1 Introduction
The pursuit of an ideal contact for a solar cell has been a long-standing challenge in photovoltaics. The ideal contact would facilitate efficient transport of a preferred type of charge carriers, while suppressing the recombination of the opposite type. Traditional solutions, such as those used in state-of-the-art p-type PERC (passivated emitter and rear cell) solar cells, have fundamental limitations due to enhanced Auger recombination in highly doped areas and high recombination at the metal-silicon interface. One way of overcoming these limitations is to form a so-called passivating contact, aiming to decouple the recombination and transport properties of the contact.
2 The Concept of TOPCon
The concept of passivating contacts was first applied in the 1980s by the group of Prof Martin Green at UNSW [1] and demonstrated in a solar cell by Tarr in 1985 [2]. However, it did not gain the deserved attention in the research community for a long time, although it has likely been used by SunPower from the 2010s in their industry-leading interdigitated back contact solar cell [3]. This changed after the re-discovery of ‘TOPCon’ contacts by Feldmann et al. in 2013 [4]. TOPCon, or Tunnel Oxide Passivated Contact, combines the advantages of heterojunctions with high-temperature processing capability.
TOPCon consists of an ultra-thin wide bandgap dielectric layer, typically silicon oxide, sandwiched between the silicon absorber and a doped polycrystalline silicon or polysilicon (poly-Si) layer. This structure provides carrier selectivity by preventing the migration of minority carriers from c-Si to the poly-Si(n) layer. The carrier selectivity is also enhanced by a shallow high-low junction (n+/n) in c-Si(n) that is typically formed during the high-temperature step required to perform doping and crystallization of the poly-Si layer. A schematic of a TOPCon solar cell is shown in Figure 1.
3 Industrial TOPCon (i-TOPCon)
The industrial version of the front junction TOPCon cells on n-type c-Si, known as industrial TOPCon (i-TOPCon) cell, is widely seen as the evolutionary upgrade to the incumbent p-PERC cells. The i-TOPCon cell design envisions a process route that benefits from the processing similarity to the PERC cell, thus requiring the integration of only a few additional process steps in the cell process chain. The cell architecture is reported to yield high efficiencies of > 25.0% in pilot-line and volume production of leading cell manufacturers, with record efficiency claims of > 26.0%.
At the time of writing of this article, there is still a lot of open questions on the best way to commercialise TOPCon solar cells as shown in Table I. The initial focus is definitely on n-type c-Si substrates and applying screen printed contacts. The deposition method for the poly-Si seems to be most open, with low pressure chemical vapour deposition (LPVCVD), plasma-enhanced chemical vapour deposition (PECVD), and physical vapour deposition (PVD) all in high-volume production at the moment. All these methods can have in-situ doping and can combine the formation of the thin oxide layer.
| Substrate | Thin oxide | Poly-Si deposition | Metalisation |
| n-type | Wet chemical | Low-pressure chemical vapour deposition (LPCVD) | Screen printing |
| p-type | UV oxidation | Plasma-enhanced chemical vapour deposition (PECVD) | Plating |
| Thermal oxidation | Physical vapour deposition (PVD) | Evaporation | |
| Plasma oxidation | Evaporation | Transparent conductive oxide |
4 Process Steps in i-TOPCon Cell Manufacturing
The process flow for manufacturing i-TOPCon cells is primarily dictated by the choice of the deposition technology to form TOPCon layers and whether the layers are in-situ doped or require an external doping. If technologically feasible, more process steps are combined in a single tool to ensure a lean process flow.
Typically, boron doping is performed using a tube diffusion process to form p+ emitters on a textured n-type c-Si substrate. This is followed by an in-line wet-chemical process for single-sided removal of rear-side emitter. During emitter removal, the borosilicate glass (BSG) layer is typically kept intact at the front side to serve as a passivation layer for the boron emitter.
5 Technological Challenges
Despite the promising potential of i-TOPCon cells, there are several technological challenges to overcome. One of the major challenges is to minimize the passivation degradation during the contact formation process. The typical firing profiles with peak temperatures in the range of 750-850°C designed to reach low contact resistivities could lead to oxide breakage and spiking of metal to c-Si. This significantly increases the recombination under the metallized area and challenges the very essence of a true passivating contact. This trade-off between fill factor (FF) and open circuit voltage (VOC) could be partly addressed by the deposition of a thicker polysilicon layer. However, it increases the process costs and short-circuit current density (JSC) losses due to the free carrier absorption in poly-Si layers.
Another industrially relevant approach is directly replacing the AlOx/SiNx stack at the rear side of a cell with poly-Si/SiOx. This approach, however, requires a careful optimization of the rear-side texture to balance the optical and electrical properties. The excessive use of silver (Ag) in metallization is a major bottleneck for solving the mass production of i-TOPCon cells in the long term [5].
6 Conclusion
The Tunnel Oxide Passivated Contact (TOPCon) solar cell technology has emerged as a promising solution to overcome the limitations of traditional solar cell contacts. It offers a potential evolutionary upgrade to the incumbent p-PERC cells with high efficiencies of > 24.0% in pilot-line and volume production. However, several technological challenges need to be addressed to realize its full potential, including minimizing passivation degradation during the contact formation process and reducing the use of silver in metallization. Despite these challenges, the future of TOPCon solar cells is bright, and it is expected to play a significant role in the next generation of solar cell manufacturing.
- Green, M.A. and A.W. Blakers, ADVANTAGES OF METAL-INSULATOR SEMICONDUCTOR STRUCTURES FOR SILICON SOLAR-CELLS. Solar Cells, 1983. 8(1): p. 3-16.
- Tarr, N.G., A POLYSILICON EMITTER SOLAR-CELL. Ieee Electron Device Letters, 1985. 6(12): p. 655-658.
- Allen, T.G., J. Bullock, X. Yang, A. Javey, and S. De Wolf, Passivating contacts for crystalline silicon solar cells. Nature Energy, 2019. 4(11): p. 914-928.
- Feldmann, F., M. Simon, M. Bivour, C. Reichel, M. Hermle, and S.W. Glunz, Efficient carrier-selective p- and n-contacts for Si solar cells. Solar Energy Materials and Solar Cells, 2014. 131: p. 100-104.
- Zhang, Y., M. Kim, L. Wang, P. Verlinden, and B. Hallam, Design considerations for multi-terawatt scale manufacturing of existing and future photovoltaic technologies: challenges and opportunities related to silver, indium and bismuth consumption. Energy & Environmental Science, 2021. 14(11): p. 5587-5610.