Photomultiplication-type all-polymer photodetectors and their applications in photoplethysmography sensor

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{2,2ʹ-((2Z,2ʹZ)-((12,13-bis(2-decyltetradecyl)-6-(2-ethylhexyl)-4,8-dimethyl-6,8,12,13-tetrahydro-4H-thieno[2ʹʹ,3ʹʹ:4ʹ,5ʹ]pyrrolo[2ʹ,3ʹ:4,5]pyrrolo[3,2-g]thieno[2ʹ,3ʹ:4,5]pyrrolo[3,2-b][1,2,3] triazolo[4,5-e]indole-2,10-diyl)bis(methaneylylidene))bis(5,5ʹ-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:2ʹ,1ʹ-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.