The two-dimensional halide perovskites exhibit a layered structure formed by splitting the three-dimensional perovskite lattice with hydrophobic spacer cations. The structural and chemical design flexibility of these naturally multiple quantum well systems has sparked intense new interest. Success has been achieved in solar cells, LEDs, lasers, and photodetectors, but their performance falls short compared to three-dimensional halide perovskite devices^1,2.

The exciton transport properties in two-dimensional halide perovskites play a pivotal role in their optoelectronic performance. They are closely associated with defects, energy landscape, and exciton-phonon coupling. However, a significant challenge currently faced is that the exciton /carrier mobility is approximately 1-2 orders of magnitude lower compared to three-dimensional perovskites^3,4. Similar to three-dimensional perovskites, two-dimensional perovskites exhibit exceptional softness. The effective coupling between excitons and lattice vibrations leads to strong exciton-phonon interactions and the emergence of the proposed polariton effects⁵.

Despite substantial progress in exciton transport, there is still a need for further exploration of the explicit correlation between exciton transport and lattice properties, particularly the impact of exciton-lattice interactions. It is crucial to adjust exciton transport characteristics in photonic and electronic applications based on 2D perovskites by modulating exciton-phonon interactions.

With our group's strong interest in ultrafast dynamics and halide perovskites, we have constructed time-resolved fluorescence microscopy and time-resolved transient absorption microscopy⁶ to observe the ultrafast exciton/charge carrier transport processes in semiconductor materials. After understanding the physical picture of electron-lattice interactions in two-dimensional halide perovskites⁷, we utilized a polymethyl methacrylate (PMMA) structure to immobilize benzylamine (BA) molecules on the surface of (BA)₂(MA)_n-1Pb_nI_3n+1. We observed the rapid diffusion of photoexcited excitons, with the mobility increasing from 8 cm²V⁻¹s⁻¹ without PMMA to 280 cm²V⁻¹s⁻¹ with PMMA, surpassing exciton mobility by 1-2 orders of magnitude (Figure 1).

Figure 1ǀ Exciton transport in exfoliated (BA)₂PbI₄ RPP flakes encapsulated by coverslip and PMMA.

We were pleasantly surprised by the significant enhancement observed, but the real challenge lies in exploring the underlying mechanisms for this improvement. For this task, a team composed of various experts is needed, dedicated to conducting steady-state and transient dynamics studies on exciton mobility enhancement, theoretical analyses, and molecular dynamics simulations. We are pleased that when all the results of different technologies point to the same conclusion, everyone's efforts are rewarded (Figure 2).

Figure 2ǀ Exciton diffusion approaching MIR limit.

Combining optical characterization and theoretical calculations, we discovered that the rigid PMMA network can anchor BA molecules on the surface, resulting in increased lattice rigidity and reduced disorder. The deformation potential for exciton-phonon scattering decreased from 4.0 ´ 10⁸ eV/cm to .9 ´ 10⁷ eV/cm, leading to a significant improvement in exciton transport properties with increased exciton scattering time and average free path. The enhanced mobility conforms to the Mott-Ioffe-Regel (MIR) criterion^8,9.

In this scenario, the localized exciton transport mechanism in the 2D RPP becomes an intermediate case between localized hopping transport and semiclassical band-like transport. It uniquely combines characteristics described by both, exhibiting delocalized exciton wavefunctions and high binding energy.

The above findings elucidate the origin of high exciton mobility in Ruddlesden−Popper perovskites, revealing a strong correlation between exciton transport, lattice rigidity, and exciton-lattice interactions. By manipulating the strength of the electron-phonon coupling, this research has achieved a significant breakthrough in exciton transport performance, offering new insights into the modulation of exciton emission and transport in two-dimensional hybrid systems.

1. Tsai H, et al. High-efficiency two-dimensional Ruddlesden–Popper perovskite solar cells. Nature 536, 312-316 (2016).
2. Yuan M, et al. Perovskite energy funnels for efficient light-emitting diodes. Nanotechnol. 11, 872-877 (2016).
3. Seitz M, et al. Halide Mixing Inhibits Exciton Transport in Two-dimensional Perovskites Despite Phase Purity. ACS Energy Lett. 7, 358-365 (2022).
4. Zhao C, et al. Trap-Enabled Long-Distance Carrier Transport in Perovskite Quantum Wells. Am. Chem. Soc. 142, 15091-15097 (2020).
5. Ziegler JD, et al. Fast and Anomalous Exciton Diffusion in Two-Dimensional Hybrid Perovskites. Nano Lett. 20, 6674-6681 (2020).
6. Shuai Yue et al. ,High ambipolar mobility in cubic boron arsenide revealed by transient reflectivity microscopy. Science 377,433-436 (2022).
7. Bo Wu et al. ,Uncovering the mechanisms of efficient upconversion in two-dimensional perovskites with anti-Stokes shift up to 220 meV. Adv. 9, eadi9347 (2023).
8. Wagner K, et al. Nonclassical Exciton Diffusion in Monolayer WSe₂. Rev. Lett. 127, 076801 (2021).
9. Glazov MM. Quantum Interference Effect on Exciton Transport in Monolayer Semiconductors. Rev. Lett. 124, 166802 (2020).