Abstract:

Photomultiplication-type all-polymer photodetectors (PM-APDs) based on structure of ITO/poly(3, 4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS)/active layer/Al were developed with wide bandgap polymer poly(3-hexylthiophene) (P3HT) as donor and narrow bandgap polymer poly{2, 2'-((2Z, 2'Z)-((12, 13-bis(2-decyltetradecyl)-6-(2-ethylhexyl)-4, 8-dimethyl-6, 8, 12, 13-tetrahydro-4H-thieno[2'', 3'': 4', 5']pyrrolo[2', 3': 4, 5]pyrrolo[3, 2-g]thieno[2', 3': 4, 5]pyrrolo[3, 2-b][1, 2, 3] triazolo[4, 5-e]indole-2, 10-diyl)bis(methaneylylidene))bis(5, 5'-3-oxo-2, 3-dihydro-1H-indene-2, 1-diylidene))dimalononitrile-alt-2, 5-dithiophene} (PTz-PT) as acceptor. A series of binary PM-APDs were prepared with P3HT : PTz-PT weight ratios of 100 : 1, 100 : 4, 100 : 7, and 100 : 10. In the dark, the holes are difficultly injected from Al electrode into the active layer due to the 0.8 eV injection barriers from the work function of Al onto the highest occupied molecular orbital (HOMO) level of P3HT. The limited PTz-PT content in the active layer results in the absence of continuous electron transport channel, leading to poor electron transport ability. The photogenerated electrons are trapped in isolated PTz-PT under light illumination due to the scarce PTz-PT content in active layer and 0.84 eV difference between the lowest unoccupied molecular orbital (LUMO) of P3HT and PTz-PT. The trapped electrons near the Al electrode induce interfacial band bending for hole tunneling injection, leading to external quantum efficiency (EQE) values exceeding 100%. The optimal binary PM-APDs based on P3HT : PTz-PT (100 : 4 wt/wt) exhibit a spectral response range from 300 to 1100 nm with EQE values over 100% at -8 V bias. The EQE spectral shape of PM-APDs is determined by the distribution of trapped electrons near the Al electrode. The shape of EQE spectra is further flattened by introducing polymer poly(2-(4, 8-bis(4-(2-ethylhexyl)cyclopenta-1, 3-dien-1-yl)benzo[1, 2-b: 4, 5-b']dithiophen-2-yl)-5, 5-difluoro-10-(5-(2-hexyldecyl)thiophen-2-yl)-3, 7-dimethyl-5H-4λ4, 5λ4-dipyrrolo[1, 2-c: 2', 1'-f][1, 3, 2]diazaborinine) (PMBBDT) as the third component. A series of ternary PM-APDs with P3HT : PMBBDT : PTz-PT weight ratios of 90 : 10 : 4 and 80 : 20 : 4 were prepared. The EQE values of ternary PM-APDs are increased in the range from 420 to 600 nm and decreased in the range from 630 to 870 nm. The flatter EQE spectra of ternary PM-APDs are derived from more uniform distribution of trapped electrons near the Al electrode. Furthermore, the ternary PM-APDs exhibit higher stability under continuous illumination and applied bias than the optimal binary PM-APDs. The optimal ternary PM-APDs exhibit EQE values of 3500% at 350 nm, 1250% at 550 nm and 1500% at 900 nm under −12 V bias, as well as specific detectivity (D_shot^*) values of 3.7 × 10¹² Jones at 520 nm and 1.9 × 10¹³ Jones at 850 nm under −10 V bias. After 170 min continuous white light illumination at −10 V bias, the optimal ternary PM-APDs possess photocurrent of 87% initial value. A photoplethysmography (PPG) sensor was successfully realized by employing the optimal ternary PM-APDs to measure human heart rate (HR), with the measured HR consistent with the normal value of human HR.

Key words: Photomultiplication, All-polymer photodetector, Hole tunneling injection, Trapped electrons, PPG sensor