Tuning band gaps with three halides: Tandem solar cells can boost solar cell efficiency by using two active layers to absorb the solar spectrum more completely, provided that the two cells are current-matched. Inorganic-organic perovskites tuned to the appropriate wide band gap (∼1.7 electron volts) as top cells that contained iodine and bromine or bromine and chlorine have short carrier diffusion lengths and undergo photo-induced phase segregation. Xu et al. now report a method for incorporating chloride that allows for fabrication of stable triple-halide perovskites with a band gap of 1.67 electron volts. Two-terminal tandem silicon solar cells made with this material had a power conversion efficiency of 27%. Science , this issue p. 1097

Efficient all-perovskite tandem cells: Organic-inorganic perovskite films can boost the output of conventional silicon solar cells in tandem geometries by utilizing more of the light at the blue end of the solar spectrum. Tandem cells that use only perovskite films have been less successful because of the lack of a suitable material with a low bandgap that can replace silicon. Tong et al. report that a mixed tin-lead organic-inorganic material containing a small fraction of guanidinium thiocyanate has a low bandgap, long charge-carrier lifetime, and efficiencies around 25%. Science , this issue p. 475

We developed lead halide perovskite quantum dot (QD) films with tuned surface chemistry based on A-site cation halide salt (AX) treatments. QD perovskites offer colloidal synthesis and processing using industrially friendly solvents, which decouples grain growth from film deposition, and at present produce larger open-circuit voltages (V_OC’s) than thin-film perovskites. CsPbI₃ QDs, with a tunable bandgap between 1.75 and 2.13 eV, are an ideal top cell candidate for all-perovskite multijunction solar cells because of their demonstrated small V_OC deficit. We show that charge carrier mobility within perovskite QD films is dictated by the chemical conditions at the QD-QD junctions. The AX treatments provide a method for tuning the coupling between perovskite QDs, which is exploited for improved charge transport for fabricating high-quality QD films and devices. The AX treatments presented here double the film mobility, enabling increased photocurrent, and lead to a record certified QD solar cell efficiency of 13.43%.

Spin injection sans magnetism: Light-emitting diodes (LEDs) that emit circularly polarized light (spin-LEDs) have potential applications in in three-dimensional displays, bioencoding, and tomography. The requisite spin polarization of the charge carriers is usually achieved with ferromagnetic contacts and applied magnetic fields, but Kim et al. report on a room-temperature spin-LED that relies instead on a chiral-induced spin selectivity organic layer. This layer selectively injected spin-polarized holes into metal halide perovskite nanocrystals, where they radiatively recombined with unpolarized electrons with an efficiency of 2.6%. Science , this issue p. 1129