Nanostructure control of Al-doped Zinc Oxide for Si thin film solar cell

Abstract

Abstract

The most fatal issue of the Si thin film solar cell is its low conversion efficiency due to the low absorption coefficient and short carrier diffusion length of the Si thin films deposited with low temperature PECVD. To overcome this weakness, light management technologies for more light absorption in Si layers has been aggressively adapted, and transparent conductive oxide (TCO) front contact has one of the most important roles among them. In this thesis, nanostructure control of the Aluminium-doped Zinc Oxide (ZnO:Al) front TCO is thoroughly studied to realize high efficiency Si thin film solar cells. At first, the overview of the backgrounds for this thesis is introduced in chapter 1. Introduction of the Si thin film solar cell and recent development of the light trapping technology is provided. Next, ZnO:Al deposition and etching process parameters for the high efficiency Si thin film solar cell, and carrier transport mechanism, that is to say, the theory about the carrier mobility of the ZnO:Al film are reviewed. The compactness and carrier mobility of the ZnO:Al films are suggested as key properties determining the performance as a front TCO, which is an important message throughout this thesis. Next, in chapter 2, the effect of oxygen controlled seed layer on the nanostructure and electrical properties of the ZnO:Al front TCO is examined for the application into the industrial manufacturing process in the future. Oxygen-controlled seed layer in Al-doped ZnO (ZnO:Al) thin films deposited by the industrially compatible dynamic DC-magnetron sputter results in both enhanced electron mobilities and appropriate etched morphologies for the Si thin-film solar cells. At the relatively low deposition temperature of 300°C, optimized ZnO:Al film grown on the seed layer has the carrier mobility of 45 cm2/Vs, and proper post-etching morphology with around 1-2 μm crater size. Reduced angular distribution of the (002) grains analyzed by the diffraction rocking curve is shown as the key structural feature for the improved carrier mobility. At last, the performance of the microcrystalline Si solar cell on the developed ZnO:Al substrate shows high efficiency potential of the tandem solar cell adapting this TCO substrate. In chapter 3, the origin of the mobility enhancement is systematically examined in terms of the nanostructural change with varied seed layer condition. The carrier mobility of the ZnO:Al films are controlled between 22 and 48 cm2/Vs by varying the ZnO:Al seed layer condition. The grain-boundary energy barrier (Eb) from Setos carrier transport model clearly exhibit linear dependence on the grain-boundary misorientation angle (ω) estimated by the (002) rocking curve. In Chapter 4, the roughness control of as-deposited and post-etching films is shown as key optimization factors for maximize the efficiency of the solar cell. Surface roughness of the as-deposited ZnO:Al film estimated by spectroscopic ellipsometry is shown to be the easy but powerful tool to optimize the deposition condition for proper post-etching surface morphology. Wet-etching time is adjusted to form the U-shaped craters on the surface of the ZnO:Al film without sharp etch-pits causing the crack-like defects in overgrown Si absorbing layers and deterioration of Voc and FF of the Si thin film solar cells. At last, The a-Si:H/a-SiGe:H/μc-Si:H triple junction Si thin film solar cells grown on the optimized ZnO:Al front TCO with anti-reflection coatings show higher than 14% initial efficiency.