Abstract
Solution-processed, planar heterojunction organic photovoltaic diodes offer severalpotential advantages over bulk heterojunction structures in relation to electrodeselectivity, reduced dark currents and suitability for fundamental studies. They have,however, received less interest in recent years, in large part due to fabricationdifficulties encountered for sequential solution deposition steps. In this study, a novelstamp transfer technique that allows ready fabrication of planar heterojunctionsfrom a variety of solution-processed organic materials is applied to constructbilayer heterojunctions from poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61 butyric acid methyl ester (PCBM). We show that whilst ‘as made’ planar heterojunctionsyield relatively poor photocurrent generation (compared to equivalent bulk heterojunctiondevices), thermal annealing improves their performance via creation of a diffuse mixedP3HT:PCBM interface layer. Good device performance with the anticipated low darkcurrent is then achieved. Spectroscopic ellipsometry allows us to monitor the changes in theinterface layer that result from annealing. We also model the external quantum efficiencyspectra and show that they are consistent with the ellipsometry data. Furthermore, it isshown that good device performance is strongly dependent on the P3HT and PCBM layerordering with respect to the electrodes, confirming the important role of electrodeselectivity. Melting of ‘incorrectly’ ordered planar heterojunction devices (with donor nextto the high work function and acceptor next to the low work function electrode) leads tothe formation of bulk heterojunction devices, thereby recovering much of the desiredperformance.
Published Version
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