Silicon nanocrystal solar cells on dielectric substrates

Abstract

Photovoltaic (PV) has developed rapidly in the recent decades, and is now considered a competitive energy source. The Shockley-Queisser limit, however, places an upper limit on the energy conversion efficiency of a solar cell. As for a single junction crystalline Si (c-Si) solar cell, the theoretical limit is about 31%. To circumvent this limit, the application of silicon nanocrystals (Si NCs) in all-Si tandem solar cells attracts great interest. This thesis focuses on the investigation of solar cells with a mesa isolated p-i-n structure based on Si NCs in a silicon dioxide host matrix fabricated on dielectric substrates. The mesa isolated structure ensures that the signal from the solar cells originates entirely from the Si NC materials and not the substrate. The thesis begins with an investigation on the electroluminescence (EL) and photoluminescence (PL) of a Si NC device, where differences between the EL (1.27 eV) and PL (1.33 eV) peak energies and their corresponding FWHWs are observed. A model categorizing the Si NCs into two subsets is discussed and verified using atom probe tomography (APT), temperature dependent EL and PL. The dopant effects in Si NC materials are also investigated. Various characterization techniques are used including PL, Raman Spectroscopy, X-Ray Diffraction (XRD) and Electron Spin Resonance (ESR). To overcome the challenge of making anode and cathode contacts on a Si NC solar cell using reactive ion etching (RIE), photolithography and lift-off are employed in an alternative fabrication method. The advantages of the photolithography method include better control of isolation mesa fabrication and the avoidance of unpredictable damages caused by the device's exposure to highly energetic particles. The PL peak observed in the device fabricated via the photolithography method is 664 meV higher than the maximum splitting between the quasi Fermi energy levels extracted from the VOC-Temperature relation. The origin of this discrepancy is discussed. Limitations and potential improvements of the Si NC cell are investigated based on its optical and electrical performance.