Charge carrier transport in solid-phase crystallized polycrystalline silicon (poly-Si) was investigated as a function of the deposition temperature, Td, the amorphous starting material and the used substrates. The samples were characterized using temperature dependent transport measurements to determine the carrier concentration, mobility, and conductivity. Samples prepared on a-SiN:H covered borofloat glass exhibit a low carrier concentration that is independent of Td. In these samples, charge transport is dominated by intra-grain scattering mechanisms. In contrast, when poly-Si is prepared on corning glass, the carrier concentration shows an inverted U-shape behavior with increasing deposition temperature. The Hall mobility is thermally activated, which is consistent with thermionic carrier emission over potential energy barriers. The change of the activation energy with experimental parameters is accompanied by a large change of the exponential prefactor by more than 4 orders of magnitude. This is indicative of a Meyer-Neldel behavior. Moreover, at low temperatures, the conductivity deviates from an activated behavior indicating hopping transport with a mean hopping distance of ≈140 Å and an energy difference of ≈82 meV between the participating states. To derive insight into the underlying transport mechanisms and to determine information on barrier energy heights and grain-boundary defect-densities, the experimental data were analyzed employing transport models for polycrystalline materials.
The influence of post-hydrogenation on the electrical and optical properties of solid phase crystallized polycrystalline silicon (poly-Si) was examined. The passivation of grain-boundary defects was measured as a function of the passivation time. The silicon dangling-bond concentration decreases with increasing passivation time due to the formation of Si-H complexes. In addition, large H-stabilized platelet-like clusters are generated. The influence of H on the electrical properties was investigated using temperature dependent conductivity and Hall-effect measurements. For poly-Si on Corning glass, the dark conductivity decreases upon hydrogenation, while it increases when the samples are fabricated on silicon-nitride covered Borofloat glass. Hall-effect measurements reveal that for poly-Si on Corning glass the hole concentration and the mobility decrease upon post-hydrogenation, while a pronounced increase is observed for poly-Si on silicon-nitride covered Borofloat glass. This indicates the formation of localized states in the band gap, which is supported by sub band-gap absorption measurments. The results are discussed in terms of hydrogen-induced defect passivation and generation mechanisms.
We report on the electrical transport properties of intentionally undoped, laser-crystallized polycrystalline silicon-germanium thin-films. The electrical transport in this material strongly depends on the alloy composition and the crystallization procedure. At low temperatures the undoped germanium-rich samples show an unexpected high p-type conductivity with a weak temperature dependence. Posthydrogenation results in a pronounced decrease in the conductivity and a change in the dominating low temperature transport behavior. The results are discussed in terms of a grain-boundary dominated transport model.
Semisquarylium dyes use a novel acyloin anchor group to strongly bind to TiO2 semiconductors. Efficient acyloin anchor mediated electron injection into nanocrystalline TiO2 is demonstrated, allowing highly efficient dye-sensitized solar cells with IPCEs > 80%. The acyloin anchor can thus be viewed as a true alternative to the standard carboxylic acid anchor group. The opto-electronic and electron injection properties of the most basic semisquarylium dye SY404 are compared to the modified semisquarylium dye DD1 and the carboxylic acid anchored indoline dye D131 using a combination of ultrafast and photoemission spectroscopy. For SY404, ultrafast injection times of ∼50 fs are found despite a small energetic driving force between dye excited states and TiO2 conduction band minimum. This is possible due to the strong electronic coupling of the semisquarylium dyes to the TiO2 surface mediated by the acyloin anchor. For a better overlap with the solar spectrum, the semisquarylium dyes are modified by substitution with a larger donor moiety (DD1). While for DD1 the overall absorption increases, the injection process slightly slows down; however, it still proves fast enough for very efficient injection. Compared to the carboxylic acid anchored indoline dye D131, the SY404 dye injects more than seven times faster despite a ∼150 meV smaller driving force.
The semisquarylium dye SY1T that is strongly bound to the surface of nanocrystalline TiO2 experiences very fast back-electron transfer of injected electrons to the SY1T cation, when the TiO2/SY1T interface is surrounded by ultrahigh vacuum. However, when located in methoxypropionitrile (MPN), which is frequently used as electrolyte solvent in dye-sensitized solar cells, the back-electron transfer is significantly retarded. Results are obtained both for picosecond and microsecond time scales using transient absorption spectroscopy. As solvent-induced interfacial energy level shifts can be excluded as possible cause, the role of TiO2 surface states in the beneficial retardation process is investigated. Highly surface sensitive synchrotron-induced photoelectron spectroscopy exhibits high densities of surface states on the pristine nanocrystalline TiO2 (nc-TiO2) surfaces. While SY1T dye-sensitization from a SY1T solution in tetrahydrofuran saturates about 30% of the surface states, the subsequent in-situ adsorption of MPN molecules at the TiO2/SY1T interface leads to further reduction by more than 50% of the remaining surface states. It is concluded that the saturation of TiO2 surface states hampers the otherwise efficient recombination of injected electrons with the SY1T dye cation.
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