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Photoinduced Stark Effects and Mechanism of Ion Displacement in Perovskite Solar Cell Materials Meysam Pazoki, T. Jesper Jacobsson, Jolla Kullgren, Erik M. J. Johansson, Anders Hagfeldt, Gerrit Boschloo, and Tomas Edvinsson ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b07916 • Publication Date (Web): 27 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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Photoinduced Stark Effects and Mechanism of Ion Displacement in Perovskite Solar Cell Materials Meysam Pazoki†,⊥ , T. Jesper Jacobsson‡, Jolla Kullgren⊥, Erik M. J. Johansson†, Anders Hagfeldt‡, Gerrit Boschloo†,* and Tomas Edvinsson∃,* †

Department of Chemistry – Ångström Laboratory, Physical Chemistry, Uppsala University, Box

523, SE 75120 Uppsala, Sweden. ‡

Laboratory of Photomolecular Science, Department of Chemistry and Chemical Engineering,

Swiss Federal Institute of Technology, Station 6, CH-1015 Lausanne, Switzerland. ⊥

Department of Chemistry – Ångström Laboratory, Structural Chemistry, Uppsala University,

Box 538, SE 75120 Uppsala, Sweden.

∃

Department of Engineering Sciences, Solid State Physics, Uppsala University, Box 534, SE

751 21 Uppsala, Sweden.

E-mail corresponding authors: [email protected] [email protected]

ACS Paragon Plus Environment

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ABSTRACT. Organometallic halide perovskites (OMHPs) have recently emerged as a promising class of materials in photovoltaic technology. Here we present an in-depth investigation of the physics in these systems by measuring the photo-induced absorption (PIA) in OMHPs as a function of materials composition, excitation wavelength and modulation frequency. We report a photoinduced Stark effect that depends on the excitation wavelength and on the dipole strength of the monovalent cations in the A position of the ABX3 perovskite. The results presented are corroborated by density functional theory calculations and provide fundamental information about the photo-induced local electric field change under blue and red excitation as well as insights into the mechanism of light induced ion displacement in OMHPs. For optimized perovskite solar cell devices beyond 19 % efficiency, we show that excess thermalization energy of blue photons play role to overcome the activation energy for ion diffusion.

KEYWORDS. Stark effect, photo-induced ion migration, perovskite solar cells. CH3NH3PbI3, mixed halide perovskites, cation dependent ion movement.

Solar cell technologies are anticipated to play a key role in clean and sustainable energy production in the future. Recently, organometallic halide perovskite materials have appeared as promising light absorber and charge transport materials, leading to the emergence of hybrid perovskite solar cells.1,2 High efficiency and low cost solution-processed fabrication methods together with spectacular material properties make hybrid perovskite solar cells a serious candidate for large-scale solar energy conversion,3 which recently was distinguished in Nature Materials research highlights.4,5


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Organometallic halide materials with general formula ABX3, where A represents an organic molecular cation, X is a halide and B is lead or tin, crystallize in the perovskite structure. These hybrid perovskites have a combination of favorable characteristics for thin film photovoltaics, such as long charge carrier lifetime, strong optical absorption, low trap densities and high dielectric constant. In solar cells, the photoactive perovskite layer is sandwiched between two selective contacts3 where only a few hundred nanometer thick active layer make a power conversion efficiencies (PCE) of more than 20% feasible.2 Perovskite solar cells have rapidly developing going from a reported efficiency of 9,7% in 20127 to a certified efficiency of 22.1% in 2016 8 very rapidly after a report for implementation of a lead-halide perovskite material as a sensitizer in a dye solar cell configuration was published in 2009 by Miyasaka et al. 6 Hybrid lead halide perovskites exhibit a plethora of interesting fundamental phenomena such as ferroelectric effects,9,10 anomalous hysteresis,

11,12

Stark effect,13 giant dielectric constant,14 giant

switchable photovoltaic effect15 and extremely slow photoconductivity response,16 which may play key-roles in the final device performance. The precise origin and importance of these effects are still largely unclear and an increased fundamental understanding of the excited state properties and charge migration processes will be important for further device optimization.17,18 Investigations of the interaction of the different perovskite cations with the photo-generated charge carriers in lead iodide perovskites is prominent for understanding the device behavior 9,16,19

, and is together with the photoinduced ion displacement the main topic of this paper.

Stark effects, spectral changes in the presence of an external electric field, have previously been observed in both quantum confined semiconductor structures20 and in dye sensitized solar cells (DSC)21 and has been used for charge compensation studies in DSCs.22,23 Stark effects in hybrid perovskite materials were reported by Listorti et al13 in 2013, who attributed the observed


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spectral changes to interfacial dipole moment at the TiO2/MAPbI3 interface. More recently, Wu et al. investigated second harmonic electro-reflectance spectra of MAPbI3 (CH3NH3PbI3, methylammonium lead iodide) and FAPbI3 (CH(NH2)2PbI3, formamidinium lead iodide). They instead attributed the observed spectral changes to changes of the light induced dipole moment in the bulk of the perovskite material.24 Here we perform a detailed investigation of photoinduced Stark effects in perovskite solar cell materials, which hold noteworthy information about the local electric fields within the material. The origin of the effect is explored by varying the monovalent cation dipole and measuring the excitation energy and modulation frequency dependence of the Stark effect using photoinduced absorption (PIA) spectroscopy combined with electro-absorption (EA) measurements. The molecular cations utilized in the hybrid lead iodide perovskites are methyl ammonium (MA) and formamidinium (FA), which have different dipole moments. To investigate the role of the dipole moment of the positive cation in the perovskite, the results were compared with an analogous system with a cation carrying no net dipole moment (Cs+). We present an experimental inspection of the photoinduced Stark shifts and thus the local electric fields in the polycrystalline perovskite films where the charge response density and the corresponding dynamics dependency on the surrounding cation dipoles under red and blue light illumination is investigated in a frequency range from 10 to 105 Hz. Time-dependent DFT calculations with photoinduced charge response as well as thermally induced ion displacements collaborate the experiments and provide insights into the fundamental thermalization mechanisms of these materials during illumination.


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RESULTS Thin films of hybrid lead iodide perovskites, MAPbI3 and FAPbI3, as well as an inorganic perovskite, CsPbI3, were deposited by spin coating on glass substrates with a thin Al2O3 nanoparticle scaffold layer (see Supporting Information for synthetic details, optical and XRD characterization). PIA measurements displayed significant signal at wavelengths near the absorption onset of the perovskite films (Figure 1). A negative absorption peak is found, which corresponds to a slight blue-shift of the absorption spectra upon the excitation. The PIA spectra closely parallel the first order derivative of the absorption spectra (Figure 1c), showing minima at 765 nm for MAPbI3, 795 nm for FAPbI3, and 700 nm for CsMAPbI3. Additional EA measurements, where an external electric field is applied across the perovskite layer, yielded very similar spectra (Figure S1). These observations provide evidences that the photoinduced optical changes in the perovskite films are due to the Stark effect where an electric field perturbs the system (Supporting Information section S4), resulting in a change in the optical absorption within the material. The amplitude of the PIA signal increases linearly with excitation power (Figure 1e), consistent with first-order kinetics of the disappearance of the photoinduced state. Recently, on the picosecond time scale, a bleach in the photoinduced absorption spectra of MAPbI3, was corroborated by spectral shifts as function of excitation energy attributed to a Burstein-Moss (BM) shift,17 i.e. a shift of the optical absorption by filling of the valence/conduction band states by charge carriers. In this study, however, no such shifts are found in the PIA spectra (Figure S2). The PIA signals found here also persists up to millisecondsecond time scale, much longer than the lifetime of free charge carriers in the perovskite,


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effectively showing another origin of the observed bleach. An external applied electric field can also cause an exponential decay of absorption coefficient below the band edge and oscillations of absorption above the band edge in a so-called Franz-Keldysh (FK) effect.25 A FK effect can be detected by oscillations in EA spectra for energies higher than the band gap of the semiconductor while no such effects are found in the PIA spectra recorded here. Hence, all data show a photoinduced Stark effect arising from photoinduced internal electric fields (see also section S3 in the Supporting Information). To determine whether the photoinduced change in absorption is an intrinsic property of the perovskite material, or caused by interfacial effects as suggested previously,13 PIA spectra were recorded for MAPbI3 films deposited on a variety of mesoporous scaffold layers, TiO2, ZrO2 and Al2O3 with different dimensions, and on bare glass. In all cases, significant photoinduced bleach was found (Figure S3 and section S1 of Supporting Information). These observations suggest that the observed photoinduced effect is intrinsic to the perovskite material and not sensitive to the proximity or type of another surface. This in agreement with the recent work of Wu et al,24 who observed a Stark effect in an electroreflectance study of MAPbI3 and FAPbI3 thin films. The effect of the monovalent “A” cation on the observed PIA of the APbI3 perovskite thin film was investigated in more detail using frequency-dependent PIA measurements (Figure 2). In this series, the dipole moment of the A-cation decreases from MA (2.22 D), via FA (0.19 D) to Cs (no dipole). Cs with no dipole delivers the strongest PIA signal where the dipoles in the material seem to contribute in shielding of the changes in the local electric fields that cause the photoinduced Stark effect. Interestingly, the wavelength of excitation has significant effect on the PIA response of perovskite thin films. The blue light excitation response is quite flat in the modulation frequency range of 10 – 105 Hz, while the response under red light excitation was


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found to be larger when the same photon flux was used for excitation, and it displayed an increase in amplitude with increasing frequency. Comparing the frequency response under red and blue excitation for the different cations (Figure 3), a shift to higher frequencies of the crossing between the bleaching response under blue and red excitation is seen for the perovskites with a low/absent dipole in the A-cation. Different features can appear in the delta absorption spectrum where the background level may shift and interfere with the Stark features. Here, experiments quantifying the signals that can be accounted for by the background in comparison to the change in absorption have to be performed. In our case, all the Stark features are clearly distinguishable from the background shifts for all frequencies except the highest frequencies where the relative intensities of the background and the Stark shifts become comparable (See Supporting Information S7 and Fig. S16). The different orders of the Stark effects can be quantified by analyzing the 1st, 2nd, and 3rd harmonics in the PIA measurements for high and low dipolar A-cations would give some extra information on an overall mechanism (see section S3 and Fig. S4-5 of Supporting Information). The second harmonic PIA spectrum, which should correspond to the second order Stark effect, interestingly changes sign for MAPbI3 and not for FAPbI3 (Figure 4) and thus reveal a reversal of the local polarizability for the material with the strongest dipolar cation. The Stark effect response is also analogous to the shape and wavelength of the first derivative of the absorption spectra (Figure 1c) giving further support that it is consistent with a photoinduced effect. This effect also correlates with the different transition dipole moment of FAPbI3 and MAPbI3, and could thus be related to both the higher binding energy of FA in the FAPbI3 structure which has a higher number of possible hydrogen bonds compared to MA,26 as well as secondary effects investigated in more detail below.


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To investigate the Stark effect in more complete devices and its relevance for understanding the device physics, state-of-the-art perovskite solar cells with efficiencies beyond 19% were fabricated (Figure 5a, here mixed perovskite27 with compositional formulae MA0.33FA0.77PbI2.5Br0.5 that shows the highest reported efficiencies so far). The Stark effect was clearly seen for the full device with mixed organic cations as well and the response closely resembles the Stark spectrum of MAPbI3 (Figure 5b) which is as expected, since the charge transporting layers have no interfering features near the wavelengths of band edge absorption.28 We can therefore conclude that the observations made for the perovskite films above also are valid for perovskite films in high efficiency devices with mixed cations. A hysteresis effect11 in the current voltage curves is observed here when the device is scanned in forward or backward scans and give an overall power conversion efficiency (PCE) of η=19.2% in the backward direction and η=16.1% in the forward direction with the largest hysteresis difference in the fill factor (FF) (Figure 5a). Encouraged by the large differences observed for blue and red light excitation in PIA measurements, we investigated the Voc decay in dark after illumination with blue or red light. Intriguing differences were found: After red light (630 nm) illumination the VOC decayed relatively rapidly, but after blue illumination (470 nm) with the same photon flux, a much slower decay was found, even though the initial voltage was approximately the same (Figure 5c). Varying the illumination time with the blue photons from 3 s to 30 s, an extended decay time is observed (Figure 5c) and complies well with an increased number of migrated ions. To further verify the interdependence of the activation energy in blue light and the drift in a photovoltaic field, a blue light illuminated solar cell was switched into dark and then into red light illumination (Figure 5c). The red light applied directly after the blue light revealed a markedly


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extended tail in Voc, in comparison to only red light illumination; its relevance to Stark spectrum and ion movement would be further discussed. Varying the illumination wavelength and monitoring the Voc decay would thus provide a promising method to pinpoint the ion movement in different perovskite compositions assisted by thermalization of charge carriers. Quantum chemical calculations, ground state DFT and time dependent (TD) DFT, were performed in order to relate the experimental observations to optical transitions, excitation wavelength dependency of charge response and implications for bond length relaxations and ion displacements in the crystal structure. Calculations were also utilized to extract the energy barriers for A-cation related ion diffusion from first principles (Fig 6, 7 and Table 1) and quantification of the iodide diffusivity in the different systems. The bottom of the conduction band (CB) of the hybrid perovskites consists of molecular orbitals dominated by Pb-states, while the valence band (VB) upper states have a strong contribution from iodide, (Figure 6) in agreement with recent studies. 29,30 The organic cations MA/FA yield energy levels more than 1 eV above the CB band edge (Figure 6a). The energetic position of the unoccupied organic states is here significantly lowered in the perovskites crystals compared to the virtual states of MA and FA in gas phase, which can be ascribed to a stabilization of the organic cations in the negatively charged inorganic framework. The virtual states of MA are high up than FA and Cs states, consistent with the relative strength of the dipole of the cations (Figure 5a, c and Figure S16). The exact determination of the absolute energetic positions of the states has to be taken with care though, due to the well-known band gap uncertainty in the generalized gradient approximation (GGA). The charge density response to an incoming light was calculated by TD-DFT to quantify the charge response in red and blue light, Figure 6c,d. Here red (730 nm) and UV/blue (395 nm) excitation were chosen to match the two distinct characters from the partial density of states


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(PDOS). Red light excitation mainly leads to an electronic charge transfer from I-based to Pbbased orbitals. In contrast, the charge response shows that higher energy excitation (blue/UV light) has a possible charge transfer to MA/FA localized states (I->MA/FA) as well as to the Pb states. Here, partial charge transfer from iodide to FA+ is clearly seen in FAPbI3 under blue light, but only minor charge transfer contribution to MA+ in MAPbI3 is observed. These results then indicate that the FA+ cation perovskite have a partial charge transfer to the organic part when illuminated with light sources including UV/blue light and thus a possibility to change of the effective dipole of the A-cation in this materials excited state in comparison to the MAPbI3 and CsPbI3 systems. As the local field also is dependent on possible ion displacement and its dependence of the type of A-cation, the energy barriers of iodide displacement in the perovskites were calculated for the MAPbI3, FAPbI3, and CsPbI3, and thus as a function of monovalent cation dipolar strength. The energy barrier for iodide displacement was determined (Table 1) by means of two methods, constrained path and nudged elastic band (NEB). To evaluate the ionic movement, a transition state model was considered in which the excess energy of the excited electrons is thermalized and thus transferred to vibrations and thus could assist the ion movement in overcoming the energy barriers of the iodide vacancy diffusion (see Table 1 and section S.9 of Supporting Information for the details). Previously reported activation energies for ionic movement in MAPbI3 have ranged from 80meV to 600meV31,32 and thus in a rather wide range, where our data comply well with recent calculations that are in the middle of that range.33 Here we find that constrained path calculations do not capture the expected decrease in the activation energy of the dipolar series in MA+, FA+, and Cs+ . The different hydrogen bonding situations during the iodide displacements thus have to be taken into account with lattice relaxations during the ion


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movement. Therefore, nudge elastic band (NEB) calculations with lattice relaxation during the iodide movement where performed showing 372, 257, and 213 meV activation energies for iodide displacements in MAPbI3, FAPbI3, and CsPbI3, respectively. Recalling the red light excitations using 630 nm and the blue light excitation using 470 nm in the experiments in Figure 3, this translates into 1.97 eV and 2.64 eV, respectively. With a band gap of 1.55 eV for MaPbI3, this implies a limiting thermalization energy of a maximum of 420 meV and 1.09 meV if relaxed to the band edge with no deconstructive lattice vibrations, and lower for the other, high band gap perovskite materials. The available thermalization energy under red light excitation is seen to barely be enough to thermally activate iodide movement in contrast to the blue excitation light that carry enough excess thermal energy after relaxation to the band edge states to allow thermally activated ion movement as discussed below. The dipolar MA+, and FA+ cations that also form I-H bonds showed a coupled motion and rotation with the movement of the iodide presented as a displacement of the center of mass with 0.14, and 0.24 Angstrom for MA+ and FA+. Interestingly, the monovalent cations Cs+ with no inherent dipole and less possibilities for directive hydrogen bonding show the largest mean displacement of 0.26 Angstrom. Here, the magnitude of the dipole and the possibility to charge compensate with a rotation seem to effectively counter balance the center-of-mass movement of the cation. As a result, a large spherical cation with no inherent dipole show a larger displacement in response to the iodide displacement (Table 1). Using a transition state model for the thermal activation of the iodide movement via the jump rate Γ= ν0·exp(-∆E/kT) 34where ν0 is the attempt frequency and the ∆E activation energy, one can calculate the effective diffusivity (D) via the Einstein-Smoluchowski relation

= Γ = (Δ/)

(1)


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Where d is the jump distance, k is Boltzmann´s constant, and T is the temperature. The attempt frequency is related to the I-Pb-I thermal motion, where one of the strongest intensity low wavenumber phonons can be found at 96 cm-1.35The energy of this mode translates to 2.88 THz and is here considered as the major contributor to the thermally activated attempts. In addition, modes at lower wavenumbers can contribute but are lower in intensity and as the attempt frequencies constitutes the pre-factor, the lower wavenumber modes as well as possible shifts of the dominating I-Pb-I phonon band with different cations in the cuboctahedra voids in between the octahedral inorganic framework are here neglected compared to the different contributions in the exponential term. The total jump distance over the NEB path is 4.614 Å, 3.977 Å, and 4.248 Å, for MAPbI3, FAPbI3, and CsPbI3, respectively. The effective iodide diffusivities for the different cations (Table 1) show a factor of 500 higher diffusivity for iodide in CsPbI3 in comparison to MAPbI3, with FAPbI3 in the middle, consistent with the relative strengths of the dipoles of the A-cations. In addition, the local structural changes including the neighboring PbI6 octahedron distortions due to the ion movement seems to be correlated to the observed absorption changes and magnitude of Stark effect as well as the dipole of the A-cation that would be further discussed (Table 1). The reported distortion factor (DF) in the table 1 is here defined as the averaged and squared difference of Pb-I bond lengths (Pb⋅⋅⋅I) within the octahedron minus the average Pb-I bond length (Pb⋅⋅⋅Iavg) with

1 = ( ⋯ − ⋯ 6

!" )

× 100 (2)


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The reported DF numbers in table 1 corresponds to the vacancy neighboring octahedron showing a rather local effect. DF for far-away octahedrons were significantly lower. DISCUSSIONS The spectral change caused by the Stark effect can be analyzed in terms of a frequency shift ∆ν of an optical transition due to an electric field E, which is related to the change in dipole moment between ground state and excited state ∆µ and the change in polarizability ∆α (Supporting Section S4):

1 h × ∆υ = − E.µ − E.∆α .E 2 (3) Where h is Planck’s constant. The resulting experimentally measured absorption change ∆A, is a function of electric field E (Supporting Section S4):

∆A × h = −

dA 1 dA 1 d2A 1 d2A 1 d3A 2 E.µ − E.∆α .E + ( E . µ ) + ( E . µ )( E . ∆ α . E ) − ( E.µ ) 3 2 2 3 dν 2 dν 2 dυ 2 dυ 6 dυ (4)

Analysis of the PIA spectra for MAPbI3 and FAPbI3 films shows that the dominating part of the observed phenomena can be attributed to the first order Stark effect described by the product of the electric field and the dipole moment change (Supporting Section S3, Figure S4-5 and Table S1). Different mechanisms seem to take part in the low and the high frequency parts of the PIA spectra and were seen to depend on the incoming photon energy. The red excitation gives features in the PIA response that gets weaker with lower frequency, whereas the stable imaginary part of blue observed in the whole frequency range is indicative of a second phenomenon happening with a delay relative to the incoming blue light.


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The frequency dependence of the Stark bleach observed in the PIA spectrum corresponds to a relaxation of the photoinduced charge response and the electric fields inside the material (equation 3). Although the lifetime of the charge carriers in the MAPbI3 film is below the millisecond time regime, the relaxation of photoinduced structural changes and photoinduced halide redistribution have been observed on considerably longer times in these systems and approach the second time scale.16,36,37 Neglecting the initial charge response contributions, the frequency dependent first order Stark effect would contain information of the electric field dynamics in the film. The behavior of the measured frequency dependent PIA for the blue excitation follows the electric field relaxation (dielectric constant) previously reported inside a semiconductor with a dipolar response 38 and further validating the ability of Stark spectroscopy as a probe for the local electric field inside a photoactive dielectric material. Excitation of hybrid lead halide perovskite films leads to the formation of free charge carriers on the ps time scale;18and the emission lifetime of the charge carriers is in the order of 100 ns for high quality perovskite films.39 Reported trap assisted recombination lifetimes are in the sub microsecond and nanosecond regimes.40 The photoinduced Stark effect observed here in is on the µs to second time scale and must therefore be caused by long-lived states rather than free charge carriers or vacancy trap states. Photoinduced trap states, PbI6-octaeder distortions, and charge trapping at interfaces together with ion displacements are also possible candidates for the origin of the Stark effect and are discussed below in relation to the experimental results presented in this study. Trapping of charge carriers can result in long-lived states in the film. If such states are randomly distributed in the film, a second order effect, which results in broadening of the absorption peak could occur, but this effect was not observed experimentally here. A net electric field can can occur if electrons and holes are trapped at different interfaces of the crystals. For


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instance, electrons can trap at Pb terminated surfaces and holes at iodide terminated interfaces. Under such conditions, an electric field will appear in the crystal in a specific orientation, and a first order Stark effect can appear. However this picture cannot explain the observed excitation light dependency of PIA spectrum and is not in agreement with expected terminated surfaces of MAPbI3 crystals.41 Rotation of the MA cation in MAPbI3 has been identified as a process occurring in devices under working conditions and suggested to affect: local metal-halogen bond length,26,42 the charge collection,9 the recombination,19 the VB and CB positions,19 as well as the hysteresis observed in current voltage measurements.43 Time constant of this rotation has been estimated theoretically19 for single MA in the lattice and there are experimental measurements evaluating the time constant in these materials44,45 but until now there have been no direct measurements of how this depends on the available total energy or spectral distribution of the incoming light. The photoinduced charges and ion displacements play important roles in and are coupled to local orientation of the MA in the lattice.42 Considering the ensemble average of the dipole orientations, any change in the orientation of the MA dipoles would change the local electric fields and relaxation dynamics of Pb-I bond length in the material and result in a local Stark effect. In contrast, the observation of a dominating first order Stark effect for the perovskite suggests a fixed angle between the photoinduced electric field and the dipole moment change. 39 Consequently, the main contribution to the Stark effect does not belong to randomly oriented organic dipoles within the film, but those could instead contribute to the second order Stark effect which depend on the change in the local polarizability. Observation of the Stark effect in CsPbI3 perovskite with no A-cation dipole further justifies this view and although the reorientation of A-cation dipole indirectly contributes to the local field change, it is not the main


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origin of the corresponding local electric fields. The observed cation dipole dependency of Stark effect (Figure 2,3) instead points towards a local movement that will shield local charge response processes in the inorganic framework of the iodide-lead octahedral (see also Table 1). Recently suggested photo-induced ion movement37 and consequent structural changes such as shortening of Pb-I bondlength26,36 presents long lived relaxation of local electric fields in the lattice that certainly can provoke the Stark effect within the films. Slightly different Pb-I bond lengths together with different orientation ordering of organic cation has previously been reported to give meaningful changes in CB and VB band edges19 and can subsequently change the dipole transition moment of the red optical transition. The latter can be another possible explanation for the observed absorption changes quantified by equation (4). Ionic movement has previously been suggested as one possible main effect responsible for the current voltage hysteresis observed in the lead perovskite solar cells31,32,46 and it has also been suggested to be responsible for variations in the low frequency part of perovskite dielectric constant (ɛ) .47 In a recent publication by De Angelis et al, it has been suggested that the ionic movement in the device is coupled with the rotation of MA dipoles that can provide local electric fields(E) through Pb-I bond length change.42 The shielding of the electric field can be formulated via the dielectric constant (ɛ) which is related to the polarizability (α) through the vacuum dielectric constant (ɛ0) by:

ε = ε0 + α (5) Therefore, ionic movement can not only create a local electric fields (E) and affect the Stark effect through the field but also influence the second order Stark effect through the changes in the dielectric constant via ∆α (equation 3). That is expected to be perturbed with a delay with


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respect to the incoming light and can thus participate in the imaginary part of the Stark effect absorption changes and dielectric constant at low frequencies (Figure 5d). The participation of ionic movement in the low frequency part of the Stark spectrum would be different for red and blue light excitations at the same photon number or the same intensity conditions (Figure 5d). As a result of the thermalization, heating the samples with more energetic phonons result in an increased ionic transport within the sample that have consequences on the hysteresis behavior of the device. A full behavior of the hysteresis effect and its dependency to the wavelength of incoming photons is under investigation and will be published in a separate study. It should also be noted that the observed PIA signal represents the ensemble average of the absorption changes of the individual octahedrons within the film but where displaced ions and also locally understoichiometric structures will experience a different local field. It worth to notice that apart from translational displacements, the photo induced local electric fields in the octahedron can also be affected by the orientation of the organic molecule, that depends on the different preferential orientations of the perovskite grains, and in turn on the illumination history and film preparation conditions.48 Based on the experimental results and the theoretical assessments, a plausible mechanism for the excitation energy dependent Stark effect can be formulated via thermalization effects from octahedral distortion and ion (A-cation and iodide) displacements (Figure 7). Upon excitation or photoinduced ionic movement37, i) the local electric field change as the bond lengths change in the inorganic octahedron and also affects the neighboring octahedrons with an induced tilting during a ligand-to-metal charge transfer similar to the mechanism for the photoinduced piezoelectric effects reported in inorganic perovskites, 49 and here amplified with a thermally activated ion movement. This in turn leads to ii), a change in the hydrogen bonding between the


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organic cation’s hydrogen atoms and the halide. This gives important information concerning the magnitude of the coupling strength between the organic cation in the cuboctahedral void and the light induced charge reorganization in the Pb-I octahedral. In the final step, iii) the organic cation rotation is initiated by a change of the charge of a neighboring iodide in an I-→Pb2+ charge transfer process and also the subsequent change in Pb-I bond length42. In UV/blue light there is also a possibility that a charge transfer accompanies this effect to the A-cation in the deep absorption (blue light) for FAPbI3 and CsPbI3 but only to a minor extent for the higher laying bands for the A-cation in MAPbI3. As the Stark effect is strongest for the Cs+ ion, without strong hydrogen bonding or inherent dipole, this reveals that the main part of the Stark effect is initiated from process i) with displacement of the iodide due to a changed bond length and possible tilting of the octahedrals, and at a longer time scales also iodide migration, changing the local electric field. The experimental data show that the cations with the stronger dipoles shield the change in local electric field more effectively. The dependence of the distortion factor and the local movement of A-cation during the ion movement (Table 1) from the density functional theory calculations are in line with the trends of the Stark effect and the dipole moment of A cation which supports this idea. The weaker 2nd order stark effect is instead dependent on process ii) and iii) where the strongest dipolar cation (MA) displays a flip and thus a reversal of the local polarizability indicative of MA rotation. On longer time scales, thermalization energy can induce ion movement and is interconnected to both steps i) to iii) and can be considered as mainly responsible to decrease the observed Stark effect on longer time scales. This is also consistent with the experimental data for the red light in Figure 3 where there is no charge compensation response at high frequencies. With lower magnitude of the A-cation dipole, and higher


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diffusivity (Table 1), the charge compensation is also shifting to higher frequencies as shown in Figure 3b and c also showing that an increase in the thermally activated ion movement under blue light excitation help in the shielding over all of the measured frequencies. The processes at longer time scales also displayed a spectral dependence where blue light excitation carry significant energy for the thermalization process to overcome the energy barrier of the iodide migration in contrast to just a small effect for red light. The spectral dependence and the frequency dependency of these effects with the dependence on the inherent dipoles moment of the cuboctahedrally coordinated cations together with first principle calculations thus given fundamental information on the mechanism of the local thermalization as well as how this is coupled to the ion movement dynamics in OMHPs. CONCLUSIONS In summary, from the data presented one can attribute the observed Stark shift in the perovskite materials to a local electric field change upon excitation. An A-cation with higher dipole in the perovskite material show a more effective shielding of the photo induced Stark effect where red light excitation (630 nm) show a frequency dependence of the bleaching in contrast to the photoinduced Stark effect under blue light excitation (470nm) Following the experimental trends and collaborating DFT calculations, an optically and thermally activated mechanism for changes in the local structure leading to less molecular interaction from the surrounding dipolar monovalent cation and, last, on longer time scale, a local change in polarizability related to the ion displacements is suggested. In agreement with our proposed mechanism, the activation energies for iodide displacements were different for the different monovalent cations following the magnitude of the dipole moments where DFT calculations showed that the iodide movement where coupled to the movements of the monovalent cations (MA+, FA+, and Cs+) and the


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rotation of the MA and FA dipoles. The change in polarizability in domains can affect the observed Stark effect on the second order, while our results for the first and second order Stark effects are in line with the picture related to the ion migration and the impacts on the I-V hysteresis behavior of the device. The experimental quantifications of the photoinduced Stark effect for different cations reveal how the local electric field depend on the molecular components where mechanisms of charge compensation as well as ion movement can be analyzed. Adding spectroscopically resolved responses and extended frequency ranges for the future bear promise of a Stark spectroscopy that can be used to analyze many aspects of dynamics of the local electric fields as well as their dependence of local order as well as thermally induced ion migration in hybrid perovskites and possibly also in photoactive semiconductor materials in general. METHODS Film preparation: Substrates of microscope glass or fluorine doped tin oxide conducting glass were ultrasonically cleaned and if needed, FTO was etched by zinc powder (and 2M HCl aqueous solution for 10 minutes with subsequent polishing and washing with DI water. Thin dense TiO2 films (blocking layers) were deposited (if needed) with either spray pyrolysis or spin coating. Different mesoscopic scaffold layers were deposited on the substrates, dried on hotplate (80 OC, 10 minutes), sintered for 30 minutes (150 OC for Al2O3) and 450 OC for the others, and transferred to the dry box. MAPbI3 was deposited on the substrates following either the two-step deposition method for obtain iodide MAPbI3 and a one-step deposition method for “mixed chloride” MAPbI3. Briefly, for the two-step deposition method, 1 M solution of PbI2 in anhydrous N,N-dimethylformamide was spin coated on the substrate. Substrates were heated to 70 OC on a hotplate placed in a dry


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box; after cooling down to room temperature and they were dipped in 10 mg/ml MAI solution in anhydrous 2-propanol for 30 seconds. Finally, the substrates were washed with 2-propanol and dichloromethane and heated to 100 OC in a dry box. In summary for mixed chloride MAPbI3, 3:1 molar ratio of MAI: PbCl2 in DMF was prepared in glovebox while stirring overnight. The solution was spin coated on the substrates in a drybox at 2000 RPM and the substrates were heated to 120 OC hotplate in a dry box. The same procedure was used for FAPbI3 material using the FAI instead of MAI in mixed halide procedure. Device fabrication procedure for mixed perovskite films is described in the supporting information as well. Film Characterizations: The prepared films were characterized by XRD, UV-Vis, PIA and EA (details in the supporting information). The PIA spectroscopy setup, utilized a modulated light produced by a blue LED (470nm) or a red LED (630 nm) connected to a wave generator and coupled to a lock-in amplifier. A white lamp produces the probe light. A Si detector together with a monochromator records the wavelength dependent absorption of the sample through the computer connected current amplifier. The PIA spectra of the bulk perovskite materials were recorded using a glass covered perovskite with an Al2O3 scaffold using halide perovskites. To certify the validity of the observed frequency dependent signal, the accuracy of measured data was extensively double-checked and corrected for any instrument response delay (Supporting text and Figure S10). For EA, a semitransparent thin layer of silver (10 nm) is applied as top electrode. For the PIA measurements the samples were excited using a blue LED (with a maximum wavelength of 470 nm), using a square wave on/off modulation with a frequency of 93 Hz where the change in light absorption was monitored using a lock-in detection system.


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For the EA measurement, the change in light absorption was measured for square wave modulation of an electric field. DFT calculations: The computations were performed on resources provided by SNIC through Uppsala Multidisciplinary Center for Advanced Computational Science (UPPMAX) under Project sinc2014-3-71 and snic2015-6-65. The Quantum Espresso package was used for DFT calculations of the MAPbI3 and FA PbI3 structure using the General Gradient Approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) pseudo-potential. Lead 5d10/6s2/6p2, Nitrogen 2s2/2p3, Iodide 5s2/5p5, Carbon 2s2/2p2 electrons considered as valence electrons. The super cells consisted of 48 atoms corresponding to four unit cells with different orientations of MA and FA dipole in order to obtain the most stable phases. Lattice parameters of the super cells were a=8.71 Å and c=12.46 Å in the tetragonal Bravai lattice while the basic unit cell of MALIP inside the super cell is near cubic with a= 8.86 Å lattice constant. Full details of DFT calculations and experimental methods can be found in the Supporting information.

ASSOCIATED CONTENT Supporting Information. Full description of theoretical and experimental methods, Experiments with different scaffolds/interfaces, XRD graphs of the perovskite films, Details of curve fitting for different harmonics of PIA spectra , Derivation of Stark effect , AC and DC bias dependency of EA spectra for FAPbI3 and MAPbI3, Light intensity dependence of photo induced Stark effect, Comment about timed PIA and calibration of frequency dependent PIA , Hopping model for iodide movement in the perovskite lattice , additional EA spectra of the perovskite films, estimated PDOS and structural geometry of the MAPbI3 and FAPbI3, and normalized UV-


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Vis absorption of the perovskite films. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] * Email: [email protected] Author Contributions M.P. initiated the work, carried out the experimental part except the device fabrication, main part of the DFT calculations, data analysis and wrote the draft. M.P. E.J, G.B, J.J., J.K,A.H. and T.E. discussed and analyzed the results in context of possible mechanisms. M.P and J.K performed the energy-barrier calculations, J.J. provided the mixed halide perovskite devices. T.E performed part of the DFT calculations, and G.B. and T.E. guided the work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources We thank the Swedish research council (VR) and the Swedish energy agency for financial support. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT MP thanks Michael Wang for his precious help in constructing the amplifier circuit for use in the high resolved PIA measurements, dedicates this paper to Dr Roghayeh Imani for her support and help, and appreciates Filippo De Angelis for his kind help regarding the DFT calculations. Uppsala Multidisciplinary Center for Advanced Computational Science (UPPMAX) is acknowledged for providing computational resources under projects snic2014-3-71 and snic2015-6-65. We thank the Swedish Energy Agency, the Swedish Research Council, and the STandUp for Energy program for financial support. REFERENCES (1)

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Table 1. Values of diffusivity, energy barriers, movement of counter-ion (A-cation) and the vicinity octahedron distortion factor for iodide vacancy diffusion in perovskite solar cell materials. Material

Constraint path barrier for Imovement(meV)

NEB barrier for Iodide movement (meV)

MAPbI3

315

372

FAPbI3

191

Iodide diffusivity using NEB (cm2s-1)

Coupled movement of counter ion (Å)

Relative Octahedron Distortion Factor

0.14

0.535

0.24

0.736

0.35·10-9 257

26.05·10-9


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CsPbI3

212

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213

0.26 173.75·10

1.963

-9

Figure 1. Schematic illustration of the experimental (a) EA and (b) PIA setup, (c) first order derivative of the absorption spectra of MAPbI3 ,FAPbI3 and CsPbI3, (d) PIA spectra of MAPbI3, FAPbI3, and CsPbI3 (e) linear dependency of observed stark effect versus light intensity for MAPbI3. The excitation wavelength in (d) is 470 nm. The presented data in (d) are representative of spectral features of the stark effect and are not normalized versus the calibration factors.


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Figure 2. Frequency dependence of the normalized photoinduced bleach (∆T/T) for OMHP films under (a) blue light excitation (470 nm) and (b) red light excitation (630 nm).

Figure 3. Frequency dependence of the observed photoinduced bleach and the effect of excitation wavelength and A-cation in OMHP thin films: (a) MAPbI3, (b) FAPbI3 , and (c) CsPbI3.The excitation wavelength for blue and red are 470 and 630 nm respectively.


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Figure 4. DFT optimized ground state structures and experimental determination of the frequency dependent Stark effect. (a) Ground state geometry of MAPbI3 in super cell calculations showing the local position of MA ; the dipole of the MA cation calculated in gas phase. (b) Ground state geometry of FAPbI3 in super cell calculations showing the local position of FA; the dipole of the FA cation calculated in gas phase. (c) PIA spectra of MAPbI3 in intermediate (1 kHz) and high frequencies (10 kHz) measured in first, second and third harmonics. (d) PIA spectra of FAPbI3 in intermediate (1 kHz) and high frequencies (10 kHz) measured in first, second and third harmonics. The excitation wavelength in (c) and (d) is 420 nm.


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Figure 5. Perovskite solar cell device: performance, photoinduced absorption and photovoltage decay. (a) Current-voltage characteristics under AM1.5G light (100 mWcm-2) (b) PIA spectrum (c) Voltage decay and (d) Low frequency response of the Stark effect. Voltage decay is measured after different illumination times by red and blue LEDs. The measured device is from the mixed perovskite family with compositional formulae MA0.33FA0.77PbI2.5Br0.5 that shows the highest reported efficiencies so far.


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Figure 6. PDOS of DFT optimized ground state structures, and charge response from TD-DFT. (a) Calculated MAPbI3 (P)DOS with energy states available upon excitation indicated with red and blue arrows. (b) TD-DFT calculations of the charge response for z-axis polarization upon excitation using UV/blue light and red light for MAPbI3. (c) Calculated FAPbI3 (P)DOS with energy states available upon excitation indicated. (d) TD-DFT calculations of the charge response for z-axis polarization upon excitation using UV/blue light and red light for FAPbI3.


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Figure 7. A schematic of suggested mechanism for the excited state charge response inferred from the DFT calculations. (a) i ) Ligand-to-Metal charge transfer with bond lengths change and tilting in the inorganic octahedron sub lattice, ii) Change of local hydrogen-iodide bonding situation for the organic ligand, and iii) Organic cation displacement or rotation in response to the new local environment, (b) Band diagram of MaPbI3 and illustration of excess thermalization energy during red and blue excitations and thermally activated ion movement in the perovskite lattice.

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Photoinduced Stark Effects and Mechanism of ... - ACS Publications

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