Towards Optimum Solution-processed Planar Heterojunction Perovskite Solar Cells
Recently, organic–inorganic hybrid perovskites have been proven to be excellent photovoltaic materials, exhibiting outstanding light absorption, high carrier mobility and facile solution processability. Besides the low-cost manufacturing of perovskite thin-films, the power conversion efficiencies demonstrated for this class of materials are already at the same level as those of poly-crystalline silicon. The pursuit of efficiency in the field of metal halide perovskite solar cells has been achieved mainly through the improvement to perovskite deposition processing and optimization of the contact materials. In this chapter, we review the commonly employed perovskite deposition techniques, with special emphasis on the morphological quality of the prepared perovskite films. Films which exhibit the largest grains and highest orientation also achieve the highest performance, as long as full surface coverage is ensured. Here, it is also important to tune the energy levels of the electron and hole acceptors, and several strategies have led to champion devices with open circuit voltages between 1.1 and 1.15 V for state-of-the-art systems. However, most of the organic materials used currently are synthesized using expensive cross-coupling reactions that require stringent reaction conditions and extensive product purification, so that they cannot be produced at a low-cost at present. For perovskite solar cells to be able to enter the photovoltaic market, their cost and stability need to be competitive with current established technologies. The development of new chemistries resulting in simple compound purification, such as those based on azomethine bonds, will be an essential part of future molecular design for perovskite solar cells.