Regular Article

Impact of RbF-PDT on Cu(In,Ga)Se₂ solar cells with CdS and Zn(O,S) buffer layers

Zentrum für Sonnenenergie- und Wasserstoff- Forschung, Meitnerstraße 1, 70563 Stuttgart, Germany

^* e-mail: ana.kanevce@zsw-bw.de

Received: 15 July 2020
Received in final form: 15 September 2020
Accepted: 17 September 2020
Published online: 21 October 2020

Abstract

Alkali-fluoride post-deposition treatments (PDTs) of Cu(In,Ga)Se₂ (CIGS) absorbers have repeatedly resulted in device efficiency improvements, observed mainly due to an open-circuit voltage (V_oc) enhancement. Replacement of the CdS buffer layer with a higher band gap alternative can increase the short-circuit current density (J_sc) and also eliminate the use of Cd. In many alternative-buffer attempts, however, the J_sc gain was accompanied by a V_oc loss, resulting in some degree of performance loss. In order to better understand the impact of RbF-PDT, we analyze a combination of experimental devices produced in the same in-line CIGS run with and without RbF-PDT in combination with chemical-bath-deposited CdS and Zn(O,S) buffers. Low-temperature current–voltage curves indicate a difference in Rb impact on the CIGS/CdS and CIGS/Zn(O,S) p-n junctions. For example, the diode-current barrier which creates a rollover often observed in RbF-treated CIGS/CdS current–voltage curves is significantly reduced for the CIGS/Zn(O,S) junction. Although the RbF-PDT had a positive impact on both junction partner combinations, the CIGS/Zn(O,S) devices' V_oc and fill factor (FF) benefited stronger from the RbF treatment. As a result, in our samples, the J_sc and FF gain balanced the V_oc loss, thus reducing the efficiency difference between cells with CdS and Zn(O,S) buffers.