Understanding the Critical Role of Sequential Fluorination of Phenylene Units on the Properties of Dicarboxylate Bithiophene‐Based Wide‐Bandgap Polymer Donors for Non‐Fullerene Organic Solar Cells

Abstract

Design and development of wide bandgap (WBG) polymer donors with low‐lying highest occupied molecular orbitals (HOMOs) are increasingly gaining attention in non‐fullerene organic photovoltaics since such donors can synergistically enhance power conversion efficiency (PCE) by simultaneously minimizing photon energy loss (Eloss) and enhancing the spectral response. In this contribution, three new WBG polymer donors, P1, P2, and P3, are prepared by adding phenylene cores with a different number of fluorine (F) substituents (n = 0, 2, and 4, respectively) to dicarboxylate bithiophene‐based acceptor units. As predicted, fluorination effectively aides in the lowering of HOMO energy levels, tailoring of the coplanarity and molecular ordering in the polymers. Thus, fluorinated P2 and P3 polymers show higher coplanarity and more intense interchain aggregation than P1, leading to higher charge carrier mobilities and superior phase‐separated morphology in the optimized blend films with IT‐4F. As a result, both P2:IT‐4F and P3:IT‐4F realize the best PCEs of 6.89% and 7.03% (vs 0.16% for P1:IT‐4F) with lower Eloss values of 0.65 and 0.55 eV, respectively. These results signify the importance of using phenylene units with sequential fluorination in polymer backbone for modifying the optoelectronic properties and realizing low Eloss values by synergistically lowering the HOMO energy levels.

Tailoring Microstructure and Morphology via Sequential Fluorination to Enhance the Photovoltaic Performance of Low‐Cost Polymer Donors for Organic Solar Cells

“…These results were ascribed from the F‐induced noncovalent S⋯F and S⋯H conformational locks, which significantly improve the coplanarity of the polymer backbone, as reported in previous reports. [ 26,42,46,51 ] Thus, PBDT‐4FBn has enhanced coplanarity than PBDT‐2FBn as depicted in the side‐view, which is expected to have marked effects on molecular packing and bulk morphologies. Meanwhile, frontier molecular orbitals energy levels profile of both polymers displayed similar electron density distribution along the backbone, where HOMO and LUMO are delocalized over the entire conjugated backbone.…”

“…Various recent works demonstrate the effectiveness of this strategy for the development of polymer donors. [42,46,[49][50][51][52] Thus, considering the above-discussed facts, we attempted to optimize the rational molecular design of PBDT-2FBnT by synergistically introducing F-BDT and 2FBn or 1,4-dibromo-2,3,5,6 tetrafluorobenzene (4FBn) units for the construction of efficient, low-cost non-fullerene OSCs, as shown in Figure 1a.…”

Structure–property relationship on insertion of fluorine- versus nitrogen substituents in wide bandgap polymer donors for non-fullerene solar cells: an interesting case study

“…Meanwhile, in the film state, although both polymers exhibit similar absorption trends as in the solutions, they showed much broader and bathochromically shifted (B50 nm) spectra, which is indicative of the strong intermolecular p-p stacking. [52][53][54] The optical energy gaps of P1-2F and P2-2N were estimated to be 2.17 eV and 2.07 eV, respectively, calculated from the corresponding absorption onsets (l onset ) located at 575 and 603 nm. Further, we measured the absorptivity coefficients of these new polymers in the film state.…”

A comparison of the positional effect of difluorination and the synergistic effect of siloxane-terminated side chains on benzodithiophene-based conjugated polymers for efficient photovoltaic application

“…Finally, all the monomers were obtained by lithiation and the subsequent reaction with chlorotrimethyltin. The chemical structures of the various difluorobenzene substituted intermediates were fully characterized and confirmed by 1 H NMR and 13 C NMR (Fig. S2-S13, ESI †).…”

“…For conjugated polymers composed of alternating donor and acceptor units, the tunable chemical structure of the building block can manipulate the optical absorption bands, molecular energy levels, as well as the three-dimensional molecular conformations. Many strategies have been developed to fine-tune the molecular structure of the polymer donors for high efficiency PSCs, such as terpolymerization of D-A polymers with different donor or acceptor building blocks as the third component, 4 the introduction of heteroatoms in the polymer backbone, [5][6][7][8][9][10][11][12][13][14][15] variation of the category or the length of the alkyl side chain [16][17][18] and introduction of electron-withdrawing substituents on 2-dimensional conjugated side chains. 19,20 In 2016, the 3,5-difluorobenzene group was introduced as the 2-dimensional conjugated side chain on the benzodithiophene (BDT) unit to compose a polymer donor (PTFBDT-BZS) by Bo et al for the first time.…”