Composition engineering of formamidinium based perovskites to improve photovoltaic performance and stability

In general, a commercial photovoltaic product requires a commitment of 25 years, but it is impossible to fully access the stability of solar cells for such a long time. Thus, accelerated tests, in which devices are subjected to high temperature and continuous illumination, are usually utilized to determine long-term stability. It should be noted that to pass this test in which solar cells need to be aged at 85°C under continuous simulated 1 sun illumination, the device performance must decrease by no more than 20% after 1000 h of testing. Hybrid organic inorganic perovskites (HOIPs) are one of the most promising next-generation photovoltaic (PV) materials as they can deliver power conversion efficiencies (PCEs) up to 25.5%, comparable to polycrystalline silicon solar cells. The high PCEs are attributed to favorable properties of HOIPs, including the direct optical band gap, high absorption coefficients, long carrier diffusion length, high charge carrier mobility, low charge trap density,  low urbach energy that corresponds to long charge carrier lifetime, small Stokes shift  and large open-circuit voltage. These HIOPs consist of ABX3 structure, where A is an organic ammonium cation ((CH3NH3) + : methylammonium (MA+), CH(NH2)2+: formamidinium (FA+), B is Pb2+ or Sn+2, and X is a halide anion.
However, there is no doubt that the stability of current PSCs is far worse than the inorganic solar cells due to the decomposition of light harvester. As normally used organic carrier transporting layer materials containing volatile ingredients and hydrophilic lithium salts have non-negligible influences on the long-term performance of device due to its own instability, the inorganic or the dopant-free carriers transporting layers should be able to help perovskite perform better stability.
However, to address the stability issues of the core perovskite layer will definitely be the most important part. Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI3) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells.In contrast to MAPbI3 which decomposes to PbI2 and volatile organic compounds upon exposure to humidity, FAPbI3 perovskite has a phase transition from the α- to δ-phase with a color change from black to yellow. As the nonperovskite δ-phase has a significantly larger bandgap and inferior charge-transport properties due to the 1D PbI6 octahedron structure, this transition is undesirable for photovoltaic applications. It usually requires a very high phase transition temperature (up to 440K) to obtain black α-FAPbI3, which is desirable for photoelectric conversion and nearly infrared (NIR) emission, while nonperovskite δ-phase is stable at about room temperature (300K).
Unfortunately, such high temperature (around 440K) can result in partial decomposition of FAPbI3 into PbI2. Moreover, the obtained black α-FAPbI3 can turn into the undesirable yellow δ-FAPbI3 under an ambient humid atmosphere. Apart from stability, non-radiative recombination process caused by the trap states(associated with vacancies and defects at crystallite surfaces) in the PSCs is one of the major reason for the energy loss which consequently decreases the efficiency.
My prime objective of research is in FA-based perovskites solar cells focuses on-
1. The stabilization of black α-FAPbI3 at relatively low temperature.
2. Better control of the trap states by improving the quality of the perovskite layer and interfaces in fully assembled device configurations.
3. Improving long-term stability of PSCs to commercialize and compete with the available Silicon solar cells.