Researchers from China, Korea and Russia have reported a molecular-level strategy to stabilize formamidinium lead iodide (FAPbI₃) perovskite solar cells, achieving a certified power conversion efficiency (PCE) of 27.6% while addressing one of the field’s most persistent challenges: phase instability.
FAPbI₃ is widely regarded as a leading perovskite absorber due to its optimal bandgap, but its performance is fundamentally limited by the instability of its ionic-covalent Pb-I octahedral framework. Under operational conditions - particularly elevated temperature and humidity - the desirable photoactive α-phase tends to transform into a non-perovskite hexagonal δ-phase. This transition is driven by structural disorder and entropy increase associated with the [PbI₆]⁴⁻ octahedra. To address this, the researchers introduced an entropy-regulating molecular-lock strategy using 1-pyridin-3-ylmethyl-piperazine hydrochloride (3-PMPCl). The additive is incorporated both within the bulk and at the surface of the perovskite layer, where it forms strong interactions with the lattice. These interactions effectively restrict the rotational freedom of the organic cations and suppress entropy-driven disorder and expansion of the Pb-I octahedra.
By reducing configurational entropy, the approach increases the phase transition energy barrier, making the α-phase thermodynamically more stable under operating conditions. The uniform distribution and strong adsorption of 3-PMPCl further enhance lattice rigidity, enabling the perovskite film to maintain structural integrity under thermal and humidity stress.
Devices fabricated using this strategy reached a certified PCE of 27.6%, placing them among the highest-performing single-junction perovskite solar cells reported to date. This level of performance approaches the theoretical efficiency limit of ~29% for commercial silicon solar cells and highlights the rapid progress of perovskite technologies, which have already surpassed the 27% threshold.
However, the study also underscores the trade-off between efficiency and long-term operational stability. High-efficiency devices employing conventional noble metal electrodes such as silver (Ag) and gold (Au) remain susceptible to degradation due to chemical interactions and ion migration.
To mitigate this, the team replaced the metal electrode with bismuth (Bi), a more chemically stable alternative. Although this substitution resulted in a modest efficiency reduction to 26.8%, it significantly improved durability: the devices retained 93.0% of their initial PCE after 1011 hours of continuous operation at 85°C under 1-sun illumination.
This work represents a shift from conventional external stabilization strategies - such as encapsulation or interface engineering - toward intrinsic stabilization via molecular design. By directly modulating entropy within the perovskite lattice, the approach provides a new pathway for improving both efficiency and stability.
"These are still results based on small-area devices, and further verification is needed for large-area uniformity, module manufacturing feasibility, and scalability," Sungkyunkwan's Professor Park explained.
The researchers suggest that this entropy-engineering concept could be extended beyond perovskites to other flexible semiconductor systems, potentially enabling broader advances in next-generation optoelectronic devices.
