Artifacts in Transient Absorption Measurements of Perovskite Films

Viewpoint Cite This: J. Phys. Chem. Lett. 2019, 10, 97−101

pubs.acs.org/JPCL

Artifacts in Transient Absorption Measurements of Perovskite Films Induced by Transient Reflection from Morphological Microstructures

Downloaded via 94.231.219.110 on January 3, 2019 at 19:06:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

O

transmitted probe light completely results from the sample absorption. On the basis of the same experimental setup, transient reflection (TR) measurements can also be carried out by using the reflected probe light as detection signal (Figure 1b). The TR signal (ΔR/R) can also be determined by the ratio of the intensity of reflected probe light with and without pump excitation (see eq S4 in the SI). Unlike the TA measurements that mainly probe the bulk property of samples, the TR signal mainly detects the photoinduced reflection variations due to the refractive index change at the sample surface. Therefore, the TR spectrum and kinetics can be significantly different from those of TA even in the same sample.13,15,17−22 For example, previous TA and TR measurements have found dramatically faster carrier recombination kinetics on the surface than in the bulk of MAPbX3 perovskite films or single crystals because of the presence of more surface defects. There is an abnormal case in the regular TA measurements particularly when performed on the films with large and heterogeneous microstructures (e.g., films with poor coverage, large grains, and pinholes) because the loss of transmitted probe light in their TA measurements likely results not only from the sample absorption but also from the reflection of the film surface or the boundary of microstructures in samples. In this case, the measured transient spectrum, though collected in the transmittance mode as in TA, can contain contributions from both TA and TR signals (see Figure 1c and eq S6 in the SI). This could lead to distorted TA spectra and thus inaccurate analysis of photoinduced kinetics. A solutionprocessed organic or inorganic halide perovskite thin film is a typical material whose morphological microstructures were found to have significant impact on device performance.12,23−25 Although the photoinduced carrier dynamics in perovskite films has been extensively studied using TA spectroscopy, the possible artifacts in TA results induced by TR signal originating from the photoinduced reflectivity variation of film surfaces and microstructures have been overlooked. Herein, in order to clarify the influence of TR signal in the regular TA measurements, we performed a careful transient spectroscopic analysis on a series of MAPbBr3 perovskite films with different microstructure morphology. Meanwhile, TR measurements on MAPbBr3 single crystals (SCs) were carried out for comparison. We confirmed that the TA spectra measured in MAPbBr3 perovskite films with large and heterogeneous microstructures do comprise non-negligible TR signals from the photoinduced reflection of microstructures, with the weight of contribution increased from

rganolead halide perovskites MAPbX 3 (MA = CH3NH3+; X = Cl−, Br−, I−) have attracted broad interest in the past 10 years1−4 for their tremendous applications in solar cells and light-emitting devices.5−10 In evaluating the quality of the perovskite materials, spectroscopic characterizations such as static and time-resolved absorption and photoluminescence measurements are essential to examine their photophysical properties. A recent report found that the correct measurement of static absorption spectra of MAPbX3 films is indeed difficult due to the strong light scattering caused by their poor surface coverage or complex microstructures.11 These morphological complexities seem to be inevitable in thin-film fabrication and should not only affect the steady-state spectroscopic measurements but also can significantly impact the time-resolved spectroscopic characterizations,7,12−14 whose results are crucial for understanding photoinduced carrier dynamics in the examined materials. Photoexcited states in semiconductor materials induce changes in the real and imaginary parts of the dielectric function. This leads to changes in absorption (imaginary part) and reflectivity (real part), which can be substantial for materials with significant values of refractive index such as lead halide perovskites. Transient absorption (TA) spectroscopy is a typical technique that has been broadly used to probe photoexcited state dynamics in perovskites and other semiconductor materials.2,15,16 In TA measurements, a pump laser pulse is used to excite the perovskite films, and the induced absorption changes (ΔA) are recorded as a function of both wavelength and time. With the transmitted light as the probe (Figure 1a), the TA signal (ΔA) is mainly decided by the ratio of the intensity of transmitted probe light with and without pump excitation (see eq S1 in the SI), assuming that the loss of

Figure 1. Simulated transient spectra in (a) normal TA and (b) normal TR measurements with the transmitted and reflected probe light as detection signals. (c) Simulated abnormal TA spectrum distorted by the TR signal due to photoinduced reflection variation on the sample surface. © 2019 American Chemical Society

Received: December 11, 2018 Accepted: December 17, 2018 Published: January 3, 2019 97

DOI: 10.1021/acs.jpclett.8b03704 J. Phys. Chem. Lett. 2019, 10, 97−101

Viewpoint

The Journal of Physical Chemistry Letters ∼20 to ∼100% as the size of the microstructure increased from 550 nm become more prominent in amplitude than those for the two-step film, in company with the bleach peak position red98

DOI: 10.1021/acs.jpclett.8b03704 J. Phys. Chem. Lett. 2019, 10, 97−101

Viewpoint

The Journal of Physical Chemistry Letters

Figure 3. TA spectra at a delay time of 5 ps measured in a series of MAPbBr3 films with different particle sizes of (a) 1−2 μm, (b) 0.5−1 μm, (c) < 500 nm, and (d) ∼200 nm. These spectra are well described using the TR spectrum of MAPbBr3 SCs and its iHT as the fitting components, yielding the percentage contribution of TR signal in the spectra. (e) Weights of the TR components in transient spectra as a function of microstructure size of different films. (f) Normalized bleach kinetics for three MAPbBr3 films with microstructure sizes of 1−2 μm, 0.5−1 μm, and ∼200 nm. The faster carrier decay kinetics as the size of the microstructure increases is due to the large TR component, which mainly probes the faster surface carrier recombination.

film increases. This trend is consistent with the larger contribution of TR signal in the film with larger microstructures. As we discussed earlier in this work, the TR signal probes the photoinduced reflection variation due to the refractive index change at the sample surface, and its kinetics can be significantly faster than that in the bulk (probed by the intrinsic TA signal) due to the presence of more surface defects.19,20,22 Therefore, the larger contribution of TR signal leads to the faster observed kinetics. However, for perovskite films with a very small size of microstructures, the TR distortion in TA spectra may not change the observed carrier kinetics because the carriers can sample both surface and bulk defects with a fast carrier diffusion process. We also tried to seek a method to reduce the distortion of photoinduced reflection in TA measurements. Because the reflectivity (R) from the sample surface is mainly related to the refractive indexes of the MAPbBr3 perovskites (n1 > 2) and surrounding air (n2 = 1) (R = ((n1 − n2)/(n1 + n2))2), reducing the difference between n1 and n2 should be able to suppress the reflection from sample microstructures. Taking the one-step MAPbBr3 film for an example, we treated the film with a drop of nonvolatile silicone oil (with negligible absorption in the visible region) whose refractive index (nsilicone oil = 1.4) is closer to that of perovskite than air. Figure 4 compares the TA spectra of the perovskite film with and without silicon oil. This result shows that the artifact induced by the photoinduced reflection variation in the TA spectra, though not completely removed, can be significantly reduced by the presence of silicon oil. In conclusion, we demonstrated that the TA spectra measured in the MAPbBr3 perovskite films with heterogeneous microstructures do comprise non-negligible TR signal originating from the photoinduced reflection variation at film surfaces or the interface of microstructures. The weight of this TR signal was found to increase as the microstructure size in the film increases and distort the shape of the TA spectra. Because TR probes the photoinduced reflection variation at the film surface, the presence of TR signal can lead to faster

Figure 4. Comparison of TA spectra at a delay time of 5 ps collected from the one-step MAPbBr3 film with and without silicone oil, showing the reduced TR-distorted artifact signals for the film with the presence of silicone oil.

observed carrier kinetics than in the bulk (probed by the intrinsic TA signal) owing to the presence of more surface defects. In principle, the artifacts resulting from TR signal in TA measurements should not only occur in perovskite films but also may occur in other film materials and suspensions (see Figure S5 in the SI; a similar TR signal was also observed in their TA measurements in CdS suspensions). Because the TA is one important technique to probe the photoinduced dynamics in semiconductors and many other materials, we pointed out in this work that special attention must be paid in analyzing TA results from samples with heterogeneous microstructures to avoid possible misinterpretation of carrier dynamics.

Junxue Liu†,‡ Jing Leng*,‡ Shiping Wang‡ Jun Zhang*,† Shengye Jin*,‡

99

DOI: 10.1021/acs.jpclett.8b03704 J. Phys. Chem. Lett. 2019, 10, 97−101

Viewpoint

The Journal of Physical Chemistry Letters †

■

(8) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (9) Xing, G. C.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X. F.; Sabba, D.; Gratzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476−480. (10) D’Innocenzo, V.; Srimath Kandada, A. R.; De Bastiani, M.; Gandini, M.; Petrozza, A. Tuning the Light Emission Properties by Band Gap Engineering in Hybrid Lead Halide Perovskite. J. Am. Chem. Soc. 2014, 136, 17730−17733. (11) Tian, Y.; Scheblykin, I. G. Artifacts in Absorption Measurements of Organometal Halide Perovskite Materials: What Are the Real Spectra? J. Phys. Chem. Lett. 2015, 6, 3466−3470. (12) deQuilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of Microstructure on Local Carrier Lifetime in Perovskite Solar Cells. Science 2015, 348, 683−686. (13) Price, M. B.; Butkus, J.; Jellicoe, T. C.; Sadhanala, A.; Briane, A.; Halpert, J. E.; Broch, K.; Hodgkiss, J. M.; Friend, R. H.; Deschler, F. Hot-Carrier Cooling and Photoinduced Refractive Index Changes in Organic-Inorganic Lead Halide Perovskites. Nat. Commun. 2015, 6, 8420. (14) Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Charge-Carrier Dynamics in Vapour-Deposited Films of the Organolead Halide Perovskite CH3NH3PbI3‑xClx. Energy Environ. Sci. 2014, 7, 2269−2275. (15) Herz, L. M. Charge-Carrier Dynamics in Organic-Inorganic Metal Halide Perovskites. Annu. Rev. Phys. Chem. 2016, 67, 65−89. (16) Zhu, H. M.; Yang, Y.; Wu, K. F.; Lian, T. Q. Charge Transfer Dynamics from Photoexcited Semiconductor Quantum Dots. Annu. Rev. Phys. Chem. 2016, 67, 259−281. (17) Chen, X.; Lu, H.; Yang, Y.; Beard, M. C. Excitonic Effects in Methylammonium Lead Halide Perovskites. J. Phys. Chem. Lett. 2018, 9, 2595−2603. (18) Sabbah, A. J.; Riffe, D. M. Femtosecond Pump-Probe Reflectivity Study of Silicon Carrier Dynamics. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 66, 165217. (19) Yang, Y.; Yan, Y.; Yang, M. J.; Choi, S.; Zhu, K.; Luther, J. M.; Beard, M. C. Low Surface Recombination Velocity in Solution-Grown CH3NH3PbBr3 Perovskite Single Crystal. Nat. Commun. 2015, 6, 7961. (20) Zhu, H.; Trinh, M. T.; Wang, J.; Fu, Y.; Joshi, P. P.; Miyata, K.; Jin, S.; Zhu, X. Y. Organic Cations Might Not Be Essential to the Remarkable Properties of Band Edge Carriers in Lead Halide Perovskites. Adv. Mater. 2017, 29, 1603072. (21) Sum, T. C.; Mathews, N.; Xing, G. C.; Lim, S. S.; Chong, W. K.; Giovanni, D.; Dewi, H. A. Spectral Features and Charge Dynamics of Lead Halide Perovskites: Origins and Interpretations. Acc. Chem. Res. 2016, 49, 294−302. (22) Yang, Y.; Yang, M.; Moore, D. T.; Yan, Y.; Miller, E. M.; Zhu, K.; Beard, M. C. Top and Bottom Surfaces Limit Carrier Lifetime in Lead Iodide Perovskite Films. Nat. Energy 2017, 2, 16207. (23) Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J. Morphological Control for High Performance, SolutionProcessed Planar Heterojunction Perovskite Solar Cells. Adv. Funct. Mater. 2014, 24, 151−157. (24) Sharenko, A.; Toney, M. F. Relationships between Lead Halide Perovskite Thin-Film Fabrication, Morphology, and Performance in Solar Cells. J. Am. Chem. Soc. 2016, 138, 463−470. (25) Zhang, W.; Saliba, M.; Moore, D. T.; Pathak, S. K.; Hörantner, M. T.; Stergiopoulos, T.; Stranks, S. D.; Eperon, G. E.; AlexanderWebber, J. A.; Abate, A.; et al. Ultrasmooth Organic−Inorganic Perovskite Thin-Film Formation and Crystallization for Efficient Planar Heterojunction Solar Cells. Nat. Commun. 2015, 6, 6142. (26) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.;

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering, China University of Petroleum, 66 Changjiang West Road, Huangdao District, Qingdao, China 266580 ‡ State Key Laboratory of Molecular Reaction Dynamics and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China 116023

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b03704.

■

Sample preparations, SEM images, TA measurements, theoretical derivation on TA and TR signals, and additional TA data of CdS suspensions PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.J.). *E-mail: [email protected] (J.L.). *E-mail: [email protected] (J.Z.). ORCID

Junxue Liu: 0000-0001-8349-1017 Shengye Jin: 0000-0003-2001-2212 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS S. Jin acknowledges financial support from the MOST (2018YFA0208704, 2016YFA0200602) and the National Nature Science Foundation of China (21725305). J. Leng acknowledges financial support from the National Natural Science Foundation of China (No. 21773237). J. Zhang and J. Liu acknowledge financial support from the National Natural Science Foundation of China (No. 21471160).

■

REFERENCES

(1) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (2) Peng, J.; Chen, Y.; Zheng, K.; Pullerits, T.; Liang, Z. Insights into Charge Carrier Dynamics in Organo-Metal Halide Perovskites: From Neat Films to Solar Cells. Chem. Soc. Rev. 2017, 46, 5714−5729. (3) Saparov, B.; Mitzi, D. B. Organic-Inorganic Perovskites: Structural Versatility for Functional Materials Design. Chem. Rev. 2016, 116, 4558−4596. (4) Snaith, H. J. Perovskites: The Emergence of a New Era for LowCost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623− 3630. (5) Tan, Z. K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; et al. Bright Light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (6) Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D. D.; Higler, R.; Huttner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; et al. High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421−1426. (7) Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. 100

DOI: 10.1021/acs.jpclett.8b03704 J. Phys. Chem. Lett. 2019, 10, 97−101

Viewpoint

The Journal of Physical Chemistry Letters et al. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains. Science 2015, 347, 522−525. (27) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Gratzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. (28) Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray-Weale, A.; Bach, U.; Cheng, Y.-B.; Spiccia, L. A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem., Int. Ed. 2014, 53, 9898−9903. (29) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476−480. (30) Huang, H.; Susha, A. S.; Kershaw, S. V.; Hung, T. F.; Rogach, A. L. Control of Emission Color of High Quantum Yield CH3NH3PbBr3 Perovskite Quantum Dots by Precipitation Temperature. Adv. Sci. 2015, 2, 1500194. (31) Dong, Q. F.; Fang, Y. J.; Shao, Y. C.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. S. Electron-Hole Diffusion Lengths > 175 Mu M in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (32) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M. J.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519−522. (33) Heo, J. H.; Song, D. H.; Im, S. H. Planar CH3NH3PbBr3 Hybrid Solar Cells with 10.4% Power Conversion Efficiency, Fabricated by Controlled Crystallization in the Spin-Coating Process. Adv. Mater. 2014, 26, 8179−8183. (34) Zhang, Z. Y.; Wang, H. Y.; Zhang, Y. X.; Hao, Y. W.; Sun, C.; Zhang, Y.; Gao, B. R.; Chen, Q. D.; Sun, H. B. The Role of TrapAssisted Recombination in Luminescent Properties of Organometal Halide CH3NH3PbBr3 Perovskite Films and Quantum Dots. Sci. Rep. 2016, 6, 27286. (35) Wu, K.; Liang, G.; Shang, Q.; Ren, Y.; Kong, D.; Lian, T. Ultrafast Interfacial Electron and Hole Transfer from Cspbbr3 Perovskite Quantum Dots. J. Am. Chem. Soc. 2015, 137, 12792− 12795. (36) Chemla, D.; Miller, D.; Smith, P.; Gossard, A.; Wiegmann, W. Room Temperature Excitonic Nonlinear Absorption and Refraction in Gaas/Algaas Multiple Quantum Well Structures. IEEE J. Quantum Electron. 1984, 20, 265−275.

101

DOI: 10.1021/acs.jpclett.8b03704 J. Phys. Chem. Lett. 2019, 10, 97−101