Triple-halide wide-band gap perovskites with suppressed phase segregation for efficient tandems.

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.

Optical modeling of wide-bandgap perovskite and perovskite/silicon tandem solar cells using complex refractive indices for arbitrary-bandgap perovskite absorbers.

Wide-bandgap perovskites are attractive top-cell materials for tandem photovoltaic applications. Comprehensive optical modeling is essential to minimize the optical losses of state-of-the-art perovskite/perovskite, perovskite/CIGS, and perovskite/silicon tandems. Such models require accurate optical constants of wide-bandgap perovskites. Here, we report optical constants determined with ellipsometry and spectrophotometry for two new wide-bandgap, cesium-formamidinium-based perovskites. We validate the optical constants by comparing simulated quantum efficiency and reflectance spectra with measured cell results for semi-transparent single-junction perovskite cells and find less than 0.3 mA/cm2 error in the short-circuit current densities. Such simulations further reveal that reflection and parasitic absorption in the front ITO layer and electron contact are responsible for the biggest optical losses. We also show that the complex refractive index of methylammonium lead triiodide, the most common perovskite absorber for solar cells, can be used to generate approximate optical constants for an arbitrary wide-bandgap perovskite by translating the data along the wavelength axis. Finally, these optical constants are used to map the short-circuit current density of a textured two-terminal perovskite/silicon tandem solar cell as a function of the perovskite thickness and bandgap, providing a guide to nearly 20 mA/cm2 matched current density with any perovskite bandgap between 1.56 and 1.68 eV.