Ion Interactions Induced p–i–n Junction in Methylammonium

Nov 15, 2017 - Interestingly, for Au/SC sample, after the Au is sputtered away, several gold complexes, such as [Au(NH2CH3)]+, [AuN]+ and [Au(C2NH7)]+...
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Metal/Ion Interactions Induced p-i-n Junction in Methylammonium Lead Triiodide Perovskite Single Crystals Ting Wu, Rupam Mukherjee, Olga S. Ovchinnikova, Liam Collins, Mahshid Ahmadi, Wei Lu, Nam-Goo. Kang, Jimmy W. Mays, Stephen Jesse, David Mandrus, and Bin Hu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10416 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Metal/Ion Interactions Induced p-i-n Junction in Methylammonium Lead Triiodide Perovskite Single Crystals Ting Wu,† Rupam Mukherjee,† Olga S. Ovchinnikova,‡,§ Liam Collins,‡,§ Mahshid Ahmadi,† Wei Lu,ǁ Nam-Goo Kang,ǁ Jimmy W. Mays,ǁ Stephen Jesse,‡,§ David Mandrus,† Bin Hu†,* †

Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, 37996, USA Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA § Institute for Functional Imaging of Materials, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

‡

ǁ

Department of Chemistry, University of Tennessee, Knoxville, Tennessee, 37996, USA

Supporting Information Placeholder ABSTRACT: Hybrid perovskites, as emerging multifunctional semiconductors, have demonstrated dual electronic/ionic conduction properties. Here we report a metal/ion interaction induced pi-n junction across slightly n-type doped MAPbI3 single crystals with Au/MAPbI3/Ag configuration based on interface dependent Seebeck effect, Hall effect and time-of-flight secondary ion mass spectrometry analysis. The organic cations (MA+) are shown to interact with Au atoms, forming positively charged coordination complexes at Au/MAPbI3 interface, while iodine anions (I-) can react with Ag contacts, leading to interfacial ionic polarization. Such metal/ion interactions establish a p-doped region near the Au/MAPbI3 interface due to the formation of MA+ vacancies, and an n-doped region near the Ag/MAPbI3 interface due to the formation of I- vacancies, consequently forming a p-i-n junction across the crystal in Au/MAPbI3/Ag configuration. Therefore, the metal/ion interaction plays an important role in determining the surface electronic structure and semiconducting properties of hybrid perovskites. Organic-inorganic hybrid perovskites have demonstrated remarkable multifunctionality in photovoltaics, light-emitting diodes, thermoelectrics and spintronics based on the solutionprocessable polycrystalline thin film.1 Within the recent two years, hybrid perovskite single crystals (SCs) have emerged with more promising intrinsic properties, such as extremely low trap density, long carrier lifetime and diffusion length.2 The single crystals of MAPbI3, MAPbBr3, and FAPbI3, have attracted extensive interests for the applications in photodetection,3 and highenergy radiation (x-ray, γ-ray) detection4 due to the large chargecarrier mobility-lifetime (µτ) product benefiting from the small effective mass of electrons and low trap density, the low thermal noise arising from the proper optical bandgap, and the high absorption cross-section owing to the presence of high-Z elements. Despite the remarkable achievements in device development, knowledge of the fundamental properties is still lacking towards further advancing the device performance and improving the long-term stability. It has been revealed that hybrid perovskites possess dual electronic and ionic conduction properties.5 The latter is owing to the low migration activation energies of the ions, such as halide vacancies, interstitials, and organic cations (MA+).5b-f Based on the ionic conduction, early studies have demonstrated switchable photovoltaic effect in hybrid perovskites

by applying electrical poling.6 However, ionic conduction also causes big concerns to the long-term stability of device performance, particularly for devices operating under large external bias. It has been shown that the metal contacts, such as Al and Ag, can be corroded by halides (I- or Br-) from hybrid perovskites through grain boundary dominant ion migration, even under inert or vacuum conditions, leading to self-degradation of device performance.7 This leads most researchers to rely on the expensive Au as electrodes for device design, especially for single crystal devices. However, a recent study demonstrated that Ag/I- reaction can form stable AgIx, which can suppress the spontaneous diffusion of I- ions, leading to improved low-resistance state retention behavior.5f Therefore, understanding the ions or ionic defects in hybrid perovskites are critical for the rational design of optoelectronic devices based on hybrid perovskites. In this work, we report an interaction between Au and organic cations (MA+) at the surface of the MAPbI3 single crystals based on study of the interface-dependent Seebeck effects and surface sensitive time-of-flight secondary ion mass spectrometry (TOFSIMS). Although Hall effect study demonstrated a slightly n-type doped character of the studied crystals, it is surprising to see that the crystals with Au and Ag contacts, denoted as Au/SC/Ag, are shown negative Seebeck effects when the Au/SC surface is used as high-temperature terminal but positive Seebeck effect when the Ag/SC surface is used as high-temperature terminal. As a reference, the crystals with symmetric Ag contacts, denoted as Ag1/SC/Ag2, only give rise to negative Seebeck effect. TOFSIMS was used to investigate the surface structure at metal/SC interface to gain an in-depth understanding of the interfacedependent Seebeck effect in the Au/SC/Ag samples. Most importantly, our works suggest that the metal/ion interactions, particularly the Au/MA+ and Ag/I- interactions, can modify surface electronic structure of MAPbI3 single crystals, leading to a p-i-n junction across the crystal in Au/SC/Ag configuration. The single crystals of MAPbI3, as shown in Figure 1a, were grown by the solution-based method7 with a modified cooling program (details are shown in Supporting Information (SI)). The trap density of the as-grown single crystals is determined to be on the order of ~1010 cm-3 by using space-charge limit current method (Figure S1 in SI). Specifically, the electron-trap density is slightly higher than hole-trap density. Temperature dependent Hall Effect study in Figure 1b indicates that our single crystals

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possess extremely low carrier density on the order of 1010~11 cm-3, therefore can be considered as an intrinsic semiconductor. In the published works, both n-type8 and p-type2a,b MAPbI3 single crystals have been reported, suggesting that the type of majority charge carriers in MAPbI3 single crystals strongly depends on the preparation conditions. In our studies, the temperature window remains below 360 K to ensure good data reproducibility. According to thermogravimetric analysis, MAPbI3 single crystals exhibit excellent thermal stability in air with negligible weight loss (less than 0.1%) within the temperature window (296 K~360 K) (Figure S2a in SI). Meanwhile, the crystal heated at~360 K in ambient condition with controlled humidity of 35~40% for 24 hours remains in black color without visible yellowish color (PbI2 phase), suggesting negligible decomposition (Figure S2b~e in SI).

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negligible for the Ag1/SC/Ag2 sample. The unexpected amplitude of ∆V∆  296 K can be detected from the Au/SC/Ag samples made of as-grown crystals (45±4 mV), polished crystals (39±3 mV), as well as the chemically etched crystals with isopropanol (30±2 mV), therefore being considered as an intrinsic effect in the Au/SC/Ag samples rather than surface contamination. The microscopic images for the sample surfaces are shown in Figure S3 (SI). Meanwhile, we observed that the amplitude of ∆V∆  T shows to gradually decrease as the temperature increases in the Au/SC/Ag samples, but remains almost zero in the Ag1/SC/Ag2 samples (Figure 2c). This suggests the presence of an interfacial polarization at the Au/SC interface.

Figure 1. (a) Image of a MAPbI3 single crystal and schematic diagram of Seebeck effect measurement. (b) Temperature dependent Hall coefficient (RH) and carrier density (n). More interestingly, we observed an anomalous interfacedependent positive and negative Seebeck effects from the slightly n-type doped single crystal with asymmetric metal contacts (Au/SC/Ag). Seebeck effect is a well-known thermoelectric effect that generates electrical potential difference (∆V) or thermovoltage (V ) with the presence of a temperature difference (∆T). Essentially, Seebeck effect originates from the diffusion of majority charge carriers (electrons or holes) driven by the entropy difference caused by ∆T. In general, positive/negative Seebeck effect can be developed by the hole/electron dominant diffusion from the high-temperature (HT) terminal to the low-temperature (LT) terminal. In this work, the samples were fabricated by depositing Au and Ag on the two parallel (100) surfaces of the MAPbI3 single crystals with the thickness around 1.5 mm, producing sample Au/SC/Ag. Meanwhile, samples with Ag-only contacts, denoted as Ag1/SC/Ag2, were studied as reference. For Seebeck effect measurements, a temperature difference ranging from -4 K to +4 K was established between the two contacts; simultaneously, the electrical potential difference was monitored and recorded when the signal becomes stabilized (Figure 1a, details are shown in SI). Figure 2a and b present ∆V as a function of ∆T for both Au/SC/Ag and Ag1/SC/Ag2 samples at T≈313 K (averaged between two contacts). We can see that the absolute value of ∆V for the two samples exhibits a linear dependence on ∆T. The linear fit to the data can determine Seebeck coefficient ( α ). For the Ag1/SC/Ag2 sample, α313 K is determined to be -0.23 mV/K and 0.47 mV/K for the Ag1/SC/Ag2 sample when Ag1/SC and Ag2/SC surfaces are used as HT terminal, respectively. Surprisingly, the Au/SC/Ag sample gives a negative α 313 K (-6.87 mV/K) and a positive α313 K (+4.61 mV/K) when Au/SC and Ag/SC surfaces are used as HT terminal, respectively. The linear relationship between ∆V and ∆T suggests that the observed interface dependent Seebeck effect for the Au/SC/Ag samples is true Seebeck effect. The underlying mechanism may be associated with the local variation of electronic structure. It should be noted that, when there is no temperature difference (Figure 2c), the Au/SC/Ag sample produces a positive potential difference at room temperature, ∆V∆  296 K=41 mV, where ∆V∆  T = V  V . As a reference, ∆V∆  296 K is

Figure 2. Anomalous Seebeck effect. Potential difference as a function of temperature difference for (a) Au/SC/Ag sample and (b) Ag1/SC/Ag2 sample at 313 K. (c) Temperature dependent potential difference when ∆T  0 K. (d) Temperature dependent Seebeck coefficients for the Au/SC/Ag sample when using Au/SC and Ag/SC surfaces as HT terminal, respectively. The reported values from reference8a,9 are also plotted for comparison. (e) Temperature dependent Seebeck coefficients for the Ag1/SC/Ag2 sample when using Ag1/SC and Ag2/SC surfaces as HT terminal, respectively. (f) Temperature dependent carrier density. Figure 2d presents the temperature dependent Seebeck coefficients for a Au/SC/Ag sample. We can see that using Au/SC surface as HT terminal generates negative Seebeck effect (α300K= 1.23 mV/K, and α355K= -15.03 mV/K); conversely, using Ag/SC surface as HT terminal gives positive Seebeck effect ( α 300K= +0.92 mV/K, and α355K= +11.89 mV/K). The giant Seebeck coefficients are on the same order with the reported values (~mV/K),8a,9 which are reasonable due to the extremely low carrier density. As a reference, the Ag1/SC/Ag2 sample only shows negative Seebeck effect ( α 300K= -0.58 mV/K, and α 355K= -1.4 mV/K, in Figure 2e). Essentially, the amplitude of α is determined by T, n, and the density of states (DOS) near the Fermi level (EF), as shown in the Mott expression:10 α~

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near EF. Figure 2f presents temperature dependent carrier density for the two samples. The carrier density of the Au/SC/Ag sample is reduced by almost one order of magnitude as compared to that of the Ag1/SC/Ag2 sample, which is consistent with the larger |α| of the Au/SC/Ag sample. This suggests that the metal contacts can influence the semiconducting properties of MAPbI3. To understand the origins of the anomalous Seebeck effect and interfacial polarization observed for Au/SC/Ag samples, we further investigated the Au/SC and Ag/SC interface by TOF-SIMS with both 2D imaging and 3D profiling of the ionic, molecular and elemental species present at the metal/crystal interface. The measurement geometry are schematically shown in Figure 3a. Specifically, a crystal partially covered by Au layer was sputtered by 1 keV Cs+ ion beam with the focus on the area right across the boundary between Au-covered and uncovered crystal surfaces (indicated in Figure 3b). The sputtering process allowed for a ~5 nm per slice removal of the metal layer.

Figure 3. (a) Schematic diagram of TOF-SIMS analysis on the metal/MAPbI3 interface. (b~e) TOF-SIMS mapping of the emitted ions or ion clusters from the crystal partially covered by Au layer. 3D profiling of the emitted ions or ion clusters from Au/crystal sample (f) and Ag/crystal sample (g,h). MAPbI3 is known to be composed of three types of ions, organic cations (MA+ or CH3NH3+), lead cations (Pb2+) and iodine anions (I-). By monitoring the MA+ or CH3NH3+, Pb+, [Cs2I]+, [CsAu]+ or [CsAg]+, we were able to detect the ions that would be generated from the metal layer, the crystal, as well as their interfacial region. Interestingly, for the Au/SC sample, after the Au is sputtered away, several gold complexes, such as [Au(NH2CH3)]+, [AuN]+, and [Au(C2NH7)]+ are emitted only from the area originally covered by the Au layer (Figure 3c~e, more information is shown in Figure S4, SI). Note that the notation of the emitted ions or ion clusters just represent the most likely elementary composition rather than the exactly chemical formula. From the constructed 3D profiling through metal/crystal interface in Figure 3f, it is clear to see a layer of [Au(NH2CH3)]+ (red area) right underneath the Au surface. It should be noted that these gold complexes are not observed at the beginning of sputtering process, suggesting that these gold complexes are not formed during sputtering process. Therefore, the TOF-SIMS analysis provides a clear evidence of the interactions between Au atoms and MA+ ions in MAPbI3. Such Au/MA+ interactions can be essentially attributed to the coordination complexes, or metal complexes, which consist of a center metal and organic ligands based on Lewis base theory.11 Although Au is known to be one of the least reactive chemical elements, both theoretical and experimental studies have demon-

strated the interactions between Au nanoparticles with DNA through amines, forming gold complexes.12 It should be noted that the coordination complexes can be neutral or charged. The positive sign of ∆V∆  in Au/SC/Ag samples at room temperature suggests that Au/SC interface forms positively charged coordination complexes, namely complex cations. The temperature dependence of ∆V in Au/SC/Ag sample without ∆T (Figure 2c) suggests that increasing temperature can weaken the interactions between Au atom and MA+. Meanwhile, by cooling to room temperature for 1 hr, ∆V can recover to the same amplitude with a variation within 2 mV. This suggests that the interaction between Au and MA+ is a reversible process. For the Ag/SC interface, the TOF-SIMS analysis did not detect coordination metal/organic complexes. Instead, from the 3D profiling, we can see a slightly overlapped area with [CsAg]+ and MA+ (Figure 3g) and a largely overlapped area with [CsAg]+ and [Cs2I]+ (Figure 3h). The former can be attributed to the penetration of Ag atoms into the crystal, while the latter suggests the migration of I- towards the Ag contact, which can form neutral compound AgIx. Now, we discuss how metal/ion interaction influences the semiconducting properties of MAPbI3 and interface dependent Seebeck effect by correlating the fundamental understanding of ions and ionic defects in MAPbI3 with our experimental studies. First principle calculation has demonstrated that MAPbI3 can be unintentionally doped by the intrinsic point defects.13 More specifically, the positively charged ions/defects (VI, MAi and Pbi) can create shallow energy levels near the conduction band minimum, acting as donors or n-type dopants. On the contrary, the negatively charged ions/defects (Ii, VMA and VPb) can form energy levels near the valence band maximum, functioning as acceptors or ptype dopants. Most importantly, both theoretical calculation and experimental studies have demonstrated low activation energies for the migration of I- (0.1~0.6 eV) and MA+ (0.46-0.84 eV) in MAPbI3.5b-e Owing to the low activation energies of ion migration, iodine ions or interstitials can readily diffuse towards the Ag contact, forming iodine vacancies (VI) near Ag/SC interface (Figure 4a). Since VI acts as acceptors, the VI-rich region becomes ntype doped near Ag/SC interface (Figure 4b). The enhanced ntype doping near the Ag/SC interface is consistent with the higher carrier density and lower |α| in Ag1/SC/Ag2 sample as compared to Au/SC/Ag samples (Figure 2). For the Ag1/SC/Ag2 sample, when the Ag1/SC and Ag2/SC sides are alternatively used as HT terminal, the difference in the amplitude of α reflects that the doping concentration is different probably due to the sequential deposition procedure. Meanwhile, the interaction between Au atoms and MA+ cations is expected to diffuse MA+ cations towards Au contacts, forming MA+ vacancies (VMA)-rich region near Au/SC interface (Figure 4a). This can lead to a p-type doped zone near the Au/SC interface. While the crystal bulk remains intrinsic with slightly n-type characteristics (Figure 4b). As a result, a p-i-n junction can be established across the crystal in the Au/SC/Ag samples due to the metal/ion interactions at the Au/SC and Ag/SC interfaces. The hypothesis of p-i-n junction in the Au/SC/Ag sample can be supported by the dark current-voltage characteristics which show a diode behavior with rectifying property (Figure 4c). The p-i-n junction established in the Au/SC/Ag sample well explains the interface-dependent Seebeck effect by taking account of the built-in field pointing from Ag/SC interface towards Au/SC interface. As schematically shown in Figure 4d, when the Au/SC side is used as HT terminal, the electron diffusion should be dominant since hole diffusion would be suppressed by the built-in field, leading to the negative Seebeck effect. On contrast, when the Ag/SC side is used as HT terminal, the hole diffusion towards Au/SC interface would become dominant since the built-in field

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would assist hole diffusion but hinders electron diffusion, consequently generating a positive Seebeck effect (Figure 4e). From the perspective of optoelectronic device design, the symmetric geometry (e.g. Ag/SC/Ag) is expected to experience serious charge recombination due to the absence of built-in field. The charge recombination can be suppressed in the Au/SC/Ag geometry, which is shown to generate photovoltaic action under white light illumination with open-circuit voltage (Voc) of 0.57 V, as well as photodetection function under the excitation close to the band edge (785 nm) (Figure S5, SI). This indicates its potential application for near-infrared light detection.

Figure 4. Schematic diagram of (a) metal/ion interactions at metal/MAPbI3 interfaces, (b) metal/ion interactions induced p-i-n junction across a MAPbI3 single crystal; (c) Dark current densityelectrical field (J-E) characteristics of Au/SC/Ag sample; (d) Diagram of negative Seebeck effect due to electron-dominant diffusion, (e) Diagram of positive Seebeck effect due to hole-dominant diffusion assisted by built-in field caused by the p-i-n junction. In summary, our studies demonstrate that metal/ion interaction can selectively drive ions migration towards specific metal contacts without using electrical bias or photoexcitation, leading to selective doping at crystal surface. In a simple Au/SC/Ag geometry, the interaction between Au and MA+ has been demonstrated to form VMA-rich region, leading to a p-doped region near Au/SC interface, while the migration of I- towards Ag leads to VI-rich region, which forms n-doped region near Ag/SC interface. As a results, a p-i-n junction is established across the MAPbI3 crystal. Our studies provide a new insight on the ions and ionic defects in hybrid perovskites and demonstrate the importance of controlling the metal/ion interactions for the interface engineering in the rational design of perovskite optoelectronic devices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section is included in the supporting information.

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT This research is supported by Air Force Office of Scientific Research (AFOSR) under grant number FA 9550–15–1–0064 and National Science Foundation (CBET-1438181). This research was partially conducted at the Center for Nanophase Materials Sciences based on projects (CNMS2016-279 and CNMS2016-R45),

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which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. B. H. acknowledges the project support from National Science Foundation of China (Gant No.21161160445; Grant No. 61077020).

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Metal/Ion Interactions Induced p-i-n Junction in Methylammonium Lead Triiodide Perovskite Single Crystals Ting Wu, Rupam Mukherjee, Olga S. Ovchinnikova, Liam Collins, Mahshid Ahmadi, Wei Lu, Nam-Goo Kang, Jimmy W. Mays, Stephen Jesse, David Mandrus, and Bin Hu

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b Ag1/SC/Ag2 JournalAu/SC/Ag of the American Chemical Page Society 8 of 10 Ag2/SC as

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HT terminal = -0.47 mV/K Ag1/SC as

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1 Au/SC as HT terminal Ag/SC as 40 terminal HT terminal = -0.23 mV/K 2 HT = -6.87 mV/K = +4.61 mV/K -3 3 30 -4 -2 0 2 4 -4 -2 0 2 4 T (K) 4 T (K) d 20 c5 45 Au/SC/Ag T=0 K 6 40 10 Ag/SC as Ref 9 7 35 HT terminal 0 Au/SC/Ag 8 10 Ref 8a Au/SC as -10 HT terminal 9 0 -10 Ag1/SC/Ag2 10 -20 300 315 330 345 360 345 360 11 300 315 T330 T (K) (K) 12 1013 f e 0.0 Ag1/SC/Ag2 13 Ag1/SC/Ag2 Ag1/SC as 14 -0.5 HT terminal 1012 15 -1.0 Ag2/SC as 16 Environment 1011 HTACS terminalParagon Plus Au/SC/Ag -1.5 17 18 300 315 330 345 360 315 330 345 360 T (K) T (K) 19

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Journal of the American Chemical Society

Figure 3. (a) Schematic diagram of TOF-SIMS analysis on the metal/MAPbI3 interface. (b~e) TOF-SIMS mapping of the emitted ions or ion clusters from the crystal partially covered by Au layer. 3D profiling of the emitted ions or ion clusters from Au/crystal sample (f) and Ag/crystal sample (g,h). 83x59mm (300 x 300 DPI)

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Figure 4. Schematic diagram of (a) metal/ion interactions at metal/MAPbI3 interfaces, (b) metal/ion interactions induced p-i-n junction across a MAPbI3 single crystal; (c) Dark current density-electrical field (JE) characteristics of Au/SC/Ag sample; (d) Diagram of negative Seebeck effect due to electron-dominant diffusion, (e) Diagram of positive Seebeck effect due to hole-dominant diffusion assisted by built-in field caused by the p-i-n junction. 338x190mm (300 x 300 DPI)

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