Photoinduced Emissive Trap States in Lead Halide Perovskite

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Photoinduced Emissive Trap States in Lead Halide Perovskite Semiconductors Silvia G. Motti,†,‡ Marina Gandini,†,‡ Alex J. Barker,† James M. Ball,† Ajay Ram Srimath Kandada,† and Annamaria Petrozza*,† †

Center for Nano Science and Technology @Polimi, Istituto Italiano di Tecnologia, via Giovanni Pascoli 70/3, 20133 Milano, Italy Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy

‡

S Supporting Information *

ABSTRACT: The recent success of lead halide perovskites is given by their optimal primary optoelectronic properties relevant for photovoltaic and, more in general, for optoelectronic applications. However, a lack of knowledge about the nature of instabilities currently represents a major challenge for the development of such materials. Here we investigate the luminescence properties of polycrystalline thin films of lead halide perovskites as a function of the excitation density and the environment. First we demonstrate that in an inert environment photoinduced formation of emissive sub-band gap defect states happens, independently of the chemical composition of the lead halide semiconductor, which quenches the band-toband radiative emission. Carrier trapping occurs in the subnanosecond time regime, while trapped carriers recombine in a few microseconds. Then, we show that the presence of oxygen, even in a very small amount, is able to compensate such an effect.

polycrystalline thin films of lead halide perovskites as a function of the excitation density and the environment. We show, for the first time, that in an inert environment (i.e., vacuum and nitrogen), emissive sub-band gap states are formed upon photoexcitation, which reduces the PLQY (photoluminescence quantum yield). Importantly, this is independent of the nature of the monovalent cation present in the perovskite structure (i.e., the organic methylammonium or the inorganic cesium) and the halide species. Finally, we show that the presence of oxygen, even in a very small amount, is able to compensate such an effect. Figure 1a shows the relative PLQY of methylammonium lead bromide (MAPbBr3) thin films obtained by normalizing the integrated PL intensity by the excitation density, plotted as a function of excitation density, in the range between 1014 and 1018 cm−3. The sample is kept either in vacuum or in ambient atmosphere. While the solid lines show the data collected with increasing excitation density, the dashed lines show the data collected when scanning the excitation density back from high to lower values. In all of the reported cases, we observe low PLQYs at low densities that increase at higher densities. It is generally accepted that at low excitation densities the carrier recombination is dominated by a monomolecular process through trapping,11,23 while with increasing excitation densities, as the trap states are filled, bimolecular band-to-band

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ead halide perovskite semiconductors with ABX3 stoichiometry, where A is generally a monovalent organic cation, (e.g., CH3NH3+, or MA), B is the divalent metallic cation (e.g., Pb2+), and X is the halogen anion, have attracted great attention lately due to their successful embodiment in photovoltaic devices1 and their promising application in light-emitting diodes and lasers.2−4 In both cases, high rates of radiative recombination of carriers are essential for efficient device operation. Such a process competes with trapping of the carriers at crystal defects, a common consequence of solution-based fabrication techniques typically used to produce polycrystalline thin films. The trap-limited recombination is usually investigated by monitoring the quantum yields and dynamics of the photoluminescence (PL).5−8 However, there is large variability in the reported PL measurements, highlighting the intrinsic material instabilities.9−11 The inconsistencies in experimental observations have been mainly related to the electronic and structural changes occurring on different time scales12,13 induced by the experimental conditions such as light soaking,14 the atmosphere,15,16, external bias,17 and the sample microstructure.18−20 In particular, recently, photoinduced ion migration healing the lattice defects has been proposed to explain observations of light-induced enhancement of PL.11,21 At the same time, PL intensity enhancement and longer PL lifetimes have also been reported as a result of interaction of oxygen with the crystallite surface.15,22 To identify the factors driving these instabilities in PL, here we systematically investigate the luminescence properties of © 2016 American Chemical Society

Received: August 13, 2016 Accepted: September 6, 2016 Published: September 6, 2016 726

DOI: 10.1021/acsenergylett.6b00355 ACS Energy Lett. 2016, 1, 726−730

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http://pubs.acs.org/journal/aelccp

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ACS Energy Letters

Figure 1. (a) Relative PLQY curves of MAPbBr3 polycrystalline thin films in air (red) and active vacuum (blue), taken with increasing (solid lines) or decreasing (dashed lines) excitation densities. (b) Time trace of integrated PL under illumination in vacuum, where excitation light is unblocked at t = 0 and the sample is exposed to ambient air at t = 1000 s. (c) Transmission (taken at 480 nm) and (d−f) PL maps of MAPbBr3 isolated crystallites on glass under N2 flow and air.

Figure 2. PL spectra of MAPbBr3 at low (red) and high (blue) excitation densities in (a) forward and (b) reverse order. (c) PL spectra after air exposure. (d) Evolution in time of PL spectra during air exposure. (e) Relative PLQY from 500 to 580 nm (blue) and from 630 to 850 nm (red).

recombination becomes the dominant process. In agreement with these previous reports, we observe that the PLQY increases until Auger-like processes kick in at densities higher than 1017cm−3. However, there are major differences in all of the reported cases, even though the measurement is performed at the same point on the sample. Under vacuum, when cycling back from high to low excitation density, we notice that the curve has hysteretic behavior with the PLQY at each density being much smaller on the reverse scan than that of the forward scan. After the complete intensity sweep, the relative PLQY drops by almost an order of magnitude at an excitation density of 1015 cm−3. This suggests that under photoexcitation there is an increase in the density of traps. It is important to point out that this decrease of PLQY occurs even at low excitation densities, that is, well below 1016 cm−3 (see Figure S1 in the Supporting Information). When the experiment is performed in air, such a hysteretic behavior is strongly attenuated, suggesting minor variation in the effective trap density.

To investigate the time scales over which trap generation occurs, we monitored the integrated PL intensity in time (seconds). Figure 1b shows the evolution of the PL intensity at an excitation density of 1016 cm−3, first under active vacuum, just after the excitation beam is unblocked (t = 0), and then followed by exposure to air. Under active vacuum, we see a quenching of the PL emission over tens of seconds immediately after illumination. As soon as the sample is exposed to air, substantial enhancement of PL emission is observed (while a very small recovery is observed after the sample is left in the dark and vacuum; see Figure S8 in the Supporting Information). Importantly, this effect is also seen when exposing the sample to dry air under illumination. When the sample is exposed to atmospheric air instead, the enhancement is followed by quenching of the PL over several minutes (see Figure S2 in the Supporting Information); thus, water may contribute to the enhancement at an early time24 but is detrimental on longer time scales. Thus, we can assume that oxygen is the main active agent in the enhancement of PL. It 727

DOI: 10.1021/acsenergylett.6b00355 ACS Energy Lett. 2016, 1, 726−730

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ACS Energy Letters

Figure 3. PL decays of the MAPbBr3 thin film with increasing (a) and decreasing (b) excitation density in vacuum and (c) in air. (d) Transient absorption (TA) spectra of the MAPbBr3 thin film, showing a long sub-gap bleach (inset). (e) TA dynamics at 530 nm in vacuum (blue) and air (red).

atmospheric and illumination conditions. In particular, Figure 3a shows the PL decays of the 530 nm band under active vacuum, from low to high excitation density. The trend, at a first glance, is consistent with the trap-limited recombination mechanism with a monoexponential PL decay at low excitation density and intensity-dependent dynamics at higher laser fluences due to larger contribution from bimolecular recombination.14,9 However, when the experiment is performed from high to low fluences (Figure 3b), the PL lifetimes remain short through the whole range of excitation densities employed. This scenario, fully consistent with the experimental data shown so far, leads to the conclusion that photoexcitation, in an inert atmosphere, induces the formation of deep trap states that quench the band-to-band emission on the hundreds of picoseconds time scale. It is worth highlighting that the shortening of the lifetime upon increased excitation intensity is a convolution of the transition toward band-to-band bimolecular recombination dynamics together with deep trapping of the carriers. Therefore, care must be given when evaluating the recombination parameters of the semiconductor using intensity-dependent studies. Figure 3c shows the PL dynamics measured after the sample is exposed to air. Data are always recorded from the same spot on the sample, from high to low excitation density, after a few minutes of stabilization of the PL enhancement, during which the sample is kept under illumination. The lifetimes after air exposure increase by over one order of magnitude, reaching several nanoseconds at low excitation density. The intensity dependence of the dynamics recovers the trend seen on a fresh sample as in Figure 3a, shifting from trap-limited monomolecular decay to bimolecular recombination with increasing excitation density. Furthermore, the observed dynamics in air become stable and independent of the order of measurement. While the PL decays provide further evidence of the increase of trap density driving the quenching of band-to-band emission and faster trapping process, they do not allow us to monitor the recombination of the trapped carriers due to their very small yield. Thus, transient absorption (TA) measurements, a spectroscopic technique that is sensitive to both emissive and

must be noted that such a process is so sensitive that all of the measurements in vacuum are performed under 10−5 mbar and with a constantly running pump as we observed that the sealed chamber could not maintain enough quality of the vacuum to completely exclude the enhancement effects. The microscopy maps shown in Figure 1 provide further visualization of this effect. In Figure 1c, we show the transmission map for light with an incident wavelength of 480 nm taken from isolated, micrometer-sized MAPbBr3 crystallites. In Figure 1d, we map the PL under N2 flow, which results in very low signal. When the crystallites are exposed to air, they gradually become more luminescent (Figure 1e). Interestingly, when the sample is brought back under N2 flow, the PL is quenched again (Figure 1f), pointing out the reversibility of the enhancement, as reported previously.22,15,16 In the above analysis, we monitored the integrated PL intensity. In Figure 2, we show the changes in the PL spectrum when increasing (Figure 2a) and then decreasing (Figure 2b) the excitation density in a range between 1015 and 1017 cm−3. We initially observe a narrow emission band peaking at 530 nm, associated with band-to-band recombination. However, when measuring in the reverse order, the PL spectrum recorded upon low excitation density reveals the presence of a broad emission band centered at around 600 nm, which becomes even more intense at lower temperatures25 (see Figure S3 in the Supporting Information). On the other hand, this broad subband gap emission is strongly quenched within a few seconds when exposed to air, while the band-to-band emission recovers its intensity (Figure 2c,d). The relative PLQY of the low-energy band as a function of the excitation density is shown in Figure 2e, and it presents different behavior with respect to the main emission band (also shown in the same figure). While the latter increases as the traps are filled, the sub-band gap band reduces in the relative intensity. This confirms that this new emission band originates from emissive trap states. To further shed light on these drastic changes in steady-state PLQY, we have also investigated the recombination dynamics over picosecond to microsecond time scales, under different 728

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ACS Energy Letters nonemissive species,25 are performed in the nanosecond to microsecond regime. Before the acquisition, the sample is stabilized in vacuum under illumination corresponding to an excitation density of 1017 cm−3. Figure 3d shows the TA spectra obtained in vacuum. They present a positive band, peaking at 525 nm, which can be assigned to the transparency (photobleach, PB) induced by the population of the bottom (top) of the conduction (valence) band by the photoexcited carriers; thus, its dynamics follow those of both the electrons and holes at the band gap. In the TA spectrum, the PB band is seen to extend as a weak tail to longer wavelengths (inset of Figure 3d) that corresponds to the filled trap states. The dynamics of the main bleach, probed at 525 nm, (Figure 3e) shows an initial decay that is completed in a few nanoseconds and a slow component extending over the experimental limit of 3 μs. While the first component follows the carrier trapping dynamics as well as the bimolecular radiative recombination dynamics, as seen in the PL decays, the slow component can be assigned to the recombination dynamics of the trapped carrier with the free carrier of opposite sign. After exposure to air, the fast component of the dynamics displays a longer lifetime than in vacuum, suggesting reduced trapping, and the slow component is quenched due to lower density of the trapped carriers. This behavior, beyond providing further evidence of the passivation of trap states by oxygen, allows us to retrieve a trapping time scale shorter than a few nanoseconds and a recombination time of trapped carriers on the order of a few microseconds. We highlight that the photoinduced formation of trap states in an inert environment (as well as their healing in oxygen) is observed not only for MAPbBr3 but also for MAPbI3 and CsPbBr3 polycrystalline films. In Figure 4, we show the trend of the relative PLQY as a

from MAPbI3 samples due to experimental limitations in the NIR spectral region. To summarize, we observe generation of carrier trap states upon photoexcitation in vacuum, and importantly, they display characteristic sub-band gap emission. The increased trap density can be perceived in both the PL and TA dynamics as an increase in trapping rate. Upon exposure to oxygen, we observe passivation of these defects leading to a substantial enhancement of the PL and slowing of the recombination dynamics. It is interesting to note that the enhancement of the PL in oxygen happens even in samples exposed to oxygen for a long time prior to the photoexcitation. This suggests that a combination of oxygen and photoexcitation is needed for effective defect passivation, which may be an effect of defect deactivation occurring by the reaction of oxygen molecules with trapped carriers. A combination of theoretical and experimental investigations to rationalize such observation is still needed. In conclusion, the data reported reveal for the first time the presence of intrinsic photoinstability of lead halide perovskite semiconductors, independent of their chemical composition, through the formation of emissive sub-band gap states under photoexcitation in an inert atmosphere. Carriers get trapped on a time scale below the nanosecond time regime and then recombine in microseconds. Importantly, this increase in trap density is attenuated if the sample is exposed to air, which suggests the possibility of treating the perovskite films under oxygen as well as applying alternative methods of encapsulation, allowing for improved device stability.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00355. Experimental methods, XRD and SEM sample characterization, and supporting steady-state and time-resolved PL measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results received funding from the European Union Seventh Framework Programme [FP7/20072013] under Grant Agreement N° 604032 of the MESO project. S.G.M. thanks the CNPq (Conselho Nacional de Desenvolvimento Cientı ́fico e Tecnológico - Brasil) for the scholarship [206502/2014-1].

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Figure 4. Relative PLQY curves of (a) MAPbI3 and (b) CsPbBr3 polycrystalline thin films in active vacuum, taken with increasing (solid lines) or decreasing (dashed lines) excitation densities.

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DOI: 10.1021/acsenergylett.6b00355 ACS Energy Lett. 2016, 1, 726−730