Ambipolar Charge Transport in Films of Methanofullerene and Poly(phenylenevinylene)/Methanofullerene Blends** By Sachetan M. Tuladhar, Dmitry Poplavskyy , Stelios A. Choulis , James R. Durrant, Donal D . C. Bradley , and Jenny Nelson* 1. Introduction Blends of conjugated polymer with the methanofullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have proven to be the most promising material combinations for all-organic solar cells, with power conversion efficiencies of over 3%. [1,2] The superior performance of methanofullerenes compared to al- ternative electron acceptors is attributed to their excellent elec- tron-transport and electron-accepting properties and their abil- ity to form a conducting network within the polymer matrix. However, some puzzles remain regarding the mechanism of photocurrent generation in the blend. In the case of the best- studied material combination, poly[2-methoxy-5-(3¢,7¢-dimethy- loctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with PCBM, optimum performance requires a higher volume fraction of ful- lerene than would be expected simply on the basis of the relative mobilities and absorption coefficients. This behavior may be ex- plained at least partly by the asymmetric morphologies of the two components in the blend. It has been shown that at large weight fractions of PCBM in the blend, estimated variously as between 60 % and 75 % [3] and over 50 %, [4] PCBM segregates out into clusters leaving a more homogenous matrix behind. The extent to which these clusters of PCBM contribute to the photo- current is not clear: Although some studies have shown an in- creased photocurrent at short wavelength at high-PCBM frac- tions, [3] other microscopic studies suggest that large PCBM clusters do not contribute significantly to photocurrent genera- tion. [5] In either case, conditions leading to PCBM segregation appear to be necessary in order to maximize photocurrent gen- eration, [3,6] which suggests that they play an important role in charge transport. Previous studies have shown that adding PCBM to MDMO- PPV [7,8] and to a red polyfluorene [9,10] improves not only the electron-transport but also the hole-transport properties of the blend. Hole transport becomes faster and less dispersive, which is surprising because fragmentation of the transport space in- creases spatial disorder and is naively expected to make trans- port both slower and more dispersive. Herein, we present results of a detailed study of hole trans- port in thin films of MDMO-PPV (Fig. 1a) blended with PCBM (Fig. 1b) and spin-coated from chlorobenzene, this being the best-studied material combination. Both electron and hole mo- bilities increased upon adding increasing amounts of PCBM to the blend. At 67 wt.-% PCBM, the hole mobility was more than two orders of magnitude greater than in the pure polymer. This was confirmed by three complementary measurement techniques. Changes in hole mobility were qualitatively similar to changes in electron mobility as PCBM was added, suggesting they arise from a similar mechanism. The proposal that holes can be transported in PCBM was confirmed by time-of-flight Adv. Funct. Mater. 2005, 15, 1171±1182 DOI: 10.1002/adfm.200400337  2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1171 ± [*] Dr. J. Nelson, S. M. Tuladhar, Dr. D. Poplavskyy, Dr. S. A. Choulis, Prof. D. D. C. Bradley Department of Physics, Imperial College London Prince Consort Road, London SW7 2BW (UK) E-mail: jenny.nelson@imperial.ac.uk Dr. J. R. Durrant Department of Chemistry, Imperial College London Prince Consort Road, London SW7 2AZ (UK) [**] We thank Amanda Chatten, James Kirkpatrick, Roberto Pacios, and Jo Wilson for useful discussions; J. C. Hummelen and M. Rispens for providing the PCBM; and Covion GmbH for providing the MDMO-PPV used in this study. The work was supported by British Petroleum Ltd and the UK Engineering and Physical Sciences Re- search Council. Herein, we report experimental studies of electron and hole transport in thin films of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) and in blends of poly[2-methoxy-5-(3¢,7¢-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) with PCBM. The low-field hole mobility in pristine MDMO-PPV is of the order of 10 ±7 cm 2 V ±1 s ±1 , in agreement with previous studies, whereas the electron mobility in pristine PCBM was found by current-density±voltage (J±V) measurements to be of the order of 10 ±2 cm 2 V ±1 s ±1 , which is about one order of magnitude greater than previously reported. Adding PCBM to the blend increases both electron and hole mobilities, compared to the pristine polymer, and results in less dispersive hole transport. The hole mo- bility in a blend containing 67 wt.-% PCBM is at least two orders of magnitude greater than in the pristine polymer. This result is independent of measurement technique and film thickness, indicating a true bulk property of the material. We therefore propose that PCBM may assist hole transport in the blend, either by participating in hole transport or by changing the poly- mer-chain packing to enhance hole mobility. Time-of-flight mobility measurements of PCBM dispersed in a polystyrene matrix yield electron and hole mobilities of similar magnitude and relatively non-dispersive transport. To the best of our knowledge, this is the first report of hole transport in a methanofullerene. We discuss the conditions under which hole transport in the ful- lerene phase of a polymer/fullerene blend may be expected. The relevance to photovoltaic device function is also discussed. FULL PAPER