Quenching of the Magnetic Field Effect on the Photocurrent and Photocurrent Detected Magnetic Resonance in Conjugated Po
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INTRODUCTION Photo-induced electron transfer in solid-state composites of conjugated polymers and fullerenes has attracted considerable scientific and technological attention [1]. The possibility of utilising this effect for photo-voltaic energy conversion is especially interesting [1]. The photo-induced forward electron transfer occurs within 100 fs and the charge-separated state is very long lived [1]. By doping the conjugated polymer matrix with a few wt % of fullerene a strong quenching of the photoluminescence and an increase of the photo-conductivity of nearly two orders of magnitude has been observed [1]. Light-induced electron-spin-resonance (LESR) studies have been reported revealing the appearance of two LESR signals [1,2]. These signals are attributed to the radical anion of the fullerene molecule and the positive polaron on the conjugated chain. For composites of MDMO-PPV/PCBM (1-(3-methoxycarbonyl)-propyl-1-phenyl(6,6)C61 g-values of g=1.9995 (PCBM -) and g=2.0025 have been reported [2]. While ordinary ESR is best suited for large volume samples it is often not sensitive enough for applications in thin layered samples and especially in active devices. One has to use other magnetic resonance techniques e.g. optically detected magnetic resonance (ODMR) or electrically detected magnetic resonance (EDMR). Usually these experiments are based on the fact that the recombination of the photo-generated charge carriers depends on their relative spin orientation. Observing a process e.g. delayed luminescence or photoconductivity ESR transitions can change the spin orientations and therefore the yield of the observed property. Especially ODMR and EDMR are well established and powerful spectroscopic techniques to study the recombination properties in inorganic semiconductors and as recently shown also in organic systems like conjugated polymers
[3]. Magnetic field effect (MFE) and photo-current detected magnetic resonance studies have already been reported for photocells based on conjugated polymers alone [4,5]. These experiments suggest that so-called polaron pairs are formed as an intermediate state after photo-excitation [4,5]. For different PPV’s a positive MFE on the photo-current has been reported [4] and a decrease of the photo-current under microwave resonance conditions [5]. We are especially interested in the physical processes occurring in a photo-voltaic device based on a blend of MDMO-PPV/PCBM. For these photocells energy conversion efficiencies of 0.7 % (under AM1.5 conditions) have been reported [6], they can be prepared on flexible substrates and are very promising for low cost applications.
EXPERIMENTAL The arrangement of the thin film devices is shown in Fig. 1. The substrates were cleaned and etched using standard procedures described elsewhere [7]. After depositing a PEDOT (poly(ethylenedioxythiophene) and a photo-active film by spincast or doctor balding technique, aluminium is evaporated as second electrode. All films were prepared from solutions holding the same concentration of MDMO-PPV (0.
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