Perfromance Improvements Of Planar Heterojunction Perovskite Solar Cells Via Solvent Engineering And Interface Optimization

Organic-inorganic halide perovskites have been attracted dramatically owing to their excellent merits in optoelectronic properties,such as high absorption coefficient,small exciton binding energy,long charge carrier diffusion length and ambipolar transport.In recent years,perovskite solar cells(PSCs)have shown the rapid development in photovoltaic technologies.Until now,the certified power conversion efficiency(PCE)of PSCs has already exceeded 22.1%.On the other hand,the PSCs with inverted planar heterojunction(PHJ)structure are considered as the most appropriate device architecture for industrial production ascribing to the simple and low-temperature processibility,and low hysteresis loss.However,the highest PCE of inverted PHJ PSCs reported in the literatures only reached 20.3%,which was much lower than that based on mesoscopic PSCs.As one of the most effective tactics in improving photovoltaic performance,the improvement on active layer and optimization of the electron and hole transporting layers(ETLs and HTLs)are presented as two very efficient methodologies.In this thesis,we controlled the nucleation and growth of the perovskite crystal via solvent engineering for pursuing the uniform,dense and large grain size perovskite films.Furthermore,we reduced the energy loss between perovskite active layer and HTLs by utilizing the interface engineering in HTLs.The main works in this thesis are shown as follows:1.Sec-butyl alcohol was used as green anti-solvent to deposit flat,pinhole-free CH3NH3PbI3 films.In this process,sec-butyl alcohol was used to induce the fast crystallization of CH3NH3PbI3 and obtain an appropriate CH3NH3I(MAI)content.Compared with the generally used anti-solvents(e.g.,toluene and chlorobenzene),sec-butyl alcohol is superior to these solvents in environmentfriendly process,repeatability,and practicability(i.e.,no supporting scaffold is needed in the process of perovskite crystallization).In addition,the Scanning Electron Microscope(SEM)and X-Ray Diffraction(XRD)were used to reveal the nucleation and growth process of perovskite crystals during the film deposition.Finally,the maximum PCE of 14.3% without hysteresis was achieved.2.Sequential thermal and solvent treatment was proposed to prepare uniform and large-grain size perovskite film.It was revealed that the film quality and grain sizes of perovskite films could be effectively controlled by varying the temperature of pre-annealing and atmosphere of post-annealing.As a result,the excellent crystal grain boundary could be obtained for all films with thickness ranging from 420 nm to 1 ?m.We also analyzed the influence of perovskite film thickness on PSC performances.The Fill Factor(FF)and PCE dropped slightly to 75% and 15.7%,respectively,even if the perovskite film thickness was increased to 1 ?m.This was possibly because the average grain size was generally larger than the film thickness.3.Polystyrene nanospheres were used as a sacrificial template to fabricate the nano-bowl porous poly(3,4-ethylenedioxythiophene): poly-(styrene sulfonate)(PEDOT:PSS)film.The growth of CH3NH3PbI3 perovskite film on the porousPEDOT:PSS substrate yields a remarkable improvement in crystallinity and an enhancement in perovskite grain sizes.Besides,the porous structure could increase interface contact between perovskite and porous-PEDOT:PSS,which promotes the rapid hole extraction and transport.When the porous structure was applied as a HTL in PHJ PSCs,the PCEs were significantly improved from 15.33% to 17.32% for the planar and porous PSCs,respectively.4.Three different materials were separately used as the HTL in PSCs,including PEDOT:PSS,poly(triarylamine)doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(PTAA:F4-TCNQ),and PEDOT:PSS/PTAA.Compared with PEDOT:PSS-PSCs,PTAA:F4-TCNQ HTL could contribute a remarkable increase of open circuit voltage(VOC)to 1.08 V,owing to the versatile energy level alignment between PTAA and perovskite film.However,F4-TCNQ would induce some intermediate impurity traps in the PTAA,which will reduce the photovoltaic performance because of the energy-losing effect.Resulting from this drawback,PEDOT:PSS/PTAA composite layer was developed as HTL for PSCs.Interestingly,the PEDOT:PSS/PTAA based PSCs showed higher PCEs than that of the F4-TCNQ-doped PSCs,due to the efficient hole extraction and fast charge transport with a thin-PTAA layer(2-4nm)by avoiding interface energy loss from the F4-TCNQ.The highest PCE was significantly improved from 18.97%(PTAA:F4-TCNQ)to 19.98%(PEDOT:PSS/PTAA),which displayed that PEDOT:PSS/PTAA composite layer could be an efficient HTL for achieving highperformance PSCs.