Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX
Anomalous Dielectric Behavior of a Pb/Sn Perovskite: Effect of Trapped Charges on Complex Photoconductivity Kento Yamada,† Ryosuke Nishikubo,† Hikaru Oga,† Yuhei Ogomi,‡ Shuzi Hayase,‡ Shohei Kanno,§ Yutaka Imamura,§ Masahiko Hada,§ and Akinori Saeki*,†,# †
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0196, Japan § Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0364, Japan # Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡
S Supporting Information *
ABSTRACT: Organic−inorganic metal halide perovskites (MHPs) exhibit prominent electronic and optical properties benefiting the performance of solar cells and light-emitting diodes. However, the dielectric properties of these materials have remained poorly understood, despite probably influencing delayed charge recombination and device capacitance. Herein, we characterize the unprecedented dielectric behavior of MHPs comprising methylammonium cations, Pb/Sn as metals, and Br/I as halides using time-resolved microwave conductivity (TRMC) measurements. At specific compositions, the above MHPs exhibit negative real and positive imaginary photoconductivities, the polarities of which are opposite those observed for conventional photogenerated charge carriers. Comparing the observed TRMC kinetics with that of inorganic perovskites (SrTiO3 and BaTiO3) and characterizing its dependence on temperature, frequency, and near-infrared second push pulse, we conclude that the above behavior is due to the trapping of polaronic holes/electrons by oriented dipoles of organic cations, which opens a hitherto unexplored route to the dynamical control of dielectric permittivity by photoirradiation. KEYWORDS: perovskite solar cell, time-resolved microwave conductivity, carrier trap, methylammonium dipole, tin−lead mixed metal, dielectric constant, Curie−Weiss law
T
has been precluded by self-doping,15,24 which is mainly ascribed to the oxidation of Sn2+ to Sn4+, and hinders optoelectronic investigations. Although the optical and electronic properties of MHPs have been intensively studied, the dielectric properties of these compounds remain underexplored,1,25,26 particularly in the high-frequency region.27−29 Notably, the mostly free rotation of methylammonium dipoles in these species at room temperature contributes to their large dielectric constants under external electric fields7,26,30 and induces the formation of ferroelectric-like domains that may aid charge separation and reduce recombination via charge carrier segregation.31 This is intuitively associated with the softness of the ionic lattice and liquid-like behavior of the permanent dipole of the A-site cation.32,33 Giant photoinduced dielectric constants34,35 and their polarity switching36 are notable examples of unique MHP properties. On the other hand, ion migration and charge accumulation at low frequency are thought to cause device hysteresis25,37 not involving the ferroelectric nature of MHPs.38
he intriguing photovoltaic properties of lead halide perovskite absorbers are profoundly rooted in their inherent nature of direct electronic transitions,1 prompt charge separation from loosely bound excitons,2 ambipolar charge transport,3,4 and high charge carrier mobility.5,6 Moreover, low carrier trap density7,8 and reduced charge recombination rates9,10 also result in increased carrier diffusion length3,4 and charge collection efficiency, making methylammonium (CH3NH3+, MA) lead triiodide (MAPbI3) one of the most promising materials11−13 for emerging photovoltaics. However, the high toxicity of Pb limits the widespread use of these materials,14 necessitating the search for less toxic alternatives.15,16 In view of the above, Sn, a lighter element of group 14, is the most plausible substitute of Pb, preserving the three-dimensional structure of ABX3 perovskites.17,18 In addition, judicious compositional engineering of A-site cations (MA, formamidinium (HC(NH2)2, FA), Cs),19−21 B-site metals (Pb, Sn),17,22 and X-site halides (I, Br, Cl)22,23 allows one to adjust the band gaps, valence band maxima (VBMs), and electronic behavior of these perovskites. So far, the boost of power conversion efficiency (PCE) of Sn-based metal halide perovskites (MHPs) © XXXX American Chemical Society
Received: April 2, 2018
A
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics
Figure 1. Complex TRMC transients of MA(Pb1−xSnx)Br3. (a) MAPbBr3, (b) MA(Pb0.7Sn0.3)Br3, (c) MA(Pb0.5Sn0.5)Br3, (d) MA(Pb0.3Sn0.7)Br3, and (e) MASnBr3. The upper and lower panels represent real and imaginary parts (Re[Δσ]: Δσ′; Im[Δσ]: Δσ″), respectively. The blue to red lines correspond to the increase of excitation photon density. Black lines denote fits obtained using a double stretched exponential function. λex = 355 nm.
recorded on a Jasco Corp. V-570 UV−vis spectrophotometer. Photoelectron yield spectroscopy (PYS) experiments were carried out on an indium tin oxide (ITO) glass substrate under vacuum ( 0.5 of MAPb1−xSnxI3). MASnI3 and FASnI3 showed normal signals irrespective of the addition of SnF2. Low-temperature TRMC measurements (220−300 K) were performed in a vacuum chamber, with the temperatures of the sample and the cavity controlled by an electronic cooler and a proportional-integral-differential unit.
Herein, the charge carrier dynamics of mixed cation (MA or Cs), metal (Pb and Sn), and halide (I and Br) perovskites were thoroughly investigated using flash-photolysis time-resolved microwave conductivity (TRMC)39−44 measurements with precise characterization of complex conductivity. Advantageously, TRMC is a contactless technique allowing the degradation effect to be minimized and excluding problematic charge transfer/transport processes in the buffer layer. The TRMC transients of mixed-Pb/Sn MHPs exhibited anomalous polarities, and the above compounds were thus investigated in terms of their composition, temperature-dependent dielectric behavior, and excitation wavelength, being subsequently compared with conventional ferroelectric perovskites.
■
EXPERIMENTAL SECTION Sample Preparation and Characterization. Lead halides (PbBr2 and PbI2) were purchased from Tokyo Chemical Industry Co., Ltd. MAI, MABr, FAI, SnBr2, SnI2, SnF2, CsBr, CsI, BaTiO3 (catalog #467634, cubic crystalline phase, 99%), and poly(triarylamine) (PTAA) were bought from Sigma-Aldrich Co., Llc. Phenyl-C61butyric acid methyl ester (PCBM) was purchased from Frontier Carbon, Inc., and dimethyl sulfoxide (DMSO) was sourced from Kanto Chemical Co., Inc. SrTiO3 (catalog #197-10712 and 358-36462: nanosized) was purchased from Wako Pure Chemical Industry, Ltd. Note that the latter nanosized sample of SrTiO3 gave no anomalous photoconductivity signal; thus the former was used throughout this work. All chemicals were used without further purification. The quartz substrate was cleaned with an aqueous detergent solution, acetone, isopropyl alcohol, and deionized water. A 0.1 M precursor solution of designated stoichiometry of MHP was prepared in DMSO in a N2-filled glovebox and drop-cast on the quartz substrate, with the obtained film subsequently annealed at 100 °C for 30 min. BaTiO3 and SrTiO3 powders were suspended in methanol (Wako Pure Chemical Industry, Ltd.) and cast on a quartz plate, followed by 2 h sintering at 500 °C. Note that BaTiO3 without sintering did not constantly give an anomalous TRMC signal, which is related to the cubic paraelectric phase and tetragonal ferroelectric phase before and after sintering, respectively.45 The crystalline size ( k2 > 0). MAPbBr3 exhibited a positive Δσ′ and negative Δσ″ over the entire time scale (10−9−10−6 s), where the decay speeds of real and imaginary parts were mostly identical, with the amplitude of the former being ∼50 times larger than that of the latter (Figure 1a). These polarities and dominant real parts were readily rationalized by the photogeneration of mobile charge carriers (holes and/or electrons) similarly to the cases of MAPbI3 and polymer:fullerene bulk heterojunctions.39−41 The observed normal signal corresponds to a small increase of the complex dielectric permittivity (positive Δε′ and Δε″). Anomalous photoconductivities giving rise to negative Δσ′ (i.e., negative Δε″) and positive Δσ″ (i.e., negative Δε′) were observed when 30 mol % of the B-site metal in MAPbBr3 was replaced by Sn (Figure 1b), with this behavior being even more pronounced for MA(Pb0.5Sn0.5)Br3 (Figure 1c). At still higher Sn contents, e.g., for MA(Pb0.3Sn0.7)Br3 (Figure 1d) and MASnBr3 (Figure 1e), the normal (negative) Δσ″ completely disappeared. More profoundly, the Δσ’s of the above two perovskites exhibited significant dynamical polarity changes with time, with the rapid decay of the normal positive signal being followed by the increase of anomalous negative signals at delay times of 10−7−10−6 s. The anomaly was observed regardless of the excitation photon density (1010−1015 photons cm−2 pulse−1) and wavelength (355 and 415−690 nm, vide inf ra) and thus is likely associated with the perovskite composition. No distinct phase separation with increasing Sn content was observed in the XRD spectra of MA(Pb1−xSnx)Br3, as confirmed by the monotonic shift of (110) and (220) peaks to higher angles (Figure S6). The crystal morphologies of the above species observed by optical microscopy varied from circular (x = 0) and cuboid (x = 0.3−0.5) to flat-leaf with crossing lines (x = 0.7−1) (Figure S7); however, no systematic changes correlated with TRMC behavior were observed. Previously, the increase of the negative Δσ″ relative to the positive Δσ′ observed for TiO2 nanoparticles was rationalized by the generation of shallowly trapped charge carriers, with the corresponding frequency dependence analyzed by the Drude− Smith−(Zener) model in both the GHz and THz regions.39 However, we herein stress that the polarity change observed for MA(PbxSn1−x)Br3 cannot be explained solely by the electroC
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics
paraelectric SrTiO3 and ferroelectric BaTiO3. SrTiO3 exhibited a normal positive Δσ′ and negative Δσ″, along with a small positive Δσ″ at the delay time (Figure S47). On the other hand, BaTiO3 featured an anomalous signal similar to that of MA(Pb1−xSnx)Br3 (Figure 3a). Figure 3b presents a bar plot of the complex Δσ of BaTiO3, also showing the end-of-pulse values of Δσ′ (Δσ′eop) and Δσ″ (Δσ″eop) together with the minima of Δσ′ (Δσ′min) and maxima of Δσ″ (Δσ′max). The polarities of these values were identical to those of anomalous mixed MHPs, while the kinetics of anomalous signal appearance was delayed by 1 order of magnitude (10−6−10−5 s). To examine frequency dispersions, TRMC evaluations of SrTiO3, BaTiO3, and MA(Pb0.3Sn0.7)Br3 were performed at microwave frequencies of 15 and 23 GHz. The obtained values of (−Δσ′min/Δσ″max), proportional to the corresponding complex permittivity ratios (Δε″/Δε′), were plotted as a function of frequency (Figure 3c, kinetics provided in Figures S48−S50). Notably, (−Δσ′min/Δσ″max) equaled zero for SrTiO3, since no negative Δσ′ was observed. In stark contrast, the Δσ′ of both BaTiO3 and MA(Pb0.3Sn0.7)Br3 increased with increasing frequency. SrTiO3 has been reported to feature a large, mostly constant real part (εr′ ≈ 300) and a small imaginary part (εr″ ≈ 0) of relative permittivity over a wide range of frequencies (100 kHz to 100 GHz) due to undergoing ionic polarization with an inverse of relaxation time corresponding to 1−10 THz.47 BaTiO3, the most extensively studied ferroelectric perovskite, shows a frequency dispersion described by the Debye model with a large εr′ of ∼3000 and nonzero εr″.47 More importantly, the relaxation time of BaTiO3 attributed to dipole and ionic polarization appeared at 1−100 GHz, which corresponded to the increase of tan(δ) (= εr″/εr′) in the frequency region of the TRMC measurements performed herein.47 The ferroelectricity of BaTiO3 is thought to originate from the response of Ti and O ions to an external electric field
Figure 2. Summary of TRMC results for A(Pb1−xSnx)X3 (A = Cs or MA, X = I and Br). (a) (−αRe,2/αRe,1) of MA-based MHPs, MA(Pb1−xSnx)I3−yBry. The height of the bar indicates the intensity of the anomalous TRMC signal, with negative values (normal signals) shown as zero. (b) (−αRe,2/αRe,1) of Cs-based MHPs, Cs(Pb1−xSnx)I3−yBry. (c) φ∑μmax, converted from the real part of photoconductivity maxima of MA(Pb1−xSnx)I3 (black circles), MA(Pb1−xSnx)IBr2 (red squares), MA(Pb1−xSnx)Br3 (blue triangles), Cs(Pb1−xSnx)I3 (orange circles), and Cs(Pb1−xSnx)Br3 (green triangles). (d) kRe,1 obtained by fitting.
Rationalization of Anomalous Photoconductivity. To provide mechanistic insight into the anomalous signals of mixed MHPs, we investigated classical inorganic perovskites, namely,
Figure 3. Rationalization of anomalous TRMC signals. (a) Normalized TRMC transients of BaTiO3 (λex = 355 nm), with orange and blue lines corresponding to real and imaginary Δσ. (b) Bar plot of Δσ′eop, Δσ′min, Δσ″eop, and Δσ″max of BaTiO3. The above quantities are defined in the main text and in (a), with the superscript “N” in the latter case representing normalized values. (c) Plots of (−Δσ′min/Δσ″max) (proportional to (Δε″/ Δε′)) vs microwave frequency for BaTiO3 (purple circles), SrTiO3 (green triangles), and MA(Pb0.3Sn0.7)Br3 (brown squares). (d) Illustration of electron trapping in BaTiO3, with green and red spheres corresponding to Ba and O ions, respectively, and Ti located inside dark blue octahedra. Arrows represent the displacements of Ti ions toward trapped electrons (with O ions concomitantly moving in the opposite direction). (e) Δσ′ and (f) Δσ″ transients of MA(Pb0.3Sn0.7)Br3 (orange line), PCBM on MA(Pb0.3Sn0.7)Br3 (purple line), and PTAA on MA(Pb0.3Sn0.7)Br3 (green line). The excitation pulse (λex = 500 nm) was applied from the back side of the film (quartz) to prevent the decrease of light intensity caused by photoabsorption in hole- and electron-transporting layers. (g) Illustration of a polaronic hole and an electron in MHPs, where the oriented MAs are supposed to cause the anomalous TRMC signal (decrease in dielectric permittivity). D
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics
“dressing” charges have a polaronic nature, preventing charge recombination and pinning the rotational motion of dipoles.35 Notably, no anomalous signal was observed for FA(Pb0.7Sn0.3)IBr2, the FA cation of which has a smaller dipole moment (0.21 D) and a larger rotational barrier (140 meV for FASnBr3) than the MA cation31 (Figure S54). Time-resolved vibrational anisotropy measurements of MA reportedly revealed that mixed-halide MAPbX3 (X = I, Br, and Cl) feature a more longlived partial immobilization of MA than pure trihalide species,51 in agreement with our observation of the anomalous signal for MA(Pb0.7Sn0.3)IBr2 being more pronounced than that for MA(Pb0.7Sn0.3)I3. The necessity of Br for the appearance of anomalous TRMC signals might be associated with a larger electron−phonon coupling constant in bromide MHPs than iodide MHPs.52 This coupling is identified as longitudinal optical phonons via the Fröhlich interaction, the possible mechanism of which includes distortions of a lattice similar to BaTiO3.32,52 The necessity of MA indicated that MA dipoles oriented toward charges became unsusceptible to the external electric field, decreasing the dielectric permittivity (Δε′, Δε″ < 0) and giving anomalous TRMC signals (Δσ′ < 0, Δσ″ > 0). Although this change is very small (a Δσ′ of 10−8 S cm−1 corresponds to a Δε″ of −10−6), and the corresponding lifetime is of the order of tens of microseconds, the anomalous dielectric behavior underscores the control over dielectric permittivity by charge photogeneration. Characterization of Charge Trapping. Furthermore, MA orientation accompanied charge trapping was examined by changing the quality of MHP films. Figure 4a and b show the real and imaginary Δσ’s of MA(Pb0.3Sn0.7)Br3 with/without 20 mol % SnF2, which is a well-known antioxidant agent used to stabilize Sn-based MHPs.18−21 Notably, the incorporation of SnF2 efficiently impeded the appearance of both Δσ′ and Δσ″ anomalous signals and resulted in a doubled end-of-pulse
(ionic and dipole polarization due to domain contribution),47 which results in slight displacements of these ions in the tetragonal phase at room temperature. Notably, the (−Δσ′min/ Δσ″max) values determined for SrTiO3 and BaTiO3 by TRMC were consistent with their frequency dispersions of tan(δ). In addition, the (−Δσ′min/Δσ″max) of BaTiO3 was negatively correlated with temperature (20−83 °C, Figure S51), in good agreement with the reported change of tan(δ) (frequency 0 and Δσ″ < 0). Trapping and charge recombination cause the decrease of normal signal intensity, and subsequently, the ordering of MA permanent dipoles (2.29 D)31 with respect to the trapped charges (formation of polaronic states)6,28,33,35 leads to the observation of anomalous signals. The orientation of MA dipoles is strongly supported by the results of previous studies, which showed that the MA rotation barrier in MAPbI3 (10−20 meV) is approximately half of the thermal energy at room temperature,6,30,31 with MA cations in octahedral Pb−Br cages being randomly oriented with a short relaxation time (∼7 ps).50 According to our firstprinciples calculation, the cubic phases of MAPbBr3, MA(Pb0.5Sn0.5)Br3, and MASnBr3 showed largely similar MA rotational barrier profiles, indicative of barrierless MA orientation in mixed Pb/Sn MHPs (Figure S53). The
Figure 4. (a) Δσ′ transients of MA(Pb0.3Sn0.7)Br3 for a drop-cast film (orange), a spin-coated film with 20 mol % SnF2 (green), and a toluene-treated spin-coated film with 20 mol % SnF2 (purple). (b) Δσ″ transients of the above films. (c, d) Pump and pump + push TRMC of MA(Pb0.3Sn0.7)Br3 using K-band microwave (22.8 GHz) and λpush = 950 nm at ∼0.7 μs after the pump pulse (red arrow). (c) Δσ′ transients obtained using [pump] (orange curve) and [pump + push] − [push] (blue curve) measurements. (d) Δσ″ transients of [pump] and [pump + push] − [push]. λex = 355 nm. E
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics intensity of the normal Δσ′. Moreover, antisolvent treatment by toluene profoundly increased the initial normal Δσ′ and Δσ″, whereas small anomalous signals were still observed after a certain delay time. Thus, the above results strongly suggested that anomalous signals were intimately associated with the density of trapping sites. To access the energetics of trapped charges, we utilized a near-infrared laser as the second push pulse for the complex photoconductivity of MA(Pb0.3Sn0.7)Br3 at the K-band (∼23 GHz, the anomalous signal is larger than the X-band). The pump−push optical/electric spectroscopy technique has been previously used to investigate the charge transfer states in a polymer:fullerene bulk heterojunction.53 Upon the application of a 900 nm push pulse (1.38 eV) at ∼0.7 μs after the pump UV pulse (3.49 eV, far above the band gap of 1.89 eV), a recovery of Δσ′ toward the positive direction was observed for MA(Pb0.3Sn0.7)Br3 (Figure 4c). Concomitantly, a slight, negative change of Δσ″ was observed after the push pulse (Figure 4d). These observations are linked to the liberation of trapped charges via a push pulse and temporal contribution of mobile charges to the normal TRMC polarity. With decreasing the photon energy of the push pulse (1.31 to 1.18 eV), the recovery of anomalous Δσ became undetectable (Figure S55). The obtained results supported the hypothesis that the presence of deeply trapped charges and oriented MA dipoles causes anomalous TRMC signals. Dependences of Δσ on Temperature and Wavelength. The complex photoconductivity of MA(Pb0.3Sn0.7)Br3 demonstrated an unusual dependence on temperature, i.e., Δσ′eop (>0) simply decreased with decreasing temperature (from 298 K to 233 K), whereas Δσ′ at 0.2 μs (Δσ′0.2 μs < 0) initially decreased and subsequently increased at 253 K (Figure 5a). The imaginary part at this delay time (Δσ″0.2 μs) also showed a similar dependence, whereas the polarity of Δσ″ progressively changed from positive to negative below 243 K (Figure 5b). The Arrhenius plot of Δσ′eop (normal signal) yielded an activation energy of 45 meV, indicating that charge carrier behavior could be explained by the hopping transport framework (Figure 5c). Notably, this Arrhenius-type dependence was consistent with the polaronic state of charge carriers depicted in Figure 3g. In sharp contrast, band-like transport associated with phonon scattering (T−3/2) was observed for MAPbI340,41 and MAPbBr3 (Figure 5d). The replacement of MA by Cs (Cs(Pb0.3Sn0.7)Br3) altered the polarities and induced a shift to band-transport behavior (Figure S56), corroborating the key role of MA in the anomalous dielectric response. Figure 5e shows plots of Δσ′0.2 μs and Δσ″0.2 μs as functions of temperature for MA(Pb0.3Sn0.7)Br3, with the negative correlation in the high-temperature region analyzed by leastmean-square fitting to the Curie−Weiss law, namely, C(T − T0)−1, where C is a constant (C < 0 for real parts and >0 for imaginary parts), and T0 is the dielectric Curie temperature.50,54 Analysis of real and imaginary parts yielded T0 = 217 and 231 K, respectively, which coincided with the phase transition temperatures of MAPbBr3 [low-T tetragonal to high-T cubic at 237 K]29 and MASnBr3 [low-T rhombohedral to high-T cubic at 195 or ∼220 K].55,56 It should be noted that a similar T−1 dependence of the real part of permittivity has been observed at low frequencies (kHz−MHz).50,54 Thus, the above results were consistent with our assignment of anomalous signals to the decrease of dielectric permittivity by MA-oriented polarons. With decreasing temperature, the anomalous signal vanished at
Figure 5. Temperature and excitation wavelength dependences of MA(Pb0.3Sn0.7)Br3. (a) Real and (b) imaginary parts of Δσ transients of MA(Pb0.3Sn0.7)Br3, with line color representing temperature. The time regions of Δσ′eop, Δσ′0.2 μs, and Δσ′′0.2 μs are indicated by a colored area. (c) Arrhenius plot for Δσ′eop of MA(Pb0.3Sn0.7)Br3. (d) Temperature dependence of Δσ′eop for MAPbBr3, with inset showing the transients of Δσ′. (e) Δσ′0.2 μs (orange circles) and Δσ″0.2 μs (blue triangles) of MA(Pb0.3Sn0.7)Br3. Black solid lines denote fits based on the Curie−Weiss law. (f) Laser-power-normalized Δσ (Δσ′eop, Δσ′min, and Δσ″max) values of MA(Pb0.3Sn0.7)Br3, superimposed on a steadystate photoabsorption spectrum (gray solid line, right axis). The colored lines are provided as guides for the eye.
255 and 275 K for the real and imaginary parts, respectively. The polarity of the imaginary part became normal (negative) below ∼250 K, presumably due to the decreased thermal energy available at low temperature for overcoming the MA rotation barrier and the increased contribution of the normal TRMC signal corresponding to mobile charge carriers. Figure 5f shows the excitation-wavelength-dependent Δσ of MA(Pb0.3Sn0.7)Br3, revealing the presence of anomalous negative Δσ′min and positive Δσ″max above the band gap (1.89 eV ≈ 656 nm), mostly in accordance with the shape of the absorption spectrum. The normal positive Δσ′eop gradually increased with increasing excitation wavelength at shorter than the absorption edge (∼ band gap). With increasing wavelength, the normalized decay speed of the normal (positive) real parts of the MA(Pb0.3Sn0.7)Br3 signal decreased (Figure S57). On the other hand, the decay of anomalous signals remained unchanged for both real and imaginary parts. The absorption coefficient decreased with increasing wavelength, resulting in a decreased density of photogenerated charges and retarded charge recombination, thus increasing Δσ′eop. This explanation was directly supported by the decay rate dependence on the excitation photon density, with higher densities accelerating the F
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics decay of the normal (positive) Δσ′ but having no influence on the slow decay of anomalous signals (Figure S58). The kinetics of the latter signals was wavelength- and excitation photon density-independent, thus corroborating our hypothesis that the anomalous signals originated from MA-oriented immobile charges. These immobile charges are classified as small polarons that are deeply trapped in the band gap and accompany MA reorientation and lattice distortion.57 In contrast, charge carriers that contribute to the photocurrent of an efficient MHP solar cell are large polarons that show band-like transport. The aspects of these polaronic states are consistent with our observations in the temperature dependences and nature of anomalous TRMC signals. Obviously, Sn-doping to a Pb-based MHP decreases its device performance due to increased trap sites; however, the polaronic state in MA-based MHPs does not directly degrade a device efficiency, because MAPbI3 and MAPbBr3 still show a high charge carrier mobility and band-like transport. We reiterate that the polaronic state (MA reorientation and decrease of complex dielectric constant) became observable when the charge carriers were immobilized by deep traps and their normal TRMC signals were decreased. We envision that such an anomaly can occur in other organic and inorganic materials that have a large dielectric constant at GHz frequency.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant-in-Aid for Scientific Research (A) (Grant No. JP16H02285) and the PRESTO program (Grant No. JPMJPR15N6) from the JST of Japan.
■
(1) Manser, J. S.; Christians, J. A.; Kamat, P. V. Intriguing Optoelectronic Properties of Metal Halide Perovskites. Chem. Rev. 2016, 116, 12956−13008. (2) Miyata, A.; Mitioglu, A.; Plochocka, P.; Portugall, O.; Wang, J. T.W.; Stranks, S. D.; Snaith, H. J.; Nicholas, R. J. Direct Measurement of the Exciton Binding Energy and Effective Masses for Charge Carriers in Organic-Inorganic Tri-Halide Perovskites. Nat. Phys. 2015, 11, 582−587. (3) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (4) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-Hole Diffusion Lengths > 175 μm in SolutionGrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (5) Valverde-Chávez, D. A.; Ponseca, C. S., Jr.; Stoumpos, C. C.; Yartsev, A.; Kanatzidis, M. G.; Sundström, V.; Cooke, D. G. Intrinsic Femtosecond Charge Generation Dynamics in Single Crystal CH3NH3PbI3. Energy Environ. Sci. 2015, 8, 3700−3707. (6) Chen, Y.; Yi, H. T.; Wu, X.; Haroldson, R.; Gartstein, Y. N.; Rodionov, Y. I.; Tikhonov, K. S.; Zakhidov, A.; Zhu, X.-Y.; Podzorov, V. Extended Carrier Lifetimes and Diffusion in Hybrid Perovskites Revealed by Hall Effect and Photoconductivity Measurements. Nat. Commun. 2016, 7, 12253/1−9. (7) Wu, B.; Nguyen, H. T.; Ku, Z.; Han, G.; Giovanni, D.; Mathews, N.; Fan, H. J.; Sum, T. C. Discerning the Surface and Bulk Recombination Kinetics of Organic−Inorganic Halide Perovskite Single Crystals. Adv. Energy Mater. 2016, 6, 1600551/1−9. (8) Wenger, B.; Nayak, P. K.; Wen, X.; Kesava, S. V.; Noel, N. K.; Snaith, H. J. Consolidation of the Optoelectronic Properties of CH3NH3PbBr3 Perovskite Single Crystals. Nat. Commun. 2017, 8, 590/1−10. (9) Manser, J. S.; Kamat, P. V. Band Filling with Free Charge Carriers in Organometal Halide Perovskites. Nat. Photonics 2014, 8, 737−743. (10) Zheng, F.; Tan, L. Z.; Liu, S.; Rappe, A. M. Rashba Spin−Orbit Coupling Enhanced Carrier Lifetime in CH3NH3PbI3. Nano Lett. 2015, 15, 7794−7800. (11) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (12) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, J. S.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591/1−7. (13) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (14) Babayigit, A.; Ethirajan, A.; Muller, M.; Conings, B. Toxicity of Organometal Halide Perovskite Solar Cells. Nat. Mater. 2016, 15, 247−251. (15) Shi, Z.; Guo, J.; Chen, Y.; Li, Q.; Pan, Y.; Zhang, H.; Xia, Y.; Huang, W. Lead-free Organic−Inorganic Hybrid Perovskites for Photovoltaic Applications: Recent Advances and Perspectives. Adv. Mater. 2017, 29, 1605005/1−28.
■
CONCLUSION Anomalous TRMC signals (negative Δσ′ and positive Δσ″) were observed for MA(Pb1−xSnx)Br3 (x > 0), exhibiting polarities opposite those of normal signals due to the photogeneration of mobile charge carriers. No anomalous signals were observed for nonpolar Cs and weakly dipolar FA cations, while being prominent in the cases of SrTiO3 and BaTiO3. Based on experimental evidence, we ascribed the timeevolved decrease of dielectric permittivity to the presence of MA-oriented immobile polaronic holes and electrons, as supported by the temperature-dependent behavior (hopping transport for the normal signal and Curie−Weiss law for the anomalous signal) of these signals, change of decay speed with excitation density/wavelength, and the dependence of the (−Δσ′min/Δσ″max) parameter on microwave frequency. The near-infrared push pulse caused a slight bleaching of the anomalous signal, possibly associated with the temporal liberation of trapped charges. Thus, our study establishes the underlying correlation between dielectric permittivity and polaronic charges with oriented MA dipoles in mixed MHPs, providing a method of controlling permittivity by photoirradiation and broadening the application range of the TRMC technique.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00422. Additional information (PDF)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (A. Saeki). ORCID
Masahiko Hada: 0000-0003-2752-2442 Akinori Saeki: 0000-0001-7429-2200 G
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics
Dielectric Constant in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 2390−2394. (35) Nie, W.; Blancon, J.-C.; Neukirch, A. J.; Appavoo, K.; Tsai, H.; Chhowalla, M.; Alam, M. A.; Sfeir, M. Y.; Katan, C.; Even, J.; Tretiak, S.; Crochet, J. J.; Gupta, G.; Mohite, A. D. Light-Activated Photocurrent Degradation and Self-Healing in Perovskite Solar Cells. Nat. Commun. 2016, 7, 11574/1−9. (36) Wang, P.; Zhao, J.; Wei, L.; Zhu, Q.; Xie, S.; Liu, J.; Menge, X.; Li, J. Photo-Induced Ferroelectric Switching in Perovskite CH3NH3PbI3 Films. Nanoscale 2017, 9, 3806−3817. (37) Eames, C.; Frost, J. M.; Barnes, P. R. F.; O’Regan, B. C.; Walsh, A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat. Commun. 2015, 6, 7497/1−8. (38) Beilsten-Edmands, J.; Eperon, G. E.; Johnson, R. D.; Snaith, H. J.; Radaelli, P. G. Non-Ferroelectric Nature of the Conductance Hysteresis in CH3NH3PbI3 Perovskite-Based Photovoltaic Devices. Appl. Phys. Lett. 2015, 106, 173502/1−5. (39) Saeki, A.; Yasutani, Y.; Oga, H.; Seki, S. Frequency-Modulated Gigahertz Complex Conductivity of TiO2 Nanoparticles: Interplay of Free and Shallowly Trapped Electrons. J. Phys. Chem. C 2014, 118, 22561−22572. (40) Savenije, T. J.; Ponseca, C. S., Jr.; Kunneman, L.; Abdellah, M.; Zheng, K.; Tian, Y.; Zhu, Q.; Canton, S. E.; Scheblykin, I. G.; Pullerits, T.; Yartsev, A.; Sundströ m, V. Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite. J. Phys. Chem. Lett. 2014, 5, 2189− 2194. (41) Oga, H.; Saeki, A.; Ogomi, Y.; Hayase, S.; Seki, S. Improved Understanding of the Electronic and Energetic Landscapes of Perovskite Solar Cells: High Local Charge Carrier Mobility, Reduced Recombination, and Extremely Shallow Traps. J. Am. Chem. Soc. 2014, 136, 13818−13825. (42) Hutter, E. M.; Sutton, R. J.; Chandrashekar, S.; Abdi-Jalebi, M.; Stranks, S. D.; Snaith, H. J.; Savenije, T. J. Vapour-Deposited Cesium Lead Iodide Perovskites: Microsecond Charge Carrier Lifetimes and Enhanced Photovoltaic Performance. ACS Energy Lett. 2017, 2, 1901− 1908. (43) Nishikubo, R.; Ishida, N.; Katsuki, Y.; Wakamiya, A.; Saeki, A. Minute-Scale Degradation and Shift of Valence-Band Maxima of (CH3NH3)SnI3 and HC(NH2)2SnI3 Perovskites upon Air Exposure. J. Phys. Chem. C 2017, 121, 19650−19656. (44) Reid, O. G.; Moore, D. T.; Li, Z.; Zhao, D.; Yan, Y.; Zhu, K.; Rumbles, G. Quantitative Analysis of Time-Resolved Microwave Conductivity Data. J. Phys. D: Appl. Phys. 2017, 50, 493002/1−13. (45) Zhao, Z.; Buscaglia, V.; Viviani, M.; Buscaglia, M. T.; Mitoseriu, L.; Testino, A.; Nygren, M.; Johnsson, M.; Nanni, P. Grain-size Effects on the Ferroelectric Behavior of Dense Nanocrystalline BaTiO3 Ceramics. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70 (024107), 1−8. (46) Ponseca, C. S., Jr.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; Wolf, J.-P.; Sundströ m, V. Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-Long Balanced Mobilities, and Slow Recombination. J. Am. Chem. Soc. 2014, 136, 5189−5192. (47) Tsurumi, T.; Li, J.; Hoshina, T.; Kakemoto, H.; Nakada, M.; Nakada, M.; Akedo, J. Ultrawide Range Dielectric Spectroscopy of BaTiO3-Based Perovskite Dielectrics. Appl. Phys. Lett. 2007, 91, 182905/1−3. (48) Curecheriu, L.; Balmus, S.-B.; Buscaglia, M. T.; Buscaglia, V.; Ianculescu, A.; Mitoseriu, L. Grain Size-Dependent Properties of Dense Nanocrystalline Barium Titanate Ceramics. J. Am. Ceram. Soc. 2012, 95, 3912−3921. (49) Ishida, N.; Wakamiya, A.; Saeki, A. Quantifying Hole Transfer Yield from Perovskite to Polymer Layer: Statistical Correlation of Solar Cell Outputs with Kinetic and Energetic Properties. ACS Photonics 2016, 3, 1678−1688. (50) Govinda, S.; Kore, B. P.; Bokdam, M.; Mahale, P.; Kumar, A.; Pal, S.; Bhattacharyya, B.; Lahnsteiner, J.; Kresse, G.; Franchini, C.;
(16) Giustino, F.; Snaith, H. J. Toward Lead-Free Perovskite Solar Cells. ACS Energy Lett. 2016, 1, 1233−1240. (17) Ogomi, Y.; Morita, A.; Tsukamoto, S.; Saitho, T.; Fujikawa, N.; Shen, Q.; Toyoda, T.; Yoshino, K.; Pandey, S. S.; Ma, T.; Hayase, S. CH3NH3SnxPb(1−x)I3 Perovskite Solar Cells Covering up to 1060 nm. J. Phys. Chem. Lett. 2014, 5, 1004−1011. (18) Zhao, Z.; Gu, F.; Li, Y.; Sun, W.; Ye, S.; Rao, H.; Liu, Z.; Bian, Z.; Huang, C. Mixed-Organic-Cation Tin Iodide for Lead-Free Perovskite Solar Cells with an Efficiency of 8.12%. Adv. Sci. 2017, 4, 1700204/1−7. (19) Ke, W.; Stoumpos, C. C.; Zhu, M.; Mao, L.; Spanopoulos, I.; Liu, J.; Kontsevoi, O. Y.; Chen, M.; Sarma, D.; Zhang, Y.; Wasielewski, M. R.; Kanatzidis, M. G. Enhanced Photovoltaic Performance and Stability with a New Type of Hollow 3D Perovskite {en}FASnI3. Sci. Adv.. 2017, 3, 1−9. (20) Marshall, K. P.; Walker, M.; Walton, R. I.; Hatton, R. A. Enhanced Stability and Efficiency in Hole-Transport-Layer-Free CsSnI3 Perovskite Photovoltaics. Nat. Energy 2016, 1, 16178. (21) Liao, W.; Zhao, D.; Yu, Y.; Grice, C. R.; Wang, C.; Cimaroli, A. J.; Schulz, P.; Meng, W.; Zhu, K.; Xiong, R.-G.; Yan, Y. Lead-Free Inverted Planar Formamidinium Tin Triiodide Perovskite Solar Cells Achieving Power Conversion Efficiencies up to 6.22%. Adv. Mater. 2016, 28, 9333−9340. (22) Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P. H.; Kanatzidis, M. G. Lead-Free Solid-State Organic−Inorganic Halide Perovskite Solar Cells. Nat. Photonics 2014, 8, 489−494. (23) Tsai, C.-M.; Wu, H.-P.; Chang, S.-T.; Huang, C.-F.; Wang, C.H.; Narra, S.; Yang, Y.-W.; Wang, C.-L.; Hung, C.-H.; Diau, E. W.-G. Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode. ACS Energy Lett. 2016, 1, 1086−1093. (24) Shao, S.; Liu, J.; Portale, G.; Fang, H.-H.; Blake, G. R.; ten Brink, G. H.; Koster, L. J. A.; Loi, M. A. Highly Reproducible Sn-Based Hybrid Perovskite Solar Cells with 9% Efficiency. Adv. Energy Mater. 2018, 8, 1702019/1−10. (25) Lin, Q.; Armin, A.; Nagiri, R. C. R.; Burn, P. L.; Meredith, P. Electro-Optics of Perovskite Solar Cells. Nat. Photonics 2015, 9, 106− 112. (26) Even, J.; Pedesseau, L.; Katan, C. Analysis of Multivalley and Multibandgap Absorption and Enhancement of Free Carriers Related to Exciton Screening in Hybrid Perovskites. J. Phys. Chem. C 2014, 118, 11566−11572. (27) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (28) Soufiani, A. M.; Huang, F.; Reece, P.; Sheng, R.; Ho-Baillie, A.; Green, M. A. Polaronic Exciton Binding Energy in Iodide and Bromide Organic-Inorganic Lead Halide Perovskites. Appl. Phys. Lett. 2015, 107, 231902/1−5. (29) Poglitsch, A.; Weber, D. Dynamic Disorder in Methylammoniumtrihalogenoplumbates (II) Observed by Millimeterwave Spectroscopy. J. Chem. Phys. 1987, 87, 6373−6378. (30) Motta, C.; El-Mellouhi, F.; Kais, S.; Tabet, N.; Alharbi, F.; Sanvito, S. Revealing the Role of Organic Cations in Hybrid Halide Perovskite CH3NH3PbI3. Nat. Commun. 2015, 6, 7026/1−7. (31) Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14, 2584− 2590. (32) Bonn, M.; Miyata, K.; Hendry, E.; Zhu, X.-Y. Role of Dielectric Drag in Polaron Mobility in Lead Halide Perovskites. ACS Energy Lett. 2017, 2, 2555−2562. (33) Anusca, I.; Balčiu̅nas, S.; Gemeiner, P.; Svirskas, Š.; Sanlialp, M.; Lackner, G.; Fettkenhauer, C.; Belovickis, J.; Samulionis, V.; Ivanov, M.; Dkhil, B.; Banys, J.; Shvartsman, V. V.; Lupascu, D. C. Dielectric Response: Answer to Many Questions in the Methylammonium Lead Halide Solar Cell Absorbers. Adv. Energy Mater. 2017, 7, 1700600/1− 12. (34) Juarez-Perez, E. J.; Sanchez, R. S.; Badia, L.; Garcia-Belmonte, G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J. Photoinduced Giant H
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
Article
ACS Photonics Pandey, A.; Sarma, D. D. Behavior of Methylammonium Dipoles in MAPbX3 (X = Br and I). J. Phys. Chem. Lett. 2017, 8, 4113−4121. (51) Selig, O.; Sadhanala, A.; Müller, C.; Lovrincic, R.; Chen, Z.; Rezus, Y. L. A.; Frost, J. M.; Jansen, T. L. J.; Bakulin, A. A. Organic Cation Rotation and Immobilization in Pure and Mixed Methylammonium Lead-Halide Perovskites. J. Am. Chem. Soc. 2017, 139, 4068−4074. (52) Wright, A. D.; Verdi, C.; Milot, R. L.; Eperon, G. E.; PérezOsorio, M. A.; Snaith, H. J.; Giustino, F.; Johnston, M. B.; Herz, L. M. Electron−Phonon Coupling in Hybrid Lead Halide Perovskites. Nat. Commun. 2016, 7, 11755/1−9. (53) Bakulin, A. A.; Rao, A.; Pavelyev, V. G.; van Loosdrecht, P. H. M.; Pshenichnikov, M. S.; Niedzialek, D.; Cornil, J.; Beljonne, D.; Friend, R. H. The Role of Driving Energy and Delocalized States for Charge Separation in Organic Semiconductors. Science 2012, 335, 1340−1344. (54) Onoda-Yamamuro, N.; Matsuo, T.; Suga, H. Dielectric Study of CH3NH3PbX3 (X = Cl, Br, I). J. Phys. Chem. Solids 1992, 53, 935−939. (55) Onoda-Yamamuro, N.; Yamamuro, O.; Matsuo, T.; Suga, H.; Oikawa, K.; Tsuchiya, N.; Kamiyama, T.; Asano, H. NeutronDiffraction Study of CD3ND3SnBr3: Semiconductor-Insulator Transition with Orientational Ordering. Phys. B 1995, 213&214, 411−413. (56) Yamada, K.; Nakada, K.; Takeuchi, Y.; Nawa, K.; Yamane, Y. Tunable Perovskite Semiconductor CH3NH3SnX3 (X: Cl, Br, or I) Characterized by X-ray and DTA. Bull. Chem. Soc. Jpn. 2011, 84, 926− 932. (57) Neukirch, A. J.; Nie, W.; Blancon, J. − C.; Appavoo, K.; Tsai, H.; Sfeir, M. Y.; Katan, C.; Pedesseau, L.; Even, J.; Crochet, J. J.; Gupta, G.; Mohite, A. D.; Tretiak, S. Polaron Stabilization by Cooperative Lattice Distortion and Cation Rotations in Hybrid Perovskite Materials. Nano Lett. 2016, 16, 3809−3816.
I
DOI: 10.1021/acsphotonics.8b00422 ACS Photonics XXXX, XXX, XXX−XXX
