Recent research progress in perovskite solar cells

Recently, all-solid state hybrid solar cells based on organic-inorganic metal halide perovskite (ABX3) materials have received much attention from the academic circle all over the world due to their unique physical and chemical properties. The perovskite materials exhibit advantages of high extinction coefficient, high charge mobility, long carrier lifetime, and long carrier diffusion distance. Furthermore, they are low cost and easily synthesized. The power conversion efficiency (PCE) has exceeded 20.8% since the PCE of 3.8% was first reported in 2009, making the perovskite solar cells the best potential candidate of the new generation solar cells to replace the high-cost and highly polluting silicon solar cells in the future. Meanwhile, because of the well-known special bipolar properties of the perovskite materials, various structures are designed such as the all-solid state mesoscopic heterojunctions, planar-heterojunctions, meso-superstructures, and HTM-free structures. In this review, we first introduce the development of the perovskite solar cells and then focus on the cell structure and its influence on the cell performance. Besides, the synthesis methods of the perovskite films and the performance characteristics and advantages of the perovskite solar cells with different cell structures are also discussed. It is found that although the perovskite crystals prepared by a one-step spin-coating method have bigger grain sizes, their morphologies are rougher and uncontrollable, which may suppress the charge carrier extraction efficiency and lead to a relatively low power conversion efficiency. Meanwhile, vapor-assisted method needs vaccum conditions, which significantly increases the manufacture cost of PSC. Compared with these methods mentioned above, solution-based sequential deposition method can not only enhance the reproducibility of PSC, but also obtain a higher PCE with a lower cost. Afterwards, the photogenerated carrier transport mechanism of the perovskite solar cells is discussed. The possible atomic interaction model and the electron structure between perovskite film and electron transport layer are proposed. There are two possible interface atomic structures at the interface of perovskite CH3NH3PbI3 and TiO2. It is supposed that the interaction between iodine atoms and titanium atoms dominates the atomic structure at the interface of CH3NH3PbI3 and TiO2, while the lead atoms are believed to bond to oxygen atoms. As is well known, charge extraction, transfer and recombination mainly occur at the interface of a cell. Therefore, the interface engineering including the design for energy level matching is important and necessary to enhance the charge transport efficiency, suppress the charge recombination and eventually improve the performance of perovskite solar cells. Moreover, the properties of the main electron transport layer (ZnO, TiO2, PCBM, Al2O3) and hole transport layer (spiro-OMeTAD, P3 HT, NiO, PTAA) and their influences on the PCE of the perovskite solar cells are discussed. The main challenges of the all-solid state hybrid perovskite solar cells such as environment pollution, the extremely small working areas and the instability are introduced. Finally, the development prospects of perovskite solar cells in the future are proposed in order to have a better understanding of the perovskite solar cells.