Organic solar cells (OSCs) have experienced rapid progress with the innovation of near‐infrared (NIR)‐absorbing small‐molecular acceptors (SMAs), while the unique electronic properties of the SMAs raise new challenges in relation to cathode engineering for effective electron collection. To address this issue, two fluorinated perylene‐diimides (PDIs), PDINN‐F and PDINN‐2F, are synthesized by a simple fluorination method, for application as cathode interlayer (CIL) materials. The two bay‐fluorinated PDI‐based CILs possess a lower lowest unoccupied molecular orbital (LUMO) energy level of ≈−4.0 eV, which improves the energy level alignment at the NIR‐SMAs (such as BTP‐eC9)/CIL for a favorable electron extraction efficiency. The monofluorinated PDINN‐F shows higher electron mobility and better improved interfacial compatibility. The PDINN‐F‐based OSCs with PM6:BTP‐eC9 as active layer exhibit an enhanced fill factor and larger short‐circuit current density, leading to a high power conversion efficiency (PCE) exceeding 18%. The devices with PDINN‐F CIL retain more than 80% of their initial PCE after operating at the maximum power point under continuous illumination for 750 h. This work prescribes a facile, cost‐effective, and scalable method for the preparation of stable, high‐performance fluorinated CILs, and instilling promise for the NIR‐SMAs‐based OSCs moving forward.
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.
Three n-OS acceptors with E g values of <1.4 eV were synthesized by introducing double-bond π-bridges into ITIC (ITVIC) and ITIC with monofluorine (ITVfIC) or bifluorine (ITVffIC) substituents on its end groups, and the structure−property relationships of the acceptors were systematically studied. The three n-OS films show broad absorption covering the wavelength range of 550−900 nm with narrow E g values of 1.40 eV for ITVIC, 1.37 eV for ITVfIC, and 1.35 eV for ITVffIC. Additionally, the fluorine substitution downshifted the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the compounds. The photovoltaic properties of the n-OS acceptors were investigated by using a medium bandgap conjugated polymer J71 as a donor. The optimized polymer solar cells (PSCs) based on J71:ITVffIC demonstrated a power conversion efficiency (PCE) of 10.54% with a high J sc of 20.60 mA cm −2 and a V oc of 0.81 V, and the highest J sc reached 22.83 mA cm −2 . The high J sc values of the devices could be attributed to the broad absorption and lower-lying HOMO energy levels of the acceptor. Considering the V oc of 0.81 V and the narrow bandgap of 1.35 eV for the acceptor ITVffIC, we found the energy loss (E loss ) of the ITVffIC-based PSCs was reduced to 0.54 eV, which is the lowest value in the PSCs with a PCE of >10%. The results indicate that ITVffIC is a promising narrow E g acceptor for application in tandem and semitransparent PSCs.
The acceptor-donor-acceptor (A–D–A) or A–DA’D–A structured small molecule acceptors (SMAs) have triggered substantial progress for polymer solar cells (PSCs). However, the high−cost of the SMAs impedes the commercial viability of such renewable energy, as their synthesis via the classical pyridine-catalyzed Knoevenagel condensation usually suffers from low reaction efficiency and tedious purifying work-up. Herein, we developed a simple and cheap boron trifluoride etherate-catalyzed Knoevenagel condensation for addressing this challenge, and found that the coupling of the aldehyde-terminated D unit and the A-end groups could be quantitatively finished in the presence of acetic anhydride within 15 minutes at room temperature. Compared with the conventional method, the high reaction efficiency of our method is related to the germinal diacetate pathway that is thermodynamically favorable to give the final products. For those high performing SMAs (such as ITIC-4F and Y6), the cost could be reduced by 50% compared with conventional preparation. In addition to the application in PSCs, our synthetic approach provides a facile and low-cost access to a wide range of D–A organic semiconductors for emerging technologies.
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