Tuning Hierarchical Cluster Assembly in Pulsed Laser Deposition of Al-doped ZnO
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Tuning Hierarchical Cluster Assembly in Pulsed Laser Deposition of Al-doped ZnO Paolo Gondoni1, Valeria Russo1, Carlo E. Bottani1,2, Andrea Li Bassi1,2 and Carlo S. Casari1,2 1 Dipartimento di Energia and NEMAS – Center for NanoEngineered Materials and Surfaces, Politecnico di Milano, via Ponzio 34/3, I-20133 Milano, Italy 2 Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, I-20133 Milano, Italy ABSTRACT The synthesis of hierarchically assembled Al-doped ZnO layers by Pulsed Laser Deposition (PLD) at room temperature was investigated. PLD was performed in a background pressure of 100 Pa O2 to deposit clusters in a low energy regime and obtain nano- and mesostructures resulting from a hierarchical assembly of nanoclusters. We here analyzed the effects of varying the gas flow rate on mesoscale morphology, mass density and optical properties. The variation of the target-to-substrate distance was also investigated, identifying its effects on mass density and film morphology. The optimization of optical properties in terms of transparency and light scattering capability is of potential interest for photovoltaic applications. INTRODUCTION Hierarchically structured materials have recently been intensively studied due to their promising functional properties for employment in several fields, from energy harvesting to gas sensing [1,2]. The possibility to employ hierarchical nanostructures as photoanodes in dyesensitized solar cells has been demonstrated for various metal oxides[3-5]. Furthermore, the benefits in device efficiency arising from the control of light trapping phenomena at the nanoand meso- scale have also been investigated [6,7]. We have recently demonstrated the growth of transparent conducting Al-doped ZnO (AZO) grown by Pulsed Laser Deposition (PLD) in oxygen atmosphere at room temperature [8] and the synthesis of hierarchically assembled AZO films with optimized light scattering properties [9]. In those works we studied the effects of the background oxygen pressure during the PLD process, and identified the optimal deposition conditions to obtain compact transparent conductors and mesoporous photon scatterers. In particular, we demonstrated how an increase in the O2 pressure allowed to control clustering phenomena in the ablation plume, decrease the kinetic energy of the ablated species and lead to the growth of forest-like structures. We identified a background pressure of 100 Pa O2 as a threshold for the synthesis of mesoporous hierarchically assembled structures. In the present work, we investigate different paths to achieve a better control of mesoscale morphology and density of hierarchically assembled AZO structures, analyzing the effects of gas flow and target-to-substrate distance on film morphology, structure and optical properties. In order to test the compatibility with organic substrates of interest for application to flexible optoelectronics and energy conversion devices, depositions were performed at room temperature also on polymer substrates.
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