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.

All-perovskite–based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite–based tandem devices has been limited by the lack of high-efficiency, low–band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs). We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low–band gap (~1.25 electron volt) perovskite films. The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers. These improved properties enable our demonstration of >20% efficient low–band gap PSCs. When combined with wider–band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite–based polycrystalline thin-film tandem solar cells.

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Two advances that address the main challenges of all-perovskite two-terminal tandem solar cell fabrication are reported. First, a nucleation layer is used to enable high-quality atomic layer deposition-based recombination layers that reduce electronic losses. Second, cation tuning is used for wide-band-gap perovskite solar cells that produce high, stable voltages. Combining these advances allows us to fabricate tandem perovskite solar cells on both rigid and flexible plastic substrates that have high efficiency and promising stability.

After attending Massey University in New Zealand as a Fulbright Scholar from 2002 to 2003, she returned to UC Berkeley to study the shape control and selective growth patterns of multimaterial heterostructures for catalytic and energy applications with Professor Yang, leading to a Ph.D. from the Department of Chemistry in 2008. Following a year of postdoctoral research on metal-semiconductor hybrid materials and the fate of nanomaterials in the environment with Dr. Taleb Mokari at the Lawrence Berkeley National Laboratory, she joined the National Renewable Energy Laboratory as a postdoctoral researcher. Her current research interests include the design of functional inks and development of solution deposition processes for photovoltaic materials.