Fast Near-Infrared Organic Photodetectors with Enhanced Detectivity by Molecular Engineering of Acceptor Materials

Organic photodetectors (OPDs) with a near-infrared (NIR) response beyond 900 nm are intriguing electronics for various applications. It is challenging to develop NIR OPDs with high sensitivity and fast response. Herein, the acceptor materials of OPDs are tuned to extend detection to ≈1100 nm with improved sensitivity. A new fused ring electron acceptor, ICS (2,2'-((2Z,2'Z)-(((4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']dithiophene-2,6-diyl)bis(4-((2-ethylhexyl)thio)thiophene-5,2-diyl))bis(methaneylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile), is developed with alkylthio thiophene as the bridge, achieving a small bandgap of 1.35 eV while decreasing dark current densities under reverse bias. By further introducing a secondary acceptor of PC₆₁ BM, the doping compensation, and unfavored hole injection blocking enable further improvement of detectivity. The PTB7-Th (Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl]): ICS: PC₆₁ BM OPDs deliver a low dark current density of 1.23 × 10^-9 A cm^-2, a high peak specific detectivity of 1.09 × 10¹³ Jones at 950 nm under -0.2 V, and a fast response speed with a -3 dB bandwidth of 720 kHz biased under -2 V. The photoplethysmography system with the PTB7-Th: ICS: PC₆₁ BM OPD can reliably monitor heartbeats under 980 nm NIR light. This study promises the development of organic NIR OPDs with high detectivity and fast response by tuning active materials.