Electron Transport in a Methanofullerene** By Valentin D . Mihailetchi, Jeroen K. J . van Duren, Paul W . M. Blom,* Jan C. Hummelen, RenØ A. J . Janssen, Jan M. Kroon, Minze T . Rispens , Wil Jan H. Verhees ,and Martijn M. Wienk 1. Introduction Photovoltaic elements based on thin films of conjugated polymer/fullerene compounds are promising candidates for solar energy conversion. [1] Organic plastic solar cells have the potential for cost effectiveness. The mechanical flexibility and low specific weight of the plastic materials opens up a wide field of applications. A promising plastic solar cell candidate is based on the donor-type conjugated polymer poly(2-methoxy- 5-(3¢,7¢-dimethyloctyloxy)-p-phenylene vinylene) (OC 1 C 10 - PPV) and acceptor molecules of [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM). [1] As a concept, a bulk heterojunction that consists of a three- dimensional interpenetrating donor±acceptor network, sand- wiched between two electrodes with different work functions, is used. This generates an electric field across the organic layer. For this kind of cells, a power conversion efficiency of 2.5% under air mass (AM) 1.5 illumination has been reported. [2,3] From photophysical studies it has been demonstrated that after absorption of a photon, ultra-fast electron transfer takes place from the excited state of a conducting polymer to acceptor molecules such as Buckminster fullerenes (C 60 ), with a quan- tum efficiency close to unity. [4,5] Subsequently, the separated charge carriers are transported via the interpenetrating net- work to the electrodes. The photogenerated current is directly governed by the charge-carrier mobility, alongside the number of photoexcited charge carriers. For the understanding of the opto-electronic properties of OC 1 C 10 -PPV/PCBM-based solar cells, knowledge about the charge transport properties of the individual components is indispensable.ForOC 1 C 10 -PPV,thetransportofholeshasbeen extensively studied due to its application in polymer light-emit- ting diodes. From current density±voltage (J±V) measure- ments, [6] transient electroluminescent measurements, [7] and im- pedancespectroscopy, [8] aholemobility l h of510 ±11 m 2 V ±1 s ±1 hasbeenobtainedforOC 1 C 10 -PPV.Thefield(E)andtempera- ture(T)dependenceoftheholemobilityinPPVisdescribedby astretchedexponentialdependence lE; T l 0 Texp cT  E p   (1) where l 0 (T) is the zero-field mobility and c(T) describes the field activation. [6±8] Inthepresentstudy,theelectrontransportinPCBMisinves- tigated.ItisdemonstratedthattheelectroncurrentinPCBMis space±charge-limited, which allows for a direct determination of the electron mobility l e from the J±V measurements. At room temperature, an electron mobility of 210 ±7 m 2 V ±1 s ±1 is obtained. Consequently, at room temperature the electron mo- bilityinPCBMisafactorof4000greaterthantheholemobility in OC 1 C 10 -PPV, making OC 1 C 10 -PPV/PCBM bulk heterojunc- tionsolarcellselectron-dominateddevices. Furthermore, it is demonstrated that the field- and tempera- ture-dependence of l e also follows the empirical law l µ exp(c  E p ). By analyzing the E- and T -dependence with a Gaussian disorder model, information about the microscopic charge transport in PCBM is obtained. 2. Results and Discussion ThedevicesunderinvestigationconsistofasinglePCBMlayer sandwiched between a hole-conducting layer of poly(3,4-ethyl- enedioxythiophene)/poly(styrenesulfonate)(PEDOT:PSS),typ- ± [*] Prof. P. W. M. Blom,V. D. Mihailetchi,Prof. J. C. Hummelen, Dr. M. T. Rispens Materials Science Centre, University of Groningen Nijenborgh4,NL-9747AGGroningen(TheNetherlands) E-mail:P.W.M.Blom@phys.rug.nl J. K. J. van Duren,Prof. R. A. J. Janssen,Dr. M. M. Wienk Laboratory of Macromolecular and Organic Chemistry EindhovenUniversityofTechnology POBox513,NL-5600MBEindhoven(TheNetherlands) Dr. J. M. Kroon,W. J. H. Verhees Energy Research Centre of the Netherlands (ECN), Solar Energy POBox1,NL-1755ZGPetten(TheNetherlands) [**] These investigations were financially supported by the Dutch Ministries of EZ,O&W,andVROMthroughtheEETprogram(EETK97115). The current±voltage characteristics of methanofullerene [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM)-based devices are investigated as a function of temperature. The occurrence of space±charge limited current enables a direct determination of the electron mobility. At room temperature, an electron mobility of l e =210 ±7 m 2 V ±1 s ±1 has been obtained. This electron mobility is more than three orders of magnitude larger than the hole mobility of donor-type conjugated polymer poly(2-meth- oxy-5-(3¢,7¢-dimethyloctyloxy)-p-phenylene vinylene) (OC 1 C 10 -PPV). As a result, the dark current in PCBM/OC 1 C 10 -PPV based devices is completely dominated by electrons. The observed field and temperature-dependence of the electron mobility of PCBM can be described with a Gaussian disorder model. This provides information about the energetic disorder and average transport-site separation in PCBM. Adv.Funct. Mater. 2003, 13, No. 1, January Ó 2003WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim 1616-301X/03/0101-0043$17.50+.50/0 43 FULL PAPER