Controlling Organization in Photovoltaic Diodes from Discotic Liquid Crystals via Anode Surface Energy Alteration
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Controlling Organization in Photovoltaic Diodes from Discotic Liquid Crystals via Anode Surface Energy Alteration Johanna P. Schmidtke1 Klaus Müllen2 Richard H. Friend1 1 Cavendish Laboratory, University of Cambridge Cambridge CB3 0HE, United Kingdom 2 Max Planck Institut für Polymerforschung, Ackermannweg 10 55128 Mainz, Germany ABSTRACT We report the control of molecular ordering of discotic liquid crystals in thin, blended films used in photovoltaic diodes. The external quantum efficiency (EQE%) of photovoltaic diodes incorporating a crystalline hexabenzocoronene (HBC) derivative is improved by lowering the surface energy of the transparent anode surface with a short alkyl chain. Upon increasing the length of the functionalizing group on the anode, we find that the relative efficiency of the HBC component in the blend improves. Evidence of changed film morphology is also presented.
INTRODUCTION Since the demonstration of an organic bilayer photovoltaic diode by Tang, the role of heterojunctions in photovoltaic diodes has been widely studied [1,2]. The role of morphology has been well described for polymer blend and small molecule photovoltaic diodes [3,4]. However, the role of surface properties in molecular organization of efficient photovoltaic systems is less well understood. Surface conditions are frequently used to manipulate the bulk organization of liquid crystals, including discotic liquid crystalline materials [5, 6]. Discotic liquid crystalline materials offer a combination of solubility, spectral absorption, self-organization, and high charge transport which is promising for optoelectronic applications. Discotic liquid crystals, including pthalocyanines and hexa-perihexabenzocoronenes (HBC), demonstrate strong self-organizing characteristics in solution, in thin films and at interfaces [7-9]. Due to strong π-π interactions, HBC derivatives form aggregates at concentrations as low as 10-9 M and demonstrate one-dimensional charge-carrier mobilities as high as 0.5 cm2 V-1 s-1 [10,11]. HBCs have also been effectively incorporated into field effect transistors and as hole-transporters in photovoltaic diodes [12, 13]. Here we report photovoltaic diodes fabricated from a blend of hexadodecyl-peri-HBC and a perylene derivative. We demonstrate control over of the molecular-scale organization of the film by altering the surface energy of the anode via functionalization with alkyltrichlorosilanes and describe the effects on device performance.
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EXPERIMENTAL DETAILS The structures of the materials used in this study are shown in Figure 1. The hexadocdecyl-peri-HBC (HBC-C12) was synthesized as reported in [14]. The perylene derivative (EPPTC) was purchased from Sensient Imaging Technologies GmbH and used without further purification. Trichlorosilanes were purchased from Aldrich. The photovoltaic diodes were prepared on glass/ITO substrates coated with 60 nm of poly(3,4ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). The ITO/PEDOT:PSS anode was functionalized with alkyl-trichlorosilanes
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