AbstractMicrocrystalline silicon films prepared over a range of deposition conditions have been investigated using dark- and photo-conductivity measurements, constant photocurrent method and transient photoconductivity. The dark conductivity was measured as a function of temperature; the photoconductivity was measured with steady state and transient excitation as a function of temperature and excitation intensity. Microcrystalline silicon films deposited at high silane concentration, near the transition from microcrystalline to amorphous growth are found to have similar optoelectronic properties to undoped amorphous silicon films. In contrast, the properties of largely crystalline films, grown at low silane concentration bear some similarities to n-type amorphous silicon.
Transient photocurrent and post-transit time of flight measurements on coplanar and pin photovoltaic structures respectively have been interpreted in terms of a multiple trapping transport model. It appears that multiple trapping analysis is applicable to transitional materials, although in highly crystalline materials significant deviations occur, especially at low temperatures. From a comparison of transient photoconductivity and time of flight results, and variations in dark- and photo-conductivity with film thickness, anisotropy in electronic properties measured in the direction of, and perpendicular to, the direction of film growth may be inferred.
The effects of ambient atmosphere on optoelectronic properties have been investigated for films prepared over a range of deposition conditions, such as silane concentration, doping and film thickness. It was found that both irreversible and reversible changes may take place. Irreversible effects, associated with oxidation process, result in a shift of the Fermi level position towards the conduction band accompanied by irreversible changes in the density of states, together with increase in dark and photoconductivity and decrease in conductivity prefactor. Thinner films are found to show more rapid irreversible changes and for these films the activation energy was found to decrease by as much as 0.4 eV over a period of one month after deposition.
The reversible effect was found to result in decrease or increase in dark conductivity, depending on material microstructure. Reversible atmospheric effects have been interpreted in terms of adsorption process that results in band bending and saturates in most cases over a period of few days. Changes in properties produced by evacuating the measurement chamber were more pronounced for films deposited at higher silane concentration, which may be explained in terms of material microstructure. A vacuum of 10−3 torr was however insufficient to prevent adsorption process. A correlation between changes in dark, steady state and transient photoconductivity over a period of time has been found and explained in terms of changes in the Fermi level position. Reversible conductivity changes measured on a series of boron-doped samples have demonstrated that the effect may be minimised or even reversed by doping. Computer simulation, which models the effect of atmospheric adsorption as a charge ‘sheet’ penetrating into the film close to the surface, gives good agreement with experimental results for the boron doped series.
Light soaking experiments on microcrystalline silicon prepared under optimal conditions for high-efficiency solar cells indicate little degradation at room temperature, while some degradation has been observed at a temperature of 50°C. The interpretation of these experiments is complicated by ‘interference’ from atmospheric effects that result in changes in dark and photoconductivity opposite in direction to light soaking. It was shown that these metastable effects may be separated using different annealing regimes.
|Date of Award||Mar 2006|