Yingquan Peng

Photoresponsivity enhancement of pentacene organic phototransistors by introducing C60 buffer layer under source/drain electrodes

Pentacene organic phototransistors (OPTs) modified by introducing C60 electrodes buffer layer were fabricated. Compared with conventional single-layer pentacene OPTs, the devices with C60 buffer layer exhibited higher performance under illumination. Maximum photoresponsivity is 4.27 A/W at zero gate bias under 0.2 mW cm(-2), which is six times higher than that of single-layer device and exceeds that of most OPTs reported recently in the same conditions. The maximum photo/dark current ratio is also higher than 5 x 10(4). Meanwhile, the C60 modified device could obtain lower threshold voltage and higher field-effect mobility. These results suggest that single-layer OPTs modified by heterojunction buffer layer will obtain better optical response

Improved Performance of Photosensitive Field-Effect Transistors Based on Palladium Phthalocyanine by Utilizing Al as Source and Drain Electrodes

In conventional photosensitive organic field-effect transistors (FETs) (OFETs) (photo-OFETs) based on p-type organic semiconductors, high work function metals such as Au are generally used as source/drain electrodes, whose photosensitivity is generally low. We report on the performance improvements of photo-OFETs based on palladium phthalocyanine (PdPc) by using Al as source and drain electrodes. It is concluded that the dark currents of the photo-OFET based on PdPc with Al as source and drain electrodes (denoted as Al-PdPc device) are about only one-thousandth of that of the photo-OFET based on PdPc with Au as source and drain electrodes (denoted as Au-PdPc device). This tremendous decrease of dark current results in about a three-order-of-magnitude increase for photosensitivity at the gate and drain voltages of -50 V. The enormous decrease of dark current is ascribed to the Schottky contacts between the Al source/drain electrodes and PdPc. In addition, the Al-PdPc devices show also larger photoresponsivity compared with the Au-PdPc devices.

Photo-Induced Balanced Ambipolar Charge Transport in Organic Field-Effect Transistors

Photo-induced balanced ambipolar transports in lead phthalocyanine organic field-effect transistor are observed. The transport mode of the device is changed from typical p-channel to ambipolar under illumination at 655 nm, 100 mW/cm2. The physical origins of this phenomenon are resulted from the photo-induced energy barrier lowering for electrons injection at the source-channel interface, which is suggested to be the downward bending of orbital level in the channel material near the source by accumulation of photo generated holes there.

Position-dependent performance of copper phthalocyanine based field-effect transistors by gold nanoparticles modification

A facile fabrication and characteristics of copper phthalocyanine (CuPc)-based organic field-effect transistor (OFET) using the gold nanoparticles (Au NPs) modification is reported, thereby achieving highly improved performance. The effect of Au NPs located at three different positions, that is, at the SiO2/CuPc interface (device B), embedding in the middle of CuPc layer (device C), and on the top of CuPc layer (device D), is investigated, and the results show that device D has the best performance. Compared with the device without Au NPs (reference device A), device D displays an improvement of field-effect mobility (μ(sat)) from 1.65 × 10(-3) to 5.51 × 10(-3) cm(2) V(-1) s(-1), and threshold voltage decreases from -23.24 to -16.12 V. Therefore, a strategy for the performance improvement of the CuPc-based OFET with large field-effect mobility and saturation drain current is developed, on the basis of the concept of nanoscale Au modification. The model of an additional electron transport channel formation by FET operation at the Au NPs/CuPc interface is therefore proposed to explain the observed performance improvement. Optimum CuPc thickness is confirmed to be about 50 nm in the present study. The device-to-device uniformity and time stability are discussed for future application.

Ultrahigh near infrared photoresponsive organic field-effect transistors with lead phthalocyanine/C60 heterojunction on poly(vinyl alcohol) gate dielectric

Performances of photoresponsive organic field-effect transistors (photOFETs) operating in the near infrared (NIR) region utilizing SiO2 as the gate dielectric is generally low due to low carrier mobility of the channel. We report on NIR photOFETs based on lead phthalocyanine (PbPc)/C60 heterojunction with ultrahigh photoresponsivity by utilizing poly(vinyl alcohol) (PVA) as the gate dielectric. For 808 nm NIR illumination of 1.69 mW cm(-2), an ultrahigh photoresponsivity of 21 A W(-1), and an external quantum efficiency of 3230% were obtained at a gate voltage of 30 V and a drain voltage of 80 V, which are 124 times and 126 times as large as the reference device with SiO2 as the gate dielectric, respectively. The ultrahigh enhancement of photoresponsivity is resulted from the huge increase of electron mobility of C60 film grown on PVA dielectric. AFM investigations revealed that the C60 film grown on PVA is much smooth and uniform and the grain size is much larger than that grown on SiO2 dielectric, which together results in four orders of magnitude increase of the field-effect electron mobility of C60 film.

Toward high performance broad spectral hybrid organic–inorganic photodetectors based on multiple component organic bulk heterojunctions

Broadening the absorption bandwidth of photodetectors by incorporating multiple absorber materials to form bulk-heterojunction active layers is an attractive method of resolving the narrow absorption of organic semiconductors. Here we report the fabrication and characterization of broad spectral hybrid organic–inorganic photodetectors through an optimized multiple component organic bulk heterojunctions route. The study clearly shows that both the two-component and three-component photodetectors have ultrahigh photosensitivity and spectral uniformity of responsivity for visible to near-infrared light. Operating at room temperature, the three-component based photodetectors achieve ultrahigh photosensitivities of 30–50 A W−1 at ∼0.1 mW incident optical power, which are almost two orders of magnitude larger than commercial Si and InGaAs photodiodes. Also, this work demonstrates that the wide variation of optical absorption of multiple component organic bulk heterojunctions will provide a strategy to fabricate facile broadband photodetectors.

Influence of donor-acceptor layer sequence on photoresponsive organic field-effect transistors based on palladium phthalocyanine and C60

Photoresponsive organic field-effect transistors (PhotOFETs) based on palladium phthalocyanine (PdPc) and C60 were fabricated with different donor-acceptor layer sequences. Both planar heterojunction devices fabricated exhibit better performance under illumination than the single PdPc device. PhotOFETs with the structure SiO2/C60/PdPc/Au exhibit a higher photosensitivity and photoresponsivity than that with the structure SiO2/PdPc/C60/Au. The origin for this is largely the high mobility of C60 and the well-matched LUMO levels of PdPc and C60. The maximum photosensitivity of the SiO2/C60/PdPc/Au device is 8 × 103, and the photoresponsivity is approximately 28 times that of the single component PdPc device.

Interface Effect on the Performance of Rectifier Based on Organic Diode

In this letter, the effect of the interface on the electrical characteristics of a rectifier based on an organic diode with pentacene as the electroactive layer is studied. It was found that the on/off ratio had a strong correlation with the interface effect (i.e., the bottom electrode material). The rectification ratio was improved to 2 × 106 at 5 V by modifying the bottom electrode with copper tetracyanoquinodimethane. The ac rectification frequency exceeded 13.56 MHz, which is a main requirement for an organic radio-frequency identification application.

Operational dynamics and architecture dependence of double-gate OFETs with balanced top and bottom channel characteristics

A double-gate organic field-effect transistor (DGOFET) utilizing thermally evaporated lithium fluoride (LiF) as the top gate dielectric and fluorinated copper-phthalocyanine (F16CuPc) as the active channel material was reported in this article. XRD and AFM analyses manifested that the fabricated LiF films on the F16CuPc channel layer were highly dense polycrystalline, uniform, and flat. A comprehensive and systemic study of operational dynamics and architecture dependence of DGOFETs was reported herein. Three different operating modes of DGOFETs were introduced and demonstrated, which indicated that controllable device performances (considering output current, threshold voltage, etc.) could be obtained by double-gate architecture, and DGOFETs working in the synchronized mode exhibited high field-effect mobility, low threshold voltage (absolute value), and large transconductance. Furthermore, the DGOFET based F16CuPc showed a better gate modulation effect, which could achieve a switch from normally-on to off-state in double-gate mode. The successful operation of the fabricated DGOFETs also indicated that LiF is a promising material as the dielectric for realizing high-performance and patterned top-gate OFETs.

Toward Ultrahigh Red Light Responsive Organic FETs Utilizing Neodymium Phthalocyanine as Light Sensitive Material

The performance of photoresponsive organic FETs (photOFETs) intimately depends on their optical absorption and charge carrier transport property. We report on red light responsive photOFETs based on various device structures utilizing neodymium phthalocyanine (NdPc2) as the light sensitive material. PhotOFETs based on planar heterojunction, bulk heterojunction (BHJ), and hybrid planar-BHJ (HPBHJ) with NdPc2 and C60 on poly(vinyl alcohol) (PVA) and SiO2 gate dielectric were fabricated and characterized. Among various device structures, HPBHJ-photOFET on PVA dielectric showed the best performance. For the 650-nm-red light illumination, an ultrahigh photoresponsivity of 108 A/W and a maximum photosensitivity of 3.75×104 were obtained. The ultrahigh enhancement of the photoresponsivity for HPBHJ-photOFET is resulted from high absorption coefficient of NdPc2 in the red light region, high dissociation efficiency of the photogenerated excitons, and high electron mobility of C60 layer grown on PVA.