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Alkali Metal Halide Salts as Interface Additives to Fabricate Hysteresis-Free Hybrid Perovskite-Based Photovoltaic Devices Lili Wang, Dhanashree Moghe, Soroush Hafezian, Pei Chen, Margaret Young, Mark Elinski, Ludvik Martinu, Stéphane Kéna-Cohen, and Richard Royal Lunt ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07368 • Publication Date (Web): 17 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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ACS Applied Materials & Interfaces

Alkali Metal Halide Salts as Interface Additives to Fabricate Hysteresis-Free Hybrid Perovskite-Based Photovoltaic Devices

Lili Wang†, Dhanashree Moghe†, Soroush Hafezian‡, Pei Chen†, Margaret Young†, Mark Elinski†, Ludvik Martinu‡, Stéphane Kéna-Cohen‡, Richard R. Lunt†§* †Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, MI, 48824 USA ‡Department of Engineering Physics, Polytechnique Montreal, Montréal (Québec), H3T 1J4, CA §Department of Physics and Astronomy, Michigan State University East Lansing, MI, 48824 USA KEYWORDS: alkali metal halide salt, iodide vacancies, hysteresis, perovskite photovoltaics, hole extraction, passivation, doping

ACS Paragon Plus Environment

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ABSTRACT: A new method has been developed for doping and fabricating hysteresis-free hybrid perovskite-based photovoltaic devices by using alkali metal halide salts as interface layer additives. Such salt layers introduced at the perovskite interface can provide excessive halide ions to fill vacancies formed during the deposition and annealing process. A range of solutionprocessed halide salts were investigated. The highest performance of methylammonium lead mixed halide perovskite device was achieved with a NaI interlayer and showed a PCE of 12.6% and a hysteresis of less than 2%. This represents a 90% improvement compared to control devices without this salt layer. Through depth-resolved mass spectrometry, optical modeling and photoluminescence spectroscopy, this enhancement is attributed to the probable reduction of iodide vacancies, passivation of grain boundaries, and improved hole extraction. Our approach ultimately provides an alternative and facile route to high performance and hysteresis-free perovskite solar cells.


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Introduction Hybrid organic-inorganic halide perovskite materials have emerged as a highly promising candidate for low-cost solar photovoltaic applications.1–7 Due to their narrow and tunable bandgap,8 high absorbance,9 low exciton binding energy,10,11 and high carrier mobility,9 the overall power conversion efficiency has been improved from a modest 3.8% in liquid-electrolyte configuration12 to over 22% in an all solid-state architecture.13 However, there are still several open issues to be addressed before it can displace other PV technologies available in the market.14 One of the most challenging issues is to understand and suppress the hysteresis phenomenon in current-voltage (I-V) characteristics,15–19 which makes it difficult to accurately evaluate the device performance and track power points. Often, the device performance is dependent on scan rate, scan direction, light soaking and external bias conditions. 20–22 These effects have been explained by a combination of factors including: charge accumulation, which is caused by trap states,

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ion migration,25,26 or unbalanced electron and hole extraction or

collection at the interfaces27 and the potential for a ferroelectric contribution.15,22,28,29 Previous work has distinguished capacitive and non-capacitive hysteresis by comparing devices with different architectures, where hysteresis caused by capacitive current dominates in typical structures with TiO2 as bottom electron selective layer. However in inverted structures fabricated with PEDOT:PSS and PCBM as carrier selective layer, the non-capactive hysteresis dominates and only occurs at slow scan rates, which is attributed to chemical interaction with the contacting layer caused by ionic migration.30 Considering these mechanism behind the hysteresis phenomenon, to solve this problem, different approaches have been investigated. The most effective methods are aimed at either improving the charge transport27,31,32 or decreasing and passivating crystal defects19,23 in the perovskite layer. Crystal defects such as self-interstitial


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atom and vacancies have been considered as one of the main reasons leading to trap states and providing ion migration pathways that provide unreliable I-V and further induce instability. One of the most probable defects in such hybrid perovskite films are iodide vacancies which have the lowest formation energy and relatively lower migration energy.33 As both methylammonium iodide and methylammonium chloride sublime at relatively low temperature, the amount of halide vacancies left behind in the perovskite film likely cannot be neglected. Therefore, a key strategy for suppressing hysteresis is to improve the crystallinity and decrease the halide vacancy defects of perovskite films. In previous work monovalent halide additives have been employed mixed with the precursor PbI2 in a two-step process to improve the perovskite film quality34. In this work, we introduce an alkali metal salt interlayer additive below the perovskite film to provide a new route to both fill vacancies generated during the perovskite growth and annealing processes, and provide alkali metal ions that can passivate the grain boundaries. This addition of an alkali metal salt layer is shown to diffuse into the active perovskite layer, suppress hysteresis, and reliably improve performance. By separating the dopant from the precursor solution we demonstrate a more flexible approach for solvent selection and concentration optimization that does not impact crystallization. This method can also provide a general strategy for interface diffusion doping and passivation in many halide perovskite devices.

Results and Discussion The perovskite device schematic and SEM image of a fabricated device are shown in Figure 1. The approach for preparing samples with alkali metal salt interlayers is shown in Figure 1c. Structural characteristics measured with x-ray diffraction (XRD) and scanning electron

microscopy (SEM) are shown in Figure 2 for samples prepared with and without the salt interlayer (NaI), respectively. Both show only a strong preferred (110) crystal orientation with no


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detectable presence of unreacted phases. Comparing Figure 2b to Figure 2c, metal halides salts are shown to help decrease the pinholes in perovskite films but otherwise do not impact morphology or crystallinity. The current density-voltage (J-V) characteristics of devices with, and without NaI, under forward and reverse scans are shown in Figure 3 along with external quantum efficiencies and detailed statistics of device performance. Table 1 provides a summary of J-V parameters and hysteresis data. By introducing a NaI layer we find there is a near doubling in the PCE from 6.8 % to 12.6 % that is accompanied by a substantial enhancement in quantum efficiency. Additionally, the hysteresis for samples is calculated by comparing the difference of PCE obtained under forward and reverse scan, respectively. This difference highlights that the hysteresis of samples prepared with NaI is reduced from 30.8% to 1.6%. This variation in hysteresis is also highlighted by the overlap of the forward current with NaI and the divergence of the forward current with scan direction without NaI. Adding NaI also significantly enhances the EQE at all wavelengths with greater enhancement at longer wavelength. Figure S1 shows the cross-section SEM image of the sample prepared without NaI, indicating that the incorporation of NaI does not significantly impact the layer thickness. Additionally, the absorption spectra of the perovskite films in both single-layers and the entire device stack are shown in Figure S2. These spectra indicate that the absorption profile with and without NaI doping is not significantly altered. To understand the mechanism by which NaI improves the device performance, TOFSIMS depth profiling was performed to determine the elemental and molecular distribution across the layers during various stages of processing. Table 2 shows selected ion profiles through the device stack obtained using positive ion detection. From these selected ion profiles


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we can track unreacted PbI2 with PbI+, the perovskite with CH3NH3+, and sodium iodide with Na2I+. We note that although PbI+ is also generated from neat perovskite layers, it is principally assigned to unreacted PbI2 based on comparisons with PbI2 controls and the dramatic reduction in PbI+ of the perovskite after annealing (with total unreacted concentrations well below the XRD detection limit). Comparing Figure 4a to Figure 4b, we find that the PbI+ peak moves towards the PCBM side after annealing. Our results further show that during the annealing process essentially all of the chloride evaporates and partially escapes from the perovskite surface35 (see Figure S3 for the detection of chloride). Furthermore, the Na2I+ peak shows that there is a broad distribution of NaI in the unannealed device, which implies that the NaI diffuses deep within the perovskite layer after its deposition, but then migrates to the PEDOT/perovskite interface during annealing. Correspondingly, the CH3NH3+ peak assigned to the perovskite has been modestly broadened by adding NaI as shown in Figure 4b and Figure 4c. The strong diffusion of NaI within the perovskite layer suggests the possible reduction in iodide vacancies within the perovskite during the annealing process. To further confirm the reduction of iodide vacancies by the introduction of the NaI interlayer, XRD patterns were collected during applied bias to investigate the crystal structure change of samples prepared with, or without, NaI as shown in Figure 7c for the (110) d-spacing (full XRD patterns under bias are provided in Figure S4). This data shows that the (110) peak shifts to smaller d-spacings in both samples when applied bias changes from 0V to +1.5V. However, for the sample prepared with NaI interlayer, the peak shift is minimal when further increasing bias from +1.5V to +4.5V, whereas the sample without NaI continues to decrease. We note that these changes in d-spacing and intensity are reversible after removing the bias. Considering Vegard’s law it is expected that an increase in the vacancy concentration would lead to smaller lattice constants particularly under electrostrictive


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bias. The lattice constant of the sample prepared with the NaI interlayer is always larger than that prepared without NaI and shows a bias dependence over a much larger range, clearly indicating that NaI interlayer helps to reduce iodide vacancies. Additionally, previous work has explored the impact of sodium incorporation into Cu(In, Ga)Se2 (CIGS) solar cells where it is standard to diffuse Na into CIGS absorber layers to achieve the highest efficiencies.36 In that system, the mechanism of enhancement by incorporating Na was assigned to an improvement of p-type conductivity as a result of increase in the effective hole carrier density37, surface/interface modification36,38, crystal orientation of CIGS39, and grain boundary passivation40,41 Therefore, the NaI may be playing a similarly multifaceted role in filling iodide vacancies and passivating grain boundaries in the perovskite layers. Indeed, improved hole extraction may also serve to reduce hysteresis. To explore this possibility, steady-state photoluminescence (PL) spectra were measured on perovskite samples coated onto ITO/PEDOT:PSS. With the addition of NaI (Figure S5), the PL peak shows a blue shift of 2 nm and an overall reduction in PL intensity. The blue-shift is attributed to defect passivation. To interpret the PL change, we also measured a perovskite sample with or without a thin PCBM layer at the interface where we also see a marked decrease in PL intensity with the PCBM consistent with other work23. Combined, these data indicate that the introduction of NaI does enhance hole extraction, which reduces the exciton formation and emission probabilities.42 To systematically study the effect of NaI on device performance, we tune the concentration of NaI, perovskite precursor solution, and the type of alkali metal salts. Figure 5 shows the J-V characteristics of devices fabricated with different NaI concentrations (which translates to different NaI thicknesses) using 0.7 M of perovskite precursor solution.


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Interestingly, with increasing concentration of perovskite precursor solution from 0.4 M to 0.7 M, the optimized thickness of NaI decreases (see Figure S6 for performance of devices fabricated with different perovskite precursor concentration). This is due to the lower solubility of NaI in perovskite precursor solutions with higher concentration which can result in an excess of NaI left at the interface. Figure 6 shows the changes of J-V parameters with the concentration of NaI for different concentration of the precursor solution. Without NaI, the efficiency is about half of that with optimized NaI concentration and both the values of JSC and FF decrease substantially during reverse scan, while VOC always increases. By adding NaI with an optimized concentration, JSC, VOC and FF essentially do not change between forward scan and reverse scan, which results in very little hysteresis in the PCE. To further understand the mechanism involved in the EQE improvement, we employ an optical transfer matrix model43–45 to study the change of position-dependent light absorption and excited state generation. Figure S7 illustrates the simulated incident light wavelength and layer position-dependent photon absorption rates under AM1.5G solar photon reflux for a complete device as shown in Figure 1. We also show the layer position-dependent exciton generation rate at 400 nm, 550 nm and 725 nm, which respectively represent shorter, medium and longer wavelengths. These data show that the generation rate is greatest near the PEDOT/perovskite interface at 400 nm and 550nm, showing an exponential decay with position (i.e. according to Beer-Lambert equation) when the absorption coefficient is large. At longer wavelength, when the absorption coefficient is smaller, the generation rate shows more complex absorption profiles where the profile still has a peak generation rate near the PEDOT/perovskite interface but also shows a substantial absorption peak near the middle of the perovskite layer. While the addition of NaI improves the EQE for all wavelengths, considering the similar absorption profile of both


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samples prepared with or without NaI (as shown in Figure S2), the greater relative enhancement of the long wavelength EQE then indicates that there is a greater relative density of defects that are eliminated by residual NaI from the interface close to glass side to the middle of the device that results in greater enhancement of long-wavelength photocurrent. This is consistent with the depth-profiled TOF-SIMS measurements, which show that even though the NaI distribution is concentrated towards the interface after annealing, there is still some distribution through most of the device. This is also consistent with wavelength dependent excitation PL data (Figure S5), where we find a similar position dependent enhancement to the hole extraction efficiency with greater relative enhancement from the center of the perovskite layer. The device modeling highlights the key importance of the PEDOT/perovskite interface, where the vast majority of excited states are generated and dissociated. For comparison to spin-coated NaI underlayer, we tested devices with the direct codeposition of NaI with perovskite from precursor solutions (see Figure S8). Importantly, the devices fabricated with codeposition do not show enhanced preformance – rather we see the emergence of an S-shape J-V data that indicates large series resistance even for the smallest concentrations (see Figure S9). Combined with the TOF-SIMS results, these data suggest that the resulting impact on the crystallization of the perovskite limit the effect of the metal halide addition. To confirm the impact of NaI co-deposition on the crystallization of perovskite film, samples prepared with NaI codeposition were characterized with XRD. As shown in Figure 7, increasing the NaI concentration in the precursor solution results in weaker intensity of (110) peak and greater full width at half maximum (FWHM). Considering the instrument limited peak breadth, the grain size in the normal direction of samples prepared without NaI is close to upper limit resolution of this measurement (200nm) and the film thickness (e.g. 350nm). In contrast,


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the grain size in the normal direction of samples prepared with NaI codeposition is found to be roughly 1/4 of the size of the film thickness. This implies that when codeposited (rather than diffused from the interface), the NaI causes poor grain formation that leads carriers to have to traverse multiple grain boundaries to reach the electrodes, in contrast to previous work with the codeposition of dopants into the perovskite precursors.34 This is then consistent with the poor and resistive device performance. This is also in agreement with previous studies on Na-doped CIGS solar cell that show that an excess of Na yields disorganized grain structure and smaller grain size.46,47 The morphology of the sample prepared with co-deposition of NaI at the lowest concentration has also been investigated (as shown in Figure S10). While the morphology of films fabricated with the direct addition of NaI into perovskite precursor solution remains similar there are a greater number of pin-holes and aggregates that are formed, which could play a role in the poorer performance, in addition to the smaller crystalline grain size. Considering the non-uniform enhancement in the quantum efficiency, it is likely that the overall enhancement is due to a combination of several mechanisms: iodide vacancy filling, grain boundary passivation, and enhanced hole collection. Indeed, previous research has demonstrated that iodide migration via vacancies leads to hysteresis.18 Such ion migration may also result in trap states which can trap charge carriers and cause charge accumulation. 22 Hybrid halide perovskites have also shown unbalanced hole and electron mobility and extraction, thus impacting charge distribution inside the device.27 Therefore, during J-V data collection, either ion accumulation caused by iodide migration, charge carrier trapped at these trap states or unbalanced charge carrier extraction or collection can be driven by the eternal bias. Such ion migration or carrier trapping/detrapping results in a modification of the internal electric field and further affect the charge carrier collection at the interface, which negatively impacts the J-V


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characteristic parameters including VOC, JSC and FF. We note that there has been previous work in which hysteresis-free organo-lead halide perovskite devices have been reported in inverted architectures.30,48 For example, Durrant et al. reported that when using dimethyl sulfoxide (DMSO) and gamma-Butyrolactone (GBL) as a mixed solvent with PbI2 and MAI as precursors to prepare perovskite films, the lack of hysteresis behavior at room temperature still became observable at low temperature.48 This is likely due to the application of PCBM as electron selective layer which passivates possible defects at grain boundaries and interfaces as studied previously.23 In our case, TOF-SIMS shows that the distribution of PCBM is concentrated close to the top of perovskite film with limited distribution into the film (as shown in Figure S11). Therefore, the defects close to the PEDOT:PSS side are not passivated effectively in the absence of the NaI interlayer. Thus, this doping approach provides another route to hysteresis-free performance. We have further tested a range of metal halide salts including NaCl, NaBr and CsI as shown in Figure S7. Among these salts, samples prepared with NaBr show the similarly high performance to NaI despite the lingering presence of some hysteresis. The reduction in performance of the remaining salts could be due to a combination of changes in solubility in DMF (used to deposit the perovskite), initial salt morphology, trap passivation capacity, doping/passivating contribution of the M+ ion, or formation of new trap states. In terms of processing, the solubility of each salt in DMF is actually an important consideration because it can lead to an excess of neat salt layers left behind that result in higher series resistance in devices. This is likely at least part of the reason NaCl film shows poorer device performance (see Figure S12), but could also be related to Cl- ions being easily lost from films during annealing shown in the TOF-SIMS data. Thus, there are a number of factors that can ultimately play a role


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in the optimization of metal halide salt additives for improving perovskite devices and the choice of perovskite solvent could ultimately impact this optimization.

Conclusion We have developed a method to suppress the hysteresis in the J-V characteristics of leadhalide perovskite devices. In this method, alkali metal salts were introduced as interlayers that distribute during perovskite formation. Utilizing this approach device performance was shown to be improved with PCEs of up to 12.6% and the simultaneous elimination of hysteresis. This was achieved by a combination of decreasing halide ion vacancy concentration, passivating grain boundaries, and enhanced hole extraction. Furthermore, such a method also provides a general route of doping perovskite-based devices, in which the dopants can be separated from precursor reactants and incorporated without disrupting the crystallization. By decoupling the dopant from the perovskite precursors, these metal halide compounds are shown to be viable dopants for significantly enhancing performance. This approach could lead to routes to controllable and multi-interface doping profiles, and provide a new strategy for the doping of a wide range of perovskite devices.


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FIGURES

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Figure 1. (a) Schematic illustration of the device architecture. (b) SEM image of the asfabricated device, where (c) the perovskite layer is spin coated onto a NaI layer which then diffuses into the perovskite layer.


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Figure 6. J-V parameters including (a) efficiency, (b) Jsc, (c) Voc and (d) FF for representative devices prepared with 0.7 M, 0.55 M and 0.4 M of precursor solution. The concentration of NaI solution used for coating NaI layer are 50 mg/mL, 100 mg/mL and 200 mg/mL, respectively. The closed symbols represent forward scan and the open symbols represent reverse scan.


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Bias (V) Figure 7. (a) XRD patterns of samples prepared with NaI codeposition by directly adding different concentrations of NaI directly into precursor solutions. (b) volume average grain size in the normal direction of perovskite films prepared with co-deposition at different NaI concentrations. The dotted line indicates the instrument resolution limit. The enlarged view of peak (110) is shown in the inset. (c) d-spacing change with applied bias in samples prepared with (blue) or without NaI (orange), the linear trendlines are guides to the eye. Photographs show the appearance of samples prepared (d) without or (e) with NaI after applying bias.


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TABLES Table 1. Parameters of J-V characteristics and hysteresis of champion devices prepared with or without NaI, respectively. JSC Scan direction (mA/cm2) with NaI without NaI

VOC (V)

FF

η

Hysteresis

(%)

(%)

Forward -22.4±2.2 0.92±0.01 0.61±0.01 12.6±1.3

-1.6%

Reverse -21.5±2.1 0.93±0.01 0.62±0.01 12.4±1.2 Forward -16.6±1.7 0.89±0.01 0.46±0.01

6.8±0.7

Reverse -15.5±1.5 0.90±0.01 0.37±0.01

5.2±0.5

-30.8%

*The hysteresis is calculated by dividing the PCE obtained under reverse scan with the PCE difference between forward scan and reverse scan.

Table 2. The selected ion with their mass and the compound they are assigned to. Ion

Center of mass (amu)

Attribution

CH3NH3+

32.0486

Perovskite

PbI+

334.889

PbI2

Na2I+

172.870

NaI


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ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website or from the author including SEM images, UV-Vis spectra, TOF-SIMS, XRD, J-V curves, PL spectra, transfer matrix optical modeling and photographs of samples. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT Work at Michigan State University was supported by the Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry (material synthesis and device fabrication), by a Michigan State University Strategic Partnership Grant (SPG) (electrical characterization), and the U.S. Department of Energy (DOE) Office of Science, Basic Energy Sciences (BES), under Award # DE-SC0010472 (structural characterization). The work at Polytechnique Montreal (composition characterization) was supported in part by the Natural Sciences and Engineering Research Council of Canada (Discovery and IRCPJ 433808-11) and the FQRNT. The authors would like to thank J. Lefebvre for assistance with TOF-SIMS measurements. EXPERIMENTAL SECTION Materials and synthesis CH3NH3Cl was synthesized by mixing CH3NH2 (40 wt%, in deionized H2O, Sigma) and HCl (36 wt%, in H2O, Sigma) in a molar ratio of 1.2:1. The mixture was stirred at 0oC for 2 hrs.49 Then the water was removed via rotary evaporation and washed by diethyl ester until white powder was obtained. The white powder was dried in a vacuum oven at 60 oC overnight and then kept in glove box for further use. CH3NH3I (Lumtec), bathocuproine (BCP, Lumtec), poly (3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS, Clevios PVP AI 4083, Heraeus Precious Metals), [6,6]-phenyl-C61-butyric acid methyl ester (PCBM, American Dye Source),


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and Al-doped ZnO (AZO) nanodispersion (Nanograde) were used as received. All the halide salts (NaCl, NaBr, NaI and CsI) were purchased from Sigma-Aldrich and used as received. Device fabrication PEDOT:PSS was spin-coated onto solvent-cleaned pre-patterned ITO substrates (Xin Yan Technology) at 6000 rpm for 30 seconds and then annealed at 140 oC for 20 mins. Subsequently, alkali metal salts (NaI, NaCl, NaBr, CsI) were spin-coated onto PEDOT:PSS film at 6000 rpm for 10 seconds and annealed at 120 oC for 10 mins. The CH3NH3PbI3-xClx precursor solutions were prepared by a procedure described previously, where PbI2, MAI and MACl were mixed with ratio of 1:1:1.5 in N,N-Dimethylformamide (DMF). The precursor solution was spin-coated onto PEDOT:PSS at 6000 rpm for 5 seconds in a nitrogen glove box. Perovskite films were annealed at 90 oC for 2 hrs unless noted otherwise. PCBM layers were coated onto the perovskite films with a solution of 20 mg/mL in chlorobenzene at 1000 rpm for 30 seconds and then annealed at 90 oC for 10 mins. A conductive AZO layer then was coated at 6000 rpm for 15 seconds using AZO nanodispersion and then annealed at 90 oC for 10 mins. Both BCP and silver electrodes were vacuum deposited at base pressures of 3×10-6 torr and the top electrode was patterned via shadowmask. The device area is 5.44 mm2. Measurement and characterization Current density (J) was measured as a function of voltage (V) under dark conditions and AM1.5G solar simulation (xenon arc lamp) in air, where the intensity was measured using a NREL-calibrated Si reference cell with KG5 filter. Devices were illuminated through metal mask and corrected for spectral mismatch (M) with values around 0.98 < M < 1.01. Roughly 80 devices were tested for each of the key architectures with all device data included in the Fig 3. Integrated photocurrents from quantum efficiencies were within 5-10% of the measured JSC, the


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uncertainty on the measured intensity. External quantum efficiency (EQE) measurements were made using chopped monochromatic light with a calibrated Newport Si detector for reference. Thin film crystallinity was characterized by using a Bruker D2 Phaser XRD instrument with a Cu K source at 30 kV and 10 mA and a Ni filter in the Bragg-Brentano configuration. The insitu XRD was taken by applying bias on the samples inside the chamber while collecting XRD spectra. The circuit was connected by using copper tape (3M Co.) and silver paste (Ted Pella, Inc.). The bias is applied using D-cells. SEM was carried out via a Carl Zeiss Auriga Dual Column FIB SEM at 20 kV accelerating voltage. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) profiles were obtained with an ION-TOF SIMS IV using a 25 kV Bi+ primary ion source used in bunched mode with an average ion current of 1.37 pA. The primary ion source was used to probe an area of 50x50 μm2 for secondary ions mass spectra. The depth profiling was performed using a Cs+ (O2-) source for positive (negative) ion depth profiles taken over an area of 500x500 μm2. Photoluminescence spectra were measured using a PTI Quanta Master 40 spectroflurometer under nitrogen atmosphere and various excitation wavelengths. A 700 nm dielectric long pass filter was used during the PL measurement to prevent wavelength doubling. UV-VIS transmission and reflection spectra were taken using Perkin Elmer UV-VIS Spectrometer. Transfer Matrix Optical Modeling Optical constants of perovskite layers used in optical modeling were determined with variable angle ellipsometry (see Figure S13). Transfer matrix modeling was performed at normal incidence using a custom Matlab code outlined elsewhere.45 For reference, the generation


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2

rate (G) is related to electric field as: G  12 c 0 n j j E j , where c is the speed of light, 0 is the permittivity of free space, nj is the index of refraction in layer j, j is the absorption coefficient in layer j. Grain Size Estimation The volume average grain size in the normal direction is estimated based on the Scherrer equation, D  K   cos , where D is the grain size, λ is the x-ray wavelength, β is peak breadth after subtracting the instrumental peak breadth, θ is the Bragg diffraction angle, and K is a dimensionless shape factor that is set to 1 by the definition of the peak breath below. Accounting for the Gaussian shape of the diffraction peaks and the instrumental resolution, the peak breadth is then  2    4ln(2)    2Meas  2Ins  , where Meas is the full-width-at-half-maximum (FWHM) for the measured sample and Ins is the instrumental FWHM.

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TABLE OF CONTENTS

Alkali metal halide salts used as interface additives to fabricate hysteresis-free hybrid perovskite-based photovoltaic devices have been investigated. These halide salts are shown to fill the halide vacancies, passivate perovskite grain boundaries and promote hole extraction during formation when diffused from the bottom interface. This work provides a new strategy for suppressing hysteresis of perovskite-based device performance and provides new additive doping strategies.

KEYWORDS: alkali metal halide salt, iodide vacancies, hysteresis, perovskite photovoltaics, hole extraction, passivation, doping


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