Theoretical Studies On Organic Small Molecules For Photovoltaic Applications

Organic photovoltaic(OPV)cells have drawn constant attention for decades because of their advantages of low cost,lightweight,flexibility,solution processability,environmental friendliness and potential applications in large-area devices.Bulk heterojunction(BHJ)is a blend of bicontinuous and interpenetrating network of donor and acceptor components.The BHJ concept greatly increases the D/A interfacial area and effectively achieves exciton dissociation at the heterojunction interface,leading to enhanced efficiency of the OPV devices.Great efforts have been dedicated to the development of both small molecules(SMs)and polymers as light-absorbing donor materials in BHJ device architectures.So far,power conversion efficiencies(PCEs)of over 10% have been achieved for polymer-based OPV devices,but the weakness of polydispersity of molecular weights and difficulty in purification for polymers may restrict their performance reproducibility.Conversely,SMs have been rapidly developing in recent years because of the advantages of well-defined structure,easier purification,less batch-to-batch variation,and good flexibility in molecular tailoring.To date,though much progress has been made for SMs with the reported PCEs over 10%.However,the current efficiencies of SMs are far behind the commercial requirements.Therefore,the high performance SMs for OPV applications are still in high demand and deserve extensive research.In this dissertation,we focus on studying organic small molecular photovoltaic materials and systematically investigate the effects of the structural modification on the photovoltaic performance by means of quantum chemical calculations.We aim to establish structure-property relationships and shed light on fundamental research on designing high performance SMs for OPV applications.Our work mainly includes two parts as follows:Part 1: The effect of terminal modulation of D-?-A small molecule for organic photovoltaic materials was studied theoretically.A class of D-?-A-A type smallmolecule donor materials used in heterojunction solar cell has been designed and investigated.To gain a better understanding of the effects of terminal acceptors on the modulation of electronic and optical properties of D-?-A molecule,the geometrical structures,light-absorbing capacities,frontier orbitals,exciton binding energies,intramolecular charge transfer(ICT)properties,and exciton dissociation rates at the interface are analyzed in detail.The calculated results indicate that the terminal modulation is an effective strategy to enhance light-absorbing capacities,ICT properties,and exciton dissociation at the heterojunction interface.The predicted PCE of designed molecules by Scharber diagram could reach up to more than 8%,which shows potential candidates with high PCE performance.Therefore,the terminal modulation could be an effective strategy for improving the performance of SMs,which sheds light on the exploration of high-performance small-molecule donors for photovoltaic applications.Part 2: A class of A-D-A type oligothiophene-based small-molecule donor materials used in heterojunction solar cell has been investigated to reveal the odd-even behavior in their photovoltaic performance.The power conversion efficiency(PCE)was affected by many factors.To gain a better understanding of the effects of odd-and evennumbered thiophene units on the PCE,the geometrical structures,light-absorbing capacities,frontier orbitals,intramolecular charge transfer(ICT)properties,exciton dissociation rates and charge mobilities are analyzed in detail by means of quantum chemical calculations.The calculated results confirm that the open circuit voltage and light-absorbing abilities of these materials do not show the odd-even feature.The intramolecular charge transfer properties,the change of dipole moments between ground state and excited state,exciton dissociation rates at the heterojunction interface and mobilities of donor materials exhibit the obvious odd-even character,thus coinciding with the observed odd-even behavior of short-circuit current in the experiment.The reason for such odd-even effects can be attributed to the molecular symmetry and intermolecular stacking.