Unlocking pathways to high performance silicon photovoltaics: novel passivating contacts, improved device architectures, and advanced characterization

Abstract

The ever-increasing global population and the demand for continuous economic and technological ad- vancement require sustainable energy sources. The current energy backbone, fossil fuels, are finite and have resulted in air pollution, smog, and climate change. Transitioning to clean renewable energy is imperative and solar photovoltaics (PV) is a key player among other renewable energy sources. Solar PV harnesses energy from the Sun, which is a virtually limitless source of power, and it provides the largest amount of energy to the Earth daily at around 1000 times more than global consumption. With its large potential and other advantages such as energy independence, distributed generation, and grid resilience, solar PV is considered one of the most practical solutions to power the future. Among di↵erent kinds of solar PV, crystalline Si (c-Si) is by far the most mature technology that occupies more than 95% of the market. The majority of the commercialized c-Si PV is based on the passivated emitter and rear cell (PERC) architecture, but the eciency of PERC is near its maximum allowable eciency at ̃24%. Some newer technologies such as poly-Si/SiOx based passivating contacts are rising and have demonstrated high eciencies of around 26%. This structure reduces recombination losses at the metal-to-semiconductor contacts and enhances carrier selectivity by heavily doping the poly-Si. Although this type of cell architecture has already entered the market, there is still room to improve to close the eciency gap between the current technology and the Shockley Queisser limit of 29.4% for a single junction c-Si solar cell. In this thesis work, we examine the poly-Si/SiOx passivating contact solar cells at di↵erent angles to improve eciency and lower cost. First, we develop both dry and wet etching methods to selectively thin down the thick front poly-Si layer required to reduce metallization damage. We achieve selective etching by using the front metal gridlines as a self-aligned mask. Following the etching, we show the measurement of layer thickness by XRD on random textured surfaces. Next, we study the poor passivation quality of the B-doped poly-Si/SiOx contacts by using Ga as an alternative dopant. We demonstrate that the low solid solubility limit of Ga in poly-Si can be overcome by a non-equilibrium method called pulsed laser melting. Then, we move to c-Si bulk by investigating conduction electrons associated with P atoms and iron impurities in industrial n-type Cz wafers using a very sensitive spectroscopy technique, Electron Paramagnetic Resonance (EPR). We also show a room-temperature surface passivation technique to allow for bulk defect studies using Nafion polymer. Finally, we report on the current e↵ort to engineer pinholes on textured surfaces using laser processing to enable a new generation of nanopinhole-based passivating contacts. The work presented in this thesis will contribute to a better understanding of the current challenges of poly-Si/SiOx based passivating contacts solar cells and facilitate their successful transition in industry.