We demonstrate open-circuit voltages exceeding 1.26 V for CH3NH3PbI3 solar cells by careful process optimization of the perovskite and its interfaces to the electron and hole transport layers. This open-circuit voltage is the highest reported so far in a full MAPI cell stack and only 64 mV below the maximum open circuit voltage that is possible for this material. We confirm these values for the open circuit voltage by independent measurements of the external photoluminescence quantum efficiency reaching values of 5 % for the fully processed solar cell. We further find exceptionally long photoluminescence lifetimes in full cells and in layer stacks involving one or two contact layers. Numerical simulations reveal that these long photoluminescence lifetimes are only possible with extremely low interface recombination velocities between absorber and contact materials.

Facile electron injection and extraction are two key attributes desired in electron transporting layers to enhance the efficiency of planar perovskite solar cells. Herein it is demonstrated that the incorporation of alkali metal dopants in mesoporous TiO 2 can effectively modulate electronic conductivity and improve the charge extraction process by counterbalancing oxygen vacancies acting as nonradiative recombination centers. Moreover, sulfate bridges (SO 4 2− ) grafted on the surface of K-doped mesoporous titania provide a seamless integration of absorber and electron-transporting layers that accelerate overall transport kinetics. Potassium doping markedly influences the nucleation of the perovskite layer to produce highly dense films with facetted crystallites. Solar cells made from K:TiO 2 electrodes exhibit power conversion efficiencies up to 21.1% with small hysteresis despite all solution coating processes conducted under ambient air conditions (controlled humidity: 25-35%). The higher device efficiencies are attributed to intrinsically tuned electronic conductivity and chemical modification of grain boundaries enabling uniform coverage of perovskite films with large grain size.

Having demonstrated incredibly fast progress in power conversion efficiency, rising to a level comparable with that of crystalline silicon cells, lead‐based organic–inorganic hybrid perovskite solar cells are now facing the stability tests needed for industrialization. Poor thermal stability (<150 °C) owing to organic constituents and interlayer diffusion of materials (dopants), and environmental incompatibility due to Pb has surged the development of organic‐free, Pb‐free perovskites and dopant‐free hole transport materials (HTMs). The recent rapid increase in efficiency of cells based on inorganic perovskites, crossing 18%, demonstrates the great potential of inorganic perovskites as thermally stable and high‐efficiency cells. Although all kinds of Pb‐free perovskites lag in efficiency in comparison to the hybrid and inorganic perovskites, they also demonstrate better structural and environmental stability. The performance of dopant‐free HTMs matching/surpassing dopant‐containing HTMs makes the former a better choice for stability. Even though the efforts to enhance the stability of Pb‐based hybrid perovskites should continue by different techniques, organic‐free and lead‐free perovskites, and dopant‐free HTMs must be pursued with greater interest for the future. This review describes the present issues and possible strategies to address them, and thus will help to improve the overall performance of robust organic‐free, Pb‐free, and dopant‐free perovskite solar cells.

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All-inorganic perovskites (CsPbI3 and CsPbI2Br), owing to their greater thermal stability compared to organic–inorganic hybrid perovskites, are becoming popular in perovskite photovoltaics, but the problem that remains with CsPbI2Br (or CsPbI3) is the humidity-assisted phase transformation. Herein, we report on the formation of CsPbI2Br α-phase and improvement of its phase stability under ambient atmosphere (20–30% relative humidity) by Pb­(II) propionate additive in the CsPbI2Br precursor. Solar cells employing a CsPbI2Br film with an optimum concentration of the additive (1 mol %) and a donor–acceptor type polymer (synthesized by us) as dopant-free hole transport material that has a better energy level matching with CsPbI2Br (compared to other polymers like P3HT, PTAA, and asy-PBTBDT) work with a champion power conversion cell efficiency of 14.58%. A continuous increase in the open-circuit voltage, reaching 1.36 V for 5 mol % Pb­(II) propionate, indicates a remarkable defect-passivation effect by the additive.