Photovoltaic (PV) systems are systems that convert light into electricity. All photovoltaic systems share a few common parts. All photovoltaic systems include a light-harvesting element, a charge-separating element, a charge-transporting element(s), and a charge collecting element(s).
A perovskite solar cell is a type of solar cell which includes a perovskite absorber (that is, light-harvesting element). A perovskite absorber has an ABX3 formula, where A is a metal atom such as lead or tin, B is a counter ion (typically an alkyl ammonium compound), and X is a halide (F, Cl, Br, or I). The name “perovskite” refers to the crystal structure of the absorber materials, which has a perovskite structure. The most commonly studied perovskite absorber is methylammonium lead trihalide (CH3NH3PbX3, where X is a halogen ion such as I—, Br—, Cl—), with a bandgap between 2.3 eV and 1.57 eV depending on halide content. Formamidinium lead trihalide (H2NCHNH2PbX3) is a recently studied newer material which shows promise, with a bandgap between 2.23 eV and 1.48 eV.
These hybrid organic-inorganic solid state solar cells with perovskite structured CH3NH3PbI3 as active layer have recently been reported with over 15% efficiency,1,2 which emerges as the most promising candidate for the next-generation solar cells. As a game changer in photovoltaics, perovskite-type material exhibits striking excellence in both light absorption3 and charge transport (1069 nm electron diffusion length and 1213 nm holes diffusion length).4 Despite its aura, the dark side of this rising star should not be ignored such as the stability issue of CH3NH3PbI3,5 the use of environment-hazardous lead,6 the cost of complex organics as hole-blocking layer,7,8 and the use of expensive noble metals as back cathode. In order to maximize the attainable open circuit voltage (Voc), low chemical potential, namely, high work function noble metals, such as gold2,9,10 and silver,1 are generally used as back cathode. Thermal evaporation of gold is a very costly and wasteful process because only a tiny portion of the gold is eventually deposited onto the devices. Therefore, replacing gold with earth-abundant elements as the cathode in perovskite solar cells while still retaining their high Voc and energy conversion efficiency is a pivotally critical step toward the cost-effective production of perovskite solar cells.