Progress in emerging solution-processed thin film solar cells - Part II: Perovskite solar cells

Perovskite solar cell (SC) is the most attractive and efficient emerging thin film SC, with a power conversion efficiency (PCE) of up to 22%, certified by the United States National Renewable Energy Laboratory (NREL), although the cell is not stable. Most layers of perovskite SCs, including the perovskite light harvesting layer, are solution-processed, and thus can be fabricated by low-cost, scalable and vacuum-free fabrication techniques, such as spray coating. Part I of this work was devoted to polymer or organic SCs, a more mature but less efficient solution-processed SC (Wang et al., Renew Sustain Energy Rev 2016;56:347-61) [1], and Part II reviews the fundamentals and recent advances in perovskite SCs, from an engineering points of view. The review starts off with an introduction to perovskite crystal structure in general, lead halide perovskite crystals as a light harvesting material, and the principle of operation of various architectures of perovskite SCs, such as mesoporous and planar. Then various components, including the light harvesting layer, electron transporting layer (ETL), hole-transporting layer (HTL), and possible materials developed for each layer are discussed. The effects of various processing parameters, such as the annealing time and temperature, solvent effect, reaction time, solution concentration, and the thickness of each layer are discussed, to find the optimum process parameters. Possible methods for the fabrication of perovskite layer and other layers are discussed, including spin-spin and spin-dip sequential methods and scalable methods, such as spray coating. The stability of perovskite SCs is also deliberated and advances made to improve the device lifetime are reviewed. The review concludes with a summary and discussion of research trends and challenges in the field. Other general issues, such as the necessity for the development of flexible substrates, indium-tin-oxide (ITO)-free devices, solution-processed back contact, and comprehensive discussion of all solution-processed techniques have been already considered in Part I of this review.