Three-dimensional (3D) organic-inorganic perovskite solar cells have undergone a meteoric rise in cell efficiency to > 22%. However, the perovskite absorber layer is prone to degradation in water, oxygen and UV light. Two-dimensional (2D) Ruddlesden−Popper layered perovskites have exhibited promising environmental stability, but perform less well in solar cells, possibly due to the inhibition of out-of-plane charge transport by the insulating spacer cations. Alternatively, moving away from methylammonium, to the mixed cation formamidinium-caesium based perovskites has led to considerably enhancement of the stability of 3D perovskite absorber layers. Here, we report highly efficient and stable perovskite solar cells based on a self-assembled butylammonium-Cs-formamidinium mixed-cation lead mixed-halide perovskite photoactive layer. Long-chain alkyl-ammonium halides added to the formamidinium-cesium based perovskite precursor solution strongly enhances the crystallinity of the 3D perovskite phase, while also inducing the formation of new layered-phases in the films. By carefully regulating the composition, we are able to achieve "plate-like" layered perovskite crystallites standing up between the host 3D perovskite grains. This spontaneously forming heterostructure allows the efficient charge carrier transport in the 3D perovskite phase, while reducing charge recombination via fortuitous grain boundary passivation. We also observe reduced current-voltage hysteresis and improved device stability, which we correlate to enhanced crystallinity and reduced crystal defects in the 3D perovskite phase. With the optimized composition, we achieved a power conversion efficiency of 20.6% (stabilised efficiency of 19.5%) from a narrow bandgap (1.61 eV) perovskite solar cell and of 17.2 % (stabilised efficiency of 17.3%) from a wider bandgap (1.72 eV) perovskite solar cell optimised for tandem applications. In addition to enhanced efficiency, the addition of butylammonium greatly enhances the long-term stability of the devices. For the first time, our cells sustain more than 80% of their "post burn-in" efficiency after 1,000 hrs of aging under simulated full spectrum sun light measured in an ambient environment without encapsulation. With additional sealing with a glass/polymer-foil/glass laminate, we extend this lifetime to close to 4,000 hrs. Our work illustrates that engineering heterostructures between 2D and 3D perovskite phases is both possible, and can lead to enhancement of both performance and stability of perovskite solar cells.

The highest power conversion efficiencies (PCEs) reported for perovskite solar cells (PSCs) with inverted planar structures are still inferior to those of PSCs with regular structures, mainly because of lower open-circuit voltages (). Here we report a strategy to reduce nonradiative recombination for the inverted devices, based on a simple solution-processed secondary growth technique. This approach produces a wider bandgap top layer and a more n-type perovskite film, which mitigates nonradiative recombination, leading to an increase in by up to 100 millivolts. We achieved a high of 1.21 volts without sacrificing photocurrent, corresponding to a voltage deficit of 0.41 volts at a bandgap of 1.62 electron volts. This improvement led to a stabilized power output approaching 21% at the maximum power point.

A major bottleneck delaying the further commercialization of thin-film solar cells based on hybrid organohalide lead perovskites is interface loss in state-of-the-art devices. We present a generic interface architecture that combines solution-processed, reliable, and cost-efficient hole-transporting materials without compromising efficiency, stability, or scalability of perovskite solar cells. Tantalum-doped tungsten oxide (Ta-WO )/conjugated polymer multilayers offer a surprisingly small interface barrier and form quasi-ohmic contacts universally with various scalable conjugated polymers. In a simple device with regular planar architecture and a self-assembled monolayer, Ta-WO -doped interface-based perovskite solar cells achieve maximum efficiencies of 21.2% and offer more than 1000 hours of light stability. By eliminating additional ionic dopants, these findings open up the entire class of organics as scalable hole-transporting materials for perovskite solar cells.

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI ) thin films, whose background hole doping density is varied through SnF addition during film fabrication. Monomolecular charge-carrier recombination exhibits both a dopant-mediated part that grows linearly with hole doping density and remnant contributions that remain under tin-enriched processing conditions. At hole densities near 10 cm , a strong Burstein-Moss effect increases absorption onset energies by ≈300 meV beyond the bandgap energy of undoped FASnI (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10 cm ), temperature-dependent measurements indicate that the effective charge-carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 10 cm and below, the charge-carrier mobility increases with decreasing temperature according to ≈T , suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 × 10 cm , charge-carrier mobilities reach a value of 67 cm V s at room temperature and 470 cm V s at 50 K. Intraexcitonic transitions observed in the THz-frequency photoconductivity spectra at 5 K reveal an exciton binding energy of only 3.1 meV for FASnI , in agreement with the low bandgap energy exhibited by this perovskite.

Reproducibility issues of perovskite materials and devices stem from their extreme sensitivity to fractional, quite possibly unintentional, deviations in the precursor solution stoichiometry.