Ultrahigh Gain, Low Noise, Ultraviolet Photodetectors with Highly Aligned Organic Crystals

Abstract

wileyonlinelibrary.comCOMMUNICATION solution methods, making them quite promising in the mass production of optoelectronic devices with low cost. [ 18 ] Organic semiconductors are appealing light detection materials due to their much stronger light absorption than most inorganic counterparts. [ 19,20 ] Nevertheless, the low carrier mobility of organic semiconductors, which is usually in a range of 10 −6 -1.0 cm 2 V −1 s −1 , limits their application for high performance photodetectors. Encouragingly, we recently demonstrated that C8-BTBT polycrystalline fi lms fabricated by an "off-centre spin-coating" method can have an ultrahigh average hole carrier mobility of 25 cm 2 V −1 s −1 . [ 21 ] This high carrier mobility provides a platform for exploring high performance organic photodetectors. In this study, we reported highly sensitive phototransistors with the highly aligned C8-BTBT polycrystalline fi lms as the active layers.The organic transistor we used here has a top-contacted bottom-gated structure ( Figure 1 a). The highly aligned C8-BTBT polycrystalline fi lms were casted from a blended solution of C8-BTBT and polystyrene (PS) using the "offcentre spin-coating" method. [ 21 ] The molecular structures of C8-BTBT and PS are shown in Figure 1 a. The C8-BTBT fi lms had an edge-on alignment on the substrate and form monoclinic crystal structure. The in-plane stacking of the C8-BTBT molecules is illustrated in Figure 1 a. Highly doped silicon wafer with a 300 nm SiO 2 layer was used as the substrate and gate electrode, which also demonstrates the direct integration of the organic phototransistors with silicon chips. The dielectric layer contains the 300 nm thermally grown SiO 2 layer and a 330 nm crosslinked poly(4-vinylphenol) (PVP) layer, [ 22 ] which has a total specifi c capacitance of 6 × 10 −5 F m −2 . We used thick dielectric layer because it can cause smaller leakage current and a larger threshold voltage shift that is needed for sensitive photodetection reported here (see Figure S1 in the Supporting Information). [ 23 ] The C8-BTBT fi lms fabricated on crosslinked PVP surface had a very high degree of crystallinity and crystal alignment, as evidenced by the clear anisotropic absorption spectra in the wavelength range of 295-375 nm (Figure 1 b). The strong dependence of the absorption with the polarization of the incident light beam indicates the presence of highly oriented C8-BTBT molecules in a macroscopic scale, since the light spot in the measurement was >3 mm 2 . [ 21,24 ] The presence of three absorption peaks (e.g., 360, 342, and 325 nm) is due to the excitation of electrons from ground state into different vibrational states with an energy step of 0.19 eV. [ 25 ] For the C8-BTBT fi lm with a thickness of 15 nm, the absorption at 354 nm is 15% or 6% when the electrical fi eld ( E ) of polarized light is parallel or perpendicular to the (100) direction of the C8-BTBT crystals (Figure 1 b), respectively, giving an average absorption of around 10% for nonpolarized light.

Photoresponse of graphene field-effect-transistor with n-type Si depletion layer gate

“…Consequently, the output current ( I DS ) of the G-FET is increased by light irradiation depending on the accumulated holes at the Si/SiO 2 interface. Similar concept of the trap controlled band modification for the enhancement of photosensitivity has been reported in organic photodiodes 24 – 28 .…”

All-Inkjet Printed Organic Thin-Film Transistors with and without Photo-Sensitivity to Visible Lights

“…Here, the transfer characteristics shift may be resulted from the slow recovery of the devices after light illumination with a time constant of 10.5 s, as shown in Figure 6b. To improve this slow recovery issue, several studies have reported using gate pulses to instantly switch the phototransistors to the on-state and remove excess charge carriers [16,25]. The responsivity and sensitivity of the TIPS-pentacene OTFT was shown in Figure 6c,d, respectively.…”

“…There have been plentiful reports on OTFTs used as photodetectors and phototransistors [8], but only sporadic demonstrations by all-printed OTFTs [14,15]. In addition, most of the reports were focused on the response to ultraviolet (UV) lights [8,15,16]. Therefore, it would be interesting to investigate the photo-sensitivity of all-inkjet printed OTFTs, in particular under the visible light exposure that has not been deeply investigated.…”

Solution-processed image sensors on flexible substrates

“…Solution-processed photodetectors have specific detectivities competitive with photodiodes in CMOS image sensors and can operate at sufficiently fast speeds for image sensing [11,15]. Additionally, the extreme flexibility (bending radius on the order of mm) of these devices fabricated on thin substrates [47,65,73,87,88] can enable a wide variety of new imaging techniques. Solution-processed TFT performance in recent years has also surpassed that of amorphous silicon, which is used in large area active-matrix displays and x-ray image sensors.…”