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The thin-film photovoltaic material Cu 2ZnSnS xSe 4–x (CZTSSe) has drawn world-wide attention due to its outstanding performance and earth-abundant composition. Until recently,[1] stateof-the-art CZTSSe thin-film solar cells were limited to 11.1% power conversion efficiency (PCE), with these performance levels being achieved via a hydrazine slurry approach.[2] Other vacuum-and non-vacuum-based deposition techniques have also been successful in fabricating CZTSSe solar cells with PCE above 8%.[3, 4] However, even record devices with PCE of 11% are still far below the physical limit, known as the Shockley-Queisser (SQ) limit, of about 31% efficiency under terrestrial conditions.[5]

For a solar cell with 1.13 eV bandgap such as the previous 11.1% champion,[2] the SQ limits for open circuit voltage (Voc) and short-circuit current density (Jsc) are 820 mV and 43.4 mA cm− 2, respectively. The previous 11.1% champion only achieved a Voc of 460 mV and a Jsc of 34.5 mA cm− 2, corresponding to about 56% and 79% of the SQ limit values. In order to boost Jsc, an optical architecture with optimized transparent conductive oxide (TCO) and CdS thicknesses has recently been reported, leading to a new CZTSSe record PCE of 12.0% and a Jsc that reaches 83% of the SQ limit.[1] Despite improvements in shortcircuit current, the Voc deficit, equal to the difference between the bandgap and Voc, is currently the biggest hurdle preventing CZTSSe devices from achieving higher efficiency.[6] Enhancement of Voc also directly improves device fill factor.[7] Although many factors can influence Voc in a solar cell, carrier generation and recombination …