This report details the Next generation SLIVER cells project aimed to maximise the likelihood of successful commercialisation of SLIVER technology.
This Next generation SLIVER cells project achieved significant improvements in SLIVER cell fabrication, demonstrating a reduction in process complexity, improved yield and substantial increases in conversion efficiency. The original goals of the project were therefore achieved however the commercial appetite for this type of technology remains to be proven.
ALD deposited Aluminium oxide (Al2O3) and Titanium dioxide (TiO2)) films and film stacks behaviour proved to be highly dependent upon exact deposition conditions and subsequent anneals. Various unforeseen problems were encountered in the development of these films including problems with film blistering, conduction or current leakage through the films; film morphology / crystallography, chemical etch resistance and compatibility with other processing requirements.
A larger than anticipated effort was required to arrive at a deposited film stack with all of the desired properties.
Sourcing reliable n-type <110> wafers proved to be a significant problem. This type of wafer is very much a non-standard wafer type and several vendors were trialled before satisfactory material was available to the project. High efficiency cell fabrication relies upon high quality material, and effective development of processes also relies upon consistent and highly-reliable material. The first 2 vendors used supplied poor quality and highly inconsistent wafer stock for the project.
SLIVER cell fabrication, laser doping and its application in particular; proved to be difficult. Several avenues for a suitable method for depositing a dopant precursor film on grooved sliver wafers were investigated with limited success. Laser doping trials revealed that, at laser settings appropriate for doping on planar silicon surfaces, the laser interaction at the corners of Sliver cell precursors resulted in significant damage and silicon removal extending some distance from the sliver corners.
Many of the new approaches developed for this project are specific to the Sliver technology, and as such are not easily and directly transferable to other projects.
There is an opportunity to significantly improve the manufacturing processes for SLIVER cells and modules, allowing substantial cost decreases and contributing to the successful commercialisation of the technology.
SLIVER cell technology was developed by the ANU Centre for Sustainable Energy Systems (CSES) and is used to manufacture thin film silicon-based solar cells and modules using less silicon material and with lower manufacturing costs.
New self-aligned technologies were introduced into the cell fabrication process in this project. This allowed a substantial simplification in process complexity, with the total number of process steps reduced from 75 to 53. In addition, the conversion efficiency was boosted, with small modules measured at 16.1%, up from just over 13% at the start of the project. The improved cells and modules were also tested for reliability and comfortably passed the reliability tests required for certification.
A second part of the project was focused on the development of the next generation of SLIVER cells based on n type silicon. A new coating based on films of aluminium oxide and titanium dioxide, deposited by atomic layer deposition was developed. A large amount of work was devoted to developing and investigating these new coatings, particularly when they are used for coating SLIVER cells and have to be incorporated into the overall cell manufacturing sequence.
A second innovation for the fabrication of n-type SLIVER cells was the incorporation of laser-based processes, which allow the creation of local features without the need for expensive photolithography the conventional technique for defining such local features. By carefully studying the laser process and its interaction with surface coatings the project was able to determine suitable values for laser power and other laser parameters to allow the selective removal of surface coatings with virtually no damage to the underlying silicon.
This work resulted in the demonstration of n-type SLIVER cells with high conversion efficiencies of 19%, as well as open circuit voltages (VOC) of more than 700mV. VOC is a measure of the overall quality of the device, and a value of 700mV indicates that excellent quality has been achieved and maintained for every surface and every region of the cell. Only a few silicon solar cells have achieved such high values of VOC.
The Next generation SLIVER cells project is making extensive use of ANU’s advanced manufacturing and characterisation equipment, which was purchased with an Australian Solar Institute Foundation Grant.
The project contributed substantially towards ensuring that cost reduction targets for SLIVER technology were met, allowing it to be potentially used in products such as lightweight, flexible modules or building integrated PV products.
The laser processes developed in this project for the selective removal of surface coatings have already been applied to other types of silicon solar cells, where efficiencies as high as 23.5% have been achieved, which is not far below the world record efficiency of 25.6% for a silicon cell. The freely available modelling program Quokka 2 is also finding widespread use for many other types of silicon solar cells, and for the investigation of other processes used in making silicon solar cells.