Ambipolar diffusion length and photoconductivity measurements on ”midgap'' hydrogenated microcrystalline silicon

Hydrogenated microcrystalline silicon (<i>µc</i>-Si:H) deposited by VHF plasma-enhanced chemical vapor deposition has recently been proven to be fully stable, with respect to light-induced degradation, when adequately used in <i>p-i-n</i> solar cells. Stable solar cells efficiencies of 7.7% have been obtained with single-junction cells, using ``midgap'' microcrystalline <i>i</i>-layers, having an optical gap of around 1 eV. In the present paper, the electronic transport properties of such microcrystalline layers are determined, by the steady-state photocarrier grating method (SSPG) and steady-state photoconductivity measurements, in a coplanar configuration. The conditions for the validity of the procedure for determining the ambipolar diffusion length, Lamb, from SSPG measurements (as previously theoretically derived in the context of amorphous silicon) are carefully re-examined and found to hold in these <i>µc</i>-Si:H layers, taking certain additional precautions. Otherwise, e.g., the prevalence of the ``lifetime'' regime (as opposed to the ``relaxation time'' regime) becomes questionable, in sharp contrast with the case of amorphous semiconductors, where this condition is almost never a problem. For the best layers measured so far, <i>L</i>_amb is about twice as high and the photoconductivity <i>σ</i>_photo four times as high in <i>µc</i>-Si:H, when compared to device quality <i>α</i>-Si:H. Until now, the highest values of <i>L</i>_amb found by the authors for <i>µc</i>-Si:H layers are around 3×10–5 cm.