Wet Pretreatment-Induced Modification of Cu(In,Ga)Se₂/Cd-Free ZnTiO Buffer Interface

Abstract

We report a novel Cd-free ZnTiO buffer layer deposited by atomic layer deposition for Cu(In,Ga)Se (CIGS) solar cells. Wet pretreatments of the CIGS absorbers with NHOH, HO, and/or aqueous solution of Cd ions were explored to improve the quality of the CIGS/ZnTiO interface, and their effects on the chemical state of the absorber and the final performance of Cd-free CIGS devices were investigated. X-ray photoelectron spectroscopy (XPS) analysis revealed that the aqueous solution etched away sodium compounds accumulated on the CIGS surface, which was found to be detrimental for solar cell operation. Wet treatment with NHOH solution led to a reduced photocurrent, which was attributed to the thinning (or removal) of an ordered vacancy compound (OVC) layer on the CIGS surface as evidenced by an increased Cu XPS peak intensity after the NHOH treatment. However, the addition of Cd ions to the NHOH aqueous solution suppressed the etching of the OVC by NHOH, explaining why such a negative effect of NHOH is not present in the conventional chemical bath deposition of CdS. The band alignment at the CIGS/ZnTiO interface was quantified using XPS depth profile measurements. A small cliff-like conduction band offset of -0.11 eV was identified at the interface, which indicates room for further improvement of efficiency of the CIGS/ZnTiO solar cells once the band alignment is altered to a slight spike by inserting a passivation layer with a higher conduction band edge than ZnTiO. Combination of the small cliff conduction band offset at the interface, removal of the Na compound via water, and surface doping by Cd ions allowed the application of ZnTiO buffer to CIGS treated with Cd solutions, exhibiting an efficiency of 80% compared to that of a reference CIGS solar cell treated with the CdS.

Charge Transfer in Mixed-Phase TiO2 Photoelectrodes for Perovskite Solar Cells

“…Today the PCE of PSCs reached 25% value on a laboratory scale in a glove-box, which is currently competitive to the efficiency of 26-27% for crystalline silicon solar cells [5,6] and 22.9% efficiency for CIGS solar cells [7]. PSCs are based on compounds with the chemical formula ABX 3 , where A = CH 3 NH 3 + , HC(NH 2 ) 2 + , Cs + ; B = Pb 2+ , Sn 2+ ; X = I − , Br − , Cl − .…”

Transparent Electrodes Consisting of a Surface‐Treated Buffer Layer Based on Tungsten Oxide for Semitransparent Perovskite Solar Cells and Four‐Terminal Tandem Applications

“…[ 3–10 ] High efficiency, low material cost, and simple solution processing make PSCs a highly promising source of sustainable energy as either single‐junction devices or in tandem structures on top of other solar cells with narrower band gap absorbers, such as silicon, chalcogenides, and Sn‐containing materials. [ 11–20 ] In particular, further improvement of silicon solar cell efficiencies through silicon‐perovskite tandems is realistic business models, considering that over 90% of the photovoltaic market is dominated by single‐junction silicon solar cells. [ 21 ]…”

ALD-ZnxTiyO as Window Layer in Cu(In,Ga)Se2 Solar Cells

“…Alternative buffer/window layers such as Zn(S,O), Zn x Mg y O, In x S y , Zn x Sn y O, and Zn x Ti y O have been applied in CIGS devices due to their wider bandgap or lower absorption coefficient achieving PCEs of 21.0%, 20%, 18.2%, 18.2%, and 12.5%, respectively. 4−8 With the introduction of heavy alkali (KF, RbF) post-deposition treatments (PDT) on CIGS absorbers the minimal thickness of CdS required for optimal PV performance was reduced from 50 nm to about 30 nm. 1,9,10 Thinner (<30 nm) CdS layer can lead to nonuniform coverage of the CIGS surface that would leave it prone to sputter damage during the subsequent ZnO/Al:ZnO window layer deposition.…”

“…13,14 To mitigate sputtering damage, plasma-free methods for the deposition of metal oxide window layers have been investigated: B:ZnO Al 2 O 3 , Zn x Ti y O, or TiO 2 were deposited by either metal–organic chemical vapor deposition (MOCVD) or ALD. 8,11,12,15…”

“…Recently a CdS-free CIGS device was presented with ALD-Zn x Ti y O in combination with a sputtered ZnO layer as HTR window layers leading to a PCE of 12.5%, which was inferior to the CdS-containing reference device of 15.4%. 8…”