Tuning 2D perovskites yields efficient, stable perovskite-silicon tandem solar cells

Researchers have engineered a molecular-level solution to one of the most stubborn efficiency bottlenecks in perovskite solar technology, achieving a certified power conversion efficiency of 33.64% in perovskite/silicon tandem solar cells. The breakthrough, published in Nature Photonics, centers on a clever chemical trick: flipping the electronic character of an ultrathin protective layer from p-type to n-type, unlocking voltages and stability figures that had previously remained out of reach for the p–i–n device architecture.

Perovskite solar cells have long dazzled the photovoltaics community with their rapid efficiency gains, but the architecture of the device matters enormously. In the conventional n–i–p configuration, in which electrons are collected at the bottom and holes at the top, engineers routinely apply a mixed-dimensional heterojunction at the illuminated surface. This top layer, typically composed of low-dimensional perovskite phases interleaved with the three-dimensional absorber, passivates defects and improves charge extraction. But when researchers invert the stack to the p–i–n layout—often preferred for tandems because of processing advantages and compatibility with silicon bottom cells—the same trick largely fails. The reason is fundamental: most reported two-dimensional perovskites are intrinsically p-type, meaning their Fermi levels and band edges are misaligned with the electron transport layer that must sit directly beneath them in these inverted devices.

The new work tackles this mismatch head-on through electronic engineering of two-dimensional Ruddlesden–Popper perovskites, a family of layered materials in which sheets of corner-sharing metal halide octahedra are separated by organic cation spacers. Rather than accepting the native p-type character of these layers, the team induced a genuine p-to-n transition using two complementary strategies. The first relies on molecular dipole tuning: by incorporating parahalogenated piperidine derivatives—piperidine rings bearing halogen atoms at the para-like position of the aromatic substitution pattern—the researchers shifted the internal electric fields and charge distribution within the layered perovskite, steering the material’s Fermi level upward into n-type territory. The second strategy involves chemically designable n-type defects, deliberate point defects introduced in controlled fashion that donate electrons to the lattice and stabilize the n-type character.

The consequences of this electronic flip cascade through the entire device. With an n-type 2D capping layer atop the wide-bandgap perovskite absorber, the energy-level alignment at the perovskite–electron transport layer interface improves dramatically. Electrons, which must cross this boundary to be collected, encounter far lower energetic barriers than before. The tailored band alignment simultaneously suppresses non-radiative recombination, the parasitic process in which photogenerated carriers annihilate at interfacial traps, releasing their energy as heat rather than light. For wide-bandgap perovskites—here approximately 1.68 electronvolts, chosen to pair optimally with silicon—non-radiative losses at this interface have been the dominant ceiling on open-circuit voltage. Removing them pays off directly in the numbers.

Indeed, the p–i–n wide-bandgap perovskite solar cells built with the n-type 2D capping layer delivered open-circuit voltages enhanced by more than 100 millivolts compared with control devices. In a field where efficiency records are often broken by fractions of a percentage point, a hundred-millivolt gain is a seismic shift, and it translated into a certified power conversion efficiency of 33.64% when the perovskite top cell was integrated with an industrial-grade silicon bottom cell. The silicon component was no laboratory curiosity: the team used 110-micrometer-thick Czochralski-grown heterojunction silicon wafers, the workhorse material of mainstream photovoltaic manufacturing. Czochralski silicon, produced by pulling a single crystal from a melt, is the industry standard precisely because it is cheap and scalable, even though it contains more defects than the float-zone silicon often used in record-setting laboratory cells. Demonstrating world-class tandem performance on such commercially realistic wafers strengthens the path from lab to factory.

Stability, the perennial Achilles heel of perovskite technology, also benefited. The molecular engineering that converts the 2D layer to n-type simultaneously enhances the material’s robustness, and the monolithic perovskite/silicon tandem cells retained 92% of their initial efficiency after 1,100 hours of continuous operation under maximum-power-point tracking. This testing protocol, which continuously extracts the maximum available power as conditions evolve, is far more demanding than simple open-circuit storage and closely mimics real-world deployment. Encapsulated tandem modules that hold more than nine-tenths of their output over well over a thousand hours of continuous illumination represent performance in the range where commercial warranties begin to look defensible.

The scientific significance of the work lies in its demonstration that the electronic character of two-dimensional perovskites is not an immutable property but a designable parameter. Ruddlesden–Popper perovskites have been prized as passivating and moisture-resistant layers since the earliest days of perovskite photovoltaics, yet their utility has been constrained by the assumption that their band structure favors hole transport. By showing that molecular dipoles and defect chemistry can be combined to invert that preference, the researchers have effectively expanded the toolkit available to device designers. The parahalogenated piperidine derivatives act as more than passive spacers; their bond dipoles reshape the electrostatic landscape of the inorganic sheets, and the rationally introduced n-type defects complete the conversion. The strategy is chemical, modular, and in principle transferable to other layered halide systems and other interfaces throughout the device stack.

For the perovskite/silicon tandem field specifically, the achievement addresses the long-standing tension between the two architectures. Tandem devices stack a wide-bandgap perovskite cell on top of a silicon cell so that each harvests the portion of the solar spectrum it converts most efficiently; the perovskite captures blue and green photons while silicon collects the red and infrared. Theoretical analyses suggest such tandems can surpass the single-junction Shockley–Queisser limit of roughly 33% by comfortable margins, and laboratory records have climbed steadily past 34%. The p–i–n configuration is attractive for manufacturing because it permits low-temperature processing on silicon and avoids some stability issues associated with high-temperature transport layers, but its performance has lagged. By resolving the interfacial energetics that held p–i–n devices back, the new results suggest that this industrially favored architecture can now compete at the very highest efficiency levels, and do so on commercially standard silicon.

The voltage gains also carry implications beyond tandems. Wide-bandgap perovskites near 1.68 electronvolts are the workhorses of tandem top cells, but in single-junction form they suffer from severe voltage deficits caused by defect-tolerant yet trap-limited carrier dynamics. The demonstration that a purpose-built n-type 2D surface layer can recover more than 100 millivolts suggests a generalizable prescription for any device where the electron extraction interface is the loss center—single-junction cells, light-emitting diodes operating in reverse, and photoelectrochemical systems alike. Because the treatment is applied through molecular additives and defect design rather than exotic processing, it should be compatible with existing coating and deposition workflows.

What remains to be seen is how the chemistry scales. Parahalogenated piperidine derivatives must be synthesized, purified and incorporated reproducibly at manufacturing scale, and long-term field testing will need to confirm that the benefits observed under laboratory maximum-power-point tracking persist through thermal cycling, humidity and ultraviolet exposure in the field. But the certified 33.64% efficiency, the industrial silicon wafer, and the 1,100-hour stability result together mark a milestone in the maturation of perovskite/silicon tandem photovoltaics. By treating the electronic type of a two-dimensional perovskite as a tunable parameter rather than a fixed constraint, the work opens a molecular-level route to optimizing interfacial energetics—one that brings the long-promised era of high-efficiency, stable, manufacturable tandem solar cells measurably closer.