Piracetam shapes wide-bandgap perovskite crystals for scalable perovskite tandems

Abstract

All-perovskite tandem solar cells (TSCs) offer exceptional performance and versatile applicability. However, a significant challenge persists in bridging the power conversion efficiency (PCE) gap between small- and large-area (>1 cm²) devices, which presents a formidable barrier to the commercialization of all-perovskite TSCs. Here we introduce a specialized crystal-modifying agent, piracetam, tailored for wide-bandgap perovskites, homogenizing top wide-bandgap subcells and enabling the construction of efficient large-area TSCs. Piracetam, featuring amide and pyrrolidone moieties, initially modulates perovskite nucleation, resulting in large-sized grains, preferred (110) orientation, enhanced crystallinity and uniform optoelectronic properties. During the subsequent annealing process, it further eliminates residual PbI₂ and facilitates the formation of one-dimensional (Pi)PbI₃ (Pi = piracetam) perovskite nanoneedles at the grain boundaries and surfaces. Consequently, single-junction 1.77 eV-bandgap solar cells achieve a certified open-circuit voltage of 1.36 V and a PCE of 20.35%. Furthermore, our monolithic two-terminal all-perovskite TSCs, with aperture areas of 0.07 cm² and 1.02 cm², yield PCEs of 28.71% (stabilized 28.55%, certified 28.13%) and 28.20% (stabilized 28.05%, certified 27.30%), respectively, demonstrating a minimal PCE loss of 0.51% when transitioning from small-area to large-area devices. In addition, piracetam demonstrates broad applicability across different perovskite compositions, increasing the PCE from 23.56% to 25.71% for single-junction 1.56 eV-bandgap counterparts. This method thus provides an effective pathway for scalable and efficient all-perovskite TSCs.