Reversible Structural Swell−Shrink and Recoverable Optical Properties in Hybrid Inorganic−Organic Perovskite Yupeng Zhang,†,⊥ Yusheng Wang,‡,⊥ Zai-Quan Xu,†,⊥ Jingying Liu,† Jingchao Song,† Yunzhou Xue,†,‡ Ziyu Wang,† Jialu Zheng,† Liangcong Jiang,† Changxi Zheng,§ Fuzhi Huang,† Baoquan Sun,‡ Yi-Bing Cheng,† and Qiaoliang Bao*,‡,† †
Department of Materials Science and Engineering, Monash University, Wellington Road, Clayton, Victoria 3800, Australia Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, and Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, People’s Republic of China § Department of Civil Engineering, Monash University, Clayton, 3800 Victoria, Australia ‡
S Supporting Information *
ABSTRACT: Ion migration in hybrid organic−inorganic perovskites has been suggested to be an important factor for many unusual behaviors in perovskite-based optoelectronics, such as current−voltage hysteresis, low-frequency giant dielectric response, and the switchable photovoltaic effect. However, the role played by ion migration in the photoelectric conversion process of perovskites is still unclear. In this work, we provide microscale insights into the influence of ion migration on the microstructure, stability, and light−matter interaction in perovskite micro/nanowires by using spatially resolved optical characterization techniques. We observed that ion migration, especially the migration of MA+ ions, will induce a reversible structural swell−shrink in perovskites and recoverably affect the reflective index, quantum efficiency, light-harvesting, and photoelectric properties. The maximum ion migration quantity in perovskites was as high as approximately 30%, resulting in lattice swell or shrink of approximately 4.4%. Meanwhile, the evidence shows that ion migration in perovskites could gradually accelerate the aging of perovskites because of lattice distortion in the reversible structural swell−shrink process. Knowledge regarding reversible structural swell−shrink and recoverable optical properties may shed light on the development of optoelectronic and converse piezoelectric devices based on perovskites. KEYWORDS: hybrid organic−inorganic perovskite, one-dimensional material, lattice distortion, optical properties, structural properties relationship in perovskites remain unanswered.17,18 In particular, current−voltage hysteresis, low-frequency giant dielectric response, a switchable photovoltaic effect, and tunable optical properties in perovskites or perovskite-based optoelectronic devices remain open questions.19−21 Thus far, several explanations have been proposed for the aforementioned phenomena, including charge traps at the interface of perovskite-based optoelectronics,22 the intrinsic ferroelectric behavior of perovskites stemming from the dipolar properties of an organic group,23 and ion migration in perovskites.24,25 Recently, ion migration caused by the poling
B
ecause of their unusual properties such as an intrinsic ambipolar character, high optical absorption coefficient, optimal band gap, and long carrier diffusion length,1−3 hybrid inorganic−organic perovskites in the ABX3 form (A = CH3NH3+, MA+; B = Pb2+ or Sn2+; and X = Cl−, I−, and/or Br−) have been demonstrated to be intriguing materials for fabricating high-performance perovskite-based solar cells with a power conversion efficiency as high as 22.1%.4 A high quantum yield efficiency and relatively high charge-carrier mobility also make these perovskites suitable for use in other optoelectronic devices such as photodetectors,5,6 light-emitting diodes,7 waveguides,8 and nanolasers.9 Despite the rapid improvements in performance associated with the evolution of synthetic methods10−12 as well as with materials modification13,14 and device architecture,15,16 many fundamental questions concerning the light−matter interaction and structure−property © 2016 American Chemical Society
Received: May 10, 2016 Accepted: July 7, 2016 Published: July 7, 2016 7031
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
Article
www.acsnano.org
Article
ACS Nano
Figure 1. (a and b) Relaxed structures for the tetragonal-phase perovskite with different MA+ contents (a: MA/Pb = 0.5; b: MA/Pb = 1). (c) Theoretical lattice parameters of the tetragonal-phase perovskite with different MA+ contents. (d) XRD patterns of the perovskite films with different MA+ contents. (e) Peak shifts in the XRD patterns. (f) Lattice parameters of perovskite films with different MA+ contents. (g−i) SEM images of perovskite films with different MA+ contents (g: MA/Pb = 1; h: MA/Pb = 1.2; i: MA/Pb = 1.3). Scale bar: 1 μm.
MA+ in perovskite are expected to exhibit similar effects because of the large radius and low diffusion barrier of MA+.30 Therefore, elucidating the influence of ion migration on the crystal structure of perovskites is not trivial. More importantly, how the change in the crystal structure will affect the band structure, absorbance, generation and transport of photoinduced charge carriers, the structural stability of perovskites, and the device efficiency is also an interesting question. However, with respect to the development of perovskite-based three-dimensional (3D) optoelectronic devices such as mesostructured or planar solar cells, concisely and effectively associating ion migration and structural changes with optoelectronic properties and light−matter interactions in perovskites at the microscale or nanoscale is difficult. This difficulty arises mainly from the inhomogeneous morphology and electric field in the vertical direction of 3D films, which may result in slow and fast channels for the transport of carriers and ions in perovskites. Cross-section mapping of the perovskitebased optoelectronic devices can reveal variation in the efficiency of charge separation and collection.31 However, the ultrathin thickness of a perovskite layer (approximately 300 nm) limits the resolution for mapping the ion migration. Lateral structure devices could be used to demonstrate the ion migration in perovskites because of the very low electric field of 0.3 V μm−1 needed for poling.27 More importantly, the variation of the spatial optical/electrical properties along the electric field direction could be directly resolved in the lateral structure. Nevertheless, similar to the vertical film structure, the ion conduction and transport in a lateral structure also mostly occur through a network of one-dimensional (1D) channels. To clarify and quantify the impact of ion migration and accumulation in perovskites at the microlevel, isolating
effect has been suggested as the most likely explanation, and substantial progress has been made toward elucidating this effect. Huang et al.26 demonstrated the giant switchable photovoltaic effect in perovskites using Kelvin probe force microscopy (KPFM) and optical microscopy, providing the evidence for electric-field-induced changes in the morphology of perovskite materials and demonstrating through the exclusion of other possible factors that the ion migration effect makes an important contribution to photocurrent hysteresis. They further applied photothermal-induced resonance microscopy to identify the subsequent migration of MA+ ions.27 Maier et al.28 reported that the ionic conductivity is higher than the electronic conductivity in perovskites. The ionic transference and trapping of electronic carriers may result in hysteresis in cyclic sweep experiments. However, the fundamental mechanism of ion migration is not yet well understood because of a lack of concise physical and fully conclusive experimental evidence for directly demonstrating the influence of ion migration on the microstructure, stability, light−matter interaction, and photoelectric properties of perovskites. Undoubtedly, thorough insight into the ion migration mechanism and related influences is strongly desired not only for the development of intrinsically stable and high-performance perovskite-based solar cells (PSCs) but also for the fabrication of optoelectronic devices based on the ion migration characteristic in perovskites. Theoretical investigations have now established a consensus that the orientation of MA+ plays a fundamental role in determining the structural and electronic properties of perovskites. For instance, if MA+ orients along a (001)-like direction, the PbI6 octahedral cage will distort and the band gap will become indirect.29 The migration and accumulation of 7032
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
Article
ACS Nano
Figure 2. (a) Schematic of the s-NSOM system. (b) SEM image of the perovskite single-wire device. Scale bar: 10 μm. (c) Corresponding I−V curves of perovskite single-wire device under dark, 532 nm laser, and white-light conditions. (d−f) Schematics of ion migration in a perovskite microwire under pristine, positive poling, and negative poling conditions, respectively. (g−i) Infrared near-field mapping images (AFM images, near-field optical images, and phase images) for a perovskite microwire under pristine, positive poling, and negative poling conditions. Scale bar: 500 nm. (j) Line profile along the white line in the AFM images before and after poling. (k) Width change of perovskite microwire upon poling measured from the AFM topography. The data points are the full width at half-maximum of 10 line profiles in the region indicated by the white box of (g). (l) Line profile along the dashed white line in (g) in the near-field optical images before and after poling.
cation orientation on the lattice structures, the MA+ are all arranged along the [100] direction. Note that relatively large changes in the lattice parameters were induced with increasing MA+ content. In particular, the length of the c-axis was increased and shear stress in the a−b plane was introduced, which changes the γ-angle (Figure 1c and Supporting Information, Table 1). The variation of the lattice constant for cubic phase perovskite is similar to that for tetragonal-phase perovskite (Supporting Information, Table 2). We further prepared the perovskite films with different MA+ contents to verify the influence of MA+ filling on the perovskite structure. The XRD patterns of these perovskite films are shown in Figure 1d. The patterns of samples with an MA/Pb ratio less than 1 show a blend phase with PbI2 and perovskite. When the ratio is greater than 1, the samples exhibit a pure tetragonal-phase perovskite structure, suggesting that the excess MA+ ions were completely incorporated into the lattices to form a solid solution.24,25 The shift of diffraction peaks to smaller diffraction angles with increasing MA+ content (Figure 1e) indicates the lattice evolution in the crystal structure (Figure 1f), which is consistent with the aforementioned theoretical calculation. However, when the ratio is greater than 1.3, the peaks shift to larger diffraction angles instead, demonstrating the instability of the perovskites. The MA+ evaporated when the content exceeded a certain amount close to saturation (approximately 30%), leading to many pinholes left in the thin film, as evidenced by the porous morphology (Figure 1g−i). As a
conduction channels from each other and confining the transport of charge carriers or ions into a 1D microstructure, is a nontrivial challenge. In this work, we fabricate a single-wire optoelectronic device based on a 1D perovskite microwire with high crystalline quality. The ion migration and related influences on a perovskite single-wire device have been identified using optical characterization, including scattering-type near-field scanning optical microscopy (s-NSOM), photoluminescence (PL) mapping, and photocurrent mapping techniques. We demonstrate that ion migration, especially the migration of MA+ ions, will induce a reversible structural swell−shrink in perovskites and affect the reflective index, quantum efficiency, lightharvesting, and photoelectric properties recoverably. Meanwhile, ion migration in perovskites could gradually accelerate the aging of perovskites due to lattice distortion in the reversible structural swell−shrink process. The results are used to construct a concise physics picture of the ion migration and related influence in perovskites.
RESULTS AND DISCUSSION First, we conducted density functional theory (DFT) calculations to elucidate the change of lattice constants with different MA+ contents in perovskites. In Figure 1a and b, the relaxed structures for the tetragonal phase of CH3NH3PbI3 are presented for two different fillings of the MA+ (Figure 1a: MA/ Pb = 0.5; Figure 1b: MA/Pb = 1). To exclude the influence of 7033
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
Article
ACS Nano
Figure 3. (a−d) PL mapping images as a function of the poling time. Scale bars: 5 μm. (e−h) PL mapping images of perovskite microwire after removing the poling voltage. Scale bars: 5 μm. (i) PL spectra at different positions in a pristine perovskite microwire. (j) PL spectra at different positions in a perovskite microwire after poling for 20 min.
laser and white-light illumination could the hysteresis effect be observed in the corresponding I−V curves, in which the current depends on the direction of the voltage sweeps between the source and drain electrodes (Figure 2c). To simulate the poling electric field in PSCs, a constant bias was applied directly between the source and drain electrodes (Figure 2d−f). The AFM images, near-field optical images, and phase images of the perovskite microwire under pristine, positive poling (+10 V), and negative poling (−10 V) conditions are presented in Figure 2g−i, respectively. As shown in the AFM images, the pristine perovskite microwire exhibits a uniform surface topography with a diameter of about 495 nm. During the poling process, the diameter of the microwire expanded about 22 nm, corresponding to approximately 4.4% in the proximity of the negative electrode, as shown in Figure 2j and k. Meanwhile, the diameter of the microwire shrank in the proximity of the positive electrode. More interestingly, when a backward poling electric field was applied to the single-wire device, the structural swell−shrink moved toward the opposite direction. Obviously, such reversible structural swell−shrink is due to the migration and accumulation of the more active MA+ ions, which has not been observed in bulk perovskite film or lateral devices.26,27 This provides another perspective for the ion migration in perovskite crystals; specifically, their lattice constants strongly depend on the MA+ ion migration and accumulation. Accordingly, we estimated the maximum ion migration quantity in perovskite by calculating the MA+ fillinginduced lattice distortion. The next question is how the reversibly structural swell− shrink affects the light−matter interaction and photoelectric properties in perovskites. In Figure 2g−i, the near-field optical images and phase images are also recorded simultaneously with the excitation of mid-IR light at 11.7 μm, which makes it possible to map the free-carriers’ distribution and refractive index5,32,33 in the perovskite microwire during the poling process. From the near-field optical images, it is clearly seen that the pristine perovskite microwire shows a uniform nearfield amplitude throughout the entire microwire, indicating a uniform local carrier density. In sharp contrast, the near-field amplitude distribution of the poled (0.5 V μm−1 for 20 min) perovskite microwire decreases gradually from the region near
tetragonal crystal, the synthesized standard perovskite lattice constants are a = 0.903 nm, b = 0.903 nm, and c = 1.268 nm. We judged the lattice distortion by calculating the XRD patterns of perovskites with different MA+ contents. When the MA+ ions exceeded 30%, the lattice constants changed dramatically to a = 0.924 nm, b = 0.924 nm, and c = 1.282. Thus, the average lattice constant of the perovskite was increased by approximately 3.1%. The theoretical and XRD results show that the MA+ filling could significantly change the lattice structures of the perovskite, which could, in turn, induce changes at the macroscale of the materials and their physical properties. In general, ion migration and accumulation will occur in perovskites under a poling electric field. It is expected that the migration and accumulation of ions in perovskites, especially MA+ ions, will play an important role in determining the structural and physical properties of perovskites. Controllable manipulation of the ion migration in perovskites and probing of the resulting effects are thus issues of strategic interest. In this regard, 1D perovskite microwires with high crystalline quality were intentionally synthesized for the fabrication of a single-wire optoelectronic device. The synthesis and characterization of 1D perovskite microwires are shown in Figure S1 of the Supporting Information. To prevent contamination from doping perovskite, a lithography-free technique with a hard Si3N4 shadow mask (Supporting Information, Figure S2) was used to fabricate a high-quality and clean single-wire device. In this work, s-NSOM was used to image the perovskite microwire during the poling process to in situ visualize the lattice distortion and local carrier redistribution upon ion migration in an on-working perovskite device (Figure 2a). Generally, optical images from s-NSOM can address a rich variety of light−matter interactions because photons in the lowenergy range can excite molecular vibrations and phonons, as well as plasmons and electrons of a metal or semiconductor. Therefore, the contrast mechanism can be easily adapted to study different properties, such as refractive index, chemical structure, local stress, and carrier density.5,32,33 A scanning electron microscopy (SEM) image of the perovskite single-wire device is shown in Figure 2b. Neither in the dark nor under 7034
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
Article
ACS Nano
indicating the saturation of ion migration in the perovskite microwire. We thus estimated the migration and accumulation length for ions to be approximately 10 μm away from the electrode, which is consistent with the results of a previous study.27 The PL spectra at different positions in the perovskite microwire are presented in Figure 3i (pristine state) and j (poling state). Obviously, the PL peak position shifts to lower wavelengths and the intensity gradually quenches along the poling direction because of the ion-migration-induced structural swell−shrink and change in the band structure. Figure 3e−h show the PL mapping images of the perovskite microwire after the poling voltage was removed. As expected, the PL images recovered gradually to a state similar to the pristine one, demonstrating that the migrated ions recovered their original arrangement in the crystal lattice. In comparison to the PL change rate in Figure 3a−d, the PL recovery rate was much lower (approximately 30 min), possibly because of the absence of reverse voltage on the device. We concluded that the origin of this switchable PL intensity in the perovskite microwire could be ascribed to reversible structural swell− shrink in the perovskite induced by ion migration; that is, the accumulation of I− may inhibit the charge carrier transport, resulting in an increase of PL intensity. Nevertheless, the accumulation of MA+ induced lattice distortion, which resulted in a further decrease of PL quantum yield. We further performed in situ PL mapping to investigate the aging dynamics of the single-wire device under the cyclic poling process, as shown in Figure 4a−d. Previous studies have
the positive electrode to the region near the negative electrode, indicating a larger local carrier density near the positive electrode (Figure 2l). Huang et al.27 demonstrated that the electric field causes the drift of I− and MA+ ions, which have low formation energies in perovskites, to the area near the electrode, forming a p−i−n structure. Obviously, the dopinginduced band bending caused by I− migration in the perovskite close to the positive electrode will increase the local carrier density. However, toward the region near the negative electrode, although the migration of MA+ ions caused by negative poling will result in n-type doping, the local carrier density will decrease instead. This is because the migration and accumulation of MA+ could induce structural expansion, which further compromises the carrier density change caused by the accumulation of MA+ ions. Nevertheless, it has significant influences on the band structure and the refractive index of perovskite. The corresponding phase images (Figure 2g−i) show that the refractive index near the negative electrode is extremely high, which occurs because the absorption of perovskites decreases with structural expansion. Notably, the characteristic of the refractive index modulated by an external electric field will make this type of perovskite a promising candidate for fabricating functional photonic devices, which deserves further investigation. To further investigate the changes in optical properties upon ion migration in perovskites, PL mapping was applied to obtain qualitative and quantitative information regarding the band structure and quantum yield efficiency for the single-perovskitewire device. The PL spectrum, as an important tool, has been widely used to study the chemical composition, electronic structure, and optical and photoelectric conversion properties of perovskites and is also useful for measuring the defects and dopants of perovskites.8,34 Using the spatially resolved PL imaging technique, we noted that the migration and accumulation of different ions resulted in distinct light emission at different parts of the perovskite microwire, as shown in Figure 3. In particular, the accumulation of I− led to an increase of PL intensity. Meanwhile, the accumulation of MA+ led to a decrease in the PL quantum yield. Generally, the optical properties of perovskites are strongly dependent on the structure and composition. Y. Yuan et al. reported a reversible conversion between CH3NH3PbI3 and PbI2 phases under a poling electric field.35 Therefore, in the single-wire device, the accumulation of I− may result in the formation of PbI2 near the positive electrode, and the presence of a proper amount of PbI2 species led to enhanced PL quantum efficiency.36,37 However, the accumulation of MA+ near the negative electrode could induce structural expansion, reducing the PL quantum yield. The PL mapping images as a function of the poling and recovery time are shown in Figure 3a−d and e−h. The relatively uniform PL intensity over the middle section of the pristine perovskite microwire suggests that the sample maintained good quality and uniformity after the fabrication process. However, PL quantum yields on both ends of the perovskite microwire were much lower than that in the middle section. Such a dramatic PL quenching results from the charge carrier transfer between the perovskite and the Au electrode. During the poling process, the corresponding PL intensity increased in the proximity of the positive electrode as a function of the poling time and decreased in the proximity of the negative electrode. Moreover, the PL mapping image obtained after electrical poling with an electric field of 0.5 V μm−1 for more than 20 min does not show a substantial change,
Figure 4. (a−d) PL mapping images for the single-wire device under the cyclic poling process. Scale bars: 5 μm. (e) PL mapping image for the single-wire device after annealing. Scale bars: 5 μm. (f) PL intensity as a function of the number of cyclic poling processes.
suggested that both MA+ and I− easily diffuse with low barriers and that the interaction of MA+ with surrounding lattice and neighboring dipoles could result in a locally preferred alignment.24,30 However, visualized evidence that shows how the ion migration accelerates the aging in perovskite is lacking. Here, a constant voltage of 10 V was applied to and removed from the perovskite microwire every 30 min. The PL mapping image quenched greatly after the 10th poling process (Figure 4d and f). The observed PL quenching subsequently demonstrated that the ion migration in perovskite could not be recovered completely, resulting in gradual lattice distortion. This lattice distortion severely affects the quantum yield efficiency and photoelectric properties of the perovskite. We speculate that grain boundaries in the perovskite microwire play 7035
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
Article
ACS Nano a critical role because the trapped ions at the boundaries can create a potential barrier, which, in turn, reduces the ionic mobility. Consequently, the accumulated ions at the grain boundaries decrease the local PL intensity. In this regard, we conclude that the ion migration in the perovskite is a critical factor that accelerates aging. In particular, Tress et al.38 recently investigated the aging of PSCs due to the presence of mobile ions. Our results further verify this argument from the viewpoint of PL emission at the nano/microscale. Postannealing treatment in an argon atmosphere at 70 °C for approximately 1 h induced the ion rearrangement and lattice relaxation of perovskite. As a result, uniform PL emission was recovered throughout almost the entire perovskite microwire, as shown in Figure 4e. Lastly, we performed photocurrent mapping to investigate the role of ion migration in photocurrent generation and transport. As mentioned in the introduction, the built-in electric field caused by ion migration in PSCs can lead to an overestimation of the device performance if the device is measured rapidly after light-soaking under open-circuit or forward-bias conditions. Therefore, qualitatively verifying the built-in electric field could also demonstrate the ion migration in perovskites.39 Herein, the photocurrent image was acquired by recording the drain current (ID‑S) at the zero drain−source bias (VD‑S), while a focused green laser (wavelength: 532 nm; spot size: approximately 300 nm) scanned the channel area, as shown in Figure 5a and b. The photocurrent mapping result for the pristine perovskite microwire device is shown in Figure 5c. Upon laser illumination, electron−hole pairs were separated and then transported in opposite directions due to local electric fields. Because the electrodes were made of the same material (Au), no preferential directionality exists for charge carrier transport. Therefore, the photocurrent close to the electrodes exhibited opposite polarity. However, a built-in electric field was clearly observed in the photocurrent mapping image after poling the single-wire device with a positive bias of 10 V for approximately 5 min. As shown in Figure 5d, the photocurrent shows a preferential directionality throughout the entire microwire because of the built-in electric field caused by ion migration. Meanwhile, the photocurrent decreases substantially after the 10th poling process (Figure 5e), and the generation of the photocurrent in the proximity of the negative electrode is relatively lower than that in the proximity of the positive electrode. The photocurrent mapping result (Figure 5f) for the single-wire device with postannealing is similar to the pristine one, except that the photocurrent intensity is slightly decreased. These results also demonstrate that the ion migration, especially the MA+ ion migration in perovskite, was not recovered completely, which, in turn, severely affected the photoelectric properties.
Figure 5. (a) Schematic of photocurrent mapping system. (b) Optical image of perovskite single-wire device. Scale bar: 10 μm. (c) Photocurrent mapping image for the pristine perovskite singlewire device. Scale bar: 2 μm. (d) Photocurrent mapping image for the perovskite single-wire device after first positive poling. Scale bar: 2 μm. (e) Photocurrent mapping image for the perovskite single-wire device after the 10th positive poling. Scale bar: 2 μm. (f) Photocurrent mapping image for the perovskite single-wire device after postannealing. Scale bar: 2 μm.
expansion in the perovskites and further reduced the absorption, quantum yield efficiency, and photoelectric conversion efficiency. In summary, we investigated the ion migration in perovskites using optical characterization, including PL mapping, s-NSOM, and photocurrent mapping techniques. Our combined experimental and computational study has provided microscale insights into the influence of ion migration on the microstructure, properties, and light−matter interaction of perovskites. We observed that ion migration, especially migration of MA+ ions, induces a reversible structural swell−shrink in perovskites and affects the reflective index, quantum efficiency, light-harvesting, and photoelectric properties recoverably. We also quantified the parameters of ion migration in perovskites; that is, the maximum ion migration quantity in perovskites can be as high as approximately 30%, resulting in a lattice swell− shrink of approximately 4.4%. Moreover, ion migration in perovskites can gradually accelerate the aging of perovskites because of lattice distortion in the reversible structural swell− shrink process. This study constructed a concise picture for understanding the light−matter interaction and structure− properties relationship in hybrid organic−inorganic perovskites, which may facilitate the development of photonic and optoelectronic functional devices.
CONCLUSION We have in situ visualized the ion migration in perovskites and then qualitatively and quantitatively investigated the role played by ion migration in the photoelectric conversion process, including the changes in the structure, optical properties, light− matter interaction, photoinduced carriers, etc. The I− and MA+ ions in perovskites migrated under an applied bias voltage, thus inducing a reversible structural swell−shrink and recoverable optical properties. The migration of I− ions caused by the poling effect induced p-type doping and then resulted in an increase of the local photocarrier density. By contrast, the migration and accumulation of MA+ ions led to structural 7036
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
Article
ACS Nano
coupled (Thorlabs P5-405BPM-FC-2) through a fiber bench (Thorlabs FBP-A-FC) into the microscope. The X−Y piezostage of the microscope was adopted to acquire two-dimensional spectral maps. The current signal was led into a preamplifier (FEMTO DLPCA-200, Messtechnik GmbH). To improve the signal-to-noise ratio, the laser beam was chopped using an optical chopper (C995, Terahertz Technologies, Inc.), and the photocurrent was collected using a lock-in amplifier (FEMTO LIA-MV-150) in the dark. The topography and near-field images were captured using a commercial sSNOM (NeaSNOM, NeaSpec GmbH). All the measurements were carried out in dry ambient condition (humidity 175 μm in SolutionGrown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (4) http://www.nrel.gov/ncpv/images/efficiency_chart.jpg. (Research Cell Efficiency Records, accessed February 2016). (5) Wang, Y.; Zhang, Y.; Lu, Y.; Xu, W.; Mu, H.; Chen, C.; Qiao, H.; Song, J.; Li, S.; Sun, B. Hybrid Graphene-Perovskite Phototransistors with Ultrahigh Responsivity and Gain. Adv. Opt. Mater. 2015, 3, 1389−1396. (6) Liu, J.; Xue, Y.; Wang, Z.; Xu, Z.-Q.; Zheng, C.; Weber, B.; Song, J.; Wang, Y.; Lu, Y.; Zhang, Y. Two-Dimensional CH3NH3PbI3 7037
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
Article
ACS Nano Perovskite: Synthesis and Optoelectronic Application. ACS Nano 2016, 10, 3536−3542. (7) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D. Bright light-Emitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (8) Wang, Z.; Liu, J.; Xu, Z.-Q.; Xue, Y.; Jiang, L.; Song, J.; Huang, F.; Wang, Y.; Zhong, Y. L.; Zhang, Y. Wavelength-Tunable Waveguides Based on Polycrystalline Organic−Inorganic Perovskite Microwires. Nanoscale 2016, 8, 6258−6264. (9) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636−642. (10) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (11) Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray Weale, A.; Bach, U.; Cheng, Y. B.; Spiccia, L. A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem. 2014, 126, 10056− 10061. (12) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent Engineering for High-Performance Inorganic−Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897−903. (13) Hao, F.; Stoumpos, C. C.; Cao, D. H.; Chang, R. P.; Kanatzidis, M. G. Lead-Free Solid-State Organic-Inorganic Halide Perovskite Solar Cells. Nat. Photonics 2014, 8, 489−494. (14) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476−480. (15) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (16) Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J. Efficient Organometal Trihalide Perovskite Planar-Heterojunction Solar Cells on Flexible Polymer Substrates. Nat. Commun. 2013, 4, 2761. (17) Manser, J. S.; Kamat, P. V. Band Filling with Free Charge Carriers in Organometal Halide Perovskites. Nat. Photonics 2014, 8, 737−743. (18) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; 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. (19) Sanchez, R. S.; Gonzalez-Pedro, V.; Lee, J.-W.; Park, N.-G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J. Slow Dynamic Processes in Lead Halide Perovskite Solar Cells. Characteristic Times and Hysteresis. J. Phys. Chem. Lett. 2014, 5, 2357−2363. (20) Kim, H.-S.; Park, N.-G. Parameters Affecting I−V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer. J. Phys. Chem. Lett. 2014, 5, 2927−2934. (21) Unger, E.; Hoke, E.; Bailie, C.; Nguyen, W.; Bowring, A.; Heumüller, T.; Christoforo, M.; McGehee, M. Hysteresis and Transient Behavior in Current−Voltage Measurements of HybridPerovskite Absorber Solar Cells. Energy Environ. Sci. 2014, 7, 3690− 3698. (22) Beilsten-Edmands, J.; Eperon, G.; Johnson, R.; Snaith, H.; Radaelli, P. Non-Ferroelectric Nature of the Conductance Hysteresis in CH3NH3PbI3 Perovskite-Based Photovoltaic Devices. Appl. Phys. Lett. 2015, 106, 173502. (23) 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. (24) Leguy, A. M.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kockelmann, W.; Law, C.; Li, X.; Foglia, F.; Walsh, A.; O’regan, B. C.
The Dynamics of Methylammonium Ions in Hybrid Organic-Inorganic Perovskite Solar Cells. Nat. Commun. 2015, 6, 7124. (25) Eames, C.; Frost, J. M.; Barnes, P. R.; O’regan, B. C.; Walsh, A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat. Commun. 2015, 6, 7497. (26) Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat. Mater. 2015, 14, 193−198. (27) Yuan, Y.; Chae, J.; Shao, Y.; Wang, Q.; Xiao, Z.; Centrone, A.; Huang, J. Photovoltaic Switching Mechanism in Lateral Structure Hybrid Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500615. (28) Yang, T. Y.; Gregori, G.; Pellet, N.; Grätzel, M.; Maier, J. The Significance of Ion Conduction in a Hybrid Organic-Inorganic LeadIodide-Based Perovskite Photosensitizer. Angew. Chem. 2015, 127, 8016−8021. (29) 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. (30) Haruyama, J.; Sodeyama, K.; Han, L.; Tateyama, Y. FirstPrinciples Study of Ion Diffusion in Perovskite Solar Cell Sensitizers. J. Am. Chem. Soc. 2015, 137, 10048−10051. (31) Bergmann, V. W.; Weber, S. A.; Ramos, F. J.; Nazeeruddin, M. K.; Grätzel, M.; Li, D.; Domanski, A. L.; Lieberwirth, I.; Ahmad, S.; Berger, R. Real-Space Observation of Unbalanced Charge Distribution Inside a Perovskite-Sensitized Solar Cell. Nat. Commun. 2014, 5, 5001. (32) Stiegler, J.; Huber, A.; Diedenhofen, S.; Gomez Rivas, J.; Algra, R.; Bakkers, E.; Hillenbrand, R. Nanoscale Free-Carrier Profiling of Individual Semiconductor Nanowires by Infrared Near-Field Nanoscopy. Nano Lett. 2010, 10, 1387−1392. (33) Radko, I.; Volkov, V. S.; Bozhevolnyi, S. I.; Henningsen, J.; Pedersen, J. Near-Field Mapping of Surface Refractive-Index Distributions. Laser Phys. Lett. 2005, 2, 440. (34) Hameiri, Z.; Mahboubi Soufiani, A.; Juhl, M. K.; Jiang, L.; Huang, F.; Cheng, Y. B.; Kampwerth, H.; Weber, J. W.; Green, M. A.; Trupke, T. Photoluminescence and Electroluminescence Imaging of Perovskite Solar Cells. Prog. Photovoltaics 2015, 23, 1697−1705. (35) Yuan, Y.; Wang, Q.; Shao, Y.; Lu, H.; Li, T.; Gruverman, A.; Huang, J. Electric-Field-Driven Reversible Conversion Between Methylammonium Lead Triiodide Perovskites and Lead Iodide at Elevated Temperatures. Adv. Energy Mater. 2016, 6, 1501803. (36) Sutter-Fella, C. M.; Li, Y.; Amani, M.; Ager, J. W.; Toma, F. M.; Yablonovitch, E.; Sharp, I. D.; Javey, A. High Photoluminescence Quantum Yield in Band Gap Tunable Bromide Containing Mixed Halide Perovskites. Nano Lett. 2016, 16, 800−806. (37) Chen, Q.; Zhou, H.; Song, T.-B.; Luo, S.; Hong, Z.; Duan, H.-S.; Dou, L.; Liu, Y.; Yang, Y. Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar Cells. Nano Lett. 2014, 14, 4158−4163. (38) Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S.; Nazeeruddin, M. K.; Grätzel, M. Understanding the Rate-Dependent J-V Hysteresis, Slow Time Component, and Aging in CH3NH3PbI3 Perovskite Solar Cells: the Role of a Compensated Electric Field. Energ. Energy Environ. Sci. 2015, 8, 995−1004. (39) 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.
7038
DOI: 10.1021/acsnano.6b03104 ACS Nano 2016, 10, 7031−7038
