Nature of Photoinduced Quenching Traps in Methylammonium Lead

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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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ACS Photonics

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Nature of Photo-induced Quenching Traps in Methylammonium Lead Triiodide

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Perovskite Revealed by Reversible Photoluminescence Decline

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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

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Chemical Engineering and Jiangsu Key Laboratory of Vehicle Emissions Control,

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Nanjing University, Nanjing, Jiangsu, 210000, China

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ABSTRACT

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Photoluminescence (PL) of organometal halide perovskite has been broadly

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investigated as a fundamental signal to understand the photophysics of these materials.

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Complicated PL behaviors have been reported reflecting to complex mechanisms

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including effects from crystal defects/traps whose nature still remains unclear. Here in

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this work we observed, besides the PL enhancement, a surprising PL decline

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phenomenon in methylammonium lead triiodide (CH3NH3PbI3) perovskite showing a

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high initial PL intensity followed by a fast decline in time scale of milliseconds to

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seconds. The similarity between the PL enhancement and PL decline suggests both

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processes are due to PL quenching traps in the material. Combining experimental and

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theoretical results, two interstitial defects of Lead and Iodide were identified to be

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responsible for the PL enhancement and PL decline, respectively. Both traps can be

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switched between active and inactive states leading to reversible process of PL

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enhancement and PL decline. This is the first time to get identification on the

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chemical nature of the PL quenching traps and take an important step towards fully

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understanding the crystal defects in these materials.

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KEYWORDS: perovskite, CH3NH3PbI3, photoluminescence, decline, nature of

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defects

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ACS Paragon Plus Environment

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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’

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concentration.12,15

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Many unusual PL behaviors have been observed in these materials. PL blinking

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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

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observed to nonlinearly depend on the excitation power indicating quantum efficiency

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dependence on the excitation.17 Atmosphere has significant effect on the PL of

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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

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could contribute to the PL emission such as excitons and traps depending on

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experimental conditions,7,20 recombination between electron and hole is still the main

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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

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behaviors.7,11,12,16,17,26–31 However, due to the unknown nature of these traps, all these

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unusual PL behaviors are still not fully understood. Experimental characterization of

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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

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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

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intensity was observed followed by a fast decline to a steady state level. The fast PL

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decline processes were completely reversible by keeping the crystal in dark for tens of

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seconds which is different from the well-known photo-degradation process in this

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material. We investigated the PL decline behaviors from different aspects including

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PL spectra and lifetime, reversibility of the PL decline as well as theoretical

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calculations. From both experimental and theoretical results, interstitial defects of I

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and Pb were identified to be responsible for the PL enhancement and PL decline,

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respectively. The total PL intensity is determined by activation/inactivation

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equilibrium between the two states of these two defects.

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EXPERIMENTAL SECTION

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Sample preparation. The CH3NH3PbI3 was prepared by the one-step method

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from reaction between equimolar methylammonium iodide (CH3NH3I) and lead

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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,

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and the solvent was then evaporated. The diethyl either was used to precipitate the

29

CH3NH3I. Pure CH3NH3I was obtained through further recrystallization. PbI2 and

30

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

5

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

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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

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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

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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).

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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

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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

25

represented by using the non-local optB86b-vdW functional which takes into account

26

van der Waals interactions.41–44 The reason why we chose this functional was given in

27

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

3

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.

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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

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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

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(Figure S3, Supporting Information). The PL behaviors of these microcrystals are

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investigated under fluorescence microscope. The PL spectra kept the same during the

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PL enhancement while the lifetime increased along the PL enhancement. These results

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are in agreement with the reported results. We thus conclude that the PL enhancement

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process in these microcrystals is due to the same processes as reported.16,47,49–52 As

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shown in Figure 1 (dash line), we observed the photo-induced PL enhancement on the

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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

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intensity as shown in Figure 1 (solid line). This fast PL decline happens in time scale

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of milliseconds to seconds and finally reaches a stable level. The PL decline rate

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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.

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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

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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

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the PL spectra, PL lifetime and reversibility as discussed in the following sections.

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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

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Figure 2a, the PL spectra kept the same during the decline with maximum at 772 nm.

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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

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recombination channel in the material which increases total recombination rate and

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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

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power density of 4 W/cm2 at 532 nm.

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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

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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

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recovery process. In this particular crystal presented, it is ~30 s to complete the

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recovery process and reach the initial state. It is worth to note that the recovery time is

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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).

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Figure 3. Reversibility of the PL decline. (a) PL decline of a CH3NH3PbI3 crystal

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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

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results, we can exclude the degradation of the material to be the reason of the PL

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decline. It seems like the sample reached a new state (with stable PL intensity) which

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has lower PL quantum yield. Such a new state can be stabilized only under light

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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

7

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

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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

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intensity decrease accompanying with spectral change.15 So the observed fast PL

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decline cannot be the same process as reported by Gottesman et al. However, slight

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structure rearrangement in nanometer scale response to ion migration or electron

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density change can still take place.

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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

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PL decline behavior. As shown in Fig. S5 in Supporting Information, no significant

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difference can be observed when the measurements were performed under nitrogen,

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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

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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

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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

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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,

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the observed reversible PL decline with correlated lifetime change can be due to

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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

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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

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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,

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clear PL decline appeared and became faster under higher excitation power. Also the

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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

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(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

7

behavior can also be different at different locations. As shown in Figure 4, we selected

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two different locations on the same crystal for analysis, and found that the PL decline

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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

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difference of PL enhancement between different samples is previously reported and

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suggested to be due to variation of the crystal defects in the sample.11,16 During the PL

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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

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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.

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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


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

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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1

!" , 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


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

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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1

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


ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

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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ACS Photonics

1

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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1

For Table of Contents Use Only:

2 3

Nature of Photo-induced Quenching Traps in Methylammonium Lead Triiodide

4

Perovskite Revealed by Reversible Photoluminescence Decline

5

Daocheng Hong , Yipeng Zhou , Sushu Wan, Xixi Hu*, Daiqian Xie and Yuxi Tian*

†

†

6 7

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

12 13

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.

30


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Nature of Photoinduced Quenching Traps in Methylammonium Lead

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