PECVD system for thin film silicon solar cells on glass
Certainly, the most crucial component of any silicon thin film solar cell is the light absorbing layer. Compared to the overall thickness of the solar cell, the absorber represents the biggest part, which is particularly true for so-called tandem and triple cells. PECVD (plasma enhanced chemical vapor deposition) is generally used to deposit the absorber layer consisting of a PIN junction from a silane hydrogen gas mixture using boron and phosphor containing dopant gases for the P and N layers, respectively. A PECVD system should feature high deposition rates of amorphous and microcrystalline silicon of the desired material quality with good film uniformities at low production costs.
PHOEBUS is the PECVD solution from LEYBOLD OPTICS for the silicon thin film solar cell industry. PHOEBUS is based on a vertical, carrier-free linear cluster approach to avoid both particle contamination issues common for horizontal systems and the elevated costs associated with vertical systems. TCO coated glass substrates of 1.4 by 1.1 meters in size are thermally conditioned for deposition and are transferred into vacuum by a vertical load-lock. A vertical fork-type transport robot picks up the glass from the inside of the gate valve of the load-lock chamber and moves it to one of up to 24 PECVD coating chambers equipped with planar parallel plate electrodes operated at 13.56 MHz. The majority of PECVD coating chambers are used to deposit either intrinsic amorphous or microcrystalline silicon while some are used for depositing thin p type (B doped) and n type (P doped) silicon films.
The design of the PECVD reaction zone and process parameters have been developed by the Institute of Photovoltaics at the Research Center in Jülich, Germany, and have been licensed to LEYBOLD OPTICS. Every glass substrate will experience three steps in different coating chambers for single junction a-Si cells or six in the case of tandem cells. During the entire process and transfer times all glass substrates are kept at a steady temperature in a thermal equilibrium within the coating chambers and the transport robot. The robot moves inside a connecting vacuum tunnel perpendicular to the center-lines of the coating chambers which are aligned parallel to each other.
After the coating processes, the glass substrates are transported to the second load-lock with active substrate cooling at the opposite end of the vacuum tunnel for transfer to atmospheric pressure. PHOEBUS is designed for in-situ plasma cleaning of the PECVD chambers with a fluorine-based gas to avoid the high maintenance costs associated with the exchange of mechanical parts. A sophisticated system software package allows for individual chambers to be taken out of the production process chain while the machine remains productive.
