Flexible Near‐Infrared Plastic Phototransistors with Conjugated Polymer Gate‐Sensing Layers

Abstract

Flexible near-infrared (NIR) light-sensing detectors are strongly required in the fast-growing flexible electronics era, because they can serve as a vision system like eyes in various innovative applications including humanoid robots. Recently, keen interest has been paid to organic phototransistors due to their unique signal amplification and active matrix driving features over organic photodiodes. However, conventional NIR-sensing organic phototransistors suffer from the limited use of organic materials because the channel layers play a dual role in both charge transport and sensing so that organic semiconducting materials with reasonably high charge mobility can be applied only. Here, it is demonstrated that a conjugated 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 exhibits no transistor performance as a channel layer, can stably detect a NIR light (up to 1000 nm) as a gate-sensing layer (GSL) when it is placed between gateinsulating layers and gate electrodes. The flexible array (10 × 10) detectors with the PEHTPPD-BT GSLs could effectively sense NIR light without visible light interference by applying visible light cut films.

Effect of Top Channel Thickness in Near Infrared Organic Phototransistors with Conjugated Polymer Gate-Sensing Layers

“…The drain current was increased under illumination with the NIR light for all cases, while the highest photocurrent was measured at t = 50 nm for all the three different wavelengths. Similar to the output curves, the photocurrent level at VD = −10 V and VG = −30 V was always lower than the drain current (in the dark) at VD = −30 V and VG = −30 V. This may reflect that the population of photogenerated charges in the PODTPPD-BT layer (GSL) at VD = −10 V is lower than that of field-effect charges in the P3HT channel layer at VD = −30 V. Particular attention is paid to the pronounced drain current increase in the off-current region by the NIR illumination, which can be attributed to the influence of photogenerated charges in the PODTPPD-BT layers [32]. As a consequence, the threshold voltage (VTH) of devices was shifted upon illumination (see green horizontal arrows in Figure 5).…”

“…This may reflect that the population of photogenerated charges in the PODTPPD-BT layer (GSL) at V D = −10 V is lower than that of field-effect charges in the P3HT channel layer at V D = −30 V. Particular attention is paid to the pronounced drain current increase in the off-current region by the NIR illumination, which can be attributed to the influence of photogenerated charges in the PODTPPD-BT layers [32]. As a consequence, the threshold voltage (V TH ) of devices was shifted upon illumination (see green horizontal arrows in Figure 5).…”

“…The thickness of the channel layer, poly(3-hexylthiophene) (P3HT), was varied from 10 to 70 nm, while the thickness of poly(methyl methacrylate) (PMMA) GIL was fixed as 550 nm. Poly[{2,5-bis-(2-octyldodecyl)-3,6-bis-(thien-2-yl)-pyrrolo [3,4-c]pyrrole-1,4-diyl}-co-{2,2 -(2,1,3benzothiadiazole)-5,5 -diyl}] (PODTPPD-BT), which has longer alkyl chains than the previously reported conjugated polymer for GSL in [32], was used as a GSL (thickness = 100 nm) in the present device structure. The photosensing characteristics of the devices were examined by employing NIR light with a wavelength (λ) of 780, 905, and 1000 nm.…”

“…The P3HT polymer (weight-average molecular weight = 51 kDa, PDI = 1.7, regioregularity >96%) was received from Rieke Metals (Lincoln, USA). The PODTPPD-BT polymer (weight average molecular weight = 8.7 kDa) was synthesized with the same method described in our previous work [32]. The PMMA polymer (weight-average molecular weight = 120 kDa) was purchased from Sigma-Aldrich (USA).…”

“…Very recently, it has been reported that efficient NIR detection can be realized by employing NIR-absorbing conjugated polymers as a gate-sensing layer (GSL) in organic phototransistors [32]. This report has shown that conjugated polymers, which have poor semiconducting property and cannot act as a channel layer, could play a NIR-sensing role in the structure of OPTRs with GSLs (GSL-OPTR).…”