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J. Phys. Chem. B 2006, 110, 9782-9784
ARTICLES Photovoltaic Properties of Au/β-Carotene/n-Si Organic Solar Cells F. Yakuphanoglu,*,† M. E. Aydin,‡ and T. Kilic¸ ogˇ lu‡ Department of Physics, Faculty of Arts and Sciences, Fırat UniVersity, 23119 Elazig, Turkey, and Department of Physics, Faculty of Science and Art, UniVersity of Dicle, 21280 Dicle, Turkey ReceiVed: February 19, 2006; In Final Form: March 29, 2006
Photovoltaic properties of Au/β-carotene/n-Si organic solar cells characterized by current-voltage and capacitance-voltage measurements have been investigated. The photocurrent in the reverse direction increases with increasing illumination intensity. The Isc increases linearly with light intensity. The Isc dependence of light intensity follows a power law Isc ∼ FR. The exponent R was found to be 1.38. This indicates a monomolecular recombination in this device. Au/β-carotene/n-Si organic solar cells give an open-circuit voltage of 0.316 V and a short-circuit current of 2.33 × 10-4 A at light intensity of 6 W/m2. The best conversion efficiency for Au/β-carotene/n-Si solar cells was found to be 23.3% at a light intensity of 6 W/m2.
1. Introduction An advantage of organic semiconductors is the processability from organic solution, such that simple deposition techniques such as doctor-blading or screen-printing can be applied.1,2 This is especially interesting for large area electronic devices such as solar cells. The organic semiconductors are suitable for use in photoelectric conversion devices due to easy fabrication of devices with low cost.3 Organic solar cells have attracted increasing research interest in the past decade, due to the advantages of their low cost, their light weight, and the capability to make flexible devices in comparison with the traditional silicon-based solar cells.4-9 In recent years, different organic materials types have been extensively used to fabricate organic solar cells.10-12 The main aim of this work is to fabricate a solar cell using β-carotene as a photovoltaic material.
evaporating Au with a diameter of about 1.0 mm (diode area ) 176 × 10-2 cm2) at about 10-5 mbar. The structure of the Au/β-carotene/n-Si device is shown in Figure 1. The capacitancevoltage (C-V) characteristics of the device were measured using a Hioki 3532-50 LCR, and the current-voltage (I-V) characteristics under different illuminations were measured using a Keithley 2400 sourcemeter at room temperature.
2. Experimental Section
3. Results and Discussion
The n-Si(100)-doped boron is used as an n-type semiconductor. To remove the native oxide surface on n-Si, the substrate was etched by HF, then was rinsed in deionized water using an ultrasonic bath for 10-15 min, and finally was chemically cleaned according to the RCA cleaning procedure (i.e., a 10 min boil in NH4 + H2O2 + 6H2O followed by a 10 min boil in HCl + H2O2 + 6H2O). An ohmic contact was formed on the n-type Si wafer by evaporating AuSb and was followed a temperature treatment at 420 °C for 30 min in N2. The film of β-carotene was prepared by evaporating the solvent from a solution of the compound with subsequent drying of the film deposited on n-Si. The solution of the compound was homogenized for 1 h and was rotated for homogeneous mixing.13 The electrical contacts were formed on the other faces by * Author to whom correspondence should be addresssed. E-mail:
[email protected]. † Fırat University. ‡ University of Dicle.
Figure 1. Au/β-carotene/n-Si structure.
3.1. Dark Current-Voltage Characteristics of Au/βCarotene/n-Si. Figure 2 shows the current voltage characteristics of Au/β-carotene/n-Si. The diode exhibits a good rectifying effect. There is an exponential increase in the forward current with applied voltage at low voltages. The current voltage characteristics of the Au/β-carotene/n-Si diode can be analyzed using the following relation14-15
qV (nkT )
I ) Io exp
for V > 3kT/q
(1)
where n is the ideality factor, V is the voltage drop across the rectifying barrier, and Io is the saturation current given by
( )
Io ) AA*T2 exp -
qφB kT
(2)
where A* is the Richardson constant (112 A cm-2 K-2 for n-type Si), A is the diode contact area, φB is the barrier height, and k
10.1021/jp0610620 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/03/2006
Au/β-Carotene/n-Si Organic Solar Cells
J. Phys. Chem. B, Vol. 110, No. 20, 2006 9783
Figure 4. Photocurrent-voltage plots of Au/β-carotene/n-Si under different illuminations.
of the semiconductor, and ND is the donor concentration given by Figure 2. Current-voltage characteristics of Au/β-carotene/n-Si.
ND )
( )
2 dV 2 qsA dC-2
(6)
Equation 6 suggests that the donor concentration, ND, can be obtained from the C-2 versus V plot. ND was determined from the slope of Figure 3 using eq 6 and was found to be 1.746 × 1013 cm-3. The barrier height can be obtained by the following relation14,15
φB(C-V) ) Vbi + Vn
where Vn is the potential difference between the Fermi level and the bottom of the conduction band (EC - EF). The Vn value can be obtained by the following relation
Figure 3. C-2 vs V plot of Au/β-carotene/n-Si.
is the Boltzmann constant. The ideality factor and barrier height of Au/β-carotene/n-Si can be determined from the following equations
n)
q dV kT ln I
(3)
and
qφB ) kT
( ) AA*T2 Io
(4)
The values of Is, n, and φB were calculated as 1.439 × 10-7 A, 2.69, and 0.70 eV, respectively. The obtained n value is greater than 1. This suggests that the diode shows nonideal I-V behavior; i.e, the diode obeys a metal-insulator-semiconductor (MIS) configuration rather than an ideal Schottky diode. The value of n greater than 1 shows the existence of the interface layer and series resistance. It is well-known that this layer may be formed during the surface preparation or metal evaporation. 3.2. Capacitance-Voltage Characteristics of Au/β-Carotene/ n-Si. The capacitance-voltage curve of Au/β-carotene/n-Si in the form of C-2 versus V is shown in Figure 3. The voltage dependence on capacitance for Au/β-carotene/n-Si can be analyzed by14
2(Vbi + V) 1 ) 2 2 C A sqND
(7)
(5)
where Vbi is the built-in potential, s is the dielectric constant
NC ) ND exp(Vn/kT)
(8)
where NC is the effective density of states in the conduction band of silicon (NC ) 2.8 × 1019 cm-3). The value of Vbi ) 0.505 V was calculated from the intercept of Figure 3. The φB(C-V) value was calculated using the Vbi and Vn values and was found to be 0.87 eV. The barrier height obtained from the C-V measurements is higher than that obtained from the I-V measurements. 3.3. Photovoltaic Properties of Au/β-Carotene/n-Si. Figure 4 shows the I-V characteristics of Au/β-carotene/n-Si under different illuminations at room temperature. The photocurrent increases with increasing illumination intensity. This suggests that photocarriers are generated by illumination. The device shows a photovoltaic behavior with a maximum open-circuit voltage Voc of 0.316 V and short-circuit current Isc of 2.33 × 10-4 A under 6 W/m2 light intensity, when illuminated. The solar cell parameters can be obtained from the following equations16
VmIm VocIsc
(9)
IscVocFF PinA
(10)
FF ) and
η)
where FF is the fill factor, Voc is the open-circuit voltage, Isc is
9784 J. Phys. Chem. B, Vol. 110, No. 20, 2006
Yakuphanoglu et al. direction increases with increasing illumination intensity. Au/ β-carotene/n-Si organic solar cells give an open-circuit voltage of 0.316 V and a short-circuit current of 2.33 × 10-4 A at a light intensity of 6 W/m2. The best conversion efficiency for Au/β-carotene/n-Si solar cells was found to be 23.3% at a light intensity of 6 W/m2. References and Notes
Figure 5. Isc and Voc dependence of light intensity for a Au/β-carotene/ n-Si device.
the short-circuit current, Im and Vm are the current and potential maxima, Pin is the intensity of incident light, and A is the cell area. The best conversion efficiency for Au/β-carotene/n-Si solar cells was found to be 23.3% under 6W/m2 (FF ) 0.11, Voc ) 0.316 V, Isc ) 2.33 × 10-4 A). Figure 5 shows the short-circuit current and open-circuit voltage dependence of the light intensity of Au/β-carotene/n-Si solar cells. The Isc increases linearly with light intensity. The Isc dependence of light intensity follows a power law Isc ∼ FR. The exponent R was found to be 1.38. This indicates monomolecular recombination in this device. 4. Conclusions Photovoltaic properties of Au/β-carotene/n-Si organic solar cells have been investigated. The photocurrent in the reverse
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