Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free Cs3Bi2I9 Perovskite Single Crystal
Item Type Article
Authors Tailor, Naveen Kumar;Maity, Partha;Satapathi, Soumitra
Citation Tailor, N. K., Maity, P., & Satapathi, S. (2022). Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free Cs3Bi2I9 Perovskite Single Crystal. The Journal of Physical Chemistry Letters, 5260–
5266. https://doi.org/10.1021/acs.jpclett.2c01369 Eprint version Post-print
DOI 10.1021/acs.jpclett.2c01369
Journal The journal of physical chemistry letters
Rights Archived with thanks to The journal of physical chemistry letters Download date 2023-11-01 08:45:52
Link to Item http://hdl.handle.net/10754/678833
Phonons Mediated Slow Hot Carrier Dynamics in Lead-Free Cs
3Bi
2I
9Perovskite Single Crystal
Naveen Kumar Tailor 1, Partha Maity, 2,* Soumitra Satapathi1,*
1 Department of Physics, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, 247667,India
2 Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia
Email- [email protected]; [email protected]
Abstract:
In this report, we study the hot carrier cooling mechanism of the Cs3Bi2I9 single crystal by using femtosecond transient reflectance (TR) spectroscopy. We find an unusual slow hot carrier cooling associated with longitudinal optical (LO) and coherent longitudinal acoustic phonons (CLAPs) emission during the deexcitation of the hot carriers. We posit the interplay between the hot-carriers and the LO and CLA phonons in sub ps to sub ns time scale, respectively, by analyzing the TR kinetics upon perturbation with excess energy. Further, we measured the CLAPs propagation velocity in Cs3Bi2I9, the crystal, ranging from 1820 to 2000 ms-1. The elastic constants and frequency of Brillouin oscillations were estimated as 20.08 GPa and 14.66 GHz, respectively. Our discovery delivers new physical insights into how the hot carriers in Cs3Bi2I9 single crystal are coupled with crystal lattice that controls the hot carrier dynamics.
Metal halide perovskites (MHPs) have emerged as the Next Big Thing in the optoelectronics research community during the last decade. The tremendous success of MHPs in photovoltaics, light emission, and detection has inspired much attention to their underlying optical and electrical characteristics.1-3 Hot carrier (HC) cooling is a fundamental photophysical process in MHPs that has a significant impact on the optoelectronic performance of these materials.4-5 The origin of this phenomenon is still under debate; however, it has been attributed to several mechanisms including the accumulation of optical phonons, optical-acoustic phonon upconversion, and polaron formation.6-10 Recently, a few exciting studies have been done to understand the effect of carrier- phonon interaction on hot carrier dynamics and their role in the temperature-dependent transport and relaxation process.11 Price et al. used the transient absorption (TA) spectroscopy technique to illustrate the hot carrier relaxation process in MAPbI3 perovskites and shown that hot carrier relaxation is significantly affected by the electron-phonon interactions.12 Li et al. demonstrated the interaction between the hot-phonon bottleneck and Auger reheating effects in MAPbBr3 bulk films and nanocrystals.13 Further, Fu et al. investigated that the Auger heating interplay hot-phonon effect is responsible for slow hot carrier cooling in MAPbI3 perovskites.8 Furthermore, Yang et al.
explained the impact of the phonon bottleneck effect generated by acoustic-optical phonon upconversion in FAPbI3 bulk film on longitudinal optical phonon emission.6 In two-dimensional (2D) Ruddlesden-Popper (RP) perovskites, how coherent acoustic phonons affect the hot carrier relaxation dynamics has been widely explored.14-16 The interaction of the hot carriers and the coherent longitudinal acoustic phonons (CLAPs) can extend the oscillation of the TR kinetics to nanoseconds, potentially resulting in higher thermal conductivities of 2D RP perovskites.
Furthermore, Guo et al. also discussed the formation and propagation of coherent longitudinal acoustic phonons in 2D RPs along the cross-plane direction.15 When compared to the 3D CH3NH3PbI3 equivalent, they significantly reduce group velocity and propagation length of acoustic phonons in 2D RPs. Although, a reasonable understanding about the hot carrier cooling dynamics and electron-acoustic phonon coupling is achieved in lead-based perovskites, the same for lead-free perovskites is almost missing and worth exploring. Moreover, it is worthwhile to mention that these lead-free perovskites have been of much interest recently as they can overcome the toxicity and poor environmental stability of lead perovskites.17-20
Among various lead-free materials, bismuth (Bi) (Bi3+ have 6s2 lone pair same as Pb2+) is promising as an alternative to lead because of similar effective ionic radius and electro-negativity to lead.21-26 Additionally, the all-inorganic lead-free perovskites are expected to offer improved stability and could tolerate a higher operating temperature and a harsh environment due to the absence of organic components.27-29 In this manner, the all-inorganic, lead-free Cs3Bi2I9 perovskite system has gained considerable attention in solar cells, photocatalysts, photodetectors, and X-ray detector applications.30-36 The Cs3Bi2I9 has zero-dimensional lattice connectivity, which results in the quantum confinement of charge carriers. Previously, our group demonstrated the structural properties, spin dynamics, hot carrier relaxation, and polaron mediated photoconductivity in the Cs3Bi2I9 system.21, 26 Ghosh et al. reported that one of the primary obstacles to achieving high efficiency in solar cells is the weak interaction between [Bi2I9]3- bioctahedra.37 The 0D Cs3Bi2I9
perovskites feature broad luminescence with high Stokes shifts, unlike 3D perovskites, which have narrow emission bands and small Stokes shifts. self-trapped excitons or halogen vacancy centers, or lattice relaxation preceding light absorption and emission processes, were previously assumed to be responsible for the large Stokes shift.37-39 However, the essential phenomena of electron- phonon coupling, which is generally the primary source of luminescence spectrum widening and charge carrier mobility, has received little attention in Cs3Bi2I9 perovskites. Previously, Nila et al.
described the exciton-phonon interactions in the Cs3Bi2I9 single crystals and found that defects in the crystal significantly affect the exciton-phonon interaction strength.40 Additionally, it was found that the overlap of excitons with phonons reduces the exciton-phonon coupling. Despite the widespread interest in this material and the numerous theoretical and experimental investigations undertaken to understand better photophysical processes, fundamental features such as electron- lattice coupling and hot carrier cooling dynamics remain poorly understood. As a result, it's vital to learn about material characteristics in their most basic form- the single crystal which exhibits long-range order and absence of grain boundary.
Herein, we explore and interpret the hot carrier cooling dynamics in Cs3Bi2I9 single crystal using state-of-the-art broadband femtosecond transient reflectance spectroscopy. The de-excitation of the hot carriers mainly occurs through the combination of optical and acoustic phonons emission.
The propagation of longitudinal optical phonons and coherent acoustic phonons are detected in the sub ps to sub ns time scale, respectively. The knowledge of the acoustic phonon properties and electron-phonon interactions will provide a deeper understanding of the microscopic phenomena ruling electron transport in Cs3Bi2I9.
The single crystals of Cs3Bi2I9 were grown using the inverse temperature crystallization (ITC) technique as described in the Supporting information.23, 26 The photograph of as-grown crystals is shown in Figure 1a, which appears to be a dark red color. The scanning electron microscopy (SEM) image shows that the Cs3Bi2I9 crystal has a smooth surface and no observable grain boundary (Figure S1a). The chemical composition of the as-grown was measured by using the energy dispersive X-ray (EDXA) analysis. The atomic percentage ratio is found as Cs: Bi: I = 2.9 : 2.1 : 9, which is almost identical to the ideal stoichiometry of Cs3Bi2I9 with measurement error (Figure S1b). To investigate the crystalline quality and structural information, we measured the powder X-ray diffraction as the obtained pattern is shown in Figure 1b. The diffraction peaks for the Cs3Bi2I9 crystal occur at 2θ = 17.47 o, 26.17 o, 43.43 o, and 52.38 o correspond to (004), (006), (0010), and (0012) planes, respectively. The Cs3Bi2I9 crystal exhibit a hexagonal crystal structure with space group P63/mmc, which exhibits 24 symmetry elements.26 The lattice parameter was obtained as a = b = 8.39 Å, c = 21.20 Å. The structure of Cs3Bi2I9 is presented in the inset of Figure 1b, which shows the hexagonal view along the c-axis. The Cs3Bi2I9 structure consists of isolated [Bi2I9]3- anions (formed by face-sharing of [BiI6]3- octahedra) filling the voids with Cs+ ions forming a zero-dimensional (0D) structure. In this structure, distinct dimer [Bi2I9]3− anions are placed in a layered configuration along the ab (00Ɩ) plane. Because of the 0D electronic
dimensionality, photoexcited charge carriers may be localized, resulting in high charge-carrier effective masses.
Figure 1. (a) Photograph and (b) Powder X-ray diffraction pattern of the Cs3Bi2I9 crystal. (c) The optical absorption spectrum and calculated bandgap using Tauc plot (inset) and (d) Urbach energy calculated from the inverse absorption tail slope of Cs3Bi2I9 crystal.
To investigate the optical properties of these single crystals, we measured the diffuse reflectance spectra and calculated the absorbance spectra. The steady-state absorption spectrum for these double perovskite single crystals is shown in Figure 1c, which shows a quite sharp absorption edge at approximately 650 nm. Further, we have calculated the bandgap using the Tauc plot method (equation S1 and S2), assuming an indirect allowed transition, and estimated it as 1.91 eV (inset of Figure 1c).26, 30, 34 Moreover, we calculated the Urbach energy using equation S3 and estimated its value as 53.27 meV (Figure 1d). The obtained value is higher than the 3D lead-halide perovskites, which can be attributed to the structural disorder and enhancement of localized state density because of the short-range interaction in this crystal.41-43 Additionally, the Jahn−Teller distortion and the Peierls-like structural instability toward lattice distortion of the semimetal Bi can lead to a higher value of Urbach energy.44-45
560 580 600 620 640 660 680 700 0.0
0.2 0.4 0.6 0.8 1.0
Absorbance (a.u.)
Wavelength (nm)
1.8 1.9 2.0 2.1 2.2 0
1 2 3 4 5
Energy (eV)
(ahn)1/2
Eg = 1.91 eV
1.9 2.0 2.1 2.2
0 1 2 3
ln(A)
Energy (eV)
EU = 53.27 meV
10 20 30 40 50 60
0.0 0.2 0.4 0.6 0.8 1.0 1.2
(0012) (0010) (006)
Intensity (a.u.)
2q (o)
(004)
a b
c d
Figure 2. (a) The survey X-ray photoelectron spectroscopy (XPS) spectrum of Cs3Bi2I9 crystal.
Core level spectra of Cs 3d (b), Bi 4f (c) and I 3d (d), respectively.
Furthermore, we have performed the XPS measurement to investigate the oxidation states of present elements in the crystal. Figure 2a shows a survey scan of a Cs3Bi2I9 crystal with prominent peaks corresponding to cesium [Cs (3d)], bismuth [Bi (4f)], and iodine [I (3d)]. The spectrum shows primarily perovskite core peaks, but it also includes some additional contributions, most likely due to surface contaminants and the sample holder's contribution.32, 46 The peaks around 284 eV, for example, can be attributed to carbon, while the peak around 529 eV can be attributed to oxygen. The Cs 3d5/2 and Cs 3d3/2 peaks are found around 723.61 and 737.68 eV, respectively, which is consistent with the conventional Cs element (Figure 2b). The energy difference between the two peaks was ~14.07 eV. As indicated in Figure 2c, the contribution of Bi 4f was identified at 163.37 eV and 157.98 eV, which corresponds to 4f5/2 and 4f7/2. The energy gap between these two peaks was measured to be ~5.39 eV, which is typical of Bi3þ species. Figure 2d shows two prominent peaks related to the core level I 3d at 629.55 and 618.07 eV, which are associated with the doublets 3d5/2 and 3d3/2 and an energy separation of ~11.48 eV. These results indicate the formation of Cs3Bi2I9 perovskite.32, 46
745 740 735 730 725 720 715
3d5/2 3d3/2
Intensity (a.u.)
Binding Energy (eV)
Cs 3d
168 164 160 156 152
4f7/2 4f5/2
Binding Energy (eV)
Intensity (a.u.)
Bi 4f
635 630 625 620 615 610
3d5/2 3d3/2
Binding Energy (eV)
Intensity (a.u.)
I 3d
1000 800 600 400 200 0
I 4p Bi 5dI 4dCs 4d
Bi 4f
C 1sBi 4dBi 4d
O 1s
I 3dI 3d
Cs 3d
Cs 3p I 3pI 3p
Binding Energy (eV)
Intensity (a.u.)
Overview Spectra
Cs 3d
a b
c d
Figure 3. (a) Two-dimensional (2D) color plot and (b) transient reflectance (TR) spectra at short time delays of Cs3Bi2I9 single crystal in response to 480 nm optical excitation.
To elucidate the carrier dynamics of Cs3Bi2I9 single crystal, we performed femtosecond transient reflectance (fs-TA) measurement using commercially available transient absorption spectroscopy.
Details about the experimental setup can be found in our previous reports.16, 47 The negative and positive TR signals are due to photobleaching (PB) and photoinduced absorption (PIA), respectively. Figure 3a portrays a 2D color plot of TR spectra of Cs3Bi2I9 single crystal in a 5.5 ns time window in response to excess energy excitation (λexcitation= 480 nm). As can be seen that a strong PB signal below 550 nm to 750 nm peaking at 600 nm, appears due to the depletion of the ground state carrier at the band edge. The PB signals oscillate periodically in the entire spectral range below the band-edge after the sub-tens ps time window. The oscillation of the TR signal at the below band edge is demonstrated in the next section. A strong PIA below 500 nm indicates a long-lived excited state (> 5.5 ns) due to a direct transition from the VB to a higher excited state in the CB. As can be seen in Figure 3b, the TR signal at bandedge gradually increases with progressing the time delay, and it reaches maxima at ~4.5 ps time. The gradual increment of the TR signal upon excess energy excitation indicates the cooling time of the photoexcited hot carrier.
To understand the mechanism of carrier cooling dynamics, we have compared the TR kinetics at below bandgap stated following 480 nm excitation. Figures 4a depict the TR kinetics below bandgap states starting from 600 nm to 890 nm. As can be seen that the TR kinetics in all wavelengths consist of an exponential recovery followed by the periodic oscillation of the TR signal, which extends up to > 5.5 ns time window. The TR kinetics at the early delay shows a rise, and it recovers exponentially. The rise time in the TR PB signal varies from 900 fs to 1.5 ps, attributed to quasi-thermal equilibrium distribution time (Figure S2). The recovery of the TR PB signal consists of two distinct parts (i) exponential recovery in sub ten ps with 4 to 7 ps time constants early, followed by (ii) periodic oscillation, which can fit using the Sine function. The sub ps time components (4 - 7 ps) during the recovery process of Cs3Bi2I9 single-crystal upon excess energy excitation can be endorsed as deexcitation of the carriers through the non-radiative
400 450 500 550 600 650 700 750 1
10 100 1000
Wavelength (nm)
Time delay (ps)
-0.08050 -0.06588 -0.05125 -0.03663 -0.02200 -0.007375 0.007250 0.02187 0.03650 DR/R
400 450 500 550 600 650 700 750 -0.02
-0.01 0.00 0.01 0.02 0.03
DR/R
Wavelength (nm)
-1 ps 200 fs 490 fs 800 fs 1.2 ps 1.7 ps 2.8 ps 4.5 ps 6 ps
Scatter Light
lex= 480 nm
a b
channel associates mainly via longitudinal optical (LO) phonons.15, 48-50 Moreover, the TR kinetics at the longer delay (Figure S3) shows a periodic oscillation like an ultrasound wave, attributed to coherent longitudinal acoustic phonons (CLAPs) propagation in the lattice during the deexcitation of carriers.15-16, 45, 51 The CLAP propagation can obtain the best fit using the Sine wavefunction, and the corresponding phonon velocity can be calculated directly from the empirical formula 𝑣 =
l
2𝑇[l] 𝑛(l), where [T(λ) is the period, 𝑣 is propagation velocity, and 𝑛(l) is the reflective index at probing wavelength, λ).16, 51 We found that CLAP velocity varies from 1820 to 2000 ms-1 in the measured energy range (1.4 to 2.1 eV) by considering the reflective index 2.2 for Cs3Bi2I9. Using this CLAP velocity value, and a mass density of ρ = 5.02 gcm-3 for the Cs3Bi2I9 crystal, we obtain the elastic constant C along the direction of propagation of the phonons using the formula 𝑣 = √𝐶𝜌, where C is elastic constant and ρ is mass density.52-53 We found that elastic constant as C = 20.08 GPa at probe wavelength of 600 nm. Additionally, propagating CLAPs are detected through Brillouin oscillations. The frequency of Brillouin oscillations can be estimated using the formula 𝑓 = 2𝑛𝑣
𝜆 , where n is the refractive index at the probe wavelength λ and v is the phonon propagation velocity. The frequency of the Brillouin oscillation was calculated as 14.66 GHz. The CLAP is further studied by comparing the fluence-dependent TR kinetics. Figure 4b compares the propagation amplitude of CLAP at 600 nm upon 1.7 and 3.2 µJcm-2 pump fluence in response to 480 nm excitation. As can be seen that the amplitude of CLAP intensity increases almost linearly, which reveals the relative contribution of thermoelastic and deformation potential stress of the Cs3Bi2I9 crystal structure.16, 51
0 50 100 150 200 250 300
DR/R
Time delay (ps)
1.7 mJcm-2
3.2 mJcm-2 lprobe= 600 nm lpump= 480 nm
Optical Acoustic
Eg hν
VB CB
0 100 200 300 400
600 nm
Time delay (ps)
890 nm
760 nm
715 nm
a b
DR/R
c
Figure 4. (a) TR kinetics probe at 600, 715, 760, and 890 nm of Cs3Bi2I9 single-crystal following 480 nm optical excitation. The black dotted and red solid lines represent the experimental and fitted data, respectively. (b) Excitation pump fluence-dependent TR kinetics probed at 600 nm following 480 nm excitation. (c) Schematic illustration of carrier cooling and propagation of phonon dynamics in Cs3Bi2I9 single crystal.
The impact of phonons emission on hot carrier cooling is illustrated in Figure 4c. At above bandedge excitation, the excited carriers are promoted to the higher electronic states, which rapidly thermalizes in the electronic states through carrier-carrier scattering.4, 8, 10 Note, the carriers at CB and VB have energy greater than the thermal energy (kBT, where kB is Boltzmann constant), known as hot electron and hot hole, respectively. Thermalization of hot carriers is a very fast process and usually occurs within less than 100 fs time, which is the limitation of our TR measurement.
Following thermalization, the hot carrier cools down to the lattice temperature through non- radiative phonons emission, i.e., lattice vibration.54-55 Two types of phonons emission are predominantly observed in superlattices during the cooling of hot carriers. The longitudinal optical phonons interact with the lattice point in one way (left side of the Figure 4c), i.e., the vibrated may occur in the same (compression) or opposite direction (stretching). As a result, the process of LO emission observes in the early time scale, and it varies in sub ps time window. Unlike LO phonons, the CLAPs propagate in crystal lattice-like sound waves with a mixture of compression and rarefaction vibration motions (right side of Figure 4c).16, 51, 56 Thus, the velocity of CLAPs is attenuated by the medium, and it takes a longer time as compared to the LO phonons.
In conclusion, we have examined the hot carrier cooling dynamics in Cs3Bi2I9 single crystal using femtosecond transient reflectance spectroscopy by exciting at excess energy. To the best of our knowledge, our fs-TR data provide the first experimental evidence of the real-time observation of hot carrier cooling in lead-free Cs3Bi2I9 single crystals perovskite. Additionally, the results demonstrate that the cooling of the hot carrier occurs at the sub ps time scale, and the deexcitation of the hot carriers is mainly coupled with LO and CLA phonons in the crystal lattice. Interestingly, we find that the lifetime of the LO and CLP phonons varies in sub ps and sub ns time scales, respectively. The results presented here provide a crucial fundamental insight into the deexcitation of hot carriers that can be beneficial for the development of hot carrier solar cells.
ASSOCIATED CONTENT
Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.
methods, scanning electron microscope image, elemental analysis, bandgap and Urbach energy calculation, normalized TR spectra at sub-ps time delays, TR kinetics probe at 600, 715, 760, and 890 nm at the longer delay
AUTHOR INFORMATION
Corresponding Author
[email protected]; [email protected]
AUTHOR INFORMATION Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest. The data that support the plots within this paper and other finding of this study are available from the corresponding author upon request.
ACKNOWLEDGMENTS
N.K.T. acknowledges UGC Fellowship.
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