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 using a tunnel oxide and a doped polysilicon layer 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. This changed after the re-discovery of ‘TOPCon’ contacts by Feldmann et al. in [3]. TOPCon, or Tunnel Oxide Passivated Contact, combines the advantages of heterojunctions with high-temperature processing capability [4].

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.

 

The key materials and process choices for an industrial TOPCon solar cell are summarised in Table 1. The substrate is n-type silicon; the tunnel oxide can be grown by wet chemical, UV, thermal, or plasma oxidation; the poly-Si layer can be deposited by LPCVD, PECVD, APCVD or PVD/evaporation; and the rear metal contact can be formed by screen printing, plating, or evaporation [5].

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

3  Evolution of TOPCon Manufacturing: TOPCon 1.0 to 4.0

As TOPCon technology has matured from a laboratory concept into a high-volume product, manufacturers have progressively refined the process sequence to improve cell efficiency, reduce cost of ownership, and enhance line throughput. Figure 3 maps the process flows of four successive manufacturing generations side by side as applied in the Chinese Industry. Each column represents a distinct industrial process architecture. The shared backbone, texturing, rear polishing, poly-Si deposition, annealing, dielectric passivation, screen printing, firing, and electrical testing, runs across all four generations, while the diffusion, patterning, contact firing, evolve substantially from one generation to the next.

3.1  TOPCon 1.0 — Baseline TOPCon

Gen1 establishes the fundamental TOPCon manufacturing sequence. After front-side pyramid texturing, a uniform boron diffusion creates the p⁺ emitter across the entire wafer surface. The boron silicate glass from the front is removed and the rear is then polished to prepare it for the passivating contact stack. First, a thin tunnelling oxide is grown using a wet chemical step or plasma oxidation, and the poly-Si layer is grown by plasma-enhanced/low-pressure chemical vapour deposition (PECVD and LPCVD) or physical vapour deposition (PVD). The poly-Si layer is either in situ doped with phosphorus or doped with phosphorus during subsequent annealing. The sample is subsequently cleaned, and an AlO_x/SiN_x stack is grown on the front side while the rear is coated with a SiN_x film. Subsequently, the silver metal contacts are screen-printed and fired. The sample receives a light or current injection treatment (typically forward-bias current injection or illuminated annealing) to optimise passivation before the cell is I-V tested. 

The simplicity of the TOPCon 1.0 flow makes it straightforward to implement on existing PERC lines with modest equipment additions. However, the homogeneous boron diffusion presents an inherent trade-off: the doping level required for low contact resistance beneath the metal fingers also increases Auger recombination in the unmetallized areas between fingers, limiting the open-circuit voltage.

3.2  TOPCon 2.0 — Selective Emitter via Laser Doping

TOPCon 2.0 addresses the homogeneous emitter limitation by splitting the boron diffusion step into three sub-processes: (1) a light boron diffusion that lowers the average doping density across the front surface, (2) laser doping that locally drives additional boron into the silicon precisely where the metal fingers will land, and (3) a second diffusion process that heals the laser damage. Together these three steps form a selective emitter, with a heavily doped region (p⁺⁺) only under the contacts and a lightly doped region (p⁺) in the active area between them.

The selective emitter reduces surface recombination in the illuminated regions while maintaining a good ohmic contact at the fingers, yielding measurable gains in open-circuit voltage (V_OC) and fill factor (FF). The principal drawback of the TOPCon 2.0 approach is process complexity; the laser doping step requires precise spatial alignment with the subsequent screen-printed finger pattern, and the additional thermal oxidation step increases cycle time.

3.3  TOPCon 3.0 — Uniform Emitter and Laser-Assisted Firing (LECO)

TOPCon 3.0 simplifies the TOPCon 2.0 front-side sequence by moving towards a uniform lightly doped boron emitter. This is enabled by the introduction of laser-assisted firing [LAF, e.g., laser enhanced contact optimisation (LECO)], which is an additional laser process added after contact firing that enables high-quality contacts to lightly diffused silicon surfaces. An optimised LAF process results in a partially contacted metallisation, thus limiting the silicon-metal contact area, and thus reducing recombination. Key demonstrations of LECO in industrial TOPCon production are provided in Fellmeth et al. and Wang et al. [6, 7]

3.4  TOPCon 4.0 — The introduction of poly fingers

The parasitic absorption of the full area TOPCon contact reduces the red response of the solar cell. This is addressed by removing the doped poly-Si as much as possible in between the rear metallisation. The most common method is laser ablation of the poly-Si layers in the non-contacted regions. A detailed review of ways to manage parasitic absorption losses in poly-Si contacts and the efficiency gains from patterning is described in Deng et al. [8]

 

References

 

1. 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.
2. Tarr, N.G., A POLYSILICON EMITTER SOLAR-CELL. Ieee Electron Device Letters, 1985. 6(12): p. 655-658.
3. Feldmann, F., M. Bivour, C. Reichel, H. Steinkemper, M. Hermle, and S.W. Glunz, Tunnel oxide passivated contacts as an alternative to partial rear contacts. Solar Energy Materials and Solar Cells, 2014. 131: p. 46-50.
4. Glunz, S.W., B. Steinhauser, J.I. Polzin, C. Luderer, B. Grübel, T. Niewelt, A.M.O.M. Okasha, M. Bories, H. Nagel, K. Krieg, F. Feldmann, A. Richter, M. Bivour, and M. Hermle, Silicon‐based passivating contacts: The TOPCon route. Progress in Photovoltaics: Research and Applications, 2023. 31(4): p. 341-359.
5. Kafle, B., B. Goraya, S. Mack, F. Feldmann, S. Nold, and J. Rentsch, TOPCon-Technology options for cost efficient industrial manufacturing. SOLAR ENERGY MATERIALS AND SOLAR CELLS, 2021. 227.
6. Fellmeth, T., H. Höffler, S. Mack, E. Krassowski, K. Krieg, B. Kafle, and J. Greulich, Laser-enhanced contact optimization on iTOPCon solar cells. PROGRESS IN PHOTOVOLTAICS, 2022. 30(12): p. 1393-1399.
7. Wang, X., J. Yuan, X. Wu, J. Nie, Y. Zhang, X. Zhang, W. Yang, F. Li, and B. Hoex, Higher-Efficiency TOPCon Solar Cells in Mass Production Enabled by Laser-Assisted Firing: Advanced Loss Analysis and Near-Term Efficiency Potential. PROGRESS IN PHOTOVOLTAICS, 2025. 33(7): p. 771-781.
8. Deng, S., Y. Cai, U. Roemer, F. Ma, F. Rougieux, J. Huang, Y. Cheng, M. Green, and N. Song, Mitigating parasitic absorption in Poly-Si contacts for TOPCon solar cells: A comprehensive review. SOLAR ENERGY MATERIALS AND SOLAR CELLS, 2024. 267.