Goldschmidt tolerance factor (t) is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 is favorable for cubic perovskite structure, and larger (>1) or smaller (<0.8) values of tolerance factor usually result in nonperovskite structures. CH­(NH2)2PbI3 (FAPbI3) can exist in the perovskite α-phase (black phase) with good photovoltaic properties. However, it has a large tolerance factor and is more stable in the hexagonal δH-phase (yellow phase), with δH-to-α phase-transition temperature higher than room temperature. On the other hand, CsPbI3 is stabilized to an orthorhombic structure (δO-phase) at room temperature due to its small tolerance factor. We find that, by alloying FAPbI3 with CsPbI3, the effective tolerance factor can be tuned, and the stability of the photoactive α-phase of the mixed solid-state perovskite alloys FA1–x Cs x PbI3 is enhanced, which is in agreement with our first-principles calculations. Thin films of the FA0.85Cs0.15PbI3 perovskite alloy demonstrate much improved stability in a high-humidity environment; this contrasts significantly with the pure FAPbI3 film for which the α-to-δH phase transition (associated with yellowing appearance) is accelerated by humidity environment. Due to phase stabilization, the FA0.85Cs0.15PbI3 solid-state alloy showed better solar cell performance and device stability than its FAPbI3 counterparts. Our studies suggest that tuning the tolerance factor through solid-state alloying can be a general strategy to stabilize the desired perovskite structure for solar cell applications.

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Though formamidinium lead triiodide (FAPbI3) possesses a suitable band gap and good thermal stability, the phase transition from the pure black perovskite phase (α-phase) to the undesirable yellow nonperovskite polymorph (δ-phase) at room temperature, especially under humid air, hinders its practical application. Here, we investigate the intrinsic instability mechanism of the α-phase at ambient temperature and demonstrate the existence of an anisotropic strained lattice in the (111) plane that drives phase transformation into the δ-phase. Methylammonium bromide (MABr) alloying (or FAPbI3-MABr) was found to cause lattice contraction, thereby balancing the lattice strain. This led to dramatic improvement in the stability of α-FAPbI3. Solar cells fabricated using FAPbI3-MABr demonstrated significantly enhanced stability under the humid air.

Organic-inorganic metal halide perovskite solar cells were fabricated by laminating films of a carbon nanotube (CNT) network onto a CH3NH3PbI3 substrate as a hole collector, bypassing the energy-consuming vacuum process of metal deposition. In the absence of an organic hole-transporting material and metal contact, CH3NH3PbI3 and CNTs formed a solar cell with an efficiency of up to 6.87%. The CH3NH3PbI3/CNTs solar cells were semitransparent and showed photovoltaic output with dual side illuminations due to the transparency of the CNT electrode. Adding spiro-OMeTAD to the CNT network forms a composite electrode that improved the efficiency to 9.90% due to the enhanced hole extraction and reduced recombination in solar cells. The interfacial charge transfer and transport in solar cells were investigated through photoluminescence and impedance measurements. The flexible and transparent CNT network film shows great potential for realizing flexible and semitransparent perovskite solar cells.

Understanding carrier recombination in semiconductors is a critical component when developing practical applications. Here we measure and compare the monomolecular, bimolecular, and trimolecular (Auger) recombination rate constants of CH3NH3PbBr3 and CH3NH3PbI3. The monomolecular and bimolecular recombination rate constants for both samples are limited by trap-assisted recombination. The bimolecular recombination rate constant for CH3NH3PbBr3 is ∼3.3 times larger than that for CH3NH3PbI3 and both are in line with that found for radiative recombination in other direct-gap semiconductors. The Auger recombination rate constant is 4 times larger in lead-bromide-based perovskite compared with lead-iodide-based perovskite and does not follow the reduced Auger rate when the bandgap increases. The increased Auger recombination rate, which is enhanced by Coulomb interactions, can be ascribed to the larger exciton binding energy, ∼40 meV, in CH3NH3PbBr3 compared with ∼13 meV in CH3NH3PbI3.

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We report the charge carrier recombination rate and spin coherence lifetimes in single crystals of two-dimensional (2D) Ruddlesden−Popper perovskites PEA 2 PbI 4 •(MAPbI 3 ) n−1 (PEA, phenethylammonium; MA, methylammonium; n = 1, 2, 3, 4). Layer thickness-dependent charge carrier recombination rates are observed, with the fastest rates for n = 1 because of the large exciton binding energy, and the slowest rates are observed for n = 2. Room-temperature spin coherence times also show a nonmonotonic layer thickness dependence with an increasing spin coherence lifetime with increasing layer thickness from n = 1 to n = 4, followed by a decrease in lifetime from n = 4 to ∞. The longest coherence lifetime of ∼7 ps is observed in the n = 4 sample. Our results are consistent with two contributions: Rashba splitting increases the spin coherence lifetime going from the n = ∞ to the layered systems, while phonon scattering, which increases for smaller layers, decreases the spin coherence lifetime. The interplay between these two factors contributes to the layer thickness dependence.

The synergistic combination of a Lewis base and a Lewis acid enables perovskite solar cells with high efficiency and stability.