Adjusting the Ga grading during fast atmospheric processing of Cu(In,Ga)Se₂ solar cell absorber layers using elemental selenium vapor

Abstract

We study the sequential fabrication of Cu(In,Ga)Se2 (CIGSe) absorber layers by using an atmospheric pressure selenization with a process duration of only a few minutes and the utilization of elemental selenium vapor from independent Se sources. This technology could proof to be an industrially relevant technology for the fabrication of thin‐film solar cells. Controlling the amount of Se provided during the selenization of metal precursors is shown to be an effective measure to adjust the Ga in‐depth distribution. A reduced Se supply for CIGSe formation leads to a more homogeneous Ga distribution within the absorber. The underlying growth dynamics is investigated by interrupting the selenization at different times. At first, CIGSe formation occurs in accordance with previously suggested growth paths and Ga segregates at the Mo back contact. Between 520 and 580 °C, the growth dynamics differs distinctly, and In and Ga distribute far more uniformly within the absorber depth. We also studied the impact of the precursor architecture. The best performing precursor in terms of efficiency of the respective solar cells was a multilayer with 22 In/CuGa/In triple layers. Simple bilayers stacks lead to films of higher roughness and correlated shunting. By optimizing the precursor architecture and the Ga in‐depth distribution in the CIGSe layer, a conversion efficiency of up to 15.5% (active area) could be achieved. To our knowledge, this is the highest reported efficiency for sulfur free CIGSe‐based solar cells utilizing fast (few minutes) atmospheric processes and elemental Se vapor. Copyright © 2017 John Wiley & Sons, Ltd.

Effects of Ammonia‐Induced Surface Modification of Cu(In,Ga)Se2 on High‐Efficiency Zn(O,S)‐Based Cu(In,Ga)Se2 Solar Cells

“…Combing the results of CGI, GGI, Se, and Na, we can speculate the following scenario: the CIGS sample grown by the three‐stage fabrication procedure inevitably consists of a non‐uniformity in the thickness direction, which not only has a Ga‐grading but also has a Cu deficiency. While such structure is globally beneficial to the device performance, imperfection still occurs. With the co‐deposition of In, Ga, and Se in the third stage of the three‐stage process, the very surface seems to lack of Cu in a high degree.…”

Two‐step selenization using nozzle free Se shower for Cu(In,Ga)Se2 thin film solar cell

“…Therefore, it is essential to control the amount of Se participating in the reaction during selenization heat treatment and maintain a uniform Se activity over a large area, which is essential for high quality CIGSe thin film synthesis. Recently, S.S. Schmidt et al reported a conversion efficiency of 15.5% using an in‐line selenization process that supplies Se vapor through a nozzle array . This result corresponded to the highest efficiency of 2‐step CIGS solar cells produced using Se vapor.…”

“…Therefore, a range of processes have been assessed. Table presents representative cases where the small area solar cell efficiency is 10% or more . On the other hand, because Se vapor has different characteristics from general gas, several problems can be encountered when it is supplied in the vapor state when attempting to obtain a uniform heat treatment result.…”

Selenization of CuInS2 by rapid thermal processing – an alternative approach to induce a band gap grading in chalcopyrite thin-film solar cell absorbers?

“…However, a reproducible control of the [Ga]/([In] + [Ga]) throughout the absorber layer prole continues to be a challenge. [13][14][15][16][17][18][19] Finding a straightforward approach to control the [Se]/([S] + [Se]) prole distribution of chalcopyrite absorbers would be an effective alternative route to produce band gap grading congurations.…”