High Bandgap Absorber for Monolithic Perovskite Silicon Tandem Solar Cells Reaching 25.1% Certified Efficiency and Ways Beyond

Perovskite silicon tandem solar cells (PSTSCs) can overcome the efficiency limitations of conventional single-junction silicon solar cells by reducing thermalization losses. To reach this goal, a perovskite absorber with an adapted high bandgap, good (photo-)stability, and high open-circuit voltage (VOC) is needed. First approaches tried halide mixing with high bromide content but compounds suffered from light-induced halide segregation causing voltage losses. In this work, a balanced tuning approach for [HC(NH2)2]1-xCsxPb(I1-yBry)3 perovskites is presented using reduced bromide content and compensating the bandgap effect by increasing the cesium(I) content. In that way, phase stability is reached while keeping the desired high bandgap. Moreover, surface passivation at the electron contact-perovskite interface highlights the important role of interfaces in the overall performance. [HC(NH2)2]0.75Cs0.25Pb(I0.8Br0.2)3 with an optical bandgap of ~1.69 eV is determined as the best candidate for tandem application. Implementation in monolithic PSTSCs with silicon heterojunction bottom solar cells enables high VOC of ~1836 mV with excellent photostability in a mesoscopic n-i-p configuration, and high fill factor of 80% and 25.1% certified stabilized efficiency in a planar p-i-n configuration. Heading towards higher efficiencies, a hybrid co-evaporation/spin coating route is applied for easy bandgap tuning and conformal perovskite film formation on µm-sized random pyramid-textured silicon.