Article pubs.acs.org/JPCC
Pronounced Exciton Dynamics in the Vacancy-Ordered Bismuth Halide Perovskite (CH3NH3)3Bi2I9 Observed by Ultrafast UV−vis−NIR Transient Absorption Spectroscopy Mirko Scholz, Oliver Flender, Kawon Oum,* and Thomas Lenzer* Universität Siegen, Physikalische Chemie, Adolf-Reichwein-Str. 2, 57076 Siegen, Germany S Supporting Information *
ABSTRACT: We present an investigation of the ultrafast charge carrier dynamics for the one-third metal-deficient lead-free perovskite (CH3NH3)3Bi2I9 on mesoporous TiO2. Excitation of the perovskite at 400 or 505 nm leads to characteristic secondderivative-type spectral features in the transient absorption spectra, suggesting substantial contributions of bound excitons, in contrast to the widely used lead-based perovskites. The immediate appearance of broad NIR absorption is assigned to TiO2 conduction band electrons formed by instantaneous dissociation of a subpopulation of excitons at the perovskite/TiO2 interface. Excitation with excess energy above the perovskite’s band gap opens up an additional fast (70 fs) exciton dissociation channel with about 26% amplitude. Antisolvent-assisted synthesis of (CH3NH3)3Bi2I9 reduces the crystallite size to about 500 nm but has only a minor effect on the carrier dynamics. The results suggest that photovoltaic applications of this material will likely require bulk-heterojunction architectures to efficiently split the excitons into free carriers.
1. INTRODUCTION Over the past years, lead-based halide perovskite materials derived from methylammonium lead iodide have reached high photovoltaic power conversion efficiencies,1 with a currently certified top value of 22.1%.2 However, two critical issues might eventually hamper the widespread use of lead-based perovskites in modules for solar light-harvesting and other applications, such as light-emitting diodes (LEDs) or laser active media. One point refers to intrinsic long-term stability problems,3,4 e.g., related to moisture,5,6 oxygen,7 thermal decomposition,8−10 and light-induced halide segregation.11 In addition, the toxicity of the water-soluble lead salts released in the case of module leakage is of great concern when it comes to outdoor applications.12−15 The unique properties of the lead-based halide perovskites, such as methylammonium lead iodide (CH 3 NH 3 PbI 3 , “MAPI”), result from their band structure which is closely linked with the 6s26p0 electronic configuration of Pb(II).16−18 Replacing the divalent lead by the isoelectronic and much less toxic Bi(III) leads to the formation of the vacancy-ordered perovskite (CH3NH3)3Bi2I9 (“MABI”) which exhibits good stability against humidity.16,19−23 This one-third metal-deficient perovskite (“CH3NH3Bi2/3I3”) features a low-dimensional 0D perovskite structure containing [Bi2I9]3− dimers of face-sharing BiI6 octahedra.24 The exciton binding energy of MABI single crystals was estimated as at least 300 meV25 and therefore substantially higher than for the lead-based MAPI perovskite (2−50 meV).26−31 Surprisingly little is known regarding the © XXXX American Chemical Society
ultrafast dynamics of MABI after photoexcitation, except for some photoluminescence20 and transient absorption (TA) results covering slower time scales. 22 However, such information is required to assess the early time dynamics and relative importance of exciton and free carrier generation. Here we present ultrafast TA results of MABI spin-coated on a mesoporous TiO2 (mp-TiO2) scaffold to identify its ultrafast charge carrier dynamics in the time range up to 2 ns with 80 fs time resolution.
2. EXPERIMENTAL SECTION 2.1. Preparation of Methylammonium Bismuth Iodide Perovskite Thin Films on Mesoporous TiO2. Cleaned glass slides were dried at 200 °C for 45 min in a dry nitrogen atmosphere. Sintered 300−600 nm thick mesoporous films of TiO2 were prepared on the slides as described in our previous publications.32−36 Bismuth triiodide (x(BiI3) = 0.04, ChemPUR, 99.999%) and methylammonium iodide (x(MAI) = 0.06, Dyenamo) were dissolved in γ-butyrolactone (Sigma-Aldrich, 99%) resulting in a 30 wt % “MABI perovskite solution”. The solution was applied while spinning the substrate at 500 rpm for 4 s under dry nitrogen in a glovebox. Then the rotation speed was increased to 3000 rpm for 30 s. In the “antisolvent Received: May 11, 2017 Revised: May 18, 2017
A
DOI: 10.1021/acs.jpcc.7b04543 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C route” the samples were additionally treated with 100 μL of chlorobenzene 14 s after the “perovskite solution” was applied. After spin-coating the samples were heated to 70 °C for 45 min. 2.2. Ultrafast Transient Absorption Spectroscopy. Two PSCP setups were employed to record femtosecond broadband transient absorption spectra in the UV−vis and NIR regions (300−700 and 850−1600 nm, respectively). They are described in detail in our previous publications.32,33,36−43 Pump pulses at 400 or 505 nm (fluence ca. 50 μJ cm−2) were produced by the second harmonic of the amplifier system and a NOPA, respectively, and chopped at 460 Hz. The pump and supercontinuum probe beams were polarized at a magic angle (54.7°). The cross-correlation of the setup was typically 80 fs with an accuracy of the zero time delay within 20 fs. The pump and probe pulses were focused into a home-built contact cell35 holding the perovskite thin film, which was constantly flushed with nitrogen (Messer, 4.6). During the measurements, the sample cell was randomly moved by an x/y piezo stage within a 2 × 2 mm2 plane perpendicular to the probe beam propagation axis. Transient spectra at each time delay were averages of three scans with 1000 pump−probe cycles. Single-shot baseline corrections were applied. Steady-state UV−vis−NIR absorption spectra were recorded on a Varian Cary 5000 spectrometer before and after each measurement showing no sign of sample degradation. For 505 nm excitation the strong scattering of the samples did not allow us to use the wavelength range 490−540 nm for analysis. 2.3. X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) Experiments. The characterization of the thin films was performed by XRD at T = 294 K using a PANalytical X’Pert MPD PRO diffractometer, as described previously.32,33 XRD patterns were simulated with GSAS44,45 using published structures.24,46 SEM pictures were recorded employing an FEI Quanta 250 FEG.
Figure 1. (A) XRD pattern of MABI on mp-TiO2 with assignments of the main Bragg reflexes (blue line: magnification of the red XRD pattern). (B) Steady-state absorption spectrum of MABI on mp-TiO2. (C) Simplified energy level diagram of the system MABI/mp-TiO2 including band gap energies and excitation wavelengths.
3. RESULTS AND DISCUSSION Figure 1(A) shows the XRD pattern of MABI on mp-TiO2 (red) including an enlargement (blue) with assignments for the most prominent peaks. A complete assignment is provided in the Supporting Information (Figure S1). The positions of the Bragg reflexes agree well with those previously reported by Kaskel and co-workers24 identifying the space group P63/mmc. The steady-state absorption spectrum of the thin film at 296 K in Figure 1(B) shows a characteristic excitonic absorption peak at 500 nm (20 000 cm−1, 2.48 eV) before the onset of the continuum absorption toward shorter wavelengths, in agreement with previous studies.20,23,25,47−49 Photoexcitation at either 400 or 505 nm in combination with broadband TA detection enables us to study the importance of excitons in the carrier dynamics of MABI. Also electron injection processes from the MABI conduction band (CB) into the CB of mp-TiO2 are evaluated. Such a process is energetically feasible as shown in the band diagram of Figure 1(C).19,50 TA spectra for the UV−vis−NIR range (300−1600 nm) from our pump−supercontinuum probe experiments for excitation at 400 are presented in Figure 2(A). Corresponding PSCP spectra for excitation at 505 nm as well as a comparison of contour plots for these two excitation conditions can be found in Figures S2 and S3 of the Supporting Information, respectively. At short times (top panels in Figures 2(A) and S2) a characteristic oscillatory triple-peak structure appears in the peak region of the steady-state absorption spectrum around 500
Figure 2. (A) Broadband transient absorption spectra after excitation at 400 nm. The noise around 400 nm is due to scattering of the pump beam light by the sample and has been removed in the third panel. The magenta and blue dotted lines in the bottom panel correspond to the inverted steady-state absorption spectrum and the smoothed second derivative of the steady-state absorption spectrum, respectively. (B) Early time kinetics of MABI in the bleach band at 350 nm (left) and in the exciton absorption band at 465 nm (right) for excitation at 400 nm (blue) and 505 nm (red).
nm. This spectral signature closely resembles the second derivative of the steady-state absorption spectrum (compare B
DOI: 10.1021/acs.jpcc.7b04543 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
into free carriers. Excitonic contributions in MAPI have been also suggested based on the results from previous TA studies.61−64 We conclude that the exciton binding energy of MAPI at room temperature must be much smaller than 26 meV and therefore in the lower range of the previously published values of 2−50 meV,26−31,65−67 probably about 5−10 meV.27,62,65,67,68 Furthermore, the PSCP spectra in Figures 2(A), S2, and S3 show a broad absorption signal in the NIR range directly after photoexcitation. We assign this signal to intraband absorption of electrons in the CB of mp-TiO2. These electrons must have been injected from MABI into the CB of mp-TiO2. Most likely excitons formed close to the MABI/mp-TiO2 interface are immediately split after photoexcitation, with hot electrons being injected into mp-TiO2 and holes remaining in the perovskite. The absorption signal in the NIR decays biexponentially (see Figure 3(B) and Table 1).
with the dotted blue line in the bottom panel). Such features have been assigned to exciton absorption in previous studies of CdSe, PbSe, and PbS quantum dots.51−53 Therefore, we conclude that a large number of stable excitons are also formed after photoexcitation of MABI. This is in marked contrast to the situation in MAPI where the dynamics is largely governed by free charge carriers.32,33,54,55 The second-derivative-like shape may be explained by substantial local fields in the perovskite, giving rise to a Stark effect. In this specific case, two energetically close, broadly absorbing electronic states must be available which is indeed supported by DFT calculations of the band structure for MABI, suggesting two overlapping absorption band onsets due to a split density of states in the CB.20 For such conditions the Stark effect will result in a repulsion of the transitions giving rise to a second-derivativetype feature in the TA spectrum ΔOD(ν̃) in the region of the band edge transitions with ΔOD(ν̃) α E2·OD″(ν̃), where E is the strength of the local electric field and OD(ν̃) is the steadystate absorption spectrum of the perovskite.51,52 The observed phenomenon is therefore consistent with a “trapped-carrierinduced Stark effect” of MABI.51,52 We note that for quantumdot experiments an alternative explanation based on biexciton effects has been put forward.56−58 The exciton signature persists up to the longest time scales studied (see the lower panels in Figures 2(A) and S2) and is consistent with the experimentally estimated exciton binding energy of at least 300 meV.23,25 The value is ten times higher than the average thermal energy at 300 K (26 meV). Therefore, excitons survive even after excitation with more than 0.6 eV excess energy above the CB edge. We note that there is a distinct difference in the kinetics for above-band-gap excitation at 400 nm and excitation directly into the exciton band at 505 nm (Figure 2(B)): Only the population with excess energy shows a distinct ultrafast decay component of the kinetics in the “exciton band” (λprobe = 465 nm). We assign this to a dissociation of a subpopulation of excitons into electrons and holes (ca. 26%), whereas excitation with virtually no excess energy does not provide access to this channel. Note that the bleach kinetics (λprobe = 350 nm) in both cases look similar, so the ultrafast excited-state decay observed in the exciton band is not associated with any corresponding carrier recombination process recovering population in the valence band (VB) of MABI. Therefore, the excitons are split into free electrons and holes with the added possibility that electrons are injected into the CB of mp-TiO2 (see below). The clear exciton features found here in MABI triggered us to screen our previously recorded TA spectra at 296 K for MAPI 32,33 and PbI 2 32 for any indications of exciton contributions. Results are provided in the Supporting Information (Figure S4). Indeed, the transients for PbI2 in Figure S4(A) also show a pronounced, long-lived secondderivative-type signal at the band edge which must arise from a fraction of excitons.59 The binding energy of excitons in the 2H polytype of PbI2 was previously estimated to be in the range 100−127 meV60 and is thus considerably higher than in MAPI (2−50 meV) but substantially lower than in MABI (>300 meV).25 Because the average thermal energy at room temperature is only 26 meV, stable excitons should be formed in MABI and PbI2, even after photoexcitation with excess energy. In contrast, the results for MAPI show that the very weak second-derivative-type feature at the band edge has disappeared by 400 fs, which is on top of a signal arising from free carriers,32,33 signaling the very fast dissociation of excitons
Figure 3. Kinetic transients at (A) 350 nm (MABI bleach band) and (B) 1250 nm (intraband absorption of TiO2 CB electrons) showing the dynamics after excitation at 400 nm (blue) and 505 nm (red), including fit lines. Fit results are provided in Table 1.
The two components τ1 and τ2 are
