Formamidine (FA)-based perovskite has bandgap close to the optimal bandgap of single junction solar cell is gradually attracting more attentions in the community. However, cracks are commonly found in FA-based perovskite films right after the spin-coating at room temperature, which has detrimental effect on the solar cell performance. Cooling down the glovebox temperature has become a general practice to solve this issue, yet the temperature effect on the nucleation and crystallization processes if still unknow. In this work, we developed in-situ photoluminescence and absorption techniques to investigate the temperature effect on the nucleation and crystallization of FA-based perovskite crystals (FAxMA1-xPbI3) during spin-coating and annealing processes. Combining with density functional theory (DFT) results, we have found that FAPbI3 framework is firstly generated, which is then followed by the incorporation of FA, MA, Pb, I ions into crystal lattice. Both the higher FA content and higher temperature would significantly increase the growth rate of perovskite, resulting in phase aggregation and stress relief. Leading to the formation of cracks. This work provides a unique and intuitive perspective towards the understanding of the FA-based perovskite growth mechanism.
Controlling the crystallization to achieve a high-quality homogeneous film of mixed halide wide bandgap (>1.7 eV) perovskites is vital for the development of tandem and semitransparent solar cells. Since nucleation from the precursor ions is sensitive and typically completed in a few seconds, monitoring the crystallization during the perovskite film formation is still challenging. Here, we demonstrate an in-situ dynamic optical-probing technique which can monitor the crystallization without affecting the process by laser excitation. It is found that the generally observed low film quality is due to the inefficient nucleation of MAPbI2Br during the spin-coating process. Once the film is transferred to the annealing process, we find that there is a fast ion-exchange of halide ions within the first few second, and large amount of PbI2 is formed simultaneously which strongly deteriorate the film homogeneity. We find that an additional preheating process at a moderate temperature is crucial to promote the nucleation of perovskite and retard the forming of PbI2. Such process can significantly increase the number of MAPbI2Br nuclei, and therefore facilitates the formation of uniformly composited perovskite film. Eventually, only a small amount of PbI2 is observed around the grain boundary which in turn passivates the perovskite. This presentation would provide insight into identifying the limiting factors which could affect the crystallization of the organo-metal halide perovskites films during the fabrication processes by in-situ optical techniques.
Organic photovoltaic cells (OPVs) using Y-series nonfullerene electron acceptors (NFA) have recently achieved record high efficiencies. However, there is still lack of experimental investigation on how individual molecules and different aggregates of these NFAs contribute to the overall charge transfer (CT) properties. We fabricated solid-solution thin films by dispersing Y-series NFA molecules in insulating polymers at various loading ratios and the CT property has been investigated using electroabsorption (EA) spectroscopy. This study provides direct experimental evidence on the origin of the strong CT character in thin film and brings insight into the correlation between charge transfer and delocalization of wavefunction.
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