Abstract
We report ‘broadband light-sensing’ all-polymer phototransistors with the nanostructured bulk heterojunction (BHJ) layers of visible (VIS) light-sensing electron-donating (p-type) polymer and near infrared (NIR) light-sensing electron-accepting (n-type) polymer. Poly[{2,5-bis-(2-ethylhexyl)-3,6-bis-(thien-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-diyl}-co-{2,2′-(2,1,3-benzothiadiazole)]-5,5′-diyl}] (PEHTPPD-BT), which is synthesized via Suzuki coupling and employed as the n-type polymer, shows strong optical absorption in the NIR region (up to 1100 nm) in the presence of weak absorption in the VIS range (400 ~ 600 nm). To strengthen the VIS absorption, poly(3-hexylthiophene) (P3HT) is introduced as the p-type polymer. All-polymer phototransistors with the BHJ (P3HT:PEHTPPD-BT) layers, featuring a peculiar nano-domain morphology, exhibit typical p-type transistor characteristics and efficiently detect broadband (VIS ~ NIR) lights. The maximum corrected responsivity (without contribution of dark current) reaches up to 85 ~ 88% (VIS) and 26 ~ 40% (NIR) of theoretical responsivity. The charge separation process between P3HT and PEHTPPD-BT components in the highest occupied molecular orbital is proposed as a major working mechanism for the effective NIR sensing.
Highlights
We report ‘broadband light-sensing’ all-polymer phototransistors with the nanostructured bulk heterojunction (BHJ) layers of visible (VIS) light-sensing electron-donating (p-type) polymer and near infrared (NIR) light-sensing electron-accepting (n-type) polymer
organic photodiodes (OPDIs) have a simple diode structure in which organic photo-sensing layers are sandwiched between two electrodes, while organic phototransistors (OPTRs) have a triode structure in which additional gate (G) electrode plays a critical role in amplifying and/or addressing photocurrent signals
Various studies have been reported for OPTRs that are fabricated with a single type of organic photo-sensing layers [i.e., individual p-type or n-type organic semiconductors]20–25
Summary
The maximum RC value was 320 ~ 450 mA/W (VIS) and 170 ~ 250 mA/W (NIR) at VG = − 80 V and VD = − 80 V, which correspond to 85 ~ 88% and 26 ~ 40% of a theoretical responsivity, respectively (see Figures S10 and S11 for the calculation of responsivity depending on the device geometry—we note that calculation of responsivity should be very careful because abnormal values overwhelming theoretical limits for typical photodetectors could be obtained if the direction (cross-sectional area) of drain current and incident light is not properly calibrated - this is why extremely high responsivity values have been reported in the couple of previous reports) Such higher RC values under illumination with VIS lights can be attributed to two reasons: (1) The VIS lights could be absorbed by both P3HT and PEHTPPD-BT components whereas the NIR lights were absorbed only by the PEHTPPD-BT component (see Fig. 1b); (2) The n-type PEHTPPD-BT component in the BHJ structure might help efficient charge separation from the excitons generated in the P3HT component so that the resulting photo-generated holes could contribute to the improved charge transport in addition to the holes generated by the electric field effect.
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