γ-Ester-Functionalized 1,1-Dicyanomethylene-3-indanone End-Capped Nonfullerene Acceptors for High-Performance, Annealing-Free Organic Solar Cells

Abstract

Modifying the end-capping groups in nonfullerene acceptors (NFAs) is an effective strategy for modulating their properties and that of the entire NFAs. This study reports the synthesis of a novel γ-ester-functionalized IC end-capping group (IC-γe) and its incorporation into the benzothiadiazole-fused central core, yielding isomer-free IC-γe end-capped NFAs, such as Y-IC-γe, Y-FIC-γe, and Y-ClIC-γe. The resultant NFAs exhibited similar absorption profiles but upshifted the lowest unoccupied molecular orbital energy level compared with those of the ester-free analogues, such as Y6 and Y7. Without thermal annealing, an excellent power conversion efficiency (PCE) of 16.4% is realized in the annealing-free OSC based on Y-FIC-γe with the PM6 donor polymer, which outperforms the OSCs based on Y-IC-γe and Y-ClIC-γe. In addition, the OSCs based on asymmetric Y-FIC-γe and Y-ClIC-γe have higher thermal stability with more than 83% PCE retention at an elevated temperature after 456 h than the symmetric Y-IC-γe case. In this study, we not only establish the structure–property relationship regarding the ester functionality and symmetricity tuning on the NFAs but also diagnose the reasons for the best-performing Y-FIC-γe-based OSCs, providing useful information for a novel high-performing NFA design strategy.

Conformational Locking Control of 2D Outer Side Chains via Fluorine Atom Positioning for Improving the Thermal Stability of Organic Solar Cells

“…56,57 Accordingly, we expected that different T g values in the FY NFAs could affect the corresponding devices' thermal stability and their active layers' morphological properties under applied continuous thermal stress. To precisely investigate the effect of FY NFA T g on the thermal stability of OSC devices, the unencapsulated devices, less the electron transport layer and top electrode, were fabricated and stored under an N 2 atmosphere with a continuous thermal stress of 85 °C for 0−360 h. 11 After thermal treatment for the specified amount of time, the electron transport layer (PDINO) and top electrode were deposited on each active layer, and the corresponding OSC devices' photovoltaic properties were measured (Figure 7). The 4FY-based OSC exhibited 88.4% PCE retention after 360 h, whereas the 2FY-based OSC retained only 79.0% PCE after 360 h. This indicated that 4FY exhibited better thermal stability in OSC devices than 2FY.…”

“…9 Since this pioneering work, considerable progress has been made in the design and synthesis of molecular structures toward superior Y-series NFAs. 4,10,11 Side-chain engineering is one of the most efficient strategies for fine-tuning NFAs. 12−141516 Y-series NFAs possess two sets of alkyl side chains, with the inner pair located on the pyrrole rings and the outer pair flanking at the 2-positions of the thienothiophene moieties.…”

“…The first representative Y-series NFA, Y6, delivered a PCE of 15.7% when paired with a wide-band-gap donor polymer . Since this pioneering work, considerable progress has been made in the design and synthesis of molecular structures toward superior Y-series NFAs. ,, Side-chain engineering is one of the most efficient strategies for fine-tuning NFAs. − Y-series NFAs possess two sets of alkyl side chains, with the inner pair located on the pyrrole rings and the outer pair flanking at the 2-positions of the thienothiophene moieties . The inner and/or outer alkyl chain alterations, such as the chain length and branching point modifications, affect the blend morphologies and phase separation/mixing properties which are directly correlated to charge separation/transfer efficiencies, in addition to the material solubility. , Moreover, replacing the alkyl chain with a two-dimensional (2D) alkylaryl group on the outer position affects the molecular backbone geometry and improves the photovoltaic parameters of Y-series NFAs because of steric hindrance and conformational locking between the core group (DA′D) and the end groups (A). ,− Optimization of the molecular structure of Y-series NFAs resulted in PCEs surpassing 18% for OSCs composed of wide-band-gap donor polymers and Y-series NFAs. ,,− …”

Building-up an interrelationship between isomeric benzyl inner side chains within nonfullerene acceptors and isomeric xylene solvents for non-chlorinated solvent-processed organic solar cells

“…Several attempts have been made for p-framework variation, including changing the heteroatom of the core DA ′ D structure, modulating the p-extension of fused rings, and attaching different end-capping units. [14][15][16] This modulation simultaneously inuences the intrinsic molecular electronic properties (such as the absorption spectrum and energy level) and intermolecular interactions. Another efficient approach for advances in Y-series NFAs is alkyl side-chain alternation, which primarily affects intermolecular interactions.…”

Combining dithieno[3,2-f:2′,3′-h]quinoxaline-based terpolymer and ternary strategies enabling high-efficiency organic solar cells

“…In the case of the NFAs, A-DA 0 D-A-type ‡ molecular frameworks are one of the most popular design strategies following the development of the Y6 acceptor. [4][5][6] For polymer donors, D-A-type copolymer designs using weak electron donor and strong/medium electron acceptor units have been adopted for high-performing wide-bandgap polymers, resulting in the appearance of representative widebandgap donors PM6 and D18. [7][8][9] For the design and combination of wide-bandgap polymer donors and NFAs, the following attributes should be considered: (i) absorption complementarity, (ii) energy level matching, (iii) suitable phase separation, and (iv) molecular orientation/crystallinity tendency between the widebandgap polymer donor and NFA.…”