Ongoing Research on Silicon Defect Characterisation for Improved Cell Efficiency

SLICE (SiLicon for Improved Cell Efficiency) is a project financed by the Malta-TÜBİTAK 2024 Joint Call for R&I Proposals. It is led by Prof. Luciano Mule’ Stagno and Dr Ing. Marija Demicoli from the Institute for Sustainable Energy (ISE) at the University of Malta.

Research support officer Mr Aarón Piculo Pérez is the main researcher working on this project at ISE.

The project is also being carried out in collaboration with the Middle East Technical University - Centre for Solar Energy Research and Applications (ODTÜ-GÜNAM), led by Dr Bülent Arikan, and the industrial partner KalyonPV, led by Nurhayat Yildirim.

The primary objective of this project is to enhance the performance of silicon-based solar cells through improved characterisation and control of defects. The project seeks to develop new knowledge and technical expertise related to the mechanisms of defect formation and growth in commercially-produced silicon crystals and their impact on the performance of high-efficiency solar cells.

Single-crystal silicon constitutes the majority of the global installed solar capacity, with state-of-the-art commercial solar panels currently achieving efficiencies of approximately 23–24%.

Further improvements in silicon solar cell efficiency are essential to fully exploit solar energy potential. Recent efficiency gains have largely been driven by advancements in manufacturing processes and the use of higher-purity materials. However, it is well established that so-called grown-in defects in silicon significantly affect solar cell performance, highlighting the need for crystal engineering strategies to suppress or mitigate defect formation and growth in silicon wafers.

During its first year, the SLICE project successfully produced gallium-doped silicon crystals at KalyonPV, resulting in the availability of samples with varying growth profiles. This achievement is essential for meeting the project’s objectives, as it enables a comparative analysis of solar cell efficiencies fabricated from silicon wafers extracted from different regions of the crystal. While previous research collaborations have primarily focused on boron-doped silicon crystals, this project is anticipated to provide new insights into the behaviour and performance of gallium-doped silicon.

Detailed polishing procedures have been developed at ISE to ensure that the samples exhibit a specular surface, which is essential for accurate and reliable material characterisation. Pre-cell material characterisation is currently underway to examine key parameters, including oxygen concentration as well as defect size and density, using a range of advanced techniques available at the Solar Research Laboratory at ISE. These techniques include Laser Scattering Tomography (LST), Fourier Transform Infrared Spectroscopy (FTIR), Photoluminescence Imaging (PLI), and carrier lifetime measurements.

In parallel, additional methods such as decorative etching techniques are also being developed for defect characterisation. Based on the outcomes of these analyses, suitable heat treatments may be designed and implemented using in-house furnaces to engineer controlled defect densities within the silicon wafers.

The next phase of the project involves the fabrication of solar cells on the characterised wafers at ODTÜ-GÜNAM to quantify solar cell yield. This approach will facilitate the establishment of clearer correlations between defect density and solar cell efficiency.

The resulting insights are expected to be of significant interest to relevant industrial stakeholders and are likely to stimulate further funding opportunities and collaborative research.