Inorganic Photo-sensitized Transparent Conductive Oxide (TCO) Nanocomposite Thin Films for Photovoltaic (PV) Energy Conv
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1211-R03-15
Inorganic photo-sensitized transparent conductive oxide (TCO) nanocomposite thin films for photovoltaic (PV) energy conversion C.G. Allen, G.H. Shih, and B.G. Potter, Jr. Materials Science and Engineering Department, University of Arizona Tucson, AZ 85721-0012 ABSTRACT Nanophase semiconductor composites are widely researched for the development of third-generation photovoltaic (PV) devices. Through quantum-size effects and phase assembly manipulation the optical absorption and carrier transport properties of nanocomposite films can be influenced. We investigate the potential for improved PV-relevant material performance by examining the photo-sensitization of indium-tin-oxide (ITO) with nanophase germanium (Ge). Nanocomposite films are produced by a sequential, RF-magnetron sputter deposition technique. Deposition control and post-deposition annealing are used to demonstrate the manipulation of the extended-assembly of the nanocrystalline Ge phase. Optical absorption characteristics were correlated to variations in the composite film structure as confirmed by transmission electron microscopy. In addition to structure-dependent variation in spectral absorption, spectrally resolved photoconductivity measurements demonstrate enhanced photoconductivity of composite films associated with the incorporation of the Ge phase into the ITO host. These results support the further evaluation of such nanocomposite TCO materials in optoelectronic devices, including PV systems. INTRODUCTION Nanostructured semiconductors, including quantum dots (QD’s) and quantum wells (QW’s), are widely investigated because of quantum-scale effects (i.e. quantum confinement) on the electrical and optical properties of these materials [1]. The energy gap of the nanophase can be influenced by reducing dimensions of the semiconductor to less than the exciton Bohr radius [2]. The potential to influence not only single-photon spectral absorption (sensitization of the heterojunction), but also to access fundamentally new operational modes (multiple exciton generation, hot carrier extraction, intermediate band absorption etc. [3-5]) within the quantum scale structure has sparked a concerted effort towards exploiting nanophase semiconductor structures to improve photovoltaic (PV) performance. Efforts have focused on the development of “third-generation” PV-devices utilizing, for example, II-VI [6] or Group IV QD’s [7]. In particular germanium (Ge) enables a wide range of quantum-size-tuned band-gap energies to be produced because of a large Bohr exciton radius (24.3 nm). Nanocrystalline Ge phases have been actively investigated in a variety of composites for optoelectronic applications that have involved silica [9], titania [10] and alumina [11] as the embedding phase for the semiconductor. In contrast to an insulating embedding phase, a transparent conductive oxide (TCO), such as indium-tin-oxide (ITO), may act as a more effective medium for electron carrier transport. The present article reports on a continued effort to examine the developme
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