Researchers from the Indian Institute of Technology Bombay and Linköping University have reported a four‑terminal (4T) silicon-perovskite tandem solar cell with a power conversion efficiency of 30.2%. The device combines an optimized transparent perovskite top cell with a monocrystalline n‑type TOPCon silicon bottom cell that delivers 25.5% efficiency on its own.
Efficient spectral utilization in tandem architectures requires careful bandgap tuning of the perovskite absorber. However, shifting the bandgap up or down often introduces open‑circuit voltage (VOC) losses, which are commonly attributed to misalignment at the charge‑transport interfaces. The study investigates whether this conventional explanation fully accounts for the observed voltage deficits.
The team focused on the hole transport layer (HTL), which governs hole extraction from the perovskite absorber. Conventional devices use spiro‑MeOTAD, an organic HTL that depends on chemical doping with LiTFSI and t‑BP and requires prolonged post‑oxidation processing, which can compromise interfacial stability. Instead, the researchers introduced a post‑oxidation‑free, ion‑modulated spiro‑MeOTAD HTL based on the TBMPTFSI salt. This modified HTL enabled controlled work‑function tuning and strongly suppressed non‑radiative recombination at the HTL/perovskite interface, effectively reducing Shockley–Read–Hall (SRH) recombination, a major source of voltage loss in perovskite devices.
Three perovskite compositions with bandgaps of 1.52 eV, 1.61 eV, and 1.72 eV were investigated. For the 1.52 eV device, replacing the conventional HTL boosted VOC by about 50 mV and raised the efficiency from 18.2% to 21.3%. For the 1.72 eV composition, efficiency improved from 14.4% to 16.1%, with a clear VOC gain. In contrast, the 1.61 eV device showed only small changes, suggesting that the interface was already close to optimal even with the standard HTL. Carrier lifetime measurements confirmed a substantial reduction in SRH recombination with the ion‑modulated HTL across these compositions.
The best‑performing transparent perovskite cells were mechanically stacked in a 4T configuration with the n‑TOPCon silicon bottom cell. Depending on the perovskite bandgap, tandem efficiencies of 30.2%, 29.4%, and 28.4% were achieved. The highest value, 30.2%, corresponds to about an 18% relative improvement over the standalone silicon cell and ranks among the top‑reported efficiencies for transparent perovskite‑based 4T tandems on active area.
The ion‑modulated spiro‑MeOTAD HTL eliminates the need for extended post‑oxidation treatments typically required by LiTFSI‑ and t‑BP‑doped systems, which can take 10–24 hours and employ hygroscopic dopants that may degrade long‑term stability. Across off‑optimum bandgaps, the approach increased VOC by 2–5% and fill factor (FF) by 6–7%, while still enabling a 4T tandem with 30.2% efficiency (~18% improvement versus silicon alone).
The work highlights a central design principle for bandgap‑tunable transparent perovskite top cells: minimizing interfacial recombination is more critical than achieving perfect energy‑level alignment. As perovskite bandgaps are adjusted to match current or optimize spectral splitting in tandem layouts, defect tolerance at the HTL/perovskite interface becomes decisive for retaining high voltage. By reducing interfacial defects and enabling defect‑suppressed, energetically favorable junctions across a range of bandgaps, the ion‑modulated spiro‑MeOTAD HTL offers a versatile route toward both highly efficient and stable silicon–perovskite tandem solar cells.
