Highly Transparent Passivating and Carrier-Selective Contact Schemes for c-Si Solar Cells

Abstract

Photovoltaics will play a pivotal role in achieving a low-carbon-emission society. Remarkable advancements in the efficiency of crystalline Si (c-Si) solar cells, combined with standardized processes along the whole value chain, have enabled cost-competitive solar electricity production. In order to further decrease the cost of photovoltaic, significant efforts must be dedicated to further enhancing the efficiency of solar cells. The implementation passivating and carrier-selective contacts in recent years has led to a remarkable increase in the conversion efficiency of c-Si solar cells. Solar cells, such as silicon heterojunction (SHJ) and poly-Si/SiOx-based technologies are prime examples of the efficacy of these contacts, as cell efficiencies above 26% have been demonstrated. These achievements can be attributed to the excellent surface passivation properties of the intrinsic amorphous hydrogenated silicon (a-Si:H(i )) and the ultra-thin SiOx interlayers, coupled with the high carrier-selectivity exhibited by the doped Si-based layers. However, a significant limitation associated with these passivating and carrier-selective contacts is related to their optical parasitic absorption losses within the layers. These losses stem from the free-carrier absorption in a-Si:H layers of SHJ solar cells and poly-Si layers, which possess relatively narrow bandgaps, making them prone to absorbing the ultraviolet (UV) portion of sunlight. Consequently, these parasitic absorption losses diminish the amount of light reaching the c-Si absorber, ultimately restricting the short-circuit current (J sc) output of the solar cell. To mitigate these losses, wide-bandgap metal oxide layers, such as MoOx and TiOx, have been proposed as promising alternatives to replace these highly doped Si-based contacts. These metal oxides possess distinct carrier-selective characteristics, primarily due to their differences in work function (WF) with respect to the c-Si, leading to induced band bending within the absorber. Despite significant progress in recent years, several challenges still exist, as metal oxide contacts often suffer from carrier-selectivity issues due to material instability and interface reactions with adjacent layers. This thesis explores several possible strategies to minimize the parasitic absorption losses in passivating and carrier-selective contacts. A considerable portion of the research is devoted to understanding and enhancing the contact properties of the MoOx layer. The focus on MoOx is driven by the challenges it faces with carrier-selectivity, stemming from its low thermal stability and its susceptibility to band alignment issues when combined with various passivating interlayers. An innovative MoOx contact is introduced, incorporating ultra-thin surface passivating interlayers based on Al2O3 films. These ultra-thin interlayers offer substantial advantages, including enhanced surface passivation, minimal parasitic absorption, and improved transport of majority carriers. Additionally, MoOx and TiOx contacts, deposited by pulsed laser deposition (PLD) are also explored for c-Si solar cells...