Application of amorphous silicon for photovoltaic silicon surface passivation
Creators: name: Mitchell, Jonathon Drew
Publisher: name: The Australian National University
Year: 2011
Subjects: subject: TK2960.M58 2011
subject: Silicon
subject: Amorphous semiconductors
subject: Photovoltaic power generation
subject: Surface preparation
subject: Silicon solar cells

description: In recent years, the application of hydrogenated amorphous silicon (a-Si:H) to crystalline silicon (c-Si) solar cells for the purpose of surface passivation has begun to move rapidly forward following early innovations by Sanyo. The bulk of the research conducted throughout this thesis has been performed prior to this new drive in the development of a-Si:H/c-Si devices. Understanding the underlying principles and the essential physics concerning the interaction of these two materials has been often overlooked, making further improvements difficult, and limiting new technological developments therein. In this light, the strategies towards merging a-Si:H with c-Si to achieve high{u00AD} efficiency, low-cost photovoltaics are studied in this thesis, with a focus on the interface and lowering interface states. Plasma-Enhanced chemical vapour deposition (PECVD) of a-Si:H has commonly been an effective method for achieving uniform coverage of the c-Si surface. However, many deposition parameters have been reported as optimal, stemming from the limited range of experimental conditions examined. In this study, a more complete range of deposition conditions are tested, with the nature of the a-Si:H across a broad array of parameters being investigated. Ideally, a-Si:H layers which are most likely to result in high quality surface passivation should be deposited using temperatures of 225{u00B0}C, applied rf-power at 4W (SlmW/cm{u00B2} ), and partial pressure of 650mT. Notably, the ranges for deposition that can be ideally utilised, are with temperatures between 200{u00B0}C and 250{u00B0}C, rf-power up to 8W (100mW/cm{u00B2}), and partial pressures between 400mT and 750mT. Although the ideal values are somewhat system specific, these broader ranges are common to many PECVD systems. Previously overlooked in many studies on a-Si:H and indeed most hydrogenated materials is the influence of hydrides on the surface passivation. A widespread belief is that layers hydrogen{u00AD} rich in their bulk are best for passivation, due to a plentiful source of hydrogen. Analysis of hydride density by IR-spectroscopy has revealed several interesting results which identify some misconceptions concerning surface passivation and the influence of hydrides. In particular, this thesis clarifies the function of the composition and distribution of hydrides throughout the layer and their influence on the quality of the surface passivation; the existence of bulk and interface regions within the a-Si:H layer; and the influence of deposition conditions on the composition and density of hydrides. Ideally, a hydride-rich interface region is shown to yield the most reliable results. The diffusion of hydrogen from within the a-Si:H bulk towards the interface with c-Si at an energy of l.SeV has been a widely accepted mechanism governing surface passivation. However, experimental evidence to support this preferential diffusion through a high-defect material such as a-Si:H has been somewhat absent. In this work, an Arrhenius relationship between temperature and surface passivation is revealed, providing evidence that disputes the a-Si:H bulk-diffusion hypothesis in favour of a surface-diffusion mechanism at the a-Si:H/c-Si interface. The thermally activated surface passivation is shown to have energy of 0.7 {u00B1} O.leV, below that required for bulk diffusion or spontaneous release of hydrogen. From this experimental study, new insight into a surface-related transport model governing the passivation of the c-Si surface by hydrogen already present at or near the interface is presented in this thesis. Determined in this physical model, is the relationship between the likelihood of hydrogen diffusion across a c-Si surface and temperature. Following from early experimentation using post-deposition thermal annealing to improve surface passivation by a-Si:H, a new plasma-enhanced chemical vapour deposition (PECVD) technique was developed as part of this work. Multi-Layer-PECVD involves the deposition of sub{u00AD} layers of a-Si:H with thermal cycling, to build up a total layer thickness. This technique of sub-layer deposition is shown to improve the control of hydride density, composition, surface coverage and reduce the inherent thin-film stresses for very thin a-Si:H layers. Comparison of layers deposited by ML-PECVD in-place of standard PECVD showed improved reliability and stability thanks to this new approach to deposition of a-Si:H. With a greater understanding for the properties of a-Si:H in passivating c-Si and improvements in deposition technique, stacked a-Si:H structures which combine n-type or p-type a{u00AD} Si:H with a thin intrinsic a-Si:H layer in a HiT-like design are investigated from the perspective of passivating c-Si. Results here show that high-quality surface passivation can be maintained, with recombination velocities and saturation current densities at the c-Si surface as low as 3cms{u207B}{u00B9} and averaged below 30fA/cm{u00B2} respectively, which are equivalent to those achieved with SiOx and SiN layers. In a world first application, the a-Si:H(i) and stacked a-Si:H layer structure have been applied in this thesis to the mc-Si surface; whereby, excellent surface passivation results are achieved using both n- and p-type mc-Si. Recombination velocities below lOOcms{u207B}{u00B9} using only a-Si:H(i) were reduced further to approximately 40cms{u207B}{u00B9} with stacked a-Si:H(i/n) or a-Si:H(i/n) layers, without a diffused emitter. In addition, low current saturation densities of 4.5 x 10{u207B}{u00B9}{u2074}Acm{u207B}{u00B2} and implied open{u00AD}circuit voltages of 670mV were achieved. In the case of 100{u03BC}m mc-Si, further reductions are shown to be possible, opening the doorway for simple, high-efficiency mc-Si based photovoltaic designs at low-cost. The work in this thesis has yielded an improved understanding relating to a-Si:H/c-Si devices. Fundamental misconceptions concerning the hydrogen passivation mechanism, hydride content and configuration have been identified and a more accurate understanding has been proposed. Although many of the principles in the Sanyo HIT design have recently been reproduced by other groups, the implications of this research remain applicable. Importantly, the research regarding the optimisation of a-Si:H, development of ML-PECVD and many of the preliminary findings of this research are focused on high-efficiency, low-cost next generation photovoltaic designs yet to be developed.