Exploring the Potential of Pure Germanium Kesterite for a 2T Kesterite/Silicon Tandem Solar Cell: A Simulation Study.

Recently, the development of tandem devices has become one of the main strategies for further improving the efficiency of photovoltaic modules. In this regard, combining well-established Si technology with thin film technology is one of the most promising approaches. However, this imposes several limitations on such thin film technology, such as low prices, the absence of scarce or toxic elements, the possibility to tune optical properties and long lifetime stability. Therefore, to show the potential of kesterite/silicon tandems, in this work, a 2 terminal (2T) structure using pure germanium kesterite was simulated with combined SCAPS and transfer matrix methods. To explore the impact of individual modifications, a stepwise approach was adopted to improve the kesterite. For the bottom sub cell, a state-of-the-art silicon PERC cell was used with an efficiency of 24%. As a final result, 19.56% efficiency was obtained for the standalone top kesterite solar cell and 28.6% for the tandem device, exceeding standalone silicon efficiency by 4.6% and justifying a new method for improvement. The improvement observed could be attributed primarily to the enhanced effective lifetime, optimized base doping, and mitigated recombination at both the back and top layers of the CZGSSe absorber. Finally, colorimetric analysis showed that color purity for such tandem structure was low, and hues were limited to the predominant colors, which were reddish, yellowish, and purple in an anti-reflective coating (ARC) thickness range of 20-300 nm. The sensitivity of color variation for the whole ARC thickness range to electrical parameters was minimal: efficiency was obtained ranging from 28.05% to 28.63%.