Al Schottky devices

ARTICLE IN PRESS

Journal of Physics and Chemistry of Solids 68 (2007) 556–560 www.elsevier.com/locate/jpcs

Determination of electrical and solar cell parameters of FTO/CuPc/Al Schottky devices K.R. Rajesha,, Shaji Vargheseb,c, C.S. Menonc a

Department of Metallurgical Engineering and Material Science, Indian Institute of Technology Bombay, Mumbai 400 076, India b Department of Physics, St. Thomas College, Kozhencherry, 689641 Kerala, India c School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, 686560 Kerala, India Received 5 August 2006; received in revised form 24 December 2006; accepted 16 January 2007

Abstract A Schottky structure is fabricated using CuPc sandwiched between fluorinated tin oxide (FTO) and aluminium electrodes. The electrical properties of the device are measured at room temperature. Permittivity of the device is calculated from capacitance measurements. The saturation current density, J 0 ¼ 5:1  104 ðAmp=m2 Þ, diode ideality factor, n ¼ 3:02 and barrier height, j ¼ 0:84 eV are determined for the Schottky juction. Reverse bias ln J versus ln V 1=2 is interpreted in terms of Schottky emission. Solar cell parameters are determined from the J–V characteristics. Power conversion efficiency, Z of 0.0024% is obtained for the cell. Band gap energy of the material is determined from UV-visible absorption spectrum. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Semiconductors; A. Thin films; B. Vapor deposition; D. Electrical properties; D. Transport properties

1. Introduction Solid-state solar cells based on organic semiconductors have attracted much interest because of the motivation for developing inexpensive, efficient and renewable energy sources [1,2]. Phthalocyanines (Pc) are organic semiconductors that have attracted lots of attention for applications in various organic electronic devices, such as solar cells [3], light-emitting diodes [4], gas sensors [5] and transistors [6]. In solar cell applications, Pc materials can be either used alone in a Schottky barrier structure [7,8] or used together with other n-type materials in a multilayer structure [9]. Although multilayer solar cells show more promising performance in terms of higher power conversion efficiency, Schottky barrier solar cells are convenient for investigation of the influence of materials properties, active layer thickness, and electrode properties on the solar cells performance. The Schottky cell is a single layer device, which exhibits optimal performance when one contact is Corresponding author. Tel.: +91 94472 89696; fax: +91 481 2730423.

E-mail address: rajthinfi[email protected] (K.R. Rajesh). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.01.023

ohmic while the other is a barrier contact. Many Pc’s show interesting photoconductive and photovoltaic responses and hence have been widely used in Schottky barrier cells [10–13]. Among these Copper phthalocyanine (CuPc) is a promising organic semiconductor for photovoltaic applications. High work function materials such as gold and ITO, have been used by various workers to make ohmic contact with Pc’s [13–16]. CuPc is a p-type organic semiconductor with direct band gap and high optical density. Aluminium is usually used as the blocking contact in Pc based Schottky devices [8,13,17,18]. The band gap and ionization potential can be tuned to the desired energies by modifying the chemical structure and solar cells made from organic semiconductors are commonly lightweight and flexible. From the available literature it is seen that not much work has been performed on the junction properties of CuPc thin films using FTO and aluminium contacts. FTO is a commonly used substrate for polymeric solar cells [19,20], since it is more resistant to the exposure to temperatures above 300  C compared to ITO [21]. In this communication we report the basic electrical and solar cell parameters of FTO/CuPc/Al devices. This work may give a general idea

ARTICLE IN PRESS K.R. Rajesh et al. / Journal of Physics and Chemistry of Solids 68 (2007) 556–560

2. Experimental details The CuPc powder obtained from Aldrich Inc. USA is used as the source material. Thoroughly cleaned FTO glass substrates with surface sheet resistance of 10 O=& are used as the substrates. For optical and structural studies precleaned micro glass slides are kept adjacent to FTO substrates. Thin film samples are prepared by thermal evaporation at a base pressure of 105 Pa. The thickness of the films are measured using the Tolansky’s multiple beam interference technique [22]. Above CuPc film, aluminium of thickness 200 nm is deposited. The thickness of the CuPc samples are in the range 250–461 nm. The rates of deposition are typically 0:5 nm s1 . The area of each sample studied is 2:25  106 m2 . Capacitance measurements are performed using a Hioki 3532 LCR Hi-tester. In the present work all the measurements are performed in air, since no rectification is observed in Schottky structures performed in vacuum [23]. UV-VIS-NIR absorption spectrum of the film is recorded using the Shimadzu 160A spectrophotometer. X-ray diffractogram is recorded using a Philips Analytical diffractometer (model PW 3710). Electrical and photosensitivity measurements are performed using the Keithley 236 Source Measure Unit. The sample is illuminated using a tungsten halogen lamp (white light). The intensity of the light source used is 100 mW cm2 with an IR filter and a water column in between to avoid heating the sample.

and antibonding molecular orbitals. The absorption peaks at higher energy region ð383 nmÞ and lower energy region ð700 nmÞ results from B (Sorret) band and Q-band, respectively. Both Q and B bands arise from p to p transitions [24]. The absorption ðaX104 cm1 Þ is related to direct band transitions [25]. The band gap is determined by plotting a2 against hn and found to be 3.09 eV. This is comparable with the reported band gap 3:10  0:02 eV for CuPc [26]. XRD pattern of CuPc thin film of thickness 461 nm deposited at room temperature is shown in Fig. 2 . It exhibits a crystalline nature. The preferential orientation of CuPc is (1 0 0) at 2y ¼ 6:60 . Grain size of the crystallites are found to be 20 nm. A typical I–V plot of FTO/CuPc/Al structure is shown in Fig. 3. It exhibits characteristic diode behavior.

800

(100)

600 Intensity (CPS)

of the role of FTO on the electrical property of organic thin film devices; thereby we can establish that it can play as competitor for ITO.

557

400

200

0 5

3. Results and discussion

10

15

20

25

The absorption spectrum of CuPc thin film deposited at room temperature is given in Fig. 1. The distinct characterized peaks in the visible region are generally been interpreted in terms of p–p excitation between bonding

Fig. 2. XRD pattern of CuPc thin film deposited on glass substrate at room temperature.

3500 0.6

3000

0.4

2000

I (nA)

Absorbance

2500

0.2

1500 1000 500

0.0 300

450

30

2 Theta (Degree)

600

750

900

Wavelength (nm) Fig. 1. Absorption spectrum of CuPc deposited at room temperature.

-4

0 -2 0 -500

2

4

6

8

V (V)

Fig. 3. I–V characteristics of FTO/CuPc/Al.

ARTICLE IN PRESS K.R. Rajesh et al. / Journal of Physics and Chemistry of Solids 68 (2007) 556–560

558

We therefore assume that the interface quality is sufficient for photovoltaic energy conversion. The forward bias direction corresponds to the situation when the bottom FTO electrode is positive. The FTO electrode having a high work function, F ¼ 4:84 eV forms an ohmic contact with the Pc layer. The rectification observed is due to the blocking contact or a Schottky barrier which is formed at the CuPc/Al interface, where the conduction and valence band edges bend downward at equilibrium. This behavior can be explained by the low work function of Al ðF ¼ 4:1 eVÞ [27] and high work function of FTO and by the p-type conduction of CuPc. The rectifying behavior of a Schottky barrier diode is assumed to follow a standard thermionic emission theory for conduction across the junction. Based on this theory the current voltage relationship can be expressed as [28] J ¼ J 0 ½expðeV =nkTÞ  1,

(1)

where J is the instantaneous current density, J 0 is the saturation current density, e is the electronic charge, V is the applied potential, n is a constant called diode ideality factor, k is the Boltzmann constant and T the absolute temperature. Since eV =nkTb1 a semilogarithmic plot of current density versus applied voltage is expected to be linear with a Y intercept corresponding to J 0 . Fig. 4 shows the ln J versus V plot for the FTO/CuPc/Al diode. From the figure it is clear that a linear relationship exist for smallapplied voltages. For large-applied voltages, i.e. above 2.2 V the graph deviates from linearity (not shown in the figure). The voltage drop across the series resistance in the natural region of the semiconductor causes this deviation. From the slope value of n is calculated. The values for J 0 and n are found to be 5:1  104 ðAmp=m2 Þ and 3.02, respectively. The barrier height j at the injected electrode is calculated from the expression for saturation

current density J 0 ¼ A T 2 expðje=kTÞ

(2)



where A is the effective Richardson constant. The value of j is estimated to be 0.84 eV. Fig. 5 shows the dependence of capacitance C on the reciprocal film thickness, 1=d. Here the capacitance measurements are made at 1 kHz. The linearity of the plot can be analyzed in terms of the capacitance of a parallel plate capacitor C ¼ eA=d,

(3)

where e is the permittivity of Pc layer, A is the area ð2:25  106 m2 Þ and d is the thickness of the sample. The value of e is estimated from Fig. 5 and found to be 3:12  1011 Fm1 . This is in good agreement with the values in the range 2.12–4:5  1011 Fm1 reported for CuPc [29–31]. The reverse bias current-voltage characteristics give information about the properties of metal–semiconductor contact. The reverse current arises due to recombination of charge carriers, release of charge carriers from trap levels, barrier lowering at high electric field or leakage. Fig. 6 show the reverse bias ln J versus ln V 1=2 for CuPc at room temperature. The linear sections of the curve can be interpreted in terms of either the Schottky emission or Pool–Frenkel emission. For Schottky emission the current density J is expressed as follows [32,33]: J ¼ A T 2 expðj=kTÞ expðebs V 1=2 =kTd 1=2 Þ,

(4)

where A is the effective Richardson constant, j is the Schottky barrier height at the injected electrode and bs is the Schottky field lowering constant. For Pool–Frenkel emission, the current density is given by J ¼ J pfo expðbpf V 1=2 e=kTd 1=2 Þ,

(5)

where bpf is the Pool–Frenkel coefficient. -4

0.28 0.26 Capacitance (nF)

lnJ (Amp/m2)

-6

-8

0.24 0.22 0.20 0.18

-10

0.16 0.14 0.0

0.5

1.0 V (V)

1.5

Fig. 4. ln J vs V for FTO/CuPc/Al.

2.0

2.0

2.5

3.0

3.5 6

-1

1/ d x10 (m )

Fig. 5. C vs 1=d for CuPc.

4.0

ARTICLE IN PRESS K.R. Rajesh et al. / Journal of Physics and Chemistry of Solids 68 (2007) 556–560

Fig. 6, Fill factor (FF) of the device is calculated and found to be 0.25. The power conversion efficiency of the solar cell is calculated using the relation [7]

0 -2

Z ¼ ðJ sc  V oc =I T Þ  FF. -4

lnJ (Amp/m2)

559

I T is the intensity of the solar light transmitted through the electrode. For CuPc solar cells using ITO and Al electrode, an FF of 0.18 at 98 mW=cm2 is reported [13]. The low FF, obtained in the present study, compared to inorganic solar cells can be attributed to the high resistance of Pc layer. Power conversion efficiency Z of the device is determined to be 0.0024 %. The reported power conversion efficiency of CuPc for white light is 0.001% [3]. Kwong et al. [13] has reported a power conversion of 0.004% for ITO/CuPc/Al device under AM 1 excitation. A higher Z is expected with FTO electrode, since it can form a low contact potential barrier with CuPc.

-6 -8 -10 -12 1.0

1.1

1.2

1.3

1.4

1.5

(7)

1.6

V1/2 (V1/2)

Fig. 6. ln J vs V 1=2 for FTO/CuPc/Al.

4. Conclusions 1

V (V) 0 J (mA/m2)

-0.2

0.0

0.2

0.4

0.6

0.8

-1

-2

illuminated dark

-3 Fig. 7. J–V curves for dark and illuminated FTO/CuPc/Al devices.

Current–voltage measurements on FTO/CuPc/Al structure show characteristics of typical Schottky-barrier devices. The rectifying behavior of the device is explained using thermionic emission theory, and the basic diode parameters are determined. From the dependence of capacitance C on the reciprocal film thickness the permittivity of the samples are calculated. The reverse bias curve is interpreted in terms of Schottky emission. In this work we have performed the solar cell characterization of CuPc using FTO and Al electrodes. The photovoltaic parameters of the device are determined. It is found that FTO electrode can enhance power conversion efficiency of the device. It is expected that the use of metals with lower work function, such as commonly used cathodes in organic light-emitting diodes, for a barrier contact would further improve the performance of the Pc based Schottky solar cells. Acknowledgments

The theoretical values of these coefficients are given by 2bs ¼ bpf ¼ ðe=peÞ

1=2

.

(6)

The theoretical values bs and bpf of are found to be 2:02  105 and 4:04  105 . The value of b calculated from Fig. 6 is found to be 2:64  105 and is in close agreement with the expected Schottky value. Thus in the case of the fieldlowering behavior, Schottky emission is identified for the voltage range in which the sample is studied. Similar results are obtained for ITO/CuPc/Al devices [13,34]. The device is illuminated through FTO electrode with the intention that more light energy is transmitted to the depletion region, and this in turn can increase the amount of exciton generation. Fig. 7 shows the J–V characteristics of dark and illuminated FTO/CuPc/Al Schottky device. We have obtained an open-circuit voltage V oc ¼ 0:44 V and short-circuit current density J sc ¼ 2:2 mA=m2 . From

The authors would like to thank Prof. K.P. Vijaya Kumar and Dr. Tenny Theresa John, Department of Physics, Cochin University of Science and Technology, India for providing experimental facilities for the photovoltaic measurements. References [1] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. [2] M. Granstro¨m, K. Petritsch, A.C. Arias, A. Lux, M.R. Andersson, R.H. Friend, Nature 395 (1998) 257. [3] C.C. Leznoff, A.B.P. Lever, Phthalocyanines, Properties and Applications, VCH Publishers Inc., New York, 1996, p. 219. [4] X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, K. Leo, Appl. Phys. Lett. 78 (2001) 410. [5] M.I. Newton, T.K.H. Starke, M.R. Willis, G. McHale, Sensors Actuators B 67 (2000) 307. [6] C.M. Joseph, C.S. Menon, Mater. Lett. 51 (2001) 200.

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