Light-sensing circuits fabricated using new phototransistors

Abstract

Since the demonstration of the first organic field-effect transistor (OFET) in the early 1980s,1 organic semiconductors have been seen as an alternative to conventional inorganic microelectronics for various low-end applications. Properties such as solution processability allow organic semiconductor devices to be fabricated at low temperatures using high-throughput fabrication methods. This makes the realization of certain organic devices potentially easier and cheaper than their inorganic counterparts. The field of OFETs has been growing rapidly. Bifunctional ambipolar light-sensing OFETs, or phototransistors,2 are a new addition to the family of organic devices, and could one day lead to the production of novel, low-cost image sensors. Unfortunately, all organic phototransistors demonstrated to date operate at relatively high voltages (>20V). This characteristic renders the technology unsuitable for portable, low-power applications. For widespread commercial use of the technology, devices with low-voltage and low-power dissipation would need to be developed. The operating voltage of organic transistors depends on the thickness and type of gate insulator used. The application of selfassembled monolayer (SAM) molecules as gate dielectrics has proven effective in reducing the gate dielectric’s thickness while retaining good insulating properties.3, 4 By combining such SAM dielectrics with hole/electron (pand n-type semiconductors) transporting organic heterojunctions, we havemade organic ambipolar phototransistors with operating voltages below 3V. A unique advantage of our approach is that hole and electron transport can be individually controlled and optimized. The spectral response of these ambipolar phototransistors can also be tuned through the use of suitable semiconductor systems. The schematic diagram of a bilayer (pentacene/[6,6]-phenylC61-butyric acid methyl ester or [60]PCBM) ambipolar lowvoltage organic phototransistor is shown in the inset of Figure 1. Under illumination (λmax= 469nm), the current flowing across Figure 1. Transfer characteristics from a low-voltage ambipolar organic phototransistor measured in the dark and under illumination using a blue (469nm) LED at different intensities in mW/cm2. Inset shows a schematic of the bilayer (pentacene/[6,6]-phenyl-C61-butyric acid methyl ester or [60]PCBM) transistor structure that was used. ID: Illumination of the current flowing between the source-drain electrodes in the ambipolar (A) phototransistor. VD: Drain voltage in volts. VG: Gate voltage.