Morphology-driven electrical and optical properties in graded hierarchical transparent conducting Al:ZnO
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Morphology-driven electrical and optical properties in graded hierarchical transparent conducting Al:ZnO P. Gondoni1, P. Mazzolini1,2, A. M. Pillado Pérez1, V. Russo1, A. Li Bassi1,2, C. S. Casari1,2 1 Dipartimento di Energia and NEMAS – Center for NanoEngineered Materials and Surfaces, Politecnico di Milano - Via Ponzio 34/3, 20133 Milano, Italy 2 Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia Via Pascoli 70/3, 20133 Milano, Italy ABSTRACT Graded Al-doped ZnO layers, constituted by a mesoporous forest-like system evolving into a compact transparent conductor, were synthesized by Pulsed Laser Deposition with different morphologies to study the correlation with functional properties. Morphology was monitored by Scanning Electron Microscopy images and by measuring the resulting surface roughness. Its effects on electrical conductivity – especially carrier mobility, which significantly decreases with increasing roughness – allow to discuss the limitations in conduction mechanisms. Significant changes in light scattering capability due to variations in morphology are also investigated and discussed to study the correlation between morphology and functional properties. INTRODUCTION The well-established importance of transparent conducting materials (TCMs) in optoelectronic and energy conversion devices has led to the development of highly performing materials, the most widely employed being Indium-Tin Oxide (ITO). [1–2] Due to the scarcity and high price of indium, ITO needs to be replaced by cheaper and more abundant materials: Aluminum doped Zinc Oxide (AZO) is among the most promising candidates and has been investigated for several years [3–4]. However, as the field of photovoltaics progresses towards hybrid and organic-based flexible devices, the optical transparency and electrical conductivity of TCMs need to be accompanied by more advanced properties, such as large effective surface area, light trapping and management, mechanical flexibility and low temperature synthesis. [5–7] By employing Pulsed Laser Deposition (PLD) as a deposition technique, it is possible to obtain control of local structure, multi-scale morphology and stoichiometry for a number of oxides even at room temperature [8–10] and thus to exploit this versatility to obtain with AZO the functional properties indicated above [11–13]. We have recently developed a functionally graded AZO structure, which is able to combine optical transparency and electrical conductivity with effective light management. [14] This structure is constituted by a bottom mesoporous layer designed to maximize light scattering, smoothly evolving into a compact transparent conducting AZO layer on top. The realization of this architecture is based on the effects of background pressure on the PLD process: at pressures of the order of 100 – 200 Pa O2 deposition takes place in the form of a porous hierarchical assembly of nanosized clusters, whose formation is favored by collisions with the background gas; as the background pressure is decreased (0
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