Solution-processed, planar heterojunction organic photovoltaic diodes offer several potential advantages over bulk heterojunction structures in relation to electrode selectivity, reduced dark currents and suitability for fundamental studies. They have, however, received less interest in recent years, in large part due to fabrication difficulties encountered for sequential solution deposition steps. In this study, a novel stamp transfer technique that allows ready fabrication of planar heterojunctions from a variety of solution-processed organic materials is applied to construct bilayer heterojunctions from poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We show that whilst 'as made' planar heterojunctions yield relatively poor photocurrent generation (compared to equivalent bulk heterojunction devices), thermal annealing improves their performance via creation of a diffuse mixed P3HT:PCBM interface layer. Good device performance with the anticipated low dark current is then achieved. Spectroscopic ellipsometry allows us to monitor the changes in the interface layer that result from annealing. We also model the external quantum efficiency spectra and show that they are consistent with the ellipsometry data. Furthermore, it is shown that good device performance is strongly dependent on the P3HT and PCBM layer ordering with respect to the electrodes, confirming the important role of electrode selectivity. Melting of 'incorrectly' ordered planar heterojunction devices (with donor next to the high work function and acceptor next to the low work function electrode) leads to the formation of bulk heterojunction devices, thereby recovering much of the desired performance. © 2008 IOP Publishing Ltd.