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Mobile Ion Induced Slow Carrier Dynamics in Organic-Inorganic Perovskite CH3NH3PbBr3 Sheng Chen, Xiaoming Wen, Rui Sheng, Shujuan Huang, Xiaofan Deng, Martin A. Green, and Anita Wing-Yi Ho-Baillie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12376 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 14, 2016

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ACS Applied Materials & Interfaces

Mobile Ion Induced Slow Carrier Dynamics in Organic-Inorganic Perovskite CH3NH3PbBr3 Sheng Chen, Xiaoming Wen*, Rui Sheng, Shujuan Huang, Xiaofan Deng, Martin A. Green and Anita HoBaillie Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable Energy Engineering, University of New South Wales Sydney, New South Wales, 2052, Australia KEYWORDS: perovskite, slow carrier dynamics, mobile ions, non-radiative recombination, light soaking

ABSTRACT: Here we investigate photoluminescence (PL) and time-resolved photoluminescence (TRPL) in CH3NH3PbBr3 perovskite under continuous illumination, using optical and electro-optical techniques. Under continuous excitation at constant intensity, PL intensity and PL decay (carrier recombination) exhibit excitation intensity dependent reductions in the time scale of seconds to minutes. The enhanced nonradiative recombination is ascribed to light activated negative ions and their accumulation which exhibit a slow dynamics in timescale of seconds to minutes. The observed result suggests that the organic-inorganic hybrid perovskite is a mixed electronic-ionic semiconductor. The key findings in this work suggest that ions are photo-activated or electroactivated and their accumulation at localized sites can result a change of carrier dynamics. The findings are therefore useful for the understanding of instability of perovskite solar cells and shed light on the necessary strategies for performance improvement.

INTRODUCTION Organometal halide perovskites have attracted great attention for photovoltaic application due to their strong light absorptions, long diffusion lengths, tunable band gaps, good carrier mobility and ease of fabrication.1-8 Compared to the other types of solar cells, perovskite solar cells have rapidly improved in their power conversion efficiency (PCE) from 6.5% to 20.1% in the past three years.9-10 Despite these attributes, one of the most challenging issues is the unstable device performance, manifested in the (i) changes of light current-voltage (I-V) characteristics during and after light soaking as well as (ii) hysteresis in the I-V characteristics displaying inconsistent I-V curves when they are measured under different voltage sweeping conditions.11-16 Light soaking was also shown to enhance or worsen cell performance.17-18 Both effects have demonstrated slow response time up to minutes, which are significant drawbacks to device performance.12, 19-20 Even though the unstable device performance has always been observed, mechanisms causing such phenomenon are still a matter of dispute. Various explanations have been proposed such as ion migration;12-13, 21-24 trapping and de-trapping of charges;25-28 and ferroelectricity.29-32 More recently, Tress et al. studied the role of charge trapping and ferroelectricity in hysteresis.24 They concluded that ion migration with similar time scale appears to contribute to the hysteresis of the perovskite solar device. Wen et al. observed the fluorescence intermittency in CH3NH3PbBr3 and proposed that mobile ions are responsible for such effect and have an impact on the carrier dynamics of perovskites.33 Xiao and co-

workers proposed that the drift of mobile ions induce a reversible formation of p-i-n junction and is responsible for the field-switchable photocurrent direction in perovskite.34-35 On the other hand, computational first principle studies 21, 23, 36 proposed that I-, CH3NH3+, Pb2+ and H+ are candidates for the slow dynamic responses due to their higher activation energy. There has been work using optical techniques such as steady-state photoluminescence (SSPL) and time-resolved PL (TRPL) and transient absorption spectroscopy (TAS) 5, 37-40 to study the carrier dynamics in perovskites including defect trapping, electron-hole recombination and Auger recombination in nanosecond scale, which are dependent on the carrier density as well as the intrinsic properties of the perovskite.4143 A rate equation 44-45 can be used to describe these carrier dynamics over a range of excitation intensity − = An + Bn + Cn , where n is the photo-excited carriers density and t is time. The first term represents defect assisted recombination. The second term refers to electron-hole recombination and the third term relates to Auger recombination. Although the ion migration is the plausible origin for the unstable device performance, the role of ion in perovskite is still unclear. Different from previous work, here we study the mechanism of mobile ions induced slow carrier dynamics. The study of time dependent SSPL and TRPL are in the timescales of seconds to minutes, appear as PL reduction relevant to excitation (laser) intensity. By associating optical imaging with or without bias voltage, we first show that only negative ions, rather than positive ions, can significantly facilitate the non-

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ACS Applied Materials & Interfaces radiative recombination of free carriers (electrons and holes), which indicates the organic-inorganic hybrid perovskite is a mixed electronic-ionic semiconductor. In addition, the timescale of the ion induced non-radiative recombination carrier dynamics observed in this work share similar timescales as the hysteresis effect and light soaking effect on I-V characteristics suggesting mobile ions are responsible for unstable device current voltage performances.

carriers. Using single exponential function, a time constant of 100 s was estimated for each of the PL time traces in Figure 1 (a). Although the degree of PL decrease is evidently dependent on the excitation intensity, it is interesting to note that the observed time constant for PL intensity decrease is independent on the excitation intensity.

RESULTS AND DISCUSSION

To investigate carrier recombination in longer time scales (seconds rather than nanoseconds), we performed a timedependent (millisecond resolution) PL for the CH3NH3PbBr3 sample. Firstly, SSPL intensity was performed which involves the monitoring of the SSPL every 28 seconds during the 400 seconds of measurement under continuous illumination. The excitation density was then increased and another set of time depending SSPL measurement was performed again. The four sets of measurements were performed under continuous illumination by a 470 nm laser with different excitation levels varying from 20 mWcm-2 to 2000 mWcm-2. Between each set of the measurement, the sample was left in the dark for 120 seconds to ensure the sample recovered from the effect of the previous illumination. The four normalized SSPL intensity plots as a function of time under various continuous excitations are shown in Figure 1 (a). It is evident that light soaking (continuous excitation) results in a decrease in SSPL intensity which worsens with increasing excitation levels. As shown in Figure 1 (b), under low excitation Iex (stars) the SSPL intensity exhibits a slight decrease by 18% within 400 seconds, while the SSPL intensity decreases by as much as 80% within 400 seconds under the highest excitation intensity. It should be noted that the PL decrease happens in a much longer timescale than the typical recombination time of photo-generated

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The samples used in this study are CH3NH3PbBr3 fabricated by vapour –assisted deposition 46 on a glass substrate in the absence of hole transport and electron transport layers. The details of the synthetic method are described in the supplementary information (SI). The x-ray diffraction (XRD) pattern of the CH3NH3PbBr3 film in Figure S1 shows good crystallinity. The SEM images in Figure S2 show a CH3NH3PbBr3 perovskite film with densely packed grains with sizes range from 200 nm to 400 nm. Both the absorption coefficient and the PL spectrum exhibit a peak at 536 nm as shown in Figure S3, consistent to other observations.47-49 There is no obvious wavelength shift at different excitation intensities, see Figure S3 (b). Both time-dependent PL and time-resolved PL were performed on a Microtime-200 (Picoquant) system, which was equipped with an inverted microscope containing an air objective (NA 0.7 20X). All the PL measurements were performed under the room temperature 296 K and humidity was less than 50 % humidity. It should be noted that CH3NH3PbBr3 film does not degrade and can be measured repeatedly for experiment that involve different excitation intensities. Figure S4 shows a result of PL recovery test for the CH3NH3PbBr3 sample that went through cycles of dark and illumination. Although PL intensity was observed to reduce during illumination, it could be recovered in the dark.

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Figure 1. (a) Normalized time-dependent SSPL under continuous excitation as a function of time measured in 400 seconds and (b) PL intensity remainder at 400 s as a function of excitation intensity.

Using time correlation single photon counting (TCSPC) technique, the TRPL of CH3NH3PbBr3 was measured consecutively for ten times (each integral time of 60 s, with an excitation of 470 nm pulsed laser at 5 MHz) under continuous excitation at constant excitation intensity. The effect of excitation intensity at 50; 90; 250; and 500 mWcm-2, was investigated and the results are shown in Figure 2 (a); (b); (c) and (d) respectively. A suitable neutral density filter was used to attenuate the emission, ensuring that single photon was detected during the high excitation measurement. Under very low excitation (50 mWcm-2), the TRPL traces are very similar through the entire illumination period of 540 seconds, as shown in Figure 2 (a). In contrast, the PL decay rate exhibits significant increase under higher continuous excitations (> 90 mWcm2) during the 540 seconds of meas-


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urement (Figure 2 (b)). The rate of PL decay increases with increasing the excitation intensity, see Figures 2 (c) and 2 (d). As the variation of the SSPL intensity and time-dependent TRPL observed occur over a period of minutes under continuous excitation, it cannot attributed to photo-generated carrier dynamics.

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The slow dynamics of perovskite based devices have been studied by electric and optical techniques and some possibilities were proposed: such as (1) trapping and de-trapping of charge;25 (2) light induced defect annihilation (curing, healing);43, 50 (3) ferroelectricity;29-32 and (4) ion migration.12-13, 22-24, 51 PL intensity enhancement has been observed by either continuous illumination 52-54 or increased laser excitation,55 ascribing to decreased defect trapping due to saturation of defects state. It usually occurs in the relatively low excitation regime, dominated by the competition between defect trapping and electron/hole recombination and Auger effect exhibit minor effect. In our case the PL intensity exhibits reduction (Figure 1(a)) and the defect trapping is only dominant under the low excitation in nanosecond timescale, Figure 2 (a), therefore (1) and (2) can be excluded. Clear evidence for the correlation between ferroelectricity and PL variation is lacking. All our investigations were in the continuous illumination condition and the sample was recoverable, Figure S4, implying no irreversible structure transformation under light soaking. Spectral response of PL to illumination intensity remains stable, Figure S3 (b), implying no irreversible structure transformation under light soaking. Similar slow behaviour was observed in light current-voltage characteristics of perovskite solar cells which exhibit hysteresis as a result of mobile ion migration in response to potential difference set up across the device by illumination.13, 24 Hence, we argue that the ultraslow carrier dynamics observed in PL variation under continuous excitation originates from the mechanism of light activated ion migration and accumulation.

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To clarify the mechanism for the observed slow dynamics, an optical imaging on a specially designed perovskite structure was performed, as shown in Figure 3 (a). CH3NH3PbBr3 was deposited using the same vapour –assisted method on the FTO layer, where a 25 µm wide ditch was made by laser etching and a bias voltage was applied on the Au electrodes. A recent publication has proposed the ions are generated from decomposition of MAPbI3 under the electric field.56 Figure 3(b) and 3(c) compared the images without and with bias (0.5Vµm-1 for 6 mins). It should be noted that the dark region in the anode side (Figure 3 (c)) represents a lower PL intensity due to ion accumulation induced nonradiative recombination.33 It is also confirmed that significantly shorter lifetime in the dark region. It is necessary to emphasize that the PL quenching only occurs under the two conditions that it happens (1) in the anode side and (2) the electric field increases to 0.5 Vμm-1. The first condition indicates only negative ion quenches the PL in this case. It should be noted that even increasing bias up to 1.0 Vμm-1, we did not observed PL quenching on the side of cathode. The latter implies that the sufficient energy is required to activate the ions to be mobile.

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Ions have been studied theoretically and experimentally in the perovskite film.21-24 Azpiroz et al. 23 suggested that the CH3NH3 and Pb vacancies (with activation energy of ~ 0.5 and


ACS Applied Materials & Interfaces 0.8 eV) are responsible for the slow processes with migration time of tens of milliseconds to minutes. In addition, Eames and co-workers proposed that I- with a diffusion coefficient of 10-12 cm2s-1 (activation energy of 0.58 eV), four orders of magnitude higher than the value of 10-16 cm2s-1 for CH3NH3+ (activation energy 0.84 eV), are likely to be the dominant species for ion induced mechanisms as CH3NH3+ negligible diffusion compared. 36 In our case, negative charges are more likely Br . When applying sufficient electric field, the negative ions can drift toward and accumulate on the anode side, which results in PL quenching. It is interesting to note that the positive ions do not result in the PL quenching effect, probably due to its much low mobility.

10V.35 These correlations suggest that ions are responsible for the observed effects in these perovskites. To obtain the recombination rate (Kion(t)) induced by the mobile ions alone, Equation (1) can be used which involves subtracting the initial recombination rate, 1/τeff (t=0), from the effective recombination rate measured at time t, 1/τeff (t). The initial effective lifetime is of photo-generated carriers with minimum influence from mobile ions. As excitation increases, so do the density of activated ions and non-radiative recombination resulting in faster PL decay:57-59 K ! "t$ =

%&& "$

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Figure 4 (b) shows the ion induced recombination rate as a function of illumination time. Under high excitation, the more ions are activated and therefore the rate of ion induced recombination is higher than those under lower excitations and increases over illumination time. In contrast, under very low excitation (50 mWcm-2), the density of activated ions are not sufficient to induce nonradiative recombination. Similarly current-voltage hysteresis and degradation under light soaking for perovskite solar cells in the same timescales have been observed and have been attributed to the migration of mobile ions20 which are activated under light illumination. These observations point to the role of ion migration in I-V hysteresis as well as slow carrier dynamics suggesting organicinorganic hybrid perovskites have the characteristics of electronic- ionic semiconductors.60

Figure 3. Schematic of (a) sample structure with bias voltage; (b) Optical image without bias voltage; (c) Optical image with bias voltage.

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lifetime as a function of illumination time for different excitation intensities. The effective lifetime does not decrease with time under low excitation (50 mWcm-2); but decreases dramatically over time under higher excitations when the effect of mobile ions becomes significant. Using a single exponential decay function we fit the 500 mWcm-2 lifetime decay trace in Figure 4 (a) and obtained a time constant of 83 seconds. This is in the same timescale as the time constant (100s) from the time-dependent SSPL as discussed above. Recently, a time constant of 90 seconds was observed from a photovoltaic effect after poling the perovskite film with a positive bias of

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Figure 4. (a) The effective lifetimes from the TRPL traces in Figure 2 and (b) the ion induced recombination rate as a function of illumination time under various excitations of 50 (black), 90 (red), 250 (blue) and 500 mWcm-2

One interpretation of the observed reduction in PL intensity under light soaking in this work is mobile ion-induced enhanced non-radiative recombination, as illustrated in Figure 5. Under dark, ions are randomly distributed and relative immobile. At the beginning of illumination (t=0 s), electrons (red dots) and holes (light green dots) in Figure 5 are generated with the minimum influence from ions as shown in our results in Figures 1 (a) and 2 (a). Increased illumination intensity and duration facilitate the activation and movement of these ions, which eventually accumulate at interfaces such as the grain boundaries.23, 33, 61 The movement and accumulating of these ions in turn induce the photo-generated carriers to recombine non-radiatively before being extracted for energy conversion. Therefore significant reduction in PL intensities (Figure 1 (b), Figures 2 (b)-(d)) and lifetimes (Figure 4 (a)) are observed. The process how the ions are activated is still an unsolved question. Basically, the defects of Br- are localized with activation energy hundreds of meV. At room temperature the activation probability with only thermal energy is extremely small. Based on our and published observation,33 we suspect that ions can obtain energy by phonon coupling. It is expected that many kinds of phonons are well activated in perovskite with illumination and thus activate the localized Br-. In the case of electric field applied, the energy barrier is decreased significantly and the probability of ion activation increases.56 It is important to note that ion-induced nonradiative recombination is characteristically slow with a time constant of around 100 second obtained from both the SSPL and TRPL. These time constants are independent of the excitation intensity as they are dependent on the ion “lifetime”. Further investigation is required for understanding the activation mechanism (that may be related to phonon coupling and/or thermal energy for example) and for quantifying the density of the ions and their impact on perovskite solar cell performance.

optical techniques. Under continuous illumination, both steady state PL intensity and time-resolved PL decay exhibit consistent variation in the timescale of seconds to minutes. The observed effect can be ascribed to the enhanced nonradiative recombination, correlated with ion migration and accumulation. The optical images with voltage bias further support this conclusion. Moreover, the optical images show evidence of the non-radiative recombination is only caused by negative ions. As long as 100 seconds of response time suggests such mobile ions, rather than photogenerated electrons and holes, are responsible for the observed carrier dynamics. The migration of these mobile ions and accumulation in turn facilitates the photogenerated electron-holes to recombine non-radiatively. It clearly shows that ions are photo-activated or electro-activated rather than pre-existed in the perovskite system. This study indicates that the organic-inorganic hybrid perovskite is a mixed electronic-ionic semiconductor. This finding suggests the photo-activated ion migration and their accumulation in the perovskites play a key role in currentvoltage hysteresis and current-voltage instability under light soaking. The findings are therefore useful for the understanding of perovskite solar cells and shed light on the necessary strategies for performance improvement.

ASSOCIATED CONTENT Supporting Information: Experimental section, SEM images of CH3NH3PbBr3 perovskite film, PL and absorption spectra and PL recovery experiment. This material is available free of charge via the Internet. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-based activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA).

REFERENCES

Figure 5. Schematically illustration of photo-generated carriers and mobile ions (a) before and after illumination under (b) low excitations & (c) high excitations

CONCLUSIONS We have investigated the carrier dynamics in a vapourassisted fabricated CH3NH3PbBr3 perovskite film in the timescale of hundreds of seconds, using PL, TRPL and electro-

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