Research On The Active Layer Crystallization And The Interfaces In Perovskite Solar Cells

Among the natural resources,solar energy attracts more and more attentions of the researchers because it is widely avaialbe green energy on earth.The solar energy can be directly converted to electricity by photoelectric conversion in photovoltaic cells.Among all different solar cells,organic-inorganic hybrid perovskite solar cells have developed rapidly in the last few years with their conversion efficiency increased from the initial 3.8% to the certified 22.7% recently.These organic-inorganic hybrid perovskite solar cells has many merts,such as long carrier diffusion lengths,wide light absorption range,adjustable band gap and simple preparation method,which are promising for the large area production and eventually their commercialization.However,there are also many issues to be addressed,such as their working mechanism,the existence of hysteresis in the devices,their large area preparation and the long-term stability.Therefore,lots of research efforts are still necessary to promote the development of perovskite solar cells.In this paper,the perovskite thin films were prepared by one step spin coating.By adjusting the precursor solution content and the anti-solvent process as well as by adding graphene oxide(GO)additives in the precursor solution,the influences of the preparation details on the perovskite films have been studied.Moreover,by evaporating organic molecules on the perovskite thin films,the interfacial energy level alignment at the interface between the perovskite thin films and the organic molecule transport layer has been investigated.The main findings in this work are as the following:1)In the preparation of perovskite films by one step spin coating,the application of dimethyl sulfoxide(DMSO)in the precursor solution and the application of anti-solvent in the spin coating process are found to significantly affect the surface morphology and crystallinity of the fabricated films.In this work,four kinds of perovskite films are prepared in ambient air using different preparation processes,namely one without DMSO in the precursor and without anti-solvent ether used in the spin coating process,one with DMSO but without anti-solvent,one without DMSO but with anti-solvent,and one with both.In-situ heating experiments were performed on these films characterized by using synchrotron based grazing incidence x-ray diffraction(GIXRD).It was found that the antisolvent ether improved the surface morphology and crystallinity of the perovskite thin films greatly.Moreover,the perovskite structure was formed quickly during the heating process,which kept quite stable during the continuous heating process.When DMSO applied in the precursor,a relatively stable intermediate phase can be formed after the spin coating,which delayed the crystallization process,but the film quality is relatively low and the perovskite structure will be rapidly decomposed with increasing heating temperature and time.The present findings could help to improve the preparation of perovskite thin films by one-step spin coating for high quality films.2)We introduced the graphene oxide into the perovskite precursor solution to study its influence on the quality of perovskite thin films.It was found that not only the grains size in the perovskite thin film become larger and the grains become more closely interconnected,but also the hysteresis effects in the devices are obviously reduced after introducing 1 vol% of graphene oxide in the precursor.It is concluded that graphene oxide not only promoted the growth of perovskite grains,but also increased the transport efficiency of carriers at the grain boundaries and surfaces.Therefore,adding graphene oxide is proven to be a simple and effective approach to achieve high efficiency perovskite solar cells which can be widely practiced.3)Organic molecule N,N'-Dipentyl-3,4,9,10-perylenedicarboximide(PTCDI-C5)has been used to replace [6,6]-phenyl-C61-butyric acid methyl ester(PCBM)electron transport layer materials in perovskite solar cells because of its good photothermal stability.However,its device efficiency has not been significantly improved.We deposited PTCDI-C5 in-situ on the perovskite films and used X-ray photoelectron spectroscopy(XPS)and ultraviolet photoelectron spectroscopy(UPS)to investigate their interfacial electronic structures.It was found that perovskite can chemically react with PTCDI-C5 core and N-C=O group.The resultant interfacial electronic structures favor the electron transfer from perovskite to PTCDI-C5,but block the hole less efficiently than PCBM.Therefore,PTCDI-C5 is not a good electron transport layer directly on the perovskite solar cells,but it may be used as a buffer layer to passivate the perovskite surface by the chemical reactions to improve the device stability.