Characterisation and defect engineering of poly-Si passivating contacts in silicon solar cells

Abstract

From a broader perspective, this thesis is concerned with the development of a novel polycrystalline silicon on oxide passivating contact technology (poly-Si/SiOx) for high efficiency silicon (Si) solar cells. A comprehensive understanding of various characteristics of poly-Si films provides a clearer picture of underlying mechanisms limiting the performance of the passivating contact. This thesis is divided into two broad categories: the characterisation and the defect-engineering by hydrogenation treatments of the technology. First, this thesis utilizes various advanced characterisation techniques available to the photovoltaic (PV) research community to study the structural, electrical, and optoelectronic properties of different poly-Si/SiOx passivating contacts. Using transmission electron microscopy (TEM), interfacial images of the poly-Si/SiOx/c-Si stack are revealed at a nanometre-scale. Atomic force microscopy (AFM) morphologies of the poly-Si surfaces demonstrate the difference in film surface morphologies of different depositing methods. Grazing incidence X-ray diffraction (GIXRD) spectra give an overview of the difference in crystallinity of the poly-Si films with various deposition conditions. Electrochemical capacitance-voltage (ECV) doping profiles show electrically active doping levels inside the poly-Si films after different diffusion processes to form the contact. Meanwhile, secondary ion mass spectrometry (SIMS) doping profiles present total doping concentrations inside the stack. The difference between ECV and SIMS profiles reflects inactive dopant concentrations. Quasi-steady state photoconductance (QSSPC) is a useful method to quantitatively evaluate the performance of the passivating contact by measuring the effective lifetime tau_eff, implied open circuit voltage iVoc, and recombination current density Jo. For the optoelectronics studies, photoluminescence spectroscopy (PLS) and Fourier transform infrared spectroscopy (FTIR) are used. The PL spectra reveal various combined features from the poly-Si film, SiOx interface, and c-Si substrate. The characteristic PL signal from a hydrogenated amorphous Si phase (a-Si:H) inside the poly-Si film can be used as a mean to trace the presence of hydrogen. Meanwhile, the sub-bandgap PL signal from the film is also used to demonstrate the passivation of nonradiative defects inside it. FTIR spectra are employed as a way to verify the presence of hydrogen by tracking the peak of the Si-H stretching modes. By combining the various methods mentioned, a comprehensive examination of the different properties of the poly-Si passivating contacts is performed, suggesting different routes to improve their performance. Second, the thesis investigates defect-engineering processes for the poly-Si passivating contacts by different hydrogenation treatments, such as forming gas annealing (FGA), and depositing hydrogen-rich dielectric layer(s) (silicon nitride (SiNx), or aluminum oxide (AlOx)) and post-annealing in FGA. Generally, after the hydrogenation treatments, the performance of the different poly-Si passivating contacts is all improved, regardless of the deposition conditions. The presence of hydrogen inside the poly-Si/SiOx passivating contacts can be detected by the characteristic luminescence peak from the hydrogenated amorphous phase (a-Si:H). It is found that only a hydrogenation treatment involving SiNx on poly-Si films formed by a plasma-enhanced chemical vapour deposition (PECVD) method gives the a-Si:H PL peak. Proposed mechanisms of the different hydrogenation methods are as follows: the two different capping layers, AlOx and SiNx, could cause different associations of hydrogen in the poly-Si films. The hydrogen from the AlOx film could be injected into the poly-Si film and form complexes with other species rather than passivate the dangling bonds of the a-Si phase. Meanwhile, the hydrogen atoms from the SiNx film both passivate the a-Si phase and form complexes. Show more