Study On Electron Transporting Layers In Photovoltaic Devices Based On Hybrid Perovskite Materials

The human development goes along with the energy improvement.Since 1960s,the petrol has been widely used all over the world.The increasing consumption speed has caused an emergency issue in energy,forcing humans to face the energy crisis.The environmental friendly and renewable energy has attracted more and more attention throughout the world.The solar energy attracts much attention because of its infinity energy supply.To use it efficiently,a lot of researches have been carried out to make the solar cells with high efficiency and low cost.In the latest decades the silicon solar cells have taken the main commercial market,while high cost and complex fabrication reduced its improvement step.In recent years,the perovskite solar cells have emerged.Organic-inorganic perovskite solar cells with low cost,high power conversion efficiency?PCE?and simple fabrication process.Currently the mesoporous structure of perovskite solar cells takes the main position.Because of its long time consumption and high temperature requirements during the fabrication process,it cannot be applied well in industrial development.The planer structure perovskite solar cells can solve these issues well.This thesis focuses on the planer structure of perovskite solar cells which have the FTO/TiO₂/CH₃NH₃PbI₃/Spiro/Au structure.As one of the most important core parts of solar cells,the electron transporting layer?ETL?plays an important role in photon-to-electron conversion efficiency.Currently the most common materials are metal oxides such as TiO₂ and ZnO.Also,because the conduction band minimum?CBM?of anatase is-4.1 eV which is lower than the lowest unoccupied molecular orbital?LUMO?of CH₃NH₃PbI₃ perovskite,it can help to improve the electron-collection.Also,TiO₂'s wide band gap can block the injection of holes which makes it more ideal and mostly used.Nowadays the TiO₂ is the main research target.The anatase TiO₂ is commonly in a nanoparticle shape,so there will be closely contact between particles during the annealing which can make the ETL compact.So far,the improvement for TiO₂ nanoparticle preparation methods have been widely carried out which provides great convenience for low-temperature perovskite fabrication.This thesis will start from low temperature perovskite devices,and mainly focus on the improvements of TiO₂ ETL compactness and reduction of ETL roughness to increase the PCE of solar cells.The chapter 1 mainly explained the history,theory and fabrication methods of perovskite solar cells.In chapter 2,experiments were carried out to compare the performance between mesoporous and planer structure perovskite solar cells.The mesoporous structure is made of compact layer/mesoporous layer/perovskite/HTL/Au electrode,and the ETL of mesoporous solar cells were made of spin coating TiO₂sol.The mesoporous layer was made by spin coating with TiO₂ paste.Perovskite layer was made by one-step of spin coating the mixture solution of MAI and PbI₂ in DMF and DMSO.The HTL was spin coated by solution of Spiro-OMeTAD dissolved in CB.The planar structure is ETL/perovskite/HTL/Au electrode,and the Titanium?IV?isopropoxide was treated as the titanium resource.Then the TiO₂ particles were dispersed in Methanol and Chloroform.Then this solution was spin coated on clean FTO glass.After annealing treatment,the uniform and compact film will be formed.To compare with the high temperature mesoporous structure,the perovskite layer and HTL were prepared in the same way.After the comparison of performance,it was found that the planar structure has great research potential for commercial application.Then to achieve better compact and uniform layer,the planar ETL film was optimized in chapter 3.The TiCl₄ was treated as the titanium resource and the TiO₂ particles were dispersed in Methanol and Chloroform.After spin coating and annealing treatment in low temperature the film was prepared.It was found that annealing at the temperature of 150?could make a more compact film which will achieve the best PCE among 100?,150?and 200?.At the same time,researches show that a second film prepared by spin-coating at a high speed above the first uncompact film would improve the device performance.In chapter 4,the ETL film was optimized by comparing the ETL performance influenced by different sizes of TiO₂ nanoparticles and the PCE was also improved.TiCl₃ was treated as the titanium resource for the nanoparticle preparation.TiO₂ nanoparticle sizes were altered by Sn ions addition to control the precipitation time.After the comparison of performance based on devices with different ETL methods,it was found that too large grain size will make the film rough while too small size particle can influence the crystallinity which will result in bad performance by poor compactness.In summary,the improvement methods are shown below.It has been proven that 150?is more suitable after the optimization of different annealing temperatures and the PCE was improved to 16.16%.It was found that spin coating twice helped to reduce the ETL electric leakage and the performance of devices can achieve 17.17%.The ETL film made by nanoparticles in the size of 15 nm was compact and smooth with the highest PCE of 17.7%.