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Enhancing Optoelectronic Properties of Low-Dimensional Halide Perovskite via Ultrasonic-Assisted Template Refinement Dejian Yu, Chunyang Yin, Fei Cao, Ying Zhu, Jianping Ji, Bo Cai, Xuhai Liu, Xiaoyong Wang, and Haibo Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12048 • Publication Date (Web): 24 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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ACS Applied Materials & Interfaces

Enhancing Optoelectronic Properties of Low-Dimensional Halide Perovskite via Ultrasonic-Assisted Template Refinement

Dejian Yu,†,§Chunyang Yin,‡,§ Fei Cao,†,§ Ying Zhu,†,§ Jianping Ji,† Bo Cai,† Xuhai Liu,† Xiaoyong Wang,‡ Haibo Zeng†*

†

Institute of Optoelectronics & Nanomaterials, MIIT Key Laboratory of Advanced

Display Material and Devices, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

‡

National Laboratory of Solid State Microstructures, School of Physics, and

Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (H. Zeng)

KEYWORDS: 2D halide perovskite, ultrasonic-assisted treatment, dimensionality construction, low-dimensional CsPbBr3, optoelectronic properties

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ABSTRACT: Low-dimensional halide perovskite (HP) has triggered lots of research attention in recent few years due to anisotropic optoelectronic/semiconducting properties and enhanced stability. High-quality low-dimensional HPs via controllable engineering is required to fulfill their encouraging promise for device applications. Here we introduce, for the first time, post-synthetic ultrasonic-assisted refinement of two-dimensional homologous HPs (OA2PbBr4, OA is octadecylamine). The solution-prepared OA2PbBr4, either in the form of large-sized microcrystal or nanosheet, obtains significantly enhanced crystallinity after ultrasonic treatment. We further show that OA2PbBr4 crystals can be used as template to construct low-dimensional CsPbBr3 with the size and morphology inherited. Importantly, we found the ultrasonic-treated OA2PbBr4 crystals, compared with pristine ones, lead to enhanced optoelectronic properties of resultant low-dimensional CsPbBr3, as demonstrated by improved photodetection performances including prolonged charge carrier lifetime, improved photostability, significant increase of external quantum yield/responsivity and faster response speed. We believe this work provides novel engineering of low-dimensional HPs beyond the reach of straightforward synthesis.


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1. Introduction Halide perovskites (HPs) have emerged as the limelight in optoelectronic and photovoltaic fields in recent few years, and have shown great potentials in a broad range of important applications such as solar cells,1-3 light-emitting diodes4-13 and photodetectors.14-15 Especially, low-dimensional HPs have received soaring research interest due to anisotropic optoelectronic/semiconducting properties and enhanced stability. Usually straightforward synthesis strategies of low-dimensional HPs are adopted, however, the formation of HPs usually suffers from ultrafast reaction dynamics, which readily leads to poor crystallinity, high defect density and less-anticipated semiconducting properties. Notorious issues such as hysteresis behaviour in solar cells,16-18 blinking in light-emitting diodes (LEDs),19-25 phase separation26 and easy decomposition were reported to be closely related to the defects within HPs. High quality is the precondition of realizing the encouraging promise of low-dimensional HPs. Recently, interactions between environmental/factitious factors and HPs including, not limited to, light, electric field and pressure were intensively studied and are now gaining increasing attention due to positive and controllable engineering over HPs. For example, W. deQuilettes et al. have reported light-induced halide redistribution in HPs and consequent decrease of defects.27 Ummadisingu et al. found that light has a strong influence on the rate of HP formation and on film morphology.28 Zhao et al. found that electronic stress drives local motion of excessive halides to fill vacancies and interstitial defects and therefore reduces nonradiative 3 ACS Paragon Plus Environment


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recombinations.29 Nagaoka et al. reported pressure-induced phase transformations of CsPbBr3 at atomic and mesoscale levels,30 and Wang et al. demonstrated pressure-induced polymorphic, optical, and electronic transitions of formamidinium lead iodide HP.31 These interactions bring innovative modulations over HPs, and therefore hold great promise for HPs engineering beyond the reach of straightforward synthesis. Being inspired, in this work we report, for the first time, ultrasonic-assisted refinement of two-dimensional (2D) homologous HPs (OA2PbBr4, OA is octadecylamine). Solution-prepared OA2PbBr4, either in the form of large-sized microcrystal or nanosheet, obtained significantly enhanced crystallinity after ultrasonic treatment. We further show that OA2PbBr4 crystals can be used as templates to construct low-dimensional CsPbBr3 with size and morphology inherited. Significantly, we found that ultrasonic-treated OA2PbBr4 crystals, compared with pristine ones, lead to the remarkably enhanced optoelectronic properties of resultant low-dimensional CsPbBr3 as evidenced by the improved photodetection performances, including prolonged charge carrier lifetime, significantly improved photostability, remarkable increase of external quantum yield (EQE)/responsivity and faster response speed. We believe the ultrasonic-treated 2D HPs with high quality will serve as ideal prototype for on-going researches, and the template refinement strategy provides innovative modulation over low-dimensional HPs.


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2. Results and Discussion



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Figure 1. Refinement of OA2PbBr4 microcrystals via ultrasonic treatment. (a) Lattice structure of OA2PbBr4. (b) Steady-state PL and absorption spectra and (c) XRD images of OA2PbBr4 microcrystals. Inset: d) XRD profiles showing FWHMs of 6 ACS Paragon Plus Environment

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pristine and ultrasonic-treated OA2PbBr4 microcrystals. (e) SEM images of pristine and (f) ultrasonic-treated OA2PbBr4 microcrystals. The ultrasonic power is 400 W, and the treating time is 30 min. (g)-(h) Planar size distributions of pristine and ultrasonic-treated OA2PbBr4 microcrystals.

To show the effect of ultrasonic treatment, we prepared large-sized OA2PbBr4 microcrystals via cooling-induced nucleation and growth approach (Figure S1). Briefly, 2:1 molar ratio of CsBr and PbBr2 were dissolved in dimethyl sulfoxide (DMSO) as precursor. The precursor was then mixed with 80 oC OA/acetic acid solution to obtain clear mixture. The mixed solution was slowly cooled at the rate of 1 o

C/min under gentle stirring, and white OA2PbBr4 precipitates gradually appeared

during the process. The formation of OA2PbBr4 microcrystals obeys following formula: 2CsBr+2OA(Ac)+PbBr2→OA2PbBr4+2Cs(Ac)

(1)

where Ac represents acetate. After cooling down to room temperature, the precipitates were extracted and purified. For ultrasonic treatment, the reaction container, kept as it was, was directly transferred to ultrasonic machine after cooling down to room temperature. During the ultrasonic treatment, shear forces and cavitation, i.e. the growth and collapse of microsized-bubbles or voids in liquids due to pressure fluctuations, act on OA2PbBr4 microcrystals. Figure 1a inset presents the schematic illustration of lattice structure of 7 ACS Paragon Plus Environment


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OA2PbBr4, [PbX6] octahedra sheets are spaced and arranged vertically by OA+ bilayer.32 2D HPs are self-organized quantum wells, therefore offer the most extreme blue-shift of photoluminescence (PL) compared with 3D counterparts.33-34 The corresponding steady-state PL and absorption spectra are shown in Figure 1b. The PL maxima is at 396 nm, which is in consistence with previous report on OA2PbBr4.35 Corresponding X-ray diffraction (XRD) image shows periodic peaks of (00l) planes with 2.5 degree spacing (Figure 1c), according to which the interlayer spacing of OA2PbBr4 can be calculated to be 3.6 nm. The result agrees with the reported relationship d(Å)=8.06+1.59×n,36 where d is the interlayer distance and n represents the number of carbon atoms of long-chain alkyl ammonium. Figure 1d presents the comparison of XRD profiles of pristine and ultrasonic-treated OA2PbBr4, a narrowed full width at the half-maximun (FWHM) is observed in that of treated OA2PbBr4, which indicates its increased crystallinity. Corresponding scanning electron microscope (SEM) images of as-prepared OA2PbBr4 microcrystals are shown in Figure 1e. Even via such high temperature-assisted and slow growth, as-formed OA2PbBr4 microcrystals are poorly-defined in shape, suggesting a non-ideal crystallization process and poor crystallinity. However, after ultrasonic treatment, the shapeless OA2PbBr4 microcrystals evolved into well-defined rectangular shape as shown in Figure 1f. According to the self-limitation property of crystals, i.e., unit cells in crystal are periodically arranged therefore the crystals easily grow into sealed polyhedrons consistent with the lattice structure, the morphology refinement of OA2PbBr4 microcrystals implies an enhancement in crystallinity, which 8 ACS Paragon Plus Environment

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is in agreement with the XRD characterization. Atomic force microscope (AFM) image (Figure S2) reveals large thickness of OA2PbBr4 microcrystals to be several hundred nanometers, and the thickness distributions are calculated in Figure S3. The thickness distribution remained almost unchanged after the ultrasonic treatment, which implies no exfoliation-like effect. The planar size distributions of OA2PbBr4 microcrystals before and after ultrasonic treatment were presented in Figure 1g and Figure 1h, respectively, no obvious alternation is observed, either, which means the refinement effect is individual behaviour so that there is no consistent trend of size evolution in the ensemble. These results are totally different from previous reports of ultrasonic treatment on 3D HPs.37-40 Time-resolved PL spectra of pristine and ultrasonic-treated OA2PbBr4 microcrystals are compared and discussed in Figure S4 and Table S1.



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Figure 2. Mechanism of energy-driven refinement. (a) Schematic illustration of morphology evolution after ultrasonic treatment. (b) Schematic illustration of thermodynamic driving force and energy barrier of the process. (c) SEM images of the products by various ultrasonic treating powers. Scale bar: 30 µm.

The morphology refinement is surely caused by ion migration within OA2PbBr4 microcrystals. Ion migration is well-known in HPs, which usually takes place under external voltage bias.41-47 Since no voltage bias is applied in above solution preparation, the driving force needs to be reconfirmed. For crystals, a higher crystallinity is correlated to a more stable thermodynamic state. Comparing the pristine and ultrasonic-treated OA2PbBr4 microcrystals, it can be concluded that the 10 ACS Paragon Plus Environment

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ultrasonic-assisted refinement is actually an energy-driven process, and the effect of ultrasonic treatment is to provide enough power to overcome the barriers of ion migration (Figure 2a) so that OA2PbBr4 microcrystals can reach the lowest energy state as depicted in Figure 2b. To confirm the hypothesis, we executed various ultrasonic powers and compared the products as shown in Figure 2c. A power threshold was found to locate between 80 W and 120 W, not until the ultrasonic power reached the threshold was the refinement effect observed. It was reported that ion migration is suppressed in 2D HPs,48 therefore a higher energy input by ultrasonic treatment is required to meet the threshold, the threshold is correlated to the activation energy of ion migration within OA2PbBr4. It’s important to note that the preset ultrasonic power is not necessarily equal to that imposed on OA2PbBr4 microcrystals, and therefore it can be very difficult to thereby infer certain ion migration paths.

Figure 3. Refinement of OA2PbBr4 nanosheets via ultrasonic treatment. (a)-(d) SEM 11 ACS Paragon Plus Environment


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images of OA2PbBr4 nanosheets after being treated for various time. The ultrasonic treating power is 400 W. Figure d inset: magnified SEM image of individual OA2PbBr4 nanosheet, the scale bar is 2 µm.

In addition to the wide applications of itself,49-52 2D HP can also be used as template for constructing low-dimensional CsPbBr3,15 which are attracting more and more interest in optoelectronic applications recently.15,

53-54

We further prepared

OA2PbBr4 nanosheets as template. The 2:1 CsBr:PbBr2 precursor was directly injected into OA/acetic acid solution at room temperature under vigorous stirring, the reaction dynamics is much faster for OA2PbBr4 nanosheets that the solution immediately turned white turbid when the precursor was added. As-formed OA2PbBr4 nanosheets were shapeless as shown in Figure 3a. Stark contrast can be observed between the margin and the interior under electron beam, which implies poor crystallinity especially at the margin. Then various ultrasonic treating time was executed, and the evolution of products was recorded as shown in Figure 3b to Figure 3d, the OA2PbBr4 nanosheets became more and more regular under prolonged ultrasonic treatment as expected. After 30 min, shapeless OA2PbBr4 nanosheets totally evolved into well-defined rectangles, suggesting significantly enhanced crystallinity.


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Figure 4. Resultant low-dimensional CsPbBr3 by pristine and ultrasonic-treated OA2PbBr4 nanosheets. (a) Schematic illustration of dimensionality construction mechanism.

(b)-(c)

SEM

images and

(d)-(e)

TEM

images

of

resultant

low-dimensional CsPbBr3 constructed with pristine and ultrasonic-treated OA2PbBr4 nanosheets, respectively. 13 ACS Paragon Plus Environment


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The dimensionality construction mechanism is illustrated in Figure 4a. Specifically, because OA2PbBr4 sheets are vertically arranged by weak Van der Waals force, selected cation such as Cs+ easily permeates into OA2PbBr4,15, 55 and exchanges with OA+. Different from OA+, Cs+ is correlated to a 3D HP crystallographic structure,56-57 therefore it assembles [PbBr6] octahedral layers to form single crystalline dimensionality-increased CsPbBr3. After mixing the precursors and subsequent ultrasonic treatment of OA2PbBr4 nanosheets, Cs+ was then forced to precipitate by adding poor solvent such as toluene, therefore triggering the cation exchange reaction. Low-dimensional CsPbBr3 fabricated with pristine and ultrasonic-treated OA2PbBr4 are denoted as CsPbBr3-p and CsPbBr3-u in the following for simplicity. The corresponding spectral characterizations and XRD image are shown in Figure S5 and Figure S6. The SEM images are presented in Figure 4b and Figure 4c. Obviously, the morphology as well as the crystallinity of parental OA2PbBr4 templates directly determines those of resultant CsPbBr3, CsPbBr3-u possesses well-defined rectangular shape, while CsPbBr3-p is shapeless the same as parental pristine OA2PbBr4 nanosheets. Close-up is presented via transmission electron microscope (TEM) characterizations (Figure 4d and Figure 4e), from the comparison of the edges the improved crystallinity in CsPbBr3-u is powerfully confirmed.


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Figure 5. Enhancement of optoelectronic properties in CsPbBr3-u. (a) Colloidal dispersions of CsPbBr3-u and CsPbBr3-p under UV exposure. (b) Cross section SEM image of drop-casted film and schematic illustration of the PD configuration. Scale bar: 30 µm. (c) Time-resolved PL spectra of CsPbBr3-u and CsPbBr3-p films. (d) I-V curves of PDs-p and PDs-u. The illumination intensity is 7.85 mW @442 nm. (e) I-t on/off switching curves of PDs-p and PDs-u, the illumination intensity is 0.856 mW/cm2@442 nm. (f) EQE and (g) responsivity of PDs-p and PDs-u as functions of wavelength. (h) Photostability comparison of PDs-p and PDs-u. The illumination intensity is 1.5 mW@442 nm.

Interestingly, we found that ultrasonic-treated OA2PbBr4, compared with pristine counterpart, leads to enhanced optoelectronic properties of subsequent 15 ACS Paragon Plus Environment


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CsPbBr3. UV-excited colloidal dispersions of CsPbBr3-u and CsPbBr3-p are shown in Figure 5a, brighter emission can be observed in the former, indicating suppressed nonradiative recombination in CsPbBr3-u. To further investigate the superiorities of CsPbBr3-u in the form of planar PDs (denoted as PDs-u and PDs-p corresponding to CsPbBr3-u and CsPbBr3-p in the following), CsPbBr3-u and CsPbBr3-p were drop-casted onto commercially available interdigital electrodes (20 µm interdigital spacing, 1.2 mm2 active area) as illustrated in Figure 5b. Thanks to the 2D morphology and large traverse size, compact and flat film could be obtained as shown by the cross-section SEM image. Time-resolved PL lifetimes of CsPbBr3-u and CsPbBr3-p films were compared in Figure 5c. The former owns a long PL lifetime of 48 ns, while the value is much shorter in the latter (11 ns). Besides, the PL decay of CsPbBr3-u can be monoexponentially-fitted, which implies suppressed defects in CsPbBr3-u. In stark contrast, PL intensity of CsPbBr3-p film shows two-channel decay, the faster recombination (τ=6.5 ns) is believed to be caused by defects. I-V curves under both darkness and illumination are presented in Figure 5d, about one order of magnitude increase of photocurrent was obtained in PDs-u, and ultrahigh on/off ratio up to ~106 can be extracted for PDs-u under illumination intensity of 7.85 mW@442 nm. I-t on/off switching curves are presented in Figure 5e. Consistently, the photocurrent of PDs-u is approximately 6 times as large as that of PDs-p. Wavelength-dependent EQE and responsivity are shown in Figure 5f and Figure 5g, the values are higher for PDs-u over all the spectra region, of which the champion values were 5.3 and 4.5 times increased compared with those of PDs-p, 16 ACS Paragon Plus Environment

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reaching up to 7280% and 25 A/W, respectively. The results confirm that CsPbBr3-u, derived from ultrasonic-treated OA2PbBr4, possesses enhanced optoelectronic conversion capability compared with CsPbBr3-p. We further tested the photostability of PDs-p and PDs-u. The photocurrents under prolonged illumination were recorded as shown in Figure 5h. Current of PDs-p experienced rapid drop in the first 5 min and continuously decreased under further illumination. Only 75% initial current was remained after 27 mins’ test. What’s worse, obvious photolysis was observed by optical microscopy as shown in Figure S7, which is probably caused by the formation of Pb according to previous report.58 In contrast, PDs-u were quite robust upon continuous illumination, 93% initial current was preserved after the test and no such photolysis was observed.

Figure 6. Rise/decay time of (a) PDs-p (2500 Hz on/off switching frequency) and (b) PDs-u (10000 Hz on/off switching frequency).

PDs-u also offer faster photoresponse speed compared with PDs-p, the on/off switching frequency of the light source (442 nm laser) was gradually elevated to approach the response limit of the PDs. Corresponding I-t periods are shown in Figure 17 ACS Paragon Plus Environment


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6a. Under 2500 Hz switching frequency the rise and recovery time, defined as time needed to rise from 10% to 90% peak current and reverse, were measured to be 45/173 µs. However, a higher switching frequency leads to triangle-like I-t curve, which means PDs-p were not able to follow that fast on/off switching. For PDs-u, the critical frequency can be promoted up to 10000 Hz, corresponding I-t period is shown in Figure 6b. Much shorter rise/decay time of 5.8/45 µs were obtained. The faster response speed is ascribed to the enhanced crystallinity of CsPbBr3-u that contributes to more efficient charge carrier transport and extraction. Compared with PDs in the literature, we found some figure-of-merits of PDs-u here, such as on/off ratio and response speed, rank top level as summarized in Table S2. It’s noteworthy that the ultrasonic treatment doesn’t refine the low-dimensional CsPbBr3 directly. The same ultrasonic treatment was applied to the resultant low-dimensional CsPbBr3, and severe fragmentation was observed rather than refinement effect as shown in Figure S8. In this regard, the template refinement strategy here provides novel modulation over low-dimensional HPs beyond the reach of commonly adopted straightforward synthesis methods.

3. Conclusion In conclusion, for the first time we have reported, ultrasonic-assisted post-synthetic refinement of 2D homologous HPs, which were found to be able to function as templates to construct low-dimensional CsPbBr3. Solution-prepared OA2PbBr4, either in the form of microcrystal or nanosheet, becomes highly crystalline 18 ACS Paragon Plus Environment

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after ultrasonic treatment, therefore contributes to enhanced optoelectronic properties of constructed low-dimensional CsPbBr3. We believe this work provides innovative guidance for HPs engineering for on-going researches.

AUTHOR INFORMATION E-mail: [email protected] (H. Zeng)

Author contribution §D. Yu, C. Yin, F. Cao and Y. Zhu contributed equally to this work.

Notes The authors declare no competing financial interest.

Acknowledgement This work was financially supported by the National Key Basic Research Program of China (2014CB931702), NSFC (51572128, 21403109, 51502139), NSFC-RGC (5151101197), the National Key Research and Development Program of China (2016YFB0401701), the Fundamental Research Funds for the Central Universities (No. 30915012205, 30916015106), Natural Science Foundation for Youths of Jiangsu Province of China (BK20140787) and PAPD of Jiangsu Higher Education Institutions, and in USA by DoD (Grant W911NF-15-1-0650).



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Supporting information Schematic illustration of OA2PbBr4 microcrystals preparation procedures, AFM images of pristine and ultrasonic-treated OA2PbBr4 microcrystals, thickness distributions of pristine and treated OA2PbBr4 microcrystals, time-resolved PL spectra of pristine and ultrasonic-treated OA2PbBr4 microcrystals, PL lifetimes of pristine and ultrasonic-treated OA2PbBr4 microcrystals, steady-state PL and absorption spectra of CsPbBr3, XRD images of constructed CsPbBr3 nanosheets, radiolysis of CsPbBr3 nanosheets, SEM image of ultrasonic-treated CsPbBr3 nanosheets, summary of PD performances in the literature.

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