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

Chalcopyrite-based solar cells have reached an efficiency of 23.35%, yet further improvements have been challenging. Here we present a 23.64% certified efficiency for a (Ag,Cu)(In,Ga)Se2 solar cell, achieved through the implementation of a series of strategies. We introduce a relatively high amount of silver ([Ag]/([Ag] + [Cu]) = 0.19) into the absorber and implement a 'hockey stick'-like gallium profile with a high concentration of Ga close to the molybdenum back contact and a lower, constant concentration in the region closer to the CdS buffer layer. This kind of elemental profile minimizes lateral and in-depth bandgap fluctuations, reducing losses in open-circuit voltage. In addition, the resulting bandgap energy is close to the local optimum of 1.15 eV. We apply a RbF post-deposition treatment that leads to the formation of a Rb-In-Se phase, probably RbInSe2, passivating the absorber surface. Finally, we discuss future research directions to reach 25% efficiency. Keller et al. use high-concentration silver alloying and steep gallium grading close to the back contact to minimize bandgap fluctuations and thus voltage losses, achieving 23.6% certified efficiency in Cu(In,Ga)Se2 solar cells.