Photon harvesting from nanostructured metallo-dielectric substrates

This report describes the results of the research collaboration between ENEA-Centro Ricerca di Portici and the Department of Phyiscs of the University of Genova (DIFI) developed in the course of 2011-12 within the MSE-ENEA protocol. The main goals of the research are related to the synthesis of nanopatterned substrates in view of Photon Harvesting applications in thin film photovoltaic devices. The activity has been mainly focused on the optimisation of the self-organised nanopatterning processes employed to prepare textured interfaces endowed with high aspect ratio nanostructures The activity was in particular oriented at the synthesis of anisotropic self-organised gratings on substrates of relevance for thin film PV applications such as Glass and transparent conductive oxides (TCOs). The nanoscale morphology has been optimised in view of (i) a broadband reduction of the reflection losses, mimicking the bio-mimtetic interfaces at the cornea of nocturnal moths, and (ii) a high scattering efficiency at large angles in order to greatly increase the Haze of the interfaces (the ratio of diffuse to total scattering). This result has been achieved recurring to a process based on defocused Ion Beam Sputtering (IBS) which employs a self-organised array of metal nanowires as a stencil mask for Ion Projection Lithography (IPL). Of relevance for future applied developments, the process is able to produce a pattern across macroscopic portions of the substrates in a single maskless step, and can be up-scaled into standard industrial coaters. The nanopatterned substrates have subsequently been employed by ENEA as supports for a:Si thin film solar cells, which have succesfully demonstrated a significant enhancement of EQE, in the order of 20%, with respect to the flat reference substrates. The general validity of the results has been demonstrated by performing similar experiments on bulk semiconductor substrates of interest in photovoltaic applications, namely on Si and GaAs wafers. Also in this case the patterned substrates exhibit a significant broadband reduction of the reflection and a remarkable increase of Haze well above 50%. The experiment performed in Genova have also demonstrated that the self-organised nanowire arrays employed as stencil masks in the IBS process, perform as semitransparent nano-electrodes with an extremely low sheet resistance in the 1 Ohm/sq. ram. This feature together with the high optical transparency, renders the metal nanowire arrays competitive with the best conventional TCOs. Additionally the nanoelectrodes support localised Plasmon resonances which make them appealing in view of photon harvesting applications in photonic devices and in biosensors.