Construction Of TiO₂-based Electron Transport Layers In Perovskite And Cuprous Oxide Solar Cells

Developing highly efficient,stable and inexpensive solar cells is one of the efficient routes to solve the issue of global energy crisis.Perovskite solar cell with the merits of low cost,high efficiency and easy fabrication,is a promising photovoltaic device for the practical application.Highly stable and low cost cuprous oxide based metal oxide solar cell with a theoretical efficiency of close to 20%is a potential photovoltaic device.Electron transport layer(ETL),as an important component in these two kinds of solar cells,performs the tasks of extracting photoelectrons(and blocking photoholes)from the active layer and delivering them towards electrode,influencing the conversion efficiency of photovoltaic devices significantly.Rational structural design and surface modification on the ETL could improve its electron collection efficiency and suppress the interfacial recombination of photocarriers,thereby enhancing the performance of solar cells.In this dissertation,we focus on the structural design and surface modification of typical TiO2 based ETLs to improve the efficiency and power output stability of perovskite and cuprous oxide solar cells,including surface modification with SnO2 quantum dots,hetero-epitaxial growth of TiO2 single crystal arrays with gradient Sn doping and structural design of mesoporous TiO2 single crystal arrays.The main results are listed as follows:The planar perovskite solar cells based on spin-coated TiO2 ETLs generally showed moderate efficiencies because of the presence of many pinholes in the TiO2 ETLs and defect trapping states at the TiO2/perovskite interface.SnO2 quantum dots(QDs)ended with chlorine ions have the capacities of filling the pinholes in the layer and passivate the trapping defects at the TiO2 ETL/perovskite interface.Meanwhile,SnO2 QDs ended with chlorine ions may promote the growth of perovskite grains.As a result of the increased interface electron collection and reduced bulk recombination,the planar perovskite solar cell with SnO2 QDs modified TiO2 ETL gives the large enhancement of average power conversion efficiency from?14%to?17%and greatly improved power output stability under the continuous light irradiation.An unusual dense film of faceted rutile TiO2 single crystals with a gradient of Sn dopant grown hetero-epitaxially on the FTO layer was rationally synthesized for constructing efficient planar perovskite solar cell.The subsequent thermal treatment in air(500?)could reduce defects density and induce a gradient Sn doping in the film due to the Sn diffusion from FTO to TiO2.A built-in electrical field was formed in the film as a result of the gradient Sn doping.Owing to the single crystalline nature,low concentration of defects,atomically smooth coherent interface with FTO and gradient Sn doping induced built-in electric field,the photoelectrons could quickly transport through the TiO2 layer and be collected by electrode.Based on such Sn doped TiO2 films as ESLs in PSCs,the average conversion efficiency was improved from?13.4%to?16.5%with open circuit voltage increasing from?0.962 V to?1.108 V.A highest efficiency of 17.2%was achieved in the optimized cell,which is among the highest values of the perovskite solar cells with rutile TiO2 films as ETLs.A mesoporous TiO2 single crystal array film was in situ epitaxially grown on the FTO substrate by using silica as template.Its pore size is enlarged and defects are reduced after a heating-etching-heating process.Following,a TiO2-Cu2O single crystal interlaced p-n heterojunction was constructed by electrochemically depositing Cu2O crystal array films through the TiO2 film.The single crystal nature is beneficial for the carrier transportation due to the absence of boundary scattering,the interlaced heterojunction could provide more carrier transferring channels and shorten the carrier transportation distance.The optimized thickness of the Cu2O film was obtained by adjusting the electrochemical deposition time.In order to further inhibit the interfacial photocarrier recombination,amorphous SnO2 layer was introduced at the TiO2/Cu2O interface via atomic layer deposition(ALD)technic.The resultant device could deliver an open circuit voltage of exceeding 0.4 V and a maximum efficiency of 1.44%,which is the highest value of the TiO2/Cu2O heterojunction based solar cells.