Electrical and Optical Response Properties of MEH-PPV 

Semiconductor Polymer Schottky Diodes

by Fred Semendy, Greg Meissner, and Priyalal Wijewarnasuriya

ARL-TR-5591 July 2011

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Army Research Laboratory

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ARL-TR-5591 July 2011

Electrical and Optical Response Properties of MEH-PPV 

Semiconductor Polymer Schottky Diodes

Fred Semendy, Greg Meissner, and Priyalal Wijewarnasuriya

Sensors and Electron Devices Directorate, ARL

Approved for public release; distribution unlimited. 
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Electrical and Optical Response Properties of MEH-PPV Semiconductor 

Polymer Schottky Diodes

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Fred Semendy, Greg Meissner, and Priyalal Wijewarnasuriya

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14. ABSTRACT

Diodes made with poly(2-methoxy-5(2’-ethyl)hexoxy-phenylenevinylene) (MEH-PPV) using indium tin oxide (ITO) as anode 

and aluminum (Al) as cathode have been fabricated and investigated for current-voltage (I-V) characteristics and optical 

responsivity. The diode exhibits non-ideal behavior with ideality factor of 0.85 eV. The I-V characteristics of the ITO/MEHPPV/Al diode without light and with different illumination intensities give an open circuit voltage (Voc) and a short circuit 

current of (Isc). This suggests that that this diode is a photovoltaic under light illumination. Variations in the responsivity 

indicate that the ITO/MEH-PPV/Al device is a photo-sensitive diode.

15. SUBJECT TERMS

MEH-PPV, polymer, diode, Schottky

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 ABSTRACT

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Fred Semendy

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(301) 394-4627

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iii

Contents

List of Figures iv

1. Introduction 1

2. Experimental 1

3. Results and Discussion 3

4. Conclusion 5

5. References 6

Distribution List 7
iv

List of Figures

Figure 1. (a) Chemical structure of MEH-PPV and (b) a schematic diagram of ITO/MEHPPV/Al Schottky diode. .............................................................................................................2

Figure 2. I-V characteristics of a ITO/MEH-PPV/Al Schottky diode............................................2

Figure 3. Plot of vs. I for the ITO/MEH-PPV/Al diode.................................................4

Figure 4. Plot of photocurrent time for a constant voltage (I-V-t) for the ITO/MEH-PPV/Al 

diode...........................................................................................................................................4

Figure 5. Plot of photocurrent vs. light intensity of the ITO/MEH-PPV/Al diode.........................5
1

1. Introduction

The usage of soluble conjugate polymers as active materials in optoelectronic applications has

opened up the possibility of fabricating many different devices. Soluble conjugated polymers, 

with their advantage of low cost, flexibility, and high absorption coefficient, have shown the 

possibility for photodetection and photovoltaic applications. Over the years, the power 

conversion efficiency has been increasing steadily and rapidly (1–4). Organic semiconductors 

have been extensively used in electronic devices as active components. Conjugated polymers, 

known as organic semiconductors with electronic properties, have a wide application in 

electronic technology. In the past few years, poly(2-methoxy-5(2’-ethyl)hexoxyphenylenevinylene) (MEH-PPV) has been considered as one of the potential and useful 

conducting polymers for various optoelectronic applications, such as sensors and organic solar 

cells, and organic light emitting diodes (OLED) because of its environmental stability and easy 

conductivity properties. The MEH-PPV acts as an electron donor (p-type semiconducting 

polymer) with a relatively low conductivity due to its low hole and electron mobilities when 

compared to inorganic semiconductor materials (5). 

The fabrication and characterization of the Schottky diodes using organic semiconductor MEHPPVs and their derivatives have commanded considerable attention in recent years (6, 7). The 

device performance of a Schottky diode depends on electrical and electronic characteristics of 

metal organic semiconductor junction. In this study, an indium-doped tin oxide (ITO)/MEHPPV/aluminum (Al) Schottky diode was fabricated. The electronic properties, such as barrier 

height, ideality factor, series resistance, interface properties and the optical response behavior of 

the diode, are reported.

2. Experimental

The semiconducting polymer MEH-PPV was purchased from Sigma Aldrich Company. The ITO 

coated on glass substrate was obtained from Merck with a sheet resistance of 20 Ω cm, a 

thickness of 100 nm, and a transmittance of about 80% in the visible. The ITO substrate was 

cleaned in an ultrasonic bath of acetone for 20 min, followed by isopropyl alcohol (IPA) rinsing 

for 20 min at room temperature, before being dried in a nitrogen gas flow. MEH-PPV solutions 

were prepared in chloroform or 1.2 dichlorobenze solvents separately at a concentration of 

15 mg/ml. The solutions were mixed for 2 h by sonicating. Before spin casting the MEH-PPV 

solution, part of the ITO was masked for future metal contacts for wire bonding. The MEH-PPV 

solution was spin cast on an ITO/glass plate at 3000 rpm for 40 s. The film was dried at 50 °C 

for 10 min on a hot plate to evaporate the solvent. Film thickness was measured using a KLA-
2

Tencor 15 and found to be ~200 nm. The film was immediately placed in CHA e-beam 

evaporator for the metal deposition. At a vacuum of 2x10–6Torr, a 200-nm-thick Al film was 

deposited on top of the MEH-PPV layer. Figure 1 shows (a) the chemical structure of MEH-PPV 

and (b) the fabricated MH-PPV diode.

(a) (b)

Figure 1. (a) Chemical structure of MEH-PPV and (b) a schematic diagram of ITO/MEH-PPV/Al Schottky diode.

Figure 2 provides the current-voltage (I-V) characteristics of the ITO/MEH-PPV/Al diode. 

Figure 2. I-V characteristics of a ITO/MEH-PPV/Al Schottky diode.

-2.00E-06

0.00E+00

2.00E-06

4.00E-06

6.00E-06

8.00E-06

1.00E-05

1.20E-05

1.40E-05

-3 -2 -1 0 1 2 3 4 5 Current (A)

Voltage ( V)

ITO

MEH-PPV

ITO/MEH-PPV/Al Polymer Schottky 

Diode

No light

10 mW

20 mW

30 mW

Al

Glass Substrate
3

3. Results and Discussion

I-V measurements shown in figure 2 were made with Agilent 4156 C parametric analyzer. The 

diode shows a rectifying behavior and the rectification ratio, which is the ratio of the forward 

current to reverse current at the certain voltage, was found to be 2.22 x 105 at ±3 V. The I-V

characteristics of the diode can be expressed by the following relation (7, 8):

I = I0 exp 

 

 

 

 , (1)

where Rs is the series resistance, V is the applied voltage, n is the ideality factor, k is the 

Boltzmann constant, T is the temperature, and I0 is the reverse saturation current given by 

I0 = AA** T

2

exp –

 

 , (2)

where is the barrier height and A is the contact area, and A

** is the Richardson constant. 

The diode indicates a non-ideal behavior due to the series resistance. For low voltages, the diode 

exhibits a linear behavior, but at higher voltages it deviates due to the series resistance and 

interface state density. To determine the diode parameters, Cheung’s method (9, 10) was used to

determine the diode’s parameters. Cheung’s functions are defined as follows:

 

 

= n

 

 

+ IRs (3)

and

H(I) = V – n

 

 

 ln 

 

 

 = IRs + n . (4)

The plots of 

 

versus I are given on figure 3 and show a straight line behavior due to the 

series resistance region. The series resistance is found to be higher due to the resistance of the 

polymer MEH-PPV organic semiconductor. The polymeric semiconductor has low mobility and 

electrical conductivity with respect to an inorganic semiconductor.
4

 

Figure 3. Plot of 

 

vs. I for the ITO/MEH-PPV/Al diode.

The photoresponse on/off characteristics of ITO/MEH-PPV/Al diode under different 

illuminations for a voltage of 0.8 V on a timed scale (I-V-t) are shown in figure 4. The diode 

exhibits a good photoconductivity response and shows a sudden change as the light turns on. 

This behavior is abrupt and the increase is noticable in very short time, and the photocurrent 

under light shows a stable plateau value. The photocurrent was found to be 2.01 μA at 0.8 V for 

an optical power of 10 mW, where as the dark current was nearly 0.26 μA without light. Thus,

the photocurrent was nearly 1.8 μA more than the dark current of the diode with the light on 

This shows that the photocurrent is drastically increased. 

 

Figure 4. Plot of photocurrent time for a constant voltage (I-V-t) for the ITO/MEH-PPV/Al diode.

0

1

2

3

4

5

-5.00E-06 1.70E-19 5.00E-06 1.00E-05 1.50E-05 2.00E-05 dV/dln((I)

Current (A)

1.50E-06

1.70E-06

1.90E-06

2.10E-06

2.30E-06

2.50E-06

2.70E-06

2.90E-06

0 10 20 30 40 50

Current ( A) 

Time (s) 

Series1

Series2

Series3

Light On/Off photocurrent response 

plot of ITO/MEH-PPV/Al diode

 Plot of dV/dln(I) vs I for ITO/MEH-PPV/Al Polymer 

Diode

10 mW

30 mW

 at 0.8 Volts

20 mW
5

Figure 5 gives the photoresponsivity for various light intensities. In each case, the calculated 

responsivity was 2 x 10–4 A/W for a 10-mW light, 1.1 x 10–4 A/W for 20-mW light, and 

0.8 x 10

–4 A/W a 30-mW light. This indicates that the responsivity is decreasing as the power of 

the light is increased. However, to increase the photoconductive properties, one may use blended 

structures with fullerene or methanofullerene 6,6-phenyl C-61-butyric acid methyl ester (PCBM)

as well as use additional light enhancement techiniques such as harnessing the plasmonic 

properties of silver or gold nanoparticles on the substrate.

 

Figure 5. Plot of photocurrent vs. light intensity of the ITO/MEH-PPV/Al diode.

4. Conclusion

MEH-PPV was used with ITO as anode and Al as cathode to fabricate diodes. The fabricated

devices were investigated for I-V characteristics and optical responsivity. The diode exhibited 

non-ideal behavior with an ideality factor of 0.85 eV. The I-V characteristics of the ITO/MEHPPV/Al diode without light and with different illumination intensities give an open circuit 

voltage (Voc) and a short circuit current of (Isc). This result suggests that that this diode is 

photovoltaic under light illumination. Additionally, variations in the responsivity indicate that 

the ITO/MEH-PPV/Al device is a photo-sensitive diode. Studies of the electrical properties 

indicate that the photoconductive properties can be further improved by using blended materials

(fullerene or PCBM) and incorporating plasmonic properties of silver or gold nanoparticles to 

enhance the absorption of light.

1.70E-06

1.90E-06

2.10E-06

2.30E-06

2.50E-06

2.70E-06

0 10 20 30 40

Photo Current(A) 

Power ( mW/cm2)

Photo current response for three 

light power of 10, 20 and 30 mW
6

5. References

1. Stallinga, P.; Gomes, H. L.; Murgia, M.; Mullen, K. Org. Electro 2002, 3, 43.

2. Lei, J.; Liang, W.; Brumlik, C. J.; Mattlin, C. R. Synth. Met 1992, 47, 351.

3. Hall, J.J.M.; Walsh, C. A.; Greenham, N. C.; Maserglia, E. A.; Friend, R. H.; Moratti, S. C.; 

Holmes, A. B. Nature 1995, 376, 498.

4. Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F. A.; Heeger, J. Science 1995, 270, 1789.

5. Aydin, M. E.; Yakuphanoglu, F.; Eom, J. H.; Hwang, D. H. Physica B 2007, 387, 239.

6. Willander, M.; Assadi, A.; Svensson, C. Synth. Met 1993, 55, 4099.

7. Rhoderick, E. H.; Williams, R. H. Metal-Semiconductor Contacts; Clarendon: Oxford, 1988.

8. Okur, S.; Yakuphanoglu, F.; Ozoz, M.; Kadayiffcilar, P. K. Microelectron. Eng 2009, 86, 

2305.

9. Cheung, S. K.; Cheung, N. W. Appl. Phys. Lett. 1986, 49, 85.

10. Pierret, R. F. Semiconductor Device Fundamentals; Addison Wesley Publishing Company:

New York, 1996
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