Abstract:The Boltzmann transport equation can be solved to give analytical solutions to the resistivity, Hall, Seebeck, and Nernst coefficients. These solutions may be solved simultaneously to give the density-of-states (DOS) effective mass ( m d * ), the Fermi energy relative to either the conduction or valence band, and a scattering parameter that is related to a relaxation time and the Fermi energy. The Nernst coefficient is essential for determining the scattering parameter and, thereby, the effective scattering mechanism(s). We constructed equipment to measure these four transport coefficients simultaneously over a temperature range of 30-350 K for thin, semiconducting films deposited on insulating substrates. We measured these coefficients for rf magnetron-sputtered zinc oxide, both doped and 2

A novel machine has been developed to measure transport coefficients in the temperature range of 50–350 K of thin films deposited on electrically insulating substrates. The measured coefficients—resistivity, Hall, Seebeck, and Nernst—are applied to solutions of the Boltzmann transport equation to give information about the film’s density-of-states effective mass, the Fermi energy level, and an energy-dependent scattering parameter. The machine is designed to eliminate or compensate for simultaneously occurring transport phenomena that would interfere with the desired measured quantity, while allowing for all four coefficients to be measured on the same sample. An average density-of-states effective mass value of 0.29±0.04me was measured on the transparent conductive oxide, cadmium stannate (CTO), over a carrier concentration range of 2–7×1020 cm−3. This effective mass value matched previous results obtained by optical and thermoelectric modeling. The measured scattering parameter indicates that neutral impurities or a mixture of scattering mechanisms may inhibit the transport of carriers in CTO.

We present a near-field scanning optical microscopy (NSOM) study of S interdiffusion in polycrystalline CdS/CdTe heterojunctions. S diffusion from CdS into CdTe leads to the formation of a CdTe1−xSx ternary phase. Because the band gap of CdTe1−xSx varies with S composition, we were able to combine NSOM with a tunable laser source to microscopically identify S-rich regions in the CdTe layer. S composition was found to be very nonuniform and frequently to be greater along grain boundaries than in the grain centers, identifying grain boundaries as locations of enhanced interdiffusion.

Admittance spectroscopy was used with a custom built temperature stage to study deep level defects in four polycrystalline thin-film CdTe solar cells that had postdeposition back contact treatments with and without Cu and CdCl2. One hole trap signature with activation energy Ea≈0.13eV was detected in all four cells and was attributed to a combination of VCd− and related complexes. A second hole trap with Ea≈0.30eV and detected only in Cu-treated cells was attributed to CuCd−. A third hole trap with Ea≈0.47eV was detected only in non-Cu-treated cells. The relationships and relative concentrations between these distinct trap levels are discussed.

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