Development of Block Copolymers with Poly(3-hexylthiophene) Segments as Compatibilizers in Non-Fullerene Organic Solar Cells

Poly(3-hexylthiophene) (P3HT)-segment-based block copolymers have been reported to deliver an effective compatibilizer function in the P3HT:PC₆₁BM bulk-heterojunction (BHJ) system to simultaneously improve performance and stability. However, as limited by the deficient optophysic properties of the P3HT:PC₆₁BM system, the resultant power conversion efficiency (PCE) of compatibilizer-mediated devices is low despite the optimized chemical structures of the P3HT-segment-based block copolymers. To better shed light on such a compatibilizer effect, the compatibilizer function of the P3HT-segment-based block copolymers is herein investigated in the emerging non-fullerene acceptor (NFA)-based BHJ systems. A P3HT analogue, poly[(4,4'-bis(2-butyloctoxycarbonyl-[2,2'-bithiophene]-5,5-diyl)-alt-(2,2'-bithiophene-5,5'-diyl))] (PDCBT), is used as the polymer donor since it shares the same backbone as P3HT to afford good compatibility with the P3HT-segment-based block copolymers and it has been proven to deliver a higher PCE than P3HT in the NFA BHJ systems. The P3HT-segment-based block copolymers (P1-P4) are manifested to offer similar compatibilizer functions for the PDCBT-based NFA BHJ systems, and the importance of their structural design is also revealed. As a result, addition of P4 delivers the largest enhancement in PCE: from 5.30 to 7.11% for the PDCBT:ITIC blend and from 6.21 to 8.04% for the PDCBT:IT-M blend. Moreover, it can also enhance the device's thermal stability, which can maintain 77% of the initial PCE after annealing at 85 °C for 120 h (for the PDCBT:ITIC blend), outperforming the pristine binary device (66% preservation). More importantly, the entire compatibilizer-mediated device exhibits an improved V_oc. Such reduced potential loss can be attributed to the improved interfacial compatibility between the photoactive components, the most important function of a compatibilizer.