The Application of Sputtered Gallium Oxide as Buffer for Cu(In,Ga)Se₂ Solar Cells

Abstract

Crystalline gallium oxide is a promising wide‐bandgap semiconductor material, especially for applications in high‐frequency and high‐power devices. With an optical bandgap energy well above 4 eV, which implies no visible light absorption, it is also a candidate for one of the front‐side layers in thin‐film solar cells. X‐ray amorphous gallium oxide (a‐Ga2O3) deposited by RF magnetron sputtering is applied as an n‐type buffer layer in substrate‐type configuration solar cells based on industry‐relevant inline coevaporated Cu(In,Ga)Se2 (CIGS) absorbers, which include an i‐ZnO/ZnO:Al bilayer as the front electrode. The cells exhibit an efficiency peak at Ga2O3 deposition temperatures in the range of 140–180 °C and show mostly a gain in short‐circuit current density compared with the CdS‐buffered reference cells, as a result of the high optical bandgap of a‐Ga2O3 in the range of 4.6–4.8 eV. The CIGS solar cells with sputtered a‐Ga2O3 buffers reach efficiencies of almost 14%, lacking in open‐circuit voltage and fill factor, whereas the CdS‐buffered cells are on a 16–17% level. Light soaking of the cells leads to a slight improvement of the fill factor, but the gap in open‐circuit voltage of 80–100 mV in contrast to the CdS‐buffered reference cells remains unaffected.

Rb Diffusion and Oxide Removal at the RbF-Treated Ga2O3/Cu(In,Ga)Se2 Interface in Thin-Film Solar Cells

“…Subsequently, Ga 2 O 3 was deposited by radiofrequency (RF) magnetron sputtering from a ceramic target at a substrate temperature of 150 °C (details of the Ga 2 O 3 sputter-deposition process can be found in refs and ), generating films with thickness d of 1, 3, 10, and 100 nm. The thicknesses were estimated from sputter-deposition rates determined by optical transmittance measurements on thick Ga 2 O 3 layers ( d > 200 nm) deposited on 1 mm highly transparent quartz-glass substrates.…”

“…Avoiding the use of CBD-CdS is motivated by reducing the environmental impact of the production process, 19,20 reducing the parasitic absorption, 21−23 and developing a "dry" process that does not interrupt a vacuum-based process chain. 20,24 Thus, alternative buffer layer materials, including In x S y , 24−28 Zn(O,S,OH), 3,28,29 (Zn,Mg)O, 30−32 Zn 1−x Sn x O y , 33,34 HfO x , 35 Al 2 O 3 , 28,36,37 Ga 2 O 3 , 23,24,38 Sn 1−x Ga x O y , 22 (In,Ga) 2 O 3 , 23 and (Al,Ga) 2 O 3 , 23 have been investigated. A particularly promising candidate among the alternative buffer layers is the wide band gap transparent oxide Ga 2 O 3 , with an optical bulk band gap between 4.4 and 4.9 eV.…”

“…In the first trial, ZSW used sputter-deposited gallium oxide as a buffer layer for CIGSe absorbers, reaching efficiencies up to ∼13.7% (compared to ∼17.6% with a CdS reference buffer). 24 Furthermore, replacing i-ZnO in a ZnO/Al/i-ZnO/ CdS/CIGSe structure by sputtered Ga 2 O 3 resulted in cell efficiencies of 20.2% (compared to 20.4% for the reference cell). 44 The absorbers were exposed to RbF-PDT as in the CdS-based ZSW process that has led to efficiencies above 22%.…”

“…Due to the wider band gap of Ga 2 O 3 , an increase in J sc compared to reference cells with CdS was observed. 24,44 In the current work, we present a detailed investigation of the chemical structure of the RbF-PDT CIGSe absorber surface, the sputter-deposited Ga 2 O 3 buffer layer, and the Ga 2 O 3 /CIGSe interface. Particular focus is placed on the investigation of the ammonia-based rinsing step and its impact on the chemical structure of the absorber surface and the buffer/absorber interface, which is substantially more complex than a simple removal of RbF.…”

“…Chemical-bath-deposited CdS (CBD-CdS) is generally used as the buffer layer in high-efficiency CIGSe-based thin-film solar cells, ,− while the current world records for mini modules and lab-scale cells have already utilized sputtered Zn­(O,S) or CBD-Zn­(O,S,OH) buffers, respectively. Avoiding the use of CBD-CdS is motivated by reducing the environmental impact of the production process, , reducing the parasitic absorption, − and developing a “dry” process that does not interrupt a vacuum-based process chain. , …”