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Role of dielectric properties in organic photovoltaic devices
Chemical Physics Letters 416 (2005) 327–330 www.elsevier.com/locate/cplett
Role of dielectric properties in organic photovoltaic devices Basudev Pradhan, Amlan J. Pal
*
Indian Association for the Cultivation of Science, Department of Solid State Physics, Jadavpur, Kolkata, West Bengal 700 032, India Received 25 July 2005; in final form 15 September 2005 Available online 14 October 2005
Abstract We have fabricated organic photovoltaic devices via layer-by-layer electrostatic self-assembly technique with varied number of donor/ acceptor interfaces. Upon illumination, the short-circuit current of the devices depended on the number of such interfaces. We have studied impedance spectroscopy of the photovoltaic cells under dark and illumination conditions. We compared the changes in bulk resistance of the device and relative dielectric constant of the active material with short-circuit current of the devices due to illumination. The results show that the photovoltaic devices should be designed in such a way that the dielectric constant of the active material exhibits large decrease under illumination. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Solar energy conversion based on organic materials has become an important field of research exhibiting substantial future perspectives. In donor/acceptor (D/A) type photovoltaic devices, photogenerated excitons are dissociated to holes and electrons at the D/A interface [1–4]. Transport of the carriers to the opposite end of the device occurs due to dissimilar work-function of the electrodes. Such transport finally results in short-circuit photocurrent at the external circuit. To dissociate most of the photogenerated excitons in a device, a D/A interface within exciton diffusion length of organic materials is naturally required. By introducing bulk [2,5,6], double [4], or multiple heterojunction [7–10] of donor and acceptor materials, a lot of progresses have been achieved in recent years. Some of the device architectures provided interfaces within exciton diffusion length enabling efficient exciton dissociation. Certain devices could however no longer provide bicontinuous pathways for holes and electrons through donor and acceptor materials, respectively, and hence resulted in charge confinement.
*
Corresponding author. Fax: +91 33 24732805. E-mail address: [email protected] (A.J. Pal).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.077
Impedance spectroscopy is sensitive to charge storage and movement in a device. It is a powerful tool to study the physical processes, which are going on in the active material of a device. Since functioning of a photovoltaic cell involves exciton dissociation, charge separation and carrier transport, the methodology involved in the device can be suitably reflected by their dielectric properties. So far, there are only a few efforts to model dye-sensitized solar cells [11]. In this Letter, to study device physics of photovoltaic cells by impedance spectroscopy, we have fabricated devices based on layer-by-layer (LbL) electrostatic self-assembly, which provided D/A interfaces in the molecular scale. We aimed to focus on the design of D/ A-type photovoltaic devices by studying dielectric properties of the active material. 2. Experimental The molecules of interest in the present work are copper (II) phthalocyanine and Rose Bengal (RB), which were used as donor and acceptor, respectively. Their molecular structures are shown in the inset of Fig. 1. Tetrasulfonic acid tetrasodium salt of the phthalocyanine (CuPc) and RB act as anions for LbL film deposition. Poly(allylaminehydrochloride), PAH (MW = 70 000 g/mol) is a polycation. The materials were purchased from Aldrich Chemical
328
B. Pradhan, A.J. Pal / Chemical Physics Letters 416 (2005) 327–330
controlled the instruments via a general-purpose interface bus (GPIB) to record the current–voltage and impedance characteristics. 3. Results and discussion
Light Dark
-6
-4 -0.2
Z" (kΩ)
Co. and were used without further purification. The details of film deposition and device fabrication procedures were reported in an earlier communication [10]. To deposit LbL films, indium tin oxide (ITO) coated glass substrates are first dipped into the polycationic electrolyte (pH 7) for 15 min followed by rinsing in deionized water baths. The slides are then dipped in an anionic bath followed by another rinsing to result one bilayer of LbL films. The dipping procedure is sequenced to get a predetermined device architecture. In the present work, we have introduced D/A interfaces in the molecular level. While keeping the thickness of donor and acceptor layers the same, the number of D/A interfaces was varied. The device structures studied were Dm/(D/A)n/Am, where m + n = 15, and the number of D/A interfaces, n varied from 1 to 11. The LbL films were annealed and aluminum (Al) was vacuum evaporated as a top electrode. Thickness of the active material in each of the cases was about 70 nm. Area of each device was 5.5 mm2. Current–voltage and impedance characteristics of the devices under dark and illumination conditions were carried out in a shielded vacuum chamber (103 Torr) at room temperature. A xenon arc lamp shined the devices through a window and the ITO electrode and provided illumination for photocarrier generation. For current–voltage characteristics, a Yokogawa 7651 dc source and a Keithley 486 picoammeter were used. Bias was applied with respect to Al electrode and changed in a step of 0.05 V with a scan rate of 50–2.5 mV/s. Real and imaginary part of complex impedance as a function of frequency were measured by a Solartron 1260 Impedance Analyzer. Amplitude of the ac test signal was 100 mV rms. Measurements were carried out in a parallel mode configuration and in the frequency range of 1–12 MHz. Short-circuit and open-circuit corrections for the external leads were carried out with standard normalization procedure. A PC
Z" (MΩ)
Fig. 1. Current density–voltage characteristics of eight devices, namely Dm/(D/A)n/Am, where m + n = 15, and the number of D/A interfaces, n varied from 1 to 11 under white light illumination (100 mW/cm2). The parameter n is represented in the figure. Molecular structures of CuPc and RB are shown in the inset.
The highest-occupied molecular orbitals (HOMO) and lowest-unoccupied molecular orbitals (LUMO) of CuPc and RB along with the metal work-functions are favorable to form a D/A-type photovoltaic device. The two materials do not form any charge-transfer complex at the interface and absorbed most of the solar spectrum. Hence, CuPc and RB satisfy the prerequisite conditions of a D/A-type photovoltaic device. The current density–voltage (J–V) characteristics of eight devices with varied number of D/ A interfaces under white light illumination condition (100 mW/cm2) are shown in Fig. 1. The figure shows that while the open-circuit voltage (VOC) remained mostly the same, the JSC depended strongly on the device architecture. The JSC actually went through a maximum with the number of D/A interfaces. The results can be explained by considering the fact that the number of such interfaces had opposing effects on exciton dissociation and charge transport in a device. The number of D/A interfaces increases the effective volume of exciton dissociation, but at the same time introduces mismatch in energy levels or barriers for electron and hole transport through the LUMO and HOMO levels, respectively. To model the photovoltaic devices and study chargeseparation kinetics, impedance spectroscopy of the devices was studied under dark and illumination conditions. As a typical case, Cole–Cole plots for D11/(D/A)4/A11 device are presented in Fig. 2. The Z 0 –Z00 plots, where Z 0 and Z00 represent real and imaginary components of complex impedance, respectively, show signs of semicircles. The magnified view near the origin of the plot exhibits the presence of another semicircle with a shift from the origin. The devices can hence be represented as a series combination of a resistor (RS) and two parallel resistor–capacitor net-
-2
-0.1
0.0 0.0
0
0.5
1.0
1.5
Z' (kΩ)
0
1
2
3
4
Z' (MΩ) Fig. 2. Z 0 –Z00 plots of a D11/(D/A)4/A11 device under dark and illumination conditions. The section near the origin is magnified in the inset.
B. Pradhan, A.J. Pal / Chemical Physics Letters 416 (2005) 327–330
Ri Rs Rp IL
Cp
Rsh Ci
Fig. 3. Equivalent circuit of a photovoltaic device under illumination. RP and CP are bulk diode resistance and capacitance, respectively, Ri and Ci are due to organic–metal interfaces, and RS is the series component of resistance. IL and RSh represent a current source and a shunt resistance, respectively.
30 Light Dark
Capacitance (nF)
25 20 15 10 5 0 0 10
10 1
10 2
10 3
10 4
10 5
10 6
Frequency (Hz) Fig. 4. Bulk capacitance of a D11/(D/A)4/A11 device versus frequency plot under dark and illumination conditions.
JSC (µA/cm2)
0.30 0.25 0.20 0.15
∆ε'
0.10 0 -2 -4
∆RP (MΩ)
works. The radii of the semicircles may represent bulk resistance (RP) of the device and a resistance due to organic/ metal interfaces (Ri). The diameters of the semicircles, specially the one due to the bulk, were larger in the dark condition than under illumination. For the latter case, dielectric measurements were carried out also with VOC as a dc bias voltage, so that no dc current could flow through the device during the measurement. The results in Fig. 2 show that the decrease in the diameter of Cole– Cole plots and hence the bulk resistance of the device was indeed due to illumination. The dissociated excitons may increase the carrier concentration in the device resulting in an increase in the mobility of charge carriers. The decrease in bulk resistance of the device could be due to the increase in carrier mobility. From the Cole–Cole plots, the electrical analogue of the photovoltaic device has been modeled (Fig. 3). A series combination of a resistor, RS and two parallel resistor– capacitor combinations are presented in the figure. Upon illumination, a current source, IL and a shunt resistance, RSh appear in the circuit. The latter is due to recombination of charge carriers at the interfaces. Due to the rectifying nature of current–voltage characteristics in the dark condition, the bulk resistance, RP is represented as a diode. In devices with different number of D/A interfaces, the resistor due to the organic/metal interfaces, Ri (500 X) did not show any variation. From the frequency of the apex of the small semicircle, we have calculated the interfacial capacitance, Ci. The Ci (60 pF) also remained invariant in the devices. Bulk capacitor (CP) and resistor (RP) varied from a device to the other. In the following, we will discuss how the CP and the RP respond to illumination in different devices. We have calculated frequency response of capacitance for all the devices. The case for D11/(D/A)4/A11 device under dark and illumination conditions is shown in Fig. 4. The plot shows that apart from the decrease in RP (Fig. 2), CP also decreases due to illumination. It is hence natural to compare JSC and changes in RP and CP due to illumination in these devices. In Fig. 5, we have correlated JSC, change in RP (DRP) and change in dielectric constant e 0 (De 0 ) as a function of the number of D/A interfaces. The JSC has been higher in devices with 3–6 D/A interfaces than the two extreme cases. Under illumination, both RP and e 0 decreased in these devices. The magnitude of decrease of
329
-6 0 -1 -2 -3
0
2
4
5
6
10
12
Number of interfaces Fig. 5. A comparison of short-circuit current density (JSC), change in bulk resistance (DRP), and change in dielectric constant at 16 Hz (De 0 ) due to illumination for devices with different number of interfaces. The lines are to guide the eye.
both the parameters was more in devices with intermediate number of interfaces. The change in RP and e 0 as up to 50% and 15%, respectively, as compared to their values in the dark condition. The plots clearly show that the JSC has a direct correlation with the change in e 0 of the active material. In other words, the devices in which illumination can decrease the dielectric constant of the material, may act as a better current source. We have speculated on the correlation between JSC and De 0 . To align fermi levels in sandwiched structures with dissimilar electrodes, space charges are accumulated near the
330
B. Pradhan, A.J. Pal / Chemical Physics Letters 416 (2005) 327–330
organic/metal interfaces. In dark conditions, space charges contribute to the capacitance of the devices. Under illumination and consequent charge separation, the contribution of space charges near the organic/metal interfaces decreases. This results in a decrease in bulk capacitance of the device and hence dielectric constant of the active material. We may hence conclude that a decrease in dielectric constant can be a good measure to determine efficient electron-hole separation in a photovoltaic device.
Acknowledgements
4. Conclusion
[1] C.W. Tang, Appl. Phys. Lett. 48 (1986) 183. [2] G. Yu, J. Gao, J. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. [3] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [4] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [5] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841. [6] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [7] M. Hiramoto, H. Fukusumi, M. Yokoyama, Appl. Phys. Lett. 61 (1992) 2580. [8] H. Mattoussi, M.F. Rubner, F. Zhou, J. Kumar, S.K. Tripathy, L.Y. Chiang, Appl. Phys. Lett. 77 (2000) 1540. [9] M.F. Durstock, R.J. Spry, J.W. Baur, B.E. Taylor, L.Y. Chiang, J. Appl. Phys. 94 (2003) 3253. [10] B. Pradhan, A. Bandyopadhyay, A.J. Pal, Appl. Phys. Lett. 85 (2004) 663. [11] L. Han, N. Koide, Y. Chiba, T. Mitate, Appl. Phys. Lett. 84 (2004) 2433.
In conclusion, we have studied dielectric properties of organic photovoltaic devices with controlled number of molecularly thin donor/acceptor interfaces. The devices have been modeled to its electrical analogue. Though the VOC remained mostly the same in these devices, the JSC was higher in the devices with moderate number of interfaces. Such dependence was due to opposing contribution of the D/A interfaces on exciton dissociation and charge transport. Upon illumination, RP of the device and e 0 of the material decreased. The variation of JSC was compared with the decrease in RP and e 0 or devices with different number of interfaces. The results showed a clear correlation between JSC and change in e 0 . We observed that the devices in which illumination could offer large decrease in e 0 of the active material would act as a better photovoltaic device.
The authors acknowledge financial supports from the Department of Atomic Energy, Government of India (Project 2002/37/14/BNRS) and the Department of Science and Technology, Government of India (Project SP/S2/M-44/ 99). References
Role of dielectric properties in organic photovoltaic devices Basudev Pradhan, Amlan J. Pal
*
Indian Association for the Cultivation of Science, Department of Solid State Physics, Jadavpur, Kolkata, West Bengal 700 032, India Received 25 July 2005; in final form 15 September 2005 Available online 14 October 2005
Abstract We have fabricated organic photovoltaic devices via layer-by-layer electrostatic self-assembly technique with varied number of donor/ acceptor interfaces. Upon illumination, the short-circuit current of the devices depended on the number of such interfaces. We have studied impedance spectroscopy of the photovoltaic cells under dark and illumination conditions. We compared the changes in bulk resistance of the device and relative dielectric constant of the active material with short-circuit current of the devices due to illumination. The results show that the photovoltaic devices should be designed in such a way that the dielectric constant of the active material exhibits large decrease under illumination. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Solar energy conversion based on organic materials has become an important field of research exhibiting substantial future perspectives. In donor/acceptor (D/A) type photovoltaic devices, photogenerated excitons are dissociated to holes and electrons at the D/A interface [1–4]. Transport of the carriers to the opposite end of the device occurs due to dissimilar work-function of the electrodes. Such transport finally results in short-circuit photocurrent at the external circuit. To dissociate most of the photogenerated excitons in a device, a D/A interface within exciton diffusion length of organic materials is naturally required. By introducing bulk [2,5,6], double [4], or multiple heterojunction [7–10] of donor and acceptor materials, a lot of progresses have been achieved in recent years. Some of the device architectures provided interfaces within exciton diffusion length enabling efficient exciton dissociation. Certain devices could however no longer provide bicontinuous pathways for holes and electrons through donor and acceptor materials, respectively, and hence resulted in charge confinement.
*
Corresponding author. Fax: +91 33 24732805. E-mail address: [email protected] (A.J. Pal).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.077
Impedance spectroscopy is sensitive to charge storage and movement in a device. It is a powerful tool to study the physical processes, which are going on in the active material of a device. Since functioning of a photovoltaic cell involves exciton dissociation, charge separation and carrier transport, the methodology involved in the device can be suitably reflected by their dielectric properties. So far, there are only a few efforts to model dye-sensitized solar cells [11]. In this Letter, to study device physics of photovoltaic cells by impedance spectroscopy, we have fabricated devices based on layer-by-layer (LbL) electrostatic self-assembly, which provided D/A interfaces in the molecular scale. We aimed to focus on the design of D/ A-type photovoltaic devices by studying dielectric properties of the active material. 2. Experimental The molecules of interest in the present work are copper (II) phthalocyanine and Rose Bengal (RB), which were used as donor and acceptor, respectively. Their molecular structures are shown in the inset of Fig. 1. Tetrasulfonic acid tetrasodium salt of the phthalocyanine (CuPc) and RB act as anions for LbL film deposition. Poly(allylaminehydrochloride), PAH (MW = 70 000 g/mol) is a polycation. The materials were purchased from Aldrich Chemical
328
B. Pradhan, A.J. Pal / Chemical Physics Letters 416 (2005) 327–330
controlled the instruments via a general-purpose interface bus (GPIB) to record the current–voltage and impedance characteristics. 3. Results and discussion
Light Dark
-6
-4 -0.2
Z" (kΩ)
Co. and were used without further purification. The details of film deposition and device fabrication procedures were reported in an earlier communication [10]. To deposit LbL films, indium tin oxide (ITO) coated glass substrates are first dipped into the polycationic electrolyte (pH 7) for 15 min followed by rinsing in deionized water baths. The slides are then dipped in an anionic bath followed by another rinsing to result one bilayer of LbL films. The dipping procedure is sequenced to get a predetermined device architecture. In the present work, we have introduced D/A interfaces in the molecular level. While keeping the thickness of donor and acceptor layers the same, the number of D/A interfaces was varied. The device structures studied were Dm/(D/A)n/Am, where m + n = 15, and the number of D/A interfaces, n varied from 1 to 11. The LbL films were annealed and aluminum (Al) was vacuum evaporated as a top electrode. Thickness of the active material in each of the cases was about 70 nm. Area of each device was 5.5 mm2. Current–voltage and impedance characteristics of the devices under dark and illumination conditions were carried out in a shielded vacuum chamber (103 Torr) at room temperature. A xenon arc lamp shined the devices through a window and the ITO electrode and provided illumination for photocarrier generation. For current–voltage characteristics, a Yokogawa 7651 dc source and a Keithley 486 picoammeter were used. Bias was applied with respect to Al electrode and changed in a step of 0.05 V with a scan rate of 50–2.5 mV/s. Real and imaginary part of complex impedance as a function of frequency were measured by a Solartron 1260 Impedance Analyzer. Amplitude of the ac test signal was 100 mV rms. Measurements were carried out in a parallel mode configuration and in the frequency range of 1–12 MHz. Short-circuit and open-circuit corrections for the external leads were carried out with standard normalization procedure. A PC
Z" (MΩ)
Fig. 1. Current density–voltage characteristics of eight devices, namely Dm/(D/A)n/Am, where m + n = 15, and the number of D/A interfaces, n varied from 1 to 11 under white light illumination (100 mW/cm2). The parameter n is represented in the figure. Molecular structures of CuPc and RB are shown in the inset.
The highest-occupied molecular orbitals (HOMO) and lowest-unoccupied molecular orbitals (LUMO) of CuPc and RB along with the metal work-functions are favorable to form a D/A-type photovoltaic device. The two materials do not form any charge-transfer complex at the interface and absorbed most of the solar spectrum. Hence, CuPc and RB satisfy the prerequisite conditions of a D/A-type photovoltaic device. The current density–voltage (J–V) characteristics of eight devices with varied number of D/ A interfaces under white light illumination condition (100 mW/cm2) are shown in Fig. 1. The figure shows that while the open-circuit voltage (VOC) remained mostly the same, the JSC depended strongly on the device architecture. The JSC actually went through a maximum with the number of D/A interfaces. The results can be explained by considering the fact that the number of such interfaces had opposing effects on exciton dissociation and charge transport in a device. The number of D/A interfaces increases the effective volume of exciton dissociation, but at the same time introduces mismatch in energy levels or barriers for electron and hole transport through the LUMO and HOMO levels, respectively. To model the photovoltaic devices and study chargeseparation kinetics, impedance spectroscopy of the devices was studied under dark and illumination conditions. As a typical case, Cole–Cole plots for D11/(D/A)4/A11 device are presented in Fig. 2. The Z 0 –Z00 plots, where Z 0 and Z00 represent real and imaginary components of complex impedance, respectively, show signs of semicircles. The magnified view near the origin of the plot exhibits the presence of another semicircle with a shift from the origin. The devices can hence be represented as a series combination of a resistor (RS) and two parallel resistor–capacitor net-
-2
-0.1
0.0 0.0
0
0.5
1.0
1.5
Z' (kΩ)
0
1
2
3
4
Z' (MΩ) Fig. 2. Z 0 –Z00 plots of a D11/(D/A)4/A11 device under dark and illumination conditions. The section near the origin is magnified in the inset.
B. Pradhan, A.J. Pal / Chemical Physics Letters 416 (2005) 327–330
Ri Rs Rp IL
Cp
Rsh Ci
Fig. 3. Equivalent circuit of a photovoltaic device under illumination. RP and CP are bulk diode resistance and capacitance, respectively, Ri and Ci are due to organic–metal interfaces, and RS is the series component of resistance. IL and RSh represent a current source and a shunt resistance, respectively.
30 Light Dark
Capacitance (nF)
25 20 15 10 5 0 0 10
10 1
10 2
10 3
10 4
10 5
10 6
Frequency (Hz) Fig. 4. Bulk capacitance of a D11/(D/A)4/A11 device versus frequency plot under dark and illumination conditions.
JSC (µA/cm2)
0.30 0.25 0.20 0.15
∆ε'
0.10 0 -2 -4
∆RP (MΩ)
works. The radii of the semicircles may represent bulk resistance (RP) of the device and a resistance due to organic/ metal interfaces (Ri). The diameters of the semicircles, specially the one due to the bulk, were larger in the dark condition than under illumination. For the latter case, dielectric measurements were carried out also with VOC as a dc bias voltage, so that no dc current could flow through the device during the measurement. The results in Fig. 2 show that the decrease in the diameter of Cole– Cole plots and hence the bulk resistance of the device was indeed due to illumination. The dissociated excitons may increase the carrier concentration in the device resulting in an increase in the mobility of charge carriers. The decrease in bulk resistance of the device could be due to the increase in carrier mobility. From the Cole–Cole plots, the electrical analogue of the photovoltaic device has been modeled (Fig. 3). A series combination of a resistor, RS and two parallel resistor– capacitor combinations are presented in the figure. Upon illumination, a current source, IL and a shunt resistance, RSh appear in the circuit. The latter is due to recombination of charge carriers at the interfaces. Due to the rectifying nature of current–voltage characteristics in the dark condition, the bulk resistance, RP is represented as a diode. In devices with different number of D/A interfaces, the resistor due to the organic/metal interfaces, Ri (500 X) did not show any variation. From the frequency of the apex of the small semicircle, we have calculated the interfacial capacitance, Ci. The Ci (60 pF) also remained invariant in the devices. Bulk capacitor (CP) and resistor (RP) varied from a device to the other. In the following, we will discuss how the CP and the RP respond to illumination in different devices. We have calculated frequency response of capacitance for all the devices. The case for D11/(D/A)4/A11 device under dark and illumination conditions is shown in Fig. 4. The plot shows that apart from the decrease in RP (Fig. 2), CP also decreases due to illumination. It is hence natural to compare JSC and changes in RP and CP due to illumination in these devices. In Fig. 5, we have correlated JSC, change in RP (DRP) and change in dielectric constant e 0 (De 0 ) as a function of the number of D/A interfaces. The JSC has been higher in devices with 3–6 D/A interfaces than the two extreme cases. Under illumination, both RP and e 0 decreased in these devices. The magnitude of decrease of
329
-6 0 -1 -2 -3
0
2
4
5
6
10
12
Number of interfaces Fig. 5. A comparison of short-circuit current density (JSC), change in bulk resistance (DRP), and change in dielectric constant at 16 Hz (De 0 ) due to illumination for devices with different number of interfaces. The lines are to guide the eye.
both the parameters was more in devices with intermediate number of interfaces. The change in RP and e 0 as up to 50% and 15%, respectively, as compared to their values in the dark condition. The plots clearly show that the JSC has a direct correlation with the change in e 0 of the active material. In other words, the devices in which illumination can decrease the dielectric constant of the material, may act as a better current source. We have speculated on the correlation between JSC and De 0 . To align fermi levels in sandwiched structures with dissimilar electrodes, space charges are accumulated near the
330
B. Pradhan, A.J. Pal / Chemical Physics Letters 416 (2005) 327–330
organic/metal interfaces. In dark conditions, space charges contribute to the capacitance of the devices. Under illumination and consequent charge separation, the contribution of space charges near the organic/metal interfaces decreases. This results in a decrease in bulk capacitance of the device and hence dielectric constant of the active material. We may hence conclude that a decrease in dielectric constant can be a good measure to determine efficient electron-hole separation in a photovoltaic device.
Acknowledgements
4. Conclusion
[1] C.W. Tang, Appl. Phys. Lett. 48 (1986) 183. [2] G. Yu, J. Gao, J. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. [3] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [4] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2003) 3693. [5] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841. [6] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 15. [7] M. Hiramoto, H. Fukusumi, M. Yokoyama, Appl. Phys. Lett. 61 (1992) 2580. [8] H. Mattoussi, M.F. Rubner, F. Zhou, J. Kumar, S.K. Tripathy, L.Y. Chiang, Appl. Phys. Lett. 77 (2000) 1540. [9] M.F. Durstock, R.J. Spry, J.W. Baur, B.E. Taylor, L.Y. Chiang, J. Appl. Phys. 94 (2003) 3253. [10] B. Pradhan, A. Bandyopadhyay, A.J. Pal, Appl. Phys. Lett. 85 (2004) 663. [11] L. Han, N. Koide, Y. Chiba, T. Mitate, Appl. Phys. Lett. 84 (2004) 2433.
In conclusion, we have studied dielectric properties of organic photovoltaic devices with controlled number of molecularly thin donor/acceptor interfaces. The devices have been modeled to its electrical analogue. Though the VOC remained mostly the same in these devices, the JSC was higher in the devices with moderate number of interfaces. Such dependence was due to opposing contribution of the D/A interfaces on exciton dissociation and charge transport. Upon illumination, RP of the device and e 0 of the material decreased. The variation of JSC was compared with the decrease in RP and e 0 or devices with different number of interfaces. The results showed a clear correlation between JSC and change in e 0 . We observed that the devices in which illumination could offer large decrease in e 0 of the active material would act as a better photovoltaic device.
The authors acknowledge financial supports from the Department of Atomic Energy, Government of India (Project 2002/37/14/BNRS) and the Department of Science and Technology, Government of India (Project SP/S2/M-44/ 99). References