In this study the photocapacitive behavior of organic semiconductor, photosensitive material copper phthalocyanine (CuPc) were investigated. Thin film of the copper phthalocyanine was deposited by vacuum evaporation on glass substrate with silver surface-type electrodes and Ag/CuPc/Ag photo capacitive sensor was fabricated. It was shown that under filament lamp illumination up to 1000 lx the capacitance of the Ag/ CuPc/Ag photo capacitive sensor increased continuously up to 20% with respect of dark condition. It is assumed that photo capacitive response of the sensor is associated with polarization due to the transfer of photo-generated electrons and holes

F.A. Khalid

I. Qazi

Kh.S. Karimov

P.H. Draper

T.A. Khan

U. Shafique

Eurasian Chemico-Technological Journal

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*corresponding  author. E-mail: khasan_karimov@yahoo.comEurasian ChemTech Journal  9 (2007) 217-221 2007 al-Farabi Kazakh National University   Printed in KazakhstanCopper Phthalocyanine Surface-type Photocapacitive SensorKh.S. Karimov1,2*, P.H. Draper2, F.A.Khalid2, I. Qazi2, T.A. Khan2 and U. Shafique2,31Physical Technical Institute of Academy of Sciences, Rudaki Ave.33, Dushanbe, 734025, Tajikistan2GIK Institute of Engineering Science and Technology, Topi, Swabi, N.W.F.P, 23640, Pakistan3International Islamic University, Islamabad, PakistanAbstractIn this study the photocapacitive behavior of organic semiconductor, photosensitive material copperphthalocyanine (CuPc) were investigated. Thin film of the copper phthalocyanine was deposited by vacuumevaporation on glass substrate with silver surface-type electrodes and Ag/CuPc/Ag photo capacitive sensorwas fabricated. It was shown that under filament lamp illumination up to 1000 lx the capacitance of the Ag/CuPc/Ag photo capacitive sensor increased continuously up to 20% with respect of dark condition. It isassumed that photo capacitive response of the sensor is associated with polarization due to the transfer ofphoto-generated electrons and holes.IntroductionAt present organic semiconductors and deviceshave attracted much due to their interesting proper-ties and potential applications for solar cells, lightemitting diodes, field effect transistors and differentkinds of detectors [1-4]. In [5] it was fabricated andinvestigated self-charging photocapacitor for directconversion and storage of solar energy. The photo-capacitor was constructed on a multilayered photo-electrode comprising dye-sensitized semiconductor(TiO2) nanoparticules/hole-trapping layer/activatedcarbon particles in contact with an organic electro-lyte solution, in which photogenerated charges arestored at the electric double layer. A high-voltagephoto-rechargeable photocapacitor of three-electrodeconfiguration, comprising a dye-sensitized mesopo-rous TiO2 electrode, two-carbon coated electrodes,and two-liquid electrolytes was investigated in [6].As far as capacitive sensors are concerned it was in-vestigated piezocapacitive sensor with poly-O-phtha-lanthracenate under uniaxial compression [7].In [8,9] it was synthesized photocapacitive organicsemiconductor material with PEPC doped by oligo-meric salt of perily (OSP). It was shown that thismaterial may be used for fabrication of the photocapa-citors sensitive in visible spectrum.Copper phthalocyanine is one of the well-studiedorganic photosensitive semiconductor [10,11]. It hashigh absorption coefficient in a wide spectrum andhigh photo-electromagnetic sensitivity at low intensi-ties of radiation. It is possible simply to deposit thinCuPc films by vacuum sublimation. Purification ofCuPc is simple and technology is economical as thesublimation occurs at relatively low temperatures(400-600°C). Recrystallization of CuPc is attainedfrom organic solutions at room temperature. The CuPcis very stable organic dye. In this paper we report oninvestigation of photo capacitive sensor with CuPc.ExperimentalFigure 1 shows the molecular structures of CuPc.For the photocapacitive sensor commercially produc-ed CuPc (Sigma-Aldrich) was used. It is known that,at least, seven crystalline polymorph states of CuPcexist: α, β, γ, R, δ, ε etc. The fabrication of CuPcfilms were in β form. The structure that character-izes the β form is a monoclinic crystal P21/a with a =19.407 Å, b = 4.79 Å, c = 14.628 Å and β = 120.93 Å[12]. It has band gap of about 1.6 eV and a conduc-tivity at T = 300 K of 5×10-13 Ω-1⋅cm [12]. The mo-Eurasian ChemTech Journal  9 (2007) 217-221Copper Phthalocyanine Surface-type Photocapacitive Sensor218lecular weight of the CuPc is 576 a.m.u. Sublima-tion temperatures varies from 400°C at 10-4 Pa to580°C at 10-4 Pa [12].Results and DiscussionFigure 4 shows relative capacitance – illumina-tion relationship for the Ag/CuPc/Ag surface-typephotocapacitive sensor at low level of illumination(up to 1000 lx). Here dark capacitance Cd = 17 pF. Itis seen from Figure 4 that photocapacitance increaseswith increase of illumination to 20%. The responseof the sensor at illumination from the face and backsides approximately is the same. For calculation ofthe relative dielectric constant εd at dark conditions,we can consider, in the first approximation, the sil-ver electrodes and gap with deposited CuPc (Fig. 3)as a plane capacitor. For the plane capacitor [14]:Cd = εdε0A/dwhere ε0 is permittivity of free space, d is distancebetween plates of the capacitor, area of capacitor'splates A = 6×10-4 mm2, d = 0.2 mm. In the result ofcalculation it was found from eq. 1 that εd = 4.Figure 5 shows relative capacitance – illuminati-on relationship for the Ag/CuPc/Ag surface-type pho-tocapacitive sensor at high level of illumination (upFig. 1. Molecular structure of copper phthalocyanine.NNNNNNNNCuThin films of CuPc, of thickness 20-40 nm, werethermally sublimed on glass substrate (of sizes of25×25×1 mm3) with silver surface-type electrodes(gap between electrodes was equal to 0.2 mm, thelength of the gap was 20 mm) at 400-450°C at 10-4Pa. and Ag/CuPc/Ag photocapacitive sensor was fab-ricated. Silver electrodes were also deposited byvacuum evaporation technique. Earlier investigationsshowed that the Ag forms ohmic contacts withCuPc.Thickness of the CuPc films was measured bycrystal-controlled thickness monitor [13].Figure 2 shows absorption spectrum of the CuPcfilm deposited by vacuum evaporation on the glasssubstrate.Fig. 2. Absorption spectrum of the CuPc film depositedby vacuum evaporation on the glass substrate.0 500 1000 15000204060Wavelength, nmAbsorption, a.u.Figure 3 shows cross-sectional view of the fabri-cated Ag/CuPc/Ag photocapacitive sensor. Measure-ment of the capacitance was done by conventionalinstruments at the frequency of 1 kHz, at room tem-perature (25±0.5°C). The capacitive sensors were il-luminated by filament lamp.Fig. 3. Schematic diagram of Ag/CuPc/Ag photocapaci-tive sensor.AgCuPcAgGlassLightLightFig. 4. Relative capacitance – illumination relationships forthe Ag/CuPc/Ag surface-type photocapacitive sensor: 1, 2illuminated from back and face sides, 3 is calculated result.0 200 100000.20.40.60.81.01.21.4400 600 8001 23Relative capacitance, /CCdIllumination, lx(1)Kh.S. Karimov et al.Eurasian ChemTech Journal  9 (2007) 217-221219to 10000 lx). It is seen that the response of the detec-tor is complicated: with increase of the illuminationphotocapacitance increases, it is saturated and finallydecreases a few. The C/Cd – illumination curve's char-acter may be simply explained by the following ap-proach. In the effect of light concentration of chargecarriers may increase exponentially. Therefore thepolarizability due to the transfer of charge carriers aselectrons and holes may increase as well. At the sametime filament lamp illumination may heat the samplethat in turn may decrease the polarizability.where αt is polarizability under illumination due tothe transfer of excess, induced by light, electron/holescharge carriers. Here we take into consideration thatconcentration of charge carriers and total polarizabili-ty (α) are illumination dependent.In general form the relationship between the rela-tive dielectric constant and molecular concentration,N, (or Nd as concentration of the electron/holes atdark conditions) and polarizability of the moleculeis determined by Clausius-Mosotti relation [15]:03)2()1(εα=+ε−ε dddd NFrom this equation it may be found relative di-electric constant as:0031321εα−εα+=εddddd NNand similarly for dielectric constant under illumina-tion (ε):0031321εα−εα+=ε NNifddd NNCCεεα−εα+=εε=0031321Actually in the eq. 6 the N may be considered asconcentration of electrons/holes under llumination.The product (Nα) depends on intensity of illumina-tion (J) of light, i.e. with increase of illuminationconcentration of electrons/holes and, probably, mo-bility of charge carriers will change as well. If weassume thatNα = Ndαd(1 + kJ)where k, we may call, photocapacitive factor. Theeq. 7 may be rewritten as the following empiric ex-pression that may be used for simulation of relativecapacitance-illumination intensity relationships ofphotocapacitive sensor:Fig. 5. Relative capacitance – illumination relationshipfor the Ag/CuPc/Ag surface-type photocapacitive sensorat high levels of illumination (up to 10000 lx): 1 and 2under illumination from back and face sides.0 2000 1000000.20.40.60.81.01.21.44000 6000 800012Relative capacitance, /CCdIllumination, lxAs is known [15] the capacitance value dependson polarizability of the material and basically thereare several sources of it as dipolar αdip, ionic αi andelectronic αe polarizability. In this case we assumethat dipolar (αdip) polarizability due to absence of visi-ble dipoles in, molecular structures of the investigatedmaterials may be neglected. Electronic polarizabilityis most universal and arises due to relative displace-ment of the orbital electrons. As the CuPc may com-prise internal charge-transfer complex we can assumethat ionic polarization takes place. The ionic and elec-tronic polarizability probably affect to dark capaci-tance at low frequency (1 kHz) measurements ofcapacitance. In [16-18] it was investigated the polar-izability due to the transfer (αtd) of charge carriers aselectrons and holes that are present at normal, includ-ing at dark conditions. Therefore we may write forthe total polarizability at dark conditions (αd)αd = αi + αe + αtdFor the illuminated samples conditions the totalpolarizability (α) may be the following:α = αi + αe + αt(2)(3)(4)(5)(6)(7)(8)Copper Phthalocyanine Surface-type Photocapacitive SensorEurasian ChemTech Journal  9 (2007) 217-221220dddddd kJNkJNCCεε+α−ε+α+=003)1(13)1(21From eq. 9 it may be calculated the product Ndαd =1.3×10-11 F/m in the case of dark conditions for theAg/CuPc/Ag sensor (J = 0, εd = 4 and Cd = 17 pF).Figure 4 shows simulated relative capacitance-il-lumination relationship compared with results ob-tained in experiment for the Ag/CuPc/Ag sensor: itis seen that both graphs show reasonable agreement.In this case k = 3.5×10-5 (1/lx).Figure 6 shows detailed (a) and simple (b) equiva-lent circuits of the surface-type photocapacitive sen-sor. Detailed circuit contains actually three basiccapacitances with three kinds of dielectric: Ca capaci-tance with air, Cd is capacitance with CuPc at darkcondition, Cph is CuPc capacitance at illumination andCg is glass capacitance. Where Gd is dark conduc-tance of CuPC, Gph is the conductance of CuPc underluminance and Gg is the glass conductance. Simplecircuit Figure 6b shows Cd is capacitance with CuPcat dark condition, and Cph is CuPc capacitance at lu-minance, Gd is dark conductance of CuPC, Gph is theconductance of CuPc under luminance. Assuming thatproperties of the surface-type capacitive detector, firstof all, depend on properties of photosensitive organicsemiconductor copper phthalocyanine, we developedsimplified equivalent circuit Fig. 6b as well. Theequivalent circuit reflects the point that photocapaci-tance and photoconductivity phenomena have a com-mon physical reason as photogeneration of electron/holes under illumination.ConclusionsThe properties of the surface-type Ag/CuPc/Agphotocapacitive sensor was investigated. It was foundthat under filament lamp illumination up to 1000 lxthe capacitance of the Ag/CuPc/Ag sensor increasedto 20% with respect of dark condition. At further in-crease of illumination the photocapacitance increaseda few and even decreased at 6000-10000 lx. It meansthe sensor show higher sensitivity at low illumina-tion. The capacitive sensor can detect from two sides:face and back sides. It is assumed that in general thephotocapacitive response of the sensor is associatedwith polarization due to the transfer of photo-gener-ated electrons and holes. Calculated results reason-ably matched with experimental. Equivalent circuitsof the sensor were developed.AcknowledgmentsWe are thankful to GIK Institute of EngineeringScience and Technology for the support extended tothis work.References1. Physics of Organic Semiconductors, Edited byW.Brutting, Wiley-VCH Verlag GmbH &Co.KGaA, Weinheim, 2005.2. C.D. Dimitrakopoulos, D.J. Mascaro, IBM J.Res. & Dev., Vol. 45, No. 1, (2001), pp. 11-27.3. Kh.S. Karimov, M.M. Ahmedov, S.A. Moiz,M.I. Fedorov, Solar Energy Materials & SolarCells, Vol. 87, (2005), pp. 61-75.(9)Fig. 6 Detailed (a) and simple (b) equivalent circuits of the Ag/CuPc/Ag surface-type photocapacitive sensor.CaCdCphGdGphCgCgaCdGdGphCphbKh.S. Karimov et al.Eurasian ChemTech Journal  9 (2007) 217-2212214. S.A. Moiz, M.M. Ahmed, Kh.S. Karimov, Japa-nese Journal of Applied Physics, Vol. 44, No. 3,(2005), pp. 1199-1203.5. T. Miyasaka, T. Murakami, Applied PhysicsLetters, Vol. 85, No. 17, (2004), pp. 3932-3934.6. T. Murakami, N. Kawashima, T. Miyasaka, Chem.Commun, (2005), pp. 3346-3348.7. Kh.S. Karimov, Kh. Akhmedov, J. Valiev, I. Ho-midov, Proc. 6-th International Symposium onAdvanced Materials (1999), pp.103-106.8. A.M. Fomin, Kh.M. Akhmedov, Kh.S. Karimov,Yu.N. Porshnev, M.G. Chauser, V.M. Misin, M.I.Cherkashin, Technology of photocapacitor fab-rication, Patent No. 1581101, USSR, 1990.9. Yu.N. Porshnev, Kh.M. Akhmedov, Kh.S. Kari-mov, M.G. Chauser, V.M. Misin, M.I. Cherka-shin, Material for photocapacitor, Patent No.1598742, USSR, 1990.10. F. Gutman, L.E. Lyons, Organic Semiconduc-tors, Robert E. Krieger Publishing Company,Malabar, Florida, 1980.11. F. Gutman, H. Keyzer, L.E. Lyons, R.B. Somo-ano, Organic semiconductors, Robert E. KriegerPublishing Company, Malabar, Florida, 1983.12. M.K. Debe, R.I. Poirer, D.D. Erickson, T.N. Tom-met, D.R. Field, K.M. White, Thin Solid Films186 (1990) pp.257-281.13. M. Madou, Fundamentals of Microfabrication,CRC Press, Boca Raton, New York, 1995.14. G.I. Epifanov, Y.A. Moma, Solid State Electron-ics, V. Shkola, Moscow 1986.15. M. Ali Omar, Elementary Solid State Physics:Principles and Applications, Published by Pear-son Education (Singapore) Pte. Ltd., 2002.16. Organic Semiconductors, Edited by V.A. Kargin,Nauka, Moscow, 1968.17. M. Iwamoto, T. Manaka, Proc. Int. Symp. Su-per-Functionality Organic Devices, IPAP Conf.Series 6, (2005), pp. 63-68.18. F. Amy, C. Chan, A. Kahn, Organic Electronics,Vol. 6, (2005), pp.85-91.Received  12 May  2006.

Copper Phthalocyanine Surface-type Photocapacitive Sensor

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Copper Phthalocyanine Surface-type Photocapacitive Sensor

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