2007 IEEE International Conference on Signal Processing and Communications (ICSPC 2007), 24-27 November 2007, Dubai, United Arab Emirates

COPPER PHTHALOCYANINE BASED ORGANIC PHOTO FIELDEFFECT TRANSISTOR AS A SENSOR FOR OPTICAL

INSTRUMENTATION

Khasan S Karimov a, Ibrahim Qazib, Umar Shafique* b MMahroof-Tahir cTauseef Ahmed Khanb, Syed Najeeb Haiderd.

a Physical Technical Institute ofAcademy of Sciences, Rudaki Ave.33, Dushanbe,734025, Tajikistan.b GIK Institute of Engineering Science and Technology, Topi, Swabi, N.W.F.P, 23640 Pakistan,.

c St.Cloud State University, 720 Fourth Avenue South St.Cloud, MN 56301-4498, USA.d Karachi Institute of Economics and Technology,75340,Karachi,Pakistan.

*Corresponding Author: umar.shafiquegieee.orgABSTRACT

A thin film of organic p-type semiconducting copperphthalocynanine (CuPc) and a semitransparent Al filmwere deposited by vacuum evaporation on a glasssubstrate with Ag film source and drain electrodes.Organic Field Effect Transistor with metal (aluminum) -semiconductor (copper phthalocyanine) Schottky typejunction was fabricated. Investigations were made for theeffect of modulated LED's irradiation on itscharacteristics at frequency ranging from 10 Hz-100 kHz.Drain current of organic phototransistor (OPT) wasobserved to increase quasi-linearly with illumination.Organic field effect transistor was observed to besensitive at frequency range from 16 Hz to 316 Hz (ACcurrent) and from 16 Hz to 10 kHz (DC current). A blockdiagram of optical instrumentation with OPT as a sensoris also proposed.

Index Terms- Organic Field Effect Transistor,Copper Phthalocyanine, Metal-Semiconductor SchottkyJunction, Phototransistor.

1. INTRODUCTION

During the past decade, organic semiconductor field-effect transistors (OFET) based on organic materials asconjugated polymers, oligomers and low molecularweight materials have been widely investigated. Lowmaterial and fabrication cost of OFET has gainedattraction/interest for their potential applications asorganic devices [1-5]. Some of the OFET showssensitivity not only to applied voltage but itself to electricfield of molecules. In [6] OFET were used to detect thecharged/uncharged chemical species in aqueous mediavia the field effect, the chemical sensitivity of the organictransistor was illustrated for protons and glucose.During the past organic phototransistors (OPT) were wellfabricated and investigated. In [7] OPT based on abiphenyl end-capped fused bithiophene oligomers were

investigated Under 380 nm UV light. The OPT's showeda similar photocurrent response to the absorptionspectrum of the organic semiconductor. It is expected thatthe OPT's can be used as highly sensitive UV sensors.Similar to the previous case in [8] the effect of ultravioletlight irradiation on the characteristics of organicphototransistor (OPT) containing sexithiophene (6-T) andpentacene was examined. The following twodistinguished responses were observed: fast and slow.The slow response was observed in several weeks. Itsuggests the possibility of application of OPT's as light-addressable field-effect transistor memory devices. Themost widely used organic semiconductors are pentacene,thiophene oligomers and regioregular polythiophenethese organic semiconductors shows good performance asto be used in OFET [1], but further improvements can bemade by using different materials, structures andfabrication technologies.

In [9] copper phthalocyanine (CuPc)/inorganicferroelectric (PbZro.2Ti0.803) heterojunction gate and aferromagnetic oxide semiconductor (Lao.87Bao. 13MnO3)channel photomemory were developed. The non-volatileand non-destructive photomemory operation weredescribed and was observed that the device could writethe light information with the combination of the lightirradiation and the negative gate bias, and can be erasedonly with the positive gate bias.

Organic semiconductors phthalocyanines [10] andespecially CuPc is one of the well-studied organicphotosensitive semiconductor materials with highabsorption coefficient in wide spectrum range. CuPc hashigh photo-electromagnetic sensitivity at low intensity ofradiation. Thin film of CuPc can be easily deposited byvacuum sublimation technology. Purification of CuPc issimple and economical as the sublimation occurs atrelatively not very high temperatures (400-6000C).Fabrication and investigation of the characteristics andphotoelectrical properties of organic-on-inorganic Ag/p-CuPc/n-GaAs/Ag photoelectric sensors at room andelevated temperatures in photovoltaic and

1-4244-1236-6/07/$25.00 © 2007 IEEE 848

photoconduction mode of operation under tungstenfilament lamp illumination were made [11,12].Photocurrent and photo-voltage spectra showed that cellis sensitive in wide spectral range of wavelengths 200-1000 nm that is from UV to visible and NIR spectrum.The numerical simulation and properties of the Schottkyjunction transistor on the base of Si were discussed [13].The results for a device with a 0.5 ptm gate length suggestthat cutoff frequencies of 10 GHz can be achieved. Thispaper reports the properties and application of OPT withCuPc and Schottky junction (Al-CuPc) that have thestructure of MESFET, unlike to MOSFETs described in[7,8].

2. EXPERIMENTAL

The CuPc was obtained from Sigma-Aldrich. Figure.1shows the molecular structure of the CuPc used as a p-type organic semiconductor [14,15]. A number ofcrystalline polymorph states of CuPc are available [14,16]. The a-CuPc form is a metastable at T = 165 °C andcan be converted thermally or with solution to the /1-

form. The a and /1 - states are the most frequentlyencountered states. X-ray diffraction data showed that thedeposited CuPc films were in a(-form [17]. Scanningelectron micrographs showed no particular crystalorientations in CuPc films [17]. A band gap of CuPc wasequal to 1.6 eV and conductivity was equal to 5 x 10-13Q 'cm at T=300K [14,15]. The sublimation temperaturevaries from 400 °C to 580 °C at 10-4 Pa [10].

On CuPc film a semi-transparent 20nm- 25nm Aluminum(Al) film with 10-15% transparency was deposited.Thickness of the CuPc was monitored by Edwards FTM5film thickness monitor. Earlier investigations showed thatthe CuPc forms an ohmic contact with Ag and Schottkytype rectifying contacts with Al [10].Figure.3 shows the cross-sectional view of the fabricatedOFET, Ag films and gate terminal with Al film. Sourceand drain contact were developed using Ag film. Length,width and thickness of the CuPc channel were 0.1mm,20mm and 60nm respectively. Red LED was used as alight source modulated using square-wave voltageoscillator at frequencies range from 10Hz- 100kHz.Conventional digital meters were used for themeasurement of voltages and currents at roomtemperature.

0.6

04r_X0

._

010.2

400 500 600 -oo 800 900 1000 11(N

Wavelength(nm).

Figure 2. Absorption spectra ofp-CuPc film deposited onglass substrate by vacuum evaporation.

hV

Ten~ind

S

Figure 1. Molecular structure of CuPc.

Thin film of CuPc was thermally sublimed on a glasssubstrate (with preliminary deposited 50 nm thicknesssilver film electrodes) at 400-4500C and _104 Pa inEdwards AUTO 306 vacuum coater with diffusionpumping system. The substrate's temperature was

maintained at -40 °C. The CuPc film was deposited on

glass substrate under the same conditions. Perkin ElmerLambda 19 UV/VIS/NIR spectrometer was used for themeasurement of absorption spectra (Figure.2). Thespectra cover the NIR- UV, i.e. 200-1000 nm

wavelengths.

Figure 3. Cross sectional view of the organic photo-transistor.

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3. RESULTS AND DISCUSSIONS

Figure 4. Drain current-illumination relationship at drain-source voltage Vds = 5 V, I(AC) and I(DC) - AC and DC

currents.

Figure.4 shows the drain current-illumination relationshipat drain-source voltage Vds = 5 V. The drain AC and drainDC currents increase with illumination (in the range of 0-102 lx) in 10 and 48 times (ratio of light ON/OFFcurrents) respectively. The observed drain current-illumination relationship is comparable with that of theOPT discussed in [8]. The frequency of modulated lightused was equal to 100 Hz.

Figure 5. Frequency response of the OPT: I(AC) and I(DC) -AC and DC currents.

Figure 5 shows the frequency response of drain AC anddrain DC current respectively. Frequency response ofOPT ranges from lower cutoff to high cutoff frequenciesthat is from 16Hz to 316Hz for AC current and from16Hz to 10kHz for DC current. Cutoff frequencies were

determined as 70.7 °0 ofmaximum currents.The two mechanisms, photoconductive and

photovoltaic behaviors were observed to be responsiblefor the photoresponse of the OPT as discussed in [7,8].

Photoconductive behavior (the increase of conductance inthe effect of light) is due to the generation of excitons bythe absorption of high energy photons with energy higherthen the band gap energy of organic semiconductor(CuPc), also due to the splitting of excitons into electronsand holes by source-drain field. Photovoltaic behavior(generation of voltage due to effect of light) is observeddue to the presence of rectifying metal-semiconductorjunction with built-in potential barrier and electric field.As a result of Metal-Semiconductor junction built-inpotential splitting of excitons into electron-hole pairs arealso observed. A decrease in electric field and increase incross-section of OPT with increase in conductivity ofchannel is observed due to the effect of incident light onmetal-semiconductor potential barrier.In [18] the optical tomography with photodiode receiversensitive in near IR was described. As the investigatedOPT is sensitive in very wide spectral range, probably, itcan be used as a receiver for optical tomography as wellOn the other hand, this sensor can be used for thedevelopment of seismic displacement transducers. Wherethe mechanical prototype of seismic displacementtransducer system was described in [19].OPT can be used as a replacement to the IR Digikey 365-1070 receiver sensor in physiological sensor system forambulatory use [20]. The IR sensor used was to measurethe arterial blood volume over the skin using thephenomenon that varying absorption is observed fromblood arteries under the skin at different blood arterialrate associated with each heart beat. The proposed OPTsensor can have benefits of high photo luminanceresponse with flexible substrate.In [21] the IR sensor for monochromatic source ofdynamic spectrum measurement in PhotoPlethysmoGraphy can be replaced by CuPc based OPT,for low cost and efficient sensing operation.

Power Supply

Digital ampermeter

3 -)4 Medium OPT

Figure 6. A block diagram of optical instrumentation with OPTas sensor.

In Figure.6 a block diagram of optical instrumentation isproposed with Organic Photo Transistor (OPT) as asignal processing sensor. Oscillator generates squarewave, triangular or sinusoidal voltage in wide range offrequencies (from 10Hz to 100KHz). Red, Infra Red,Green or Blue LED's can be used as a modulated lightsource, as OPT is sensitive in UV-Visible- IR spectrarange. The power supply output is equal to 5V, DigitalDC/AC ampere meters can be calibrated directly tomeasure values. Human's body, free space or opticalfibre can play the role ofmedium.

850

0.6 II C) x100.5 -4

00.4

"0.1 0~~~~~~~~0

-0.1 6 50 100 150

Illumination (lIx)

0.25 -

0.2 _I(D)

0.15- I

- 0.1

n 0.05- I I(ACx10

OI- A)x, O2 4 6

-0.05log f (Hz)

4. CONCLUSION

CuPc based Organic Photo Transistor/sensor withSchottky junction (Al-CuPc) was fabricated andinvestigation of electrical characteristics were madeunder illumination of modulated LED's. It was observedthat photoconductive and photovoltaic effects areresponsible for increase in drain current underillumination. It was concluded that the organic phototransistor was sensitive to frequency range from 16 Hz to316 Hz (AC current) and from 16 Hz to 10 kHz (DCcurrent) It was proposed that the transistor can easily befabricated on flexible substrate and can be used foroptical instrumentation in particular, biomedical, seismicinstruments.

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