Optical and morphological investigations of non-homogeneity in poly¯uorene blends J. Morgado a,b , E. Moons c , R.H. Friend a , F. Cacialli a,* a Cavendish Laboratory, Cambridge University, Madingley Road, Cambridge CB3 0HE, UK b Instituto Superior Te Âcnico, Av. Rovisco Pais, Lisboa, Portugal c CDT Ltd., Madingley Road, Cambridge CB3 0HJ, UK Abstract We report investigations of organic light-emitting diodes with an active layer consisting of poly9,9-dioctyl¯uorene), PFO, blended with a green emitting derivative F8BT). We used indium tin oxide ITO) anodes with and without a hole transporting layer, consisting of doped poly3,4-ethylene dioxythiophene). We also studied the incorporation of an oxadiazole based hole-blocking/electron transporting layer, HBL, namely 2-4-biphenylyl)-5-butylphenyl-1,3,4-oxadiazole, PBD, between the emissive layer and the Ca/Al cathodes. The presence of this layer reduces the range of radiative recombination in a region closer to the HBL interface, and therefore, allows investigation of depth- dependent structural non-uniformities of the emissive layer. We ®nd marginal alterations of the electroluminescence spectra depending on theweightconcentrationoftheF8BTintherange5±25%.Ef®cientenergytransferfromthehostPFO)totheguestF8BT)ensuresthatthe emission spectra are dominated by the green component. However, we ®nd that blue emission from the PFO host is much more intense for the structures incorporating the HBL than for those without it. We propose that phase separation phenomena of the blend induced by the spin-coating of the PBD-based layer, are responsible for the higher weight of the blue component. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Poly¯uorene; Blends; Phase separation; AFM 1. Introduction Poly¯uorenes constitute a family of interesting organic semiconductors for application in light-emitting diodes, LEDs). They display electroluminescence EL) [1], and photoluminescence PL) and a high ionisation potential. Recently, circularly polarised emission has been demon- strated by embedding oligo¯uorenes in a glass forming chiral nematic liquid crystal [2]. Thepossibilityofpreparingusefuloptoelectronicmateri- als by mixing different semiconductors in solution is an important advantage of organic over inorganic semiconduc- tors. This approach allows decoupling of the electronic transport and luminescence functionalities, and therefore, a greater freedom in the choice of optimum structures. The EL ef®ciency and the value of both turn-on and operating voltage can be improved effectively with this strategy [3], which also allows for harvesting of both ¯uorescence and phosphorescence, where one of the components displays ef®cient phosphorescent emission [4,5]. For polymeric blends, control over the active layer morphology is a very important issue, as in general the small value of the entropy of mixing leads to phase separation of polymeric compo- nents, even if chemical differences are minimum. These phenomena enrich and complicate the physics of devices, withadditionofnewpropertieswhichneedtobeunderstood for optimum design. Here, we report the fabrication of LEDs which make use of poly9,9-dioctyl¯uorene), PFO, blended with a green- emitting derivative, namely: poly9,9-dioctyl¯uorene-alt- benzothiadiazole), F8BT. Ef®cient energy transfer from thehostPFO)totheguestF8BT)ensuresthattheemission spectraaredominatedbythegreencomponent.However,we ®nd that blue emission from the PFO host is much more intense for structures incorporating a hole-blocking layer HBL), between the cathode and the emissive blend ®lm. 2. Experimental Wepreparedthreedifferentblendswith25,15and5%by weightofF8BTinPFO,whichwerefertoasF75,F85,and Synthetic Metals 124 2001) 63±66 * Corresponding author. Tel.: 44-1223-333-318; fax: 44-1223-353-397. E-mail address: fc10004@cam.ac.uk F. Cacialli). 0379-6779/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII:S0379-677901)00423-4