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Optoelectronics & Communications

Light-sensing circuits fabricated using new phototransistors

Recent low-voltage, organic ambipolar phototransistors could lead to inexpensive, large, and flexible image-sensor arrays.
31 August 2009, SPIE Newsroom. DOI: 10.1117/2.1200908.1768

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.

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.

The operating voltage of organic transistors depends on the thickness and type of gate insulator used. The application of self-assembled 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 (p- and n-type semiconductors) transporting organic heterojunctions, we have made 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]-phenyl-C61-butyric acid methyl ester or [60]PCBM) ambipolar low-voltage organic phototransistor is shown in the inset of Figure 1. Under illumination (λmax= 469nm), the current flowing across the channel between the source-drain electrodes, ID, increases. This is believed to be due to photogeneration of carriers at the p-n interface (formed by joining p- and n-type semiconductors). By varying the incident light's intensity, the channel current's magnitude can be modulated (see Figures 1 and 2). Hence, the device can be used as a photosensor. The responsivity (the photo-induced current divided by the incident optical power) of these phototransistors depends on the magnitude and sign of the applied gate field, and it decreases with increasing incident optical-power density.

Figure 2. Dynamic response of a low-voltage ambipolar organic phototransistor to pulsed incident light. Current and intensity are in arbitrary units (a.u.).

We integrated a number of low-voltage ambipolar organic phototransistors, to fabricate light-sensing optoelectronic circuits, such as complementary-like inverters (see Figure 3 inset). By varying the incident light's intensity, we found that the inverter's operating characteristics change (see Figure 3). This is due to the difference of each organic phototransistor's responsivity under dissimilar biasing conditions. In particular, by maintaining the device at constant supply (VDD) and input (VIN) voltages, we noted that the inverter's trip voltage can be modulated and the circuit can be used as an optical sensor.

Figure 3. Transfer characteristics obtained from a low-voltage, complementary-like inverter in the dark and under illumination at different intensities in mW/cm2. Inset shows the circuit of the complementary-like inverter employed. Photo-FET is the photo-field-effect transistor. VDD: Constant supply voltage. VIN: Input voltage. GND: Ground pin.

In summary, by using self-assembled monolayer gate dielectrics and p-n-type, bilayer organic heterostructures, we have demonstrated low-voltage (<3V) ambipolar phototransistors and used them to fabricate light-sensing integrated circuits. The present work is a significant step towards low-cost and low-power light-sensor arrays. Our future work in this field aims to demonstrate organic phototransistors that are able to resolve incident light color, hence paving the way toward full-color image sensors.

John Labram, Donal Bradley, Thomas Anthopoulos
Department of Physics
Imperial College London
London, UK

John Labram graduated from the University of Warwick in July 2008. He is currently studying for a PhD within the Experimental Solid State Physics group under the supervision of Thomas Anthopoulos. His research interests include ambipolar organic field-effect transistors and light-sensing organic field-effect transistors.

Donal Bradley is the Lee-Lucas Professor of Experimental Physics and deputy principal of the faculty of Natural Sciences. He is a fellow of the Royal Society and the Institute of Physics.

Thomas Anthopoulos holds a degree in medical engineering and a PhD in physical electronics from Staffordshire University. After two postdoctoral appointments at the University of St. Andrews (UK) and Philips Research Laboratories (Netherlands), he joined Imperial as an engineering and physical sciences research council/research council UK fellow.