Breakthrough Efficiency from Targeted PbI₂ Removal
China’s Northwestern Polytechnical University has announced a major leap in solar technology: a two-terminal (2T) perovskite-silicon tandem cell31.71% power conversion efficiency (PCE). According to pv magazine, the team’s innovation lies in a precise interfacial engineering method that removes residual lead(II) iodide (PbI₂) from the perovskite layer without damaging its crystalline lattice—unlocking performance gains and stability improvements critical for commercial viability.
Why PbI₂ Removal Matters
In wide-bandgap perovskite solar cells, PbI₂ forms due to incomplete precursor conversion during crystallization. While trace amounts can passivate defects, excessive PbI₂ at surfaces and grain boundaries introduces nonradiative recombination pathways, facilitates ion migration, and destabilizes the perovskite phase. This translates into reduced efficiency, operational instability, and undesirable hysteresis effects.
By selectively eliminating excess PbI₂, the Northwestern Polytechnical team mitigated these losses, enabling uniform interfaces and consistent electronic properties across the cell stack.
The DMSO–Chlorobenzene Chemical Polishing Technique
The researchers devised a chemical polishing process using a mixed solvent of dimethyl sulfoxide (DMSO) and chlorobenzene (CB). At an optimal ratio of 1.6:100, the mixture effectively dissolved residual PbI₂ while preserving surface integrity and grain compactness. Advanced characterization—such as X-ray photoelectron spectroscopy (XPS)—confirmed reduced Pb content and a slight binding energy shift, indicating chemical modification without structural damage.
This process also improved optoelectronic quality: lower trap-state density, enhanced grain boundary fusion, stronger photoluminescence, and extended carrier lifetimes. Such improvements are vital for tandem cells, where both subcells must operate in perfect optical and electrical harmony.
Device Architecture and Performance Metrics
The high-efficiency tandem cell integrates:
- A 2.5 cm × 2.5 cm heterojunction silicon (HJT) bottom cell
- Wide-bandgap perovskite top cell on indium tin oxide (ITO)
- Hole transport layer (HTL) based on Me-4PACz
- SnO₂ buffer layer
- Transparent indium zinc oxide (IZO) contact
- Silver (Ag) top electrode
Under standard test conditions, the device achieved:
- Open-circuit voltage: 1.839 V
- Short-circuit current density: 21.04 mA/cm²
- Fill factor: 81.95%
Importantly, the cell maintained over 90% of initial PCE after 700 hours of continuous simulated sunlight—a clear indication of improved operational stability and reduced degradation.
Advancing the Tandem Frontier
This achievement builds on a rapid succession of efficiency records in perovskite-silicon tandem PV. While conventional silicon cells are capped by a theoretical limit near 29.4%, tandem designs can exceed 40% in theory by better splitting the solar spectrum between subcells. In practice, the leap from the high-20s into the 30%+ range signals that lab-scale innovations are approaching commercial readiness.
The targeted PbI₂ removal method is not only effective but scalable, offering a path toward mass production without complex or high-cost processing steps.
Implications for Solar and Storage Markets
For battery system integrators and off-grid enthusiasts, higher-efficiency PV directly impacts system design:
- Smaller footprint for the same power output
- Lower balance-of-system costs—fewer panels, wiring, and mounting hardware
- Higher energy yield per unit area, critical for space-constrained installations
These benefits cascade into storage sizing, enabling more compact and efficient solar-plus-storage systems. If stability improvements hold in real-world environments, such cells could accelerate adoption in building-integrated photovoltaics (BIPV) and mobile applications where weight and surface area are at a premium.
Looking Ahead
As noted in recent research updates, achieving and sustaining efficiencies above 30% with robust stability is the tipping point for market disruption. Continued advances in interfacial chemistry—like Northwestern Polytechnical’s DMSO–CB polishing—will be central to this trajectory.
For now, this 31.71% milestone stands as a proof-of-concept that thoughtful materials engineering can resolve long-standing barriers in perovskite-silicon tandems, bringing them closer to the rooftops, facades, and devices of everyday life.
