Doping Profile Recognition Applied to Silicon Photovoltaic Cells Using Terahertz Time-Domain Spectroscopy

Here we demonstrate for the first time that terahertz time-domain spectroscopy (THz-TDS) can be used to monitor doping profiles in standard industry silicon wafers and commercial multi-crystalline silicon (mc-Si) solar cells. The proof-of-concept results presented here demonstrate the suitability of the technique for in-line, or close to real-time, process and quality control applications in the photovoltaic (PV) industry. The experimental results show that THz pulses can detect changes in implant dosage in both the time and frequency domains. It is also shown that the spectral content of THz pulses can be used to distinguish between doping profiles with identical implant dosages but different profile shapes (resulting from different diffusion times). The reliability and repeatability of these measurements is demonstrated. It is found that the average change in THz spectrum (12.3%) corresponding to the smallest doping profile difference detected is an order of magnitude bigger than the average day-to-day measurement error (1.1%). The THz measurement approach is shown to be fully compatible with the surface morphology (texturing) of commercial mc-Si solar cells.