Study On The Photoresponsive Organic Field-effect Transistors Based On Pentacene

2001, the first photoresponsive organic field-effect transistor(Phot OFET) was reported by Narayan et al. Phot OFET have attracted extensively attention owing to its unique properties, such as low-cost, easy-prepared, compatible with flexible substrates and high gain and signal to noise ratio. Phot OFET has broad application prospects in the field of optical detectors. In this paper, pentacene-Phot OFET based on a bottom-gate top-contact was studied. The main work of this paper is the following aspects:(1) For the source and drain electrode material of the conventional P-type Phot OFET is metal with high work function generally, although the responsivity R is high, the photo/dark current ratio P is low. In order to increase P, metal with low work function is used as the source and drain electrodes, increasing the carrier injection barrier, reducing the dark current of the channel. In experiment, Au(5.1 e V), Cu(4.65 e V), Ag(4.26 e V) and Mg: Ag(10:1)(3.7 e V) were used as source and drain electrodes to compare the character under the simulating illumination at an intensity of 100 m W/cm2. The results showed that the Pho OFET based Ag and Mg: Ag, since the HOMO value of pentacene and the work function is not match, resulting a very small channel current in darkness, so that the maximum photo/dark current ratio P based on the Phot OFET of Ag and Mg: Ag were 619 and 2.89×103 respectively, which were much larger than the Phot OFET based on Au and Cu electrode. To further validate the theory, the carrier injection layer Mo O3 was added into pentacene and Ag, Mg: Ag, the results showed that Mo O3 improve the hole injection capability, then the maximum photo/dark current ratio decreased to 153 and 260 respectively.(2) We study the impact of device performance for Phot OFET with different insulation layers. In experiment, PVA, PMMA and PS were used as insulating material, wherein the underlying insulating layer was PVA, PMMA and PS were used as the modified layer. Device A based on PVA(500 nm), device B based on PVA(500 nm) / PMMA(50 nm), device C based on PVA(500 nm) / PS(50 nm) were illuminated under the simulating illumination at an intensity of 100 m W/cm2 to compare their performance. The results showed that the performance of the device B and C were superior to the device without adding the modified layer. This large difference is attributed to two reasons. On the one hand, the dielectric constant of PVA is higher than PMMA and PS. PVA has strong polarity effect which may prevent the transmission of interface carrier. On the other hand, the grain size of pentacene film in Device B and C is larger than device A, which is good for the carrier transmission.(3) To increase the stability and extended the life of Phot OFET devices in the air, paraffin was used to encapsulate the device. First, we explore the methods of encapsulation to select the feasible one. Then the encapsulated and unencapsulated devices were exposed to the air, the performance of the device was tesed after 0 h, 100 h and 500 h. The results showed that the on/off current ratio and mobility of unencapsulated devices were reduced 93% and 40% after 500 h, while the value of the encapsulated devices were 32% and 11% respectively. Finally we noted that paraffin encapsulation can increase the stability and extended the life of the device.