. Based on JVT measurements, CIGSe cells with RbF-PDT present non-ideal current- voltage (J-V) characteristics especially at low temperatures such as a roll-over of the diode current, a cross-over between the dark and illuminated curves, a saturation of the open-circuit voltage (Voc) and, examined in detail here for the first time, a discrepancy between dark and light J-Vs. These negative and positive (typically an increased Voc) effects of PDT may derive from different physical mechanisms since these non-idealities are not necessary linked to the high efficiency presented in some measured solar cells. The open-circuit voltage (Voc) can be influenced by the phototransistor effect visible at low or in some cases even at higher temperatures, reducing its value and extrapolating to lower activation energies than expected without any Voc clamping. Analysis of recombination mechanisms showed that typical CIGSe solar cells without or with RbF-PDT present dominant recombination within the bulk therefore contradicting those studies that assume a transition from interface to recombination in the bulk due to changes especially observed at or near the absorber surface after PTD. The behavior of the deduced diode quality factors as a function of the temperature suggested a recombination in the space charge region with an exponential defect distribution, and a transition towards recombination in the quasi neutral region in samples with RbF-PDT. The influence of electronic material parameters on the blocking of the forward diode current across the heterojunction and at the back contact of the device was studied by numerical simulations. These models are focused on barriers at the heterointerfaces of the solar cell device in order to consider some of the effects of RbF reported in previous studies such as the alkali diffusion from the absorber into the buffer or window layer and their interfaces. It was found that a low-doped ZnO window layer, the presence of acceptor defects at the buffer/window interface or a high band offset at that interface all lead to a similar diode current limitation as a result of a low carrier concentration in the buffer layer. The reduction of the Na content especially at the back contact of the absorber with long Rb deposition times revealed in previous studies is consistent with the introduction of a back barrier in numerical simulations, exhibiting the aforementioned non-idealities and describing those cells which present a strong Voc saturation at low temperatures. This work provides a better understanding of the electronic parameters that may have an influence on the blocking mechanisms when alkalis are deposited on CIGSe absorbers.