Role of Cu deficient layer in chalcopyrite based solar cells

Chalcopyrites are a demonstrated material platform for realizing efficient thin-film photovoltaics, with the most well-known Cu(In,Ga)Se₂ (CIGS) based solar cells exceeding 23%. The presence a Cu-poor phase layer between the absorber and buffer layers in CIGS solar cells is known to enhance device performance, however, the overall properties and role of very thin layer remain poorly understood. Using first principles calculations based on density functional theory with screened hybrid functionals, we explore the electronic structure and stability of a series ordered vacancy compounds (or OVCs) as a model for the Cu-poor phase layer. We calculate band offsets between the OVCs and defect-free chalcopyrite Cu and Ag based compounds (ABX₂ where A = Cu, Ag; B = In, Ga, Al and X = S, Se). Using AB₃X₅ and AB₅X₈ stoichiometries as model OVC systems, we report on the variation of the band gap with A/B ratio and discuss the trends in other Cu and Ag-based chalcopyrites beyond CuInSe₂. We additionally perform device-level simulations to understand the implications of formation of OVCs, finding that valence band offsets of OVCs are favorable for hole transport, while the conduction band offsets of chalcopyrites beyond CuInSe₂-based absorbers may be detrimental to device performance.