Optimization of hydrogenated amorphous silicon p–i–n solar cells with two-step i layers guided by real-time spectroscopic ellipsometry

Hydrogenated amorphous silicon (a-Si:H) p–i–n solar cell performance has been optimized using a two-step i-layer growth process. This effort has been guided by real-time spectroscopic ellipsometry (RTSE) studies of the nucleation and growth of a-Si:H films by plasma-enhanced chemical vapor deposition at 200 °C using a variable ${\mathrm{H}}_{\mathrm{2}}$ -dilution gas flow ratio $\mathrm{R=[}{\mathrm{H}}_{\mathrm{2}}\mathrm{]/[}{\mathrm{SiH}}_{\mathrm{4}}\mathrm{].}$ RTSE studies during film growth with $\text{R>15}$ reveal a transition from the amorphous to microcrystalline $\mathrm{(a\to \mu c)}$ phase at a critical thickness that decreases with increasing R. From such results, the optimum two-step process was designed such that the initial stage of the i layer (∼200 Å) is deposited at much higher R than the bulk to ensure that the film remains within the amorphous side of the $\mathrm{a\to \mu c}$ phase boundary, yet as close as possible to this boundary at low i-layer thicknesses.