Fullerene-Based Photoactive A-D-A Triads for Single-Component Organic Solar Cells: Incorporation of Non-Fused Planar Conjugated Core

Abstract

Two acceptor-donor-acceptor (A-D-A) single-component (SC) photovoltaic triad molecules, P3T4Rh-C6-PC61BM and P3T4Rh-C10-PC61BM, were synthesized. A conformation-locked planar conjugated core, 1,4-bis(thiophenylphenylthiophene)-2,5-difluorophenylene (P3T4), with intrachain noncovalent coulombic interactions was coupled with two fullerene derivatives, [6,6]-phenyl-C61 butyric acid propargyl ester, via copper (I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition. The D-A separation was varied by modulating the spacer alkyl chain length (C6 and C10). Both SC triads exhibited maximum absorption by the P3T4 core at λabs = 507-510 nm, as well as absorption by PC61BM at ∼300 nm. Because of the broken conjugation between the P3T4 core and PC61BM termini, the highest occupied molecular orbital (−5.58 to −5.59 eV) was determined by the P3T4 moiety, and the lowest unoccupied molecular orbital (−3.89 to −3.92 eV) was determined by PC61BM in the SC structures. In diluted solution, both SC triads showed significant photoluminescence quenching, indicating efficient intramolecular charge transfer between the P3T4 and PC61BM moieties. However, the semicrystalline packing of the P3T4 core was severely disrupted by the incorporation of a bulky PC61BM moiety at each terminus, which degraded the carrier transport and diode characteristics of SC organic solar cells (SCOSCs) based on P3T4Rh-C6-PC61BM and P3T4Rh-C10-PC61BM, as indicated by poor power conversion efficiency (∼0.4%). No clear spacer length effect was observed. To improve the performance of SCOSCs, a design strategy is needed that enhances the intermolecular packing and ordering of the D and A moieties, which are important prerequisites for the development of optimal SC photoactive molecules.