Frank Juergens, Director Sales & Marketing at INDEOtec SA, tells us more about the innovative top and bottom side deposition concept of the OCTOPUS system…
Motivation
High-efficiency solar cells of the heterojunction or IBC type, which are composed of bulk crystalline silicon and thin film layers, have the potential to gain a substantial share in the PV cell market in the future. Industry leaders like Panasonic currently hold the world-record in efficiency of 25.6% for the HIT cell type at R&D scale and 22.5% in mass production . Moreover, the cell manufacturing technology promises the costs will fall well below 0.30 USD/Wp. A study, which investigated the cost reduction roadmap for heterojunction cell types, predicts a level of 0.28 USD/Wp for SHJ, with a corresponding cell efficiency level of 24.9% .
Symmetrical cell architectures, such as the standard heterojunction (SHJ) cell type, have a simplified manufacturing procedure with less process steps in the state-of-the-art technology. One of the key, but critical processes is the deposition of a-Si:H layers on the top and the bottom side of the textured c-Si bulk material, which at first, passivates the Si surface and, secondly, creates the emitter by adding n- and p-type dopant layers. Nowadays, the common method of depositing a-Si:H films is PECVD in RF mode. The current industry knowledge base for depositing amorphous thin films by PECVD reactors is quite good, since the material was used, on a commercial scale, for PV thin-film cell devices in the past. In contradiction to this, the SHJ device requires the a-Si:H deposition on both sides of the bulk wafer, which sets a new challenge for the equipment suppliers, in terms of handling and costs. Also, the films are extremely thin (approx.10 nm) and hence the deposition time is very short, which means that special care has to be taken to ensure the correct plasma conditions immediately after ignition. A simple adaption of the state-of-the-art PECVD reactor technology from large substrate deposition to the new parameter set for HIT cells is therefore relatively risky.
