Controlling the active layer composition in organic electronic devices represents one of the major challenges in their fabrication. In particular, the composition of mixed donor/acceptor active layers for photosensitive device applications is known to strongly influence device performance. Here, an alternative approach for the preparation of organic heterojunction photoactive layers by successive spray deposition of the donor material, poly(3-hexylthiophene) (P3HT), and acceptor material, [6,6]-phenyl C61-butyric acid methyl ester (PCBM), is reported. Optical absorption spectra, X-ray reflectivity, and cross-sectional transmission electron microscopy investigations are used to indicate the penetration of PCBM into a previously deposited P3HT layer and the spontaneous formation of a bulk heterojunction (BHJ) within the active layer, which provides the large interfacial area needed for efficient exciton dissociation. It is shown that organic photodiodes composed of photoactive layers prepared using this fabrication method exhibit a performance comparable to conventional BHJ devices in which the active layer is rigorously blended in advance. Moreover, separate handling of the individual materials and their deposition from distinct solutions enables an enhanced control of the active layer composition and hence increases the ability of tuning device characteristics.
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