Passivating Contacts for High-Efficiency Silicon Solar Cells Based on Poly-Si/SiO_x Structures

The basic function of photovoltaic devices can be distinguished in two fundamental processes. First the electron and holes are generated by the incident photons. In the second step these electrons and holes have to be separated and transferred to either the n- or p-electrode of the cell. This is achieved by a layer or layer system with a high carrier selectivity placed under the p- and n-electrode. The carrier-selective layers of classical crystalline silicon solar cells are created by near-surface n- or p-type doping of the silicon absorber, resulting in diffused pn- and high-low-junctions. Silicon solar cells using this approach have achieved a maximum efficiency of 25%. A major limitation of such cells is the increased Auger-recombination due to the high doping concentration in the diffused regions. The principal idea to overcome this limitation is to decouple the contact layer system from the absorber. For other types of photovoltaic devices, such as organic solar cells, this idea is not new at all. In fact, decoupled selective contacts are required for many PV technologies since the formation of diffused pn-junctions is technically not feasible. For crystalline silicon solar cells, however, it took more than 50 years after the first diffused pn-junctions were created in the Bell labs that this approach gained significant importance. This switch to passivating contacts allowed efficiencies beyond 25%.<br/>In principle we can distinguish four types of passivating contacts for crystalline silicon solar cells:<br/>(i) a-Si/c-Si heterojunctions (SHJ), (ii) transparent layers with high or low work functions as metal oxides, (iii) metal-insulator silicon solar cells (MIS) and (iv) doped polycrystalline silicon on SiO_x (SIS).<br/>In this contribution, we will focus on the last option, i.e. passivating contacts based on poly-Si/SiO_x structures also known as TOPCon (tunnel oxide passivating contacts). They have a great potential to increase the efficiency of crystalline silicon solar cells, leading to efficiencies higher than 26%. Along with a-Si/c-Si heterojunction solar cells, the TOPCon technology is considered by the photovoltaic industry as one of the main routes for the next generation of industrial solar cells. The efficiency potential of TOPCon cells and its compatibility with the state-of-the-art PERC process flow make this technology very interesting for a transfer to industrial production. Indeed, efficiencies higher than 25% have been achieved on large areas. However, it is well known that mass production of solar cells has a lot of stringent requirements. Therefore, for mass production of industrial TOPCon (i-TOPCon) cells, the evaluation of reliable and high-throughput technologies for the cell fabrication is essential. For example, the question of the best deposition technique is still open: LPCVD, PECVD, APCVD, and sputtering are viable technological options with their specific pros and cons.<br/>This contribution will give an overview of the historical development of poly-Si/SiO_x contact structures which have started already in the 1980s and describes the current state-of-the-art in laboratory and industry. In order to demonstrate the great variety of scientific and technological research, four different research topics are addressed in more detail: (i) the superior passivation quality of TOPCon structures made it necessary to re-parametrize intrinsic recombination in silicon, (ii) the control of diffusion of dopants through the intermediate SiO_x layer is essential to optimize passivation and transport properties, (iii) single-sided deposition of the poly-Si layer would reduce process complexity for industrial TOPCon cells, and (iv) silicon-based tunnel junctions for perovskite-silicon tandem cells can be fabricated using the TOPCon technology.