Novel electron-selective passivating contacts for silicon solar cells by atomic layer deposition

Abstract

This doctoral thesis presents a comprehensive investigation into the potential of passivating contacts based on transition metal oxides (TMOs) to address existing limitations in traditional crystalline silicon (c-Si) solar cells. These transparent contacts show potential as a viable approach to realizing cost-effective, high efficiency and low-temperature-processed c-Si photovoltaic devices. The research encompasses four key contributions related to the development and understanding of TMO-based electron-selective passivating contacts, and a fifth contribution relating to the electrical characterization of c-Si solar cells employing SiOx/poly-Si rear passivating contacts: The first contribution focuses on the development of innovative passivating contact structures using TMOs. Specifically, stacks comprising Al-alloyed TiOx (AlyTiOx) and pure TiOx are explored as transparent electron-selective contacts for c-Si surfaces. The research reveals that the efficacy of these layers strongly depends on the choice of metal precursor, with TiCl4 yielding superior results. Notably, it is found that a significant portion of the passivation mechanism can be attributed to the accumulation of chlorine (Cl) at the silicon surface, where it appears to passivate interface defects. This mechanism, which is shown to be applicable also across various interlayers, underscores the essential role of residual chlorine in silicon surface passivation, with passivation capabilities akin to those of hydrogen. Through a strategic combination of earth-abundant, wide-bandgap materials, a highly transparent electron-selective passivating contact structure is systematically engineered. The resulting highly transparent AlyTiOx/ZnO/TiOx stack exhibits exceptional surface passivation and carrier transport characteristics. Compared to the previously investigated AlyTiOx/TiOx stack, this stack achieves an improved balance between efficient silicon surface passivation and carrier transport. The thesis proposes a pioneering approach involving a multilayer structure to resolve these issues. By incorporating conductive barrier layers and capping materials, this strategy effectively addresses degradation concerns associated with metallization, sputtering, ambient conditions, and high temperatures. This innovative approach is shown to result in greatly improved stability and consistent passivation performance, heralding a significant advancement in TMO-based contact technology.

In conclusion, this doctoral thesis comprises a comprehensive exploration of passivating contacts for c-Si solar cells based on transition metal oxides, offering insights and advancements across various dimensions. By pushing the boundaries of contact performance, elucidating passivation mechanisms, refining stack properties through systematic interface engineering, and addressing stability challenges, this research paves the way for notable progress in TMO-based photovoltaic technology. Indeed, the novel contact structures presented here hold the potential to replace Si-based contacts in commercial silicon solar cells. In addition, this thesis explores the electrical characterisation of c-Si solar cells employing SiOx/poly-Si contacts using impedance spectroscopy to unravel their static and dynamic characteristics. Measurements are performed at a range of frequencies and biases, as well as under illumination and in darkness, elucidating the cell's behavior under diverse conditions. Show more