Theoretical Study On Photoelectric Properties Of Three Types Of Organic Hole Transporting Molecules

Solar energy is one of the most promising new energy resources,which can effectively solve both the increment of environmental pollution and depletion of fossil fuels.As one of the most promising photovoltaic technologies,solar cell can generate electric energy through the conversion of photoelectric energy.Up to date,solar cells are still one of the most attractive research topics,including silicon-based solar cells,quantum dot solar cells,and perovskite solar cells?PSCs?.Since the pioneer report by Kojima et al.,PSCs have attracted numerous attentions because of their simple structure,low manufacture cost,and excellent photovoltaic performance.More importantly,PSCs have presented an unprecedented increase of power conversion efficiency?PCE?from 3.8%to 23.7%within decade,which is unbelievable for other solar cells.Hole transport materials?HTMs?is one of the most critical components in PSCs,which plays a vital role in facilitating hole migration from perovskite to electrode and inhibiting charge recombination.Therefore,the ideal HTMs plays an important role in improving the efficiency of perovskite solar cells.Until now,2,2'7,7'-tetrakis?N,N-p-dimethoxy-phenylamino?-9,9'-spirobifluorene?Spiro-OMeTAD?is the most popular organic HTMs applied in PSCs.However,it suffers from the low mobility,difficult synthesis and purification along with the expensive price,which hinder its potential large scale application in commercial photovoltaics.Therefore,it is necessary to develop the alternatives.The developed HTMs include inorganic,polymeric,and organic HTMs.Organic HTMs stand out from them due to the following advantages including variable structures,abundant and raw materials,simple purified process and others.Currently,there are still some problems for organic HTMs including low mobility,unstability and high cost.Therefore,organic HTMs still can not completely replace Spiro-OMeTAD.For aforementioned problems,three types of organic HTMs,including spiro-based,linear-based and star-based HTMs,are investigated in this thesis by both first-principle and molecular dynamics?MD?simulations with the goal to uncover the structure-property relationship.As compared with the experimental molecules,the organic HTMs with excellent properties could be explored.The specific research content is as follows:1.On the basis of X59,other two HTMs,X59-P and X59-T are theoretically designed by spiro[fluorene-9,9'-xanthene]ascoreandp-methoxytriphenylamine/N,N-bis?4-methoxyphenyl?-5-methylthiophe-2-amine as donor group,respectively,with the ultimate goal to improve the hole transporting mobility by enhancement of?-?stacking.The electronic structures,energy levels of aforementioned three molecules at the B3LYP/6-31G?d,p?level,and their absorption properties were simulated by the TD-PBE0 method.Then,the hole transporting mobility is investigated by combination of first principle and MD associated with Marcus theory and Einstein equation.As compared with X59,the highest occupied molecular orbital?HOMO?energy level of X59-P is more close to the valence band of perovskite,which is beneficial to improving the open-circuit voltage(V_OC).The lowest unoccupied molecular orbital?LUMO?energy levels of all three studied molecules are higher than conduction band of perovskite to efficiently block the electron recombination.Combination of the smaller reorganization energy and the larger transfer integral,the larger hole transporting mobility is obtained for X59-P.Moreover,two new molecules have the better solubility and hydrophobic property,which is favorable to producing the device in solution.The synthesized pathways are proposed for them,which would be completed in a relatively benign condition.Our studies introduce a possible pathway to explore the efficient HTMs by suitable combination mode rather than development of new groups.2.On the basis of HT1,two derived HTMs,HT1-P and HT1-OP,are designed by employment of different substituted groups.The influence of substituted groups on properties was investigated for three organic hole transport materials,HT1,HT1-P,and HT1-OP,by combination of first principle and molecular dynamics.The frontier molecular orbital,absorption spectrum,and hole transporting mobility are investigated.In addition,the dimer and HTM@perovskite adsorbed system are also considered along with the densities of states?DOS?and projected density of states?PDOS?were calculated to deeply analyze the adsorption properties.The HOMO is more distributed over the whole molecule,while the LUMO is only localized over the central fluorene group,which is similar for all three molecules.As compared with HT1 and HT1-OP,HT1-P not only has the lower HOMO energy level indicating the larger V_OC but also the more suitable absorption region indicating the better light harvesting.Among them,HT1-P has the larger hole mobility than HT1 and HT1-OP.After adsorbed on the CH₃NH₃PbI₃?110?surface,there is weak physisorption between HTMs and CH₃NH₃PbI₃?110?surface,and the energy difference between HOMO energy of HTMs and valence band?VB?of CH₃NH₃PbI₃ is further enlarged after adsorption,which will facilitate hole extraction.In general,HT1-P presents better performance than HT1.The properties of HTMs would not definitely be changed by employment of different substituted group or core group,which is related with their electronic structure.It is necessary to screen them by theoretical method before synthesis rather than repeat error and trial.3.On the reported TCP-OH,other two star-shaped molecules are designed by replacement of side group by N,N-di?4-methoxyphenyl?aniline for TPAP-OH and oxygen-bridged triarylamine for TBOPP-OH.Their potential to be hole transport material in perovskite solar cells without dopants is evaluated by multiscale simulations.The properties of isolated molecules are estimated by the absorption spectrum,frontier molecular orbital,and hole transporting mobility.After that,the HTM@CH₃NH₃PbI₃adsorbed system is studied to consider the influence of adsorption on the performance of HTMs.Besides the primary judgment,the glass transition temperature?T_g?is also simulated to determine the stability of amorphous film.TPAP-OH has the matched energy level and suitable absorption region.More importantly,TPAP-OH has the largest hole mobility.After three molecules are adsorbed on the CH₃NH₃PbI₃ surface,TPAP-OH has the largest energy difference between HOMO of HTMs and VB of CH₃NH₃PbI₃ in three studied molecules.The T_g of TPAP-OH is the larger than that of both TCP-OH and Spiro-OMeTAD suggesting the stability to form amorphous film,which is also an important item to affect the stability of devices.TPAP-OH has superior performance in all aforementioned items,it is a promising HTMs.