Fabrication and optimization of single-junction nc-Si:H n–i–p solar cells using Si:H phase diagram concepts developed by real time spectroscopic ellipsometry

Abstract

In this study, we have developed and applied deposition phase diagrams in the plane of the bulk layer thickness db and the H2-dilution ratio R=[H2]/[Si2H6] for Si:H materials deposited by 70MHz VHF PECVD from [H2]+[Si2H6] mixed gases on c-Si/(native-oxide)/n-layer substrates. To establish the phase diagrams, series of Si:H depositions having different R values over the range of 60–150 were measured in real time using a rotating-compensator multichannel ellipsometer. Using phase diagram concepts for guidance, we have fabricated high efficiency single-junction nc-Si:H n–i–p solar cells with ∼3Å/s intrinsic layers using the VHF PECVD process. We have found that the nc-Si:H solar cells with the best performance are obtained by incorporating i-layers deposited in the single-phase nanocrystalline silicon regime near the transition boundary to mixed-phase (a+nc)-Si:H. Applying insights from real time spectroscopic ellipsometry moreover, we have investigated in detail the effects of the phase of the underlying n-layer on the phase evolution of the overdeposited Si:H i-layer and on the overall device performance. With the strategy developed here, a stabilized efficiency of η=9.46% (Voc=0.516V, Jsc=24.65mA/cm2, FF=0.744) has been achieved for nc-Si:H solar cells (0.25cm2 in active area) fabricated with an i-layer deposition rate of ∼2.2Å/s.