In order to reach thermal cost reduction as well as environmental safety, new approaches are necessary in the silicon solar cells industry. The aim of the present project is to investigate a new continuous manufacturing line based on low thermal budget processing steps relying on an optical energy transfer to the sample. The general goal of the LOWTHERMCELLS project, carried out by five laboratories (CNRS-PHASE, FhG-ISE, IMEC, ENEA and INSA) in association with industrial companies (AST, ASE and SOLTECH), is the replacement of all conventional thermal processing steps in solar cell manufacturing by Rapid Thermal (RT) steps using lamp furnaces. The main objective is a reduction of the total number of steps. In particular, for homogeneous emitter solar cells, the goal is to perform an entirely passive 'npp+ structure in a single thermal cycle and to suppress masking and photolithographic steps for selective emitters.
This project concerns, for three of the five tasks, the development of the cell structure. Rapid thermal diffusion is used for a simultaneous formation of the emitter and back surface field (BSF) from different doped sources such as glasses, SiO2 or polysilicon layers deposited by spin-on, screen-printing or CVD processes. For surface passivation, rapid thermal oxidation, PE-CVD and doped or un-doped glass deposition are to be investigated together with a rapid thermal sintering of screen printed contacts. For selective emitter solar cells, an additional laser treatment is used to over-dope the regions under the contacts and to perform the grooving of buried contacts. The two other tasks concern the characterisation and production of the solar cells as well as the conceptual design and evaluation of the process by the industrial partners. They will test the stability under encapsulation of the cells (Soltech), design a continuous processing line integrating all the RT steps (AST) and perform an accurate economic evaluation of the LOWTHERMCELLS process (ASE). As preliminary results, 16.3 and 14.1% conversion efficiencies have respectively been obtained by FhG-ISE for 5 x 5 cm2 CZ and by IMEC for 10 x 10 cm2 multicrystalline silicon solar cells.
Expected Achievements and Exploitation
The main output of this project is a simplification and reduction of the duration and number of thermal manufacturing steps of high efficiency silicon solar cells.
The measurable goal is to achieve for 10 x 10 cm2 industrial cells a conversion efficiency of 17.0% on CZ silicon and 15.5% on multicrystalline substrates as well as 17.5 and 16.0% on small 2 x 2cm2 laboratory cells, respectively.
In order to be able to propose at the end of this project a new concept for a manufacturing process line to the industry, preliminary stability tests under encapsulation of the structures will be performed, as well as an economic evaluation of these low-level thermally processed cells.
Funding SchemeCSC - Cost-sharing contracts