Halide Perovskite Nanopillar Photodetector - ACS Nano (ACS

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Halide Perovskite Nanopillar Photodetector Do Hyung Chun, Young Jin Choi, Yong Jae In, Jae Keun Nam, Yung Ji Choi, Sangeun Yun, Wook Kim, Dukhyun Choi, Dongho Kim, Hyunjung Shin, Jeong Ho Cho, and Jong Hyeok Park ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04170 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Halide Perovskite Nanopillar Photodetector Do Hyung Chun1, Young Jin Choi2, Yongjae In3, Jae Keun Nam1, Yung Ji Choi4, Sangeun Yun4, Wook Kim5, Dukhyun Choi5, Dongho Kim4, Hyunjung Shin3, Jeong Ho Cho*,2 and Jong Hyeok Park*,1

1

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea 2

SKKU Advanced Institute of Technology, Department of Nano Engineering, Sungkyunkwan University, Suwon 440-746, Korea 3

Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea

4

Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea. 5

Department of Mechanical Engineering, Kyung Hee University, 1732, Deogyeong-daero, Giheunggu, Yongin-si, Gyeonggi-do 17104, Republic of Korea

E-mail: [email protected], [email protected]

ABSTRACT

Numerous studies have reported the use of halide perovskites as highly functional lightharvesting materials. The development of optimized compositions and deposition approaches has led to impressive improvements; however, no noticeable breakthrough in performance has been observed for these materials recently. Here, a breakthrough that enables the fabrication of vertically grown halide perovskite (VGHP) nanopillar photodetectors via a nanoimprinting

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crystallization technique is demonstrated. We used engraved nanopatterned polymer stamps to form VGHP nanopillars during the pressurized crystallization of the softly baked gel state of a methylammonium lead iodide (CH3NH3PbI3, denoted MAPI) film. The VGHP films exhibit much lower defect density and higher conductivity, as supported by current-voltage (I-V) characteristic measurements and conductive atomic force microscopy (c-AFM) measurements. Ultimately, two-terminal lateral photodetectors based on the VGHP nanopillar films show a greatly enhanced photoresponse compared with flat film-based photodetectors. We expect that the deposition method presented here will help surpass the technical limits and contribute to further improvements in various halide perovskite-based devices.

KEYWORDS: perovskite; photodetector; nanopattern; nanopillar; nanoimprinting


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Halide perovskites have received tremendous attention as next-generation photoelectronic materials.1-5 Halide perovskites exhibit ideal photophysical properties, including tunable optical absorption (bandgap), charge-carrier separation/transport, and slow recombination kinetics.6-9 Heretofore, most studies regarding halide perovskite materials have focused on engineering of their composition, deposition, and interfacial properties.10-17 As a result, solar cells with efficiencies over 22% have been fabricated by employing the-state-of-the-art halide perovskite films. However, growth in the solar cell efficiency has lagged by approximately 20%, which may represent an ultimate hurdle. In this regard, a universal methodology is required to tailor the photophysical properties of halide perovskites to eventually break the technical limit and achieve ultrahigh efficiency.3,10,11 The soft imprinting lithography method has been widely applied to modify the morphologies of various soft materials, including photoresponsive semiconducting polymers, from which their photophysical properties can be easily tailored.18,19 In this regard, several attempts have been made to apply the imprinting lithography method to fabricate of halide perovskite-based devices.20-23 For instance, Wang et al.20 applied nanoimprinting lithography to tailor the surficial structure of a

MAPI perovskite film and succeeded in fabricating a

photodiode, although some defects were present at the surface of the perovskite film after the imprinting process. To obtain defect-free nanoimprinted perovskite films, Pourdavaud et al.21 conducted the post-treatment of a fully crystallized perovskite to passivate the surface defects and revealed the photonic effect of the two-dimensional (2D) photonic crystal structure and profiled the general photophysical properties. However, the studies reported so far tried to obtain nanopatterned films by merely physically deforming the totally crystallized pre-formed MAPI film. Further, they utilized rigid nanopatterned silicon molds, which are expensive and not


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suitable for flexible substrates. To solve this problem, Jeong et al.22 introduced a solvent-assisted gel printing method to control the crystalline structure of perovskite thin films. In this paper, we report, the simple formation of vertically grown halide perovskite (VGHP) nanopillar films by stamping the pre-annealed MAPI. A previous study by our group verified that the pressure spatially induced by the nanopatterned polymer stamps changes the crystallinity of the halide perovskites films.23 Here, we designed halide perovskites with a onedimensional (1D) geometry and observed advantages such as reduced defect density and improved charge-carrier mobility in the MAPI films, which are beneficial for harvesting charges induced by photons. 1D nanoarrays are expected to greatly improve the electronic properties, including fast charge transport, over those of the corresponding bulk film, and the photophysical properties of the VGHP film were immensely improved, indicating their applicability to perovskite-based photoelectronic devices.24-27 Moreover, the nanopillar structure has a greatly reduced channel thickness, which is critically important for obtaining much smaller dark currents. As a result, VGHP photodetectors, developed along with the emergence of state-of-theart perovskite films, are successfully demonstrated. The VGHP photodetectors exhibited greatly improved figure-of-merit parameters, such as responsivity and detectivity, over those of conventional flat film-based photodetectors.

RESULTS AND DISCUSSION Structural Analysis of Vertically Grown Halide Perovskite Films. The schematic diagram in Scheme 1 demonstrates the experimental procedures to form the VGHP film by the nanoimprinting method. First, hexagonal nanohole-arrayed polyurethane


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acrylate (PUA) molds (Figure S1a) were prepared through a UV-curing procedure with an unclenching UV-curable resin on hexagonal nanopillar-arrayed PUA master molds (Figure S1b).28 The light scattering pattern of the PUA film, shown in the Figure S1, shows a photonic crystal effect, indicating the existence of a nanopattern on the polymer mold.19-21 After that, the MAPI film was spin-coated on a precleaned indium-doped tin oxide (ITO)-coated substrate following the methodology reported elsewhere under an atmosphere with controlled humidity.14,29 A substrate with the intermediate state of MAPI, i.e., the pre-annealed film, was formed by the spin-coating process. The residual solvent in the intermediate MAPI film was further dried by stamping at a pressure of 16 kgf/m2 for a few seconds with the intaglio PUA mold while being baked at 100 ℃ and continuously annealed for complete crystallization. During this process, the intermediate MAPI film crystallized, and the mold shape was imprinted under the applied vertical stress. The pressurizing process was simulated to investigate the effective and uniaxial stress (z-axis) distribution on the perovskite layer during stamping, and the crosssectional views are presented in Figure 1a,b, respectively. When the perovskite layer was compressed by the nanohole-arrayed PUA mold, symmetric stresses were formed in x and y directions. The lowest effective stresses of 50 kPa were formed in the internal space of the hexagonal array, as shown in the perspective view. (Figure S2a) The largest effective stresses of 534 kPa were formed in the hexagonal array due to tensile deformation, which resulted from the compressive load and nanohole patterning. The cross-sectional distribution over the yz plane was obtained to observe the inner effective stresses of the perovskite layer (Figure 1a). Due to the nanohole pattern, the stress concentration occurred in the hexagonal array. Since the perovskite layer was compressed by a z-axis load, we divided and observed the z-axis stress component, as shown in Figures 1b and S2b. As expected, relatively strong tensile stress of 370 kPa formed in


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the hexagonal array. Uniform compressive stress was formed in the z direction throughout the entire perovskite layer, and a balanced pressure distribution on the VGHP film can contribute to the uniform crystallization of MAPI.30,31 By pressurizing and crystallizing the intermediate state of the MAPI film, we greatly reduced the pressurizing time and the magnitude of pressure compared with those in previously reported imprinting strategies utilizing pre-formed halide perovskite films.20,21 Although perovskite crystallization occurred in the restricted space of the PUA nanoholes during the imprinting process, MAPI residue on the PUA surface is negligible even after 100 reuses without any surface treatment (Figure S3). Furthermore, the large-area PUA mold can easily be prepared with a UV-curing process, as exhibited in Figure S4a. Using this large-area mold, a 3 × 3 cm2 perovskite film was successfully fabricated by our methodology (Figure S4b). The morphological evolution on the VGHP film was observed by scanning electron microscopy (SEM). The SEM image of the VGHP film (Figure 1c) explicitly exhibits a columnar perovskite morphology, while the as-prepared MAPI film (Figure 1d) shows a flat surface. The 1D vertically aligned nanostructure has a beneficial surface-to-volume ratio,32 which can improve light absorption, as supported by UV-visible absorption spectroscopy measurements (Figure S5). The atomic force microscopy (AFM) image (Figure 1e) of the VGHP film also supports the successful morphological modification compared with the pristine MAPI film (Figure 1f). The height profile extracted from the AFM data (Figure S6) clearly shows that the extruded columnar perovskite film has uniform height of approximately 130 nm. The rounded shape of the tips of the perovskite columns allows us to envision that the intermediate perovskite composites were extruded into the restricted area of the polymer pore pattern by capillary forces during the crystallization process, which is supported by the difference in the


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contact angle of ethylene glycol (EG) on MAPI (30°) and PUA (53°).33 (demonstrated in Figure S7a,b, respectively) To investigate of the intrinsic structural properties of the perovskite pillars, X-ray diffraction (XRD) (Figure S8) was performed. The pattern shows almost an identical distribution of diffraction peaks, except for the enhanced intensity of the peak at 14.08°, which corresponds to the (110) peak of tetragonal MAPI. The enhanced intensity of the (110) peak can be understood as due to the improved crystallographic orientation, which can increase the device performance.22,23,34 Vertically Grown Halide Perovskite Nanopillar Photodetectors Figure 2a shows a three-dimensional (3D) schematic image of the two-terminal photodetector based on the VGHP film. ITO-coated glass was used as the substrate. The ITO electrodes were patterned via conventional photolithography followed by wet etching. The channel length and width were 1000 and 50 µm, respectively. The VGHP film was formed by the afore-mentioned method. Figure 2b shows the current-voltage plot of the perovskite photodetectors in the dark and illuminated states. Light with a wavelength of 520 nm and an optical power with 500 µW was illuminated onto the perovskite channel region. The black lines represent the curves for the reference device without a nanopillar structure, while the blue lines represent the curves for the device with VGHP. The linear plot of the curves under both conditions clearly show linear behavior (Figure S9), indicating Ohmic contact between the perovskite and ITO. Under light illumination, photon absorption generated electron-hole pairs in the channel layer, which subsequently dissociated into holes and electrons under the electric field applied between the two electrodes, which resulted in a dramatic enhancement in the


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photocurrent. The VGHP photodetectors exhibited lower dark current (Idark) than the reference devices. The lower Idark could be attributed to the reduced channel thickness (~130 nm) and decreased defect density induced by the pressure applied during imprinting.30,35 Importantly, a higher photocurrent (Iphoto) was observed, even though the VGHP photodetectors have lower Idark. This result is due to the enhanced light absorption and high-quality crystallization of MAPI. Note that the application of the nanopillar structure in the perovskite channel layer not only increased Iphoto but also decreased Idark. The optical power-dependent photoresponse of the perovskite photodetectors was investigated by illumination by light with an optical power ranging from 1 to 500 µW. Figure 2c shows the I-V plot of the VGHP photodetectors under different optical powers. The current increased gradually as the optical power increased because more photoexcited excitons were generated under higher optical power. Both the photoresponsivity (R) and specific detectivity (D*), which are important performance parameters of photodetectors,36,37 were extracted from the plot in Figure 2c. R is defined as the photocurrent (Iphoto – Idark) divided by optical power of the light. D* is defined as R/(2qJdark)0.5, assuming that the noise of the detected signal is predominantly due to dark current, where q is the element charge and Jdark is the dark current density.38 Considering these relationships, improved photodetector performance could be achieved by both increasing Iphoto and suppressing Idark. Figure 2d shows R and D* as functions of optical power, exhibiting that both values decreased as the optical power of the light increased. R and D* were calculated from the Idark and Iphoto values at V = 50 V. The nanopillar structure of the VGHP channel enhanced both R and D* by an order magnitude, which resulted from both the increased Iphoto and suppressed Idark. For example, the values of R and D* for the VGHP devices were approximately 1 A/W and 5 × 109 Jones, respectively, at an optical power of 1 µW. Finally,


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the temporal response of the photodetectors was investigated, as shown in Figure 2e. The laser was turned on for 5 s with an interval of 10 s per cycle, and the current was measured at V = 10 V. The current increased promptly under illumination (on state) and then returned to a low value (off state) when the light was turned o℃. The on/off current ratio increased with optical power and was over 100 for 100 µW switching. The rise and fall times were found to be less than 100 ms, which were much faster than those of the reference devices (Figure S10). This result could be attributed to the improved charge-carrier mobility, as detailed in the next section. Reduced Defect Density and Increased Exciton Lifetime To investigate the defect densities in the VGHP and MAPI nanopillar films, the dark I-V profiles were measured, as shown in Figure 3a. The defect density was calculated by measuring the slope in the trap-filled limited (TFL) region (blue region in Figure 3a), and slope increased in the I-V graph with the formula of ்ܸி௅ = ݊௧௥௔௣ ‫ܮݍ‬ଶ ∕ 2ε (where ݊௧௥௔௣ is the trap density of the perovskite, q is the elementary charge, L is the length of the perovskite film, ்ܸி௅ is the TFL voltage and ε is the dielectric constant of the perovskite, which is 32 for polycrystalline MAPI).7,9 VTFL was 4.08 and 21.55 V for the VGHP and pristine MAPI films, respectively. The defect density of the VGHP film was reduced by approximately one-fifth, i.e., 5.27 × 1017 to 9.81 × 1016, and these values are almost the same as those of a typical polycrystalline MAPI film.7 The pressurizing process leads to the shrinkage of the perovskite grain boundaries, which behave as defect sites after the nanoimprinting process.32,39 For this reason, considering that defects are one of the major obstacles to suppress Idark, especially in polycrystalline perovskite photodetectors, the reduced defect density in the VGHP film can clearly support the decreased Idark and enhanced Iphoto, finally inducing responsivity to the device.4


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Since the reduced defect density extended the carrier diffusion length and enhanced the charge-carrier mobility,7,34 time-resolved photoluminescence (PL) spectroscopy was examined to probe the correlation between defect density and carrier lifetime in the VGHP film (Figure 3b). The time-resolved PL measurement was monitored under the wavelength of peak emission derived from the steady PL data, which exhibited that the PL intensity in the VGHP film increased over that of the as-prepared MAPI film (Figure 3c). The time-resolved PL exhibited biexponential decay that included fast (τ1) and slow (τ2) components. Notably, the fast and slow components are attributed to nonradiative and radiative recombination, respectively.7 The measured data show an increased average carrier lifetime (τ2) over that in the pristine MAPI film, from 38.6 to 43.5 ns (Table 1). Considering that an extended carrier lifetime can contribute to minimizing the energy loss of generated carriers and improve the extraction of electrons and holes to the electrodes, this result indicates the increased photocurrent in the photodetector.9,20 The ratio of τ2 to τ1 also increased from 49.7 to 85.3%, and the measured data are given in Table 1. A reduction in the τ1 component, indicating suppressed nonradiative recombination in the perovskite film, can positively affect the function of the photodetector under illumination. The change in the intrinsic properties of the MAPI film induced by generating the 1D nanopillar array structure, as confirmed by the defect state density and PL profile, clearly supports the enhancement in the figure-of-merit parameters of the photodetectors. Investigation of Conductivity with Conductive Atomic Force Microscopy Analysis c-AFM was performed to investigate the electrical properties of the perovskite thin films. The perovskite film samples for c-AFM measurement were prepared on compact TiO2-deposited FTO substrates to detect the extracted holes, which can clearly exhibit the conductivity of a film. The topography and current profile were obtained by c-AFM measurement, which showed the


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morphology (Figure 4 upper left) and spatial current profile, respectively (Figure 4). The topography of the as-prepared and VGHP films indicates the high quality of each film. c-AFM images illustrating current flow at certain areas were obtained at the same positions at which the topography was observed. The c-AFM image of the VGHP film under 0.1 V and illumination conditions (Figure 4a) showed brighter contrast than the reference perovskite film (Figure 4b), which supports the conclusion that the formation of the 1D nanopillar structure on the halide perovskite film contributes to the enhancement in the conductivity of the perovskite film along the thickness direction.40 To examine the improved conductivity, the I-V profile was analyzed (Figure S11). The I-V data were obtained by sweeping the voltage over the range of -1 to 1 V, approximately corresponding to the open-circuit voltage (Voc) of MAPI-based solar cells. The I-V data exhibited more conductive current flow at the perovskite pillars than at the flat area, which corresponds well to the c-AFM image of the tailored perovskite film under light illumination. This enhanced conductivity supports the increase in the photocurrent of the device.27,41-44 c-AFM current scanning analysis without illumination was also conducted to clarify the depression of Idark under biased conditions, directly exhibiting the difference from the Idark of the devices5,45 (Figure 4c,d). The c-AFM image under dark conditions was monitored over the same area at which illuminated image was taken. A lower Idark was detected for the VGHP film, even though the conductivity of the VGHP film was enhanced. According to the c-AFM data measured under both illumination and dark conditions, the enhanced Iphoto and suppressed Idark were visualized by spatial current images, and the measured data clearly indicate that the VGHP film is an ideal candidate for perovskite photodetectors. As nanoimprinting lithography technique was reported as a strategy to switch ferroelectric property,46-49 the pressure-induced crystallization process is also expected to have


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high functionality in perovskite-based ferroelectric applications. Since tailored structure of halide perovskite film depends on the used nanostructure of polymer molds, developing high-density nanopillar structure is experimentally limited by the density of silicon master mold. Our future work is to study the physical and chemical dependence on the diameter, spacing and height of nanostructured-halide perovskites and designing apparatus for precise and scalable compression on halide perovskite film will be followed.

CONCLUSIONS In summary, we developed a VGHP film via a pressure-induced crystallization method. The strategy we utilized was derived from the distinctiveness of the intermediate state of a preannealed MAPI film, referred to as a composite of MAI-DMSO-PbI2 or a Lewis base adduct.11,29 Our process is easy to follow, and various advantages, including physical and photoelectrical properties essential to the performance of photoelectric devices, can be realized. Here, because of its simple structure and working system, a two-terminal lateral photoconductor was fabricated to examine the effect of the tailored nanostructure of MAPI.27,50-52 The VGHP photodetector showed improved performance in terms of photophysical and electronic properties over the flat film-based photodetector, ultimately leading to high responsivity and detectivity. Furthermore, the polymer master mold was reusable and could be used to fabricate large-area devices, and thus, the method we demonstrated can be expected to contribute to the commercialization of other halide perovskite-based devices in the future. Considering that various nanostructures can be easily tailored as a strategy to achieve desired intrinsic properties, the nanoimprinting process utilizing the intermediate state of a pre-crystallized halide perovskite is a potential methodology that can be applied to various photoelectronic devices.


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METHODS VGHP Photodetector Preparation: UV-curable PUA resin (Norland Optical Adhesive 63) was purchased from Norland Products Inc. The PUA resin was squeezed on a PUA nanodot master mold prepared with e-beam patterned silicon wafer. The PET film was positioned above the PUA resin, and clustered UV resin was unclenched to cover the whole area of the nanopillarshaped mother mold. The PUA resin was cured by a UV lamp with a wavelength of 265 nm for few minutes.25 Thereafter, solidified PUA was detached from the triangular nanodot master mold, and PET film substrate was removed after the UV-curing step. N,N-Dimethylformamide (DMF, 99.8% Sigma-Aldrich) and dimethyl sulfoxide (DMSO, 99.5% Sigma-Aldrich) were prepared. A stoichiometrically balanced 0.8 M MAPI precursor was prepared by mixing MAI and PbI2 powder in a mixture of DMF and DMSO at a volume ratio of 9:1. The prepared precursor was heated at 75 ℃ for 30 min with vigorous stirring for complete dissolution. The perfectly stirred precursor was filtered with hydrophobic filter (Adventec, JP050AN) before deposition. The MAI and PbI2 powders are purchased from Dyesol and Sigma-Aldrich, respectively. The etched indium-doped tin oxide (ITO) substrates were washed by a 20 min sonication process with a mixture of acetone and ethanol. Then, UV/ozone (Altech LTS) treatment was performed for 30 min. The MAPbI3 film was deposited on the UV/ozone-treated ITO substrates by a two-step spin-coating method, which was demonstrated elsewhere.11,26 When spin coating was finished, the intermediate perovskite film was compressed at a pressure of 1.6 kgf/cm2 with the nanohole-array PUA mold for a few seconds while annealing at 100 ℃ to form a pillar structure on the device. The 100 ℃ annealing process was continued for 15 min for the complete crystallization of VGHP.


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Device Measurement: The electrical properties of the photodetectors under dark and illumination conditions were characterized by a source measure unit (Keithley 4200). A 520 nm-wavelength monochromatic dot laser (Susemicon) was utilized as a light source for illumination. The incident light power was measured with an optical power meter (NDC-50C-4M, Thorlabs) and modulated with an optical power attenuator (PM100D, Thorlabs). MAPI Film Characterization: The surficial morphology was observed by field-emission scanning electron microscopy (FE-SEM, JSM-7001F, JEOL) and atomic force microscopy (AFM, Nano Wizard, JPK Instrument). The light absorption data were acquired by a UV-VisNIR spectrophotometer (Agilent, Cary5000). The contact angle was measured using a CCD camera (Prosilica GX) and the Onyong protractor software. The effective stresses (i.e., von Mises stress) and uniaxial stresses in the perovskite layer were calculated using Midas NFX, a commercially available finite element method (FEM) software. To investigate the crystallographic characteristics, X-ray diffraction (XRD, SmartLab, Rigaku) was performed. Steady-state photoluminescence spectra were recorded on a photoluminescence spectrometer (F2500, Hitachi). Time-resolved photoluminescence was detected using a time-correlated singlephoton counting (TCSPC) technique to measure spontaneous photoluminescence decay. The excitation light source was a mode-locked Ti:sapphire laser (MaiTai BB, Spectra-Physics, Santa Clara, CA, USA) that provides ultrashort pulses (80 fs at full width at half maximum) at a high repetition rate (80 MHz). This high repetition rate can be slowed to 1 MHz to 800 kHz using a homemade pulse picker. The pulse-picked output pulse was frequency doubled by a 1 mm-thick BBO crystal (EKSMA, Vilnius, Lithuania). The photoluminescence was collected by a microchannel plate photomultiplier (R3809U-51, Hamamatsu, Hamamatsu, Japan) with a thermoelectric cooler (C4878, Hamamatsu) connected to a TCSPC board (SPC-130, Becker &


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Hickl GmbH (Berlin, Germany)). The overall instrumental response function was ~25 ps (full width at half maximum). A pump pulse vertically polarized by a Glan-laser polarizer was used to irradiate the samples, and a sheet polarizer, set at an angle complementary to the magic angle (54.7°), was placed in the photoluminescence collection path to obtain the polarizationindependent photoluminescence decay. The current images were measured by a commercial atomic force microscope (SPA-400, SII, Japan) using a Pt/Ir-coated Si probe tip (CONTPt, Nanoworld, Inc.) with a spring constant of 0.2 Nm-1 and resonance frequency of 13 kHz. Both the current and topographic measurements were performed in ambient conditions. All images were obtained with a contact force of -0.5 nN and bias voltages of 0 and 0.1 V at a scan rate of 1.0 Hz while the Pt tip was grounded. The samples illuminated by a white light-emitting diode (LED, W42180-08, Seoul semiconductor, Korea) with a maximum intensity of 3.5 mWcm-2 at a distance of 2 cm and angle (α) of 0° when 2.7 V is applied. In this measurement, the white LED was 2 cm away from the sample at an angle (α) of ~30°, in which case the LED intensity was 2.8 mWcm-2, as determined from the data sheet.


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Table 1. Time-resolved PL decay data of pristine and vertically grown perovskite films. The data are fitted with a bi-exponential function. VGHP

Pristine MAPI

τ

τ1

τ2

τ1

τ2

[ns]

3.3

43.5

3.3

38.6

14.7

85.3

50.2

49.7

Weight [%]


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Scheme 1. Schematic illustration of VGHP film preparation.


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Figure 1. Cross-sectional view of (a) the effective stress and (b) the z-axis stress on the perovskite film while stamping with a negative patterned PUA mold. SEM images of the (c) VGHP and (d) pristine MAPI films. AFM images of (e) VGHP and (f) pristine MAPI films (2 × 2 µm2).


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Figure 2. (a) Schematic illustration of the VGHP photodetectors. (b) Current-voltage curves of the reference and VGHP photodetectors under dark and light illumination conditions. (c) Current-voltage curves of the VGHP photodetectors under light illumination at various illumination powers. (d) Responsivity and detectivity of the reference and VGHP photodetectors. (e) Temporal photoresponse of the VGHP photodetectors (1, 10, and 100 µW).


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Figure 3. (a) I-V measurement of the vertically grown (red circles, upper) and as-prepared (black circles, lower) MAPI films. The I-V curve is divided into two areas, the Ohmic (red) and trapfilled limit (blue) regimes, at the point where the gradient of the graph changes rapidly. (b) Timeresolved PL profile of the VGHP (black line) and pristine MAPI (blue line) films. (c) Steady PL intensity of the VGHP (black line) and pristine MAPI (blue line) films. The peak wavelengths were 773 nm for pristine MAPI and 789 nm for VGHP. Both samples were prepared on glass.


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Figure 4. c-AFM images of the (a,c) VGHP and (b,d) pristine halide perovskite films under (a,b) illumination and (c,d) dark conditions at 0.1 V bias. The topography of each scanned image is shown at the top left of the image. The current scale bar ranges from 0.00 (dark) to 0.10 (bright) nA. The size of both the c-AFM and topography images is 2 × 2 µm2. The measured samples are prepared as MAPI/TiO2/FTO to detect holes, which are extracted toward the c-AFM tip.


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ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. SEM images, Effective stresses simulation results, Photographic images, UV-vis light absorption, Height profile, Contact angle, X-Ray diffraction, Extra data of devices, c-AFM I-V profile.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (J. H. Park) *E-mail: [email protected] (J. H. Cho) Author Contributions D. H. C. and Y. J. C. contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No. 20163010012450, 20173010013340). This work was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2016R1A2A1A05005216, NRF2017R1A2B2005790)

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