Maximizing Photocurrent via Current Matching Towards Efficient All-perovskite Tandem Solar Cells

Abstract

In the current age where climate change is no longer a threat of a distant future, there is a definite imminence to the clean energy transition initiative. Solar energy has been a steadily rising candidate for the transition, with photovoltaics attracting much attention for their outstanding optical and electrical properties, allowing for fabrication of highly efficient devices. From the wide variety of photovoltaics, perovskites have been at the center of focus for their realization of highly efficient devices that have the possibility of surpassing their silicon counterparts due to their tunable bandgap and wide window of applications. The flexibility in processing, from low-temperature to solution- and solid-state manufacture, coupled with flexibility in choice of substrates make them viable for various applications, such as their incorporation into tandem structures. Therefore, researchers have researched numerous methods of raising the efficiency of perovskite-based photovoltaics, ranging from compositional engineering to additive incorporation, to reach efficiencies closer to the limit of ~33.16% power conversion efficiency (PCE) set by the Shockley-Quiesser limit (SQ Limit) for single-junction photovoltaics. While current records have made great efforts to reach the aforementioned limit, with the current record for single-junction photovoltaics set at 27% PCE, the current world record efficiency for two-junction perovskite/silicon tandem photovoltaics is set at 34.9% PCE, highlighting the immense potential of multi-junction devices toward ultra-high efficiency photovoltaics. The typical tandem structure consists of two or more sub-cells that are stacked on top of each other, for example, a sub-cell with a wide bandgap (WBG; top) paired with another sub-cell with a very narrow bandgap (NBG; bottom). The top sub-cell is typically perovskite-based, while the bottom sub-cell is interchangeable between perovskite, organic, and silicon. While perovskite/silicon and perovskite/organic tandem structures show record and moderate efficiencies, respectively, perovskite/perovskite tandem structures show most promise in attaining the best balance between high performance and low-cost manufacture, while also unlocking flexibility in applications due to the versatility in substrate choice and tunability of both sub-cells. Research is mostly focused on optimization of each sub-cell to raise the efficiency of perovskite/perovskite tandem structured photovoltaics, with the goal of achieving better current- matching and higher voltage output through various methods of engineering. Studies revolving around current matching, raising voltage output and substitution of the interconnecting layer have led to the attainment of high efficiencies reaching up to 30.1% PCE. Herein, we propose the immense significance of achieving better current matching for high quality, ultra efficient all-perovskite tandem photovoltaics using manipulation of layer thicknesses and addition of additives. Manipulating the thickness of each sub-cell through changes in the fabrication process to increase their thicknesses led to increased current output and better current matching as proved by the heightened PCE of the resultant tandem photovoltaics. Specifically, the addition of stabilizing additives in the NBG sub-cell led to a dramatic increase in voltage output, leading to greater voltage output in the resultant tandem photovoltaics. Modification of each respective layer made it possible to realize highly efficient perovskite/perovskite tandem photovoltaics and subsequent modules. This led to the fabrication of all-perovskite tandem photovoltaics with properly optimized sub-cells. Overall, this research has provided future researchers with proof-of-concept current matching capabilities and serves to emphasize the significance of this work towards the eventual development of commercializable all-perovskite tandem modules ready for real life operation.