Quaternary Sputtered Cu(In,Ga)Se₂Absorbers for Photovoltaics: A Review

Abstract

Quaternary sputtering is a promising alternative to more established deposition methods for the fabrication of Cu(In,Ga)Se 2 (CIGS) thin films for photovoltaics (PV). In this technique, a single sputtering target containing all four constituents is employed to deposit the CIGS film. Quaternary sputtering offers several advantages over other deposition methods, including excellent uniformity over large areas, high material usage, and less reliance on toxic Se precursors such as H 2 Se. Despite these advantages, several drawbacks remain. To date, devices fabricated by quaternary sputtering without additional selenization have been limited in efficiency to about 11%, and realizing bandgap grading in order to match the performance of the best evaporated devices presents a challenge. We discuss the prospects for quaternary sputtering as a fabrication technique for CIGS and highlight areas of research that may result in improved performance. Target fabrication and usage is reviewed. We also present results for films and devices including data for the optical constants of sputtered CIGS. Some recent previously unpublished results, including a study of impurities in CIGS sputtering targets and the first demonstration of a CIGS device on a flexible glass substrate, are discussed Index Terms-Cu(In,Ga)Se 2 (CIGS), impurities, photovoltaic (PV) cells, sputtering.

Over 14% Efficiency of Directly Sputtered Cu(In,Ga)Se2 Absorbers without Postselenization by Post‐Treatment of Alkali Metals

“…Instead of employing a postselenization process, we have previously reported that Se deficiency can be overcome by sputtering from a Se‐rich CIGS target or by extra Na doping . However, the cell efficiency of the single‐target sputtering process without postselenization seems to be limited to 10%–11% . Further development is required so that it can compete with other processes.…”

Gallium In-Depth Profile in Bromine- Etched Copper–Indium–Galium–(Di)selenide (CIGS) Thin Films Inspected Using Raman Spectroscopy

“…As expected for samples grown with vacuum methods, not based on a three-stage approach, these samples do not host metal grading. 12…”

Effects of annealing atmosphere on the performance of Cu(InGa)Se2films sputtered from quaternary targets

“…In the industrial production of Cu(InGa)Se 2 (CIGS) photovoltaic devices, absorbers are usually produced by a two-step process comprising sputter deposition of a Cu-In-Ga alloy precursor, followed by post-selenization and sulfurization [ 1 , 2 ]. Sputtering has great advantages when the technology is transferred from laboratory-scale solar cells to production-scale panels, because it produces large-area film homogeneity [ 3 – 5 ]. Another promising method is based on the sputtering of CIGS quaternary targets and post-annealing; it involves high materials usage and less reliance on toxic selenium powder or H 2 Se [ 6 , 7 ].…”

“…Various Se-free fabrication routines have been reported [ 8 – 10 ]. For example, Frantz et al [ 5 ] and Chen et al [ 6 ] obtained CIGS device conversion efficiencies of 8%–10% by using one-step sputtering at high substrate temperatures with no post-treatment. However, the efficiency of a solar cell prepared in a Se-free atmosphere is far less than those prepared in a Se-containing annealing atmosphere.…”

“…It is unclear why CIGS device efficiency is lower when prepared by Se-free annealing. Franz et al found that the upper limit of the device efficiency is most likely the result of impurities in the sputtering target, which leads to electronic defects in the absorber [ 5 ]. Park et al reported that the formation of Se vacancies (V se ) on a CIGS film surface during Se-free treatment is the main limiting factor of the device efficiency [ 9 ].…”