Systematic Merging of Nonfullerene Acceptor π-Extension and Tetrafluorination Strategies Affords Polymer Solar Cells with >16% Efficiency

Abstract

The end-capping group (EG) is the essential electron-withdrawing component of nonfullerene acceptors (NFAs) in bulk heterojunction (BHJ) organic solar cells (OSCs). To systematically probe the impact of two frequent EG functionalization strategies, π-extension and halogenation, in A-DAD-A type NFAs, we synthesized and characterized four such NFAs: BT-BIC, LIC, L4F, and BO-L4F. To assess the relative importance of these strategies, we contrast these NFAs with the baseline acceptors, Y5 and Y6. Up to 16.6% power conversion efficiency (PCE) in binary inverted OSCs with BT-BO-L4F combining π-extension and halogenation was achieved. When these two factors are combined, the effect on optical absorption is cumulative. Single-crystal π–π stacking distances are similar for the EG strategies of π-extension. Increasing the alkyl substituent length from BT-L4F to BT-BO-L4F significantly alters the packing motif and eliminates the EG core interactions of BT-L4F. Electronic structure computations reveal some of the largest NFA π–π electronic couplings observed to date, 103.8 meV in BT-L4F and 47.5 meV in BT-BO-L4F. Computed electronic reorganization energies, 132 and 133 meV for BT-L4F and BT-BO-L4F, respectively, are also lower than Y6 (150 meV). BHJ blends show preferential π-face-on orientation, and both fluorination and π-extension increase NFA crystallinity. Femto/nanosecond transient absorption spectroscopy (fs/nsTA) and integrated photocurrent device analysis (IPDA) indicate that π-extension modifies the phase separation to enhance film ordering and carrier mobility, while fluorination suppresses unimolecular recombination. This systematic study highlights the synergistic effects of NFA π-extension and fluorination in affording efficient OSCs and provides insights into designing next-generation materials.

Unveiling structure-performance relationships from multi-scales in non-fullerene organic photovoltaics

“…As for the (010) π–π stacking peak, it was located at q = 1.73 Å −1 ( d = 3.63 Å) for both PM6:BTP-S7 and PM6:BTP-S8 blends, but at q = 1.74 Å −1 ( d = 3.61 Å) for PM6:BO-4Cl blend and q = 1.75 Å −1 ( d = 3.59 Å) for PM6:BTP-S9 blend. Above distinct features of molecular packing from the acceptors in the blends might also reveal the molecular packing information in the single crystals of relevant NFAs, as demonstrated in previous works 39 , 44 . Comparing with neat acceptor films, only PM6:BTP-S9 blend could maintain the position of π–π stacking peak as that of neat acceptor, after introducing polymer donor PM6, while the other three blends would all be affected.…”

Exploiting Novel Unfused‐Ring Acceptor for Efficient Organic Solar Cells with Record Open‐Circuit Voltage and Fill Factor

“…The maximum absorption coefficient of neat film is 0.87×10 5 for BTCD‐IC and 1.30×10 5 cm −1 for BTCD‐2FIC. The red‐shifted absorption band and higher absorption coefficient of BTCD‐2FIC relative to BTCD‐IC is attributed to the end‐group fluorination effect, [35,36] which is beneficial to obtain higher J sc in the device. According to the equation: E g opt =1240/ λ onset , the optical bandgaps ( E g opt ) of BTCD‐IC and BTCD‐2FIC are calculated to be 1.46 and 1.45 eV, respectively.…”

Near‐Infrared Absorbing Nonfullerene Acceptors for Organic Solar Cells

“…[ 58 ] By asymmetrically replacing the end group on one side of Y6 with a thiophene‐fused end group, Luo et al reported an asymmetric Y6 analog called BTP‐2F‐ThCl (Figure 5), which balances the V OC and J SC and fine‐tunes the morphology of blends to realize an over 17% PCE (Table 2). [ 59 ] Such successful cases by optimizing end groups to improve photovoltaic performance of Y6 derivatives can be seen in many reports, such as BTP‐ClBr, [ 60 ] BT‐BO‐L4F, [ 61 ] and BTIC‐2Cl‐γCF 3 . [ 62 ]…”