The Dawn of a New Era in Solar Efficiency
In a remarkable advancement for renewable energy technology, researchers at the National University of Singapore (NUS) and the Solar Energy Research Institute of Singapore (SERIS) have achieved a certified power conversion efficiency (PCE) of 32.76% in a perovskite-silicon tandem solar cell. This breakthrough, developed in collaboration with China's JinkoSolar, represents one of the highest efficiencies for monolithic perovskite/tunnel oxide passivated contact (TOPCon) tandem solar cells to date.
This achievement not only pushes the boundaries of photovoltaic (PV) performance but also addresses key commercialization hurdles, making high-efficiency solar cells more viable for industrial production. For Singapore, a nation committed to its ambitious renewable energy targets, this innovation underscores the pivotal role of higher education institutions in driving sustainable development.
Demystifying Tandem Solar Cells
Tandem solar cells, also known as multi-junction solar cells, stack multiple photovoltaic materials with different bandgaps to capture a broader spectrum of sunlight. Traditional silicon solar cells, which dominate the market, have a theoretical efficiency limit of around 29% (known as the Shockley-Queisser limit). By layering a wide-bandgap top cell, such as perovskite, on a narrow-bandgap bottom cell like silicon, tandem configurations can theoretically exceed 45% efficiency.
Perovskite solar cells (perovskites derive their name from the crystal structure resembling the mineral calcium titanate perovskite) emerged in 2009 and rapidly climbed to over 26% efficiency standalone. When paired with silicon in tandems, they leverage perovskites' superior light absorption in high-energy wavelengths and silicon's proven stability and manufacturability.
The Crystallization Conundrum in Industrial Production
One major obstacle in scaling perovskite-silicon tandems is the perovskite layer's deposition on thin industrial silicon wafers. TOPCon silicon cells, with their ultra-thin tunnel oxide passivation, feature reduced thermal mass and high thermal conductivity. During annealing (a heating step to crystallize the perovskite), rapid heat transfer causes uncontrolled, fast crystallization, leading to voids, defects, halide segregation (separation of iodine/bromide ions), and high non-radiative recombination losses.
These defects degrade voltage and fill factor, limiting PCE and stability. Previous lab-scale efforts used thick glass substrates, masking this issue on real-world silicon.
Introducing the MBT Additive Revolution
The NUS-SERIS team introduced 2-mercaptobenzothiazole (MBT), a small molecule with dual functional groups: a heterocyclic nitrogen atom for hydrogen bonding and a thiol (-SH) group for electrostatic interactions with formamidinium (FA) cations in the perovskite precursor solution. This dual-mode binding stabilizes intermediate phases, retards crystallization kinetics, and promotes uniform, void-free films.
Step-by-step process:
- Precursor Preparation: MBT added to perovskite ink (e.g., FAPb(I0.85Br0.15)3).
- Deposition: Solution-coated on TOPCon silicon (130 µm thick industrial wafer).
- Annealing: Controlled crystallization at ~150°C, forming compact grains with reduced grain boundaries.
- Characterization: Confirmed via SEM, GIWAXS, NMR, DFT simulations showing MBT-FA interactions.
Outstanding Performance Metrics
The resulting two-terminal monolithic tandem cell delivered a champion PCE of 33.62% (Voc 1.97 V), with certified stabilized 32.76% over 0.925 cm². Trap density dropped dramatically, non-radiative voltage loss minimized to 0.39 V. Under accelerated aging (MPPT at 25°C, 85% RH), it retained 91% efficiency after 1,700 hours—far surpassing typical perovskite stability.
| Metric | With MBT | Without MBT |
|---|---|---|
| PCE (%) | 32.76 (certified) | <30 |
| Voc (V) | 1.97 | Lower |
| Stability (1700h) | 91% retention | Poor |
| Trap recombination (cm s-1) | 4.3 × 104 | 3.2 × 105 |
The Brains Behind the Breakthrough: NUS and SERIS Team
Led by Assistant Professor Yi Hou from NUS's Department of Chemical and Biomolecular Engineering and SERIS, the team includes co-first authors Qilin Zhou, Renjun Guo (NUS/SERIS), Nengxu Li (NUS/SERIS), and Menglei Xu (JinkoSolar). Yi Hou, a Presidential Young Professor, has a track record of tandem cell innovations, including prior records in perovskite-organic tandems.
SERIS, NUS's solar research powerhouse, has authored Singapore's Solar PV Roadmap, projecting solar LCOE at SGD 0.038-0.045/kWh by 2050. Their work supports national goals, with Singapore now targeting 3 GWp solar by 2030—enough for 350,000 households.
Industry Synergy: Partnering with JinkoSolar
This lab-to-fab transition exemplifies academia-industry ties. JinkoSolar provided industrial TOPCon wafers, validating scalability. The additive strategy integrates seamlessly with solution processing lines, bypassing costly vacuum methods. For details, see the full Nature Energy paper.
Benchmarking Against Global Leaders
- LONGi: 34.85% (2025, lab-scale perovskite-Si tandem).
- JinkoSolar prior: 33.24% TOPCon tandem.
- NUS/SERIS-Jinko: 32.76% certified on industrial wafer—key for mass production.
This positions it among top certified stabilized efficiencies for TOPCon tandems.
Singapore's Solar Ambitions and NUS's Pivotal Role
Singapore, import-dependent for energy, aims for net-zero by 2050 via renewables (solar 20%+ mix). SERIS contributions include floating PV pilots and efficiency records, accelerating deployment. This breakthrough aligns with the updated Solar PV Roadmap, forecasting cost parity by 2030.EMA's 3 GWp target announcement highlights urgency.
Global Impacts and Pathways Forward
Beyond efficiency, enhanced stability reduces levelized cost of energy (LCOE), enabling tandem adoption. Challenges remain: lead toxicity mitigation, large-area uniformity. Future: Triple-junctions, flexible tandems. NUS's REC@NUS lab (S$77M) accelerates commercialization.
Stakeholder views: Yi Hou emphasizes, "This bridges perovskite with silicon industry." Implications for climate: Tandems could cut PV costs 30%, aiding Paris goals.
Career Opportunities in Singapore's Solar Frontier
This milestone spotlights demand for experts in materials science, PV engineering at NUS/SERIS. Singapore's green push creates jobs in research, fabrication.