Photovoltaic Efficiency Enhancement of Indium-Free Wide-Bandgap Chalcopyrite Solar Cells via an Aluminum-Induced Back-Surface Field Effect

Wide-bandgap chalcopyrite materials are attractive candidates for a wide variety of energy conversion devices such as the top cell of tandem-type photovoltaic devices and photoelectrochemical water splitting hydrogen evolution devices. Nevertheless, simultaneous realization of high open circuit voltage (<i>V</i>_OC) and high fill factor (FF) values has been challenging, and thus, the photovoltaic performance has been limited. In this article, high <i>V</i>_OC and high FF values of wide-gap chalcopyrite CuGaSe₂ thin-film solar cells are simultaneously demonstrated using an aluminum-induced back-surface field effect. An independently certified photovoltaic efficiency of 12.25% was obtained from an elemental In-free CuGaSe₂:Al (bandgap energy ∼1.7 eV) solar cell (<i>V</i>_OC: 0.959 V, short circuit current density: 17.64 mA cm^-2, FF: 72.5%). In addition, an over 1 V-<i>V</i>_OC CuGaSe₂ cell (FF: 74.5%) was obtained even with the use of a conventional CdS buffer layer. Although incorporation of aluminum often leads to degradation of the chalcopyrite solar cell performance, the addition of a small amount of aluminum is found to be effective in enhancing wide-gap chalcopyrite photovoltaic performance.