No abstract

In this contribution, we used a practical method to synthesize 3-fluorothiophene as π-bridge units on the polymer backbone via constructing 3-fluoro-2-iodothiophene to decrease synthesis steps and lower costs. Fluorine atoms introduced into the polymer backbone can improve polymer planarity, packing, crystallinity, and hole mobility via extensive noncovalent interactions such as F−H, F−Cl, F−S, and F−π. When compared to the analogue PBZ-Cl without any fluorine substituent on the thiophene unit, our fluorinated polymer J52ClF exhibited red-shifted absorption of roughly 42 nm with a narrower band gap (E g opt ) of 1.82 eV, a low-lying highest occupied molecular orbital (HOMO) energy level, and a highly coplanar molecular configuration in the backbone. The optimal device based on J52ClF:IT-4F achieved a desired power conversion efficiency (PCE) of 14.59%, a V OC of 0.93 V, a J SC of 22.67 mA cm −2 , a fill factor (FF) of 69.22%, and an E loss of 0.57 V, all of which were significantly superior to those of PBZ-Cl:IT-4F (PCE = 9.7%). It demonstrated that fluorinating π-conjugated bridges utilizing 3-fluoro-2-iodothiophene is a practical strategy that deserves greater attention for increasing photovoltaic performance.

Solution-processed polymer solar cells (PSCs), as a potential next-generation clean-energy conversion technology that harnesses inexhaustible solar light, have drawn substantial attention in both academia and industry due to the merits of low cost, lightweight, semitransparency, flexibility, and roll-to-roll largearea preparation. [1][2][3][4] The active layers of bulk heterojunction (BHJ)-PSCs, which are made of a p-type conjugated donor and an n-type organic semiconductor (n-OS) acceptor, have become one of the most effective device architectures in the organic photovoltaic (OPV) field. [5][6][7][8] With advancements in photovoltaic/interfacial materials and device manufacturing techniques, the power conversion efficiencies (PCEs) of nonfullerene PSCs have continuously grown and have recently surpassed 18% in binary devices, indicating considerable promise for practical applications. [9][10][11][12][13][14][15][16][17][18][19][20][21] However, compared with other photovoltaic technologies like c-Si and perovskite cells, these results are still less impressive. [22][23][24] Organic semiconductors have intrinsically low dielectric properties and relatively high electron-hole pair (exciton) binding energies. This is due to the fact that incident light forms tightly bound excitons with high coulombic binding energy, requiring additional energy to dissociate them into spatially free charges. [25][26][27] The chemical structures of active layer materials are one of the most influential factors in determining the properties. It is generally agreed upon that cascading energy levels in the polymer donor and acceptor are specifically needed to work as a driving force for excitons to dissociate at the heterojunction interface. Specifically, the highest occupied molecular orbital (HOMO) level and the lowest unoccupied molecular orbital (LUMO) level of the donor should be greater than those of the acceptor, and the ΔE LUMO (difference between the LUMO levels of the donor and acceptor), as well as ΔE HOMO, should be higher than the commonly accepted 0.3 eV. [28][29][30][31] The energy-level alignments are efficiently fine tuned to apply to the polymer donors via molecular structure design to provide sufficient exciton dissociation and prevent significant energy loss (E loss ), making them commonly adopted in boosting the devices' V OC and photovoltaic performance.Almost all molecular design strategies for high-efficiency polymer donors are based on the pattern of the design of "monomer units," which include electron-rich blocks (known as D-units), electron-deficient blocks (known as A-units), and π-bridge units. [3,16,[32][33][34][35][36][37][38][39] For example, Hou et al. developed two chlorinated-conjugated polymer donors, PCl(3)BDB-T and PCl(4)BDB-T, to examine the structure-property relationship from the perspective of modifying the π-bridge unit.

No abstract