CH₃NH₃PbX₃ (MAPbX₃) perovskites have attracted considerable attention as absorber materials for solar light harvesting, reaching solar to power conversion efficiencies above 20%. In spite of the rapid evolution of the efficiencies, the understanding of basic properties of these semiconductors is still ongoing. One phenomenon with so far unclear origin is the so-called hysteresis in the current–voltage characteristics of these solar cells. Here we investigate the origin of this phenomenon with a combined experimental and computational approach. Experimentally the activation energy for the hysteretic process is determined and compared with the computational results. First-principles simulations show that the timescale for MA⁺ rotation excludes a MA-related ferroelectric effect as possible origin for the observed hysteresis. On the other hand, the computationally determined activation energies for halide ion (vacancy) migration are in excellent agreement with the experimentally determined values, suggesting that the migration of this species causes the observed hysteretic behaviour of these solar cells.

Metal halide perovskites are the first solution processed semiconductors that can compete in their functionality with conventional semiconductors, such as silicon. Over the past several years, perovskite semiconductors have reported breakthroughs in various optoelectronic devices, such as solar cells, photodetectors, light emitting and memory devices, and so on. Until now, perovskite semiconductors face challenges regarding their stability, reproducibility, and toxicity. In this Roadmap, we combine the expertise of chemistry, physics, and device engineering from leading experts in the perovskite research community to focus on the fundamental material properties, the fabrication methods, characterization and photophysical properties, perovskite devices, and current challenges in this field. We develop a comprehensive overview of the current state-of-the-art and offer readers an informed perspective of where this field is heading and what challenges we have to overcome to get to successful commercialization.

We present a facile mechanochemical route for the preparation of hybrid CH ₃ NH ₃ PbI ₃ (MAPbI ₃ ) perovskite particles with the size of several hundred nanometers for high-efficiency thin-film photovoltaic devices.

We present the design and synthesis of a promising hole transporting material (HTM) using the 4,4′-spirobi[cyclopenta[2,1-b:3,4-b′]dithiophene] derivative (spiro-CPDT) as the core and triarylamines as terminal units. The implementation of the new HTM in CH₃NH₃PbI₃-based perovskite solar cells exhibited an excellent overall power conversion efficiency (PCE) of 13.4% without the use of any dopants and additives which is comparable to 15.0% obtained using p-doped spiro-MeOTAD-based devices. Furthermore, the device based on the new HTM generated a slightly higher open circuit voltage (V_OC) of 971 mV compared to a spiro-MeOTAD (V_OC = 951 mV) based device. The present results demonstrate that spiro-CPDT could be an excellent building block to prepare dopant-free HTMs for perovskite solar cells and holds promise to replace the p-doped spiro-OMeTAD, which is important for the fabrication of cost-effective devices in the future.