Fabrication of thin film silicon solar cells on plastic substrate by very high frequency PECVD

The paper describes the way to transfer process technology of state-of-the-art high efficiency thin film silicon solar cells fabrication on cheap plastic (such as PET or PEN) substrates, by two completely different approaches: (i) by transfer process (Helianthos concept) of thin film silicon cells deposited at high substrate temperature, Ts (∼200 °C) and (ii) direct deposition on temperature sensitive substrates at low Ts (∼100 °C). Adaptation of the process parameters and cell processing to the requirement of the flexible/plastic substrate is the most crucial step. In-situ diagnosis of the plasma has been done to understand the effect of inter-electrode distance, substrate temperature and hydrogen dilution on the gas phase conditions. Whereas, for the transfer process, the inter-electrode distance is a critical deposition condition that needs to be adapted for the flexible substrates, the direct deposition on plastic substrates has an added issue of loss in material quality and the deposition rate due to depositions at low Ts. Our studies indicate that ion energy is crucial for obtaining compact films at low temperature and high hydrogen dilution helps to compensate the loss of ion energy at low substrate temperatures. Efficiencies of ∼5.9% and 6.2% have been obtained for n–i–p type a-Si cells on PET and PEN substrates, respectively, using direct deposition. Using an adapted inter-electrode distance, an a-Si/nc-Si tandem cell on plastic (polyester) substrate with an efficiency of 8.1% has been made by Helianthos cell transfer process.