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Tracking Iodide and Bromide Ion Movement in Mixed Halide Lead Perovskites during Photoirradiation Seog Joon Yoon, Sergiu Draguta, Joseph S. Manser, Onise Sharia, William F. Schneider, Masaru Kuno, and Prashant V. Kamat ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00158 • Publication Date (Web): 20 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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ACS Energy Letters

Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation

Seog Joon Yoon,1,2 Sergiu Draguta,2 Joseph S. Manser,1,3Onise Sharia,3 William F. Schneider,2,3 Masaru Kuno,2 and Prashant V. Kamat1,2,3*

Radiation Laboratory, Department of Chemistry and Biochemistry and Department of Chemical and Biomolecular Engineering University of Notre Dame, Notre Dame, Indiana, 46556, United States

*Corresponding author: [email protected] 1

Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana, 46556, United States

2

Department of Chemistry and Biochemistry, University of Notre Dame

3

Department of Chemical and Biomolecular Engineering, University of Notre Dame

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Abstract Mixed halide lead perovskites (e.g.CH3NH3PbI3-xBrx) undergo phase segregation creating iodide-rich and bromide-rich domains when subjected to visible irradiation. This intriguing aspect of halide ion movement in mixed halide films is now being tracked through excited state behavior using emission and transient absorption spectroscopy tools. These transient experiments have allowed us to establish the time scale with which such separation occurs under laser (405 nm, 25 mW/cm2−1.7 W/cm2) irradiation as well as dark recovery. While the phase separation occurs with a rate constant of 0.1-0.3 s-1, the recovery occurs over a time period of several minutes-hour. The relative photoluminescence quantum yield observed for Br-rich regions (em. max 530 nm) is nearly two orders of magnitude lower than that of I-rich regions (em. max 760 nm) and arises from the fact that I-rich regions serve as sinks for photogenerated charge carriers. Understanding such cascading charge transfer to localized sites could further enable the design of gradient halide structures in mixed halide systems.

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As we continue to explore the organic lead halide perovskites for photovoltaic and photonics applications,1-7 mixed halide perovskites have emerged as attractive candidates for continuous tuning of the bandgap.8-10 For example, the bandgap of methylammonium lead iodide/bromide (CH3NH3PbBrxI3-x) can be tuned between 1.55 eV and 2.43 eV by varying the composition of Br and I (x=0 to 3).11-12 The use of mixed halide perovskites in solar cell and lasing applications motivates further investigation of the underlying optical and electronic properties of such systems.4, 13-14 The halide ions of perovskite films are chemically “softer” materials as compared to other semiconductors employed in photovoltaics since large ionic displacements have been noted at room temperature.15-16 For example, halide ions are easily exchangeable by subjecting the iodide form of the film to other halides. Ion exchange from solution-based as well as gas-phase exposure to halogens12, 17-19 can also yield different forms of lead halide perovskites. A common method employed in designing mixed halide lead perovskites is to spin-coat a N,N-dimethylformamide (DMF) solution of stoichiometric Pb2+ and halide (I-+Br-) on a suitable substrate followed by annealing.12, 20-21 However, as shown in our recent study, the lead halide complexation chemistry dictates the nature of binding between Pb2+ and Br- and Pb2+ and I-, especially when excess halide ions are present.22 Walsh and coworkers23 explored mixing energies in the CH3NH3PbBrxI3-x system with density functional theory. They found that mixing energies tend to be small and positive, that alloying is entropy-driven, and thus predict the system to exhibit a miscibility gap. Similar calculations on the cesium lead mixed halide system come to the same conclusion.24 These results are consistent with a facile halide exchange and with a tendency for phase segregation to occur under appropriate stimulation. Anomalous alloy properties of mixed halide perovskites have also been studied using first-principle calculations together with cluster-expansion methods.24 Another interesting aspect is the migration of halide ions and associated defect in perovskite films with relatively activation energies as low as ∼0.1 eV.

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This effect predominantly seen in mixed halide

perovskite (e.g. CH3NH3PbBrxI3-x ) films as they undergo phase segregation when subjected to visible light

irradiation,19,

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consistent with a small mixing energy. Hoke and coworkers observed trap induced

emission in the lower energy region as the input of photons resulted in iodide-rich minority and bromideenriched majority domains.19 These segregation effects are reversible as the original spectral features were recovered after stopping the illumination. Even without light irradiation, Sadhanala and coworkers observed that the emission peak position of CH3NH3PbBr1.2I1.8 films changed from 1.68 eV to 1.94 eV after 2 weeks under inert, dark conditions.30 This behavior is consistent with the gradual and spontaneous room temperature phase segregation of an as-prepared halide composition within the miscibility gap.23


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However, illumination can provide energy to overcome underlying energetic barriers so that phase segregation occurs within seconds. Later, Hoke and coworkers also found similar trap-induced emission in lower energy regions in CsPbBrxI3-x.31 Regardless of whether the monovalent cation is CH3NH3+ or Cs+, the mobility of halide ions within the perovskite crystallites is important in determining the optical and electronic properties of mixed halide perovskites since phase segregation can have significant implications in the operation of photovoltaic devices. Thus far, trap-induced emission within mixed halide compounds has been reported by a number of researchers.9, 11, 32-35 However, the majority of these efforts relied on steady state band edge emission or X-ray diffraction studies before and after irradiation to explore halide ion movement. In order to track segregation effects in mixed halide films, we have now conducted steady state and transient absorption studies, enabling determination of the kinetics of segregation and recovery and their dependence on the intensity and duration of laserirradiation. Tracking the Phase Segregation through Changes in the Absorption. In the present study, we prepared CH3NH3PbBrxI3-x films by mixing stoichiometric amounts of Pb2+, CH3NH3+ and halide ions in DMF and spin coating the solution onto glass slides. Unless otherwise stated, the fraction of Br (x value) in CH3NH3PbBrxI3-x

films was maintained at 1.3. The absorption spectrum of this film, which is shown as spectrum a in

Figure 1A, exhibits a peak around 625 nm. The bandgap of 1.89 eV for this mixed halide perovskite (corresponding emission maximum at 655 nm) lies within the bandgap range of pristine CH3NH3PbBr3 (2.43 eV) and CH3NH3PbI3 (1.55 eV) perovskites. Upon subjecting this film to 405 nm continuous-wave (CW) laser irradiation (636mW/cm2) for 10 minutes, we see a depletion of the 625 nm absorption with concurrent increase in the absorption at higher and lower wavelengths. The difference between the two spectra, which is shown in Figure 1B, highlights the changes in the optical property in response to light irradiation. Upon stopping the laser irradiation, the original absorption is restored (spectrum c in Figure 1A), thus confirming the reversibility of the system (Figure S1). These results are in agreement with absorption changes observed earlier under steady state light irradiation.19


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A

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C

Figure 1. (A) Absorption spectra of CH3NH3PbBr1.3I1.7 film (a) before and (b) after 10 min laser irradiation (405 nm CW laser, 636 mW/cm2), and (c) absorption spectra after storing the film for 1 hr in dark conditions. (B) Absorption changes following 10 min irradiation of CH3NH3PbBr1.3I1.7 film with 405 nm CW laser at different laser powers. (C) Absorption-time profiles of CH3NH3PbBr1.3I1.7 film showing absorbance changes at (a) 625 nm and (b) 725 nm before, during and after laser irradiation. Changes observed at other irradiation intensities (25 mW/cm2 and 1.7 mW/cm2) are presented in the supporting information (Figure S1).

The bleaching at 625 nm along with increased absorption in the low and high energy region confirms that the CH3NH3PbBrxI3-x film undergoes segregation to form I-rich and Br-rich domains. nCH3NH3PbBrxI3-x+ hν → (n – 2m)CH3NH3PbBrxI3-x + mCH3NH3PbBrx-yI3-x+y+ mCH3NH3PbBrx+yI3-x-y

(1)

The fraction, ‘m,’ refers to segregated phase and x-y and x+y correspond to the changes in the individual halide concentrations. Based on the known absorption coefficient (2 x 104 cm-1 at 720 nm)19 and the observed changes in the absorption at 720 nm, we estimate that only a small fraction undergoes phase segregation(~2%). However, the decrease in absorbance at 625 nm corresponding to CH3NH3PbBrxI3-x (from 0.028 to 0.023) amounts to relatively a larger fraction (18%) of segregation. This value is in agreement with the relatively larger fraction of segregation (23%) observed through XRD measurements.19 It is likely that variance in halide substitution will introduce tail absorption in the 650740 nm region and thus distort the fraction determined through change in the iodide rich phase. The most iodide rich phase, which in the present case is characterized through the absorption in the 720 nm region, amounts only to ~2% in the segregated phase. As will be discussed later in this paper, this small fraction of I-rich domain energetically favors the capture of charge carriers and hence dictates the emission of the film following phase segregation. We also followed the phase segregation by monitoring the absorbance change at 625 and 725 nm (Figure 1C, Figure S3). The change in the absorbance at both these wavelengths is rapid, suggesting that the


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movement of halide ions to create iodide and bromide-rich domains occurs within 10-15 seconds. On the other hand, the recovery in the dark is a slower process and is complete in 40-60 minutes. The reversibility of the halide ion movement indicates that the free energy of mixing in the presence and absence of light are different and hence dictate the segregation process. Emission Changes during the Phase Segregation. An alternate way to track the movement of halide ions during irradiation of CH3NH3PbBrxI3-x film is to monitor its emission. Figure 2A shows emission spectra of aCH3NH3PbBrxI3-x film before (spectrum a) and after irradiation with 405 nm (200 mW/cm2) laser for 1 minute. This time scale provides minimum time of laser irradiation required to achieve phase segregation (Figure 1C, 2B). The emission band of CH3NH3PbBrxI3-x at 655 nm decreases with laser irradiation giving rise to a prominent emission band at 755 nm. The appearance of this new emission band at lower energies is the result of formation of iodide-rich domains. Note that pristine CH3NH3PbI3 films emit at 768 nm36 (A typical emission spectrum of CH3NH3PbI3 film is also show in Figure S4). If CH3NH3PbBrxI3-x films segregate to form iodide- and bromide-rich regions, one expects to see emission arising from Br–rich region on the high energy side. However, identification of Br-rich region was elusive and was not possible to track in previous studies.19 In the present study, when we carefully inspected the emission in the 500-600 nm region, we identify a weak emission band with maximum at 530 nm (Figure 2A, inset). The weak emission

A

B

Figure 2. (A) Emission spectra of CH3NH3PbBr1.3I1.7 film (a) before, and (b) after 1 min of laser irradiation (405 nm CW laser, 200 mW/cm2). (Inset: Enhanced scale of spectrum b in the 450-600 nm region) (B) Changes in emission as monitored at (a) 655, (b) 530, and (c) 755 nm respectively and the monoexponential kinetic fit (a: CH3NH3PbBr1.3I1.7 film, b: bromide-rich region, c: iodide-rich region).


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at 530 nm is similar to the emission arising from CH3NH3PbBr3 (Figure S5). The emission from the Br-rich region is significantly weaker (nearly two orders of magnitude less) than the iodide-rich region, suggesting that charge carriers are quickly transported and accumulated at the iodide-rich region. In other words, Irich regions serve as the primary charge carrier recombination sites irrespective of the carrier generation site in the mixed halide system. Similar arguments of charge transfer between Br-rich and I-rich regions as well as trap initiated recombination have been proposed in earlier studies.19 We also probed the emission growth from the Br-rich (530 nm) and I-rich (755 nm) regions to obtain insight into the time scale of the segregation (Figure 2B). From the pseudo-first order kinetic analysis, we obtain a rate constant 0.09 s-1 for Br-rich and 0.11 s-1 for I-rich regions respectively. However, it should be noted that the rate constant for the growth of I-rich region and Br-rich regions are strongly dependent on the film quality and laser intensity. As noted in Table S1, the apparent value of rate constant for bulk segregation varies in the range of 0.1 - 0.3 s-1 for . It should be noted that this value represents an apparent value corresponding to bulk segregation effect. Localized segregation processes within the crystallites are likely to occur on faster time scales. Interestingly the decay of parent emission (655 nm) of the mixed film is faster compared to the growth of I-rich and Br-rich regions (Figure S2). Such a discrepancy in the rate of disappearance and growth of emission corresponding to segregated region indicates the formation of an intermediate state as part of the overall segregation process.


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Transient Absorption Measurements to Probe Phase Segregation Events. Processes resulting in phase segregation were further analyzed using transient absorption spectroscopy. This technique allows us to record the transient absorption spectra at different times following excitation with a pump pulse so as to directly probe the excited state behavior. By introducing an additional CW laser to excite the probed

Figure 3. (A) Schematic illustration of sample excitation in a pump-probe transient spectrometer. (B)-(D) Time resolved difference absorption spectra of CH3NH3PbBr1.3I1.7 film recorded following 387nm laser pulse (pump) excitation (B) before 405 nm CW laser irradiation, (C) after subjecting the sample to 1 min CW laser irradiation, and (D) after subjecting the sample to 40 min CW laser irradiation (405 nm CW laser with 1.7 W/cm2). region, we induce phase separation while simultaneously monitoring changes in difference absorption spectra (Figure 3A). In the absence of the CW laser, CH3NH3PbI3-xBrx shows a transient-bleach at 625 nm following the 387 nm laser pulse excitation (Figure 3B). The bleaching maximum at 625 nm, which arises from charge separation, matches well with the absorption peak seen in the ground state absorption of CH3NH3PbI3-xBrxfilms (Figure 1A). Since the effective masses of electrons and holes are similar,37 both of them contribute to the bleaching and the bleaching recovery represents the charge carrier recombination. Details on the


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transient behavior of excited CH3NH3PbI3, CH3NH3PbBr3 and CH3NH3PbI3-xBrx films have been discussed in earlier studies.19, 21, 38-40 The energy of the pump laser in these experiments was kept below the threshold fluence required for phase segregation (4 μJ/cm2). Even when the 387 nm pump laser excitation was extended for several minutes, we could not observe any noticeable phase segregation. The fact that the transient bleach at 625 nm remained unperturbed during the time period of 1 ns (note the peak position of the spectra recorded at different monitoring times) confirms our ability to probe the excited state events without inducing any segregation (Figures 3B and S6). Figures 3C and D show time-resolved transient absorption spectra recorded following 1 min and 40 min irradiation with 405 nm CW laser (1.7 W/cm2). Although the short term (1 min) laser irradiation was sufficient to achieve phase segregation we also wanted to probe long term (40 min) laser irradiation effects. The bleaching of the irradiated CH3NH3PbI3-xBrx film in both these cases shows a maximum around 710 nm, a feature that is distinctively different than the bleaching maximum seen without CW irradiation in Figure 3B. The shift of bleaching to lower-energy region is indicative of the formation of excited I-rich regions. The results also confirm that irradiation times as low as 1 min are sufficient to achieve phase segregation. Whereas a small dip in the absorption around 530 nm is seen in spectrum a of Figure 3C (1 min irradiated sample), it becomes more pronounced as a bleach in the 40 min irradiated sample (Figure 3D). As discussed earlier, this characteristic bleach with maximum at 530 nm arises from the spectral changes associated with CH3NH3PbBr3 film. Thus, the spectral profiles in Figure 3D identify the Br-rich and I-rich regions formed following the photoinduced segregation of CH3NH3PbI3-xBrx films. There is a clear distinction between the lifetime of two bleaches corresponding to Br-rich and I-rich regions. The bleaching corresponding to Br-rich region recovers quickly (within ~10 ps) where as that of Irich region persists even after 1 ns. It should be noted that the excited behavior of preformed CH3NH3PbI3 and CH3NH3PbBr3 films remains distinctly different. Both these films when excited separately show prompt appearance of the charge separated state with a lifetime greater than 1 ns. This difference in kinetic behavior indicate that an additional deactivation pathway for the Br-rich region is operative to create the excited I-rich region through charge transfer (Figure S7). In fact the initial decay of Br-rich region during first 10 ps after the pump pulse match with the growth in the I-rich region. It is expected that more energetic charge pairs from Br-rich regions are transferred to I-rich sites with a rate constant of ~3x1011 s−1.This argument of localizing charges at the lower energy defect sites induced by the I-rich region is in agreement with earlier proposed model for observing emission in the red region.19


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In another experiment, we recorded transient spectrum at 5 ps delay after 387 nm laser pulse excitation during the CW irradiation (1.7 W/cm2). The experimental conditions were the same as in Figure 3A. However, instead of waiting for the full duration of laser irradiation for segregation to complete, we recorded transient spectra at 1s time intervals during CW laser irradiation. This experimental arrangement

A

B i

a

i

a

Figure 4.(A) Femtosecond transient absorption spectra of CH3NH3PbBr1.3I1.7 film with 5 ps pumpprobe delay under irradiation of CW laser(405 nm, 1.7 W/cm2). (B) Change of ΔA at different wavelengths under the presence of irradiation of CW laser. allowed us to monitor the initial phase segregation events through the characterization of the excited state (Figure 4A). The difference absorption spectrum recorded immediately after the 387 nm pulse before CW irradiation (t=0 s) shows an intense bleach (620-660 nm region) corresponding to separated electrons and holes in the mixed halide film. The spectra recorded at different times following the CW laser irradiation shows continuous decrease in this bleached absorption as we extend the irradiation time. (Note that the spectra were recorded at 1 s time intervals of irradiation and selected spectra are shown in Figure 4A). During the same irradiation period we see the growth of new bleach between 700-740 nm. The bleach in the 700-740 nm region corresponds to the appearance of I-rich regions in the film. As expected, with continued CW irradiation we see less of the mixed halide excited state as it undergoes phase separation. The absorption time profiles monitored at these two bleached bands (Figure 4B) were fitted to pseudo-first order decay and growth kinetics respectively. This rate constant of 0.3 s-1 (Table S2) further agrees with the results discussed in Figure 2 indicating the phase segregation rate constant to be 0.1-0.3 s-1. The phase segregation observed under CW laser irradiation is reversible as the initial composition can be reverted upon stopping the laser irradiation. The recovery of the bleaching at 625 nm in a steady state


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absorption measurement provided kinetic details of the remixing process. The overall recovery in dark took place over a longer period of time as compared to the photoinduced segregation process and extended over the course of several minutes (Figure 5A and S8). The recovery time was dependent on the irradiation time and intensity employed for phase segregation. Normalized recovery traces and the bimolecular kinetic fit for one experiment are shown in Figures 5A and B respectively. When irradiated for a short duration of 1 min with 405 nm CW laser (636 mW/cm2) we see recovery within minutes (τ= 267 s). On the other hand, films irradiated for 30 min required nearly an hour to recover fully (τ=1390 s). With increased irradiation time we expect an increased number of crystallites to undergo phase segregation. Since the segregated pairs are likely to spread spatially with increased irradiation time, one would expect it to slow down the full recovery in dark.

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Figure 5. (A) Absorption changes representing recovery of CH3NH3PbBrxI3-x film following the CW laser irradiation. (B)Second order kinetic analysis of the recovery traces in (A)

The time-resolved changes in the emission spectra and transient absorption spectra confirm the formation of segregated phases following irradiation of CH3NH3PbI3-xBrx films. As confirmed from both the emission and transient absorption spectroscopy measurements, the phase segregation in mixed halide perovskites occurs with a rate constant of 0.1-0.3 s-1. As shown for several lead halides, halide ion migration constitutes an important property in delivering ionic conductivity.41 The activation energy for halide ion movement is as low as 0.1 eV.23, 42 25-28 Based on the temperature dependence of growth rate of I-rich domains, Hoke and coworkers have obtained activation energy of 0.27 eV for CH3NH3PbI3-xBrx


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films.15, 19 The mechanism of light induced segregation in mixed halide perovskite is a topic of current debate.10 The disappearance of the absorption of parent CH3NH3PbI3-xBrx in the present study (Figure 1A) shows that

Scheme 1. (A) Energy band diagram for mixed halide and phase segregated pairs. Energy level estimates for CH3NH3PbI3 and CH3NH3PbBr3 are based on reference 43. (B) Charge separation in phase segregated pairs and transfer of charge carriers to CH3NH3PbI3 sites. I-rich domains serve as charge recombination sites and thus contribute to the observed emission. at least 18% of the film undergoes transformation to form localized Br-rich and I-rich domains as a result of the movement of halide ions. The mechanism proposed by Hoke et al. for phase segregation involves hole stabilization by I-rich domains which, in turn, provides a driving enthalpy for halide segregation upon illumination.19 Such a large fraction of segregation was also noted through XRD measurements in the same study. However, when we track the absorption at 720 nm (using extinction of 2 x 104 cm-1),19 only about 2% I-rich domains are accounted for. The discrepancy between the two measurements suggests that the segregated regions have different halide composition within the irradiated region. The observed long tail absorption in the red region further supports the argument. Interestingly, we see the localization of charges at the I-rich region with lowest bandgap, thus inducing sharper and brighter emission band at 755 nm (Scheme 1). The low emission yield of Br-rich region and the absence of emission in the 600-700 nm region further confirms the dominance of I-rich region as the charge recombination site. The conduction band in lead halide perovskite, which is formed through a network of Pb 6p orbitals, is less sensitive to the halide ion. UPS measurements indicate the conduction band for the three lead halide perovskites to be around -3.7 eV.43-44 The estimated conduction band energies for the for CH3PbI3 and CH3PbBr3 vary by only 0.09 eV.44-45 Since the conduction band remains isoenergetic, it allows free


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movement of electrons within the network of different halide perovskites. On the other hand the valence band values range from -5.7 to -6.3 eV vs. vacuum. A valence band energy level of -5.4 and -5.9 eV vs. vacuum for CH3PbI3 and CH3PbBr3 and 5.82 for CH3PbCl3 has been observed experimentally for of CH3PbCl3

nanocrystals. 43-44, 46 However the holes get localized in the I-rich sites (represented by CH3NH3PbI3

in scheme 1) as the valence band of CH3NH3PbI3is the lowest among the three perovskite systems. The valence band orbitals are sensitive to halide ion change from 4p to 5p as we transition from Br to I. Walsh and coworkers have determined energies for the highest occupied orbital (or valence band maximum) for the three perovskites as 3.64eV (CH3NH3PbI3) and 3.94 eV (CH3NH3PbBr3) respectively.44 We expect the valence band of the mixed halide (CH3NH3PbI3-xBrx) to lie in between these values. Thus, the low lying Irich state serves as a recombination center. Such a phenomenon is not just unique to lead halide perovskites. For example, transfer of holes from CdS to lower energetic valence band in CdSe in CdSe@CdS core/shell nanorod structures results in the exclusive emission arising from CdSe core.47 Increased length of CdS shell resulted in increased emission from CdSe, thus confirming the electron-hole recombination at the lowest band energy site of CdSe. Barbara and coworkers48 have also observed electronic energy transfer along the polymer chain to a localized defect site, thus funneling excited energy into a single site. The emission originating at 755 nm region of the segregated film confirms that that lowest energy site of I-rich phase serves as radiative recombination center. Such recombination site becomes active only under irradiation and they revert back to the original mixed halide phase upon stopping the illumination. It has been proposed that mixed halide perovskites offer the possibility of continuous tuning of the bandgap by varying the ratio of Br:I. Such a possibility remains attractive in designing gradient perovskite structures or tandem cells for maximizing effective capture and conversion of sunlight in a photovoltaic device. However, one needs to take into account photoinduced segregation effects while evaluating mixed halide perovskites for solar cells or photonics applications and see their impact on long term performance. The localization of charge carriers at the low band energy/trap site remains a major factor dictating overall photoconversion efficiency.


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ASSOCIATED CONTENT Supporting Information. Experimental methods including film preparation and various characterizations (UV-Vis. absorption, photoluminescence, and femtosecond transient absorption spectroscopy) are presented in the Supporting Information. AUTHOR INFORMATION Corresponding Author *Address correspondence to this author:[email protected] twitter: @kamatlabND Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge ND Energy, University of Notre Dame for the seed funding of the project. PVK and MK acknowledges 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 DE-FC02-04ER15533 and DESC0014334. JM acknowledges the support of King Abdullah University of Science and Technology (KAUST) through the Award OCRF-2014-CRG3-2268. This is contribution number NDRL No. 5123 from the Notre Dame Radiation Laboratory. We thank Dr. Donghoon Han for his assistance for AFM measurements and Mr. Weixin Huang for X-ray photoelectron spectroscopic characterization.


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