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organic photovoltaics; discotics; liquid crystalline; coronene
Abstract:
Two different types of soluble discotic liquid crystalline materials and a crystalline perylene dye have been used to create, directly from solution, photovoltaic devices which are compared in this work. Self-organisation of the soluble electron-accepting perylene derivative and the soluble liquid crystalline (LC) discotic material which is stable in a LC phase at room temperature (HBC-PhC12) leads to segregated structures optimised for charge separation and transport in photovoltaic device structures. High external quantum efficiencies up to 34% near 490 nm have been reached. The high efficiencies result from efficient photo-induced charge transfer between the materials as well as effective transport of electrons and holes to the cathode and anode through segregated perylene and the discotic peri-hexabenzocoronene p- system. Atomic force microscopy and device characteristics suggest that the driving force for phase separation and surface energy effects during spin coating of the HBC-PhC12:perylene blend result in a spontaneous vertical segregation of the HBC and the perylene normal to the plane of the spun film. This represents a nearly ideal, self-organised structure in which vertical segregation of charge transport layers coexist with a high interfacial area between the two charge transfer components. This vertical segregation has not been observed in the spin-coated blends where the HBC-PhC12 is replaced by HBC- C-8(*). One probable reason for this may be the different phase stability of the LC phase in the HBCs, which leads to different film-forming properties and film morphologies. (C) 2002 Elsevier Science B.V. All rights reserved.
