The Role of Excitation Energy in Photobrightening and

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Energy Conversion and Storage; Plasmonics and Optoelectronics

The Role of Excitation Energy in Photobrightening and Photodegradation of Halide Perovskite Thin Films Wolf-Alexander Quitsch, Dane W. deQuilettes, Oliver Pfingsten, Alexander Schmitz, Stevan Mihajlo Ognjanovic, Sarthak Jariwala, Susanne Koch, Markus Winterer, David S Ginger, and Gerd Bacher J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00212 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018

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The Journal of Physical Chemistry Letters

The Role of Excitation Energy in Photobrightening and Photodegradation of Halide Perovskite Thin Films Wolf-Alexander Quitsch1, Dane W. deQuilettes2, Oliver Pfingsten1, Alexander Schmitz1, Stevan Ognjanovic3, Sarthak Jariwala2,4, Susanne Koch2, Markus Winterer3, David S. Ginger2 and Gerd Bacher1* 1.

Werkstoffe der Elektrotechnik and CENIDE, University of Duisburg-Essen, Bismarckstraße 81, 47057 Duisburg, Germany.

2.

Department of Chemistry, University of Washington, Box 351700, Seattle, WA 981951700, USA. 2 Clarendon Laboratory. 3.

Nanoparticle Process Technology and CENIDE, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany.

4.

Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195-1700, USA.

ACS Paragon Plus Environment

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Corresponding Author * E-mail address:

[email protected]

Tel.:

+49-203-379-3406

Fax:

+49-203-379-3404


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ABSTRACT

We study the impact of excitation energy on the photostability of methylammonium lead triiodide (CH3NH3PbI3 or MAPI) perovskite thin films. Light soaking leads to a transient increase of the photoluminescence efficiency at excitation wavelengths longer than 520 nm while light-induced degradation occurs when exciting the films with wavelengths shorter than 520 nm. X-ray diffraction and extinction measurements reveal the light-induced decomposition of CH3NH3PbI3 to lead iodide (PbI2) for the high energy excitation regime. We propose a model explaining the energy dependence of the photostability, involving the photoexcitation of residual PbI2 species in the perovskite triggering the decomposition of CH3NH3PbI3.

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Over the past few years, metal halide perovskites have become one of the most promising materials for a wide range of photovoltaic and optoelectronic applications. Processed out of a solution at low temperatures, low cost production of large area devices became feasible1. Within a short period of time, perovskite-based solar cells with efficiencies exceeding the 10 % mark have been reported2-6, culminating in a current record efficiency of more than 22 %7. Hence, perovskites represent one of the most efficient solution-processed material systems for photovoltaic applications. Besides this remarkable progress, perovskite crystals offer high internal photoluminescence (PL) quantum yields8-11, enabling optically pumped lasing12-15 and room temperature single photon emission16. In addition, their widely tunable emission color, which can easily be adjusted by structural or chemical modification17-21, paved the way for a new class of light-emitting devices, the so-called perovskite-LEDs (PeLEDs)22. A variety of devices emitting from the blue to the near infrared spectral range have been presented23-27, even fully printed on flexible substrates as reported recently28. In addition, perovskites were introduced as a promising competitor to the typically used phosphors as converter materials29-31 in LED applications due to their narrow bandwidth and their color tunability32-35.

A critical issue for such applications is the long-term stability of the materials under illumination. However, various reports in the literature describe both decreases and increases in photoluminescence intensity upon photoexcitation of halide perovskite semiconductors, whereas an agreement on the mechanistic origin of the different light soaking effects has not been reached yet.36 For instance, using a white light source with a power density of < 0.01 Wcm-2, Gottesman et al. observed a decrease of the photoluminescence intensity in a CH3NH3PbI3 (MAPbI3) film37. Similarly, Sutter-Fella et al. reported an intensity decrease in a mixed halide system at high


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excitation power density (9 Wcm-2) using an Argon ion laser at 488-514 nm for excitation, yet the emission efficiency was constant over the same period at a moderate excitation power density (0.2 Wcm-2)11. In contrast, drastic enhancements of the PL efficiency under illumination – socalled photobrightening – have been observed in similar material systems: independent of the power density (0.3 Wcm-2 and 1 Wcm-2), an increased PL efficiency was observed by Tian et al. using the 514 nm line of an Argon ion laser as excitation source38,39. In addition, deQuilettes et al. and Stranks and coworkers report on photobrightening for an excitation power density range between 0.06 Wcm-2 and 0.3 Wcm-2 at an excitation wavelength of 532 nm40,41 and photobrightening was even seen at 640 nm excitation wavelength38 with a power density of 0.02 Wcm-2. Just lately, Kheraj et al. demonstrated a transient increase of the peak luminescence of encapsulated lead(II)iodide-rich MAPbI3 films during illumination with 532 nm at about 0.015 Wcm-2, whereas unencapsulated films showed decreasing intensity.42

In this letter, we demonstrate that the photostability of MAPbI3 layers critically depends on the excitation wavelength. We find a transition between photobrightening and photodegradation at about 520 nm, which we correlate to the bandgap of resident PbI2. Extinction and X-ray diffraction (XRD) measurements confirm the decomposition of MAPbI3 into PbI2 for excitation energies above the PbI2 bandgap. We suggest photolysis of residual PbI2 and/or charge transfer from excited PbI2 residuals to the perovskite as possible sources for the formation of I2, which triggers the decomposition of MAPbI3. Time-dependent PL measurements under different atmospheric conditions reveal the role of water in photoinduced degradation.


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In a first experiment, we performed time-dependent PL measurements on a MAPbI3 layer under continuous illumination using a stable, high-performance blue light-emitting diode at λ = 457 nm, i.e. a wavelength well below those for which photobrightening was reported38-41. In a reference experiment, a green laser (λ = 532 nm) was used as the excitation source. Figure 1a shows the PL spectra during an illumination period of 60 s using the blue LED at 0.1 Wcm-2 in lab air, while the inset depicts the analogue experiment for the green laser at 0.4 Wcm-2. Note, that all spectra are scaled relatively to the maximum intensity of the respective initial measurement.

A fundamental difference between the experiments is observed: illumination with the blue light source leads to an intensity decrease with time as indicated by the blue arrow, whereas a clear enhancement of the PL intensity with illumination time is observed for excitation with the green laser source (green arrow). Note, that we exclude sample heating as a possible mechanism for the transient changes in intensity since we observe no variations in shape or energetic position of the spectra with time for both experiments (see Figure 1a and Figure S1). The variation of the relative efficiency with time under continuous illumination is depicted in Figure 1b. Within the first ten seconds, a rapid increase of the PL intensity (around 50 %) is achieved for green illumination (green circles). On a similar timescale, a decrease of roughly 50 % can be observed for blue light illumination. The slopes of both curves then slowly start to saturate at around 65 % above (below) the initial intensity after about 60 s.

In Figure 1c a single run PL experiment in lab air for several minutes is shown, where the excitation source was switched every 60 s. For the whole run, an efficiency decrease is observed


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when the blue LED is used as the illumination source, while the mechanism changes to photobrightening instantaneously after switching to the green laser source and vice versa.

Figure 1. Time-dependent PL intensity of a MAPbI3 film for different excitation conditions measured in air. a) Time-dependent PL spectra of the MAPbI3 film using a LED with a peak wavelength of 457 nm at 0.1 Wcm-2 as illumination source. The spectra are normalized with respect to the spectrum obtained after one second of illumination time. Inset: Time-dependent PL spectra of the MAPbI3 film using a 532-nm laser at 0.4 Wcm-2 as illumination source. The


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apparent double band shape of the spectra is attributed to the involvement of surface defect states, resulting in an additional contribution of a slightly redshifted emission with respect to the band gap luminescence.43,44 b) Relative change of the PL intensity with illumination time for blue (blue circles) and green (green circles) excitation. c) Continuous PL experiment switching the light source every 60 s. The green circles represent the data during green illumination, while the blue ones represent the data during blue excitation. Note, that the sample position was kept constant throughout the experiment. Figure 1 shows unambiguously that the transient change in PL intensity is dependent on the excitation photon energy. These findings strongly suggest two separate mechanisms may be responsible for photodegradation and photobrightening, respectively. Note, that a partial recovery of the PL intensity between two illumination periods with blue light can also be seen without green illumination by just storing the sample in the dark (see Figure S2).

In order to gain deeper insight into the wavelength-dependent photostability, PL intensity time trace measurements at various excitation wavelengths between 465 nm and 540 nm have been performed using a Xenon lamp and monochromator setup. The slit width used in the experiment resulted in a 14-nm linewidth of the excitation source and each data set was taken at 5 nm intervals. The power density was kept well below one sun (0.02 Wcm-2 – 0.04 Wcm-2) to exclude possible heating and/or the influence of high carrier densities in the sample. In addition, the sample was kept in the dark for 15 minutes in between each time trace to lower the overall stress on the sample.


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The relative changes of the PL intensity with time for the different excitation wavelengths in air are shown in Figure 2a. Starting at the highest excitation wavelength (i.e. 540 nm), a rapid transient increase of the intensity is observed, as expected when compared to the measurements with the green light source (see Figure 1b). With decreasing wavelength (< 540 nm), the photobrightening effect becomes less prominent, until the PL intensity is virtually constant over the whole illumination period at 520 nm. Such a constant efficiency over illumination time was also observed by Sutter-Fella et al. at a similar excitation wavelength and with a moderate excitation power density11. For excitation wavelengths below 520 nm, the photobrightening effect is no longer dominant and the PL intensity starts to decrease with illumination time. The wavelength dependence of the transient PL intensity thus shows a clear change of the mechanism in the range of (520 ± 7) nm, switching from photobrightening to photodegradation towards shorter excitation wavelengths.

Figure 2. Transient PL efficiency change for different excitation wavelengths. a) Time traces of the PL intensity of a MAPbI3 film sample for 60 s illumination at each excitation wavelength between 540 nm and 465 nm and measured in air. Periods of 15 minutes in the dark were kept in


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between each measurement to reduce the overall stress on the sample. Emission was detected at the PL peak position (778 nm). b) Extinction of a degraded MAPbI3 layer with increased PbI2 content in the spectral region around the PbI2 band gap.

Note, that the different extinction of the sample and the variance in excitation power at the respective wavelengths do not change the fundamental mechanism which is dominant at each wavelength. While the efficiency increase using wavelengths > 520 nm remains dominant up to ~ 1.6 Wcm-2 (see Figure S3a), we observed photodegradation over a large range of excitation densities for blue excitation starting from ~ 0.1 Wcm-2 down to 0.0076 Wcm-2 during blue LED illumination (λ = 457 nm; see Figure S3b).

Importantly, the observed threshold-wavelength for degradation of the MAPbI3 of ~ 520 nm (2.38 eV) corresponds well with previously reported band gap energies for PbI2 (2.3 eV 2.5 eV)45,46. To determine if there is a correlation or coincidental relationship between the onset of the changes in photostability and PbI2 absorption, Figure 2b shows the extinction spectrum of an almost fully degraded MAPbI3 sample, illuminated for one hour at 0.1 Wcm-2 at a wavelength of 457 nm. A clear absorption edge can be identified starting at about 520 nm for which we estimate an optical bandgap energy of ~ 2.38 eV using a Tauc plot. This indicates a strong correlation of the PbI2 absorption and the photostability of the MAPbI3 film. Note, that PbI2 residuals are either inadvertently or intentionally present in many MAPbI3 thin films and devices processed from several different synthesis routes even before degradation47-50 and recent findings highlight the role of excess PbI2 in solar cell degradation48.


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To exclude the influence of the sample processing route applying lead(II) chloride (PbCl2) as a precursor, reference experiments with samples processed from PbI2 and lead acetate (Pb(Ac)2) precursors have been carried out using excitation wavelengths below and above the PbI2 band gap. The data are presented in Figure S4 of the Supporting Information. These samples show the same overall trend, exhibiting photodegradation only during illumination with blue light. Due to their lower initial PbI2 content (compare XRD data in Figure 3b and Figure S4d), photobrightening dominates the PL evolution on short timescales even in ambient air, whereas the long-term behavior is controlled by photodegradation. Further, corresponding experiments have been conducted using a sample equipped with a PMMA capping layer as shown in Figure S5. The experiment reveals the same general behavior showing photodegradation during blue light illumination, while green light excitation only leads to transient photobrightening of the sample.

Various photochemical mechanisms have been introduced during the last years to explain the beneficial effect of light soaking of the perovskite in terms of photobrightening. Among them, Tian et al. suggested the passivation of surface and bulk defects in oxygen containing environments by photoinduced reactions with oxygen species and temporary trap filling to cause irreversible and reversible photobrightening, respectively38. Similarly, Galisteo-Lopez et al. proposed surface and grain boundary passivation by oxygen promoted reactions, such as the formation of lead oxides51. Recently, Brenes et al. reported photobrightening of MAPBI3 films by exposure to a 532-nm laser which was attributed to the light-induced formation of superoxide species, annihilating shallow surface states52. DeQuilettes et al. showed the photoinduced brightening of MAPbI3 films to be correlated to the migration of surface iodide species from


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illuminated regions into dark regions of the sample, indicating the reduction of trap states to be related to either interstitial iodide ions or iodide vacancies being annihilated by the migrating species both at the surface and in the bulk40. In addition, it was shown that the observed photobrightening effect correlates with the transient improvement of the open-circuit voltage of a MAPbI3-based solar cell, highlighting the importance of the photoinduced changes for device application40.

In contrast, only few reports present photoinduced decreases of the PL intensity. Ito et al. ascribed the decomposition of the perovskite in a TiO2/MAPbI3 film stack to the oxidation of the perovskite’s iodide ions by electron transfer processes between TiO2 and MAPbI353, while Gottesmann et al. suggested photoinduced structural changes of the perovskite crystals to induce a reversible reduction of the PL intensity37. To get an insight into the chemical processes occurring during photodegradation observed in this work, illumination-time-dependent extinction of a MAPbI3 film was measured under ambient conditions (see Figure 3a). Two main features characterize the extinction spectrum: A sharp optical feature at 780 nm related to the bandgap transition of MAPbI3 and an absorption edge at around 520 nm stemming from PbI2 residuals in the film. The sample was excited with the blue LED (457 nm) at 0.1 Wcm-2 for 15 minutes for multiple cycles, while the extinction was measured between each pumping cycle. For consistency between the PL and absorption measurements, each extinction spectrum was recorded at the exact same spot on the sample. A strong transient reduction of the perovskite extinction is observed, while the PbI2 absorption edge becomes more and more pronounced with illumination time. These findings strongly indicate the decomposition of the perovskite into PbI2


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during blue light excitation. Note, that the extinction measured above 780 nm is likely caused by scattering effects due to the surface not being fully covered by large crystals54.

Figure 3. Extinction spectra and X-ray diffractograms for illuminated and reference samples. a) Extinction spectra of a MAPbI3 film before (blue) and after multiple 15-minute long exposure cycles in air with blue light of 457 nm at a power density of 0.1 Wcm-2. The measurements were done at the exact same spot on the film. b) X-ray diffractograms of a reference film (black) and a film illuminated for three hours with white light at a power of 0.1 Wcm-2 (blue). The peaks expected for PbI2 and MAPbI3 are indicated by green (PbI2) and red (MAPbI3) dashed lines, respectively55,56.

Additionally, a long-time illumination experiment using a white light source at 0.1 Wcm-2 for 3 hours was performed while a reference sample was kept in the dark. When comparing the X-


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ray diffractograms for the illuminated sample and the reference, the same trend as observed for the extinction experiment is obtained (see Figure 3b): While the reference sample shows the typical Bragg reflections of MAPbI3 and a smaller contribution from PbI2, as commonly observed in literature53, a vastly decreased contribution of the MAPbI3 signal is found for the illuminated sample (blue), indicating the degradation of the film. At the same time, the XRD signal of PbI2 rises, demonstrating increased PbI2 content after white light illumination, thus further confirming the findings above.

Degradation of MAPbI3 to PbI2 has been subject to numerous studies during the last years which have further elucidated the influence of illumination53,57-59 and atmospheric conditions, i.e. humidity and oxygen content60-64. Thereby, various possible pathways for the chemical decomposition of the perovskite have been proposed, often involving the evaporation of volatile methylamine (CH3NH2) and hydrogen iodide (HI) or molecular hydrogen (H2) and iodine (I2)53,65,66. In particular, recent reports highlight the importance of the presence or formation of I2 in the perovskite film on photoinduced degradation. Ito et al. proposed the extraction of electrons from the perovskite at a MAPbI3/TiO2 interface to lead to the formation of I2 and thus deconstruction of the perovskite crystal53. Wang and coworkers exposed MAPbI3 to iodine vapor and demonstrated rapid degradation of the perovskite especially under illumination conditions, which was ascribed to a series of chemical chain reactions. Herein, under illumination conditions, molecular iodine is expected to be split up into atomic iodine by photolysis, which subsequently forms I2- by reacting with iodide ions from the perovskite. This highly reactive species eventually reacts with the perovskite’s methylammonium ion to form volatile


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methylamine, molecular iodine and molecular hydrogen, leaving only PbI2 behind and resulting in a cycle reaction, as molecular iodine is preserved throughout the whole process.65

These findings must be kept in mind when discussing the excitation wavelength dependence of the photostability of MAPbI3 films in the presence of PbI2 as observed in our series of experiments. From the well-known photophysics of PbI2, photolysis of residual PbI2 species in the perovskite film can generate molecular I2 in the samples via Equation (1):67 h PbI ⇄ Pb + I

(1)

As photolysis requires PbI2 to be excited by the incident light, the origin of the observed threshold-wavelength for photodegradation of the MAPbI3 (see Figure 2) becomes obvious. The initial generation of I2 in the perovskite sample could then subsequently lead to the decomposition of the crystal by the evaporation of organic compounds53,65. During chemical decomposition, the photogenerated I2 from photolysis is preserved65, such that after blue light illumination is turned off, photolysis can at least partially be reversed (see Equation (1)). Thus, initiated by the photolysis of PbI2 for excitation above the PbI2 bandgap, MAPbI3 can be chemically decomposed. This further increases the amount of PbI2 in the sample, in agreement with the observations depicted in Figure 3. While other mechanisms might also explain the energy dependence – for instance, the photooxidation of iodide in the perovskite lattice might have a sharp energy threshold – we favor the role of direct absorption by PbI2 here due to the good match between the onset of the PbI2 absorption and the crossover between photobrightening and photodegradation observed in our experiments.


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Optical excitation of PbI2 does not only result in photolysis but also in the generation of charge carriers in the lead iodide. This process might provide an additional pathway for the initiation of the photodegradation of MAPbI3 by a charge extraction process. While the ionization energy of PbI2 exceeds that of MAPbI3 by approximately 0.8 eV, both materials feature similar electron affinities46,68-71. This energetic alignment allows for an efficient transfer of photogenerated holes from PbI2 to the MAPbI3 valence band upon optical excitation of PbI2 in the sample. Such carriers are expected to exhibit significantly longer lifetimes compared to photo-charges directly excited in the MAPbI3, as they lack recombination partners in the perovskite and might thus induce the formation of I2 from the perovskite iodide53 and/or iodide interstitials in the film40,72,73 via Equation (2): 2 I + 2 h → I

(2)

This process, likewise, would result in the formation of molecular I2 inside of the perovskite film and subsequently induce chemical decomposition of the sample as outlined above. Note, that upon excitation, photogeneration of charge carriers in PbI2 is expected to happen alongside photolysis, as the photolysis quantum yield is < 167. We hence assume photolysis of PbI2 to be supported by the proposed hole transfer process in the formation of I2 and thus in the photodegradation of MAPbI3.

The chemical decomposition pathway of the perovskite film, triggered by I2 formation, involves the evaporation of organic compounds, while only PbI2 is left behind, and is thus considered to be irreversible in principle. However, evaporation of volatile decomposition products can be hindered within the crystalline film itself due to its finite thickness and/or by encapsulating layers or charge transport layers in full devices. In this case, the perovskite


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structure could in theory be regenerated from the trapped species after illumination is turned off. Additionally, after illumination, migration of organic species from undamaged (not illuminated) areas of the sample into degraded (illuminated) regions is expected, as it has been suggested e.g. for excess iodide in MAPbI3 before40. Both these mechanisms will lead to a partial recovery of the perovskite crystal in the degraded areas after illumination and might thus explain the partial reversibility of the photodegradation (see Figure S2).

As recent reports highlight the importance of the measurement atmosphere, i.e. the presence of humidity60,74 and oxygen62,63 on the degradation of MAPbI3, we additionally performed photostability measurements at different excitation wavelengths (405 nm and 532 nm) in varying atmospheres. For green light excitation, only photobrightening can be observed in vacuum (< 5·10-6 mbar) and lab air even at long illumination times (Figures S6a,b), while the effect appears to be promoted in lab air conditions. Such behavior has also been reported by Brenes et al., showing amplified photobrightening in both oxygen and humid environments compared to a dry N2 atmosphere52. Analogous experiments with blue light excitation show a different behavior. On a short timescale (~ 90 s), photodegradation seems to only occur in (humid) lab air, while illumination in dry air (20 % O2, 80 % N2) and in vacuum shows photobrightening (see Figure S6c). Yet, upon longer excitation in vacuum, photobrightening starts to saturate and the PL intensity eventually declines with ongoing illumination time (see Figure S6d). These observations point towards the coexistence of (wavelength-independent) photobrightening and (wavelength-dependent) photodegradation effects in MAPbI3 upon excitation above the PbI2 bandgap. Thereby, photobrightening dominates the illumination time response of the PL intensity on short timescales in dry surroundings (O2, N2 and vacuum) but can be


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overcompensated by photodegradation after it saturates. In humid atmospheres, however, photodegradation is boosted and dominates the transient PL behavior even at short illumination times. Hence, a competition between photobrightening and photodegradation is likely to take place in the perovskite under low wavelength illumination and might favor either effect depending on e.g. the measurement atmosphere (see Figure S6), the fabrication method or the sample composition (i.e. the initial PbI2 content, see Figure S4).

In conclusion, the role of excitation wavelength on the long-term photostability of MAPbI3 perovskite films was investigated. A systematic change from photodegradation to photobrightening is observed around the bandgap of the residual PbI2 in the perovskite films when measured in air. Extinction and XRD measurements showed an increased PbI2 content in the films after photodegradation, proving the chemical decomposition of the perovskite upon excitation with blue light. We suggest a model ascribing the photoinduced degradation of MAPbI3 to the formation of I2 by photolysis of PbI2 and/or by hole transfer between the photoexcited species. Studies under different atmospheric conditions highlight the role of water for photoinduced chemical decomposition. Our findings implicate an intrinsic vulnerability of perovskite films to low wavelength illumination in the presence of PbI2 and might thus be crucial for the applicability of MAPbI3-based devices operating under sunlight or exposure of light with wavelengths < 520 nm, like e.g. solar cells or converters for blue LEDs. Based on these findings, the optimization and development of new fabrication routes towards totally PbI2-free perovskite films as demonstrated recently47,75 might represent a promising perspective for improving the lead halide perovskite’s long-term stability.


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Experimental Methods a) Synthesis of MAPbI3 Perovskite Crystals Methylammonium iodide (MAI) was prepared by reacting methylamine, 33 wt% in ethanol (Sigma-Aldrich), with hydroiodic acid (HI) 57 wt% in water (Sigma-Aldrich), at room temperature. HI was added dropwise while stirring. MAI was precipitated out of solution by heating the solution at 100 °C overnight to drive off the solvent. The crude MAI was either used without further purification or recrystallized in a mixed solvent of ethanol and ether. To form the non-stoichiometric MAPbI3 precursor solution, MAI and lead(II) chloride (PbCl2; 99.999 %, Sigma-Aldrich) were dissolved in anhydrous N,N-dimethylformamide (DMF) at a 3:1 molar ratio of MAI to PbCl2, with final concentrations 0.88 M lead chloride and 2.64 M MAI. This solution was stored under a dry nitrogen atmosphere.

b) Fabrication of MAPbI3 Thin Films Glass substrates were cleaned sequentially in 2 % Micro-90 detergent in water, acetone, propan-2-ol, and then air plasma. To form the perovskite layer for spectroscopy measurements, the MAPbI3 precursor solution was spin-coated on a plasma-cleaned glass substrate in a nitrogen-filled glovebox, at 2000 rpm for 60 s. After spin-coating, MAPbI3 films were left at ambient temperature in the glovebox for 30 min and then annealed on a hotplate in the glovebox at 90 °C for 150 min. The thickness of the layer was determined using a profilometer to be around 120 nm. The substrates are homogeneously covered, showing a rather rough surface of the perovskite.


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c) Characterization of MAPbI3 samples Transient PL measurements were recorded using a µ-PL setup. The laser (λ = 532 nm) was coupled in trough a 10x (NA = 0.9) microscope objective. The PL spectra were recorded with a monochromator/CCD setup. The time dependent spectra were recorded fully automatic to ensure exact time steps in between single measurements. For the excitation with the blue LED (peak wavelength 457 nm; spectral bandwidth at half maximum 21 nm), the chip was mounted 5 mm below the film, using the same detection path as for laser excitation. For the measurements under different atmospheres and temperatures, the sample was mounted inside a cryostat, while the laser sources (λ = 532 nm, λ = 405 nm) were focused using a lens with a focal length of 2.5 cm. The wavelength dependent time traces were carried out using a Horiba FluoroLog-3 system, containing a Xenon lamp (P = 450 W) and a monochromator for excitation. For the measurements, the monochromator slit was adjusted to get a spectral linewidth of 14 nm. Absorption measurements were carried out with a Shimadzu UV-2550 UV-VIS spectrometer. Unless stated otherwise, all optical measurements were carried out under laboratory conditions (lab air) at a temperature of (21.5±0.5) °C with a humidity of (42±2) %. XRD measurements were performed in grazing incidence in θ-θ geometry with ω = 1° in a PANalytical X’Pert PRO MPD system using Cu-Kα radiation.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Tel.: +49-203-379-3406 Fax: +49-203-379-3404


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Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT D.W.D. acknowledges support from an NSF Graduate Research Fellowship (DGE-1256082). A.S. acknowledges funding through the International Max Planck Research School (IMPRS) on Surface and Interface Engineering of Advanced Materials (SurMat). D.S.G. acknowledges support for perovskite sample preparation by D.W.D from the DOE BES DE-SC0013957.

ASSOCIATED CONTENT Supporting Information Available: Temperature dependent PL spectra of perovskite films, reversibility of PL decrease, photostability tests in different environments and with samples processed by different routes, time-resolved PL.

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PL Intensity Change

e Journal of PhysicalPage Chemistry 32 of 35 Lett Δt λexc. > 520 nm

MAPbI Photobrightening 1 2 Time Δt 3 λ < Environment 520 nm ACS Paragon Plus 4 Photodegradation MAPbI 5 3

exc.

3

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Figure 1. Time-dependent PL intensity of a MAPbI3 film for different excitation conditions measured in air. a) Time-dependent PL spectra of the MAPbI3 film using a LED with a peak wavelength of 457 nm at 0.1 Wcm-2 as illumination source. The spectra are normalized with respect to the spectrum obtained after one second of illumination time. Inset: Time-dependent PL spectra of the MAPbI3 film using a 532-nm laser at 0.4 Wcm-2 as illumination source. The apparent double band shape of the spectra is attributed to the involvement of surface defect states, resulting in an additional contribution of a slightly redshifted emission with respect to the band gap luminescence.43,44 b) Relative change of the PL intensity with illumination time for blue (blue circles) and green (green circles) excitation. c) Continuous PL experiment switching the light source every 60 s. The green circles represent the data during green illumination, while the blue ones represent the data during blue excitation. Note, that the sample position was kept constant throughout the experiment. 151x299mm (300 x 300 DPI)




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Figure 2. Transient PL efficiency change for different excitation wavelengths. a) Time traces of the PL intensity of a MAPbI3 film sample for 60 s illumination at each excitation wavelength between 540 nm and 465 nm and measured in air. Periods of 15 minutes in the dark were kept in between each measurement to reduce the overall stress on the sample. Emission was detected at the PL peak position (778 nm). b) Extinction of a degraded MAPbI3 layer with increased PbI2 content in the spectral region around the PbI2 band gap. 68x31mm (300 x 300 DPI)



Figure 3. Extinction spectra and X-ray diffractograms for illuminated and reference samples. a) Extinction spectra of a MAPbI3 film before (blue) and after multiple 15-minute long exposure cycles in air with blue light of 457 nm at a power density of 0.1 Wcm-2. The measurements were done at the exact same spot on the film. b) X-ray diffractograms of a reference film (black) and a film illuminated for three hours with white light at a power of 0.1 Wcm-2 (blue). The peaks expected for PbI2 and MAPbI3 are indicated by green (PbI2) and red (MAPbI3) dashed lines, respectively55,56. 88x103mm (300 x 300 DPI)


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The Role of Excitation Energy in Photobrightening and

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