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

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 λ_abs = 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.[Figure not available: see fulltext.]