Tracking Transformative Transitions: From CsPbBr3 Nanocrystals to

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Tracking Transformative Transitions. From CsPbBr3 Nanocrystals to Bulk Perovskite Films Rebecca A. Scheidt, Corey Atwell, and Prashant V. Kamat ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00001 • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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ACS Materials Letters

Tracking Transformative Transitions. From CsPbBr3 Nanocrystals to Bulk Perovskite Films Rebecca A. Scheidt, Corey Atwell, Prashant V. Kamat* Radiation Laboratory and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States

*Address correspondence to this author [email protected] Email: [email protected]

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Abstract: The control of grain size and surface properties is an important parameter in controlling the optoelectronic and photovoltaic properties of metal halide perovskites. When CsPbBr3 nanocrystal (~10 nm in diameter) films were annealed at 100 – 125 ºC they grow in size to produce ~400 nm diameter crystallites while transforming into bulk perovskite films. Characteristic changes in the optical properties were noted when such transformation occurred from nanocrystals into bulk. By tracking absorbance and emission spectra and morphological changes of CsPbBr3 films at different annealing times and temperature, we were able to establish the mechanism of particle growth. The presence of nanocrystals and larger crystals during the intermediate annealing steps and narrowing size distribution confirmed the Ostwald ripening mechanism for the crystal growth. The energy of activation of crystal growth as determined from the temperature dependent optical properties was estimated to be 75 kcal/mole.

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Metal halide perovskites have emerged as a promising material to be used in next generation light harvesting and emitting devices1. Their attractive qualities, such as variable device architectures2–4, tunable band gaps5–8, and long charge carrier diffusion lengths9–12, have made this material an epicenter of energy research. Of particular interest is their ability to deliver high efficiency solar cell devices and light emitting diodes13–20. In our previous study, we employed a layer-by-layer deposition method utilizing CsPbX3 nanocrystals (NCs) to create bulk perovskite films through an annealing process21. This approach of annealing nanocrystals at higher temperature is similar to the one employed to design other semiconductor films from nanocrystals22–24. This technique allows for facile, controllable film formation in which NCs sinter to become bulk crystallites which has enabled the creation of solar cells created with efficiencies of over 5%25. Many research laboratories have pointed out the importance of grain size and grain boundaries in dictating optical and photovoltaic properties of methyl ammonium lead halide films26–29. A thorough understanding of the growth mechanism of nanocrystals is essential to engineer metal halide perovskite films with fewer defects. The CsPbBr3 system offers the convenience to track the changes in the optical properties as it can be prepared as nanocrystals (of diameter 5-10 nm) and grown into bulk films. By reducing the annealing temperature from the original study, we were able to control the growth process and probe the optical and physical changes associated with the transformation. Here we discuss how the growth process was elucidated from absorption and emission measurements coupled with SEM and discuss the identified growth mechanism. CsPbBr3 NCs were synthesized using a previously described method.5 A high concentration solution of CsPbBr3 NCs (~53.6 µM) in hexane was spun cast onto microscope (2.5 cm × 2.5 cm) slides. Each glass slide with spun cast film was cut into thirds and then annealed at a constant temperature. The slides were periodically removed (each slide at different time interval) and were analyzed with optical and microscopy measurements (all measurements were carried out at room temperature (~25°C) after the samples were cooled to room temperature). These sets of experiments allowed us to monitor the morphological changes that occur when annealed at a constant temperature. The SEM micrographs in Figure 1, show the morphological changes observed during the annealing temperature of 125 ºC at predetermined time intervals between 0 and 60 minutes. With increasing time, we see the growth of crystal size and the disappearance of NCs. For example, the SEM image presented in Figures 1A shows a uniform layer of small (~10 nm diameter) nanocrystals of pristine spun cast film (t = 0 minutes). Higher magnification of this samples could not be obtained because the samples degraded at electron beam voltages needed for higher magnification (The TEM images of colloidal solution is given in the supporting information, Figure S1). The 15 minute annealed sample (Figure 1B) shows a similar uniform layer of NCs but with larger crystallites thus indicating that the conversion process has already begun. The growth in grain size is more evident in Figures 1C,E and G, recorded with annealing



intervals of 30, 45, and 60 minutes at 125 °C, respectively. Defined grain boundaries become visible in the 30 and 45 minute samples while NCs are still observable. By 60 minutes, though, all of the NCs have transformed into a bulk crystals. Particle sizes in the 30, 45, and 60 minute annealed samples were measured and histograms showing particle sizes were created (Figures 1D, F, and H). The average particle size increased from 264 nm to 472 nm during annealing from 30 to 60 minutes, respectively. Additionally, the distribution of particle size broadens from 30 to 45 minutes but narrows in the interval between 45 and 60 minutes. The morphological changes in SEM images (Figure 1) is indicative of a growth mechanism involving Ostwald ripening. This argument is consistent with the previously made observation in an in-stu TEM analysis in which the growth of NCs was recorded with a heated TEM grid25. In Ostwald ripening, smaller particles are sacrificed to help the growth of larger particles, which are more energetically favorable. The disappearance of small particles by the end of the annealing process points to this conclusion. The narrowing of the distribution over time also indicates Ostwald ripening as the preferred mechanism due to the crystal sizes becoming more uniform as the growth process progresses. Once the particles are fully grown, the differences in energetic favorability between particles diminishes, thus halting the growth process. It should also be noted that the final average particle size reported here (472 nm) is much larger than in our previous study (50 nm)25. This can be attributed to our lower annealing temperature which allowed for a more ordered growth of the crystals and therefore larger particles overall. To confirm that a change in crystal structure did not occur along with crystal growth, powder X-ray diffraction (PXRD) was performed (Figure S2 in the supporting information). PXRD shows broad diffraction peaks where the (0,0,1) and (2,0,0) crystal planes for cubic perovskite should be. These broad peaks are expected of NCs due to the relationship between peak width and crystal size30. The diffraction peaks narrow for the 30 and 60 minute sample without shifting in the peak position, demonstrating that no change in crystal phase has occurred. The absence of other expected crystal plane diffraction peaks is due to the preferred orientation that the NCs take when they are spun cast.


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1 μm

2 μm

H

Figure 1: SEM micrographs after (A) 0 minutes, (B) 15 minutes, (C) 30 minutes, (E) 45 minutes, and (G) 60 minutes of annealing at 125 °C. (A) and (B) show nanocrystals which were unable to be resolved due to their size and SEM resolution. (D), (F), and (H) show histograms made from 30, 45, and 60 minute annealed samples. The average particle size increases from 264 to 472 nm. The distribution of the particles at 45 minutes broadens with respect to the 30 minute sample before becoming narrower again at 60 minutes.



t=0 min

t=30 min

t=60 min

Scheme 1. Schematic illustration of growth of CsPbBr3 nanocrystals into bulk film during annealing process

The morphological changes indicate that we should be able to observe the optical changes such as a loss of quantum confinement and emission behavior associated with a growth of bulk crystals. Scheme 1 qualitatively illustrates the growth process.

UV-Vis absorbance spectra were recorded at selected time intervals (Figure 2) as the samples were subjected to annealing. A broad excitonic peak at 486 nm (2.55 eV), indicative of monodisperse NCs, is present at early times (≤10 min). However, a lower energy excitonic peak (520 nm, 2.38 eV) appears with increased time of annealing. Such a shift in the absorption is consistent with previous reports for the growth of CsPbBr3 NCs into large NCs31 as well as for bulk CsPbBr3 films21. With continued annealing time, this low energy peak becomes prominent and remains steady as the sample gets converted to bulk crystals after ~50 minutes. The existence of two distinctly different excitonic band edges 0.6 (e,f,g) (a) 0 min shows the evolution of (b) 10 min NCs into bulk crystals with (c) 20 min (d) 30 min different excited state (e) 40 min 0.4 characteristics. (f) 50 min (d) (g) 60 min Specifically, the appearance of a sharper (c) (b) excitonic peak in the bulk 0.2 samples indicates the (a) growth of very homogenously sized large 0.0 particles compared to the 400 500 600 700 800 ensemble of small Wavelength (nm) particles in the NC film samples. The slow Figure 2: Absorbance spectra of CsPbBr3 nanocrystal film evolution of the bulk recorded during over annealing time (60 min) at 125 °C as it crystal characteristics transforms to become a bulk film. The 0 minute trace (a, during 50 min of annealing red) shows a relatively sharp excitonic peak at ~490 nm shows that the transition corresponding to monodisperse nanocrystals. The emergence from NC to bulk is not an of a lower energy, sharper excitonic band edge peak emerges instantaneous process but at the 20 minute mark (c, yellow) and continues to grow until

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40 minutes (e, light blue).


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evolves slowly through disappearance of smaller NCs (viz., Ostwald ripening). To elucidate the change in other optical properties through the annealing process photoluminescence spectra of samples annealed at different times were recorded as well. Figure 3 shows the PL spectra taken over time while the sample was annealed at 125 °C. A single fluorescence peak can be observed at 505 nm for unannealed film, corresponding to NC fluorescence. This fluorescence peak (2.45 eV) decreased for samples with increased annealing time. The fluorescent nature of perovskites has been studied in depth32 and has been attributed to surface recombination33–35. Thus, the decrease in PL, indicating a loss of photoluminescent properties of the material and subsequent increase in non-radiative recombination, is due to the loss of ligands and the creation of surface defect recombination centers during the annealing process. After 10 minutes of annealing, a second, lower energy emission peak at 522 nm emerges. The lower energy emission peak (2.37 eV) becomes prominent at the 15 minute mark, which matches with the changes seen in the absorbance spectra indicating the presence of a bulk crystal peak with simultaneous loss of quantum confinement. After 55 minutes of annealing, only the bulk emission peak at 522 nm is retained. The original fluorescence peak of CsPbBr3 NCs completely disappears by 60 minutes of annealing, indicating the complete transition from NC to bulk film. As the annealing process proceeds, the Stokes shift decreases from ~100 meV to 60 meV. These values are consistent with previous reports for both the bulk film36 and the NC5,37. This transformation also shows the previously reported change of the Stokes shift from being an intrinsic confined hole state in NCs to primarily from a lattice induced carrier stabilization independent of the grain size38. The intensity of both peaks continues to diminish with increasing annealing time until the signal becomes negligible. To better visualize the shifting trend in the photoluminescence, we normalized each PL spectra to its corresponding peak maximum for each time point (Figure 3B). While the lower energy peak (corresponding to bulk CsPbBr3) becomes dominant at later annealing times, the contributions of both NCs and bulk could be seen during samples annealed at intermediate times from the broadness of the emission spectrum. After 40 minutes of annealing the PL becomes narrow and stops exhibiting a shift in emission peak. These trends, which parallel the spectral changes observed in the absorbance spectra (Figure 2), help us to track the crystal growth process of CsPbBr3 films.



B

Figure 3: Photoluminescence spectra illustrating the change in fluorescence with increasing annealing time at 125 °C. (A) Fluorescence of a CsPbBr3 film from 0 minutes (a, red) to 55 minutes (l, purple). The emergence of a second, lower energy fluorescence peak appears full by the 15 minute time point (d, yellow) indicating the presence of a bulk species. Both species decrease in intensity until they no longer fluoresce. (B) The PL spectra from (A) normalized to the maximum intensity to show the gradual shift in emission peak wavelength.

The results discussed so far involved CsPbBr3 films annealed only at a temperature of 125 °C, with crystal growth being completed in under an hour. When annealing was performed at 225 °C, the growth process as monitored from the emission intensity took only about three minutes to transform into bulk21. To further investigate the temperature dependence of this process, PL measurements were carried out at temperatures ranging from 100 – 125 °C. By tracking the change in intensity of the nanocrystal emission band, we were able to find the rate of change from CsPbBr3 NCs into bulk crystals in the film. Samples were annealed at temperatures in the range of 100 to 125 °C and the PL spectra of films at different annealing times were recorded (SI 3). In order to extract the emission maximum of the NCs, the overall PL spectra needed to be deconvoluted in order to exclude contributions from the bulk crystal emission peak (522 nm). Each spectrum was fit with a PseudoVoigt curve, with one or two peaks depending on the broadness of the emission peak. An example of a post deconvolution spectrum can be found in SI 4. The absence of a second peak at early times further confirms that bulk crystals were not present in the original sample and that they were only formed through annealing. Additionally, the higher energy NC peak blue shifts slightly over time. This indicates that the residual NCs become even smaller over time as they are scavenged for material to grow large particles, in accordance to the Ostwald ripening mechanism.


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B

A

Figure 4: (A) The peak intensity of the deconvoluted higher energy peak (corresponding to NC emission) plotted over time for 100 (red), 105 (orange), 110 (yellow), 115 (green), and 125 (blue) °C. All data is shown from 0 to 90 minutes but data was collected for longer time periods for 100 and 105 °C since the process occurred much more slowly. Each point shown is an average of the three trials for each temperature. Monoexponential decay fits are shown by a solid line in the corresponding color. (B) An Arrhenius plot based off of the rate of change of PL intensity (k) versus 1/temperature. The slope was used to find the energy of activation of crystal growth which was 75 kcal/mole. Once deconvoluted, the emission peak intensity for the CsPbBr3 NC fluorescence was determined for each annealing time at the temperatures studied (Figure 4A). The photoluminescence peak intensity was plotted versus time for each annealing temperature studied (note that all emission measurements were done at room temperature after removing slides from the hot plate at different annealing times.) Each trace was fit with a monoexponential decay curve, which is typical of a of a pseudo first order reaction that reflects the dependence of the rate of change on the concentration of CsPbBr3 NCs in the film. The unambiguous temperature dependence of the rate of change of PL intensity on annealing temperature exhibited an Arrhenius relationship (equation 1), 𝐸𝐸

Equation 1

ln(𝑘𝑘) = ln(𝐴𝐴) − 𝑅𝑅𝑅𝑅𝑎𝑎

where k is the rate constant of a physical phenomenon, A is a pre-exponential factor, Ea is the energy of activation, R is the universal gas constant, and T is the temperature in Kelvin. The plot of the natural log of the rate constant against 1/T exhibited a linear relationship thus, confirming the validity of Arrhenius relationship for the growth process. Based on the slope of this plot we determined the energy of activation for the CsPbBr3 crystal growth to be 75 kcal/mole.



A

B

Figure 5: Charge carrier recombination as monitored through (A) emission decay (excitation at 350 nm) and (B) recovery of transient bleaching (excitation at 387 nm) of CsPbBr3 films during the annealing process at 125 °C from 0 min to 60 min. The emission (A) was monitored at emission maximum as presented in Figure 3. The bleaching recovery (B was monitored at 520 nm. Perovskites have been lauded as a promising material for photovoltaic and light emitting applications partially because of their long charge carrier diffusion length9–12. Grain boundaries between crystals, however, can act as recombination centers which shortens the average charge carrier diffusion length39,40. Since crystal growth and increasing grain size were actualized in the SEM images, time correlated single photon counting (TCSPC) was used to find the emission lifetime for samples annealed over time. The lifetimes, shown in Figure 5A, show a shorter emission lifetime with increased annealing time at 125 °C. As the CsPbBr3 NC films are annealed, large particles are formed and protective ligands are lost. The annealing process is likely to create more surface states, adding additional non-radiative decay pathways for charge carrier recombination. This data is further supported by the excited state lifetimes found from ultrafast transient absorption spectroscopy (Figure 5B). Change in absorption data was taken at the bleach absorption maxima (~520 nm, see SI 5 for example transient spectra). These bleach recovery traces were analyzed using a biexponential kinetic fit, representing a fast decay component and a slow decay component. These values were then averaged (see SI for detail) to find the average excited state lifetimes. Between 10 and 20 minutes of annealing, the slow decay component becomes a larger contributor to the average excited state lifetime. For all time points, though, the average lifetime continuously decreased from 1709 ps to 682 ps (SI 6). While this is a significant decrease in the charge carrier lifetime for the bulk film, our charge carrier lifetimes are still consistent with previously reported values for both the NC41 and bulk films42.


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In summary, CsPbBr3 NCs were annealed at temperatures ranging from 100 – 125 °C in order to study the growth mechanism of the NCs into a bulk perovskite film. Physical changes monitored with SEM, showed an increase in average particle size. Optical properties were observed and exhibited a loss of confinement and a concomitant shift in the excitonic band edge and emission maxima. An Arrhenius relationship was found to exist and the energy of activation for crystal growth was found to be 75 kcal/mole. An overall decrease in excited state lifetime was found to occur, possibly arising from crystal defects created during the growth process. Overall, the growth mechanism for CsPbBr3 NCs is now more fully understood and characterized.

Acknowledgment: P.V.K. and C.A. acknowledge the support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, through award DEFC02-04ER15533. C.A. would also like to acknowledge the support from ND Energy through the Slatt Fellowship for Undergraduate Research. R.A.S. acknowledges the support of the Arthur J. Schmitt Leadership Fellowship as well as the Division of Materials Sciences and Engineering Office of Basic Energy Sciences of the U.S. Department of Energy through Award DE-SC0014334 for carrying out the analysis and discussion of results. This is contribution number NDRL No. 5204 from the Notre Dame Radiation Laboratory. The authors would also like to thank Prof. Ken Kuno for helpful discussions, the Notre Dame Integrated Imaging Facilities, and Dr. Allen Oliver for discussions on powder XRD.

Supporting Information: The supporting information contains synthetic and annealing procedures, fitting information, excited state lifetime average information, instrumentation (UV-Vis absorbance, fluorescence, TSCPC, transient absorption, SEM, and PXRD), nanocrystal TEM, PXRD, PL spectra at different temperatures, transient absorption spectra, and excited state lifetimes.



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