Light Induced Defect Creation Kinetics in Thin Film Protocrystalline Silicon Materials and Their Solar Cells
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Light Induced Defect Creation Kinetics in Thin Film Protocrystalline Silicon Materials and Their Solar Cells C. R. Wronski, J. M. Pearce, R. J. Koval, X. Niu, A. S. Ferlauto, J. Koh, and R. W. Collins Center For Thin Film Devices, The Pennsylvania State University, University Park, PA ABSTRACT Using real time spectroscopic ellipsometry to characterize the microstructure and evolutionary growth of Si:H materials deposited with and without hydrogen dilution, phase diagrams were developed which clearly defined and established growth in the protocrystalline regime. Guided by these phase diagrams thin films and intrinsic layers in p-i-n cell structures were grown which consist solely of the protocrystalline phase so that the bulk uniform properties of the material could be characterized with confidence. Studies were carried out on the light induced changes in these films and cell structures that include the annealing out of defects as well as their creation under 1 sun illumination at temperatures from 25°C to 100°C that include the attainment of a degraded steady states (DSS). Defect states were characterized in films with electron mobility lifetimes (µτ), and subgap absorption at 1.2eV (α(1.2)); and in the i material of the p-i-n cells by the bulk limited fill factor (FF). The contributions of the different gap states to SWE are identified and characterized. The absence of direct correlations between α(1.2) with µτ and FF present in undiluted and diluted materials also found in protocrystalline Si:H. Similarities, on the other hand, are found between the µτ products and the FFs including the striking changes in the kinetics that occur at ~40°C. Direct correlations between the changes in µτ and FF at different temperatures are presented. The reason for this correlation and lack of it for α(1.2) are briefly discussed with direct correlation of the α(1.2) to cell characteristics being presented, be it not with the FF. INTRODUCTION Significant progress has been made in improving the performance and stability of a-Si:H based pi-n and n-i-p solar cells by fabricating the intrinsic layers using plasma-enhanced chemical vapor deposition with moderate hydrogen dilution of silane [1-4]. Using real time spectroscopic ellipsometry (RTSE) first developed by Collins et al. [5,6] to characterize thin film growth and microstructure, researchers at The Pennsylvania State University have shown that the thin film Si:H prepared under moderate-to-high H2-dilution conditions evolves from the amorphous phase to a mixed amorphous + microcrystalline phase [(a+µc)-Si:H] with the accumulated thickness of the layer. The thin film material in the amorphous regime of growth has been called “protocrystalline” Si:H and exhibits a higher degree of ordering than materials deposited under similar conditions without H2-dilution [5-8]. Furthermore they showed that the phase evolution of this material with thickness and, in particular, the transition to the mixed-phase (a+µc)-Si:H material, depends not only on the H2-dilution ratio, R = [H2]/[SiH4], but al
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