Study On Solution Processed Cadmium Telluride Nanocrystal/Oxide Heterojunction Solar Cells

With the development of nanocrystal synthesis technology,nanometer-scale semiconductor crystals capped with surfactant molecules and dispersed in solution,have provided a powerful platform for the development of numerous classes of solution-processed optoelectronic devices.In the field of solar cells,II-VI?such as CdTe,CdS,etc.?,III-V?such as GaAs,InP,etc.?,IV-VI?such as PbS,PbSe,etc.?binary or multi-nanocrystalline solar cells developed rapidly.CdTe with a direct bandgap compound?1.5 eV?,has a good match with the solar spectrum and becomes an ideal photovoltaic semiconductor material,with the device efficiency exceeding 10%.However,the research shows that the open circuit voltage of solution processed CdTe nanocrystal solar cell devices is generally low?0.4 0.5 V?,which greatly affects the performance of the device.The underlying cause is that the presence of the interface barrier between the ITO and CdTe active layers and the thickness effect on the transport of heterojunction charges.In order to solve the above problems,Au,Ni,MoOx and WO3 were introduced into CdTe / ZnO heterojunction devices on the basis of optimizing the synthesis and thin film preparation of CdTe nanocrystals.Through the test and analysis,we find that the introduction of Au?1 nm?can effectively improve the interface barrier and reduce the interface recombination,so that the open circuit voltage of the device is greatly increased to 0.71 V,and the energy conversion efficiency of the device is enhanced.Joining the vacuum preparation MoOx film and solution treatment MoOx film into the devices,the open circuit voltage is almost no change,but the short circuit current has improved.However the device performance with the introduction of MoOx interface material is less than device without MoOx film.WO3 can increase open circuit voltage and short circuit current.However,due to the small fill factor the efficiency is reduced.The fill factor is related to the internal resistance and leakage current of the film.After optimizing the thickness of Au,the best device had an open circuit voltage of 0.65 V,the efficiency of 5.23%.Moreover,the stability of the device had also been greatly improved.On the other hand,we introduced an TiO₂ layer to prepare the inverted heterojunction devices,which not only avoids direct contact between ITO and CdTe,but also makes the incident light side closer to the heterojunction,reducing the loss of blue light and photogenerated carrier recombination.We can improve the carrier mobility by controlling the band position with Sb element doping TiO₂ thin films,which makes the formation of better heterojunction quality and promotes the separation of excitons.Finally,the device?FTO / ZnO / Sb-TiO₂ / CdTe / Au?prepared by 3% Sb-TiO₂ thin film obtains the open circuit voltage up to 0.74 V,which is the maximum value of CdTe nanocrystal solar cell.However,due to the serious lattice mismatch between TiO₂ and Cd Te,the device fill factor is lower and the efficiency of the device needs to be further improved.