High-Energy Photon Spectroscopy Using All Solution-Processed

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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High-Energy Photon Spectroscopy Using All Solution-Processed Heterojunctioned Surface-Modified Perovskite Single Crystals Suneel G. Joglekar,† Mark D. Hammig,*,‡ and L. Jay Guo*,† †

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Department of Electrical and Computer Engineering, University of Michigan, 1301 Beal Avenue, Ann Arbor, Michigan 48109, United States ‡ Department of Nuclear Engineering and Radiological Sciences, University of Michigan, 2301 Bonisteel Blvd., Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: Organic−inorganic hybrid perovskites have been intensively studied for their use in optoelectronic devices due to their utilization of lowcost, earth-abundant precursors that are solution-processed at low-temperatures into high-quality devices. Despite this progress, interdevice variability and long-term stability have prevented the widespread commercial adoption of perovskite devices, especially for high-energy photon detectors. Using methylammonium lead iodide perovskite single crystals grown via inversetemperature crystallization, we demonstrate a facile solution-based technique to coat the single-crystalline bulk with a micrometer-scale thick surface layer comprised of a wider band gap two-dimensional Ruddlesden−Popper (RP) hybrid perovskite. The resulting perovskite room-temperature γ-ray detector devices exhibit greatly improved device yield and repeatability from run-to-run and device-to-device within a given processing run. With an energy resolution of under 15% (12.0 keV) for incident 81 keV photons, this solution-based technique resolves interdevice variability concerns and could pave the way for low-cost, scalable manufacturing of optoelectronic devices based on RP hybrid perovskite films. KEYWORDS: perovskite, solution-processed, surface engineering, radiation detection, 2D, Ruddlesden−Popper

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INTRODUCTION Organolead halide perovskite semiconductor materials have attracted intense interest due to their favorable optical and electronic properties as well as the ease with which single and polycrystalline forms can be fabricated.1−6 Most notably, perovskite-based photovoltaic (PV) cells have achieved a rapid rise in power conversion efficiency (PCE) from just 2.2% in 20072,5 to over 22% in 2017,7 an order of magnitude increase in 10 years. The excellent optical absorption properties of perovskite3,4 have also led to their use in photodetectors with greatly improved performance when inorganic perovskite quantum dots are used in combination with a two-dimensional (2D) MoS2 absorbing substrate.8 The relativistic suppression of radiative recombination due to Rashba splitting9 as well as the low density10−12 and location of trap states outside the band gap3,12 in bulk monocrystalline methylammonium lead iodide (MAPbI3) allow large carrier diffusion lengths11 and long carrier lifetimes in this material.10 Though perovskite’s heavy elements (Pb, I) improve its radiation absorption cross-section compared to other semiconductor detector materials, spin-coated layers are far too thin to sufficiently absorb high-energy (>50 keV) photons, leading to polycrystalline spray-coated perovskite12 as well as perovskite single-crystal (PSC)-based14 X-ray detectors. Although Kovalenko15 and Huang16 have demonstrated working solution-grown PSC-based spectroscopic γ-ray © XXXX American Chemical Society

detectors, the energy resolution is considerably worse than even scintillation-based detectors.17,18 A critical, unaddressed aspect of these devices is the quality and electronic properties of the PSC surface. Although the density of bulk trap states in PSCs is low and near that of monocrystalline silicon (Si) as determined from firstprinciples3 as well as experimentally,10 the density of trap states at the perovskite crystal surfaces is much higher, on the order of 1016 cm−3 when studied theoretically 19 and empirically,20 inducing band-bending at the perovskite−metal interfaces leading to increased leakage current and reduced bulk field strength.21 The band-bending can even induce chemical degradation of the perovskite material itself, decreasing the useful device lifetime, and inducing additional constraints on the perovskite device structure.22 Although the presence of trap states can both: (1) increase current recombination within current-mode optoelectronic devices (e.g., reducing the short-circuit current for PV devices)1,3,23−25 and (2) reduce dose-specific sensitivity in Xray imagers,13 transient trapping and detrapping in optoelectronic devices also introduces random telegraph noise (RTN) with a Brownian noise power spectrum (1/f)26 that can Received: May 29, 2019 Accepted: August 9, 2019

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DOI: 10.1021/acsami.9b09381 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) γ-ray spectra collected from 31.55 MBq 133Ba for four different semiconductor detectors: CdZnTe “CZT” (teal), CdTe (blue), Si (fabricated using methods from the literature,42,43 red), and a 2.2 × 1.7 × 0.85 mm MAPbI3 PSC using point contacts (black). The inset shows the detail of the higher energy peaks. (b) γ-ray spectra of (a), focusing on the 81 keV peak. (c) γ-ray spectra collected from the above point-contacted PSC at varying biases, indicated in the legend except in the case of the highest bias of 100 V (navy blue), using the linear energy calibration from the 40 V bias curve to gauge peak shifting at varying applied biases. The inset shows an image of a PSC in the radiation measurement test setup, showing components illustrated in (e). (d) γ-ray spectra simulated and collected (red) from the above point-contacted PSC biased to 100 V. The “unbroadened” spectrum (blue) reflects the energy deposition derived from the MCNP 6 simulation, and the “simulated” distribution (black) includes the effects of energy broadening and charge transport, including photonic backscatter from the Al test chamber and elemental X-ray escape (“XRE”) features. (d-inset) Probability density for three photonic energies as a function of the interaction depth.37 (e) Schematic of the test setup used to measure radiation spectra of PSCs, showing the PSC under test, InGa eutectic contacts, stainless steel shim, test chamber Al base plate, stainless steel probe, and readout electronics: a charge-sensitive amplifier “CSA”, pulse-shaping amplifier “PSA”, and multichannel analyzer “MCA”. Additional details are provided in the Supporting Information (SI), Section S1.

resolution of PSC detectors of under 15% for 81 keV 133Ba γrays, outperforming scintillation-based detectors and performing similarly to cadmium−zinc−telluride (CZT) without depth correction.17,18,28 Furthermore, the high repeatability and consistency of the SAAT PSC detectors could enable the deployment of spectroscopic imaging systems at over an order of magnitude lower cost relative to CZT, paving the way for greater ubiquity of these systems for nondestructive testing as well as security, medical, and astronomy applications.

broaden the peak resolution of spectroscopic devices and diminish their performance.17,18,27,28 One way to reduce the density of surface trap states is by surface passivation.1,24,25 The use of organoamine capping ligands has been demonstrated for the synthesis of fluorescent perovskite nanoparticles,29 with the alkyl chain length affecting their surface energy.30 We report on inverse-temperature crystallization (ITC)grown PSCs that are capped with a 2−3 μm thick surface zone of higher-band gap 2D Ruddlesden−Popper (RP) hybrid perovskites31,32 deposited by a solution-based alkylamine treatment (SAAT). The resulting devices exhibit improved stability, performance repeatability, and a measured energy

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RESULTS AND DISCUSSION Response to γ-ray Impingement. The intrinsic response of the MAPbI 3 PSC can be gauged, irrespective of B


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Figure 2. (a) Best energy resolution spectra from 81 keV 133Ba peak for PSCs by the category of surface treatment and contacting strategy, showing Gaussian fit lines superimposed on the measured data (black dots). (b) Box plot of 81 keV 133Ba peak energy resolutions by category vs detector yield of an untreated (1,2) and SAAT-treated (3,4) PSC, measured using Al point (1,3) and InGa eutectic (2,4) contacts. The box outlines show the 25th (bottom) and 75th (top) percentile energy resolution, the middle line shows the median resolution, and the central dot shows the mean resolution, and the crosshairs show the outliers. (c) Schematic of γ-ray absorption in PSCs through the photoelectric effect (left) and carrier relaxation to the PSC band edges (right). The schematic is not representative of the number of carriers produced per photon absorption event, which number in the thousands depending on the photon energy.17,18

applied bias (Figure 1c), where the PSC was irradiated and held under bias at each potential for 1 h or longer (including 12 h at 50 V bias), exhibits the spectral degradation due to potential ion motion and space-charge accumulation within the solid under bias and irradiation. Though small, this polarization effect17,18,27,28 motivates our desire to prevent surface diffusion and the subsequent formation of a concentration gradient in the bulk. Whether untreated or surface-treated, degradation in the spectral features can occur if a high electric field strength or a reactive contact is utilized. The crystals themselves can be stored in the dry air for at least 2 years without performance degradation, as revealed by uncompromised spectra taken from 2 years old untreated PSCs (e.g., Figure S10). If the electric field is uniformly low, then stable device performance can be realized (e.g., Figure S11a,b); however, if a bias beyond ∼100 V mm−1 is applied, then rapid degradation in the leakage current can occur (e.g., Figure S11c). It is, thus, imperative to eliminate high electric field gradients through an electrode and guard ring design that maximizes the uniformity of the field throughout the bulk, a design effort that is part of an on-going study. For the sake of evaluating the effect of passivation on the charge-transport properties in the material and to further improve the performance uniformity of the devices, SAAT post-processing and larger-area contacts (using liquid InGa eutectic) were employed. To quantify the effects of these strategies on the device performance, the responses from 36 PSCs were evaluated. The 36 crystals were derived from six separate batches of precursor solutions for use in seven separate crystallization runs. The details are enumerated in Table S1. The crystals, categorized by surface treatment (either

nonuniformities in the surface structure, by point-contacting the solid along one of the smooth surface regions. The resulting γ-ray spectrum following the impingement by 133Ba γrays is shown in comparison with spectra derived from the standard semiconductor detectors composed of silicon, cadmium telluride (CdTe), and CZT in Figure 1a,b. The fairest comparison is between the MAPbI3 and CZT because both are simply contacted without surface passivation, guardrings, or single-polarity sensing schemes such as using a Frisch Collar33 or co-planar grid,34 whereas the CdTe and Si have heavily engineered surfaces that minimize the surface leakage and utilize Schottky barriers to reduce the bulk leakage current. These crystals are all biased to full depletion, and the signal is optimized. A relatively large MAPbI3 feature near 276 keV (Figure 1a inset: focused on the region near the 356 keV photopeak) is due to the confluence of the 133Ba 276.4 keV γ-ray35 as well as X-ray escape peaks from lead and iodine.36,37 The prominence of the feature is due to the fact that high-energy X-rays can readily escape the mm scale depth of the PSC, shown in the interaction probability vs depth plot of the inset of Figure 1d. In contrast to CdTe and CZT, which require high bias (>700 V) to reveal clear spectral features, PSCs can show strong spectral response at low bias, reflecting the excellent material purity and higher mobility-lifetime (μτ) product of MAPbI3 compared to the cadmium chalcogenide materials.13−16 Modeling of a PSC’s spectral response using γ-ray interaction locations (using Monte Carlo N-Particle Code (MCNP 6.1)) and subsequent charge transport (from the Shockley−Ramo Theory)28 agreed with the measured PSC spectrum, especially when X-ray escape was accounted for (Figure 1d). However, the slight shifting peak position vs C


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Figure 3. Top-facet SEM images of an untreated (a) and a SAAT-treated (b) PSC, showing surface secondary growth (SSG) regions in the former that are lacking in the latter, which has a smooth surface. False-color cross-sectional SEM images of a cleaved untreated (c) and a SAAT-treated (d) PSC. A sizeable (∼200 μm2) cross-section of secondary growth, with a visible grain boundary and different grain orientation from the bulk, is shown in orange in (c), whereas the bulk PSC is uncolored. The surface zone in the SAAT-treated PSC, with a platelet-like structure, is shown in green in (d), whereas the bulk PSC is shown in brown. Scale bars are shown on the images.

Figure 4. (a) Single-crystal X-ray diffraction (SC-XRD) pattern comparing untreated (red) and SAAT-treated (blue) PSCs, showing characteristic bulk MAPbI3 peaks (200), (224), and (400) at 2θ ≈ 19.9, 40.4, and 40.6°,53 respectively. (b) Background-subtracted SC-XRD showing the presence of RP hybrid perovskite materials (n = 2), along with their characteristic peaks (020), (040), and (060) at 4.75, 9.44, and 12.75°, respectively, in SAAT-treated PSCs as well as their absence in untreated PSCs. (c) Reflection-mode FTIR measurements of an untreated and a SAAT-treated PSC. (d) The comparison between the photoluminescence of untreated (red) and SAAT-treated (blue) PSCs, using a measurement step size of 5 nm (dots) across the entire wavelength range (400−850 nm) as well as a finer step of 1 nm (lines) in their specific regions of interest.

untreated or SAAT-treated) and contact type (Al point contact or InGa eutectic contact), were compared according to the best energy resolution peak from each PSC, which is itself

dependent on measurement conditions and charge-sensitive/ pulse-shaping amplifier (CSA/PSA) settings. D


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ACS Applied Materials & Interfaces Table 1. Measured vs Expected Material Characterization Propertiesa measurement photoluminescence (PL) peak

material property bulk MAPbI3 band gap surface RP band gap

reflection-mode FTIR peaks

single-crystal XRD

octylamine −CH2 symm. octylamine −CH2 asymm. −CH3 symm. MAPbI3 (200) peaks (2θ) MAPbI3 (400) peaks (2θ) RP RP RP RP

(n (n (n (n

= = = =

2) 2) 2) 2)

(020) (040) (060) (020)

peak (2θ) peak (2θ) peak (2θ) lattice spacing

expected value

measured value

820 nm (Eg = 1.5 eV)1−6,10,11 570 nm (Eg = 2.2 eV)31 2858 cm−138 2927 cm−138 2958 cm−138 19.9731° 20.022°53 40.5876° 40.6902°53 4.488°c 8.983°c 13.491°c 19.67 Åd

760 nm (Eg = 1.6 eV)b 510 nm (Eg = 2.4 eV) 2850.3 cm−1 2921.6 cm−1 2949.6 cm−1 19.94° 20.04° 40.58° 40.69° 4.75° 9.44° 12.75° 18.65 Å

a

References used for the expected value data are provided in the table. Expected values for single-crystal XRD peaks were determined by simulating the crystal model (.cif file) in VESTA,40,41 using the literature data for MAPbI3 peaks53 bThe measured MAPbI3 PL peak deviates by over 60 nm (0.1 eV) from the expected value due to declining PMT sensitivity in the measurement setup at wavelengths longer than 700 nm. cDetermined by simulating the octylamine-substituted RP (n = 2) crystal model (.cif) file derived from the butylamine (n = 2).cif file.31 dFound from expected (020) peak and Bragg’s Law.

The measured 133Ba spectrum of the highest-performing PSC in each category is shown in Figure 2a. The percent yield was calculated as the number of PSCs that produced a visible peak or peak edge divided by the total number of measurable PSCs (with initial leakage current < 5 nA) in the category. The distribution of the peak resolution of PSCs is reflected in the error bars of the Figure 2b box plot, the box edges corresponding to the 15th and 75th percentiles, with the detector yield for each category being shown on the abscissa. The yield is significantly improved by using the SAATtreatment (Figure 2b), especially in combination with an InGa contact, resulting in a 100% yield in this category. Although the improvement in the highest-energy resolution for each category is minimal (14.80% for SAAT-treated, eutecticcontacted PSCs vs 16.91% for untreated, point-contacted PSCs), SAAT-treatment resulted in a much higher yield (100%), average resolution (24.9%), and repeatability (standard deviation of 6.58%) compared to untreated, pointcontacted PSCs (43.8% yield, 39.9% average resolution, and 22.6% standard deviation), as shown in Figure 2b. The optimization of the crystallization and post-treatment process led to an increased performance and consistency of PSC detectors, with optical and electronic characterization studies indicating improved carrier lifetimes and reduced surface trapping of SAAT-treated PSCs compared to untreated PSCs. Material Characterization. To gain insight on why SAAT post-processing substantially improved device performance uniformity, we performed bulk and surface material characterization of PSCs using optical (Figure S2) and scanning electron microscopy (SEM) (Figures 3 and S3), X-ray diffraction (XRD), and Fourier-transform infrared (FTIR) spectroscopy (Figure 4). The bandgaps of the bulk PSC and SAAT surface coating were determined through photoluminescence (PL) to be approximately 1.6 and 2.4 eV, respectively (Figure 4d). The surface regions of the PSCs, in both their morphology and electronic behavior, were markedly impacted by the SAAT post-growth treatment. From a morphological perspective, untreated, as-grown PSCs exhibited secondary growth during

drying after the bulk PSC was formed, as evinced by decreasing specular reflection (surface dulling) of the PSC facet within minutes of removal from the growth solution. Compared with the smooth surface for the SAAT-treated PSC (Figure 3b), the facet-view SEM micrographs of the untreated PSC (Figures 3a and S3d−f) indicate that a surface secondary growth (SSG) forms from additional nucleation and growth due to the presence of the PSC precursor solution on the surface as the high surface tension solvent evaporated. A cross-sectional SEM micrograph of a cleaved untreated PSC (Figure 3c) confirmed that the SSG consisted of a central microcrystal surrounded by post-secondary perovskite crystals located within a slight depression of the bulk PSC, as indicated by the dissimilar grain orientations and the visible boundary between the bulk and SSG regions. In contrast, untreated PSC regions without the SSG were quite smooth (Figure S4f). From the analysis of optical micrographs of untreated PSCs (Figure S3), approximately 10% of the surface area was covered by SSG regions, varying in size but having a characteristic length of ∼0.1 mm and an area of ∼0.01 mm2. The surface morphology of PSCs after SAAT post-treatment is drastically different. Most notably, the surface is free of the large-area SSG prevalent on the untreated PSCs. The surface of the treated PSCs also has a different color, suggesting an altered composition at the surface of the crystal. An optical micrograph (Figure S2d) showed that the surface morphology consisted of regions < 10 μm long that was confirmed by a SEM micrograph (Figure S3g,h) as having a plate-like structure at the surface region. A cross-sectional SEM micrograph (Figure 3d) of the surface showed that the surface material consisted of multiple layers of platelets, with a total thickness of 2−3 μm. The powder XRD (PXRD) pattern (Figure S4a) and singlecrystal X-ray diffraction pattern (Figure 4a) both confirm the monocrystalline nature of the bulk PSCs and show their high purity. The PXRD data contained all of the peaks corresponding to the MAPbI3 perovskite structure while lacking common impurity peaks.10,11 The monocrystallinity of the bulk PSC was discerned from the analysis of the singleE


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Figure 5. Mott−Gurney analysis of the I−V data (black dots, after moving-average smoothing with a width of 7) curves for the untreated (a) and SAAT-treated (b) PSC showed characteristics of the three regimes of charge transport, as well as threshold biases between them. (c) Transient photovoltage (TPV) signal of an untreated (red dots, left scale bar) and SAAT-treated (blue dots, right scale bar) PSC in response to a light pulse signal (gray line, normalized), used to calculate the carrier lifetime. (d) Normalized excess carrier concentration vs time of an untreated (red dots) and a SAAT-treated (blue dots) PSC, along with their corresponding exponential decay fits (red and blue lines) used to calculate carrier lifetimes, shown with the normalized light pulse signal (gray line). (f-inset): Setup for measuring the TPV signal of a PSC, shown with a 460 nm lightemitting diode (LED), probes, and an InGa-contacted PSC. The TPV measurements were taken using a 660 nm LED. This image is for illustration purposes only.

chemical formula of (C8H17NH3)2CH3NH3Pb2I7. Initial (unrelaxed) modeling of this material’s crystal structure (Figure S5), based on the expanded-lattice and RA-substituted model for (C4H9NH3)2CH3NH3Pb2I731 showed these same low-angle powder XRD peaks when simulated using visualization for electronic and structural analysis (VESTA).40,41 The simulated peak positions agreed with the single-crystal XRD measured peak positions (Figure 4b) of 2θ = 4.75, 9.44, and 12.75° for the (020), (040), and (060) lattice planes, respectively, to within one degree, confirming the crystal structure. The measured Bragg’s Law (020) lattice plane spacing of 18.65 Å was close to the modeled value, with a better agreement for this and the peak positions expected after the relaxation of the initial model. A comparison of expected and measured material characterization parameters is shown in Table 1. Electric and Optoelectronic Characterization. Electrical and optoelectronic testing of pre- and post-SAAT PSCs showed that the post-treatment caused an increased surface resistivity and reduced surface trapping, resulting in reduced leakage current and improved detector performance.16,42,43 Current−voltage (I−V) measurements of InGa-contacted PSCs (Figure S6) showed the increased bulk resistivity of SAAT-treated PSCs vs untreated PSCs, revealing the impact of the surface layer on the total resistance. The bulk resistivity (measured from the Ohmic region I−V curves (Figure S6)) increased from 463 ± 116 to 497 ± 199 MΩ cm with SAAT post-treatment. Using Mott−Gurney (M−G) analysis (Figure 5a,b), I−V measurements were used to estimate the carrier mobility and bulk trap density of the PSCs. To find the carrier mobility

crystal XRD (SC-XRD) plot, which shows only two tall and narrow (FWHM < 0.04°) peaks corresponding to the (110) and (220) lattice planes, confirming the ability of ITC methods10,11 to produce mesoscopic single crystals. However, the coloration, morphology, and crystal planes of the surface region indicated the formation of a different material at the surface. Reflection-mode Fourier-transform infrared (FTIR) measurements (Figure 4c) revealed the different surface chemistry between SAAT-treated and untreated PSCs, with the former showing peaks characteristic of the octylamine precursor at approximately 2840, 2920, and 3020 cm−1 that correspond to the −CH2 symmetric, −CH2 asymmetric, and −CH3 symmetric vibrational modes.38 The absence of these peaks in transmission-mode FTIR showed that the presence of the octylamine-derived material was located primarily at the surface rather than in the bulk of the PSC. The PL spectra (Figure 4d) reveal that for the untreated sample, the emission peak of the solid, at 760 nm, resides close to the band gap of the bulk material (1.55 eV)10,11,39 as expected. However, the surface layer caps the solid from this underlying emission resulting in a photoluminescent emission that peaks at 510 nm, an emission consistent with the formation of a hybrid two-dimensional Ruddlesden−Popper (RP) halide perovskite having a characteristic chemical formula of (RA)2(CH3NH3)n−1PbnI3n+1, with RA representing the longchain ammonium cation and n representing the number of interconnected PbI4 layers.31 Specifically, the presence of three low-angle single-crystal XRD peaks (Figure 4b) correspond to the (020), (040), and (060) lattice planes of an n = 2 RP halide perovskite, with a F


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similar techniques and PSC surface quality,51 the SRV for a SAAT-treated PSC was 307 cm s−1, less than half that of the untreated PSC’s SRV of 620 cm s−1, agreeing with SRV values for PSCs14 and showing the significant improvement of PSC surface transport properties after SAAT post-processing, accounting for the improved performance and reliability of SAAT-treated PSC-based radiation detectors compared to those using untreated PSCs.

(which is difficult to derive from Hall measurements due to the PSC’s small size and rough surface),10 the M−G power law44 is fitted to the I−V data in the space charge limited current (SCLC) regime μ=

J 8 1 × 2 × εrε0 9 Vbias

(1)

where εr and ε0 are the relative vacuum dielectric constants, respectively. Using εr = 26,3 the mobility was determined to be 39.4 cm2 V−1 s−1, comparable to the literature10,15 and values derived from time of flight (TOF) measurements (detailed in the SI, Section S4). M−G analysis can also be used to estimate the bulk trap density, which is detailed in the SI (Section S6). The reduction in the calculated bulk trap density from untreated to SAAT-treated PSCs (from ∼1.33 × 1010 cm−3 to ∼2.46 × 109 cm−3) indicates a reduction in the near-surface trap density in otherwise nearly identical PSCs. Although M−G analysis revealed the reduction in nearsurface trap states causing poor detector resolution,17,18,28 transient photovoltage (TPV) characterization was used to quantify the near-surface carrier lifetime, which was then used to compare the near-surface trap density in the PSC. TPV signals are produced from photogenerated carriers, which decay as these carriers recombine.45,46 Due to the high optical absorption of MAPbI3, especially at wavelengths below 800 nm,1−7 most photogenerated carriers are located within 1 μm of the perovskite surface. The relation between excess carrier concentration and TPV for an intrinsic material can be derived from semiconductor physics47,48 resulting in a single-term or double-term exponential decay for the untreated PSCs or SAAT-treated PSCs, respectively: ΔnUT(t ) = (Δns|t = 0 ) × e−t / τ

(2)

ΔnSAAT(t ) = (Δns|t = 0 ) × e−t / τ + A e−t / τ2

(3)

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CONCLUSIONS A surface alkylamine treatment of ITC-grown PSCs greatly improves their performance and repeatability in spectroscopic γ-ray detection. Solvent post-processing of PSCs in octylamine was chosen due to process compatibility considerations. First, octylamine is a liquid at the SAAT processing temperature, and it is miscible with γ-butyrolactone. Second, the PSC precursor solutes either dissolve completely or form rapidly diffusing nanocolloidal suspensions in octylamine, minimizing rough precipitate deposition on PSCs as well as preventing the formation of secondary microcrystals compared to other posttreatment solutions investigated, including alkylammonium iodide solutions dissolved in isopropanol and toluene. Finally, the octylamine is a precursor to the surface-coated zone, reacting with the PSC precursor solutes to form a Ruddlesden−Popper hybrid perovskite surface zone. We have demonstrated a facile, repeatable, and scalable process for the solution-based surface alkylamine treatment of solution-grown methylammonium lead halide perovskite single crystals as a means to significantly reduce their surface and overall bulk trap density. Our simulations of γ-ray detection by MAPbI3 agreed with our measured results, confirming the viability of this material for radiation detection. This surface treatment strategy resulted in one of the best energy resolutions for 81 keV 133Ba γ-ray detection by a lowtemperature solution-processed PSC. Furthermore, SAAT post-treatment greatly improved the consistency and repeatability of PSC detectors, a major step on the pathway toward low-cost, high-resolution PSC-based detection and imaging systems. Additionally, the demonstration of a facile, solutionbased fabrication of smooth, high-quality surface coatings of Ruddlesden−Popper hybrid perovskites, a class of 2D materials, could have numerous applications in their own regard, including as both stand-alone as well as bulk MAPbI3coated 2D electronic and optoelectronic devices.

where Δns is the near-surface MAPbI3 excess carrier concentrations due to photogeneration and τ is the carrier lifetime. The second term of the two-term exponential fit in eq 3 is generic, with a magnitude A and decay constant τ2. Upon fitting eqs 2 and 3 to the excess carrier concentration (Figure 5d) derived from the TPV signals (Figure 5c) for SAATtreated and untreated PSCs, the near-surface carrier lifetime of 488.3 μs for the SAAT-treated PSC was significantly higher than the 322.2 μs for the untreated PSC, using a 660 nm LED source (for which the RP layer is transparent).31 The second decay constant of τ2 = 3.639 ms for the double-term exponential fit for the SAAT-treated PSC was due to the resistor−capacitor (RC) time constant of the RP surface layer itself, which is further explained in the SI (Sections S7 and S8). The measured and bulk carrier lifetimes can be used to calculate the surface recombination velocity (SRV), assuming that it is low49 2s 1 1 = + r τeff τB d

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EXPERIMENTAL SECTION

Inverse-Temperature Crystallization of PSCs. MAPbI3 perovskite single crystals were formed using inverse-temperature crystallization derived from the literature.10,11 A 1.2 M precursor solution containing PbI2 (Fisher Scientific, 99.9985%), MAI (DyeSol, Inc., unknown purity) of slight (99%) was mixed overnight at 60−75 °C. This solution was poured into a flat-bottom octadecyltrichlorosilane-treated hydrophobic glass dish (Figure S1a), and the temperature was raised to a precrystallization point lying at 80 ± 5 °C. After 1−2 h, the solution temperature was then increased, typically by 5 ± 1 °C, to induce nucleation (Figure S1b). Once an appropriate number of observed seed PSCs had formed (surface coverage density of 1−2 cm−2, (Figure S1c)), the temperature was reduced by 3 ± 1 °C to stop further nucleation. The temperature was then continually adjusted to maintain PSC growth while preventing additional nucleation (Figure S1d,e), with GBL added periodically to maintain the solution level.

(4)

where τeff and τB are the measured and bulk carrier lifetime, respectively, sr is the surface recombination velocity (SRV), and d is the distance between contacts. Reported values for τB are heavily dependent on the measurement technique and PSC quality,50 ranging from the sub-micro-second range for timeresolved photoluminescence characterization10,11 to 2.6 s using TPV-derived techniques.51 Using the higher value due to G



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After reaching a length of 2−5 mm, PSCs were either removed from the dish for SAAT-treatment or kept in the dish. The remaining solution was drained, and the PSCs were left to dry overnight at 85 ± 5 °C. SAAT Post-Treatment of PSCs. After removal from the crystallization dish, PSCs were dried for ∼5 min on a hotplate set at 82.5 ± 2.5 °C. The PSCs were then immersed first in octylamine (Sigma-Aldrich, 99%) (Figure S1f) for 20 s then in diethyl ether (Sigma-Aldrich, 99%) for 60 s under agitation. Following immersion, the PSCs were further rinsed with diethyl ether (Figure S1g). The SAAT-treated PSCs were then left to dry on a hotplate at 82.5 ± 2.5 °C overnight. Radiation Spectroscopy Measurement. A PSC was contacted on the top and bottom facets either directly with Al probes or using InGa eutectic (Sigma-Aldrich, 99.99%) drops and placed in a vacuum chamber. A 31.55 MBq 133Ba source was placed 2 cm above the PSC, and the chamber was closed and evacuated. The probes connected to a charge-sensitive amplifier (CSA, ev550 from Kromek, Inc.), itself connected to a pulse-shaping amplifier (PSA, Ortec Model 572A) and DC power supply (ISEG SHQ 224 M). The PSA output connected to a multichannel analyzer (MCA, Ortec EasyMCA), with the measured spectrum as well as measurement conditions being recorded. Material Characterization. Both powder-mode and single-crystal XRD were performed using a Rigaku SmartLab 150 tool using a Cu K-α source (λ = 1.5406 Å) at a bias of 44 kV and current of 40 mA, 5 mm slit width, Bragg−Brentano goniometry, and a D/teX Ultra 250 Si detector. Photoluminescence of PSCs was measured using a 325 nm helium−cadmium (He−Cd) source laser and detection system comprising a monochromater (0.5 m SPEX 500 m), a photomultiplier tube (Hamamatsu R928), and a lock-in amplifier (Stanford Research Systems SR850). The Fourier-transform infrared (FTIR) response of PSCs was measured using an Agilent uFTIR Microscope (Cary 620) and Bench (Cary 670). Electric and Optoelectronic Characterization. Current− voltage (I−V) measurements were performed on PSCs by contacting the top and bottom facets with InGa eutectic drops, with probes connected to a Keithley 4200 Semiconductor Analyzer system contacting the drops, applying a sweeping bias and measuring the current. For transient photovoltage (TPV) measurement, a PSC was connected on opposite side facets with InGa drops, contacted to probes feeding into an oscilloscope (Agilent DSO7054A). A function generator (HP 3314A) provided a square wave signal to a 660 nm LED and the oscilloscope.

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.D.H.). *E-mail: [email protected] (L.J.G.). ORCID

Suneel G. Joglekar: 0000-0001-7405-3701 Author Contributions

S.G.J., M.D.H., and L.J.G. conceived of the project and wrote the manuscript. S.G.J. performed PSC growth, SAAT posttreatment, and material characterization. S.G.J. and M.D.H. performed radiation testing of the PSC devices. S.G.J. and L.J.G. performed the optoelectronic response analysis. Funding

Funding for this work was provided by the US Department of Homeland Security, Domestic Nuclear Detection Agency (2015-DN-077-097), and the Michigan Energy Institute Seed Fund. This support does not constitute an express or implied endorsement on the part of the government. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank D. K. Wehe and M. Wrobel for helpful discussion regarding nuclear radiation detection and M. Jeong for insight regarding device design and fabrication. We thank A. F. Aiello, A. Pandey, and P. K. Bhattacharya for their assistance and use of their PL measurement setup. We also thank Y. Qi of the J. D. Hanawat X-ray MicroAnalysis Laboratory, T. Chambers of the Van Vlack Laboratory, and P. Herrera-Fierro of the Lurie Nanofabrication Facility, all of the University of Michigan, Ann Arbor, for their help in materials measurement and characterization.

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ABBREVIATIONS BAMA, butylammonium methylammonium CSA, charge-sensitive amplifier CZT, cadmium zinc telluride (Cd1−xZnxTegenerally with x ≤ 0.1) eut, eutectic FTIR, Fourier-transform infrared spectroscopy FWHM, full width at half-maximum GBL, γ butyrolactone ITC, inverse-temperature crystallization I−V, current−voltage MA, methylammonium (CH3NH3+) MAI, methylammonium iodide (CH3NH3I) MAPbI3, methylammonium lead(II) iodide (CH3NH3PbI3) MCA, multichannel analyzer MCNP, Monte Carlo N-Particle Code (ver. 6.1) M−G, Mott−Gurney; μτ,mobility-lifetime product (in cm2 V−1) OAMA, octylammonium methylammonium PCE, power conversion efficiency PL, photoluminescence PMT, photomultiplier tube PSA, pulse-shaping amplifier PSC, perovskite single crystal (i.e., monocrystalline perovskite) PV, photovoltaic PXRD, powder-mode X-ray diffraction RA, r-group ammonium cation (R-NH3+)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b09381. Radiation spectroscopy modeling, I−V analysis, RP crystal structure modeling, TOF carrier mobility determination, explaining measured I−V oscillations, I−V analysis to estimate bulk trap density, explanation of photovoltage generation during TPV measurement, and double-exponential fitting of TPV data for SAAT-treated PSCs; supporting table contains detailed information about each measured PSC, including solution and crystallization batch, type of measured peak, and the peak resolution; supporting figures are: PSC formation and growth, optical micrographs, SEM micrographs, XRD plots, RP crystal structures, I−V measurements, band diagrams of PSCs under illumination, equivalent circuit models, and stability of PSCs under storage and under bias; and 17 references (PDF) H


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(13) Yakunin, S.; Sytnyk, M.; Kriegner, D.; Shrestha, S.; Richter, M.; Matt, G. J.; Azimi, H.; Brabec, C. J.; Stangl, J.; Kovalenko, M. V.; Heiss, W. Detection of X-ray Photons by Solution-Processed Lead Halide Perovskites. Nat. Photonics 2015, 9, 444−449. (14) Wei, H.; Fang, Y.; Mulligan, P.; Chuirazzi, W.; Fang, H.-H.; Wang, C.; Ecker, B. R.; Gao, Y.; Loi, M. A.; Cao, L.; Huang, J. Sensitive X-ray Detectors Made of Methylammonium Lead Tribromide Perovskite Single Crystals. Nat. Photonics 2016, 10, 333−339. (15) Yakunin, S.; Dirin, D. N.; Shynkarenko, Y.; Morad, V.; Cherniukh, I.; Nazarenko, O.; Kreil, D.; Nauser, T.; Kovalenko, M. V. Detection of Gamma Photons Using Solution-grown Single Crystals of Hybrid Lead Halide Perovskites. Nat. Photonics 2016, 10, 585−589. (16) Wei, H.; DeSantis, D.; Wei, W.; Deng, Y.; Guo, D.; Savenije, T. J.; Cao, L.; Huang, J. Dopant Compensation in Alloyed CH3NH3PbBr3−xClx Perovskite Single Crystals for Gamma-ray Spectroscopy. Nat. Mater. 2017, 16, 826−833. (17) Lutz, G. Semiconductor Radiation Detectors: Device Physics, 1st ed.; Springer: NY, 2010. (18) Knoll, G. F. Radiation Detection and Measurement, 4th ed.; Wiley: Hoboken, NJ, 2010; p 830. (19) Uratani, H.; Yamashita, K. Charge Carrier Trapping at Surface Defects of Perovskite Solar Cell Absorbers: A First-Principles Study. J. Phys. Chem. Lett. 2017, 8, 742−746. (20) Han, C.; Wang, K.; Zhu, X.; Yu, H.; Sun, X.; Yang, Q.; Hu, B. Unraveling Surface and Bulk Trap States in Lead Halide Perovskite Solar Cells using Impedance Spectroscopy. J. Phys. D: Appl. Phys. 2018, 51, No. 095501. (21) Chen, Y. F.; Tsai, Y. T.; Bassani, D. M.; Clerc, R.; Forgács, D.; Bolink, H. J.; Wussler, M.; Jaegermann, W.; Wantz, G.; Hirsch, L. Evidence of Band Bending Induced by Hole Trapping at MAPbI3 Perovskite/Metal Interface. J. Mater. Chem. A 2016, 4, 17529−17536. (22) Si, H.; Liao, Q.; Zhang, Z.; Li, Y.; Yang, X.; Zhang, G.; Kang, Z.; Zhang, Y. An Innovative Design of Perovskite Solar Cells with Al2O3 Inserting at ZnO/Perovskite Interface for Improving the Performance and Stability. Nano Energy 2016, 22, 223−231. (23) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P. A.; Mohammed, O. F.; Sargent, E. H.; Bakr, O. M. Low Trap-state Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519−522. (24) Yip, H.-L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994−6011. (25) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, Tb; Duan, H. S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (26) Pioro-Ladrière, M.; Davies, J. H.; Long, A. R.; Sachrajda, A. S.; Gaudreau, L.; Zawadzki, P.; Lapointe, J.; Gupta, J.; Wasilewski, Z.; Studenikin, S. Origin of Switching Noise in GaAs/AlxGa1−xAs Lateral Gated Devices. Phys.Rev. B 2005, 72, No. 115331. (27) Kasap, S. O. X-ray Sensitivity of Photoconductors: Application to Stabilized a-Se. J. Phys. D: Appl. Phys. 2000, 33, 2853−2865. (28) He, Z. Review of the Shockley−Ramo Theorem and its Application in Semiconductor Gamma-ray Detectors. Nucl. Instrum. Methods Phys. Res., Sect. A 2001, 463, 250−267. (29) Aharon, S.; Etgar, L. Two Dimensional Organometal Halide Perovskite Nanorods with Tunable Optical Properties. Nano Lett. 2016, 16, 3230−3235. (30) Aharon, S.; Wierzbowska, M.; Etgar, L. The Effect of the Alkylammonium Ligand’s Length on Organic−Inorganic Perovskite Nanoparticles. ACS Energy Lett. 2018, 3, 1387−1393. (31) Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G. Ruddlesden−Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chem. Mater. 2016, 28, 2852−2867. (32) Beznosikov, B. V.; Aleksandrov, K. S. Perovskite-Like Crystals of the Ruddlesden−Popper Series. Crystallogr. Rep. 2000, 45, 792− 798.

RC, resistor−capacitor (associated with a characteristic time constant) RP, Ruddlesden−Popper RTN, random telegraph noise SAAT, solution-based alkylamine treatment SCLC, space charge limited current SC-XRD, single-crystal X-ray diffraction (X-ray diffraction of a crystal’s facet) SEM, scanning electron microscopy SPV, surface photovoltage SRIM, the stopping and range of ions in matter52 SRV, surface recombination velocity SSG, surface secondary growth TFL, trap-filled limited TOF, time of flight TPV, transient photovoltage TRPL, time-resolved photoluminescence VESTA, visualization for electronic and structural analysis (software package)38,39 XRD, X-ray diffraction XRE, X-ray escape

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REFERENCES

(1) Gao, P.; Gratzel, M.; Nazeeruddin, M. K. Organohalide Lead Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 2448−2463. (2) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (3) 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. (4) Hodes, G. Perovskite-Based Solar Cells. Science 2013, 342, 317− 318. (5) Snaith, H. J. Perovskites: The Emergence of a New Era for LowCost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623− 3630. (6) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-performance Perovskite-sensitized Solar Cells. Nature 2013, 499, 316−319. (7) Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Grätzel, M.; Hagfeldt, A. The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 710−727. (8) Wu, H.; Si, H.; Zhang, Z.; Kang, Z.; Wu, P.; Zhou, L.; Zhang, S.; Zhang, Z.; Liao, Q.; Zhang, Y. All-Inorganic Perovskite Quantum Dot-Monolayer MoS2 Mixed-Dimensional van der Waals Heterostructure for Ultrasensitive Photodetector. Adv. Sci. 2018, 5, No. 1801219, 1-9 . (9) Azarhoosh, P.; McKechnie, S.; Frost, J. M.; Walsh, A.; van Schilfgaarde, M. Research Update: Relativistic Origin of Slow Electron-Hole Recombination in Hybrid Halide Perovskite Solar Cells. APL Mater. 2016, 4, No. 091501, 1-8 . (10) Saidaminov, M. I.; Abdelhady, A. L.; Murali, B.; Alarousu, E.; Burlakov, V. M.; Peng, W.; Dursun, I.; Wang, L.; He, Y.; Maculan, G.; Goriely, A.; Wu, T.; Mohammed, O. F.; Bakr, O. M. High-quality Bulk Hybrid Perovskite Single Crystals Within Minutes by Inverse Temperature Crystallization. Nat. Commun. 2015, 6, No. 7586. (11) Liu, Y.; Yang, Z.; Cui, D.; Ren, X.; Sun, J.; Liu, X.; Zhang, J.; Wei, Q.; Fan, H.; Yu, F.; Zhang, X.; Zhao, C.; Liu, S. F. Two-InchSized Perovskite CH3NH3PbX3(X = Cl, Br, I) Crystals: Growth and Characterization. Adv. Mater. 2015, 27, 5176−5183. (12) Wu, X.; Trinh, M. T.; Niesner, D.; Zhu, H.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X. Y. Trap States in Lead Iodide Perovskites. J. Am. Chem. Soc. 2015, 137, 2089−2096. I


Research Article


Absorption at Ambient Conditions. Acta Crystallogr., Sect. B: Struct. Sci., Cryst. Eng. Mater. 2016, 72, 716−722. (54) Ziegler, J. F.; Ziegler, M. D.; Biersack, J. P. SRIMthe Stopping and Range of Ions in Matter (2010). Nucl. Instrum. Methods Phys. Res., Sect. B 2010, 268, 2853−2865.

(33) Jeong, M.; Jo, W. J.; Kim, H. S.; Ha, J. H. Characteristics of a Frisch Collar Grid CdZnTe Radiation Detector Grown by LowPressure Bridgman Method. J. Korean Phys. Soc. 2015, 66, 1669− 1674. (34) Luke, P. N. Unipolar Charge Sensing with Coplanar ElectrodesApplication to Semiconductor Detectors. IEEE Trans. Nucl. Sci. 1995, 42, 207−213. (35) Be, M. M.; Dulieu, C.; Chiste, V. NUCLEIDE-LARA, a Library for Alpha, X and Gamma Emissions Sorted by Increasing Energy; CEA/ LNE-LNHB: Saclay, France, 2008. (36) Thompson, A.; Vaughan, D.; Gullikson, E.; Howells, M.; Kim, K. J.; Kirz, J.; Kortright, J.; Lindau, I.; Liu, Y.; Pianetta, P.; Robinson, A.; Scofield, J.; Underwood, J.; Williams, G.; Winick, H. X-ray Data Booklet; Center for X-Ray Optics and Advanced Light Source: Berkeley, CA, 2009. (37) Berger, M. J.; Hubbell, J. H.; Seltzer, S. M.; Chang, J.; Coursey, J. S.; Sukumar, R.; Zucker, D. S.; Olsen, K. XCOM: Photon Cross Sections Database; NIST, PML, Radiation Physics Division, 2010. (38) Jung, M.; Shin, T. J.; Seo, J.; Kim, G.; Seok, S. I. Structural Features and their Functions in Surfactant-Armoured Methylammonium Lead Iodide Perovskites for Highly Efficient and Stable Solar Cells. Energy Environ. Sci. 2018, 11, 2188−2197. (39) Dittrich, T.; Awino, C.; Prajongtat, P.; Rech, B.; Lux-Steiner, M. C. Temperature Dependence of the Band Gap of CH3NH3PbI3 Stabilized with PMMA: A Modulated Surface Photovoltage Study. J. Phys. Chem. C 2015, 119, 23968−23972. (40) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional Visualization of Crystal, Volumetric and Morphology Data. J. Appl. Crystallogr. 2011, 44, 1272−1276. (41) Momma, K.; Izumi, F. VESTA: a Three-Dimensional Visualization System for Electronic and Structural Analysis. J. Appl. Crystallogr. 2008, 41, 653−658. (42) Jeong, M.; Jo, W. J.; Kim, H. S.; Ha, J. H. Radiation Hardness Characteristics of Si-PIN Radiation Detectors. Nucl. Instrum. Methods Phys. Res., Sect. A 2015, 784, 119−123. (43) Hammig, M. D.; Jeong, M. In Fabrication Methods by which the Intrinsic Noise is Reduced in Si and CZT Nuclear Radiation Detectors, 2011 IEEE Nuclear Science Symposium Conference Record; IEEE, 2011; pp 356−362. (44) Takeshita, S. Modeling of Space-Charge-Limited Current Injection Incorporating an Advanced Model of the Poole-Frenkel Effect; Clemson University, 2008. (45) MacKenzie, R. C. I.; Shuttle, C. G.; Chabinyc, M. L.; Nelson, J. Extracting Microscopic Device Parameters from Transient Photocurrent Measurements of P3HT:PCBM Solar Cells. Adv. Energy Mater. 2012, 2, 662−669. (46) Lee, J. Y. Ultra-thin Intrinsic Amorphous Silicon (a-Si) Hybrid Structure with Inorganic/Organic Materials and Its Applications; University of Michigan. Library. Deep Blue, 2014, http://hdl.handle. net/2027.42/110489. (47) Sze, S. M.; Ng, K. K. Physics of Semiconductor Devices, 3rd ed.; Wiley-Interscience: Hoboken, NJ, 2007; p 815. (48) Van Zeghbroeck, B. V. Principles of Semiconductor Devices and Heterojunctions, 1st ed.; Prentice Hall: Upper Saddle River, NJ, 2010; p 450. (49) Schroder, D. K. Semiconductor Material and Device Characterization, 3rd ed.; IEEE Press; Wiley: Hoboken, NJ, 2006; p 779. (50) Bi, Y.; Hutter, E. M.; Fang, Y.; Dong, Q.; Huang, J.; Savenije, T. J. Charge Carrier Lifetimes Exceeding 15 μs in Methylammonium Lead Iodide Single Crystals. J. Phys. Chem. Lett. 2016, 7, 923−928. (51) 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. (52) Williams, S. T.; Zuo, F.; Chueh, C.-C.; Liao, C.-Y.; Liang, P.W.; Jen, A. K. Y. Role of Chloride in the Morphological Evolution of Organo-Lead Halide Perovskite Thin Films. ACS Nano 2014, 8, 10640−10654. (53) Arakcheeva, A.; Chernyshov, D.; Spina, M.; Forró, L.; Horváth, E. CH3NH3PbI3: Precise Structural Consequences of Water J


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