Wide–band gap metal halide perovskites are promising semiconductors to pair with silicon in tandem solar cells to pursue the goal of achieving power conversion efficiency (PCE) greater than 30% at low cost. However, wide–band gap perovskite solar cells have been fundamentally limited by photoinduced phase segregation and low open-circuit voltage. We report efficient 1.67–electron volt wide–band gap perovskite top cells using triple-halide alloys (chlorine, bromine, iodine) to tailor the band gap and stabilize the semiconductor under illumination. We show a factor of 2 increase in photocarrier lifetime and charge-carrier mobility that resulted from enhancing the solubility of chlorine by replacing some of the iodine with bromine to shrink the lattice parameter. We observed a suppression of light-induced phase segregation in films even at 100-sun illumination intensity and less than 4% degradation in semitransparent top cells after 1000 hours of maximum power point (MPP) operation at 60°C. By integrating these top cells with silicon bottom cells, we achieved a PCE of 27% in two-terminal monolithic tandems with an area of 1 square centimeter.

A new monolithic perovskite/silicon tandem solar cell architecture is proposed based on double-side-textured silicon cells with sub-micrometer pyramids. These pyramids are rough enough to scatter light within silicon nearly as efficiently as large pyramids but smooth enough to solution process a perovskite film. A bladecoated perovskite film planarizes the textured silicon cell. With a textured lightscattering layer added to the top to reduce front-surface reflectance, a monolithic perovskite/silicon tandem cell reaches an efficiency of 26%.

The rapid rise in efficiency and tunable bandgap of metal-halide perovskites makes them highly attractive for use in tandems on silicon. Recently we demonstrated a perovskite–silicon monolithic two-terminal tandem with 23.6% power conversion efficiency. Here, we present work on optical optimization to improve light harvesting that includes thinning out the top transparent electrode to reduce front-surface reflection and parasitic absorption; introducing metal fingers to minimize series resistance losses; and further minimizing reflection loss with a polydimethylsiloxane (PDMS) stamp with random, pyramidal texture. Additionally, to reduce voltage loss while achieving current matching, we employ poly­[bis­(4-phenyl)­(2,4,6-trimethylphenyl)­amine] (PTAA) as a hole transport material instead of NiO x and a wider 1.68 eV bandgap perovskite composition. These optimizations boost the open-circuit voltage to 1.77 V and the short-circuit current density to 18.4 mA/cm2, culminating in a 25% efficient perovskite–silicon tandem with a 1 cm2 active area.

Significant effort has focused on controlling the deposition of perovskite films to enable uniform films, enabling efficiencies to climb dramatically. However, little attention has been paid to the evolution of thin-film stresses during deposition and the consequent effect on film morphology. While a textured surface topology has potential benefits for light scattering, a smooth surface is desirable to enable the pinhole-free deposition of contact layers. We show that the highly textured morphology made by popular antisolvent conversion methods arises because of in-plane compressive stress experienced during the intermediate phase of film formation where the substrate constrains the film from expandingleading to energy release in the form of wrinkling, resulting in trenches that can be hundreds of nanometers deep with periods of several micrometers. We demonstrate that the extent of wrinkling is correlated with the rate of film conversion and that ultrasmooth films are obtained by slowing the rate of film formation.

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