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.

We report the first-principles electronic structure of BiVO 4, a promising photocatalyst for hydrogen generation. BiVO 4 is found to be a direct band gap semiconductor, despite having band extrema away from the Brillouin zone center. Coupling between Bi 6s and O 2p forces an upward dispersion of the valence band at the zone boundary; however, a direct gap is maintained via coupling between V 3d, O 2p, and Bi 6p, which lowers the conduction band minimum. These interactions result in symmetric hole and electron masses. Implications for the design of ambipolar metal oxides are discussed. © 2009 American Chemical Society

Extrinsic ions (e.g., Li+) migrate across perovskite solar cells and modify the TiO2 layer, affecting device performance and hysteresis.

Lead thiocyanate in the perovskite precursor can increase the grain size of a perovskite thin film and reduce the conductivity of the grain boundaries, leading to perovskite solar cells with reduced hysteresis and enhanced fill factor. A planar perovskite solar cell with grain boundary and interface passivation achieves a steady-state efficiency of 18.42%.

The unsatisfactory performance of low‐bandgap mixed tin (Sn)–lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all‐perovskite tandem solar cells. By analyzing dark‐current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed Sn–Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two–three orders of magnitude. By optimizing the Br concentration, low‐bandgap (1.272 eV) mixed Sn–Pb halide perovskite solar cells are fabricated with open‐circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The best‐performing device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low‐bandgap mixed Sn–Pb halide perovskite solar cells.

When CdTe solar cells are doped with Cl, the grain boundaries no longer act as recombination centers but actively contribute to carrier collection efficiency. The physical origin of this remarkable effect has been determined through a combination of aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and first-principles theory. Cl substitutes for a large proportion of the Te atoms within a few unit cells of the grain boundaries. Density functional calculations reveal the mechanism, and further indicate the grain boundaries are inverted to n type, establishing local p-n junctions which assist electron-hole pair separation. The mechanism is electrostatic, and hence independent of the geometry of the boundary, thereby explaining the universally high collection efficiency of Cl-doped CdTe solar cells.

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Over the past decade, the global cumulative installed photovoltaic (PV) capacity has grown exponentially, reaching 591 GW in 2019. Rapid progress was driven in large part by improvements in solar cell and module efficiencies, reduction in manufacturing costs and the realization of levelized costs of electricity that are now generally less than other energy sources and approaching similar costs with storage included. Given this success, it is a particularly fitting time to assess the state of the photovoltaics field and the technology milestones that must be achieved to maximize future impact and forward momentum. This roadmap outlines the critical areas of development in all of the major PV conversion technologies, advances needed to enable terawatt-scale PV installation, and cross-cutting topics on reliability, characterization, and applications. Each perspective provides a status update, summarizes the limiting immediate and long-term technical challenges and highlights breakthroughs that are needed to address them. In total, this roadmap is intended to guide researchers, funding agencies and industry in identifying the areas of development that will have the most impact on PV technology in the upcoming years.