Optical metrology of thin film solar cells from 0.2 to 30 µm

Spectroscopic ellipsometry (SE) from the ultraviolet (UV) to mid-infrared (IR) has been applied to analyze thin film solar cell structures deposited on transparent conducting oxide (TCO) coated glass substrates. Two structures were studied here, chosen from two different thin film photovoltaic (PV) technologies, a hydrogenated amorphous silicon (a-Si:H) p-i-n and a CdS/CdTe heterojunction, both without back contact processing. The mid-IR capability was used to study TCO free carrier absorption in the actual solar cell device configuration, which was further analyzed to extract free carrier properties. In addition, network vibrational absorption bands due to the wagging and stretching modes of hydrides in a-Si:H were also measurable in the device configuration. These results can be used to characterize properties such as H content, its bonding configurations, and amorphous/crystalline content. By combining film side and glass side measurements in the UV-visible range, the ability to obtain structural parameters of multilayer devices can be enhanced. The associated optical property determinations yield insights into disorder in amorphous films and grain structure and strain in micro/polycrystalline films.