We fabricated and studied the electrical and photovoltaic properties of organic solar cell based on poly (3-hexylthiophene) (P3HT) as an electron donor blended with the acceptor [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester (modified fullerene). The active layer composed of (3:1, w/w) mixture of P3HT and the modified fullerene was sandwiched between indium tin oxide (ITO) and aluminum (Al). The ideality factor n and barrier hight b values were determined from the dark current density-voltage characteristics and found as 2.45 and 0.78 eV, respectively. The device shows photovoltaic behavior with an open circuit voltage of 400 mV, short circuit current of 22.9 A/cm2 and fill factor 0.32 under 2.8 mW/cm2 light intensity.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3194

Omer, B.M.

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 5 No 3, 03011(4pp) (2013) Том 5 № 3, 03011(4cc) (2013) 
 
 
2077-6772/2013/5(3)03011(4) 03011-1  2013 Sumy State University 
Photovoltaic Properties of Poly (3-Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-
Ethylthiophene Thin Films  
 
B.M. Omer1,2,* 
 
1 Department of Physics, College of Science and Arts-Ranyah, Taif University, Kingdom Saudi Arabia 
2 Department of Applied Physics, Faculty of Applied Science and Computer, Omdurman Ahlia University,  
P.O.Box 786, Omdurman-Sudan 
 
(Received 15 February 2013; published online 12 July 2013) 
 
We fabricated and studied the electrical and photovoltaic properties of organic solar cell based on poly (3-
hexylthiophene) (P3HT) as an electron donor blended with the acceptor [6, 6]-Phenyl C61-Butyric Acid 3-
Ethylthiophene Ester (modified fullerene). The active layer composed of (3:1, w/w) mixture of P3HT and the 
modified fullerene was sandwiched between indium tin oxide (ITO) and aluminum (Al). The ideality factor n 
and barrier hight b values were determined from the dark current density-voltage characteristics and found 
as 2.45 and 0.78 eV, respectively. The device shows photovoltaic behavior with an open circuit voltage of 
400 mV, short circuit current of 22.9 A/cm2 and fill factor 0.32 under 2.8 mW/cm2 light intensity. 
 
Keywords: Poly3-Hexylthiophene, Modified fullerene, Organic solar cell, Open circuit voltage. 
 
  PACS numbers: 78.66.Qn,78.40.Me 
 
 
                                                                
* bushra_omer4@yahoo.com  
1. INTRODUCTION 
 
Since the discovery of the conjugated polymers, rec-
tifying junctions such as p-n junction and Schottky 
junctions have been studied as to explore the possible 
application of conjugated polymers [1-4]. Polymer solar 
cells have attracted considerable attention due to their 
low cost, lightweight [5], as well as their potential ap-
plication in flexible large area devices [6]. 
Despite the low cost, the power conversion efficiency 
of bulk hetrojunction devices is low compared to inor-
ganic solar cells. Recently Yong Cao et al reported an 
efficiency of 9.2 % for a photovoltaic cell with a bulk 
heterojunction inverted structure [7]. Bulk heterojunc-
tion photovoltaic consists of an intimate blend of a con-
jugated polymer and an electron acceptor that form an 
interpenetrating network with percolated paths to both 
electrodes [8]. When light is incident on the device, 
excitons are created on the polymer chains. If an exci-
ton is able to diffuse within its lifetime to an electron 
acceptor, it will be quenched with near unit quantum 
yield, creating a spatially separated electron-hole pair 
with the hole localized on the polymer chain and the 
electron on the acceptor molecule. Under the influence 
of the built-in electric field, these initially bound charg-
es can then separate and migrate to the electrodes, 
thus producing a photocurrent. 
So far, research efforts in solution-processed organic 
photovoltaic materials have been dominated by the use 
of one particular fullerene derivative, [6, 6]-Phenyl C61-
Butyric acid methyl ester (PCBM), as the electron ac-
ceptor. But PCBM is not necessarily the optimum ac-
ceptor for solution processed organic photovoltaic ma-
terials. The preparation of different derivatives of C60 
fullerenes and their use in bulk heterojunction polymer 
solar cells was reported by different authors [9-11].  
To our knowledge there is no published study on the 
photovoltaic properties of P3HT: [6, 6]-Phenyl C61-
Butyric Acid 3-Ethylthiophene Ester (Modified full-
erene) thin films. In this paper, we report the fabrica-
tion and characterization of bulk heterojunction solar 
cell based on Poly (3-Hexylthiophene) and [6, 6]-Phenyl 
C61-Butyric Acid 3-Ethylthiophene Ester with (3:1) 
composite ratio. 
 
2. EXPERIMENTS 
 
The organic photovoltaic cell in this study consists of 
a layer of poly (3-Hexylthiophene) / [6, 6]-Phenyl C61-
Butyric Acid 3-Ethylthiophene Ester blend thin film 
sandwiched between transparent anode indium tin oxide 
(ITO) and metal cathode aluminum (Al). The p-type do-
nor polymer P3HT and the n-type acceptor modified full-
erene in the active layer were purchased from sigma-
Aldrich Corporation and American Dye Source, Inc. re-
spectively; the chemical structures are shown in Fig. 1. 
All materials were used as received without further puri-
fication. Before device fabrication the ITO coated glass 
substrate was first etched and patterned to serve as an-
ode. The substrate was cleaned with Isopropyl alcohol 
and acetone in an ultrasonic bath each for 15 min se-
quentially. To make a P3HT/modified fullerene (3:1, 
w/w) solution, P3HT (16.26 mg) and modified fullerene 
(5.48 mg) were dissolved in 1,2 dichlorobenzene (concen-
tration 21.74 mg/ml). The blend was spin coated onto the 
substrates with atypical spinning speed and time at 
2000 rpm for 40 s. Finally Al metal was vacuum-
deposited to form the cathode. The active area of the 
device, defined by shadow mask, was 0.07 cm2 .The 
structure of the device is shown in Fig. 2. 
For the UV-vis absorption measurements, the solu-
tions were spin coated onto quartz substrates, the 
properties of the relevant films were obtained on a 
Lambda-900 UV/vis/NIR spectrophotometer (Perkin 
Elmer). The current-voltage (I-V) curves were meas-
ured with a Keithley 238 source measurement unit. All 
thin films preparation and measurements were carried 
out at room temperature in air.  
 B.M. OMER J. NANO- ELECTRON. PHYS. 5, 03011 (2013) 
 
 
03011-2 
     
 
Fig. 1 – Chemical structure of (a) poly (3-Hexylthiophene) and 
(b) [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester 
 
 
 
Fig. 2 – Schematic of the polymer solar cell device structure 
 
3. RESULTS AND DISCUSSIONS  
 
Fig. 3 shows the absorption of thin films made from 
pure poly (3-Hexylthiophene), pure [6, 6]-Phenyl C61-
Butyric Acid 3-Ethylthiophene Ester and (3:1) poly (3-
Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-
Ethylthiophene Ester composite at room temperature. 
All the thin films were spin coated from 
1,2 dichlorobenzene. The absorption maximum ( max) of 
P3HT can be observed at 556 nm (2.23 eV) and two 
shoulders appeared at 532 nm and 605 nm. The optical 
band gap of P3HT is determined with a tangential 
through the turning point of the low energy side of the 
spectra, leading to estimated value of 1.89 eV. The  
absorption maximum of P3HT in the composite films 
was found to be around 574 nm (18 nm red shifted 
compared to pure P3HT), the shoulders appeared at 
542 nm and 623 nm. From the absorption spectra of the 
composites it was very clear that the two peaks at 
(281 nm and 354 nm) are belong to the modified fuller-
ene. It was noticeable that the absorption spectra of the 
composites are just superpositions of the absorption 
spectra of the individual components.  
 
 
 
Fig. 3 – Absorption spectra of pure P3HT, pure modified Full-
erene and P3HT: modified Fullerene (3:1) composite films 
 
Fig. 4 shows the current density-voltage character-
istics of the device in dark; we observe that the forward 
current is exponentially increased with applied voltage; 
this is an indication of the diode characteristics. The 
device shows a high rectification ratio ( ) of about 392 
at (  1.5 V), the diode is regarded as well rectified. 
 
 
 
Fig. 4 – Current density-voltage characteristics under dark 
for (3:1) P3HT / modified fullerene device structure 
 
Fig. 5 shows a semi-logarithmic plot of the current 
density versus the applied voltage of the device, in dark 
at room temperature. The general diode equation, 
 
 J  J0 [exp (qV / nKT) – 1], (1) 
 
describes the J-V characteristics of both p-n junctions 
and Schottky diodes. Here, V is the applied voltage, n is 
the ideality factor, q is the electron charge, K is the 
Boltzmann constant, T is the temperature and J0 is the 
saturation current derived from the straight line inter-
cept of ln J at V  0 and is given by: 
 
 J0  A* T2 exp (– q b / KT), (2) 
 
where A*  120 A/cm2 K2 is the effective Richardson 
constant for an electron in free space and b the zero-
bias potential barrier height. The ideality factor n is 
unity in the Shockly theory for p-n junctions in the ab-
sence of recombination, as well as for thermionic emis-
sion theory and diffusion theory for Schottky diodes. 
The slope of the logarithmic plot is related to the ideali-
ty factor n through: 
 
 1 / n  (KT / q) (d (ln J) / dV), (3) 
 
Once J0 is found, we can find the barrier height per 
unit charge using the relation: 
 
 Φb  (KT / q) ln(A* T2 / J0), (4) 
 
From ln J versus V characteristics in the dark, 
shown in Fig. 5 and making use of equations (1), (2), (3) 
and (4) the value of the saturation current determined 
as 3.16  10 – 9 A/cm2. The ideality factor n and the bar-
rier hight b was found to be 2.45 and 0.78 eV, respec-
tively. The deviation of the ideality factor of the dark 
current from unity may be due to a trap-assisted re-
combination. 
At high bias the outflow rate of the injected carriers 
is reduced greatly and charges accumulate in the bulk 
of the semiconducting organic layer and that is due to 
low charge carrier mobility as well as the presence of  
ITO Coated Glass 
Photoactive layer 
Al 
a b 
 PHOTOVOLTAIC PROPERTIES OF POLY (3-HEXYLTHIOPHENE)… J. NANO- ELECTRON. PHYS. 5, 03011 (2013) 
 
 
03011-3 
 
 
Fig. 5 – Semi-logarithmic plot of Current density-Voltage 
characteristics in the dark 
 
traps. In this case the transport flow is dominantly 
controlled by the space charge limited current (SCLC) 
resulting into saturation of current. The relation be-
tween the current density and voltage in free-traps 
SCLC is given by the Childs law [12]: 
 
 J  (9/8)(ε  / d3)V2 (5) 
 
where  is the charge carrier mobility and d is the film 
thickness. 
In presence of traps with exponential trap distribu-
tion the SCLC takes a power law dependence of the 
form: 
 
 J  K Vm + 1 (6) 
 
where m is defined in terms of a characteristics trap 
distribution constant. 
Plot of the current density versus voltage at high 
forward bias, beyond 1 V is shown in Fig. 6. The cur-
rent density depends super linearly on the forward 
voltage. From the fit we could find the relation: 
 
 J  5.5  10 – 4V2.8 (7) 
 
The correlation coefficient was good (R  0.998). 
Comparing equation (7) with (6) we get K  5.5  10 – 4 
and m  1.8, this is indicative of electrical transport 
that is dominated by SCLC with exponential trap dis-
tribution. The overall parameters for the J-V curve 
under dark were tabulated in Table 1. 
 
Table 1 – The photovoltaic parameters under dark 
 
 
Fig. 7 shows the current density versus the voltage 
curves in dark and when the device was illuminated 
through the ITO (front) side with an incident light in-
tensity of about 2.8 mW/cm2 from a projector lamb. The 
current in the reverse direction is strongly increased by 
the illumination. This indicates that the light illumina-
tion increases the production of electron-hole pairs. At 
higher forward bias, the dark and photocurrent are 
quite similar, indicating that photoconductivity does 
 
 
Fig. 6 – Current density-Voltage characteristics at high bias 
in dark 
 
not play an important role in the device. The device 
shows photovoltaic behavior with an open circuit volt-
age of 400 mV, short circuit current of 22.9 A/cm2, fill 
factor of 0.32, and a power conversion efficiency of 
0.105 %. The short circuit current of our device is small 
because the work function of the ITO has a boor match-
ing with the highest occupied molecular orbital level of 
the P3HT. This in turn expected to decrease the whole 
transport and collection at the anode. According to the 
metal-insulator-metal (MIM) model for non-ohmic con-
tacts devices, the open circuit voltage is determined by 
the work function difference of the materials used for 
the electrodes, Al (4.3 eV) [6, 12] and ITO (4.7 eV) [6, 
12] results Voc in the range of 0.4 V, which agree very 
well with the experimentally observed open circuit 
voltage. 
 
 
 
Fig. 7 – Current density-voltage characteristics of 
P3HT / modified fullerene blend, 3:1 ratio 
 
4. CONCLUSIONS  
 
In summary, we have fabricated Poly (3-
Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-
Ethylthiophene photovoltaic cell. The dark current 
density-voltage characteristics indicates a non-ideal 
diode behavior with ideality factor n (2.45), barrier 
Parameters J0(A/cm2) Φb (eV)  n 
Value 3.16  10 – 9 0.78 392 2.45 
 B.M. OMER J. NANO- ELECTRON. PHYS. 5, 03011 (2013) 
 
 
03011-4 
height b (0.78 eV) and a rectification ratio of about 
392 at  1.5 V. Under illumination the device gave an 
open circuit voltage of 400 mV, short circuit current of 
22.9 A/cm2 and fill factor of 0.32. 
 
 
AKNOWLEDGEMENTS 
 
Author is grateful to the school of Applied physics 
and the Institute of Microengineering and Nanoelec-
tronics (IMEN), University of Kebangsaan Malaysia for 
unlimited support. 
 
 
REFERENCES 
 
1. N.S. Sariciftci, D. Braun, C. Zhang, V.I. Srdanov, A.J. Heeger, 
G. Stucky, F. Wudl, Appl. Phys. Lett. 62, 585 (1993). 
2. Abay Gadisa, Bantikassegn Workalemahu, Synthetic Met. 
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3. L.S. Roman, M. Berggren, O. Inganas, Appl. Phys. Lett. 
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4. Lionel Sicot, Bernard Geffroy, Andre Lonin, 
Paul Raimond, Carole Sentein, Jean Michel Nunzi,  
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5. G. Dennler, C. Lungenschmied, H. Neugebauer, 
N.S. Sariciftci, A. Labouret, J. Mater. Res. 20, 3224 (2005). 
6. Maher Al-Ibrahim, H. Klaus Roth, Uladzimir Zhokhavets, 
Gerhard Gobsch, Steffi Sensfuss, Sol. Energ. Mat. Sol. C. 
85, 13 (2005). 
7. Zhicai He, Chengmei Zhong, Shijian Su, Miao Xu, 
Hongbin Wu, Yong Cao, Nature Photonics 6, 591 (2012). 
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Science 270, 1789 (1995). 
9. Daisuke Sukeguchi, Surya Prakash Singh, Mamidi 
Ramesh Reddy, Hideyuki Yoshiyama, Rakesh A. Afre, Ya-
suhiko Hayashi, Hiroki Inukai, Tetsuo Soga, Shuichi 
Nakamura, Norio Shibata, Takeshi Toru, Beilstein J. Org. 
Chem. 5, No. 7 (2009). 
10. C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, 
M.T. Rispens, L. Sanchez, J.C. Hummelen, T. Fromherz, 
Thin Solid Films 403-404, 368 (2002). 
11. Floris B. Kooistra, Joop Knol, Fredrik Kastenberg, Lacra-
mioara M. Popescu, Wiljan J.H. Verhees, Jan M. Kroon, 
Jan C. Hummelen, Org. Lett. 9, 551 (2007). 
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English

Photovoltaic Properties of Poly (3-Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Thin Films

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 5 No 3, 03011(4pp) (2013) Том 5 № 3, 03011(4cc) (2013)   2077-6772/2013/5(3)03011(4) 03011-1  2013 Sumy State University Photovoltaic Properties of Poly (3-Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Thin Films   B.M. Omer1,2,*  1 Department of Physics, College of Science and Arts-Ranyah, Taif University, Kingdom Saudi Arabia 2 Department of Applied Physics, Faculty of Applied Science and Computer, Omdurman Ahlia University,  P.O.Box 786, Omdurman-Sudan  (Received 15 February 2013; published online 12 July 2013)  We fabricated and studied the electrical and photovoltaic properties of organic solar cell based on poly (3-hexylthiophene) (P3HT) as an electron donor blended with the acceptor [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester (modified fullerene). The active layer composed of (3:1, w/w) mixture of P3HT and the modified fullerene was sandwiched between indium tin oxide (ITO) and aluminum (Al). The ideality factor n and barrier hight b values were determined from the dark current density-voltage characteristics and found as 2.45 and 0.78 eV, respectively. The device shows photovoltaic behavior with an open circuit voltage of 400 mV, short circuit current of 22.9 A/cm2 and fill factor 0.32 under 2.8 mW/cm2 light intensity.  Keywords: Poly3-Hexylthiophene, Modified fullerene, Organic solar cell, Open circuit voltage.    PACS numbers: 78.66.Qn,78.40.Me                                                                   * bushra_omer4@yahoo.com  1. INTRODUCTION  Since the discovery of the conjugated polymers, rec-tifying junctions such as p-n junction and Schottky junctions have been studied as to explore the possible application of conjugated polymers [1-4]. Polymer solar cells have attracted considerable attention due to their low cost, lightweight [5], as well as their potential ap-plication in flexible large area devices [6]. Despite the low cost, the power conversion efficiency of bulk hetrojunction devices is low compared to inor-ganic solar cells. Recently Yong Cao et al reported an efficiency of 9.2 % for a photovoltaic cell with a bulk heterojunction inverted structure [7]. Bulk heterojunc-tion photovoltaic consists of an intimate blend of a con-jugated polymer and an electron acceptor that form an interpenetrating network with percolated paths to both electrodes [8]. When light is incident on the device, excitons are created on the polymer chains. If an exci-ton is able to diffuse within its lifetime to an electron acceptor, it will be quenched with near unit quantum yield, creating a spatially separated electron-hole pair with the hole localized on the polymer chain and the electron on the acceptor molecule. Under the influence of the built-in electric field, these initially bound charg-es can then separate and migrate to the electrodes, thus producing a photocurrent. So far, research efforts in solution-processed organic photovoltaic materials have been dominated by the use of one particular fullerene derivative, [6, 6]-Phenyl C61-Butyric acid methyl ester (PCBM), as the electron ac-ceptor. But PCBM is not necessarily the optimum ac-ceptor for solution processed organic photovoltaic ma-terials. The preparation of different derivatives of C60 fullerenes and their use in bulk heterojunction polymer solar cells was reported by different authors [9-11].  To our knowledge there is no published study on the photovoltaic properties of P3HT: [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester (Modified full-erene) thin films. In this paper, we report the fabrica-tion and characterization of bulk heterojunction solar cell based on Poly (3-Hexylthiophene) and [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester with (3:1) composite ratio.  2. EXPERIMENTS  The organic photovoltaic cell in this study consists of a layer of poly (3-Hexylthiophene) / [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester blend thin film sandwiched between transparent anode indium tin oxide (ITO) and metal cathode aluminum (Al). The p-type do-nor polymer P3HT and the n-type acceptor modified full-erene in the active layer were purchased from sigma-Aldrich Corporation and American Dye Source, Inc. re-spectively; the chemical structures are shown in Fig. 1. All materials were used as received without further puri-fication. Before device fabrication the ITO coated glass substrate was first etched and patterned to serve as an-ode. The substrate was cleaned with Isopropyl alcohol and acetone in an ultrasonic bath each for 15 min se-quentially. To make a P3HT/modified fullerene (3:1, w/w) solution, P3HT (16.26 mg) and modified fullerene (5.48 mg) were dissolved in 1,2 dichlorobenzene (concen-tration 21.74 mg/ml). The blend was spin coated onto the substrates with atypical spinning speed and time at 2000 rpm for 40 s. Finally Al metal was vacuum-deposited to form the cathode. The active area of the device, defined by shadow mask, was 0.07 cm2 .The structure of the device is shown in Fig. 2. For the UV-vis absorption measurements, the solu-tions were spin coated onto quartz substrates, the properties of the relevant films were obtained on a Lambda-900 UV/vis/NIR spectrophotometer (Perkin Elmer). The current-voltage (I-V) curves were meas-ured with a Keithley 238 source measurement unit. All thin films preparation and measurements were carried out at room temperature in air.   B.M. OMER J. NANO- ELECTRON. PHYS. 5, 03011 (2013)   03011-2       Fig. 1 – Chemical structure of (a) poly (3-Hexylthiophene) and (b) [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester    Fig. 2 – Schematic of the polymer solar cell device structure  3. RESULTS AND DISCUSSIONS   Fig. 3 shows the absorption of thin films made from pure poly (3-Hexylthiophene), pure [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester and (3:1) poly (3-Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Ester composite at room temperature. All the thin films were spin coated from 1,2 dichlorobenzene. The absorption maximum ( max) of P3HT can be observed at 556 nm (2.23 eV) and two shoulders appeared at 532 nm and 605 nm. The optical band gap of P3HT is determined with a tangential through the turning point of the low energy side of the spectra, leading to estimated value of 1.89 eV. The  absorption maximum of P3HT in the composite films was found to be around 574 nm (18 nm red shifted compared to pure P3HT), the shoulders appeared at 542 nm and 623 nm. From the absorption spectra of the composites it was very clear that the two peaks at (281 nm and 354 nm) are belong to the modified fuller-ene. It was noticeable that the absorption spectra of the composites are just superpositions of the absorption spectra of the individual components.     Fig. 3 – Absorption spectra of pure P3HT, pure modified Full-erene and P3HT: modified Fullerene (3:1) composite films  Fig. 4 shows the current density-voltage character-istics of the device in dark; we observe that the forward current is exponentially increased with applied voltage; this is an indication of the diode characteristics. The device shows a high rectification ratio ( ) of about 392 at (  1.5 V), the diode is regarded as well rectified.    Fig. 4 – Current density-voltage characteristics under dark for (3:1) P3HT / modified fullerene device structure  Fig. 5 shows a semi-logarithmic plot of the current density versus the applied voltage of the device, in dark at room temperature. The general diode equation,   J  J0 [exp (qV / nKT) – 1], (1)  describes the J-V characteristics of both p-n junctions and Schottky diodes. Here, V is the applied voltage, n is the ideality factor, q is the electron charge, K is the Boltzmann constant, T is the temperature and J0 is the saturation current derived from the straight line inter-cept of ln J at V  0 and is given by:   J0  A* T2 exp (– q b / KT), (2)  where A*  120 A/cm2 K2 is the effective Richardson constant for an electron in free space and b the zero-bias potential barrier height. The ideality factor n is unity in the Shockly theory for p-n junctions in the ab-sence of recombination, as well as for thermionic emis-sion theory and diffusion theory for Schottky diodes. The slope of the logarithmic plot is related to the ideali-ty factor n through:   1 / n  (KT / q) (d (ln J) / dV), (3)  Once J0 is found, we can find the barrier height per unit charge using the relation:   Φb  (KT / q) ln(A* T2 / J0), (4)  From ln J versus V characteristics in the dark, shown in Fig. 5 and making use of equations (1), (2), (3) and (4) the value of the saturation current determined as 3.16  10 – 9 A/cm2. The ideality factor n and the bar-rier hight b was found to be 2.45 and 0.78 eV, respec-tively. The deviation of the ideality factor of the dark current from unity may be due to a trap-assisted re-combination. At high bias the outflow rate of the injected carriers is reduced greatly and charges accumulate in the bulk of the semiconducting organic layer and that is due to low charge carrier mobility as well as the presence of  ITO Coated Glass Photoactive layer Al a b  PHOTOVOLTAIC PROPERTIES OF POLY (3-HEXYLTHIOPHENE)… J. NANO- ELECTRON. PHYS. 5, 03011 (2013)   03011-3   Fig. 5 – Semi-logarithmic plot of Current density-Voltage characteristics in the dark  traps. In this case the transport flow is dominantly controlled by the space charge limited current (SCLC) resulting into saturation of current. The relation be-tween the current density and voltage in free-traps SCLC is given by the Childs law [12]:   J  (9/8)(ε  / d3)V2 (5)  where  is the charge carrier mobility and d is the film thickness. In presence of traps with exponential trap distribu-tion the SCLC takes a power law dependence of the form:   J  K Vm + 1 (6)  where m is defined in terms of a characteristics trap distribution constant. Plot of the current density versus voltage at high forward bias, beyond 1 V is shown in Fig. 6. The cur-rent density depends super linearly on the forward voltage. From the fit we could find the relation:   J  5.5  10 – 4V2.8 (7)  The correlation coefficient was good (R  0.998). Comparing equation (7) with (6) we get K  5.5  10 – 4 and m  1.8, this is indicative of electrical transport that is dominated by SCLC with exponential trap dis-tribution. The overall parameters for the J-V curve under dark were tabulated in Table 1.  Table 1 – The photovoltaic parameters under dark   Fig. 7 shows the current density versus the voltage curves in dark and when the device was illuminated through the ITO (front) side with an incident light in-tensity of about 2.8 mW/cm2 from a projector lamb. The current in the reverse direction is strongly increased by the illumination. This indicates that the light illumina-tion increases the production of electron-hole pairs. At higher forward bias, the dark and photocurrent are quite similar, indicating that photoconductivity does   Fig. 6 – Current density-Voltage characteristics at high bias in dark  not play an important role in the device. The device shows photovoltaic behavior with an open circuit volt-age of 400 mV, short circuit current of 22.9 A/cm2, fill factor of 0.32, and a power conversion efficiency of 0.105 %. The short circuit current of our device is small because the work function of the ITO has a boor match-ing with the highest occupied molecular orbital level of the P3HT. This in turn expected to decrease the whole transport and collection at the anode. According to the metal-insulator-metal (MIM) model for non-ohmic con-tacts devices, the open circuit voltage is determined by the work function difference of the materials used for the electrodes, Al (4.3 eV) [6, 12] and ITO (4.7 eV) [6, 12] results Voc in the range of 0.4 V, which agree very well with the experimentally observed open circuit voltage.    Fig. 7 – Current density-voltage characteristics of P3HT / modified fullerene blend, 3:1 ratio  4. CONCLUSIONS   In summary, we have fabricated Poly (3-Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene photovoltaic cell. The dark current density-voltage characteristics indicates a non-ideal diode behavior with ideality factor n (2.45), barrier Parameters J0(A/cm2) Φb (eV)  n Value 3.16  10 – 9 0.78 392 2.45  B.M. OMER J. NANO- ELECTRON. PHYS. 5, 03011 (2013)   03011-4 height b (0.78 eV) and a rectification ratio of about 392 at  1.5 V. Under illumination the device gave an open circuit voltage of 400 mV, short circuit current of 22.9 A/cm2 and fill factor of 0.32.   AKNOWLEDGEMENTS  Author is grateful to the school of Applied physics and the Institute of Microengineering and Nanoelec-tronics (IMEN), University of Kebangsaan Malaysia for unlimited support.   REFERENCES  1. N.S. Sariciftci, D. Braun, C. Zhang, V.I. Srdanov, A.J. Heeger, G. Stucky, F. Wudl, Appl. Phys. Lett. 62, 585 (1993). 2. Abay Gadisa, Bantikassegn Workalemahu, Synthetic Met. 129, 179 (2002). 3. L.S. Roman, M. Berggren, O. Inganas, Appl. Phys. Lett. 75, 3557 (1999). 4. Lionel Sicot, Bernard Geffroy, Andre Lonin, Paul Raimond, Carole Sentein, Jean Michel Nunzi,  J. Appl. Phys. 90, 1047 (2001). 5. G. Dennler, C. Lungenschmied, H. Neugebauer, N.S. Sariciftci, A. Labouret, J. Mater. Res. 20, 3224 (2005). 6. Maher Al-Ibrahim, H. Klaus Roth, Uladzimir Zhokhavets, Gerhard Gobsch, Steffi Sensfuss, Sol. Energ. Mat. Sol. C. 85, 13 (2005). 7. Zhicai He, Chengmei Zhong, Shijian Su, Miao Xu, Hongbin Wu, Yong Cao, Nature Photonics 6, 591 (2012). 8. G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger,  Science 270, 1789 (1995). 9. Daisuke Sukeguchi, Surya Prakash Singh, Mamidi Ramesh Reddy, Hideyuki Yoshiyama, Rakesh A. Afre, Ya-suhiko Hayashi, Hiroki Inukai, Tetsuo Soga, Shuichi Nakamura, Norio Shibata, Takeshi Toru, Beilstein J. Org. Chem. 5, No. 7 (2009). 10. C.J. Brabec, A. Cravino, D. Meissner, N.S. Sariciftci, M.T. Rispens, L. Sanchez, J.C. Hummelen, T. 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Photovoltaic Properties of Poly (3-Hexylthiophene): [6, 6]-Phenyl C61-Butyric Acid 3-Ethylthiophene Thin Films

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