Chinese researchers have achieved a power conversion efficiency of 29.71% in a four-terminal perovskite/copper indium gallium selenide (CIGS) tandem solar cell, advancing a class of photovoltaic devices that combine two light-absorbing materials to capture a broader portion of the solar spectrum than either technology can manage independently.

Coordination Engineering as the Core Innovation

The performance gains stem from the application of bis(2-pyridylmethyl) sulfide, known by the abbreviation 2PyS, as a coordination engineering agent within the wide-bandgap perovskite subcell. Wide-bandgap perovskites are a necessary component of tandem architectures, but they carry well-documented reliability challenges that have historically limited the commercial viability of such devices.

By introducing 2PyS into the perovskite layer, the research team targeted three specific degradation mechanisms simultaneously: defect formation, halide migration, and phase segregation. Each of these processes can independently reduce the efficiency and operational lifetime of a perovskite cell, and their combined suppression represents a meaningful step toward making high-bandgap perovskite materials more stable under real-world conditions.

Why Perovskite-CIGS Tandems Matter

The pairing of perovskite and CIGS subcells is of particular interest to the photovoltaic research community because CIGS is a commercially mature thin-film technology with favorable absorption characteristics in the lower-energy portion of the solar spectrum. When stacked in a tandem configuration, the wide-bandgap perovskite layer handles higher-energy photons while the CIGS layer captures lower-energy light that would otherwise pass through or be wasted as heat in a single-junction device.

The four-terminal architecture used in this design keeps the two subcells electrically independent, allowing each to operate at its own optimal voltage. This approach avoids the current-matching constraints imposed by two-terminal monolithic tandems, offering greater design flexibility at the cost of added optical and fabrication complexity.

Efficiency in Context

A certified or reported efficiency of 29.71% positions this device competitively within the broader landscape of perovskite-based tandem research, a field that has seen rapid record progression over the past several years. Perovskite/silicon tandems have attracted the largest share of industry attention and have posted higher efficiency figures, but perovskite/CIGS combinations offer a distinct advantage in applications where lightweight, flexible substrates are required, since both perovskite and CIGS layers can in principle be deposited on non-rigid materials.

The stability improvements demonstrated through 2PyS coordination engineering are as consequential as the efficiency figure itself. Phase segregation in halide perovskites — a process in which the material separates into regions of differing composition under illumination or applied voltage — has been one of the principal obstacles to deploying wide-bandgap perovskites in tandem cells outside of laboratory settings. Suppressing halide migration alongside phase segregation addresses the root causes of this instability rather than managing its symptoms after the fact.

Implications for Thin-Film Tandem Development

The work, conducted by researchers in China, contributes to a growing body of evidence that molecular-level surface and bulk passivation strategies can bring perovskite performance closer to the thresholds needed for commercial deployment. Coordination chemistry, which involves forming bonds between a molecular agent and metal ions within the perovskite lattice, has emerged as a productive avenue for defect passivation because it can be integrated into existing solution-processing workflows without requiring major changes to device architecture.

Whether the efficiency and stability gains demonstrated at the laboratory scale can be reproduced in larger-area modules remains an open question common to virtually all perovskite research at this stage. Scaling solution-processed thin films while maintaining uniform coverage and defect suppression across a full module area is a persistent engineering challenge that the field has not yet fully resolved. The 29.71% result nonetheless establishes a new data point for four-terminal perovskite/CIGS tandems and illustrates the potential of sulfide-based coordination agents as a tool for managing the degradation pathways that have constrained wide-bandgap perovskite stability.