Mapping amorphous silicon p-type layers in roll-to-roll deposition: Toward spatially resolved PECVD phase diagrams

Spectroscopic ellipsometry was applied to analyze the roll-to-roll deposition of thin film hydrogenated silicon (Si:H) n-i-p solar cells on back-reflector-coated flexible plastic substrates. Real time SE (RTSE) was used for probing along the substrate center line during deposition, and ex situ SE was used for mapping over the substrate area after deposition. The current work focuses on the topmost p-layer of the n-i-p solar cell, with the goal being to develop a thin protocrystalline Si:H film growth process for uniformity across the web and thus optimum performance of the resulting amorphous Si:H (a-Si:H) solar cells and modules. The first step toward this goal is the determination of spatially resolved deposition phase diagrams, which show Si:H phase transitions superimposed onto a bulk p-layer thickness map. Two p-layers have been fabricated by plasma-enhanced chemical vapor deposition in test runs at web speeds of 0.015 and 0.020 cm/s. The lower speed run reveals both amorphous-to-(mixed-phase) Si:H and (mixed-phase)-to-(single-phase) nanocrystalline Si:H p-layer transitions whereas the higher speed reveals only an amorphous-to-(mixed-phase) transition, as confirmed by both RTSE and mapping SE.