Photoconductive AFM Study And Plasmonic Tuning Of Interfaces In Perovskite Solar Cells

Perovskite Solar Cells(PSCs)are a new class of solar cells utilizing organicinorganic hybrid metal halide perovskite materials as the light-absorbing layer.With the great endeavor by researchers all over the world,PSCs have achieved a tremendous development with power conversion efficiency(PCE)reaching 25.7%from the initial 3.8%.PSCs have excellent photoelectric properties,with strong light absorption,long carrier lifetime,high carrier mobility,adjustable band gap and facile as well as low-cost preparation processes,and are regarded as the "game changer" in the field of solar cells,showing great commercial potential.How to ensure the stability of PSCs while pursuing high efficiency is currently the main research direction in this field,and interface issues are regarded as crucial to the ultimate efficiency and stability of devices.Focusing on the influence of interfaces on stability and efficiency in PSCs,methods for characterizing and controlling interfaces were developed in this thesis to investigate in-depth the benign and detrimental effects of perovskite grain boundaries,the screening of charge transport materials,and the plasmon-enhanced efficiency of PSCs.First,the conductivity and stability of grain boundaries and interface of perovskite thin films under working conditions were investigated by employing photoconductive atomic force microscopy(pc-AFM),and then a method for screening interfacial charge transport materials was attempted to be established with the aid of AFM probes.In addition,the appropriate plasmonic nanomaterials were introduced to improve light absorption and interfacial carrier transport for improving the efficiency of PSCs.The main achievements are outlined as follows:1.Construction of AFM probe-assisted local photocells for studies under close to working conditions.With the aid of AFM probe,local photoelectric conversion device configuration(free of hole transport materials)was constructed so that pc-AFM and KPFM could be employed to systematically investigate the carrier transport behaviors of(FAPBbI3)0.85(MAPbBr3)0.15 mixed-cation mixed-halide perovskite under close to working condition,especially the benign and detrimental effects of perovskite grain boundaries in carrier transport and interfacial degradation decomposition,and the bias dependency.Local I-V curves and two-dimensional photocurrent results show that the grain boundary conductivity of the perovskite films is no higher than that of the grains at a bias of 0.7 V(near the open circuit),while the grain boundary conductivity is significantly stronger than that of the grains at a bias of 0.5 V(the maximum power point)or below.Bias-induced irreversible decomposition of the pristine perovskite would occur at smaller bias of 0.1 V or below,which is accelerated by light illumination,leading to the formation of an interfacial layer that restricts carrier transport and thus causes current degradation.Meanwhile,bias-dependent morphology and photocurrent images reveal the double-edged roles of grain boundaries to photovoltaic performance of PSCs:grain boundaries appear to enhance carrier transport at larger bias,but serve as breakthrough sites for perovskite decomposition at smaller bias.Furthermore,methylammonium iodide passivation promotes decomposition,while potassium iodide passivation greatly relieves the decomposition which can effectively inhibit ion migration.This work provides valuable guidance for defining an appropriate operation condition of PSCs.2.A method based on AFM probe-assisted local photocells for screening hole transport materials have been established.Combining with the above established method for constructing AFM probe-assisted local photocells with AFM tip modification,a device configuration that was closer to the actual PSC was constructed,and local I-V curves were measured to rapidly screen hole transport materials with various characteristics.First,the deposition of inorganic hole transporting materials such as CuSCN and CuI coated on AFM probes by electrochemical or chemical reaction was attempted,but the deposition uniformity of the materials on the probe needs to be further improved.Furthermore,4-mercaptobenzoic acid(4-MBA)or 4methylbenzenethiol(4-MBT)molecules were modified on AFM probes,respectively,and by comparing the difference of local I-V curves and combining with results of complete macro-PSCs devices,with perovskite films modified by the corresponding molecules,we have developed a convenient,fast and effective method for screening modified molecule in PSCs for improving efficiency of optimizing PSCs.3.An optical tuning approach based on plasmon-enhanced PSCs by molecularly isolated gold nanorods for improving the efficiency have been established.With the aid of freeze-drying method,two molecularly isolated gold nanorods(Au NRs),namely Au@MBA(4-mercaptobenzoic acid)NRs and Au@MBT(4-methylbenzenethiol)NRs were successfully introduced onto the surface of perovskites for efficient optical tuning to further improve PCE of PSC devices,and the mechanism of Au NRs and molecular isolation layer was investigated systematically.Bearing relatively stronger scattering ability,the molecularly isolated Au NRs with appropriate LSPR(localized surface plasmon resonance)peaks locating at the wavelength range of 600-850 nm which perovskites absorb weakly can scatter the incident light in this wavelength range back to the perovskites to improve light harvesting,resulting in boosting short-circuit current.The two isolated molecules can maintain maximum possible localized electromagnetic field enhancement of naked Au NRs for accelerating charge separation;meanwhile the refractive index of 4-MB A is larger than that of 4-MBT,and the near-field enhancement effect of Au@MBA NRs is stronger,which is more conducive to the generation and separation of carriers.Equally important is that 4-MBA also provides better matched energy level at interfaces,which facilitates more efficient charge separation and transport across the interface,resulting in an increase in PCE from about 20%to 22%.However,Au@MBT NRs cause significant decrease of open-circuit voltage and fill factor due to the energy level mismatch of 4-MBT.This work reveals the profound influences of the LSPR effects of Au NRs,the refractive index and energy level alignment of isolated molecules on the efficiency improvement.Such an optical approach is promising to further improving the efficiency of single-junction perovskite solar cells toward the Shockley-Queisser limit.In addition,preparation of lead-free perovskite films on FTO/SnO2 electrode by electrodeposition was attempted.Currently,MASnI3 perovskite of small area can be obtained by one-step electrodeposition on FTO/SnO2 substrate,and it is difficult to get large areas of MASnI3.The difficulty in large-area preparation and the improvement method need to be further investigated.