High-efficiency IBC-SHJ devices processing (WP5)

Objective
The final goal of WP5 was to demonstrate an IBC-SHJ device with > 26.0 % efficiency using lab-scale processes. Fulfilling this ambitious goal required to continuously improve the various IBC-SHJ processes already developed by the partners in their background knowledge. To support this, WP5 has been working in close collaboration with WP4 and WP8, so as to integrate the novel materials and layers developed in the frame of the former, as well as the outcomes of the numerical simulations performed in the latter. More specifically, in relation to the IBC-SHJ devices processing itself, it was duly identified at the proposal stage already that the choice of the methods used to pattern the various active layers of the devices (namely the a-Si:H-based fingers, the TCO and the metal electrodes) was of paramount importance. Indeed, a given patterning technique not only determines the properties of the patterned features (such as their shape, aspect ratio, material quality, etc.), but it also constrains the complete processing of the IBC-SHJ devices downstream, and possibly its potential upscaling. Therefore, a specific task of WP5 was fully dedicated to investigate in details and compare on a fair basis several patterning techniques: photolithography, laser-based patterning, and shadow-mask patterning. The first of these methods, photolithography, was known to provide accurate patterning, and was considered as the reference process, against which the two other challengers – laser and shadow mask – were benchmarked.

Results
Regular progress in conversion efficiency of the IBC-SHJ devices were recorded with all patterning schemes and all process flows investigated within the NextBase consortium during the project duration. Eventually however, the top-runner of all these investigated approaches was found to be the combination of shadow-mask patterning with a tunnelling electron contact. Champion IBC-SHJ devices processed with this method demonstrated conversion efficiencies up to 25.4 %, both on 25 cm² FZ wafers and on 92 cm² Cz wafers. Remarkably, these impressive efficiencies are obtained in only 9 process steps, which is probably the leanest process flow for IBC-SHJ devices to date.

Impact
The NextBase’s WP5 allowed to bridge the gap between the best-in-class IBC devices worldwide and those processed in the European research institutes and industrial facilities. In-depth insights into the technological requirements and the scientific know-how for top-efficiency IBC-SHJ solar cells were provided. Remarkably, IBC-SHJ devices with up to 25.4 % efficiency were demonstrated, which is the European record to date for such devices. Additionally, WP5 resulted in several publications in highly-ranked peer-reviewed journals as well as several oral presentations in renowned international conferences.

Lessons learnt
One of the major issues encountered in WP5 was the difficulties in putting together layers or process steps from different partners that would – at least on paper – have been interesting in pushing the IBC-SHJ solar cell efficiency further. For instance, combining the front side layers from one partner with the rear side patterning scheme of another one actually resulted more complicated than anticipated an the time of the proposal writing. Such hurdles could have been better identified in the early stage of the project, and alternative solutions might have been sought for.