The development of microcrystalline silicon (mc-Si:H) for solar cells has made good progress with efficiencies better than those of amorphous silicon (a-Si:H) devices. Of particular interest is the absence of light-induced degradation in highly crystalline mc-Si:H. However, the highest efficiencies are obtained with material which may still include a-Si:H regions and lightinduced changes may be expected in such material. On the other hand, material of high crystallinity is susceptible to in-diffusion of atmospheric gases which, through adsorption or oxidation, affect the electronic transport. Investigations are presented of such effects concerning the stability of mcSi:H films and solar cells prepared by plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition.

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We have measured transient photocurrents on several p-i-n solar cells based on microcrystalline silicon. For two of these samples, we were able to obtain conclusive hole drift-mobility measurements. Despite the predominant crystallinity of these samples, temperature-dependent measurements were consistent with an exponential-bandtail trapping model for transport, which is usually associated with noncrystalline materials. We estimated valence bandtail widths of about 31meV and hole band mobilities of 1–2cm2∕Vs. The measurements support mobility-edge transport for holes in these microcrystalline materials, and broaden the range of materials for which mobility-edge transport corresponds to an apparently universal band mobility of order 1cm2∕Vs.

The influence of the preparation conditions in hot wire chemical vapour deposition (HWCVD) on the electronic properties of microcrystalline silicon is investigated in view of application of the material in thin film solar cells. Poor grain boundary passivation, as a result of hydrogen etching at strong hydrogen dilution of the process gas or thermal desorption of hydrogen at high deposition temperatures, is considered a main obstacle for material optimisation. We conclude that optimum μc-Si:H solar cell material, both from HW-CVD and from plasma enhanced CVD, is not necessarily obtained with largest grain sizes and apparent highest crystalline content, but rather by a material prepared under conditions which yield a compact morphology with an effective grain boundary passivation.

Electron spin resonance and conductivity measurements were used to study adsorption and oxidation effects on microcrystalline silicon with different structure compositions ranging from porous, highly crystalline to compact, mixed phase amorphous/crystalline. We found a correlation between active surface area and the magnitude of observed meta-stable and irreversible effects.