Research On The Hole-transporting Layers And Interfaces In Perovskite Solar Cells

Perovskite solar cells（PSCs）have attracted significant attention due to their high efficiency and easy fabrication.The typical PSC is composed of a perovskite layer,electron-transporting layer,hole-transporting layer（HTL）and electrodes.HTL plays fundamental role in efficiency,hysteresis,stability and cost of PSCs.Therefore,it is vital to study on the HTL for the fabracation of high-efficiency,hysteresis-less,stable and cost-effective PSCs.In this doctoral dissertation,HTL was systematically investigated,mainly focusing on the influence of novel hole-transporting materials,versatile dopants and doping strategy on performances of PSCs.Moreover,impact of a well-known dopant,4-tert-butyl pyridine（t-BP）has also been investigated as an interface modifier for TiO₂ films.The main contents of this dissertation are briefly discussed in following paragraphs.Two novel triphenylamine-based small molecules were designed and synthesized as hole-transporting materials for PSCs.The only difference of two molecures is that replacing phenothiazine in LHTM-1 with carbazole to construct LHTM-2,aiming to investigate the effect of minor structural discrepancy on the photovoltaic performances.The results showed that as molecular structure changes from phenothiazine to carbazole significantly improved the molecular planarity,hole mobility and shifted the highest occupied molecular orbital level（HOMO）downward.Consequently,comparing with LHTM-1,CH₃NH₃PbI₃-based PSCs employing carbazole-based LHTM-2 doped by Li-TFSI/t-BP/FK209 as HTL achieved higher power conversion efficiency（PCE）of14.81%(J_SC=19.72 mA cm^-2,V_OC=1.058 V,FF=0.72),which reaches about 92%of the PSCs based on the Spiro-OMeTAD（PCE=16.07%）.Fluorine-containing hydrophobic F4-TCNQ has been introduced into Spiro-OMeTAD as an efficient dopant to replace conventional Li-TFSI/t-BP.The UV-Vis absorption spectra revealed that the F4-TCNQ could oxidize Spiro-OMeTAD thereby with no need for air oxidation process,which was beneficial to shorten preparation time of PSCs.Meanwhile,F4-TCNQ is an efficient dopant for Spiro-OMeTAD to decrease series resistance,facilitate hole extraction and transport,resulting in significantly enhanced J_SC,FF and PCE simultaneously.The best-performing PSCs,using 1.5 mol%F4-TCNQ doped Spiro-OMeTAD,achieving PCE of 12.93%with J_SC of 19.04 mA cm^-2,V_OC of 0.925 V and FF of 0.73,which was comparable to that of 14.32%for reference device with Li-TFSI/t-BP doped Spiro-OMeTAD.Furthermore,the lower J-V hysteresis and superior stability were observed for the PSCs based on F4-TCNQ doped Spiro-OMeTAD compared to Li-TFSI/t-BP as dopants.The hydrophobic Lewis acid dopant（LAD）was developed and employed as an effective dopant for PTAA.It has been observed that PCE of PTAA-LAD based PSCs was enhanced to a superior value of 19.01%which was significantly higher than 17.77%for the control device based on Li-TFSI/t-BP doped PTAA.Experimental analysis showed that the high efficiency of the PSCs using the LAD dopant attributes to the doped PTAA with LAD would enhance hole mobility,down-shift HOMO level,improve hole transport and extraction as well as reduce charge recombination.Besides,the LAD-based PSCs displayed lower J-V hysteresis and much better long-term stability.Time-of-flight secondary-ion mass spectrometry（TOF-SIMS）results suggested that the reduced hysteresis can be ascribed to the non extrinsic Li⁺-ion migration.In order to analyze and elucidate the outstandingly environmental stability of the LAD based PSCs,synchrotron-based two dimensional grazing incidence X-ray diffraction（2D-GIXD）techniques have been conducted to investigate the phase change of perovskite degradation process in complete devices with different HTL dopants.The novel approach of infiltrated diffusion gradient doping（IDGD）was reported toward PTAA doped with LAD.The femtosecond transient absorption（fs-TA）and photoluminescence spectra results demonstrated that PTAA doped with LAD by IDGD strategy could improve hole extraction from perovskite layer to PTAA layer,ultimately reducing charge recombination.Meanwhile,the pressure-assisted solution processing（PASP）strategy was used to precisely control the perovskite nucleation and growth hence controllable fabrication of highly crystallized perovskite films with micron-sized grains and low defects density.PSCs fabricated by combining these two novel techniques IDGD and PASP showed superior PCE of 20.32%(J_SC=23.06 mA cm^-2,V_OC=1.12 V,FF=0.79).t-BP was used as an interfacial layer to modify the TiO₂ surface for fabrication of PSCs.It demonstrated that V_OC and FF of PSCs could be significantly improved from1.03 to 1.09 V and from 0.71 to 0.75,respectively,by modifying TiO₂ with t-BP,resulting in improvement of PCE from 16.14%to 17.58%for the best performing devices.The results revealed that t-BP modification improved efficiency through two separate mechanisms.First,t-BP interfacial dipole layer could tune Fermi energy level and work function of TiO₂.Second,reduced defect states of TiO₂ by specific binding of t-BP at defect sites,resulting in reduction of charge recombination.Further,the low defect sites on the TiO₂ surface by t-BP modification correspondingly led to reduced J-V hysteresis of PSCs.