Optimizing the π-Bridge of Non-fullerene Acceptors to Suppress Dark Current in NIR Organic Photodetectors

Recently, the rapid development of non-fullerene acceptors (NFAs) has laid the foundation for performance improvements in near-infrared (NIR) organic photodetectors (OPDs). However, reducing the bandgap of NFAs to achieve strong absorption in the shorter-wave region usually leads to increased dark current density (J(d)) and decreased responsivity (R), severely limiting the detectivity (D*) of NIR-OPDs. To date, it remains challenging to manipulate the J(d) of NIR-OPDs through rational structure engineering of NFAs. Herein, three NIR-NFAs, namely bis(2-decyltetradecyl)4,4 '-(2 ',7 '-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b ']dithiophene-4,9 '-fluorene]-2,6-diyl)bis(6-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate) (TSIC-4F), bis(2-decyltetradecyl)6,6 '-(2 ',7 '-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b ']dithiophene-4,9 '-fluorene]-2,6-diyl)bis(4-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate) (STIC-4F), and 2,2 '-((2Z,2 ' Z)-(((2 ',7 '-di-tert-butylspiro[cyclopenta [2,1-b:3,4-b ']dithiophene-4,9 '-fluorene]-2,6-diyl)bis(2,3-bis(5-(2-butyloctyl)thiophen-2-yl)thieno[3,4-b]pyrazine-7,5-diyl))bis(metha-neylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (TPIC-4F), were designed using the thieno[3,4-b]thiophene (TT) and thieno[3,4-b]pyrazine (TPy) derivatives as the pi-bridge. Owing to the intramolecular S-S and S-N interactions, STIC-4F and TPIC-4F exhibited smaller backbone distortions than TSIC-4F. A significantly red-shifted absorption with a peak at 1015 nm was observed in TPIC-4F film, larger than that (ca. 960 nm) for TSIC-4F and STIC-4F films. Moreover, OPDs operating in a photovoltaic mode were successfully fabricated, and TPIC-4F-based OPDs achieved the lowest J(d) of 3.18x10(-8) A/cm(2) at -0.1 V. Impressively, although TPIC-4F-based OPDs exhibited the lowest R, higher shot-noise-limited specific detectivity (D-sh*) in 1000-1200 nm could be achieved due to its lowest J(d). This study underscored the effectiveness of optimizing the pi-bridge structure of NFAs to suppress J(d), ultimately attaining higher D-sh* in the NIR region.