Manipulating the donor:acceptor (D:A) energetics, e.g. the highest occupied molecular orbital (HOMO) offset, is the key to balancing the charge separation and charge recombination for high-performance organic solar cells (OSCs)....
For polymer solar cells (PSCs), the mixture of polymer donors and small‐molecule acceptors (SMAs) is fine‐tuned to realize a favorable kinetically trapped morphology and thus a commercially viable device efficiency. However, the thermodynamic relaxation of the mixed domains within the blend raises concerns related to the long‐term operational stability of the devices, especially in the record‐holding Y‐series SMAs. Here, a new class of dimeric Y6‐based SMAs tethered with differential flexible spacers is reported to regulate their aggregation and relaxation behavior. In their polymer blends with PM6, it is found that they favor an improved structural order relative to that of Y6 counterpart. Most importantly, the tethered SMAs show large glass transition temperatures to suppress the thermodynamic relaxation in mixed domains. For the high‐performing dimeric blend, an unprecedented open circuit voltage of 0.87 V is realized with a conversion efficiency of 17.85%, while those of regular Y6‐base devices only reach 0.84 V and 16.93%, respectively. Most importantly, the dimer‐based device possesses substantially reduced burn‐in efficiency loss, retaining more than 80% of the initial efficiency after operating at the maximum power point under continuous illumination for 700 h. The tethering approach provides a new direction to develop PSCs with high efficiency and excellent operating stability.
With the power conversion efficiency of binary polymer solar cells dramatically improved, the thermal stability of the small-molecule acceptors raised the main concerns on the device operating stability. Here, to address this issue, thiophene-dicarboxylate spacer tethered small-molecule acceptors are designed, and their molecular geometries are further regulated via the thiophene-core isomerism engineering, affording dimeric TDY-α with a 2, 5-substitution and TDY-β with 3, 4-substitution on the core. It shows that TDY-α processes a higher glass transition temperature, better crystallinity relative to its individual small-molecule acceptor segment and isomeric counterpart of TDY-β, and a more stable morphology with the polymer donor. As a result, the TDY-α based device delivers a higher device efficiency of 18.1%, and most important, achieves an extrapolated lifetime of about 35000 hours that retaining 80% of their initial efficiency. Our result suggests that with proper geometry design, the tethered small-molecule acceptors can achieve both high device efficiency and operating stability.
Nonradiative recombination loss is the key factor to be responsible for low open-circuit voltage (V oc ) in organic solar cells (OSCs), which can be reduced via tuning the chemical structure of conjugated materials. However, the intrinsic correlation between them was rarely studied. In this work, we were able to build a strong connection between chemical structure and nonradiative recombination loss, which was then used to lower the voltage losses in OSCs. The studies start from designing several double-cable conjugated polymers with rigid phenyl linkers, which guarantee the precise distance between donor (D) backbone and acceptor (A) side units. In addition, the number of phenyl linkers was changed from one to three, so as to provide different D/A distances. The universal studies of solar cells, morphology, and voltage losses showed that longer D/A distance provided lower nonradiative recombination losses and hence higher V oc in singlecomponent OSCs. Our results demonstrate that extending the D/A distance via rigid phenyl linkers is an efficient way to reduce the voltage losses in OSCs.
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