| Organic solar cells(OSC)have been widely studied due to the advantages of aboundant sources,low cost,good flexibility,easy processability and potential application for large-area production.The structure of bulk heterojunction have attracted much attention in recent years.This structure is a blend of donor and acceptor,which increases the contact area of donor and acceptor materials and achieves more effective exciton dissociation,leading to the improved power-conversion efficiencies(PCEs)of the device.Polymers and small molecues(SMs)are used as donors based on fullerenes as acceptors in the bulk heterojunction solar cells.So far,the PCEs based polymers as donor have exceeded 14%.Compared to their polymeric counterparts,SM donor materials have drawn constant attention due to the well-defined struceture,easy purification,and easy tailoring,and the PCEs over 11% have been achieved.It is believed that there is still room for improving the PCEs of SMs in OSCs by further optimizing their chemical structures.Therefore,SMs deserve more extensive research to improve their performance in low-cost commercialized OSCs.In this dissertation,we investigate the structure-property ralationships of organic small-molecule photovoltaic materials by quamtum chemistry calculation.The impact of structural modulation on photovoltaic properties is explained,and theoretical guidance for the design and synthesis of new ideal materials in the experiment is provided.Our work mainly includes two parts as follows:Part 1: Modulation of electron-donating ability in D-A-A small molecules for application in heterojunction solar cell was studied theoretically.Comparative studies of a class of dithienopyrrole(DTP)-based D-A-A type small-molecule donor materials based on fused benzene and fused thiophene series have been carried out.The amine group in the DTP is substituted with the conjugated electron-withdrawing units,which will be expected to modulate the electron-donating ability of SMs.To gain a better understanding of the effects of structural modification of DTP on the photophysical properties and the photovoltaic performance,the geometrical and electronic structures,absorption spectra,energy loss,intramolecular charge transfer(ICT),exciton dissociation and charge recombination rates at the interface,and hole-transport properties are analyzed in detail to establish structure-property relationships.This work provides an intuitive explanation why the modification of DTP could modulate the frontier orbital levels,light-absorbing capacities,charge-transfer properties,and interfacial exciton dissociation and charge recombination.The calculated results also demonstrate that the fine tuning of the electron-donating ability of small-molecule donor material could effectively improve the cell performance.This study could give theoretical guidelines for molecular design and pave the way to synthesize efficient small-molecule donor materials.Part 2: The effect of extended ?-conjugation of terminal indenedione group on the properties of A-?-D-?-A small molecules was investigated.A series of small molecules were designed by extending ?-conjugation of the acceptor unit.The ground-state geometrical structures,absorption spectra,frontier orbital levels,ICT,energy loss,exciton binding energy,exciton dissociation and charge recombination rates are analyzed in order to establish the structure-property relationships.The calculation results show that extending terminal ?-conjugation incerases the open-circuit voltage of the molecule,widens the absorption spectral range,achieves better intramolecular charge transfer properties,and obtains more effective exciton.The predicted PCE of the investigated SM reaches up to 8%(in underestimated conditions)by Scharber diagram,it could be a potentially efficient material.Therefore,it is an effective design strategy to extend the ?-conjugation of the terminal acceptor group.This result could provide some reference for the experimental design and synthesis of novel promising materials. |
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