Effects of Organic Cation Length on Exciton Recombination in Two

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Letter

Effects of Organic Cation Length on Exciton Recombination in Two-Dimensional Layered Lead Iodide Hybrid Perovskite Crystals Lu Gan, Jing Li, Zhishan Fang, Haiping He, and Zhizhen Ye J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02083 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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The Journal of Physical Chemistry Letters

Effects of Organic Cation Length on Exciton Recombination in Two-Dimensional Layered Lead Iodide Hybrid Perovskite Crystals Lu Gan, Jing Li, Zhishan Fang, Haiping He,* Zhizhen Ye* State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (H. P. He); [email protected] (Z. Z. Ye).

ACS Paragon Plus Environment

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ABSTRACT. In recent years, two-dimensional (2D) layered organic-inorganic lead halide perovskites have attracted considerable attention due to the distinctive quantum confinement effects as well as prominent excitonic luminescence. Herein, we show that the recombination dynamics and photoluminescence (PL) of the 2D layered perovskites can be tuned by the organic cation length. 2D lead iodide perovskite crystals with increased length of the organic chains reveal blue-shifted PL as well as enhanced relative internal quantum efficiency. Furthermore, we provide experimental evidence that the formation of face-sharing [PbI6]4- octahedron in perovskites with long alkyls induces additional confinement for the excitons, leading to 1D-like recombination. As a result, the PL spectra show enhanced inhomogeneous broadening at low temperature. Our work provides physical understanding of the role of organic cation in the optical properties of 2D layered perovskites, and would benefit the improvement of luminescence efficiency of such materials.

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Over the past decade, three-dimensional (3D) lead halide hybrid perovskites (e.g., CH3NH3PbI3) has emerged as one of the most promising absorber candidates for solar energy harvesting1, 2 because of the impressive power conversion efficiency (PCE) up to 22.1%3. Owing to their outstanding optical properties, other optoelectronic applications of these materials have also been reported, such as light emitting diodes4, lasers5, and photodetectors6. The rapid progress in 3D perovskites and the methodology developed in preparing high-performance optoelectronic and photonic devices has led to extended research focus on their two-dimensional (2D) counterparts due to the significantly improved environmental stability as well as prominent exciton recombination. For instance, the 2D structure was first introduced into perovskite solar cells (a quasi-2D structure) in Tsai’s work7, and a photovoltaic efficiency of 12.52% as well as greatly improved stability was achieved. As for light-emitting devices, LED based on selforganized quasi-2D multiple quantum wells (MQWs) exhibits very high EQE of 11.7% with good stability8. Unfortunately, for pure 2D layered perovskites, PL efficiency at room temperature is rather low, possibly due to the strong exciton-phonon coupling9. Therefore, it is of great significance and urgency to improve the PL efficiency for high-performance optoelectronic devices based on 2D perovskites. In this regard, Yang’s group have reported the solution growth of single- and few-unit-cell-thick (C4H9NH3)2PbBr4 nanosheets with PLQY~26%10. However, relevant studies on PL efficiency enhancement as well as the mechanism is still rare, and demands further investigation. Generally, the 2D layered perovskites can be regarded as quantum wells (QWs). In such structures, carriers are confined in the inorganic part because the organic layer has wider bandgap and lower dielectric constant11-13. These features lead to the existence of excitons with large binding energy (typically a few hundred meV)14-16, and induce pronounced excitonic


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effects such as photo- and electro-luminescence17, 18, large third-order optical nonlinearity19, and bi-exciton lasing20. Obviously, the exciton recombination plays a key role in the PL efficiency. In semiconductors, it is well known that photo-generated carriers/excitons decay by recombination, usually both radiatively and nonradiatively. Radiative recombination means that electrons and holes recombine and emit photons, while in the nonradiative case, the carriers lose energy through phonons. Therefore, it is critical to study the recombination dynamics of excitons in 2D layered perovskites. What’s more, the size of the organic cation plays critical roles in altering the inorganic framework (the [PbX6]4- octahedron) and determining the band gap21-23. Coincidently, enhancing the length of the organic ligands can cause structural reorganization, which gives rise to an effectively one-dimensional (1D) quantum confinement behavior. For instance, Kamminga et al24 synthesized phenylalkylammonium lead iodide 2D perovskite single crystals. They found that the fabricated 2D perovskites with short organic cations consist of inorganic layers of corner-sharing [PbI6]4- octahedron separated by organic cations, while for those with long organic cations the inorganic layers contain both corner- and face-sharing [PbI6]4- octahedron. According to their calculations, an additional confinement effect is induced by the presence of face-sharing [PbI6]4- octahedron to form an effective 1D structure, which strongly affects the band gap. However, to our knowledge, such 1D confinement effect is still lacking direct experimental evidence, and the luminescence origin and excitonic performance of 2D perovskite QWs is also demanding further investigation. In this letter, 2D organic-inorganic hybrid perovskite crystals with diverse length of organic ligands were synthesized through a layered solution technique first reported by Mitzi25. Three types

of

phenylalkylammonium

cations

were

utilized:

benzylammonium,

2-


4

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phenethylammonium, and 4-phenyl-1-butylammonium, i.e., C6H5(CH2)nNH3+ with n = 1, 2, and 4. These organic ligands are abbreviated as PMA, PEA, and PBA, and their corresponding perovskite crystals are abbreviated as PMPI, PEPI, and PBPI, respectively. The simple layered structure of PMPI and PEPI crystals as well as co-existence of corner-sharing and face-sharing [PbI6]4- octahedron in PBPI was characterized by XRD measurements. Both temperature- and excitation density-dependent photoluminescence (PL) analysis were carried out to examine the excitonic behavior and luminescent nature. Besides, temperature dependence of exciton radiative lifetime was calculated to probe the quantum confinement effect induced by the structural reorganization. Structural illustration of the as-synthesized 2D PMPI, PEPI, and PBPI crystals are displayed in Figure 1a. PMPI and PEPI crystals adopt the widely reported (PMA)2PbI426-28 and (PEA)2PbI42931

structures, which consist of alternating organic cations (C6H5CH2NH3+ and C6H5C2H4NH3+,

respectively) and only corner-sharing [PbI6]4- octahedron inorganic layers. Since all of these ligands are long-chain organic cations, pure 2D structures with only a single layer of [PbI6]4octahedrons are formed (see Supplementary Note 1 for details). Their powder X-ray diffraction (PXRD) patterns exhibit a series of intense sharp (0 0 2l) (l=1-6) diffraction peaks (Figure 1b), indicating the perfect periodicity of the stacking along the direction as well as high crystallinity. In contrast, PXRD pattern of PBPI crystals presents a secondary phase (typical peak labeled with “★”) besides the main hybrid phase (typical peak labeled with “◆”), consistent with Kamminga’s study. Revealed by the XRD results, the PBPI crystal possesses a disparate structure as the inorganic sheets combine corner-sharing with face-sharing [PbI6]4- octahedron (shown in Figure 1a as well). On the other side, all the 2D crystals present clear contours as well as layered structure, and the size of the 2D perovskite crystals is over several tens of micrometers


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(Figure S1 in the supporting information). No obvious difference is observed in the X-ray photoelectron microscopy (XPS) spectra (Figure 1c). Triumphant preparation of all 2D perovskite crystals is confirmed by the N 1s peak at 401.7 eV, corresponding to the ammonium salts32. Coincidentally, the spectra of Pb and I elements also exhibit good agreement with several literatures on 2D organic-inorganic hybrid perovskites33, 34. Additionally, it is worth to pointing out that Pb clusters exist in all perovskite samples as evidenced by the peaks around 141.3 and 136.4 eV for metallic Pb.

Figure 1. Schematic structure of PMPI, PEPI, and PBPI (a) with their corresponding structural characterizations: powder XRD pattern (b) and XPS core-level spectra of N1s, I3d, and Pb4f (c). For XPS spectra, C 1s peak at 284.6 eV is used as the reference. As mentioned above, band gap of the 2D perovskites varies with the enhanced organic cation length. Such variation is reflected by the blue-shift in the room-temperature PL and absorption spectra (Figure 2a and Figure S2), in which the major emission peak (labelled as M1, E1, and B1) shifts visually from 2.34 eV to 2.37 eV, and finally 2.40 eV as systematically lengthened the


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organic ligand from PMA to PBA. Similar trend represents in the whole temperature range (Figure 2c), and is confirmed by repetitive measurements (inset of Figure 2a). This significant blue-shift also agrees well with the slight loss of perovskite crystal color: orange for PMPI, yellow-orange for PEPI, and yellow for PBPI, respectively (Figure S1a). In addition, the spectra of PMPI and PEPI show a sideband on the low energy side (Figures S3 and S5), which likely originate from self-trapped excitons35 (see Supplementary Note 2 and Figure S8).

Figure 2. PL characterization of PMPI, PEPI, and PBPI crystals under 405 nm excitation. (a) Room-temperature steady state PL emission spectra. The inset presents energies of M1, E1, and B1 peak for multiple crystals, in which the compound 1, 2, and 4 refers to PMPI, PEPI, and PBPI, respectively. (b) Integrated PL intensity as a function of excitation density at 15 K. The data are plotted in logarithm scale, and show the power-law dependence with k = 1.01, 1.00, and


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0.99 for peak M1, E1, and B1, respectively. (c) Temperature dependence of peak position of peak M1, E1, and B1. (d) Temperature dependence of FWHM for peak M1, E1, and B1, respectively. Generally, 2D perovskites exhibit attractive excitonic properties14-16 due to the strong dielectric and quantum confinement effects. In our case, the excitonic nature of the major emissions is demonstrated by their excellent power-law dependence of k ≈ 1.0 (Figure 2b) as well as the little changed peak position within the whole regime of excitation power densities (Figure S4, and discussed in detail in Supplementary Note 3). It is worth mentioning that for 2D perovskites with large exciton binding energy (~170 meV) and prominent exciton effects, one would expect generation of both excitons and free charges upon above-gap photo-excitation, whose concentrations are described by Saha equation36, 37. For the subsequent recombination, however, the excitation power dependent PL results (Figure 2b and Figure S4) show slopes very close to 1, in excellent consistence with that predicted for excitons. According to the well-established theory38, recombination of free charges would have a slope close to 2. The Lorentz shape of the PL is also a signature of exciton recombination (Figure S3). The results strongly suggest that the contribution of free charge recombination to the PL can be neglected. This may be owing to the high oscillator strength of quantum confined excitons and slow bimolecular recombination rate of perovskites39. On this basis, the excitonic identities of peak M1, E1, and B1 are further identified by temperature-dependent PL (Figure S5). We note that there is no structural phase transition within the temperature range of PL measurements, because no abrupt shift in the optical gap are observed.14 In Figure 2c, peak position of emission M1 and E1 remains almost constant with only a slight unconventional blue-shift as the temperature goes up. While as for peak B1, the emission keeps


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this trend within the whole temperature range except for a mild red-shift when the temperature exceeds 240 K. We note that the above blue-shifting behavior is attributed to the negative Varshni parameter40, whereas the slight red-shift at high temperatures of peak B1 is possibly caused by the electron-phonon coupling effect raised by self-trapped excitons. Among the three emissions, peak E1 is the most widely studied and generally established to origin from free exciton recombination14, 41. When it refers to our results, both the excitonic luminescent nature and the almost unchanged peak energy provide solid evidence for free exciton emission, consistent with those reported references. Taking the possession of both features into consideration, peak M1 and B1 are ascribed to result from free exciton recombination as well. Besides, full width at half-maximum (FWHM) of the three emissions at different temperatures are obtained by spectral fitting (Figure S3) and plotted in Figure 2d. Despite the almost monotonously increasing trend for peaks M1 and E1, FWHM of peak B1 owns a decrease tendency between 15 and 80 K, while broadens again above 80 K. These features are regarded as typical signature of localized excitons42. At the beginning, excitons are randomly trapped in the localization states at low temperatures. Then, they are thermally activated and become mobile within the band of localized states as the temperature increases. Such thermal activation leads to the distinct narrowing of the linewidth. Finally, at even higher temperatures, the excitons are forced to populate higher-energy states by thermal distribution, and thus the line width broadens again43. What’s more, enhanced length of organic chains achieves higher flexibility, which may induce some structural disorders. Obviously, peak B1 possesses higher FWHM than peak M1and E1, possibly results from the structural disorder. The emission quenching behavior is reflected by its temperature dependence of integral PL intensity. As clearly described in Figure 3, peak B1 exhibits different quenching behavior with


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peaks M1 and E1 due to the negative thermal quenching (NTQ) effect, thoroughly discussed in the supporting information. On this account, peaks M1 and E1 follow a typical thermal quenching process, and the integrated PL intensity can be expressed as

I=

I0 m

1 + ∑i=1 ai exp(− Ei / k BT )

(1)

where I0 is the peak intensity at T = 0 K, kB is the Boltzmann constant, T is the temperature, Ei represents the nonradiative activation energies, and ai describes a relative contribution of the different activation processes, which can be expressed as44: a = τ r / τ 0 ,where τr is the radiative lifetime, and τ0 stands for the nonradiative lifetime at 0 K. The best fit of peak M1 yields two ai and Ei values: a1 = 399.5, E1 = 93.1 meV, and a2 = 0.1, E2 = 7.0 meV, while peak E1 achieves one a1 of 12.4, and one E1 of 53.5 meV. We note that the quite small activation energy of 7.0 meV (E2) for PMPI crystals at low temperature may not represent a genuine thermal activation energy, but just result from the temperature-dependent capture cross section of the carriers at the recombination centers45. On the other side, the integrated intensity of peak B1 is fitted with an NTQ model46:

( 1 + ∑ a exp(− E w

I = I0

) /k T)

1 + ∑q=1 bq exp − Eq' / k BT m j

j

j

(2)

B

where the Eq’ stands for the activation energies for processes increasing the PL intensity, whereas the bq describes the corresponding relative contribution of the different nonradiative channels. The best-refinement gives one Ej and one Eq’ value: a1 = 6.5, E1 = 29.0 meV, and b1 = 2.2, E1’ = 13.0 meV. All the obtained activation energies (E1) are much smaller than the exciton binding energy, which is generically above 200 meV for 2D layered organic-inorganic perovskites47. Similar phenomenon appears in Gauthron’s work16, in which they performed parallel


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temperature-dependent PL measurements on several PEPI samples prepared at different days, the corresponding nonradiative activation energy was found to vary between 30 and 80 meV depending on the samples. The different E values suggest different nonradiative channels in each sample, for examples, carrier recombination on impurities in the barrier layer, or at the interface between two domains of the inhomogeneous sample16.

Figure 3. Relative IQE at different temperatures for peak M1, E1, and B1, respectively. Additionally, relative internal quantum efficiency (IQE) can also be estimated from the temperature dependence of integral PL intensity. If the nonradiative channel is assumed to be inactive at low temperature (i.e., η = 1), the relative IQE can be obtained by48

η (T ) =

I (T ) I0

(3)

where I is the integrated PL intensity, and I0 is taken as the integral PL intensity at 15 K for PMPI and PEPI, while 80 K for PBPI due to the NTQ effect. We note that the method have frequently been used in studies of recombination dynamics in semiconductors, and is reasonable in the present case because at low temperature the trap density in our samples is quite low (Figure S6). In this regard, Figure 3 also displays the relative IQE of peak M1, E1, and B1 at


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various temperatures. Due to the thermal activation of nonradiative recombination channels, the relative IQE gradually decreases with increasing temperature, and is calculated to be 6.0%, 29.7%, and 62.2% for peak M1, E1, and B1 at room temperature. Furthermore, perovskite crystals with longer organic cations achieve higher relative IQE at all temperatures. Such enhanced relative IQE is due to the increased radiative recombination rate, as evidenced by the decreased a1 values from PMPI to PBPI. What’s more, the weaker self-trapping possibly caused by lattice distortions in the PBPI crystals may also benefit the relative IQE (discussed in detail in Supplementary Note 2). In Kamminga’s study24, theoretical calculations show quantum confinement in one and two dimension for the shorter and longer alkyls, respectively. In other words, although generally 2D exciton governs in the self-assembled QW structure, the formation of face-sharing [PbI6]4octahedron can cause another 1D confinement, leading to significant changes in the band structures. On the other side, the spatial confinement directly determines the temperature dependence of the exciton radiative lifetime. For zero dimensional (0D), 1D, and 2D excitons, the radiative lifetime shows independence, square root dependence, and linear dependence on temperature, respecitively49. For PBPI crystals, the localized excitonic nature is revealed by the temperature dependence of FWHM. In this regard, the additional 1D confinement induced by the face-sharing [PbI6]4- octahedron is expected for a square root temperature dependence of their exciton radiative recombination lifetime. Hence, the PL lifetime measurement was performed to probe the 1D confinement. Low temperature time-resolved PL spectra of emission M1, E1, and B1 are displayed in Figure S7a, in which all the three emissions, especially peak M1, exhibit fast PL decay. For peak M1, PL lifetime above 160 K is unavailable due to the instrument detection limit (Figure S7b). On


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this basis, the exciton radiative lifetime (τr) was calculated from the temperature-dependent timeresolved PL data (see Supplementary Note 5 for details), and plotted in Figure 4 by green dots.

Figure 4. Temperature dependence of exciton recombination lifetime (black dots), nonradiative lifetime (red dots), and radiative recombination lifetime (green dots) of peak M1 (a), E1 (b), and B1 (c), respectively. The inset in (c) provides fitting of the radiative recombination time data between 120 and 300 K. As clearly illustrated in Figures 4a and b, τr of peak M1 and E1 exhibit almost no temperature dependence at all, corresponding to the 0D case in Rosales’s study49. Thus, both peak M1 and E1 are regarded as recombination of 0D-like excitons, possibly localized around the Pb clusters as suggested by XPS results (Figure 1c). At high temperatures (>180 K), τr decreases slightly, which is likely due to the increase of dielectric constant. On the contrary, the temperature dependence of τr for peak B1 is much more complicated (Figure 4c). From 15 K to 100 K, its τr follows T0 trend very well, very similar to peaks M1 and E1. However, when the temperature exceeds 100 K, it donates an observable increase tendency. Revealed by the inset of Figure 4c, the τr data matches well with T0.5, implying a typical feature of 1D exciton. Note that the absorption peak of PBPI is ~2.66 eV (Figure S2), showing a large Stokes shift (inset in Figure 2a), which is consistent with the localization nature of the PL emission. Therefore, peak B1 is


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considered as caused by QD-like excitons at low temperatures, while affected by 1D localized exciton recombination at high temperatures, resulting from the face-sharing [PbI6]4- octahedron. In this context, the effect of 1D confinement induced by face-sharing [PbI6]4- octahedron in PBPI crystals is verified. In summary, comprehensive PL studies on 2D organic-inorganic hybrid perovskite crystals with diverse length of organic cations are carried out. The synthesized perovskite crystals yield 2D structures containing only corner-sharing [PbI6]4- octahedron with short alkyls, while both corner- and face-sharing [PbI6]4- octahedron with long alkyls. A blue-shift in PL as well as significantly enhanced IQE is induced by the length increase of the phenylalkylammonium chains from PMA to PBA induces. Analogously, all the three types of 2D perovskites share the luminescence nature of QD-like free excitons in their major emissions. Moreover, 1D-like exciton recombination takes place in PBPI crystals, resulting from the structural rearrangement with enhanced length of organic ligands. This result provides the first experimental evidence for the additional confinement of excitons induced by the face-sharing [PbI6]4- octahedron in perovskite crystals, which is previously predicted by theoretical calculations24. Our systematical PL study offers a new insight into the excitonic properties of this series of hybrid organicinorganic semiconductor. To the best of our knowledge, this is the first time that an important factor, i.e., organic cation, that impacts the recombination dynamics of 2D perovskites is discovered and elucidated. In addition, 2D perovskite crystals with long organic cations (PBPI) exhibit higher relative IQE, which may provide a new and efficient approach to tune the luminescence properties of 2D perovskites. Our work may shed light upon the luminescence mechanism and device application for organic-inorganic hybrid perovskites.


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Experimental Methods Materials All chemicals were purchased from Sigma-Aldrich of analytical grade and used as received without further purification. Crystal growth The 2D crystals were synthesized at room temperature by using a long glass tube. First, 1.0 mmol (0.461 g) PbI2 (Aldrich; 99.999%) was weighed and added to the tube under inert atmosphere (an argon-filled glove box, with oxygen and water levels maintained below 1 ppm). Then, 7.5 ml concentrated (45 wt%) aqueous HI (Aldrich; 99.999%) was injected with a syringe to dissolve the PbI2. Next, 15 ml methanol (Aldrich; anhydrous, 99.8%) was syringed onto the HI/PbI2 solution, and thus a relatively sharp interface was created between the solvent layers. Finally, a few drops (2 mmol) of phenylalkylamine (benzylamine for PMPI, 2-phenethylamine for PEPI, and 4-phenyl-1-butylamine for PBPI) was carefully syringed on the top of the solution. After standing for over at least one week, orange or yellow crystals appear at the solution interface. The crystals were filtrated and dried in vacuum at 55 oC. Characterization The structure of the crystals was analyzed by powder XRD using an X’pert PRO diffractometer (PANalytical) with Cu Kα radiation (λ=0.15406 nm). XPS measurements were carried out on an ESCALAB_250Xi spectrometer using Al Kα radiation (1486.6 eV) as the excitation source. Scanning electron microscope (SEM) images were taken by using an FEI Quanta 200FEG instrument. Ultraviolet-visible near-infrared absorption (UV) measurements of the perovskites were performed at room temperature in a UV3600 UV-vis-NIR spectrophotometer (Shimadzu, Kyoto, Japan). The PL measurements were conducted on a FLS


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920 fluorescence spectrometer (Edinburgh Instrument), and a 405 nm laser with a wavelength of 404.2 nm and pulse width of 58.6 ps was used as the excitation source. The excitation fluence was ~4 nJ cm-2, corresponding to photocarrier density of ~1014 cm-3. For measurements with various excitation density, the laser operated at 20 MHz and a neutral attenuator was used to tune the light power. PL lifetime was measured by time-correlated single-photon counting technique, using the same laser operating at 200 kHz. The PL lifetime is obtained by deconvolution exponential fit considering the instrumental response function (IRF). For temperature-dependent measurements, the samples were placed in a closed-cycle helium cryostat.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China under Grant No. 91333203, and Natural Science Foundation of Zhejiang Province under Grant No. LY17A040008. ASSOCIATED CONTENT Supporting Information. Discussion on excitonic emission determination, NTQ behavior in PBPI, calculation of exciton radiative recombination lifetime, self-trapping bands in PMPI and PEPI crystals, and extended data of Figure S1-S8. AUTHOR INFORMATION Notes The authors declare no competing financial interests.


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