Heterojunction vs. TOPCon in perovskite-silicon tandem

From pv magazine Global

Heterojunction (HJT) solar cells generally achieve slightly higher open-circuit voltage than TOPCon devices, reflecting differences in surface passivation and recombination losses.

The advantage of HJT stems from its use of intrinsic amorphous silicon layers, which provide excellent surface passivation and reduce carrier recombination. Although TOPCon technology has significantly narrowed the gap through advanced tunnel oxide passivated contacts, a slight difference in performance remains, contributing to HJT’s marginally higher efficiency potential.

With this in mind, researchers at the Australian National University (ANU) have sought to quantify this distance and provide a realistic roadmap for TOPCon to remain competitive as a bottom cell technology in the emerging perovskite-silicon tandem segment.

Open-circuit voltage gap

“We analyzed recent incremental innovations in TOPCon technology and show that the traditional open-circuit voltage advantage of HJT cells is rapidly diminishing, now approaching a gap of less than 10 mV,” the research’s corresponding author, Rabin Basnet, told pv magazine. “Building on this, we presented quantitative modelling of tandem efficiency potential, benchmarking TOPCon and HJT bottom cells under realistic assumptions. This enabled us to identify the origins of the current performance gap and the conditions under which TOPCon-based tandems can become competitive.”

“Our review provides a systematic analysis of device-level trade-offs, including surface passivation, optical performance, and contact recombination specifically in the context of TOPCon-based tandem architectures,” he went on to say. “We also propose a technology roadmap tailored to TOPCon tandems, highlighting key research directions such as double-side texturing, poly-Si thickness reduction, and contact optimization, along with their expected impact on device performance.”

In the paper “TOPCon-based bottom cells for perovskite/silicon tandem solar cells,” published in Joule, Basnet and his colleagues explained that, in the last two years, TOPCon cells were able to narrow the open-circuit voltage gap with HJT counterparts to under 10 mV, with the laser-assisted firing (LECO) process enhancing front-side contact passivation and enabling a 740 mV open-circuit voltage in recent mass-produced TOPCon cells.

Furthermore, they noted that innovations in contact optimization and metallization have increased TOPCon fill factors above 84%, approaching HJT performance. However, simulations of two-terminal (2T) perovskite/silicon tandems still indicate that HJT-based tandems achieve higher power conversion efficiencies due to superior fill factor and short-circuit current.

Mass production

The research team stressed that TOPCon cell mass production is less demanding in wafer quality due to poly-Si gettering, whereas HJT requires high-purity wafers and pre-gettering steps to mitigate defects, increasing cost and complexity. It also highlighted that TOPCon fabrication involves 8–10 steps, including boron-diffused front emitters and n⁺ poly-Si rear contacts with silicon oxide (SiOx) interlayers, while HJT involves only 4–6 low-temperature steps, although it also requires amorphous and doped silicon layers, TCO deposition, and low-temperature metallization.

“Despite involving fewer process steps, HJT cell fabrication remains more costly than TOPCon,” the academics said. “The Capex per GW for TOPCon manufacturing lines is approximately two to three times lower than that of HJT lines and remains competitive with that of previous mature PERC technology based on p-type wafers. This cost advantage stems primarily from equipment differences: TOPCon relies on low-cost low-pressure chemical vapor deposition (LPCVD) tools, whereas HJT requires relatively expensive plasma-enhanced chemical vapor deposition (PECVD) tools, which account for the majority of HJT line Capex.”

LCOE

Sustainability and scalability are also presented as additional constraints for HJT, especially due to indium-based transparent conductive oxidse (TCOs), which may limit production beyond 40 GW. TOPCon’s combination of lower cost, industrial compatibility, and high efficiency makes it more suitable for large-scale perovskite/Si tandem manufacturing, according to the researchers.

Moreover, levelized cost of energy (LCOE) modeling showed that, despite slightly lower efficiency, TOPCon-based tandems can achieve comparable or lower LCOE than HJT-based tandems due to reduced fabrication costs. “These findings are reflected in the strategic focus of some of the leading PV manufacturers, such as JinkoSolar, Trinasolar, and Hanwha Qcells, on the development of tandem solar cells utilizing TOPCon as the bottom cell,” the academics emphasized.

Despite these promising developments, some challenges remain for TOPCon as the silicon bottom cell in tandem architectures. These include maintaining effective passivation on textured surfaces, mitigating parasitic absorption in poly-Si contacts, and ensuring stability during high-temperature integration of the top cell. Overcoming these challenges is critical to fully realize the optical and electrical gains needed to boost the efficiency of 2T tandems and will require innovative approaches in material engineering, such as sub-micron texturing, optimization of thinner doped poly-Si layers, and incorporation of hydrogen-rich capping layers.

“Overall, our work reframes TOPCon as a realistic, industry-compatible pathway for the scalable manufacturing of perovskite/silicon tandem solar cells,” Basnet concluded.

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