Symmetry-Induced Ordered Assembly of a Naphthobisthiadiazole-Based Nonfused-Ring Electron Acceptor Enables Efficient Organic Solar Cells

Abstract

Nonfused-ring electron acceptors (NFREAs) have received increasing attention for use in organic solar cells (OSCs) because of their synthetic simplicity and tunable optical spectra. However, their fundamental molecular interactions and the mechanism by which they govern the property–function relations of OSCs remain elusive. Here, to investigate the effects of the structural symmetry of NFREAs, two acceptor–donor–acceptor′–donor–acceptor (A–D–A′–D–A)-type NFREAs, 2,2′-(((naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole-5,10-diylbis(4,4-bis(2-butyloctyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-6,2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (NTz-4F) and 2,2′-(((benzo[c][1,2,5]thiadiazole-4,7-diylbis(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-6,2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (BT-4F), are designed and synthesized. They have different A′ cores: NTz-4F has a modified centrosymmetric NTz core, whereas BT-4F has a modified axisymmetric BT core. In pristine films, the NTz-4F, which has a centrosymmetric core, shows substantially enhanced intermolecular interaction and microstructural crystalline ordering compared with BT-4F, which has an axisymmetric core. Even in blends with poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8,-dione))] (PBDB-T), NTz-4F retains its highly crystalline structure, whereas BT-4F loses crystalline packing. These changes in NTz-4F result in increased electron transport and suppressed nonradiative voltage loss, resulting in a power conversion efficiency of 9.14% for PBDB-T:NTz-4F vs 7.18% for PBDB-T:BT-4F. This work demonstrates that centrosymmetric-structured cores are promising building blocks for high-performance NFREA-based OSCs.

Tuning the LUMO levels of non-fullerene acceptorsviaextension of π-conjugated cores for organic solar cells

“…10 In addition, non-fused-ring electron acceptors (NFREAs) with A-D-A 0 -D-A-type structures have been investigated because of their facile synthesis. [17][18][19][20] Chen et al reported DF-PCIC, the first NFREA-based OSC with a PCE greater than 10%, in 2018. 21 DF-PCIC exhibits a nearly planar structure as a result of HÁ Á ÁF noncovalent interaction locks between its 2,5difluorobenzene core and cyclopentadithiophene (CPDT) D units; thus, its conjugation degree is high.…”

High Luminescence Stability Enables Reduced Burn‐In Voltage Loss in Organic Solar Cells

“…Inspired by the of the Y6:PCBM‐based devices, we analyzed the using Fourier‐transform photocurrent spectroscopy (FTPS) and EL measurements ( Figure a). The of a device can be divided into three terms [ 28–30 ] …”

“…These results indicate that PCBM forms smaller aggregates than Y6 both in blends and in pristine films. [ 28 ] Thus, the smooth surface morphologies of the Y6:PCBM‐based blends with >20% PCBM might result from a concurrent increase in the number of PCBM aggregates and a decrease in the number of Y6 aggregates. The combination of GIWAXS and AFM results confirms that PCBM is mainly dispersed in the mixed phase when the PCBM content is <20% but forms aggregates when the PCBM content is >20%.…”

“…Inspired by the E loss of the Y6:PCBM-based devices, we analyzed the E loss using Fourier-transform photocurrent spectroscopy (FTPS) and EL measurements (Figure 5a). The E loss of a device can be divided into three terms [28][29][30]…”

Design and synthesis of non-fused non-fullerene acceptors containing naphthobisthiadiazole for organic solar cells

“…We note that the overall photovoltaic performances of the cells derived from these two acceptors are moderate, and they might need to be integrated with more appropriate electron-donating polymers to enhance the photovoltaic parameters. 44,45 The detailed photovoltaic parameters are summarized in Table 2. The slightly higher J SC of the device based on NTIC-4Cl can be further confirmed by the current density values integrated from the external quantum efficiency (EQE) spectra (Fig.…”