Ion Migration Heals Trapping Centers in CH3NH3PbBr3 Perovskite

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Ion Migration Heals Trapping Centers in CHNHPbBr Perovskite Supriya Ghosh, Suman Kalyan Pal, Khadga Jung Karki, and Tõnu Pullerits ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00577 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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ACS Energy Letters

Ion Migration Heals Trapping Centers in CH3NH3PbBr3 Perovskite Supriya Ghosh,a Suman K. Pal,a* Khadga J. Karki,b* Tönu Pulleritsb* a

School of Basic Sciences and Advanced Material Research Center, Indian Institute of

Technology Mandi, Kamand 175005, H.P, India. b

Department of Chemical Physics and NanoLund, Lund University, Box 124, 22100, Lund,

Sweden. AUTHOR INFORMATION Corresponding Authors *[email protected], *[email protected], *[email protected]

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Abstract. We investigate the local changes in photophysics at different micro-regions of a methylammonium lead bromide (MAPbBr3) perovskite crystal under illumination. Our results show that the emission from the structurally homogenous region is blue shifted compared to the emission from the inhomogeneous regions. The yield and spectrum of the emission from the structurally homogeneous region does not vary with the illumination time, whereas distinct light induced changes are seen in the spectra from the inhomogeneous region. The changes in the spectra at long illumination time suggest that ion-migration inhibits the emission from the inhomogeneous regions. The measurements of the emission lifetime suggest that the emission from the inhomogeneous regions is dominated by the defect related emission at short illumination times and the band-to-band emission at the longer illumination times. Our work provides a direct evidence for the light induced healing of the defect centers, which is important in the design of photoactive devices of MAPbBr3. TOC

Because of their high photo conversion efficiency1-3 (which already exceeds 22%)4, 5 and easy low cost fabrication,6, 7 lead halide perovskite semiconductors have attracted a lot of attention as highly promising solar cell material.8-11 Such high solar cell efficiencies are attributed to the excellent properties of this material, such as large absorption coefficient, direct band to


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band transitions,1, 3, 12, 13 long carrier lifetimes,14-18 long diffusion lengths19, 20 and high carrier mobility.21,

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Moreover, in recent years, lead halide perovskites have also emerged as

promising materials for potential applications in other optoelectronic devices such as light emitting diodes, photodetectors and lasers.21,23 For approaching highest photovoltaics performance at the Shockley-Queisser limit,24 all nonradiative loss channels have to be minimized in order to obtain high photoluminescence quantum efficiency (PLQE) at the band edge.13,

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Thus, in recent years a number of investigations have focused on improving the

PLQE in methyl ammonium lead halide perovskites by reducing the trap density.26-30 Previous studies have revealed that PLQE increases in the presence of argon as well as nitrogen, and it is further boosted in the presence of oxygen.29, 31, 32 This behavior has been assigned to the passivation of the traps due to the interaction of photogenerated charge carriers and oxygen related species. On the other hand, more recent work by Dane et al.,26 has shown that oxygen may not be important for the passivation. Accordingly, the passivation of the traps, and consequently the enhancement of PLQE, has been attributed to the migration of interstitial halide ion to the defect centres.27 The passivation of the traps as a consequence of the ion migration has also been supported by the observation of a positive correlation between chlorine concentration and the regions of brighter PL in the mixed halide MAPbI3i(Cl)i

system.33 A two photon microscope can be used to selectively excite the different

regions to obtain the corresponding PL spectra. Using two photon microscope to selectively excite different regions of a perovskite crystal we have previously observed variations in the PL spectra at different regions of the crystal. The variations have been attributed to the different contributions from the band edge emission and the emission from the self-trapped excitons.34 Here, we utilize a combination of wide-field microscopy, two-photon photoluminescence spectroscopy (2PPL) and time resolved photoluminescence (TRPL) to investigate the local


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changes in photophysics at different regions of the methyl-ammonium lead bromide (MAPbBr3) perovskite crystal under illumination. We identify structurally homogeneous and inhomogeneous regions in a crystal of MAPbBr3 in the microscope images. We observe that the yield and spectrum of the emission from the structurally homogeneous region does not change with the illumination time, whereas the inhomogeneous region shows distinct light induced changes. Our observations infer that the emission from the homogeneous region is dominated by the recombination of the band edge carriers while the emission from the inhomogeneous region most likely arises from the radiative recombination of trapped carriers. The changes in the photoluminescence yield and the spectrum in the inhomogeneous region can be explained by the quenching of the emission from the trapped carriers and the subsequent enhancement of the band-to-band emission over long illumination time. The decrease in the emission from the carriers in the inhomogeneous regions could be associated with an order of magnitude reduction of the trap density in the inhomogeneous region due to the light induced slow migration of interstitial halide ions to the defects centers. Our work provides a direct evidence for the light induced annihilation of the defect centres leading to enhanced band-to band-PL. Spectrally resolved 2PPL studies. We have used two-photon excitation to selectively excite different regions in a crystal. Since two-photon absorption is a nonlinear process, the PL originates mainly from the focus spot. This allows us to probe the time variation of the PL from highly localized volume. Figure 1(a) shows the schematics of the microscope setup and 1(b) shows the optical image of a crystal surface. In the image, the homogeneous regions, which do not scatter the light, appear dark while the inhomogeneous regions that scatter light appear bright. Figure 1(c) shows the PL spectra scanned along the line indicated in (b). In all the crystals we have measured, the PL spectra from the inhomogeneous regions are distinctly red-shifted compared to that from the


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homogeneous regions. Here, we designate the initial emission from the inhomogeneous region, which has the peak at about 2.23 eV, as low energy emission and the emission from the homogeneous region with the peak at 2.28 eV as high energy emission. We first analyze the PL spectra from the inhomogeneous region acquired after short and long illumination times. Figure 2(a) demonstrates the evolution of the spectra with the illumination time and Figure 2(b) compares normalized PL spectra acquired after 1 min (green curve) and 60 min (blue curve) of continuous illumination. As can be seen in the figure, the PL spectra from the inhomogeneous region after 60 min of illumination resembles that from the homogeneous region with the PL maximum at 545 nm (2.28 eV).

Figure 1. (a) Schematics of the setup for microspectroscopy. (b) A wide field image of a crystal of MAPbBr3. The dark and the bright areas in the image are structurally homogeneous and inhomogeneous regions, respectively. (c) Normalized PL spectra scanned along the line indicated in (b). The spatial resolution of the setup is less than 2 µm.

Figure 2(a) shows the non-normalized PL spectra after 1 to 70 minutes of continuous illumination. The spectra show strong quenching of the low energy emission peak. After 30 minutes of light illumination the emission reaches its minimum, which is followed by the enhancement of high energy emission peak as shown in inset of Figure 2(a). The high energy


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emission stabilizes after 60 minutes of illumination. The total intensity of the 2PPL emission integrated over the spectral range of 500 – 600 nm is shown in Figure 2(d). The PL spectrum from the homogeneous region, on the other hand, does neither show the large initial decrease nor the increase over the long time illumination (Figure 2(c)). Moreover, the spectrum as well as the intensity of the PL is similar to that of the PL from the inhomogeneous region that has been illuminated for more than 60 minutes. As the PL from the inhomogeneous region after long illumination time is dominated by the high energy emission, we conclude that the PL from the homogeneous region has the same origin as PL from the inhomogeneous region after long illumination time. The absence of initial decrease in the PL intensity also indicates that formation of the low energy emissive states is negligible in the homogeneous regions. More importantly, our results also show that photo-response of the inhomogeneous region after a long illumination time is similar to that of the homogeneous region. The large decrease in the PL intensity from the inhomogeneous region before reaching the stable emission indicates that the carriers in low energy states in MAPbBr3 are highly emissive. This is in stark contrast to the emission observed in MAPbI3, where only the increase in the PL yield as a result of light soaking has been observed.29 However, highly emissive low energy “layer-edge-states” have been reported in two-dimensional perovskite thin films.35 Thus, it appears that the low energy emissions are significant only in some of the systems of the halide perovskites.


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Figure 2. Changes of PL over time under illumination at different region of the crystals. (a) Quenching of low energy emission and enhancement of high energy emission with constant irradiation of light over time at inhomogeneous region. Inset shows the enhancement of high energy emission after 30 min of irradiation. (b) Normalized PL acquired at 1 min (green) and 60 min (blue) of constant irradiation at inhomogeneous region. (c) A series of PL measured (1-70 min) at the homogeneous region under light irradiation over time. (d) Integrated PL under illumination over time at inhomogeneous region (black) and homogeneous region (red). Filled 3D circles are data points at inhomogeneous (orange) and homogeneous region (blue).


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Time resolved PL kinetics. We have also studied the different recombination kinetics in MAPbBr3 by using TCSPC. In this experiment, the signal is collected over larger area without separating the homogeneous and inhomogeneous parts of the crystal. In order to distinguish the PL kinetics at different illumination times, measurements are done by recording the traces of the PL over a data acquisition time of 20s, 40s, 60s, 80s, 100s,120s, 180s, 260s, 420s and 600s. Since the light induced changes in the PL dynamics that we observe are reversible, we have recorded the kinetics repeatedly by keeping the sample in dark for about 5 minutes before each recording. The time resolved kinetics with different acquisition times are shown in Figure 3(a). As the total number of photons detected in each measurement is different, we have normalized the kinetics by the illumination time for comparison. The recombination dynamics at the different illumination times show clear differences. In the PL kinetics with the shortest (20 s) illumination time, we observe an initial rise followed by a much slower decay. We have used exponential functions to quantify the growth and the decay time constants. We have only used the data after 1 ns of pulsed excitation since within the apparatus response time a rapid decay with small amplitude, most likely arising from the scattering of the excitation beam, can be observed. From the exponential fit we obtain a growth time constant of about 15 ns and a decay time constant of about 200 ns (Table 1). We assign the growth to the formation of the bound electron-hole pairs at low energy states from the initially excited free carriers. The subsequent decay of the PL is due to the radiative recombination of paired carriers in the low energy states. The growth component, however, does not appear in the kinetics that has been measured with longer illumination time. This is consistent with our steady state measurements, where we observed quenching of the low energy emission with the increase in illumination time. Here, we note that the rise in the signal in the PL kinetics has not been reported before since the illumination times in the previous reports were much longer than 20


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s. Thus, we emphasize that the growth and the decay of the low energy emission can be observed only if the illumination time during the measurements is sufficiently short. The kinetics of the PL with the illumination time of more than 120 s show two decay components. The fast component of about 1 ns and the slow in the 100 ns range. The lifetimes and the amplitudes of the different components obtained from the fitting are given in Table 1. The two decay components have also been observed previously. The short component has been attributed to the recombination of the carriers at high energy states, while the long component to the radiative recombination of the carriers at low energy states. Similar short and long lived emissions for high energy and low energy states, respectively have also been reported in 2D perovskite thin films of MAPbI3.35 The short component is not apparent in early time of illumination. This can be due to the dominant low energy emission at early time illumination. In our measurements, we also observe an increase in the amplitude of the short component and a concomitant decrease in the amplitude of the long component with the illumination time. This further supports the assignment that the fast decay is due to the recombination of the carriers at high energy states and the slow decay is due to the recombination of the carriers at low energy states. We also observe decrease in the PL intensity with the illumination time in our time resolved measurements. Figure 3(b) shows the plot of the average number of photons counted by the detector every second with the illumination time. In the beginning the count rate is 3 × 104 s-1, which decreases monotonically to 2.5 × 103 s-1 after 250 seconds.


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Figure 3. (a) A series of time resolved PL decays from MAPbBr3 film measured under irradiation over time at 20 s (black), 40 s (green), 60 s (blue), 80 s (magenta), 100 s (dark yellow), 120 s (purple), 180 s (olive), 200s (orange), 420 s (cyan) and 600 s (dark gray). (b) Changes of photon counting rate with illumination time. The measurements of the PL spectra and the kinetics of MAPbBr3 at the different illumination times show that the emission from the low energy states is reduced with the time. This is a novel finding, which has not been reported before. Although the dynamics of PL in MAPbBr3 is in contrast to the reported increase in the PL intensity in MAPbI3, 26, 27 where the Table1. Fitting Parameters of PL Kinetics of MAPbBr3 Perovskite at Different Illumination Time Irradiation Time (Sec.)

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observation results from the passivation of the traps by ion migration. The sites that contain low energy states in the perovskites mainly comprise of the halide vacancies with interstitial halide ion.26, 27, 36-40 The structurally inhomogeneous regions have high density of vacancies. Under photo-excitation many of the electrons get rapidly trapped in the vacancies. The trap filling generates an electric field, which changes the equilibrium and induces migration of the halide ions to the positively charged defect centers in organo-halide perovskites and subsequently reduces the trap density.25, 38 Now, whether we observe an increase or a decrease in the PL intensity depends on the emissive nature of the traps. The increase in the PL intensity with the illumination time, which has been observed in the crystals of MAPbI3, indicates that the traps do not emit.25-27,

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However, highly emissive traps on the edges of

layered 2D MAPbI3 have recently been reported.35 Contrary to the general belief, these traps shield the carriers from non-radiative recombination and enhance the PL yield.35 It appears that inhomogeneous regions of MAPbBr3 crystals contain similar kind of highly emissive traps. Hence, the passivation of such traps leads to the decrease in the PL intensity. Based on these discussions, we speculate that the low energy emission in MAPbBr3 arises from the trapped carriers. It is also noticed that the PL quenching or trap passivating rate is faster in Figure 3(b) compare to Figure 2(d). This can be explained by higher beam diameter (100 µm) of pulsed diode laser in TCSPC experiment compare to the beam diameter (1 µm) of pulsed laser source in 2PPL experiment. Higher beam diameter covers a larger area at the surface of the crystal during irradiation causing generation of more electrons, which can fill more traps producing a larger electric field to drive the halide ion from interstitial position to defect centre. This leads to faster trap passivation and PL quenching. On the other hand, Richter et al.42 reported that the dominant recombination channel for bromide perovskite is bimolecular


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band-to-band radiative recombination of carriers even at excitation density

Ion Migration Heals Trapping Centers in CH3NH3PbBr3 Perovskite

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