2, Department of Chemistry & Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina, United States
Perovskite solar cells (PSCs) have attracted enormous attention due to their potential for low-cost fabrication photovoltaic devices on flexible or rigid substrates. While many approaches have been used to control perovskite formation, thermal annealing has been a typical procedure for the state-of-the-art perovskite solar cells, giving rise to additional costs and challenges for applying perovskites in flexible and/or tandem photovoltaics. Recently, thiocyanate containing additives, such as MASCN, have been shown to be candidates for highly efficient room-temperature perovskite processing.  Nevertheless, the mechanisms of perovskite formation and crystallization pathways involved in MASCN-additive-processing approach are unclear. Using time-resolved grazing incidence wide-angle x-ray scattering (GIWAXS), we investigate the perovskite formation in situ during spin coating and the subsequent drying process, aiming at revealing the mechanisms of additive-assisted-perovskite-formation.
Time-resolved monitoring of the perovskite thin film formation process reveals the formation of precursor phases on the route of perovskite formation, whereas perovskite formation is dominated by a sol-gel process. Our findings reveal that the nature of the precursor phase and their formation/dissociation dynamics have an impact on the extent of nucleation and growth of perovskite phase affecting the microstructure of the perovskite film. We show that a DMSO-precursor phase is obtained in the as-cast film for the MASCN-free films, whereas an additional (presently unidentified) phase is obtained when adding MASCN to the precursor solution. The latter precursor phase is less stable than the DMSO precursor phase and dissociates shortly upon applying N2 flow on the film leading to fast room-temperature conversion to perovskite. Moreover, MASCN aids in the dissociation of DMSO-precursor phase which decays faster with the presence of MASCN in the wet film. The combination of two precursors with fast and slow decay rates may contribute to the formation of micron-sized perovskite crystals, through seeding perovskite nuclei combined with the slow growth of the perovskite phase. Understanding the mechanism of room-temperature-additive-processing will pave the way for more facile control of perovskite formation, while the use of N2 flow provides the suitability for forming perovskite directly on roll-to-roll processing at room temperature without the need for subsequent separate steps.
 Q. Han et.al, Energy Environ. Sci., 2017, 10, 2365—2371.