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A low-cost and 2D semi-paracrystalline conjugated polymer PTQ10 yields higher efficiency and stretchability in dual donor ternary solar cells.

Active layer morphology is a key factor that dictates the device performance and stability of polymer solar cells (PSCs). Herein, we investigate the morphology evolution of the active layer and the impacts on device performance and stability upon varying the molecular weights of polymer donor (PBDB-T4Cl5S) from 17 to 98 kg mol−1. When blended with a leading non-fullerene acceptor (Y6), both the short-circuit current density and fill factor of the PSCs rise gradually with increasing the polymer molecular weights. Detailed morphology analysis showed that the reduced phase separation scale and improved phase purity accounted for these enhancements. Interestingly, the best device stability was achieved by the polymer with a medium molecular weight of 30 kg mol−1, which is most likely caused by the nearly-optimal miscibility between the polymer donor and Y6. These results demonstrate that the optimization of molecular weights of polymer donors is crucial to achieve high-efficiency and high-stability simultaneously for non-fullerene PSCs.

Due to the simple structure and excellent scalability, regioregular poly(3-hexylthiophene) (P3HT) has become much more popular as a low-cost donor polymer partner with the emerging nonfullerene acceptors in recent years. Presently, the application and the molar mass dependence of direct arylation polycondensation (DArP) prepared P3HT in fullerene-free organic solar cells have not been explored yet. In this work, five batches of P3HT with weight-average molar mass ranging from 8.9 kg mol−1 to 89.1 kg mol−1 were successfully prepared via DArP, with the aim to detail the interaction–performance relationships in the state-of-the-art P3HT:nonfullerene photovoltaic blend, namely P3HT:ZY-4Cl. Our thermodynamic analysis revealed that the amorphous–amorphous interaction parameters strongly depend on the P3HT molar mass and can be varied by a factor of 2.75. Owing to the best π–π stacking order deduced from grazing incidence X-ray diffraction and the smallest domain size, the blend based on the medium molar mass batch (25.5 kg mol−1) delivered the highest current density, fill factor, and power conversion efficiency up to ∼10% among the five systems. Our study confirmed that DArP is an effective protocol for the synthesis of P3HT for efficient non-fullerene polymer solar cells and the established relationships can be applied to screen the best batch for P3HT-based organic photovoltaics.

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) based on non-fullerene acceptors are still limited by large energy loss (ΔEloss). Herein, we report an easy and feasible way to reduce the ΔEloss, specifically the energy losses caused by non-radiative recombination of OSCs via introducing 3,4-dicyanothiophene (DCT), a structurally simple building block with strong electron-withdrawing ability, into a state-of-the-art donor polymer PBDB-TF to form ternary copolymers. The effects of DCT content in the ternary copolymers on the optical properties, energy levels, film morphology, charge transport, photovoltaic performance, and energy losses are studied comprehensively. Significant efficiency improvement in OSCs has been achieved by the DCT-based ternary copolymers as compared to the benchmark polymer PBDB-TF mainly because of a higher open-circuit voltage enabled by the reduced non-radiative recombination energy loss. A high PCE of 16.6% and a low non-radiative recombination energy loss (0.22 eV) were achieved in ternary OSCs with the DCT-based terpolymer as the electron donor and Y6-BO and PC71BM as the electron acceptors.