Photo-induced effects in organic thin film transistors based on dinaphtho [2,3-b:2′,3′-f] Thieno[3,2-b′] thiophene (DNTT)

Abstract

We have investigated the photoresponse of organic thin film transistors (OTFTs) based on evaporated films of dinaphtho [2,3-b:2',3'-f] thieno[3,2-b'] thiophene (DNNT) as the active semiconductor and spin-coated polystyrene as the gate insulator. Both during illumination and in subsequent measurements in the dark after long periods under illumination, transfer characteristics shift to more positive gate voltages. The greatest photoresponse was achieved at 460 nm, near the absorption maximum of DNTT. The maximum photosensitivity and photoresponsivity measured were ~10 4 and 1.6 A/W respectively. The latter is the highest reported for an organic semiconductor on a polymeric gate insulator and by suitable adjustments to device geometry could be increased to match the highest reported, ~10 5 A/W, for organic semiconductors. Weaker responses were also obtained when exposed to light from the long-wavelength tail in the absorption spectrum. At these longer wavelengths, the response arises entirely from a shift in flatband voltage caused by deep interface trapping of photo-generated electrons. At 460 nm, however, the positive shift, VON, in turn-on voltage is much greater than the shift, VT, in threshold voltage suggesting that ~3.5 x 10 11 electrons/cm 2 are trapped at the interface at the start of the gate voltage sweep, but ~60% are neutralised by holes from the channel as the device begins to turn on. While the resulting change in subthreshold slope could be interpreted as a change in the density of states (DoS) in the DNTT, this is discounted. Gate bias stress measurements made under illumination, reveal that positive bias enhances interface electron trapping while negative bias reduces the effect owing to the simultaneous trapping of holes from the accumulation channel.

Pushing On-Chip Photosensitivity Forward Using Edge-Driven Vertical Organic Phototransistors

“…Figure c shows the output curves acquired during illumination with different light colors at V GS = −3.6 V. As shown in Figure c, the devices present significantly higher photoswitching properties under blue light illumination ( I DS is almost two magnitude orders higher than I dark ). Following the present results, previous studies have demonstrated that DNTT OFETs exhibit larger photoresponsivities to blue light. ,,, This effect is explained by the fact that OSCs absorb light with a specific wavelength depending on their conjugated bonds and band gap. , For DNTT, phonons with a wavelength of around 450 nm have enough energy to promote exciton formation (electron–hole pairs), which dissociate into holes and electrons during interactions with defects or electric fields. ,, A detailed study of this light-dependent effect was presented by Milvich et al, who showed a strong and slow positive V th shift for DNTT-based transistors under proper biasing and blue light illumination . This photoresponse was explained by a charge trapping mechanism, i.e., the increase in photo-generated electrons and simultaneous trapping of these free charge carriers at structural or interfacial defects.…”

“…17,58 For DNTT, phonons with a wavelength of around 450 nm have enough energy to promote exciton formation (electron−hole pairs), which dissociate into holes and electrons during interactions with defects or electric fields. 9,58,59 A detailed study of this light-dependent effect was presented by Milvich et al, who showed a strong and slow positive V th shift for DNTTbased transistors under proper biasing and blue light illumination. 14 This photoresponse was explained by a charge trapping mechanism, i.e., the increase in photo-generated electrons and simultaneous trapping of these free charge carriers at structural or interfacial defects.…”

“…Particularly at the dielectric/semiconductor interface, the electrons are captured by interfacial traps (hydroxyl groups, OH−). Both the dynamics introduced by illumination may cause a shift in V th toward more positive values. ,− …”

Organic N-type Electrets for Field-effect Transistors, Photo Memories and Artificial Synapses

“…For as-prepared OFETs, onset voltage (Von) is positively shifted from -4 V to 10 V, and the maximum Ids, referring the Ids at Vgs = -60 V, is increased twice in magnitude under light illumination (Fig. 3a), which is a sign of photoconductivity 27 . To quantitatively evaluate photoconductivity, the shift of onset voltage (∆Von) and Iphoto/Idark ratio will be discussed.…”

Flexible fully organic indirect detector for megaelectronvolts proton beams

“…Photocurrent in organic semiconductors has been related by several authors to an increase of trapping of minority carriers induced by light exposure 8,[54][55][56][57] . The measurements reported in this work for the OPTs are consistent with this interpretation.…”