Red-shifted Photoluminescence from Crystal Edges Due to Carrier

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C: Physical Processes in Nanomaterials and Nanostructures

Red-shifted Photoluminescence from Crystal Edges Due to Carrier Redistribution and Re-absorption in Lead Triiodide Perovskite Daocheng Hong, Jun Li, Sushu Wan, Ivan G. Scheblykin, and Yuxi Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03647 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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The Journal of Physical Chemistry

Red-shifted Photoluminescence from Crystal Edges Due to Carrier Redistribution and Re-absorption in Lead Triiodide Perovskite Daocheng Hong1, Jun Li2, Sushu Wan1, Ivan G. Scheblykin2,*, Yuxi Tian1,* 1Key

Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing, Jiangsu, 210023, China 2Chemical

Physics and NanoLund, Lund University, PO Box 124, 22100, Lund, Sweden

Corresponding Author *Email: [email protected] *Email: [email protected]

ACS Paragon Plus Environment

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Abstract Photoluminescence of CH3NH3PbI3 perovskite depends strongly on sample preparation, atmosphere, crystal size, etc. However, the origin of these dependencies is always misunderstood due to the co-works of many different factors. Herein, we prepared hexagonal-shaped single crystals with tens of micrometers in size and observed a red-shifted PL emission (800 – 830 nm) mainly from the crystal edges besides the usual band-to-band transition (760 nm) from the central regions. Also, significantly different time-resolved dynamics and excitation power dependencies were observed. To summary, we conclude that the red-shifted component comes from the depth of the crystal where monomolecular recombination occurs involving photogenerated charges and unintentional doped charges, while the normal PL is emitted by bimolecular recombination from the surface layers. These results showed the significance of pure optical effects in perovskite crystals and would promote detailed understanding of the charge dynamics and recombination in perovskite crystalline materials of different geometries and sizes.


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Introduction Organometal halide perovskites (OMHPs) attracted lots of attention as very promising materials for photovoltaics and optoelectronics.1–4 Over the last several years, the fundamental electronic processes in these semiconductors have also been investigated with a great variety of methods.5–15 Due to the direct connection between PL and recombination of photo-generated charges as well as the convenience of detecting, the PL spectroscopy and imaging have been widely applied to OMHPs to investigate the photophysical properties and processes in these materials.8–10,16–19 Based on the band gap of CH3NH3PbI3 perovskite to be about 1.6 eV,20,21 the observed normal PL has energy slightly lower than the bandgap corresponding to the wavelength of about 760 - 770 nm.22,23 However, the PL spectra of these materials often show extra bands in addition to the normal emission. Coexistence of different phases, structure transformation, degradation or re-absorption and photon recycling effect can lead to shift or broadening of the PL spectra.11,24–33 In this work, we report on an investigation of the PL spectra of individual CH3NH3PbI3 microcrystals with hexagonal shape by spatially resolved PL micro-spectroscopy and imaging. We will show that total reflection effect and the redistribution of the charge carrier can also significantly affect the PL spectra and PL dynamics.

Experimental Methods Sample preparation: The CH3NH3PbI3 was prepared by the one-step method from equimolar γ-butyrolactone solution of methylammonium iodide (CH3NH3I) and lead iodide.34 CH3NH3I was purchased from Heptachroma. PbI2 and γ-butyrolactone were purchased from Sigma and Aladdin and used as received. PbI2 powder was dissolved into γ-butyrolactone and stirred for 60 min at 80 ℃. Then CH3NH3I was added into the PbI2 solution (molar ratio 1:1) and


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the solution was kept under stirring for 60 min at 80℃. The crystals were grown by drop casting of 30 l of the diluted CH3NH3PbI3 (0.3 mol/L) solution on cleaned glass cover slips and followed directly by an annealing process for 20 min at 80 ℃. The crystals were formed on the edge of the solution droplet during the evaporation of the solvent. The sample preparations were carried out under ambient conditions. XRD measurements: XRD measurements were carried out on a Shimadzu XRD-6000 powder X-ray diffractometer with Cu Kα radiation (λ=1.5418 Å) between 5° and 50° at a scanning rate of 5° min-1. PL measurements: The PL of the crystals were analyzed by a home-built wide-field microscope. Briefly, a 532 nm diode CW laser was used as the excitation source and focused above the sample plane by a dry objective lens (Olympus LUCPlanFI 40×, NA=0.6) leading to a 70 μm sized laser spot at the sample plane. The luminescence light was collected by the same objective lens and detected by an EMCCD camera (Andor Ixon U888) after passing through a 550 nm long-pass filter (ET550lp, Chroma). The images were taken with the exposure time of 100 ms and an accumulation of 100 frames. And band-pass filters FF01-755/35-25 (Semrock), FB790-10 (Thorlabs) and FB810-10 (Thorlabs) were used for spectrally resolved imaging. A transmission grating (Newport, 150 lines/mm) was put in front of the camera for the spectral measurements. PL lifetime measurements were done using a single photon counting system (TCSPC, picoharp 300) with 532 nm excitation light from super continuous laser separately operating at 2 MHz and 1 MHz repetition rate (Fianium SC-400). The atmosphere was controlled by purging a sample chamber with oxygen or nitrogen.


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Results and Discussion When a single hexagonal crystal was excited by wide-field illumination, the emission intensity was not uniform over the whole crystal (Figure 1e-g) but the crystal morphology as well as chemical composition was homogeneous, as measured by SEM and SEDX, respectively (Figure S1a-c). There are regions with bright PL close to the crystal edge, as well as close to its center. With the aid of two band-pass filters (one for normal PL and the other for the red-shifted PL), the location dependence of red-shifted emission and the normal (maximum at 760 - 770 nm) emission can be clearly visualized in Figure 1a and Figure 1b. After summing up the two images, almost no difference was observed between Figure 1c and Figure 1d (total PL intensity) showing that the chosen spectral bands allow for correct representation of the PL spectral features and their spatial location. From the PL spectra of the two locations in one crystal, we can see that the red-shifted emission from the edge was at 805 nm and the emission from the middle of the crystal had the maximum at 773 nm corresponding to the normal band-to-band radiative recombination in CH3NH3PbI3 perovskite. In some hexagonal crystals, even both peaks were observed in one location (Figure S2a-d, Supporting Information). It is interesting to note that while spatial distribution of the normal PL is less structured over the crystal, the red-shifted PL shows clear structures in the hexagonal crystal, compare a) and b) in Figure 1.


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Figure 1. Photoluminescence images of the crystal were taken through two different band-pass filters: (a) band-pass from 738 nm to 773nm. (b) band-pass from 805 nm to 815 nm. (c) Sum of the images (a) and (b). (d) and (f) Photoluminescence image of the crystal without filters. (e) and (g) show the PL emission spectra taken from the locations marked by blue and red rectangles in the image (f). The PL emission spectra marked by violet, olive and dark cyan were measured with the corresponding filters as indicated.

Red-shifted PL spectra of CH3NH3PbI3 perovskites have been reported by many groups. Among proposed reasons, we consider the following: 1) inclusion of a different crystal phase;27,29,35 2) atmosphere effect and degradation of the sample;31,32,36 3) re-absorption effects;37 4) radiative recombination involving shallow trapping levels.38 To find the real origin of the effect we carried out a series of experiments as described below. Since the fabrication and characterization processes were all carried out at room temperature, the XRD characterization was first done to see if there was any other crystal phase different from the existing tetragonal phase. Figure S3a showed the corresponding sharp diffraction peak at 14.13°, 28.46° and 43.05° belonging to the lattice plane (110), (220) and


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(330) confirming the tetragonal phase39 of the material and excluding the presence of impure phases in the crystals. We also did not observe any change on the PL spectra when changing the atmosphere from air to pure oxygen and nitrogen as shown in Figure S3b. The excitation power dependence was also measured (Figure S3c) and it showed no difference in PL spectra indicating that photo-degradation and photochemical transformation of the material can be ruled out as the origins. Because the red-shifted emission was always observed at the edge of the crystal, we further excited the sample locally using a focused laser beam, which is contrary to the wide-field excitation regime discussed above, and detected the PL emission at different places over the crystal. As shown in Figure 2a and Figure 2b, when we excited and detected at the same place on the crystal (does not matter at the center or at the edge), no red-shifted PL emission was observed. The red-shifted PL can be only observed when we excited the crystal at one place (e.g. center) and detected the emission at another place (e.g. edge) (Figure 2c). The spectrum of this emission was exactly similar to the red-shifted emission from the edges observed under widefield excitation (Figure 1). The PL emission, which was consistent with band gap transition, from the direct excitation at the edge can help to rule out the band gap variation due to structure distortion.40 Combined with the results measured in different atmospheres, we can also exclude the possibility of compositions difference or structural inhomogeneity at the edge and center of the crystal which was recognized to induce the PL shift.41 As a result, we proposed that the redshifted spectra can only be due to re-absorption (or self-absorption) 25,37,42 effect in the crystals.


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Figure 2. PL under focused excitation in the epi-fluorescence geometry. (a) and (b) PL excited and detected from the same local region. (c) PL detected from the edge while the middle of the crystal was excited. The excitation and detection locations are marked by green circles and red squares, respectively.

Re-absorption effects in CH3NH3PbBr3 single crystals with a size of several millimeters have been verified via depth-resolved cathodoluminescence and the transmitted luminescence methods.43 In order to verify our hypothesis, the transmitted luminescence of CH3NH3PbI3 single crystals was also studied. We excited the crystals by wide-field excitation from different sides (from the top and from the bottom) and compared the PL emission properties detected from the bottom. Compared to the results measured in Figure S4a and Figure S4b, we found that the blue part of the overall emission spectrum at the center was significantly decreased by re-absorption effect. However, the PL spectra from the edge and the center were still different when excited and detected from the different side. This is because the center of the crystal is not thick enough (about 6 m at the edge, as measured by AFM in Figure S5) to completely re-absorb the blue part of the emission after propagating through the crystal from the top to the bottom directly.


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Therefore, the light coming from the edges must have passed a much longer distance inside the crystal.

Figure 3. (a) The optical image of a large crystal. The green circle represents the focused excitation spot and the red arrow indicates the direction along which photoluminescence is detected. (b) The photoluminescence spectral map along the red arrow shown in (a). (c) Effect of re-absorption on the PL spectra (color lines) calculated using the absorption spectrum (black line) from the literature.33

Then we further investigate the light propagating processes in a large crystal. When the size of the crystal was large enough, we were able to choose the detection area using a slit of our spectrometer and detect the photoluminescence spectrum at different distances from the excitation spot. A clear red-shift was observed upon going away from the excitation spot (Figure 3a and Figure 3b) which should be due to the total internal reflection on the perovskite/air and perovskite/glass interfaces. High refractive index44,45 (about 2.5 at 750 nm) of CH3NH3PbI3 leads to the critical angle at the perovskite/air interface as small as 23.5 degrees. It means that a large


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amount of the light emitted in 4 solid angle inside the crystal is not able to leave the crystal directly. Such emission has to experience multiple reflections and scattering by the interfaces until it hits a surface with a suitable angle to escape from the crystal. This condition naturally occurs at the edges of the crystals because there are more surfaces at different angles and also due to the imperfections of the crystal shape. This process increases the average optical path much above the crystal dimensions leading to the substantial red-shifted PL spectra by reabsorption. Quantitative analysis of the red-shifted spectra as a function of the propagating distance can be found in Figure S6 (Supporting Information).

Figure 4. PL decay dynamics of CH3NH3PbI3 crystals. (a) PL decays measured from different locations of the MAPbI3 crystal; (b) The proportion of different components in different


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locations fitted from (a). (c) PL decays measured via different wavelength ranges: 738 - 773 nm, 785 - 795 nm and 805 - 815 nm; (d) The proportion of different components in corresponding wavelength ranges. (e) Excitation power dependence of the PL decays, the data was collected from the edge of the crystal, (f) Dependence of the two components on the excitation power fitted by bi-exponential function. Repetition rate of the pulsed laser used in (a) and (c) was 2 MHz and in (e) was 1 MHz.

Besides the propagating distance dependence, the wavelength of the red-shifted emissions from the micro-crystal also showed a dependence on the crystal size (Figure S7, Supporting Information). In order to see if the effect of re-absorption can explain so large shift of the emission spectrum, we modeled the re-absorption effect using absorption coefficients of CH3NH3PbI3 material available in the literature.44 As shown by the simulated results in Figure 3c, to have a red-shifted emission at 810 nm, light path of at least 5 m is needed. Comparing the shape and the position of the calculated PL bands with the experimentally observed emission from the edges of the crystals (with thickness  6 m), re-absorption effect can be responsible for the red-shifted emission. Even much larger propagation length can be easily reached in the case of several internal total reflections. We also investigated the PL decay dynamics at different locations under wide-field excitation. Very different PL decay dynamics were observed in different detection locations (Figure 4a and Figure 4b) with the largest difference between the center (normal PL spectrum) and the edge (spectrum with red-shifted component) of the crystal. The decay dynamics at the edge of the crystal were also measured separately through different filters. If the red-shifted emission is just the normal emission which is spectrally filtered due to internal absorption inside the crystal, there should not be any location dependence and wavelength dependence of the PL decay. However, As shown in Figure 4c and Figure 4d, the decay of the red-shifted emission was


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evidently longer. Most importantly, the excitation power dependence in different stages showed totally opposite tendencies for the normal emission and the red-shifted emission as shown in Figure 4e. The lifetimes obtained from the bi-exponential fitting are shown in Figure 4f. This result clear show that the red-shifted emission cannot be simply explain by the re-absorption effect because otherwise the excitation power dependence should follow the same trend. To explain these differences, we proposed a model involving light excitation, charge carrier diffusion, re-absorption, and trap-assisted recombination processes. The scheme of the model is shown in Figure 5. In the first stage which happens within the first 10 ns, a high concentration of charge carriers are generated due to the excitation of a thin layer of the crystal (~200 nm which is the absorption depth at the excitation wavelength of 532 nm). Most of the carriers should recombine within a short period of time via bimolecular recombination. The higher the excitation power is, the higher the concentration of photo-generated carriers is and the faster the initial PL decays. A fraction of this initially generated PL will emit out directly from the front surface of the crystal keeping the spectral band at 773 nm free from the influence of re-absorption. However, as explained before, due to the high refractive index of the perovskite, most of the initially emitted PL photons will propagate towards the depth of the crystal after first total internal reflection on the flat surface of the crystal. Since the absorption coefficient at the PL maximum is low (see Figure 3c), the propagation length of the PL will be quite long. Thus in the second stage, after multiple total reflections from the top and the bottom surface, the reabsorption of the photons will occur quite uniformly all over the whole volume of the crystal. This uniform re-absorption generates charge carriers with low concentration over the whole crystal volume. In this condition, monomolecular recombination between photo-generated carriers and the opposite carriers existed due to non-intentional doping will dominate leading to


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mono-exponential decay of PL.9 The PL decay dynamics in the second stage depends on the interplay between the excess charge carriers and deep traps. Filling of the deep traps at higher excitation power leads to slowing down of the non-radiative monomolecular recombination, increasing of the PL quantum yield and increasing of the PL lifetime. Since most of the PL generated in the second stage originates from the interior of the crystal, it needs to pass through a substantial thickness of the material before detected. Of course, the emission will be reflected by the top and bottom flat surface of the crystal due to the high refractive index. The light can go out of the crystal only when it reach a surface with appropriate angle which will happen at the edge of the crystal. Therefore, re-absorption effect at the second stage becomes significant leading to red-shift of the PL spectra (Figure 4b). This origin of the red-shifted emission is also supported by measuring the PL spectra and decay dynamics of small perovskite microcrystals. As shown in Figure S8, no red-shifted emission was observed in small microcrystals and the PL decay rate decreased with increasing of the excitation power in good agreement with the bimolecular recombination process.


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Figure 5. The scheme of the model to explain different excitation power dependencies of PL with normal and red-shifted spectra.

Conclusions In conclusion, a red-shifted PL emission with the maximum in the range from 800 to 830 nm from hexagonal-shaped CH3NH3PbI3 crystals was observed showing significantly different properties in comparison with the normal PL of this material. Red-shifted PL was present at the edges of the crystals under wide-field excitation or under focused excitation at the center. The red-shifted PL possessed mono-exponential long decay dynamics and the opposite excitation power dependence in comparison to that for the usual PL detected from other parts of the crystals. Analysis of the spatially-resolved PL properties and their excitation power dependencies allows us to propose that the different PL properties result from a combination of pure optical effects (light propagation, reflection and self-absorption) with concentration dependent charge dynamics and charge diffusion in perovskites. Contrary to the previously suggested in the literature, there is no evidence that the red-shifted emission from crystal edges is related to any trap states or other chemical differences in the crystal. Our study shows that PL of crystalline perovskite materials is very strongly influenced by peculiarities of light propagation in individual crystals (determined by their shape and size) and charge carrier diffusion from the initially excited surface layers to the depth of the material.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.


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SEM image and SEDX mappings of CH3NH3PbI3 hexagonal crystals; Variation of the PL spectra measured at the edges from different crystals; XRD characterization, atmosphere dependence and excitation power dependence characterization; Excitation direction dependence of the crystal; AFM characterization; Modeling of the PL spectra in large perovskite crystals with re-absorption; Size-dependence of the red-shifted PL in the crystal; Excitation power dependence of the PL decay measured for small CH3NH3PbI3 crystals.

AUTHOR INFORMATION Corresponding authors: *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors are grateful to the Swedish Research Council (Sweden-China collaboration grant number 2017-00748). This work is supported by National Natural Science Foundation of China (NSFC No. 21673114 and No.51711530161), and the Natural Science Foundation of Jiangsu Province (No. BK 20160645). We are also thankful for the support from Chinese university undergraduate innovation program and Fundamental Research Funds for the Central Universities (No.020514380104 and No.021014380116).


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Red-shifted Photoluminescence from Crystal Edges Due to Carrier

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