Approaching High Performance Inverted Planar-Heterojuction Perovskite Solar Cells Through Interfacial Engineering

In the past years,perovskite solar cells?PSCs?have attracted tremendous attentions due to their distinct physical properties,such as extremely long charge carrier diffusion lengths,good electrical properties and high extinction coefficient.The power conversion efficiencies?PCEs?of PSCs over 20%have been reported.Although significant progress has been made over the past nine years,in the development of perovskite solar cells,significant challenges still exist.These challenges include increasing the reproducibility of efficient devices,eliminating hysteresis during operation,improving long-term PSCs device stability,as well as improving charge-selective contacts.In the dissertation,we focus on fabricating efficiently inverted planar-heterojunction?PHJ?PSCs with different hole extraction layers?HELs?and improving perovskite layers.We find a simple way to fabricate VO_X thin film from pure-water solution,as HEL for PSCs.Furthermore,an aminopropanoic acid?APPA?interfacial layer is used to modify VO_X thin film for improving the surface morphology of VO_X thin film and reducing the charge carrier recombination rate.Moreover,we find that the chemical interaction between APPA and VO_X thin film reduces the work function of VO_X thin film,The low lying WF of VO_X/APPA thin film facilitates the holes to be efficiently extracted from CH₃NH₃PbI₃ layer to the VO_X/APPA HEL.When CH₃NH₃PbI₃ thin film is coated on the top of VOx/APPA thin film,CH₃NH₃PbI₃ thin film possesses full of interconnected plate-like shape grains with full surface coverage and the number of pinholes is significantly reduced.As a result,the PSCs with the VO_X/APPA HEL exhibits better device performance than that of the PSCs with the VO_X HEL and the PSCs with PEDOT:PSS HEL.Moreover,the PSCs with the VO_X/APPA HEL exhibits hysteresis-free behavior.All these results indicate that we report a simple approach to realize high performance inverted PHJ PSCs.Based on previous results,we fabricate the VO_X:yCs thin films by adding cesium hydroxide monohydrate into metavanadate precursor solution.It is found that the VO_X:yCs?where y is the mole ratio of Cs versus V,and y=0.1,0.3 and 0.5?thin films possess better electrical conductivities than that of pristine VO_X thin film.The addition of Cs element in the VO_X thin films can change their microstructural film morphology.Moroever,the VO_X:0.3Cs thin film exhibits enlarged surface roughness,resulting in enhanced interfacial adhesion between the HEL layer and the CH₃NH₃PbI₃ layer.As a result,the PSCs incorporated with the VO_X:yCs HEL exhibit large fill factors,high short-circuit currents,with a consequently high PCEs,which is more than 30%enhancement as compared to that with pristine VO_X HEL.The VO_X:0.3Cs HELs can effectively suppress nongerminate recombination process in PSCs.The trap-assisted charge-recombination process can almost be suppressed in the PSCs VO_X:0.1Cs and PSCs VO_X:0.3Cs.Our studies provide a facial way to enhance the electrical conductivity of the hole extraction layer for boosting device performance of perovskite solar cells.In the third section of this dissertation,composite HELs are prepared by combining VO_X or VO_X:0.3Cs with poly?triaryl amine??PTAA?,which are utilized to fabricate inverted PHJ PSCs as HELs.The PSCs incorporated with the composite HELs obtain a reduced E_loss and further an enhanced V_OC as compared to that of the PSCs with corresponding single HELs.It is shown that the enhanced V_OC in PSCs is attributed to the improved energy level alignment and the suppressed charge-recombination of the PSCs with composite bilayer HELs.By incorporating with composite HELs,a V_OC as high as 1.09 V and a PCE of 19.64%are realized.The PSCs with composite HELs exhibit hysteresis-free characteristics.The results show that the proper energy level alignment and the reduction of charge-recombination at the peroskite/HELs interface are of significance to find ideal selective contacts toward high V_OCC and PCE.In the fourth section of this dissertation,we fabricate perovskite thin films by adding aminopropanoic acid?APPA?into perovskite precursor solution.It is found that the addition of APPA in the perovskite layer can efficiently reduce energy loss and further enhance V_OC of PSCs.The enhanced V_OC in APPA-treated PSCs is attributed to the improved film morphology of CH₃NH₃PbI₃ layer,the suppressed non-radiative and trap-assisted recombination.By adding APPA into perovskite layer,a V_OC as high as 1.11 V and a PCE of19.23%are realized.The APPA-treated PSCs exhibits hysteresis-free characteristics and enhanced photo-stability.The results show that the addition of APPA in perovskite layer is a feasible way to control the morphology of the perovskite films,reduce energy loss and charge carriers recombination.