Optical monitoring and control of three-stage coevaporated Cu(In1−xGax)Se2 by real-time spectroscopic ellipsometry
Real-time spectroscopic ellipsometry (RTSE) has been applied for in situ monitoring and control of thin-film copper-indium-gallium-diselenide, i.e., Cu(In1−xGax)Se2 (CIGS), deposition by high vacuum coevaporation in the three-stage process used for efficient photovoltaic devices. Initial studies have been performed on a ∼0.7-µm CIGS layer deposited on crystal silicon to minimize surface roughness and to develop an accurate structural/ optical model of the Cu-poor-to-Cu-rich and Cu-rich-to-Cu-poor transitions that define the ends of the second (II) and third (III) stages of growth, respectively.With a better understanding of the surface achieved through this model, correlations can be made between the surface state and the unprocessed RTSE data {ψ(t), Δ(t)}. During deposition in the solar cell configuration with 2- µm-thick CIGS on a Mo-coated glass substrate, indications of the Cu poor-to-rich and Cu rich-to-poor transitions appear clearly in {ψ(t), Δ(t)}, enabling direct control of stage II and III transitions. The transition times deduced optically are in good agreement with those identified from the film/substrate emissivity by tracking the substrate heater power. It is clear, however, that RTSE can provide higher sensitivity to these transitions and is, therefore, suitable for improved control of three-stage CIGS deposition.
Effect of Alkali on the Efficiency and Reliability of Cu(In,Ga)Se2 Solar Cells
EFFECT OF ALKALI ON THE EFFICIENCY AND RELIABILITY OF Cu(In,Ga)Se2 SOLAR CELLS Shankar Karki Old Dominion University, 2019 Co-Directors: Dr. Desmond Cook Dr. Sylvain Marsillac The incorporation of alkali metal has contributed tremendously in a bid to realize greater than 20% efficient Cu(In,Ga)Se2(CIGS) solar cells. Achieving high efficiency is one key parameter for the success of a photovoltaic technology but so is its longterm stability. In this thesis, the relationship between the performance of alkali treated Cu(In,Ga)Se2 solar cells and their physicochemical, electronic and structural properties are explored through a comparative study between standard devices and alkali (K, Rb) treated devices. The alkali treated devices tend to have a lower concentration of Ev+0.98 eV trap, higher majority carrier concentration and improved minority carrier lifetime, contributing to the experimentally observed improvement in open circuit voltage. Critical changes in alkali elemental profile occur throughout the film, while no other major physicochemical or structural properties are modified. Furthermore, we explored the long-term stability of CIGS solar cells due to damp heat treatment. We specifically study the influence on the molybdenum back contact and the CIGS absorber layer itself, with an emphasis on the role played by sodium. Molybdenum thin films showed drastic micro-structural, surface morphology, electrical and optical properties deterioration leading to the degradation of the solar cell performance. In the case of bare CIGS thin films, we observed surface oxidation and degraded electronic properties, also leading to degradation of solar cell performance. In both cases, alkali migration is responsible for the most part of the degradation, along with surface oxidation.
The General Theory of Relativity
THE WORD “SPECIAL” IN “SPECIAL THEORY OF RELATIVITY” means that the theory is restricted to those frames of reference for which the law of inertia holds, that is, inertial frames of reference.* Einstein sometimes referred to them as Galilean frames of reference. At first glance, it does not seem reasonable to expect that the principle of relativity can be extended to non-inertial frames. The reasons are fairly obvious. The principle of relativity that we have been using states that the laws of physics have the same form in all frames of reference to which the principle applies. In other words there should be no way to determine by an experiment which of two frames of reference is in motion relative to the other.