High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency

Title	High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency
Authors	Jan Keller, Klara Kiselman, Olivier Donzel-Gargand, Natalia M. Martin, Melike Babucci, Olle Lundberg, Erik Wallin, Lars Stolt, Marika Edoff
Magazine	Nature Energy
Date	02/19/2024
DOI	10.1038/s41560-024-01472-3
Introduction	Chalcopyrite solar cells have achieved efficiencies up to 23.35%, but further enhancement has proven difficult. This work details the attainment of a 23.64% certified efficiency for a (Ag,Cu)(In,Ga)Se2 solar cell, accomplished through several key approaches. These include integrating a significant silver content ([Ag]/([Ag] + [Cu]) = 0.19) into the absorber layer and applying a distinct gallium grading profile. This profile features a high gallium concentration near the molybdenum back contact, decreasing to a constant, lower concentration closer to the CdS buffer layer. This precise elemental distribution effectively mitigates lateral and in-depth bandgap variations, thereby curtailing open-circuit voltage losses. Furthermore, the resulting bandgap energy is optimally set at approximately 1.15 eV. An additional strategy involves a Rubidium Fluoride (RbF) post-deposition treatment, which facilitates the creation of a Rb–In–Se phase, likely RbInSe2, that effectively passivates the absorber surface. The study also outlines prospective research avenues aimed at achieving 25% efficiency.
Quote	Jan Keller, Klara Kiselman and Olivier Donzel-Gargand et al. High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency. Nat. Energy. 2024. DOI: 10.1038/s41560-024-01472-3
Element	Silver (Ag)