Enhancing photocurrent collection in wide-gap ACIGS solar cells

Abstract Chalcopyrite solar cells utilising (Ag,Cu)(In,Ga)Se ₂ (ACIGS) absorbers have demonstrated cell efficiencies approaching 24% and adjustable band gaps between 1.0 and 1.7 eV, rendering them a promising material for tandem solar cells. The increased band gap obtained by increasing the compositional [Ga]/([Ga] + [In]) ratio has a detrimental impact on the absorber quality, resulting in open circuit voltage, fill factor and short-circuit current below expectations. The latter is due to a poor photocurrent collection, which is particularly evident in reduced external quantum efficiency (EQE), accompanied by substantial losses in the near infrared. This study identifies a previously unrecognised mechanism reducing the photocurrent collection in wide-gap ACIGS solar cells processed on transparent back contact. Manipulations at the ACIGS/CdS interface, particularly KCN etching and variations in ammonia rinsing duration of the absorber, effectively restores both the EQE maximum and the near-infrared response. The observed inverse correlation between doping density and EQE maximum suggests an electron barrier caused by a highly doped p ⁺ layer near the ACIGS/CdS interface, a trend reproduced by drift-diffusion simulations. Raman spectroscopy and x-ray diffraction analyses demonstrate no correlation between ordered vacancy compounds and the photocurrent collection issue. Mitigation strategies significantly improve photocurrent collection and device performance, notably reduced post deposition treatment associated with reduced doping level, thicker CdS buffer layer through extended chemical bath deposition, and absorber annealing after rinsing. The resulting devices with efficiency reaching 12.9% are close to the most efficient semi-transparent wide-gap ACIGS solar cells to date.