Theoretical Study of the Charge Transfer Exciton BindingEnergy in Semiconductor Materials for Polymer:Fullerene-Based BulkHeterojunction Solar Cells

摘要

Recent efforts and progress in polymer solar cell research have boosted the photovoltaic efficiency of the technology. This efficiency depends not only on the device architecture but also on the material properties. Thus, insight into the design of novel semiconductor materials is vital for the advancement of the field. This paper looks from a theoretical viewpoint into two of the factors for the design of semiconductor materials with applications to bulk heterojunction solar cells: the charge transfer exciton binding energy and the nanoscale arrangement of donor and acceptor molecules in blend systems. Being aware that the exciton dissociation of local excitons in charge transfer states initiates the charge generation process, the excited state properties of four oligomers (one donor-type: PEO–PPV; and three donor–acceptor-types: PTFB, PTB7, and PTB7–Th) and two fullerene derivatives ([60]PCBM and [70]PCBM), previously reported in the literature as having high electrical conductance, are studied. With such a study, the donor molecules, either of donor-type or donor–acceptor type, arescreened as candidates for [60]PCBM- and/or [70]PCBM-based bulk heterojunctions.The charge transfer energy and charge transfer exciton binding energyof suitable donor:acceptor bulk heterojunctions, some of them notyet fabricated, are studied. Further, the charge transfer excitonbinding energies of [60]PCBM- and [70]PCBM-based blends are compared.A combination of molecular dynamics simulations with calculationsbased on Kohn–Sham density functional theory (KS-DFT) and itstime-dependent extension (KS-TDDFT) is used. An important featureof this work is that it incorporates the effect of the environmentof the quantum chemical system in KS-DFT or KS-TDDFT calculationsthrough a polarizable discrete reaction field (DRF). Our predictionsin terms of the influence of the nanoscale arrangement of donor andacceptor molecules on the performance of organic solar cells indicatethat bulk heterojunction morphologies for donor–acceptor-typeoligomers lead to their lowest excited states having charge transfercharacter. Further, we find that in terms of favorable charge transferexciton binding energy, the PTB7–Th:[70]PCBM blends outperformthe other blends.