Study On The Window Layer Of PbS Quantum Dot Solar Cells

To solve the energy crisis and the serious environment problems,more and more people are paying greater attention to develop green energy.Solar energy is regard as an inexhaustible energy of a renewable and green energy,developing and utilizing it is receiving great support and positive response by our community.Especially,solar cells which can directly convert solar energy into electricity have been attracted great attention by researchers.Quantum dots have a special place in photovoltaic community for their characteristics of low-temperature solution synthesis,size-dependent tunable bandgap,as well as the multiple exciton generation effect.Recently,most of the research efforts focus on the synthesis method,new strategy of ligand passivation and film preparation,however,there are very few reports about window layer,which make as a heterojunction part and plays a key role in carriers dissociation and transportation.PbS is a material of mature development system of quantum dot solar cells,the highest conversion efficiency of the PbS solar cells has reached 11.3%,this is also the highest efficiency in the system of quantum dot solar cells.The classic device structure of PbS solar cells is:ITO/ZnO/PbS-TBAI/PbS-EDT/Au.The widely used ZnO film get less research,but it will have the major impact on ZnO-PbS devices.Thus,optimizing and investigating ZnO widow layers are main focus of this part of project.The thickness of ZnO films and corresponding PbS solar cells performace have been systematically studied.It was found that the thickness and correspond thermal treatment of ZnO films will have a significant impact on the surface defects,doping concentration,carrier recombination and depletion width in PbS layers,leading to affect the performance of the device.The optimized thickness of ZnO film is about 90 nm.In order to further improved device performance,it is our first time to introduce the new window layer about SnO₂ as window layer in PbS solar cells.After a serious characteristics for SnO2 films and SnO2-PbS devices,it is found that the SnO2 films has larger bandgap,faster mobility for electron and lower trap densities than ZnO films.The SnO₂-PbS solar cells also show a better performance than ZnO based devices.As a new window layer with simple synthesis,low temperature process and low cost,SnO₂ is expected to enrich full-solution processed CQD device research.Furthermore,it has been redefined the conception of window layer,which is not just a function layer,but also can be made as a subcell in tantem solar cells.We employed PMDPP3T:PCBM organic based front subcell as window layer for PbS based back subcell to assemble a tandem solar cells.After a series optimization of device structure,the tandam solar cells not only compensate the valley absorption of PbS quantum dots film,but also improve the open circuit voltage with good charge recombination layer and interface.The new approach about tandem solar cells offers a solution to optimizing PbS solar cells performance.