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Nature of Photo-induced Quenching Traps in Methylammonium Lead Triiodide Perovskite Revealed by Reversible Photoluminescence Decline Daocheng Hong, Yipeng Zhou, Sushu Wan, Xixi Hu, Daiqian Xie, and Yuxi Tian ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 11 Mar 2018 Downloaded from http://pubs.acs.org on March 11, 2018
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Nature of Photo-induced Quenching Traps in Methylammonium Lead Triiodide
2
Perovskite Revealed by Reversible Photoluminescence Decline
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Daocheng Hong†, Yipeng Zhou†, Sushu Wan, Xixi Hu*, Daiqian Xie and Yuxi Tian*
4 5
Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and
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Chemical Engineering and Jiangsu Key Laboratory of Vehicle Emissions Control,
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Nanjing University, Nanjing, Jiangsu, 210000, China
8 9
ABSTRACT
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Photoluminescence (PL) of organometal halide perovskite has been broadly
11
investigated as a fundamental signal to understand the photophysics of these materials.
12
Complicated PL behaviors have been reported reflecting to complex mechanisms
13
including effects from crystal defects/traps whose nature still remains unclear. Here in
14
this work we observed, besides the PL enhancement, a surprising PL decline
15
phenomenon in methylammonium lead triiodide (CH3NH3PbI3) perovskite showing a
16
high initial PL intensity followed by a fast decline in time scale of milliseconds to
17
seconds. The similarity between the PL enhancement and PL decline suggests both
18
processes are due to PL quenching traps in the material. Combining experimental and
19
theoretical results, two interstitial defects of Lead and Iodide were identified to be
20
responsible for the PL enhancement and PL decline, respectively. Both traps can be
21
switched between active and inactive states leading to reversible process of PL
22
enhancement and PL decline. This is the first time to get identification on the
23
chemical nature of the PL quenching traps and take an important step towards fully
24
understanding the crystal defects in these materials.
25 26
KEYWORDS: perovskite, CH3NH3PbI3, photoluminescence, decline, nature of
27
defects
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Due to excellent performance as photo-voltaic and photo-emitting materials,
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organometal halide perovskites (OMHPs) have become one of the hottest materials
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and attracted broad interest in the last several years.1–3 Besides improving the power
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conversion efficiency and performance of the devices, the materials have also been
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investigated to understand their fundamentals. During the investigations, many
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different technologies have been employed including transient absorption,
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time-resolved terahertz, microwave conductivity, and photoluminescence (PL).4–14
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Comparing to other methods, the PL signal is easy to detect and directly relates to the
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photo-generated species in the materials. Thus the PL of OMHPs has been used as an
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important signal to investigate the fundamental photophysical properties and
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processes after one-photon or two-photon excitation, e.g. the estimation of charge
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diffusion length,4,5,13,14 the measurement of the exciton binding energy,8 the
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mechanism of the photo-degradation and the estimation of the defects’
14
concentration.12,15
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Many unusual PL behaviors have been observed in these materials. PL blinking
16
with extremely large amplitude of CH3NH3PbI3 nanoparticles showed that a single
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trap can control the PL of entire nanocrystal with 105 nm3 due to efficient charge
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diffusion.12 Light soaking effect was observed showing large enhancement of the PL
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intensity under light irradiation.11,16 The PL intensity of OMHPs has also been
20
observed to nonlinearly depend on the excitation power indicating quantum efficiency
21
dependence on the excitation.17 Atmosphere has significant effect on the PL of
22
OMHPs and can even control the PL of the materials.16,18,19 Although the
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photophysical mechanism of the PL is still not fully understood, different species
24
could contribute to the PL emission such as excitons and traps depending on
25
experimental conditions,7,20 recombination between electron and hole is still the main
26
source of the PL emission in CH3NH3PbI3 under room temperature.8,21–25 Crystal
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defects/traps are believe to play important roles in most of these PL
28
behaviors.7,11,12,16,17,26–31 However, due to the unknown nature of these traps, all these
29
unusual PL behaviors are still not fully understood. Experimental characterization of
30
the chemical nature of the defects is very difficult because the concentration of the 2
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defects is extremely low. Theoretical calculation contributed the most and listed many
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possible crystal defects including deep and shallow defects which are believed to be
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efficient and inefficient quenching defects, respectively.32,33 However, the range of the
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candidates is still too broad due to limited experimental parameters. More
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experimental results related to the crystal defects are necessary to further approach
6
their nature.
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In this work, by using a home-built wide-field fluorescence microscope, we
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investigated the PL of CH3NH3PbI3 micro-crystals with hexagon shape. Besides the
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PL enhancement effect which was also called “light soaking” or “light brightening” as
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reported by several groups,11,16,34,35 we found a new interesting phenomenon in
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CH3NH3PbI3 micro-crystals. When the excitation light was switched on, high PL
12
intensity was observed followed by a fast decline to a steady state level. The fast PL
13
decline processes were completely reversible by keeping the crystal in dark for tens of
14
seconds which is different from the well-known photo-degradation process in this
15
material. We investigated the PL decline behaviors from different aspects including
16
PL spectra and lifetime, reversibility of the PL decline as well as theoretical
17
calculations. From both experimental and theoretical results, interstitial defects of I
18
and Pb were identified to be responsible for the PL enhancement and PL decline,
19
respectively. The total PL intensity is determined by activation/inactivation
20
equilibrium between the two states of these two defects.
21 22
EXPERIMENTAL SECTION
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Sample preparation. The CH3NH3PbI3 was prepared by the one-step method
24
from reaction between equimolar methylammonium iodide (CH3NH3I) and lead
25
iodide as reported.36 Briefly, the CH3NH3I was synthesized by dropping hydroiodic
26
acid (Sigma-Aldrich, 57wt% in water) into the stirred solution of methylamine
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(Sigma-Aldrich) in ethanol at 0℃. After that, the mixture was stirred for about 1 h,
28
and the solvent was then evaporated. The diethyl either was used to precipitate the
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CH3NH3I. Pure CH3NH3I was obtained through further recrystallization. PbI2 and
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γ-butyrolactone were purchased from Sigma-Aldrich and Aladdin, respectively, and 3
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used directly. First we dissolved PbI2 powder into γ-butyrolactone solution and stirred
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for 60 min at 80 ℃. Then we added equimolar CH3NH3I into the PbI2 solution and
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kept stirring for 60 min at 80℃. The crystals studied in this work were prepared by
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direct drop-casting of the diluted CH3NH3PbI3 (0.5 mol/L) solution on cleaned glass
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cover slips followed by an annealing process for 20 min at 80℃.
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PL measurement. The PL of the crystals was analyzed by a home-built
7
wide-field microscope (see Figure S1 in Supporting Information). Briefly, a 532 nm
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diode CW laser was used as the excitation source and focused above the sample plane
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by a dry objective lens (Olympus LUCPlanFI 40×, NA=0.6). The luminescence signal
10
was collected by the same objective lens and detected by an EMCCD camera after
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passing through a 550 nm long-pass filter (ET550lp, Chroma). The PL traces were
12
measured by taking movies consisting of 500-10000 frames and the exposure time per
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image was 100 ms. A transmission grating (Newport, 150 lines/mm) was put in front
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of the camera for the spectra measurement. PL lifetime measurement was done by
15
using a single photon counting system (TCSPC, Picoharp 300) with 532-nm
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excitation light from super continuous laser (Fianium SC-400). The atmosphere was
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controlled by putting the sample in a sample chamber which will be filled with
18
different gases flow (oxygen or nitrogen).
19
Computational details. All the theoretical computations were performed by
20
using the Vienna Ab initio Simulation Package (VASP) based on plane-wave density
21
functional theory (DFT) method.37–39 The interaction between the ionic cores and
22
electrons was described using the projector-augmented wave (PAW) method,40 and the
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Kohn-Sham valence electronic wavefunction was expanded in a plane-wave basis set
24
with a kinetic energy cut-off of 520 eV. The exchange-correlation effects were
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represented by using the non-local optB86b-vdW functional which takes into account
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van der Waals interactions.41–44 The reason why we chose this functional was given in
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the section 2 of the Supporting Information. Valence states included the 6s, 6p, and 5d
28
states of Pb, 5s and 5p states of I, 2s and 2p states of C and N, and 1s state of H.
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Brillouin-zone integrations were performed using Monkhorst-Pack grids of special
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points with 4×4×4 and 8×8×8 meshes for the tetragonal CH3NH3PbI3 unit cell 4
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calculations of structural and electronic properties, respectively. For the calculation of
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defective system, a 2×2×2 supercell, including 384 atoms, was constructed with the
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optimized lattice constants. And a Γ point was used for supercell structural and
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electronic calculation to ensure that the defect levels and transition energy levels can
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be correctly described.45 The initial structure of the CH3NH3PbI3 unit cell has been
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determined by relaxing both cell and ions positions while the supercell geometry was
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optimized with all atomic positions relaxed. The structural optimization was
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considered converged for the unit cell (or supercell) if the forces acting on the atoms
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were less than 0.01 (or 0.05) eV/Å.
10 11
RESULTS AND DISCUSSION
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Photoluminescence enhancement and decline. In OMHPs, PL enhancement has
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been observed by many different groups from different samples.46–50 The PL
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enhancement can be induced by light or chemical treatment.7,11,16,18,26 It is well
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accepted that the PL enhancement is related to the change of crystal structure and ion
16
migration.16,47,49–52 In this work, we prepared monocrystalline CH3NH3PbI3
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microcrystals with hexagon shape (Figure 1 inset). The morphorgies and lattice
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diffraction of the micro-crystals characterized by scanning electron microscopy (SEM)
19
and transmission electron microscopy (TEM) revealed the relative high crystal quality
20
(Figure S3, Supporting Information). The PL behaviors of these microcrystals are
21
investigated under fluorescence microscope. The PL spectra kept the same during the
22
PL enhancement while the lifetime increased along the PL enhancement. These results
23
are in agreement with the reported results. We thus conclude that the PL enhancement
24
process in these microcrystals is due to the same processes as reported.16,47,49–52 As
25
shown in Figure 1 (dash line), we observed the photo-induced PL enhancement on the
26
CH3NH3PbI3 microcrystals (Figure 1 inset) similar as reported. Surprisingly, besides
27
the PL enhancement, we observed a fast PL decline following the initial high PL
28
intensity as shown in Figure 1 (solid line). This fast PL decline happens in time scale
29
of milliseconds to seconds and finally reaches a stable level. The PL decline rate
30
depends on the excitation power and also varies from crystal to crystal, or even it can 5
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be different from place to place on the same microcrystal.
2 3
Figure 1. Photoluminescence enhancement (dashed line) and decline (solid line) in a
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hexagonal CH3NH3PbI3 microcrystal.
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PL intensity decrease has been observed in many studies due to photo-induced
7
degradation of the materials. Although the stability of these materials is one of the
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main problems to be solved, the degradation processes are still in much longer time
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range normally over hours or days depending on the experimental conditions
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(atmosphere and humidity).53–55 Spectral shift was also observed during the
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degradation which was not observed in this work15 and the degradation is normally
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irreversible. To fully exclude the possibility of the photo degradation, we measured
13
the PL spectra, PL lifetime and reversibility as discussed in the following sections.
14
PL spectra and lifetime during the decline. To find out the origin of the PL
15
decline, we checked the spectra and lifetime during the PL decline. As shown in
16
Figure 2a, the PL spectra kept the same during the decline with maximum at 772 nm.
17
No significant spectral change was observed. In Figure 2b we compared the PL
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lifetime at the initial position and after the PL decline. One can clearly see that the PL
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lifetime at the stable PL level is significantly decreased comparing to that at the initial
20
state. Both spectra and lifetime results suggest the formation of extra non-radiative
21
recombination channel in the material which increases total recombination rate and
22
reduces the PL intensity (i.e. dynamic quenching of the PL). 6
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Figure 2. Photoluminescence spectra and lifetime during the PL decline. (a) PL
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spectra of the CH3NH3PbI3 crystal during the PL decline. (b) PL lifetime of the
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CH3NH3PbI3 crystal at the initial state (black) and the stable level (red) with excitation
5
power density of 4 W/cm2 at 532 nm.
6 7
Reversibility of the PL decline. It is well known that CH3NH3PbI3 perovskite can
8
degrade under light irradiation with PL intensity dropping.11 To distinguish from the
9
sample degradation, we checked the reversibility of the PL decline. As shown in
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Figure 3a and 3b, when the PL intensity reached the stable level, we switched off the
11
excitation light. The PL decline can reappear when the excitation light was switched
12
on again. This process can be repeated for many times until the sample was degraded.
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By varying the time duration of the dark period, we could estimate the rate of the
14
recovery process. In this particular crystal presented, it is ~30 s to complete the
15
recovery process and reach the initial state. It is worth to note that the recovery time is
16
different for different crystals but mostly in the time range of tens of seconds. When
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the sample has been fully recovered, longer duration time in dark does not have any
18
effect on the PL behaviors as shown in Figure S4 in supporting information. And the
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final stable PL intensity level kept constant for all the repeating cycles (excluding the 7
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PL enhancement effect).
2 3
Figure 3. Reversibility of the PL decline. (a) PL decline of a CH3NH3PbI3 crystal
4
after being kept in dark for different duration time (2 s, 6 s, 10 s, 20 s, 30 s, 40 s); (b)
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The recovered PL intensity (Normalized to the first peak) plotted versus the duration
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time which kept in dark.
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From the reversibility of the PL decline, together with the spectra and lifetime
9
results, we can exclude the degradation of the material to be the reason of the PL
10
decline. It seems like the sample reached a new state (with stable PL intensity) which
11
has lower PL quantum yield. Such a new state can be stabilized only under light
12
irradiation, or in other words, with high density of charge carriers. The properties (PL
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quantum yield) of the new state depend on the sample preparation and the excitation
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light condition.
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PL dynamic quenching in time scale of seconds to minutes has been reported by
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Wen et al. in CH3NH3PbI3 film with extra hole transport layer.56 The fast PL 8
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quenching was then explained by accumulation of ions on the interface between
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CH3NH3PbI3 and hole transport layer inducing electric field in the material. We
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exclude this effect because there is no interface in our case and the PL decline and
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recovery processes are too slow comparing to the charge accumulation process.
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Gottesman et al. has reported slow reversible PL decline which required several hours
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for recovery in dark.57 Structure transformation (rearrangement of the crystal scaffold
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or ion motion) of the material under light illumination was proposed to be the reason.
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In our work, the possibility of the rearrangement of the crystal scaffold cannot be
9
completely excluded, but the time scale of the PL decline is too short for such
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rearrangement. In addition, no spectral change was observed in this work during the
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PL decline while reorganization of the crystal structure was reported to cause PL
12
intensity decrease accompanying with spectral change.15 So the observed fast PL
13
decline cannot be the same process as reported by Gottesman et al. However, slight
14
structure rearrangement in nanometer scale response to ion migration or electron
15
density change can still take place.
16
Atmosphere and dimension effects on the PL decline behavior. As all known,
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organometal halide perovskite materials are very sensitive to the environments (e.g.
18
atmosphere, oxygen and moisture.)18,58 We thus checked the atmosphere effect on the
19
PL decline behavior. As shown in Fig. S5 in Supporting Information, no significant
20
difference can be observed when the measurements were performed under nitrogen,
21
oxygen or air. So the PL decline cannot be due to chemical reaction between the
22
materials and the atmosphere. Since the oxygen and moisture in air can readily react
23
with perovskite during the synthesis processes leading to complex chemistry variation
24
in perovskite materials such as introducing potential traps,19,59 we also prepared the
25
samples under Argon atmosphere in glove box and then did the optical measurements
26
under Argon atmosphere. As shown in Fig. S6 in supporting information, we did not
27
observe significant difference comparing to the result obtained in air.
28
It is also reported that crystals with different size and morphology show different
29
PL behaviors. Here in this work, we did not see clear correlation between the PL
30
decline behavior and the dimension or morphology of the crystals. As shown in Fig. 9
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S6 in the supporting information, we can see polycrystalline films that show clear PL
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decline behavior. We also observed hexagonal monocrystals that does not have PL
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decline. For nanocrystals with dimension smaller than the diffraction limit, we
4
observed strong PL blinking which does not allow us to distinguish the PL decline.
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Excitation power dependence of the PL decline behavior. PL quenching defects
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have been proposed to be one of the important factors which significantly affect the
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PL intensity of the materials.7,33 Such defects can work as efficient charge traps
8
strongly quenching the PL intensity and shortening the PL lifetime. Different PL
9
variation behaviors have been explained by the quenching traps including PL
10
enhancement and PL blinking.11,12,16 The quenching traps can be generated or cured
11
under light irradiation depending on different experimental conditions. In this work,
12
the observed reversible PL decline with correlated lifetime change can be due to
13
variation of the quenching traps. It is reported that the photo-induced PL enhancement
14
strongly depends on the excitation light intensity.11,16 We thus checked the excitation
15
power dependence of the PL decline behavior by varying the excitation power density
16
from 0.05 W/cm2 to 7 W/cm2. As shown in Figure 4, both the PL decline rate and
17
decline amplitude clearly depend on the power density of the excitation light. When
18
the excitation power is low, no PL decline is observed. The PL intensity kept constant
19
or slightly increased depending on the samples. When the excitation power increased,
20
clear PL decline appeared and became faster under higher excitation power. Also the
21
relative decline amplitude increased as excitation power increased.
22 23
Figure 4. Photoluminescence decline at different excitation power density. (a) and (b) 10
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show the PL decline behaviors from two different locations on the same crystal
2
(position “1” and “2” as indicated in the inset image), respectively. The arrow
3
indicates the increase of excitation power (from 0.05 W/cm2 to 7 W/cm2).
4 5
It is worth to note that the PL behavior strongly depends on the samples. Different
6
behaviors were observed for different crystals. Even on the same crystal, the PL
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behavior can also be different at different locations. As shown in Figure 4, we selected
8
two different locations on the same crystal for analysis, and found that the PL decline
9
curves are significantly different (comparing Figure 4a and 4b). In Figure 4a, clear PL
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enhancement can be seen at location “1” at low excitation power, while no PL
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enhancement at location “2” was observed (Figure 4b). More importantly, the PL
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decline rate and amplitude are also different for different samples and locations. The
13
difference of PL enhancement between different samples is previously reported and
14
suggested to be due to variation of the crystal defects in the sample.11,16 During the PL
15
decline, the spectra kept the same, and the lifetime decreased with the PL intensity
16
drop. This is normally due to formation of an extra non-radiative decay channel of the
17
excited species. In perovskite materials, such new non-radiative decay channel can
18
only be formation of quenching traps/defects. This is opposite to the PL enhancement
19
process which removes quenching traps/defects in the materials and show increase of
20
the PL lifetime and intensity.
21
So there must be two different quenching traps existing in these materials which
22
control the PL enhancement and PL decline, respectively. These two different traps
23
are independent with each other. To make it easy, we call these two traps TrapE (the
24
trap which controls the PL enhancement) and TrapD (the trap which controls the PL
25
decline). Since both the PL enhancement and the PL decline are reversible or at least
26
partly reversible, both traps must be able to switch between two different states
27
(activated and inactivated). The PL intensity of the materials will be controlled by the
28
total concentration of active TrapE and TrapD. Actually the activation and inactivation
29
of the quenching traps have already been observed in perovskite nanocrystals showing
30
PL blinking.12 The switch between different states of the traps is a photo-induced 11
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process proving that some photo-generated species have been involved. It is not
2
surprising that the rate of both processes strongly depends on the excitation power.
3
Since it is well accepted that the photo-generated species efficiently separate into
4
charges in these materials in time scale of femtoseconds. We believe that the balance
5
between the two states should be controlled by the concentration of the
6
photo-generated free charges:
7
∗ ne- ⇆
(1)
8
me- ⇆ ∗
(2)
9
Where * indicates the activated states of the traps, n and m indicates the number
10
of electron involved in the reactions, “other” includes all other factors e.g.
11
atmosphere.
12
13 14
Figure 5. Scheme of the light induced transformation between different states of the
15
quenching traps.
16
With these two simple reversible reactions, we can explain all the possible PL
17
traces shown in Figure S7 in Supporting Information. Different PL behaviors are
18
because of different concentration of the traps. For example, if CTrapE* (concentration
19
of TrapE*)≫ CTrapD (concentration of TrapD), only PL enhancement will be observed
20
as reported by different groups (black line in Figure S7). On the other hand, if
21
CTrapD≫ CTrapE*, only PL decline can be observed (red line in Figure S7). Once the 12
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concentrations of these two traps are comparable, both processes will be observed. Of
2
course, the PL intensity is controlled not only by the concentrations of the traps but
3
also by their quenching efficiency. The PL traces will become complex depending on
4
the reaction rate of the two reactions. If r1 (rate of reaction 1) > r2 (rate of reaction 2),
5
the PL intensity will increase followed by a PL decline (blue line in Figure S7). While
6
if r2 > r1, the PL decline comes first followed by a PL enhancement (magenta line in
7
Figure S7). It is worth to note that the PL decline was usually observed when the
8
excitation power is high, implying that this process is more difficult than the PL
9
enhancement. It is reported by deqQuilettes et al.60,61 that the trap density can be
10
estimated based on the PL decay curves. We estimated the PL quenching trap densities
11
at the initial and stable state and got 3×1015/cm3 and 8×1015/cm3 before and after
12
light irradiation, respectively. Although the trap density varies for different crystals,
13
the density at the stable state is always higher than that at the initial state. These
14
results support the formation of new quenching traps which is in good agreement with
15
the proposed mechanism.
16
Theoretical calculation. Presence of different crystal defects/traps in the
17
perovskite materials is well accepted in this field. However, the nature of the traps is
18
still not clear. Various types of intrinsic defects have been proposed based on
19
theoretical calculations.62,63 The energy level of the defects and the formation energies
20
were all compared.33 It is believed that deep level defects can work as efficient PL
21
quenchers while shallow level defects will not significantly affect the PL. Based on
22
the PL behaviors presented in this work, both traps (TrapD and TrapE) show: 1)
23
reversible between active and inactive states; 2) efficient PL quenching ability at
24
active states; 3) strong dependence on the light irradiation, or in other words, presence
25
of free charge carriers. To meet all above conditions, we propose that most probably
26
the active/inactive states of the same trap are due to different valence states. From all
27
proposed crystal defects in previous theoretical studies, we find the most possible
28
candidates could be interstitial defects of Pb and I because only these two interstitials
29
may introduce different energy levels to realize the transformation between deep and
30
shallow levels as their charge states happen to change, while all other defects can 13
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merely introduce a same type of energy level (deep or shallow) no matter what
2
charge states they are.32,64–66 And the formation energy of Pb and I interstitial defects
3
was relative low compared with antisite defects indicating that they are more possible
4
to form during the fabrication processes. To further confirm our assumption, we also
5
performed theoretical calculations on the defect energy levels together with transition
6
energy levels of Pb and I interstitial defects in details.
7
8 9 10
Figure 6. Band structures for (a) Pb with 0, +1, +2 charge states and (b) I with -1,
11
0, +1 charge states. The first plot named “Perfect” refers to the same supercell without
12
defect. The zero of energy in all plots is referred to valence band maximum.
13 14
Through the band structure calculations of chosen defective systems (Figure 6),
15
we can note that the energy levels of Pb and I are different for different valence
16
states. For Pb , only the neutral charge state is deep level defect, while only the +1
17
charged state of I has deep energy level in the band gap which should have efficient
18
PL quenching ability. All the other states of both Pb and I are defects with shallow
19
level which have little effect on the PL of the materials. To understand the properties
20
of the charge transition for a certain defect further, its thermodynamic transition levels
21
need to be studied. The defect transition energy level ⁄ is the Fermi energy 14
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!" , when the formation energy of defect # with charge state q is equal to that of
2
another charge state .45 Following the established method,45,66 ⁄ relative to
3
the valence band maximum (VBM) of the defect-free supercell (!$%& ) can be defined
4
as
5
⁄ =
,) * ,) + ) +*)
− !$%& ，
（ 3）
6
where ! #, is the total energy for a supercell which contains the relaxed defect #
7
with charge state q. The calculated transition levels between different charge states of
8
Pb and I defects are shown in Figure 7. One can see that Pb and I have the
9
negative-U behavior,67 implying that these two defects are not stable at +1 and neutral
10
charge states, respectively. Take Pb as an example, the +1 charge state (Pb-. is
11
metastable because the (2+/0) transition level is located between its (2+/+) level and
12
(+/0) level. For the same reason, the neutral charged state of I is unstable. So the
13
active/inactive states of these two different defects must be Pb0/Pb+2 and I+1/I-1,
14
respectively. The (2+/0) transition level for Pb (1.46 eV above the VBM) obtained
15
from this work shows good agreement with the previous reports.32,66 However, the
16
(+/-) transition level for I (0.09 eV above the VBM) is significantly lower than the
17
value reported by Du.65,68 We believe the discrepancy would be derived from using
18
different functionals and a larger supercell in our work which allows better
19
relaxation.67 Anyway, it is obvious that (2+/0) transition level for Pb is significantly
20
higher than (+/-) transition level for I , which indicates the activation/deactivation of
21
the Pb defect should be more difficult than that of I defect.
15
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Figure 7. Calculated thermodynamic transition levels for the interstitials Pb and I .
3 4
PL enhancement has become a common phenomenon for different kinds of
5
perovskite materials controlled by the TrapE. It must be a deep level defect at the
6
active state. DeQuilettes et al. discovered the redistribution of iodide element during
7
the PL enhancement suggesting iodide must be involved.50 So the defect which
8
controls the PL enhancement behavior (TrapE) is more likely to be I .32 Active state
9
of I (I-.) was formed during the sample preparation significantly quenching the PL.
10
With light irradiation, the photo-generated free electrons could react with I-.
11
producing I*. which is the inactive state.
12
PL decline was seldom reported and the defects which control the PL decline have
13
never been discussed previously. From the above discussion, only Pb can be the
14
defect which is responsible for the PL decline (TrapD). The active and inactive states
15
of the TrapD should be Pb/ and Pb-0 , respectively. During the sample preparation,
16
only Pb-0 are formed because the formation energy of Pb/ is too high. Under light
17
irradiation, free electrons are generated in the material and would react with Pb-0
18
then produce Pb/ . The Pb/ is an efficient trap and will strongly quench the PL
19
emission of the materials reducing both the PL intensity and PL lifetime.
20
The total PL intensity of the CH3NH3PbI3 crystal is determined not only by the 16
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intrinsic properties of the material but also the content of PL quenching traps. For the
2
same crystal/location studied in this work, the PL intensity will be controlled by the
3
content of Pb/ and I-. depending on the equilibrium of reaction (1) and (2). The
4
shift of the equilibrium reactions was controlled by light irradiation and the
5
thermodynamic transition level between the two stable charge states of I and Pb .
6
From the theoretical calculation, the thermodynamic transition level between I-. and
7
I*. is shallow (close to the VBM), which indicates the transformation from I-. to
8
I*. is easy to happen. While for Pb , the deep (2+/0) transition level causes it more
9
difficult to change from Pb-0 to Pb/ . These results are in very good agreement with
10
the experiment. The PL enhancement is very easy to be observed and has been
11
reported in many different studies because only very low excitation power (or in other
12
words, low concentration of electrons) was needed. However, the PL decline
13
phenomenon is seldom observed because high concentration of electrons was
14
necessary to shift the equilibrium of reaction (2) towards the right direction. In
15
addition, if we decrease the concentration of electrons, Pb/ can easily transform into
16
Pb-0 but I*. is difficult to turn back into I-. . Therefore, when the light is switched
17
off, the PL decline can recover very fast in the timescale of tens of seconds but the PL
18
enhancement needs more time to reappear in timescale of hours as observed in our
19
experiments. Furthermore, we can estimate that I-. and Pb-0 are stable when the
20
Fermi level is low while I*. and Pb/ are stable when the Fermi level is high on the
21
basis of our defect formation energy calculations (details in the section 8 of the
22
Supporting Information) and the reported formation energies32,64. In addition, for Pb ,
23
the formation energy is higher than I at the I-rich growth condition while close to I
24
at the I-poor growth condition. It means that the type and concentration of stable
25
defects strongly depend on the Fermi level and preparation condition, which
26
eventually lead to different PL behaviors for different samples.
27
PL decline vs I-V hysteresis. The hysteresis behaviors in the I-V curve
28
measurements of perovskite-based solar cells have been widely investigated showing
29
clearly different power conversion efficiency for forward scan and backward scan.69–
30
72
Stepwise measurements discovered a fast process when the voltage switched to a 17
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new value which takes some time to reach the stable states in a time range from
2
milliseconds to seconds.69,73,74 If we compare the PL decline observed in this work
3
(Fig. 3a) and the time-dependent photocurrent response during the stepwise voltage
4
sweep (Fig.3d in ref. 68), similar decline behaviors were observed in both
5
reversibility and time scale. Such similarity leads us to question: are these two
6
phenomena from the same physical nature? Many different mechanisms have been
7
proposed to explain the I-V curve hysteresis including (a) transient capacitive
8
effect,75–77 (b) trapping/detrapping processes of charge carriers,78,79 (c) ion
9
migration56,80–82 and (d) ferroelectric polarization.83,84 However, the mechanism is still
10
in debate. Different or even contrary experimental results were reported. For example,
11
Chen et al. found capacitive effect to be one of the main reasons for the hysteresis73
12
while Almora et al. claimed there is no capacitive effect.85 The situation presented in
13
this work is much simpler than the I-V measurement because we are working on an
14
individual CH3NH3PbI3 crystal without any extra layers or interfaces. The PL decline
15
can only be due to the properties of the material itself. If the PL decline is due to
16
formation of TrapD, which is an efficient non-irradiative recombination site for charge
17
carriers, such TrapD will also reduce the power conversion efficiency of the solar cells.
18
However, we cannot directly correlate these two phenomena right away. Simultaneous
19
measurement of the I-V curve and PL on devices made from such a micro-crystal
20
would be very helpful to fully understand the mechanism of the hysteresis which will
21
be investigated in the future by collaboration with other groups.
22 23
CONCLUSIONS
24
In this work, we observed both PL enhancement and PL decline on the same
25
CH3NH3PbI3 crystal. Strong excitation power dependence and reversibility suggest
26
that these two processes are related to the photo-induced activation/deactivation of the
27
traps. The complex PL variation comes from the contribution of two independent
28
quenching defects depending on their concentration, quenching ability and activation
29
reaction rate. Combining experimental and theoretical results, interstitial defects of
30
Lead and Iodide are identified to be responsible for the PL decline and PL 18
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enhancement, respectively. The activation process is transition between different
2
valence states of 12 and I , i.e. Pb/ /Pb-0 and I-. /I*. . Although we cannot fully
3
connect the PL decline to the I-V hysteresis, it provides possible evidence for the
4
charge trapping mechanism. The observation of the PL decline calls attention to
5
possible mistake during all PL quantum yield measurement and helps us to identify
6
the chemical nature of the defects. The nature of the crystal defects shows the
7
direction to further understand the role of defects in the photophysical processes and
8
provides suggestions for improving their properties.
9 10
ASSOCIATED CONTENT
11
Supporting information
12
The supporting information is available free of charge:
13
Scheme of the home-built wide-field microscope; Lattice parameters and band
14
structures of CH3NH3PbI3 obtained from optimized structures; SEM,TEM and XPS
15
characterization of CH3NH3PbI3 micro-crystals; PL intensity reversibility over longer
16
duration time kept in dark; PL decline under different atmosphere; PL behaviors of
17
different samples; Different PL intensity traces observed for different samples; Defect
18
formation energy calculations.
19
AUTHOR INFORMATION
20
Corresponding Author
21
*E-mail:
[email protected],
[email protected] 22
Author Contributions
23
D. Hong and Y. Zhou contributed equally to this work.
24
Notes
25
The authors declare no competing financial interest.
26 27
ACKNOWLEDGEMENTS
28
This work is supported by National Natural Science Foundation of China (NSFC
29
No.21673114 (YT) and No.91641104 (XH)), and the Natural Science Foundation of
30
Jiangsu Province (No.BK20160645). 19
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For Table of Contents Use Only:
2 3
Nature of Photo-induced Quenching Traps in Methylammonium Lead Triiodide
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Perovskite Revealed by Reversible Photoluminescence Decline
5
Daocheng Hong , Yipeng Zhou , Sushu Wan, Xixi Hu*, Daiqian Xie and Yuxi Tian*
†
†
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Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and
8
Chemical Engineering and Jiangsu Key Laboratory of Vehicle Emissions Control,
9
Nanjing University, Nanjing, Jiangsu, 210000, China
10 11
TOC
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The TOC shows the light induced transformation between different valence states of
14
the interstitial defects in MAPbI3 a microcrystal. Under light irradiation, the Pb-0
15
defects will switch to Pb/ defects that work as efficient PL quencher. Thus PL
16
intensity is significantly reduced. This process is completely reversible as shown by
17
the arrows.
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