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Concentration dependence of photovoltaic properties of photodiodes based on polymer–fullerene blends
Materials Science and Engineering B 137 (2007) 5–9
Concentration dependence of photovoltaic properties of photodiodes based on polymer–fullerene blends Hui Jin, Yan-Bing Hou ∗ , Xian-Guo Meng, Feng Teng Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China Received 12 April 2006; received in revised form 29 September 2006; accepted 30 September 2006
Abstract Photovoltaic properties of photodiodes based on the blends of poly[2-methoxy-5-(2 -ethylhexoxy-1,4-phenylenevinylene) (MEH-PPV) and fullerene (C60 ) are investigated. The experimental results show that the open-circuit voltage (Voc ) declines with the increasing concentration of C60 . A big variation of ∼400 mV in the Voc for the devices with the concentration of 0% and 50% indicates the electron affinity potential of C60 can strongly influence the resulting Voc . At the bias of −1.5 V, the photosensitivity of the device with the concentration of 50% is 55.6 mA/W under the illumination intensity of 16.7 mW/cm2 and its photogain sharply rises by two orders of magnitude than that of the undoped MEH-PPV device. © 2006 Elsevier B.V. All rights reserved. Keywords: Photovoltaic; Polymer; Fullerene
1. Introduction The use of soluble conjugated polymers as the active materials in optoelectronic applications significantly simplified the fabrication process of devices. Soluble conjugated polymers, with their advantages of low cost, flexibility, and high absorption coefficient, have shown potential for photodetector and photovoltaic applications. In these years, power conversion efficiencies of thin-film polymer photovoltaic cells have been increased steadily and rapidly [1–4]. These improvements are partly attributed to the discovery of the ultrafast photo-induced charge transfer from conjugated polymers to fullerene (C60 ), which brought new possibilities for photovoltaic devices using a combination of polymer and C60 [5]. The best performance was currently obtained with polymer–fullerene bulk heterojunction solar cells, yielding power conversion efficiency of typically 2.5% under 1 sun (100 mW/cm2 ) AM 1.5 simulated solar illumination [6]. Different concentrations of C60 can make different effects on the morphology of blend films [7], or even the different transport mechanism of charge carriers [8], both of which may lead to dramatic changes in photovoltaic properties. In this paper,
∗
Corresponding author. Tel.: +86 10 51688605; fax: +86 10 51688018. E-mail address: [email protected] (Y.-B. Hou).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.09.026
C60 concentration dependence of photovoltaic properties in the MEH-PPV/C60 blends has been reported. Due to the addition of C60 , the current–voltage characteristics of the devices based on C60 doped MEH-PPV both in the dark and under illumination are very different from that of the device made from undoped MEH-PPV.
2. Experimental In our experiments, MEH-PPV and C60 were used as the polymer host and the electron acceptor, respectively. The charge transfer from MEH-PPV to C60 can efficiently dissociate excitons generated in MEH-PPV chains. The molecular structures of the two materials are shown in Fig. 1. The blends of MEHPPV and C60 at weight ratios (the weight of C60 /the weight of MEH-PPV) of 1%, 2%, 5%, 10%, 20% and 50% were prepared in toluene solvent. Photovoltaic cells were fabricated using a traditional sandwich structure of ITO/PEDOT–PSS/MEHPPV–C60 /LiF/Al. PEDOT–PSS was spin-coated on top of cleaned ITO-coated glass substrates as polymer anode, followed by thermal treatment leaving a 20 nm-thick film. The composition of MEH-PPV and C60 was deposited on top of PETOD–PSS by spin-coating with a thickness of around 100 nm.The cathode of LiF/Al was deposited in sequence through a shadow mask, defining a device area of 3.5 mm2 , on top of the active layer
6
H. Jin et al. / Materials Science and Engineering B 137 (2007) 5–9
Fig. 1. Molecular structures of poly[2-methoxy-5-(2 -ethylhexoxy-1,4phenylenevinylene] (MEH-PPV) and C60 .
by thermal evaporation in a vacuum lower than 10−5 Torr. The thickness of LiF layer was about 1 nm. All devices were encapsulated in a glove box filled with dry nitrogen. In the glove box, the concentrations of both O2 and H2 O were lower than 5 ppm. The device based on undoped MEH-PPV was also fabricated in the same process for comparison. Devices were illuminated through ITO electrodes, and forward bias was defined as positive voltage applied to the ITO electrode. Current–voltage curves were recorded by a Keithely 2410 Source Measure Unit both in the dark and under illumination. The incident light from a Xe lamp was passed through a monochromator to select a wavelength at 500 nm with an intensity of 16.7 mW/cm2 . 3. Results and discussion Fig. 2 shows the current–voltage (I–V) curves of six photodiodes based on MEH-PPV–C60 blends with different concentrations (1%, 2%, 5%, 10%, 20% and 50%) of C60 in the dark. Fig. 3 compares the I–V characteristics of an undoped MEH-PPV device and the blend film devices with various concentrations both in the dark and under illumination on single-log scale. As shown in Fig. 2, the forward dark I–V curves exhibit two regions: a very small current (<0.05 A/cm2 ) and an exponential increase by more than two orders of magnitude within the range of 0.5 V. These two regions present different transport behaviors of carriers. The small current is dominated by leakage through the device, and much small leakage current indicates good quality of devices. Once the current increases exponentially with applied voltage, carrier injection occurs [9,10]. Between these two regions, there is a turn-on voltage. It is noted that, in Fig. 3, the turn-on voltage in the dark current becomes unclear as the concentration of C60 increases. Due to the insolubility of C60 , C60 molecules only disperse, but not dissolve into the MEHPPV solution. At high concentration, inhomogeneous dispersion
Fig. 2. Dark current–voltage characteristics for blend film devices with the concentrations of 1% (solid), 2% (dash), 5% (dot), 10% (dash dot dot), 20% (short dash dot) and 50% (short dash).
and congregation of C60 molecules may result in the increase in the leakage current. Concomitantly, Fig. 2 obviously shows the decline of the turn-on voltage with the increasing concentration of C60 , i.e., the built-in potential for carrier injection into the device declines. Consequently, both the rise in leakage current and the reduction of potential for carrier injection make the turn-on voltage at high concentration unclear. As well known, the voltage required for carrier injection (the turn-on voltage) almost equals to the open-circuit voltage (Voc ) of devices, since carrier injection occurs once applied forward bias overcomes the built-in potential. So, the Voc declines with the increasing concentration of C60 , which is demonstrated by Fig. 3. According to the Voc of devices with various concentrations listed in Table 1, the Voc is reduced from 1.02 V for the device made from undoped MEH-PPV to 0.608 V for the device based on the composite with C60 concentration of 50%. This result that the Voc reduces with the increasing concentration of C60 is coincident with the result obtained by Yang and his coworkers [11]. Mihailetchi et al. [12] proposed that the loss of the Voc was caused by the band bending created by accumulated charges at an ohmic contact. Snaith et al. [13] attributed the reduction of the Voc to the balance between the drift current and the diffusion current. Brabec et al. [14] have investigated origin of the Voc using a series of fullerene derivatives with varying acceptor strengths. They suggested that the Voc was strongly affected by the acceptor strength of the fullerenes, while it was insensitive to variations of the work function of the cathode. In our work, a maximal variation of almost 400 mV in the Voc for devices with different concentrations is shown, which indicates that the Voc correlates directly with the concentration of C60 . Our explanation for the change of the Voc is that the
Table 1 Open-circuit voltage (Voc ) and Short-circuit current (Isc ) of devices with different C60 concentrations
Voc (V) Isc (A/cm2 )
MEH-PPV
1%
2%
5%
10%
20%
50%
1.020 8.28
0.870 14.87
0.855 11.47
0.795 9.05
0.735 9.46
0.720 26.87
0.608 91.81
H. Jin et al. / Materials Science and Engineering B 137 (2007) 5–9
7
Fig. 3. The current–voltage characteristics of an undoped MEH-PPV device and blend film devices with different C60 concentrations (1%, 2%, 5%, 10%, 20% and 50%) in the dark (circle) and under illumination (square) are plotted on single-log scale. The illumination light is monochromatic at the wavelength of 500 nm with the intensity is 16.7 mW/cm2 .
interaction between the electron affinity potential of C60 and the work function of Cathode determines the final value of the Voc . Fig. 4 illustrates the variation of the Voc with the electron affinity potential of C60 and the electrode work function in ITO/PEDOT–PSS/MEH-PPV–C60 /LiF/Al blend film devices. For the device made from undoped MEH-PPV, the Voc is essentially determined by the difference of work function between anode (ITO/PEDOT–PSS) and cathode (LiF/Al). In Fig. 4, it is
expressed by Voc1 . For the composite devices, the doping of C60 reduces the Voc . The higher concentration of C60 is, the lower the Voc is. Between the concentrations of 1% and 50%, the variation of 270 mV in the Voc is found. Recent results showed that there was considerable charge transfer at fullerene/metal interfaces [15]. The extent of the energy alignment of the metal to the LUMO level of C60 relies on the strength of charge transfer between C60 molecules and the metal electrode. In such a case,
8
H. Jin et al. / Materials Science and Engineering B 137 (2007) 5–9
Fig. 4. Schematic variation of Voc with the electron affinity potential of C60 and the electrode work function in ITO/PEDOT–PSS/MEH-PPV–C60 /LiF/Al blend film devices as a function of concentration of C60 . For an undoped MEHPPV device, the Voc is dominated by the different work function between the anode and the cathode (Voc1 ). With the increasing concentration of C60 , the limitation for resulting Voc corresponding to a MEH-PPV–C60 composite device at negative electrode gradually shifts towards the LUMO energy level of C60 (Vocn ). When the LUMO of the acceptor (C60 ) completely acts as the limitation for Voc , the resulting Voc can be expressed by Voc2 .
the work function of the metal is pinned to the Fermi level of the organic semiconductor [16], which provides a new limitation at the cathode for the Voc of MEH-PPV–C60 blend devices (Vocn ). As the concentration of C60 increases, more C60 /metal interfaces are provided. Thus, stronger charge transfer leads to the lower Voc . When the concentration of C60 is high enough, the strong charge transfer can result in that the lowest unoccupied molecular orbital (LUMO) energy level (∼4.5 eV) of C60 totally replaces the lower work function of LiF/Al (<4.3 eV) [17,18] as a new limitation for the Voc (expressed by Voc2 in Fig. 4). Above explanation manifests the direct correlation between the Voc and the strength of charge transfer at C60 /metal interfaces. The doping of C60 obviously improves the photosensitivity and the photogain (photogain = light current/dark current) at reverse bias, compared with those of the device made from undoped MEH-PPV. Fig. 5 shows the photosensitivity at −0.5 V, −1.0 V and −1.5 V and the photogain at −1.5 V,
Fig. 5. The photosensitivity at −0.5 V (square), −1.0 V (circle) and −1.5 V (triangle) and the photogain at −1.5 V (star) are shown for the undoped MEHPPV device and the blend film devices with C60 concentrations of 1%, 10%, 20% and 50%.
respectively, for the undoped MEH-PPV device and the blend devices at the concentrations of 1%, 10%, 20% and 50%. At −1.5 V, the photosensitivity of the device with the concentration of 50% is 55.6 mA/W under the illumination intensity of 16.7 mW/cm2 . The corresponding external quantum efficiency (EQE) is about 14%. These features make photodetectors based on polymer–fullerene blend systems attractive for specific applications. The photogain increases with the increasing concentration of C60 at −1.5 V. And for the device with the concentration of 50%, the photogain sharply rises by two orders of magnitude than that of the undoped MEH-PPV device and reaches to ∼6200. 4. Conclusions In summary, the photovoltaic properties of devices based on MEH-PPV–C60 blends are investigated. The decline of the opencircuit voltage with the increasing concentration of C60 can be attributed to the increasing strength of charge transfer between C60 /metal interfaces. Photodiodes made from MEH-PPV–C60 blend show remarkable photosensitivity and photogain at reverse bias. The polymer–fullerene systems open the new way to the development of high sensitivity photodetectors. Acknowledgement The work is supported by Trans-Century Training Program Foundation for the Talents of Natural Science by the State Education Commission, the Key Project of the Ministry of Education of China under Grant No. 105041, the National Natural Science Foundation of China under Grant Nos. 90401006, 10434030 and 90301004, and the National Key Basic Research and Development Programme of China under Grant No. 2003CB314707. One of authors (Hui Jin) is grateful to the Doctor Innovation Foundation of Beijing Jiaotong University. References [1] J.J.M. Hall, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [2] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. ¨ [3] M. GranstrOm, K. Petritsch, A.C. Arias, A. Lux, M.R. Andersson, R.H. Friend, Nature 395 (1998) 257. [4] H. Jin, Y.B. Hou, X.G. Meng, F. Teng, Chin. Phys. Lett. 23 (2006) 693. [5] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [6] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841. [7] A. Ltaief, J. Davenas, A. Bouazizi, R.B. Chaˆabane, P. Alcouffe, H.B. Ouada, Mater. Sci. Eng. C 25 (2005) 67. [8] C.H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N.S. Sariciftci, A.J. Heeger, F. Wudl, Phys. Rev. B 48 (1993) 15425. [9] G. Yu, C. Zhang, A.J. Heeger, Appl. Phys. Lett. 64 (1994) 1540. [10] G. Yu, A.J. Heeger, J. Appl. Phys. 78 (1995) 4510. [11] J. Liu, Y.J. Shi, Y. Yang, Adv. Funct. Mater. 11 (2001) 420. [12] V.D. Mihailetchi, L.J.A. Koster, P.W.M. Blom, Appl. Phys. Lett. 85 (2004) 970. [13] H.J. Snaith, N.C. Greenham, R.H. Friend, Adv. Mater. 16 (2004) 1640.
H. Jin et al. / Materials Science and Engineering B 137 (2007) 5–9 [14] J.B. Christoph, A. Cravino, D. Meissner, N.S. Sariciftci, T. Fromherz, M.T. Rispens, L. Sanchez, J.C. Hummelen, Adv. Funct. Mater. 11 (2001) 374. [15] T.R. Ohno, Y. Cehn, S.E. Harvey, G.H. Kroll, J.H. Weaver, R.E. Haufler, R.E. Smalley, Phys. Rev. B 44 (1991) 13747.
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[16] V.D. Mihailetchi, P.W.M. Blom, J.C. Hummelen, M.T. Rispens, J. Appl. Phys. 94 (2003) 6849. [17] G.P. Kushto, W. Kim, Z.H. Kafafi, Appl. Phys. Lett. 86 (2005) 093502. [18] P. Peumans, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 126.
Concentration dependence of photovoltaic properties of photodiodes based on polymer–fullerene blends Hui Jin, Yan-Bing Hou ∗ , Xian-Guo Meng, Feng Teng Key Laboratory of Luminescence and Optical Information, Ministry of Education, Institute of Optoelectronic Technology, Beijing Jiaotong University, Beijing 100044, China Received 12 April 2006; received in revised form 29 September 2006; accepted 30 September 2006
Abstract Photovoltaic properties of photodiodes based on the blends of poly[2-methoxy-5-(2 -ethylhexoxy-1,4-phenylenevinylene) (MEH-PPV) and fullerene (C60 ) are investigated. The experimental results show that the open-circuit voltage (Voc ) declines with the increasing concentration of C60 . A big variation of ∼400 mV in the Voc for the devices with the concentration of 0% and 50% indicates the electron affinity potential of C60 can strongly influence the resulting Voc . At the bias of −1.5 V, the photosensitivity of the device with the concentration of 50% is 55.6 mA/W under the illumination intensity of 16.7 mW/cm2 and its photogain sharply rises by two orders of magnitude than that of the undoped MEH-PPV device. © 2006 Elsevier B.V. All rights reserved. Keywords: Photovoltaic; Polymer; Fullerene
1. Introduction The use of soluble conjugated polymers as the active materials in optoelectronic applications significantly simplified the fabrication process of devices. Soluble conjugated polymers, with their advantages of low cost, flexibility, and high absorption coefficient, have shown potential for photodetector and photovoltaic applications. In these years, power conversion efficiencies of thin-film polymer photovoltaic cells have been increased steadily and rapidly [1–4]. These improvements are partly attributed to the discovery of the ultrafast photo-induced charge transfer from conjugated polymers to fullerene (C60 ), which brought new possibilities for photovoltaic devices using a combination of polymer and C60 [5]. The best performance was currently obtained with polymer–fullerene bulk heterojunction solar cells, yielding power conversion efficiency of typically 2.5% under 1 sun (100 mW/cm2 ) AM 1.5 simulated solar illumination [6]. Different concentrations of C60 can make different effects on the morphology of blend films [7], or even the different transport mechanism of charge carriers [8], both of which may lead to dramatic changes in photovoltaic properties. In this paper,
∗
Corresponding author. Tel.: +86 10 51688605; fax: +86 10 51688018. E-mail address: [email protected] (Y.-B. Hou).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.09.026
C60 concentration dependence of photovoltaic properties in the MEH-PPV/C60 blends has been reported. Due to the addition of C60 , the current–voltage characteristics of the devices based on C60 doped MEH-PPV both in the dark and under illumination are very different from that of the device made from undoped MEH-PPV.
2. Experimental In our experiments, MEH-PPV and C60 were used as the polymer host and the electron acceptor, respectively. The charge transfer from MEH-PPV to C60 can efficiently dissociate excitons generated in MEH-PPV chains. The molecular structures of the two materials are shown in Fig. 1. The blends of MEHPPV and C60 at weight ratios (the weight of C60 /the weight of MEH-PPV) of 1%, 2%, 5%, 10%, 20% and 50% were prepared in toluene solvent. Photovoltaic cells were fabricated using a traditional sandwich structure of ITO/PEDOT–PSS/MEHPPV–C60 /LiF/Al. PEDOT–PSS was spin-coated on top of cleaned ITO-coated glass substrates as polymer anode, followed by thermal treatment leaving a 20 nm-thick film. The composition of MEH-PPV and C60 was deposited on top of PETOD–PSS by spin-coating with a thickness of around 100 nm.The cathode of LiF/Al was deposited in sequence through a shadow mask, defining a device area of 3.5 mm2 , on top of the active layer
6
H. Jin et al. / Materials Science and Engineering B 137 (2007) 5–9
Fig. 1. Molecular structures of poly[2-methoxy-5-(2 -ethylhexoxy-1,4phenylenevinylene] (MEH-PPV) and C60 .
by thermal evaporation in a vacuum lower than 10−5 Torr. The thickness of LiF layer was about 1 nm. All devices were encapsulated in a glove box filled with dry nitrogen. In the glove box, the concentrations of both O2 and H2 O were lower than 5 ppm. The device based on undoped MEH-PPV was also fabricated in the same process for comparison. Devices were illuminated through ITO electrodes, and forward bias was defined as positive voltage applied to the ITO electrode. Current–voltage curves were recorded by a Keithely 2410 Source Measure Unit both in the dark and under illumination. The incident light from a Xe lamp was passed through a monochromator to select a wavelength at 500 nm with an intensity of 16.7 mW/cm2 . 3. Results and discussion Fig. 2 shows the current–voltage (I–V) curves of six photodiodes based on MEH-PPV–C60 blends with different concentrations (1%, 2%, 5%, 10%, 20% and 50%) of C60 in the dark. Fig. 3 compares the I–V characteristics of an undoped MEH-PPV device and the blend film devices with various concentrations both in the dark and under illumination on single-log scale. As shown in Fig. 2, the forward dark I–V curves exhibit two regions: a very small current (<0.05 A/cm2 ) and an exponential increase by more than two orders of magnitude within the range of 0.5 V. These two regions present different transport behaviors of carriers. The small current is dominated by leakage through the device, and much small leakage current indicates good quality of devices. Once the current increases exponentially with applied voltage, carrier injection occurs [9,10]. Between these two regions, there is a turn-on voltage. It is noted that, in Fig. 3, the turn-on voltage in the dark current becomes unclear as the concentration of C60 increases. Due to the insolubility of C60 , C60 molecules only disperse, but not dissolve into the MEHPPV solution. At high concentration, inhomogeneous dispersion
Fig. 2. Dark current–voltage characteristics for blend film devices with the concentrations of 1% (solid), 2% (dash), 5% (dot), 10% (dash dot dot), 20% (short dash dot) and 50% (short dash).
and congregation of C60 molecules may result in the increase in the leakage current. Concomitantly, Fig. 2 obviously shows the decline of the turn-on voltage with the increasing concentration of C60 , i.e., the built-in potential for carrier injection into the device declines. Consequently, both the rise in leakage current and the reduction of potential for carrier injection make the turn-on voltage at high concentration unclear. As well known, the voltage required for carrier injection (the turn-on voltage) almost equals to the open-circuit voltage (Voc ) of devices, since carrier injection occurs once applied forward bias overcomes the built-in potential. So, the Voc declines with the increasing concentration of C60 , which is demonstrated by Fig. 3. According to the Voc of devices with various concentrations listed in Table 1, the Voc is reduced from 1.02 V for the device made from undoped MEH-PPV to 0.608 V for the device based on the composite with C60 concentration of 50%. This result that the Voc reduces with the increasing concentration of C60 is coincident with the result obtained by Yang and his coworkers [11]. Mihailetchi et al. [12] proposed that the loss of the Voc was caused by the band bending created by accumulated charges at an ohmic contact. Snaith et al. [13] attributed the reduction of the Voc to the balance between the drift current and the diffusion current. Brabec et al. [14] have investigated origin of the Voc using a series of fullerene derivatives with varying acceptor strengths. They suggested that the Voc was strongly affected by the acceptor strength of the fullerenes, while it was insensitive to variations of the work function of the cathode. In our work, a maximal variation of almost 400 mV in the Voc for devices with different concentrations is shown, which indicates that the Voc correlates directly with the concentration of C60 . Our explanation for the change of the Voc is that the
Table 1 Open-circuit voltage (Voc ) and Short-circuit current (Isc ) of devices with different C60 concentrations
Voc (V) Isc (A/cm2 )
MEH-PPV
1%
2%
5%
10%
20%
50%
1.020 8.28
0.870 14.87
0.855 11.47
0.795 9.05
0.735 9.46
0.720 26.87
0.608 91.81
H. Jin et al. / Materials Science and Engineering B 137 (2007) 5–9
7
Fig. 3. The current–voltage characteristics of an undoped MEH-PPV device and blend film devices with different C60 concentrations (1%, 2%, 5%, 10%, 20% and 50%) in the dark (circle) and under illumination (square) are plotted on single-log scale. The illumination light is monochromatic at the wavelength of 500 nm with the intensity is 16.7 mW/cm2 .
interaction between the electron affinity potential of C60 and the work function of Cathode determines the final value of the Voc . Fig. 4 illustrates the variation of the Voc with the electron affinity potential of C60 and the electrode work function in ITO/PEDOT–PSS/MEH-PPV–C60 /LiF/Al blend film devices. For the device made from undoped MEH-PPV, the Voc is essentially determined by the difference of work function between anode (ITO/PEDOT–PSS) and cathode (LiF/Al). In Fig. 4, it is
expressed by Voc1 . For the composite devices, the doping of C60 reduces the Voc . The higher concentration of C60 is, the lower the Voc is. Between the concentrations of 1% and 50%, the variation of 270 mV in the Voc is found. Recent results showed that there was considerable charge transfer at fullerene/metal interfaces [15]. The extent of the energy alignment of the metal to the LUMO level of C60 relies on the strength of charge transfer between C60 molecules and the metal electrode. In such a case,
8
H. Jin et al. / Materials Science and Engineering B 137 (2007) 5–9
Fig. 4. Schematic variation of Voc with the electron affinity potential of C60 and the electrode work function in ITO/PEDOT–PSS/MEH-PPV–C60 /LiF/Al blend film devices as a function of concentration of C60 . For an undoped MEHPPV device, the Voc is dominated by the different work function between the anode and the cathode (Voc1 ). With the increasing concentration of C60 , the limitation for resulting Voc corresponding to a MEH-PPV–C60 composite device at negative electrode gradually shifts towards the LUMO energy level of C60 (Vocn ). When the LUMO of the acceptor (C60 ) completely acts as the limitation for Voc , the resulting Voc can be expressed by Voc2 .
the work function of the metal is pinned to the Fermi level of the organic semiconductor [16], which provides a new limitation at the cathode for the Voc of MEH-PPV–C60 blend devices (Vocn ). As the concentration of C60 increases, more C60 /metal interfaces are provided. Thus, stronger charge transfer leads to the lower Voc . When the concentration of C60 is high enough, the strong charge transfer can result in that the lowest unoccupied molecular orbital (LUMO) energy level (∼4.5 eV) of C60 totally replaces the lower work function of LiF/Al (<4.3 eV) [17,18] as a new limitation for the Voc (expressed by Voc2 in Fig. 4). Above explanation manifests the direct correlation between the Voc and the strength of charge transfer at C60 /metal interfaces. The doping of C60 obviously improves the photosensitivity and the photogain (photogain = light current/dark current) at reverse bias, compared with those of the device made from undoped MEH-PPV. Fig. 5 shows the photosensitivity at −0.5 V, −1.0 V and −1.5 V and the photogain at −1.5 V,
Fig. 5. The photosensitivity at −0.5 V (square), −1.0 V (circle) and −1.5 V (triangle) and the photogain at −1.5 V (star) are shown for the undoped MEHPPV device and the blend film devices with C60 concentrations of 1%, 10%, 20% and 50%.
respectively, for the undoped MEH-PPV device and the blend devices at the concentrations of 1%, 10%, 20% and 50%. At −1.5 V, the photosensitivity of the device with the concentration of 50% is 55.6 mA/W under the illumination intensity of 16.7 mW/cm2 . The corresponding external quantum efficiency (EQE) is about 14%. These features make photodetectors based on polymer–fullerene blend systems attractive for specific applications. The photogain increases with the increasing concentration of C60 at −1.5 V. And for the device with the concentration of 50%, the photogain sharply rises by two orders of magnitude than that of the undoped MEH-PPV device and reaches to ∼6200. 4. Conclusions In summary, the photovoltaic properties of devices based on MEH-PPV–C60 blends are investigated. The decline of the opencircuit voltage with the increasing concentration of C60 can be attributed to the increasing strength of charge transfer between C60 /metal interfaces. Photodiodes made from MEH-PPV–C60 blend show remarkable photosensitivity and photogain at reverse bias. The polymer–fullerene systems open the new way to the development of high sensitivity photodetectors. Acknowledgement The work is supported by Trans-Century Training Program Foundation for the Talents of Natural Science by the State Education Commission, the Key Project of the Ministry of Education of China under Grant No. 105041, the National Natural Science Foundation of China under Grant Nos. 90401006, 10434030 and 90301004, and the National Key Basic Research and Development Programme of China under Grant No. 2003CB314707. One of authors (Hui Jin) is grateful to the Doctor Innovation Foundation of Beijing Jiaotong University. References [1] J.J.M. Hall, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Nature 376 (1995) 498. [2] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270 (1995) 1789. ¨ [3] M. GranstrOm, K. Petritsch, A.C. Arias, A. Lux, M.R. Andersson, R.H. Friend, Nature 395 (1998) 257. [4] H. Jin, Y.B. Hou, X.G. Meng, F. Teng, Chin. Phys. Lett. 23 (2006) 693. [5] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [6] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841. [7] A. Ltaief, J. Davenas, A. Bouazizi, R.B. Chaˆabane, P. Alcouffe, H.B. Ouada, Mater. Sci. Eng. C 25 (2005) 67. [8] C.H. Lee, G. Yu, D. Moses, K. Pakbaz, C. Zhang, N.S. Sariciftci, A.J. Heeger, F. Wudl, Phys. Rev. B 48 (1993) 15425. [9] G. Yu, C. Zhang, A.J. Heeger, Appl. Phys. Lett. 64 (1994) 1540. [10] G. Yu, A.J. Heeger, J. Appl. Phys. 78 (1995) 4510. [11] J. Liu, Y.J. Shi, Y. Yang, Adv. Funct. Mater. 11 (2001) 420. [12] V.D. Mihailetchi, L.J.A. Koster, P.W.M. Blom, Appl. Phys. Lett. 85 (2004) 970. [13] H.J. Snaith, N.C. Greenham, R.H. Friend, Adv. Mater. 16 (2004) 1640.
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