A plasma approach to rare-earth based solar spectrum conversion

Abstract

Solar spectrum conversion concepts are undoubtedly necessary to further enhance the solar cell efficiency. According to this approach, the solar spectrum can be efficiently modified by shifting the photons towards a wavelength range where the solar cell has a higher response. Therefore, by placing a spectrum conversion layer on the front surface (i.e. down convertors) or at the back of the solar cell (i.e. up convertors), the existent optical losses can be quantitatively reduced. One of the light conversion mechanisms, namely the luminescent down shifting (LDS) mechanism, has been implemented for different luminescent materials embedded in different host matrices showing their potential for a wide range of PV technologies. In general, the LDS mechanism can be activated by incorporating specific rare-earth (RE) ions in a host material due to their special optical properties, i.e. UV absorption followed by visible emission. Currently, thin amorphous layers such as silicon nitride or silicon oxide, are extensively applied in c-Si as well as thin film solar cell technologies. In this PhD thesis a plasma-based deposition approach to obtain LDS layers has been applied. In particular, a hybrid method combining two plasma- aided techniques, namely an expanding thermal plasma and a magnetron sputtering unit, has been developed to obtain novel amorphous RE doped materials, i.e. a-SiNx:Eu, a-SiO2:Eu,. The novelty of this approach consists in combining three functionalities in one amorphous layer, i.e. antireflection, bulk/surface passivation of the absorber and spectral conversion properties of the rare earth elements. The goal is to demonstrate whether the LDS mechanism proposed to increase the solar cells efficiency can be achieved. The challenge in achieving this goal is to control the incorporation of RE elements in the amorphous layer through a proper control of the plasma process parameters. In order to determine the ion flux available for the RE target sputtering, ion flux measurements have been carried out. A pulse-shaped capacitive probe has been implemented for the first time in a high rate deposition (2-20 nm/s) ETP system fed with depositing gas mixtures, showing the probe tolerance towards the presence of insulating layers. The method is based on the discharge of an external capacitor, Cp, by changing the slope of the applied bias signal. This method makes the ion probe an accurate and relatively simple tool to determine ion fluxes under harsh conditions. Moreover, for high SiH4 flows (> 1 sccs), an interesting behaviour in the downstream region of the plasma at 5 cm off the axial position of the reactor, has been observed. We have hypothesized on the presence of the local ion flux enhancement: it is suggested that cluster formation occurs in the recirculation volume of the ETP at the boundaries between the plasma jet and the reactor walls. The response of the clusters to pressure, thermophoretic and electrostatic forces, has been investigated. Therefore, the ion probe technique has been proposed as suitable tool to detect the presence of clusters developed in depositing plasmas. Moreover, the ion flux measurements performed in front of the substrate in combination with in situ film growth monitoring by spectroscopic ellipsometry (SE) during the deposition process has been used to understand the influence of ETP plasma on magnetron sputtering rate when the sources are simultaneously operated. The successful incorporation of Eu in both oxide and nitride matrices has been confirmed by Rutherford Backscattering (RBS) measurements whereas the valence state of Eu (i.e. Eu2+and Eu3+) has been identified by X-ray Photoelectron Spectroscopy (XPS). In order to confirm whether the Eu incorporated in the amorphous matrix is optically active, photoluminescence (PL) measurements have been performed. We have observed Eu3+ line emissions corresponding to (J = 1, 2, 3, 4) transitions superimposed on an Eu2+ band emission corresponding to transition from 400 to 800 nm. However, in addition to radiative emission, a contribution of nonradiative emission indicated by the fast decay time values for the doped dielectric layers has been observed. Moreover, Transmission Electron Microscopy (TEM) top view and cross sectional measurements show the presence of Eu clusters with an average diameter of 3 nm, suggesting clustering phenomena occurring in the dielectric layer. In conclusion, by combining the short decay time values we measured and the clusters observed in the TEM images, a hypothesis for the nonradiative decay has been drawn as it follows: the energy transfer between the luminescent active centres within a cluster becomes very efficient, which will then be transferred to a defect in the vicinity of the cluster and therefore lost. Both plasma and material characterization insights obtained through the investigation of RE doped materials are particularly useful to further explore the possibility of using the ETP-PVD hybrid technique in the synthesis of novel materials for solar cell applications.