A Study Of Growth Regulation Of The Active Layer And Its Interfacial Electronic Structures In Perovskite Solar Cells

In recent decades,environmental issues and the energy crisis have become more and more prominent.Solar energy has become one of the most promising solutions to solve the dilemma between the continuous development and the protection of the environment because of its clean and renewable nature.Photovoltaics is becoming more important because it converts solar energy into electricity convenient to use.Organicinorganic hybrid perovskites are gaining more and more attentions due to their excellent photovoltaic performance with their power conversion efficiency(PCE)increased from 3.8% to 25.5% in just a decade,which makes them the most promising candidate in the photovoltaic market.Perovskites present lots of merits,including simple preparation methods,suitable and tunable band gap,long carrier diffusion length,and high optical absorption.However,there are still many problems to be solved in perovskite solar cells(PSCs),such as complex working mechanism to be fully understood,poor environmental stability(against water,oxygen,and illumination),the control of the crystallization dynamics,the optimization of the film quality,difficulty in large area fabrication,and the optimization of the interfacial energetics.In order to obtain stable and efficient PSCs,researchers mainly focus on two aspects: 1)preparing high-quality perovskite films to improve the conversion efficiency of solar energy and photogenerated carrier lifetime;2)improving the carrier extraction and transport efficiency by improving the interfacial contact between semiconductor materials.There have been many researches about doping of additives to regulate the perovskite film growth to improve the film quality.However,few of them studied how the additives affect the growth and crystallization during the growth in real time.Therefore,this thesis focuses on the regulation of the perovskite crystallization dynamics as well as the interfacial energetics of perovskite.On one hand,different organic small molecules were deposited on perovskite thin films to study the interfacial electron structures between perovskite thin films and hole transport layer.The change of the electronic structure and the hole mobility of the hole transport layer after Li TFSI/t BP doping and subsequently air exposure was further investigated as well.On the other hand,we added additives into the precursor solution to regulate the growth and crystallization of perovskite thin films leading to improved film quality.The influences of additives were characterized by optical and electrical characterization techniques as well as synchrotron based grazing incidence wide-angle X-ray scattering(GIWAXS).The main results are as follows: 1.Organic small molecules N2,N7-bis(4-methoxyphenyl)-N2,N7-di(spiro[fluorene-9,9’-xanthen]-2-yl)spiro[fluorene-9,9’-xanthene]-2,7-diamine(X55)and N2,N2,N2’,N2’,N7,N7,N7’,N7’-octakis(4-methoxyphenyl)spiro[fluorene-9,9’-xanthene]-2,2’,7,7’-tetraamine(X60)were used to replace conventional hole transport layer material 2,2’,7,7’-tetra-(N,N--methoxy-phenylamine)spirobifluorene(spiro-OMe TAD).We used in situ thermal evaporation in combination with X-ray photoelectron spectroscopy(XPS)and ultraviolet photoelectron spectroscopy(UPS)to investigate the interfacial electronic structures between perovskite and X55(X60).Based on that,we further investigated how Li TFSI/t BP doping and then oxidation of X55(X60)transport layer will affect the interfacial electronic structures,its carrier mobility,and the performance of the resulting PSCs.It is shown that both molecular films present interfacial energetics favoring hole transfer/injection,but the poor hole mobility in both films leading to poor performance in the resulting PSCs.The Li TFSI/t BP doping and then oxidation significantly increase the hole mobility in the X55(X60)transport layer step by step.This enhancement facilitates the transport and extraction/collection of holes,which effectively suppresses the charge accumulation and non-radiative combination of carriers at the perovskite/hole transport layer interface.Thus,the fabricated PSCs exhibited significantly improved photoelectric conversion efficiency step by step after these two treatments.In contrast,the changed interfacial energetics after Li TFSI/t BP doping and then oxidation can not explain the dramatically improved PSC performance.This study suggests that high carrier mobility in the transport layer may be more decisive in high-performance PSCs than the interfacial energetics.2.Methylamidine lead bromide(MAPb Br3)was added as additive into the precursor solution of formamide-lead iodide(FAPb I3)perovskite to regulate the crystallization process of perovskite films.The scattering and diffraction signals of the perovskite precursor during the process from solution to gel during spin-coating and then to polycrystalline film were detected in real time by in situ synchrotron grazing incident wide angle X-ray scattering(GIWAXS)to analyze the crystallization kinetics in the perovskite films during spin-coating and annealing processes.With help of the literature,we speculate that the additive can effectively dope into the perovskite lattice,thus inhibiting the formation of δ-FAPb I3;In addition,the additive can effectively lower the phase transition energy barrier from δ-FAPb I3 to α-FAPb I3,prompting the black phase to appear at room temperature.Meanwhile,optical and electrical characterization indicated that the Br ions in the additive can slow down the crystallization of perovskite,leading to smaller grains with lower defect state density.Thus,the carrier extraction efficiency and lifetime was improved,which in turn leads to significant improved photoelectric conversion efficiency and stability.3.The growth and crystallization of perovskite films were regulated by adding strong electron accepting material N,N’-bis(dimethylaminopropyl-N’’’-oxide)-perylene-3,4,9,10-tetracarboxidiimide(PDINO)into the perovskite precursor solution.Using GIWAXS,it was demonstrated that the crystallinity of the perovskite films was improved,which effectively enhanced the extraction and separation of excitons in perovskite.Using ultrafast spectroscopy,the effect of optimal charge separation within the perovskite under low-light conditions was systematically analyzed.The resulting optimal perovskite solar cell exhibited improved conversion efficiency of indoor power from 33.8% to 37.9% under low light of 1000 lux.This study provides a novel approach to improve charge transfer with increased photovoltaic conversion efficiency under low light,which is instructive for rapidly expanding indoor PV applications.