Three-dimensional (3D) organic–inorganic hybrid perovskite has become a promising photoelectric conversion material that is widely used in solar cells [1,2], light-emitting diodes [3–8], and semiconductor lasers [9,10]. Although 3D perovskite materials have achieved outstanding photoelectric conversion efficiency, the instability under atmospheric moisture hinders their large-scale commercial application [11–14]. To date, reduced-dimensional (two-dimensional, 2D) perovskites are considered to be the most successful attempt to solve the stability problem [11–13,15–19]. By involving hydrophobic organic cations, 2D perovskite destroys the crystal symmetry of 3D perovskite and breaks the limitation of the tolerance factor [20]. More importantly, its dielectric confinement and quantum confinement effects can lead to a large exciton binding energy and special photophysical characteristics [21–24].

It is known that the dielectric confinement effect exists in quantum well structures [21–25], as shown in Fig. 1(a); here, εw and εb represent the dielectric constants of potential well materials (corresponding to 3D perovskites) and potential barrier materials (corresponding to the hydrophobic organic cations of 2D perovskites), respectively. Gray ovals represent the polarized charges, which can reduce the Coulomb interaction between the electrons and holes. The barrier materials exhibit a smaller dielectric constant and a larger band gap compared to the well materials [14,26–30]. Both the effective dielectric constant of the whole system (corresponding to 2D perovskites) and the polarized charges are reduced [25,26] due to the penetration of the electric field. Consequently, the Coulomb interaction between the electrons and holes is enhanced and the exciton binding energy is increased, accompanying the emergence of dielectric confinement.

Dielectric confinement not only affects the exciton binding energy, but also adjusts the optical band gap [31,32]. More importantly, it significantly boosts the emission efficiency of 2D perovskite materials [31,32]. Recently, a few reports have emphasized the relationship between dielectric confinement and the properties of quasi-2D perovskites, such as the carrier mobility [33], photoluminescent properties [31], hot carrier cooling [34], electron–phonon coupling [34–36], and Auger recombination [32]. However, as a general guiding principle for the design of quantum well structural materials, research on the dielectric confinement effect is still insufficient. For example, it remains unclear how dielectric confinement affects coherent phonon dynamics and how to improve the efficiency. The real-time observation of coherent phonons can provide important insights into electron–phonon coupling [37–39] and exciton dynamics [40]. Besides, the systematic investigation on the influence of dielectric confinement on the physical mechanism in 2D perovskite materials is inadequate.

In this work, we explore the exciton dynamics in quasi-2D Ruddlesden–Popper perovskite (RPP) films containing organic spacers with different dielectric constants, ethanolamine (EA+), p-fluorophenethylammonium (p-FPEA+), and phenethylammonium (PEA+) [31–33], by combining femtosecond pump-probe transient absorption (TA) spectroscopy [41,42] and temperature-dependent PL spectroscopy. The dielectric constant tends to decrease, and the schematic molecular structures are shown in Fig. 1(b). To provide a quantitative comparison, the dielectric constants of EABr, p-FPEABr, and PEABr solid powder materials were measured. At 1 GHz, the dielectric constants of EABr, p-FPEABr, and PEABr powders are 3.04, 2.90, and 2.74, respectively. The observed trend is consistent with the theoretical predictions [33,43,44]. As expected, the coherent phonon dynamics, the energy transfer process, and the Auger recombination can be tuned by the dielectric confinement effect. Using ligands with small dielectric constants enables the formation of quasi-2D film with a weaker dynamic disorder, resulting in a more complete energy transfer and the inhibition of trap-mediated nonradiative recombination. Furthermore, the increase of the bulk-ligand dielectric constant reduces the corresponding exciton binding energy, thereby suppressing the Auger recombination. Our study will not only improve the radical understanding of exciton dynamics in hybrid layered perovskites but also emphasize the significance of the dielectric confinement effect for exciton dynamics.

Quasi-2D RPPs possess the formula of B2An−1PbnX3n+1, where B represents an aromatic or aliphatic alkylammonium cation; A+ can be an organic or inorganic cation such as methylammonium cation (MA+) or Cs+; and X− is a halide anion. The n-values represent the number of inorganic [PbX6] octahedra sandwiched between organic barriers [13,26,27,32,36]. To precisely explore the influence of the dielectric constant on the coherent phonon dynamics [40] of quasi-2D RPPs, we fabricated ⟨n⟩=1EA2PbBr4, p-FPEA2PbBr4, and PEA2PbBr4 perovskite films [Fig. 1(c)] for investigation. (Note: “n” stands for the species with a fixed composition and “⟨n⟩” represents the average number of perovskite layers in the film.) Details of the sample preparation are presented in Appendix A. Figure 1(d) shows the normalized UV-vis absorption and steady-state PL spectra of EA2PbBr4, p-FPEA2PbBr4, and PEA2PbBr4 perovskite films. The results indicate that the photophysical properties of 2D RPP films are influenced by organic cations with different dielectric constants. The three ⟨n⟩=1 films show the characteristic of single exciton absorption peak, indicating a pure phase for these materials. The absorption peak wavelength is blueshifted in the order of 417.5 nm (EA2PbBr4)>406.0nm(p-FPEA2PbBr4)>402.5nm (PEA2PbBr4). The corresponding PL peak wavelengths reveal a similar trend (412.8 nm for p-FPEA2PbBr4>408.9nm for PEA2PbBr4), except that the emission of EA2PbBr4 is too weak to measure. Figure 1(e) depicts the X-ray diffraction (XRD) patterns of EA2PbBr4, p-FPEA2PbBr4, and PEA2PbBr4 perovskite films. All of the 2D RPP films are characteristic of a set of (0 0 k) diffraction peaks [31–33], demonstrating the formation of a layered structure.

The broadband pump-probe TA spectroscopy was used to investigate the ultrafast dynamics of 2D RPP EA2PbBr4, p-FPEA2PbBr4, and PEA2PbBr4 films. The corresponding TA spectra and dynamics are depicted in Figs. 2(a)–2(c). Note that the orange coordinate scale corresponds to TA dynamics of a representative probe wavelength, and the black coordinate scale corresponds to TA spectra. As shown in Fig. 2(a), the TA spectrum is comprised of one ground state bleaching (GSB, ΔA<0) peak at 418 nm correlating with the linear absorption spectra and one photoinduced absorption (PIA, ΔA>0) band at the blue side of the GSB band. Similar phenomena are also observed in p-FPEA2PbBr4 [Fig. 2(b)] and PEA2PbBr4 [Fig. 2(c)] films. Unlike the EA2PbBr4 film, obvious coherent oscillations decaying with time due to electron-optical phonon coupling [39,40,45,46] are present in the TA kinetics of p-FPEA2PbBr4 and PEA2PbBr4 films, as shown in black dots of Figs. 2(b) and 2(c). To precisely evaluate the variation trend of coherent phonon dynamics, we fitted TA kinetics with multi-exponential functions [ΔA(t)=a1exp(−t/τ1)+a2exp(−t/τ2)+…+anexp(−t/τn)−c1exp(−t/τet), where a1,a2,…,an, c1 are amplitudes; τ1,τ2,…,τn are decay time constants, and τet is formation time constant], and then obtained the coherent phonon dynamics by subtracting the exciton kinetics, as shown in the inset of Fig. 2(d). The background exciton dynamics is related to rapid electronic excited state relaxation. The τ1 corresponds to the defect trapping process. The τ2 is the result of hot carrier cooling and exciton formation processes. The τ3 is associated with carrier population decay caused by various radiative and nonradiative recombination processes [40,46]. The coherent oscillation time of 2D RPP EA2PbBr4 film is short and the modulation depth is weak. Furthermore, the PEA2PbBr4 film exhibits superior coherent phonon modulation depth compared to p-FPEA2PbBr4.

In the process of exploring the relationship between coherent phonon dynamics and luminescence performance, it was observed that the amplitude of phonon dephasing dynamics is positively correlated with photoluminescence quantum yield (PLQY). To further investigate the relationship between coherent phonon dynamics and the luminescence mechanism, a TA kinetic analysis was conducted. As shown in Fig. 2(e), the bleaching kinetics of EA2PbBr4, p-FPEA2PbBr4, and PEA2PbBr4 films exhibit a fast decay (τ1, corresponding to the defect trapping process [47]) under the same experimental conditions. Detailed fitting data for the three films are described in Table 1. As shown in Table 1, the proportion of τ1 gradually decreases and the lifetime gradually increases as the dielectric constant of the bulk ligand decreases, indicating that trap-assisted recombination is inhibited. Based on these observations, we propose a mechanism for defect screening in the presence of a coherent phonon. Perovskite films contain numerous grain boundaries where most defects are located [35], as shown in Fig. 2(f). When decreasing the dielectric constant of bulk ligand, Coulomb shielding is tuned, and the dynamic disorder is weaker. Excitons are strongly coupled with optical phonons [40], enabling them to screen charge defects and overcome the trap-mediated nonradiative recombination [48,49]. In the case of 3D lead halide perovskites, it has been proposed that electron–phonon coupling (polaron effect) protects photogenerated carriers from lossy scattering paths involving defects and Auger-like processes [45,50–52]. These observations indicate the coherent phonon of quasi-2D perovskites that can also potentially shield the excitons from trap-mediated nonradiative pathways.Table 1.

Fitting Parameters of the GSB Kinetics of ⟨n⟩=1 Quasi-2D RPP Films

Based on the discussion above, ⟨n⟩=42D RPP films, with excellent emission properties according to previous reports [32], were fabricated to further investigate exciton dynamics. Figure 3(a) shows the PL and absorption spectra of the ⟨n⟩=4 thin films using the three dielectric constant bulk ligands. Unlike the ⟨n⟩=12D RPP films, multiple absorption peaks appear in the steady-state absorption spectra of ⟨n⟩=4 films, indicating the distribution of a series of quantum wells instead of a pure phase [44]. The large-n quantum well absorption peak of the EA2MA3Pb4Br13 film is blueshifted compared to p-FPEA2MA3Pb4Br13 and PEA2MA3Pb4Br13 perovskite films, which is attributed to the difference in the domain distribution [29,32,44]. The characteristics of steady-state PL spectra are consistent with those of steady-state absorption spectra. The XRD patterns of EA2MA3Pb4Br13, p-FPEA2MA3Pb4Br13, and PEA2MA3Pb4Br13 are presented in Fig. 3(b). The (0 0 k) diffraction peaks derived from n=1 species can be observed in ⟨n⟩=4 perovskites, demonstrating the formation of a layered structure. In addition, the diffraction peaks of high n space appear, which confirms the mixed phase within the quasi-2D perovskite films [32]. Furthermore, temperature-dependent PL measurements were performed to quantitatively extract the Eb [Figs. 3(c)–3(e)] [32,53]. With the increase in temperature, the PL spectra exhibit features of intensity reduction and spectral line broadening. The exciton binding energy of the perovskite film is evaluated by the temperature dependence of the PL-signal integral intensity [43], and is fitted using [16,32,43] I(T)=I01+Ae−EbkBT,(1)where I0 is the integrated PL intensity extrapolated at 0 K, A is a constant, Eb is the exciton binding energy, and kB is the Boltzmann constant. The extracted Eb is estimated to be 57.4, 75.3, and 135.9 meV for EA2MA3Pb4Br13, p-FPEA2MA3Pb4Br13, and PEA2MA3Pb4Br13, respectively. The results confirm that decreasing the dielectric constant of the ligand will increase Eb [32,33]. Basically, organic ligands possessing small dielectric constants exhibit reduced polarity, which diminishes the dielectric screening of electron–hole interactions and consequently leads to dielectric confinement. Thus, it becomes feasible to reduce Eb by attenuating the dielectric confinement.

To investigate the ultrafast exciton dynamics in quasi-2D RPPEA2MA3Pb4Br13, p-FPEA2MA3Pb4Br13, and PEA2MA3Pb4Br13 films, TA measurements, as shown in Figs. 3(f)–3(h), were conducted. The orange coordinate scale corresponds to the TA dynamics of a representative probe wavelength, and the black coordinate scale corresponds to the TA spectra. With the exception of EA2MA3Pb4Br13 films, the coherent phonon dynamics similar to ⟨n⟩=1 films were observed in ⟨n⟩=4 quasi-2D perovskite films, and the strongest TA oscillatory kinetics of the three spectra were extracted, respectively. The variation trend of phonon dephasing dynamics is consistent with that of ⟨n⟩=1 films, in which the material with the lowest dielectric constant has the largest modulation depth of a coherent phonon.

Auger recombination, as an important factor in exciton dynamics, has a crucial impact on the performance of infrared detectors, the efficiency roll-off of light-emitting diodes, and the transport properties of topological insulators [54,55]. The recombination dynamics of GSB under different pump fluences were investigated to explore the Auger recombination kinetics. Notably, a fast decay component grows rapidly as the pump fluence increases, which is consistent with the characteristic of biexciton recombination [56–58]. The biexciton Auger recombination lifetime (τ) was obtained by performing a two-by-two subtraction (P2–P1, P3–P2, and P3–P1) from the TA population dynamics at three different pump fluences (P1, P2, and P3) and then averaging the three fitted time constants. As shown in Fig. 4(e), the time constants of EA2MA3Pb4Br13, p-FPEA2MA3Pb4Br13, and PEA2MA3Pb4Br13 perovskite films are 50.49, 19.81, and 14.83 ps, respectively. This analysis confirms that the increase in the bulk-ligand dielectric constant reduces the corresponding exciton binding energy, thereby suppressing the Auger recombination [54,56,59]. These results demonstrate the potential use of the dielectric constant as a regulator of exciton dynamics in perovskite materials.

In summary, we selected three large cations with different dielectric constants as the ligands of quasi-2D RPPs and studied the corresponding coherent phonon dynamics, energy transfer process, and Auger recombination dynamics of ⟨n⟩=1 and 4 thin films. By correlating the dielectric confinement effect with exciton dynamics, we found that the use of ligands with a smaller dielectric constant can reduce the screening of Coulomb interaction and weaken dynamic disorder, resulting in coherent phonons with a higher modulation depth. The TA kinetics under different dielectric constant ligands and with different ⟨n⟩ indicate that the coherent phonons can effectively screen the trap-mediated nonradiative relaxation path, increase the energy transfer rate, and obtain a higher PLQY. Note that the transfer rate increases to 35.5%, 54%, and 92.8% after the modulation depth of the coherent phonons increases. Furthermore, the TA GSB kinetics under different pump fluences of ⟨n⟩=4 films with various dielectric constants confirm that Auger recombination can be suppressed by increasing the bulk ligand dielectric constant. The time constants of biexciton recombination kinetics are 50.49, 19.81, and 14.83 ps for EA2MA3Pb4Br13, p-FPEA2MA3Pb4Br13, and PEA2MA3Pb4Br13 perovskite films, respectively. By increasing the dielectric constant, the contribution of the Auger recombination can be reduced for realizing stable devices under high current density. This work emphasizes the importance of the dielectric constant in quasi-2D perovskite material design and may provide valuable insights into the exciton dynamics of organic–inorganic hybrid layered perovskite relevant for emergent optoelectronics development.