Revisit Of Amorphous Semiconductor InGaZnO₄:A New Electron Transport Material For Perovskite Solar Cell

Compared with other cells,perovskite solar cells?PSCs?have attracted great interest recently due to their simple device structure,high effiency?23.3%?,and low cost,etc.The planar structure of PSCs consists of ITO bottom electrode,electron transporting layer?ETL?,perovskite absorber layer,hole transporting layer?HTL?,gold top electrode,etc.The electron transporting layer can improve the separation and transporting of photogenerated carriers,thereby improving the photoelectric conversion efficiency of the cells.Up to date,the materials used as the electron transporting layer are mainly binary semiconductor oxides,such as TiO₂,SnO₂,Nb₂O₅,and ZnO.Among them,TiO₂ is the widely used electron transporting layer,but it often needs high temperature treatment,and its catalytic effect is hard to be suppressed especially in the ultraviolet region;ZnO is easy to form various nanostructures to tune device performances.But ZnO is polycrystalline even prepared at room temperature,and the existence of defects at grain boundary affects the performance of devices.However,if doped with In and Ga,the crystallization of ZnO will be inhibited.The previous study shows that amorphous IGZO has high carrier concentration and high electron mobility which can be used to improve the electrical properties of thin films.As the start-of-the-art amorphous transparent oxide semiconductor,IGZO was mainly used in transistors,while it has not been reported as an electron transporting layer yet.This paper proposes to use amorphous IGZO thin film as an electron transport layer for perovskite solar cells.The IGZO ETL has advantages that it possesses an amorphous structure,can be physically deposited,and could be prepared at a relatively low temperature.In this paper,IGZO films with excellent performances were prepared by pulsed laser deposition and solution process,respectively.The influences of oxygen pressure of deposition,annealing temperature,ratio of metal ions and other deposition processes were analyzed.Through the optimization of film and device preparation process,we have achieved perovskite solar cells with a high photoelectric conversion efficiency and high stability.The specific research results are listed as follows:?1?For the pulsed laser deposition,the oxygen pressure and annealing temperature have a great influence on the microstructure and photoelectric property of the films.As the deposition oxygen pressure gradually increased from 10^-33 Pa,10^-22 Pa,10^-11 Pa and 4 Pa,the IGZO film remain still amorphous.Since it is amorphous,the roughness and surface morphology of the film are basically unchanged compared with ITO glass.However,the carrier concentration in the film reduced gradually,but the hall mobility increased at the beginning,then decreased,it will reach the highest electron mobility when the deposition oxygen pressure is 10^-22 Pa.When the annealing temperature gradually increases from 100?C,200?C 300?C,400?C,and 500?C,the IGZO films are amorphous,and the carrier concentration in the film gradually decreases,but the hall mobility increases firstly and then decreases.The highest electron mobility will be achieved with annealing temperature at 400?C.By controlling the deposition oxygen pressure and annealing temperature to adjust the oxygen vacancy defect concentrations of the amorphous IGZO film,IGZO films with excellent photoelectric properties will be prepared,and it was used for the electron transport layer of the perovskite solar cell to obtain a photoelectric conversion efficiency of 15.12%.Analysis of XPS and hall test results shows that the photoelectric properties in amorphous IGZO film are mainly caused by oxygen vacancy defects in the film.When the deposition temperature and oxygen pressure are low,the carrier concentrations in the film are high,but the high concentration of oxygen vacancy defects will form traps for electron transportation,thus hindering the smooth transmission of electrons.As the deposition oxygen pressure and annealing temperature increase,the oxygen vacancy defects in the film decrease,the carrier concentrations of the film were also reduced,resulting in lower electron mobility and affecting the performances of films and cell device.?2?When the IGZO films are prepared by solution process,the annealing temperature and the ratio of metal ions have a great influence on the microstructure and photoelectric properties of the film.As the annealing temperature increases from 100?C,200?C,300?C,400?C,500?C,The IGZO films remained still amorphous,and the roughness and surface morphology of the films do not change obviously.A-IGZO thin films with different metal ions ratio were fabricated and the corresponding devices were investigated.The optimized composition of a-IGZO for PSCs should be denoted as InGaZnO₄.Amorphous IGZO films with excellent photoelectric properties are prepared and can be used for the electron transporting layer of perovskite solar cells,it has achieved a photoelectric conversion efficiency of 17.35%.