Study On The Preparation And Interfacial Modification Of SnO₂ Electron Selective Layers For Efficient Perovskite Solar Cells

Solar cells,converting solar energy into electricity,have been regarded as a kind of highly attractive solar energy utilization device.Metal halide perovskites emerging as promising photovoltaic materials have outstanding optoelectronic properties,and the efficiency of perovskite solar cells(PSCs)has rapidly increased to over 25%in the past decade.Meanwhile,the low cost precursors and simple synthesis of perovskite materials make PSCs highly promising for the commercial application.Although the outstanding optoelectronic properties can enable perovskites with high capacity of carrier transportation and separation,it still needs to develop highly efficient carrier selective layers with high quality interfaces to obtain the matched carrier extraction ability for high performance PSCs.For the regular(n-i-p)structure PSCs with high efficiencies,electron selective layers(ESLs)perform the tasks of both selectively collecting electrons(blocking holes)from perovskite layer and serving as the substrate of growing the perovskite layer in n-i-p planar PSCs.Accordingly,controlling the properties of ESL itself along with the interface between ESL and perovskite layer is crucial to achieve high performance PSCs.To date,SnO2 has become a dominant ESL component of high performance PSCs due to its high electron mobility and proper band alignments with perovskite materials.To further promote the commercial application of PSCs,it is still desirable to develop new techniques for the simple synthesis of uniform,low cost and huge size SnO2 ESLs,and produce high quality perovskite layers with less bulk and interface defects for the performance and durability improvement of the devices.Based on the above analysis,the thesis is focused on the synthesis and modification of SnO2 ESLs,exploiting simple techniques for the scalable synthesis of SnO2 ESLs and developing efficient interface modification methods for the removal of defects in devices,to build highly efficient and stable PSCs with less hysteresis behavior.The main contents of the thesis are listed as follows:1)Based on the fact that SnO2 is the matrix of the fluorine doped tin oxide(FTO)substrate,we creatively developed an electrochemical reduction technique followed by a thermal oxidation process for the selective removal of fluorine dopants at the surface layer of FTO substrates,in situ conformly forming an ultrathin SnO2 ESL atop the FTO.Compared with the ESL free device based on the pristine FTO substrate,which delivers an efficiency of?14%under the reverse scan and shows a strong hysteresis loop with its forward scan efficiency of 9%,the ESL free device with the modified FTO substrate can give an improved efficiency of 15.7%under the reverse scan and it shows a much weaker hysteresis behavior with a forward scan efficiency of 15.1%.2)We developed a photo-assisted Cl doping method to prepare Cl modified SnO2 by introducing a HAuCl4 involved photocatalytic reaction process.Surface defects of SnO2 can be effectively passivated by the Cl atoms released from HAuCl4 in the photocatalytic process.The Cl modified SnO2(Cl-SnO2)ESL not only favors the formation of high quality perovskite films with less Pb-I anti-site defects formation at interfaces,but also has proper band alignments with the perovskite film for efficient electron collection from perovskite.Accordingly,compared with SnO2 based PSC,the PSC based on Cl-SnO2 ESL gives the enhanced efficiency of 18.94%,little hysteresis and much better stability.3)We originally adopted alkali metal oxalates as a new modifier of SnO2 ESLs for efficient PSCs.The carboxylate group in oxalates is capable of passivating surface oxygen vacancies of SnO2 ESLs,and the alkali metal cations as the common components of perovskites can promote the nucleation and growth of high quality perovskite active films with less defects in bulk and at interfaces.Accordingly,the electron collection ability of SnO2 ESLs can be much improved,and thus PSCs based on the SnO2 ESLs with alkali metal oxalate modifiers can give the improved efficiency of 19.5%(compared to 17.7%),little hysteresis and much better stability.