Transverse Electrical Transport in Pentacene Photodiodes
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Transverse electrical transport in pentacene photodiodes Cristobal Voz1, Joaquim Puigdollers1, Marta Fonrodona1, Isidro Martin1, Albert Orpella1, Michael Vetter1, Francisco Fabregat2, Germa Garcia2, Juan Bisquert2 and Ramon Alcubilla1 1 Departament d’Enginyeria Electronica, Universitat Politecnica de Catalunya, Campus Nord, Barcelona E-08034, Spain 2 Departament de Ciencies Experimentals, Universitat Jaume I, Campus Riu Sec, Castelló de la Plana E-12080, Spain ABSTRACT The microstructure of pentacene thin films deposited by thermal evaporation is studied by X-ray diffraction. The transmittance of these films evidences different molecular orbital levels and their related excitonic states. Pentacene photodiodes have been also fabricated on ITO-coated glass substrates with aluminium top electrodes. The current voltage characteristics of such devices are discussed paying special attention to the strongly marked space-charge limited regime. This has been related to trapping in an exponential distribution of localised states in the gap of pentacene. The analysis of the characteristic offers valuable information about such distribution of traps. Finally, the external-quantum-efficiency of these photodiodes shows antibatic features, which evidence the importance of excitonic states in the photovoltaic conversion in pentacene. INTRODUCTION Organic devices could be competitive for inexpensive electronic applications requiring large area coverage and low-temperature processing compatible with flexible or weightless substrates. In particular, conjugated polymers have attracted great attention in the last years. Among them, pentacene has allowed the fabrication of high-performance thin-film transitors (TFT) with organic semiconductor channel [1]. Furthermore, integrated circuits incorporating more than 1800 pentacene TFTs have been fabricated very recently [2]. However, although pentacene has evidenced significant photosensitivity [3], very few studies about its use in optoelectronic devices have been reported. It was not until last year that first significant organic solar cells incorporating pentacene as donor layer have been reported [4,5]. Despite the intense research to improve the processability of pentacene devices, optical absorption and electronic transport mechanisms are not completely understood yet [6,7]. In this work, we offer an insight in structural and optical properties of pentacene samples. The electrical characteristics of rectifying indium-tin-oxide/pentacene/aluminum structures are discussed in detail. Finally, measurements of the external quantum efficiency yield valuable information about the carrier photogeneration in evaporated pentacene films. EXPERIMENTAL Pentacene films were deposited by thermal evaporation in a high vacuum chamber with base pressure 10-6 mbar. The pentacene source is commercially available from Aldrich Chemical with 98% purity. Pentacene films were grown at two different substrate temperatures (30 ºC and 90 ºC) with moderate deposition rates (
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