Naphthalene-diimide-based all-conjugated block copolymer as an effective compatibilizer to improve the performance and thermal stability of all-polymer solar cells

Abstract

A new donor–acceptor-type all-conjugated block copolymer (BCP), P(NDI-DBDT)-block-P(NDI-2T), is synthesized through the coupling of a naphthalene diimide (NDI)-bithiophene (2T) segment-based polymer (P1) and an NDI-dithienylbenzodithiophene (DBDT) segment-based polymer (P2). P1 has a prevailing backbone for NDI-based polymer acceptors, while P2 is modified to contain a p-type DBDT moiety to facilitate miscibility with the DBDT-based polymer donors. When adding 1 wt% BCP (the optimized blending amount) into the PBDB-T:N2200 blend that possesses similar constituent units to the BCP, the derived device can deliver an enhanced power conversion efficiency (PCE) of 7.07%, outperforming the control device (with a PCE of 5.71%). This improvement is found to arise from the more optimized bulk-heterojunction (BHJ) morphology as a result of facilitated intermixing between the photoactive components. This leads to reduced charge recombination and potential loss for the device. Moreover, the more optimized BHJ morphology endows the device with improved thermal robustness. It demonstrates longer T₈₀ lifetimes when stored under constant thermal conditions at high temperatures. Consequently, the device lifetime can be extended to ∼2 years when stored under inert conditions at 50 °C. These results demonstrate that the rationally designed all-conjugated BCP can effectively modulate the BHJ morphology of an all-polymer blend to simultaneously enhance its photovoltaic performance and stability.