fullerene composites

Current Applied Physics 9 (2009) 1315–1317

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Charge transport in low-concentration MEH-PPV conjugated polymer/fullerene composites K.W. Lee, K.H. Mo, J.W. Jang, N.K. Kim, W. Lee, I.-M. Kim, Cheol Eui Lee * Department of Physics, Korea University, Seoul 136-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 7 November 2008 Accepted 11 November 2008 Available online 6 March 2009 PACS: 78.66.Qn 78.66.Sq 78.66.Tr 72.40.+w

a b s t r a c t Charge transport was studied in composites of poly[2-methoxy-5-(20 -ethyl-hexyloxy)-p-phenylenevinylene] (MEH-PPV) conjugate polymer and low-concentration fullerenes (C60) below the percolation threshold. The electron mobility showed a linear increase with the fullerene concentration, to which the hole mobility was insensitive. Our results indicate that fullerene–polymer networks provide a conduction path to the electrons, whereas the holes are transported through the polymer-only paths. The microscopic environments of the two distinct conduction paths in the composites as revealed by the electric field dependence of the mobilities are also discussed. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Charge transport Low-concentration MEH-PPV/fullerene composites

1. Introduction During the last decade there has been a great interest in the conjugated-polymer–fullerene bulk heterojunctions [1–10]. The polymer (donor)-to-fullerene (acceptor) photo-induced electron transfer in the excited state has been well evidenced [1,3–5,9]. The photo-excited species are dissociated at the polymer–fullerene junction, the electrons being transferred to the fullerenes. Since the charge transfer takes place much faster than the radiative and/or nonradiative decay of the photoexcitation [1,5], the quantum efficiency for charge transfer and charge separation is nearly unity, which would give rise to very high photovoltaic efficiencies. However, the photovoltaic quantum efficiency can be limited by the charge collection efficiency, as the separated charges must be collected prior to the recombination. The photo-generated current is directly governed by the carrier mobility as well as by the number of photo-generated charge carriers [10–18]. The maximum device efficiency of the bulk heterojunctions was obtained at around 80 wt% fullerenes, above which concentration reduced light absorption deteriorates the device performance [18]. In the bulk heterojunctions, two thresholds have been found in the PCBM concentration, corresponding to the open* Corresponding author. Tel.: +82 2 3290 3098; fax: +82 2 927 3292. E-mail address: [email protected] (C.E. Lee). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2008.11.010

ing up of the percolation pathways of PCBM at around 20 wt% and the formation of PCBM-rich aggregates at about 60 wt%, respectively [10]. For low fullerene concentrations, a number of free electrons produced by the exciton dissociation at the polymer–fullerene interface were taken to be localized on the isolated fullerene clusters and to form geminate pairs of an electron at the fullerene and a hole at the polymer [10,18]. The light absorption generating bound electron–hole pairs and the dissociation efficiency of the pairs greatly affect the device performance [18]. In a recent study of the carbon nanotube (CNT)/polythiophene composites, a very low threshold of about 0.005% CNT was found, above which concentration the CNT-polymer networks are believed to arise [19]. Another threshold, apparently corresponding to the CNT percolation, was found at about 1% CNTs. Above this concentration the CNTs are closely packed without polymer links, and there is no polymer contribution to the charge transport [20]. The MEH-PPV conjugate polymer, a PPV derivative, is known to be a hole transport material, its hole mobility being several orders of magnitude higher than its electron mobility [12]. Photoinduced electron transfer in the MEH-PPV/fullerene composites is well known, whereas no interaction between MEH-PPV and fullerenes has been found in the ground state [1,3]. In this work, the electron and hole transport has been studied in composites of MEH-PPV and fullerenes below the fullerene percolation threshold.

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2. Experiment The MEH-PPV conjugated polymers were purchased from Aldrich and the fullerenes from SES Research Inc. The MEH-PPV/fullerene composite solutions were prepared by blending the MEHPPV with the fullerenes in chlorobenzene and dispersing them by a magnetic stirrer. The photovoltaic devices were fabricated by spin-coating the composite solution onto the ITO glasses of 1 cm  1 cm to a thickness of about 200 nm. For a cathode metal contact, a thin Al layer was deposited on the sample by thermal evaporation. Thus, devices fabricated in a thin sandwich configuration, ITO/composite/Al, were obtained. For the time-of-flight (TOF) mobility measurements, 355-nm Nd-YAG laser pulses with a duration of 7 ns were illuminated on one of the electrodes. The photo-created carriers drifted across the sample under a reverse bias field to be collected at the opposite electrode. The incident photon energy (355 nm) was chosen well above the bandgap of the polymer. The optical absorption coefficient was large enough to ensure the incident light to be absorbed within a very shallow penetration depth near the illuminated surface. The hole mobility was measured by illuminating the cathode (Al electrode) by the laser pulses, whereas the electron mobility was measured by illuminating the anode (ITO electrode). 3. Results and discussion Fig. 1 shows the logarithmic plot of the electron and hole mobilities measured at 0.1 wt% and 10 wt% as a function of square root of the electric field. At 0.1 wt%, the electron mobility is lower than the hole mobility, strongly depending on the electric field, whereas the hole mobility shows a very weak field dependence. On the contrary, at 10 wt%, the electron mobility is much higher than the hole mobility, depending very weakly on the electric field, whereas the hole mobility exhibits a strong field dependence. The logarithmic plot in Fig. 1 indicates that the carrier mobility l follows a Poole–Frenkel form, a universal behavior in the conjugated polymers,

pffiffiffi

l ¼ l0 expðc EÞ;

ð1Þ

where l0 is the zero-field mobility and c the electric field coefficient [13–15]. In the pristine MEH-PPV, the fits according to Eq. (1) gave l0h  8  107 cm2/Vs and ch  9  104 V1/2 cm1/2 for the holes, and l0e  8  109 cm2/Vs and ce  7  104 V1/2cm1/2 for the electrons. In a previous work [13], l0h  2  107 cm2/Vs and ch  3.4  103 V1/2 cm1/2 were obtained by the TOF measurements on MEH-PPV. The slightly greater zero-field hole mobility obtained in this work is compatible with the slightly smaller electric field coefficient, also indicating less energetic disorder [14]. Fig. 2 shows the zero-field mobilities obtained by the fits in Fig. 1 as a function of the fullerene concentration. The zero-field electron mobility shows a linear increase with the fullerene concentration, in contrast to the zero-field hole mobility showing only a very weak dependence. The electrons cannot be transported without the fullerene–polymer paths in the low fullerene concentration range investigated in this work, below the fullerene percolation threshold of about 20 wt% [6,10]. In other words, the fullerene–polymer networks can provide a conduction path to the electrons on the isolated fullerene clusters. Thus, the linear increase in the zero-field electron mobility with increasing fullerene concentration can readily be ascribed to the three-dimensional fullerene–polymer networks serving as the electron conduction paths below the fullerene percolation threshold. On the other hand, the insensitivity of the zero-field hole mobility to the fullerene concentration indicates that the holes are transported through the polymer-only paths, whereas the slight decrease in the zero-field hole mobility with increasing fullerene concentration may be ascribed to an increase in the energetic disorders, corresponding to an increase in the electric field coefficient as discussed below. Fig. 3 shows the electric field coefficient obtained from the fits in Fig. 1 as a function of the fullerene concentration. In the pristine MEH-PPV, energetic disorders of the polymer chains are responsible for the electric field coefficients of both charge carriers, giving nearly the same values of ce and ch. In the MEH-PPV/fullerene composites, ch can still be attributed to the energetic disorders of the polymer chains, whereas ce will be due to the fullerene–polymer networks. At 0.1 wt% fullerenes, ch is an order of magnitude smaller than that in the pristine MEH-PPV, while ce remains almost un-

10 10 hole electron

μ0 (10 cm /Vs)

h, 10 wt.%

1

2

1

h, 0.1 wt.% e, 10 wt.%

-5

-5

2

μ (10 cm /Vs)

e, 0.1 wt.%

0.1

0.1 600

700

800 1/2

1/2

900

1000

1/2

E (V /cm ) Fig. 1. Logarithmic plot of the electron (square) and hole (circle) mobilities vs square root of the electric field. Solid and open symbols correspond to the fullerene concentrations of 0.1 wt% and 10 wt%, respectively. The solid lines are fits to Eq. (1).

0.1

1

10

Fullerene concentration (wt.%) Fig. 2. Electron (open symbols) and hole (solid symbols) zero-field mobilities, obtained from the fits in Fig. 1, as a function of the fullerene concentration.

K.W. Lee et al. / Current Applied Physics 9 (2009) 1315–1317

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showed little sensitivity, indicating that the electrons are transported through the three-dimensional fullerene–polymer networks whereas the holes are transported through the polymer-only paths. An increase in the fullerene concentration in the polymer matrix is shown to decrease the energetic disorder in the fullerene conduction path whereas it decreases that of the polymer matrix.

hole electron

1/2

cm )

10

γ (10 V

-1/2

Acknowledgements

-4

This work was supported by the Korea Ministry of Education, Science and Technology (NRL Program R0A-2008-000-20066-0, User Program of Proton Engineering Frontier Project, KRF-2006005-J03601). This work was also supported by the Seoul Research and Business Development Program (Grant No. 10583). The measurements at the Korean Basic Science Institute (KBSI) are acknowledged.

1

References 0.1

1

10

Fullerene concentration (wt.%) Fig. 3. Electric field coefficients, obtained from the fits in Fig. 1, as a function of the fullerene concentration. Open and solid symbols correspond to the electrons and the holes, respectively.

changed. As the energetic disorders in the polymer chains can be attributed to the conformational disorders [14], a small fullerene concentration appears to improve the polymer chain conformation. With further increase in the fullerene concentration, ch shows an increase, which is believed to arise from the fullerene aggregation promoting conformational disorders of the polymer chains [10]. In contrast to the case of ch, ce shows a gradual decrease with increasing fullerene concentration. Charge carriers in the disordered organic conductors are transported by hopping in a Gaussian site-energy distribution, the electric field coefficient being determined by the width of the Gaussian distribution and the intersite spacing [14]. Thus, in view of the electrons being transported through the fullerene–polymer networks and the separation between the fullerenes (or between fullerene aggregates) being reduced with increase in the fullerene concentration, the decrease in ce may be attributed to that in the intersite spacing. In summary, we have studied the charge transport in the MEHPPV/fullerene composites. The electron/hole mobilities were obtained at various fullerene concentrations below the percolation threshold as a function of the electric field and analyzed according to the Poole–Frenkel model. The electron mobility increased linearly with the fullerene concentration, while the hole mobility

[1] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [2] G. Yu, J. Gao, J.C. Hummelen, F. Fudl, A.J. Heeger, Science 270 (1995) 1789. [3] B. Kraabel, J.C. Hummelen, D. Vacar, D. Moses, N.S. Sarciftci, A.J. Heeger, F. Wudl, J. Chem. Phys. 104 (1995) 4267. [4] 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. [5] L. Smilowitz, N.S. Sariciftci, R. Wu, C. Gettinger, A.J. Heeger, F. Wudl, Phys. Rev. B 47 (1993) 13835. [6] C.J. Brabec, F. Padinger, N.S. Sariciftci, J.C. Hummelen, J. Appl. Phys. 85 (1999) 6866. [7] L.J.A. Koster, E.C.P. Smits, V.D. Mihailetchi, P.W.M. Blom, Phys. Rev. B 72 (2005) 085205. [8] J.G. Müller, J.M. Lupton, J. Feldmann, U. Lemmer, M.C. Scharber, N.S. Sariciftci, C.J. Brabec, U. Scherf, Phys. Rev. B 72 (2005) 195208. [9] J. De Ceuster, E. Goovaerts, A. Bouwen, J.C. Hummelen, V. Dyakonov, Phys. Rev. B 64 (2001) 195206. [10] T.J. Savenije, J.E. Kroeze, M.M. Wienk, J.M. Kroon, J.M. Warman, Phys. Rev. B 69 (2004) 155205. [11] R.C. Haddon, A.S. Perel, R.C. Morris, T.T.M. Palstra, A.F. Hebard, R.M. Fleming, Appl. Phys. Lett. 67 (1995) 121. [12] B.K. Crone, I.H. Campbell, P.S. Davids, D.L. Smith, Appl. Phys. Lett. 73 (1998) 3162. [13] I.H. Campbell, D.L. Smith, C.J. Neef, J.P. Ferraris, Appl. Phys. Lett. 74 (1999) 2809. [14] H.C.F. Martens, P.W.M. Blom, H.F.M. Schoo, Phys. Rev. B 61 (2000) 7489. [15] V.D. Mihailetchi, J.K. van Duren, P.W.M. Blom, J.C. Hummelen, R.A.J. Janssen, J.M. Kroon, M.T. Rispens, W.J.H. Verhees, M.M. Wienk, Adv. Funct. Mater. 13 (2003) 43. [16] S.A. Choulis, J. Nelson, Y. Kim, D. Poplavskyy, T. Kreouzis, J.R. Durrant, D.D.C. Bradley, Appl. Phys. Lett. 83 (2003) 3812. [17] R. Pacios, J. Nelson, D.D.C. Bradley, C.J. Brabec, Appl. Phys. Lett. 83 (2003) 4764. [18] V.D. Mihailetchi, L.J.A. Koster, P.W.M. Blom, C. Melzer, B. de Boer, J.K.J. van Duren, R.A.J. Janssen, Adv. Funct. Mater. 15 (2005) 795. [19] X.-Z. Bo, C.Y. Lee, M.S. Strano, M. Goldfinger, C. Nuckolls, G.B. Blanchet, Appl. Phys. Lett. 86 (2005) 182102. [20] K.W. Lee, S.P. Lee, H. Choi, K.H. Mo, J.W. Jang, H. Kweon, C.E. Lee, Appl. Phys. Lett. 91 (2007) 023110.