In an organic photovoltaic device, a layer consisting of two organic compounds, one functioning as an electron acceptor and the other as an electron donor, is sandwiched between two electrodes. Sufficient concentration of the electron acceptor is required at the interface to the low work function electrode as well as sufficient concentration of the electron donor at the interface to the high work function electrode. Frequently used compounds are poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). It is reported in the literature that blends of these materials show a gradient throughout the polymer layer within devices due to phase separation. The direction of the gradient is dependent on the surface energy of the substrates onto which the polymer solutions are spincoated. This is assumed to be detrimental in the usual device structure of photovoltaic devices. In this report the surface of the blend was studied with a combination of a depth profiling technique sensitive for elemental composition and two electron spectroscopy techniques differing in their surface sensitivity. After spin coating, a thin layer of PCBM with a thickness of approximately a single monolayer was found just below the blend surface. The implication for organic photovoltaic devices is discussed.
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