Influence of Compact, Inorganic Surface Ligands on the

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The Influence of Compact, Inorganic Surface Ligands on the Electrophoretic Deposition of Semiconductor Nanocrystals at Low Voltage Andrew D. Dillon, Shawn Mengel, and Aaron T. Fafarman Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00787 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on August 2, 2018

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The Influence of Compact, Inorganic Surface Ligands on the Electrophoretic Deposition of Semiconductor Nanocrystals at Low Voltage Andrew D. Dillon, Shawn Mengel, and Aaron T. Fafarman* Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA 19104 USA Keywords: Cu2ZnSnS4; copper chalcogenide; solution processing; Rayleigh-Taylor instability; photovoltaics; ligand exchange; quantum dots; gravity

Abstract: For electrophoretic deposition (EPD) to achieve its potential as a method for assembling functional semiconductors, it will be necessary to understand both what governs the threshold voltage for deposition and how to reduce that threshold. Herein we demonstrate that post-synthetic modification of the surface chemistry of all-inorganic copper-zinc-tin-sulfide (CZTS) nanocrystals (NCs) enables EPD at voltages as low as 4V—a three-fold or greater reduction over previous examples of non-oxide semiconductors. The chemical exchange of the original surfactant-based NC-surface ligands with selenide ions yields essentially bare, highly surface-charged NCs. Thus, both the electrophoretic mobility and electrochemical reactivity of


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these particles are increased, favoring deposition. In situ imaging of the reactor during deposition provides a quantitative measure of the electric field in the bulk of the reactor, yielding fundamental insights into the reaction mechanism and mass transport in the low-voltage regime. A crossover from mass transport-limited to reaction rate-limited EPD is observed. Under the latter conditions, the influence of gravity can result in boundary-layer instabilities that are severely deleterious to the uniformity of the deposited film, despite the gravitational stability of the colloids in the absence of electric fields. This knowledge is applied to deposit thick, uniform, and crack-free films without sintering from stable, well-dispersed colloidal starting materials.

Introduction The widespread adoption of photovoltaics as a renewable energy source is currently constrained by the costs associated with manufacturing the devices themselves. While current technologies require high temperature and/or vacuum during fabrication, significant study has gone into developing colloidal semiconducting-nanocrystal-based precursors for photovoltaics1– 9

. Such colloidal ‘inks’ can be condensed to form thin films via potentially low-cost, high-

throughput solution processing methods10,11. However, to date, spincasting or dipcasting are the dominant modes for preparing lab-scale photovoltaics from nanocrystal solutions, both of which suffer from limited scalability1–8,12. Several researchers have explored electrophoretic deposition (EPD) as a possible scalable alternative13–25. Electrophoresis is the movement of a charged particle suspended in solution due to an externally applied electrical field. Once reaching one of the electrodes, a separate deposition process must occur, which may occur through different mechanisms depending on the conditions. The classic theory from Hamaker posits that the pressure applied by arriving particles will overcome the repulsive forces between charged particles, allowing van der Waals forces to


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dominate. In contrast, for strongly electrostatically stabilized colloidal particles, such as those utilized for the present study, the magnitude of the electric field necessary to overcome the interparticle repulsion is orders-of-magnitude higher than the observed field at which deposition occurs26. Instead, it is posited that deposition occurs in these cases due to electrochemical reactions at the depositing electrode, such as solvent electrolysis, which neutralize the surface charge on the arriving particles 26–31. EPD has proven technological merit, having been used commercially for decades for ceramics, metals and polymers29,32,33, but has only been applied to semiconductor nanocrystals in academic studies. On the lab scale, EPD has been used to fabricate functional solids from wide band gap oxides of zinc, tin, and titanium. These depositions are typically performed at 101 – 103 volts, but occasionally at voltages below 10 V20–23,34. In contrast, for the chalcogenide semiconductors [CdTe, CuIn(Ga)Se2, etc], the most common absorber layers in commercially relevant thin film solar cells, the applied voltages in previous studies exclusively range from 10’s to 100’s of volts15–18,35,36. In addition to generating unwanted heat and convective flows in the EPD bath, high voltage poses the risk of driving poorly controlled electrochemical reactions that can alter the properties of the deposit. For example, high deposition voltage degrades the photoluminescence quantum efficiency of CdTe nanocrystals37. Conversely, electrochemical transformations of the accreting material during EPD can potentially be harnessed for enhancing the properties of the deposit38. In either case, it is advantageous to lower the voltage of the process to gain maximum control over any adventitious electrochemical reactions that do occur. Following the example set by a few prior studies19,39,40, we apply EPD to nanocrystalline copper-zinc-tin-sulfide (Cu2ZnSnS4 or CZTS). CZTS possesses an appropriate bandgap for use in single-junction solar cells (1.4-1.5 eV) and it is comprised of non-toxic, earth-abundant


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materials19. Previous examples of EPD of non-oxide semiconductors involve surfactant-coated nanocrystals, resulting in a deposit of inorganic cores embedded in an organic matrix of insulating surfactant molecules, necessitating some post-deposition processing to achieve an allinorganic thin film. By contrast, in the present study, no organic surfactant capping groups were present on the NC surface; instead, the original insulating NC organic ligands were replaced with selenide ions via solution-exchange with sodium selenide41 prior to EPD. The resulting colloids are extremely stable due to their high surface charge, which also results in a highly increased electrophoretic mobility, in turn reducing the voltage and time required for electrophoresis. In this study, EPD was performed at 4 V, the lowest voltage reported for deposition of intermediate-bandgap (i.e. non-oxide) semiconductors to our knowledge. 4 V represents an important milestone, as it exists within the electrochemical stability window for the solvent, N,N-dimethylformamide (DMF)42. However, using in situ monitoring, we can infer that the decreased deposition voltage used in this study slows the reactions occurring during deposition such that the process transitions from being transport-limited to being reaction rate-limited at high particle concentrations. At this low voltage, it is possible for un-deposited particles to buildup near the electrode surface and screen the electric field, thus reducing the electrophoretic force felt by particles in the center of the cell. As transport is slowed or even stalled, the action of gravity on the interface between particle-rich and particle-depleted regions of the deposition cell leads to instabilities, observed in situ, that are severely deleterious to the formation of uniform films. This gravitational instability is entirely a function of the applied voltage—in its absence, the colloids are stable for weeks. Finally, methods for circumventing this settling phenomenon and depositing thick, uniform, and crack-free films without sintering are shown. Experimental


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Materials Tetrahydrofuran

(anhydrous,

99.85%),

naphthalene

(99%),

1-dodecanethiol

(98%),

isopropanol (anhydrous, 99.8%), chloroform (anhydrous, 99.9%), DMF (anhydrous, 99.8%), copper (II) acetylacetonate (98%), and hexanes (anhydrous, 99.9%) from Acros Organics; formamide (99.9%) from Fisher Scientific; oleylamine (OLAm, 70% technical grade) and selenium (99.5%) from Sigma-Alrich; zinc acetate dihydrate (98%) from Ricca Chemical Company; tin (II) chloride dihydrate (98%) from Strem Chemicals; and sodium sticks from Alfa Aesar. Copper Zinc Tin Sulfide (CZTS) NC Synthesis For the present work, copper zinc tin sulfide (CZTS) NCs were synthesized via a temperatureramp method. In a three-neck flask, 520 mg of copper (II) acetylacetonate, 290 mg of zinc (II) acetate dihydrate, 230 mg of tin (II) chloride dihydrate, 3 mL of 1-dodecanethiol, and 40 mL of oleylamine were mixed. This solution was heated to 110°C on a Schlenk line under vacuum for 2 hours with stirring. Afterwards, the solution was placed under N2 and heated to 280°C for 1 hour and then cooled to room temperature. The NC solution was transferred via Schlenk tube to a N2filled glovebox for washing. NC washing was achieved through antisolvent precipitation. Briefly, NCs were split evenly between two centrifuge tubes. 15 mL of isopropanol was added to each tube in order to flocculate the NCs. These suspensions were then centrifuged at 4000 RCF for 6 minutes, and the resulting supernatant discarded. To each NC pellet, 5 mL of hexanes was added to redisperse the NC pellet. This washing step was repeated once, reducing the isopropanol used to 5 mL, after which the NC solutions were combined and stored. CZTS NC Ligand Exchange


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In this work, the original organic ligands were replaced with selenide capping groups in solution via sodium selenide in formamide, using a modified literature procedure41. Preparation of sodium selenide (Na2Se) was done in a dry, N2 filled glove box, during which 1440 mg of selenium, 1200 mg of sodium, 460 mg of naphthalene, and 50 mL of tetrahydrofuran (THF) were combined with stirring in a sealed flask, following a modified procedure reported elsewhere43. CAUTION: sodium metal reacts violently with water; wastes from this process must be handled carefully as they can cause spontaneous combustion. After 48 hours, the slurry was separated from unreacted sodium, split in half, and centrifuged at 4000 RCF for 5 minutes. The supernatant was discarded and the resulting pellet was washed with 10 mL of THF. After a second centrifugation and decanting, the Na2Se pellet was dried under vacuum for 30 minutes and then stored in dry N2 atmosphere. In a typical ligand exchange, half of a synthesis yield (~200 mg of CZTS NCs) was dispersed in 16 mL of hexanes in a dry N2 glovebox. Separately, formamide was dried using vacuum for 2 hours and then transferred to the glovebox, where 16 mL of 21.2 mg/mL Na2Se in formamide solution was prepared. The CZTS and Na2Se solutions were combined and mixed for 10 minutes, after which 10 mL of isopropanol was added. The solution was centrifuged at 4000 RCF for 10 minutes, resulting in a NC pellet and two liquid phases. The supernatants were discarded, and the NCs washed with 5 mL of formamide followed by 5 mL of isopropanol. This wash was repeated with 5 mL of N,N-dimethylformamide (DMF) and 10 mL of chloroform. After centrifuging and discarding the supernatant, the NCs were dispersed in 5 mL of DMF and stored for later use. CZTS Characterization Methods The synthesis yield was measured as the dry weight of particles from a known volume-fraction of the total synthesis product, as follows. Half of the final volume of solution described above


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was set aside, flocculated by addition of antisolvent and centrifuged; the supernatant was discarded. The resulting pellet was then dried under vacuum overnight and weighed. From the other aliquot of the synthesis product, a portion was used to measure the absorbance of these CZTS NCs in solution using a PerkinElmer LAMBDA 35 UV-vis spectrophotometer, from which an extinction coefficient at 600 nm was calculated on a mass basis. The extent of ligand exchange was calculated using the height of the C-H stretching peak at ~2900 cm-1. Using pristine CZTS in hexanes and ligand-exchanged CZTS in DMF, films were spincast onto quartz rounds for absorbance measurements. Using a ThermoScientific Nicolet iS50R FTIR, film absorbance was measured from 20000 cm-1 to 2000 cm-1 (500 to 5000 nm; Figure S1). To compensate for differences in film thickness, the spectra were normalized using absorbance at 600 nm, beyond the onset of band-gap absorption. The extent of exchange was calculated by the ratio of the thickness-normalized absorption at 2920 cm-1, which indicated that at least 86% of the original ligands were removed (Figure S1, inset). Particle hydrodynamic radius and electrophoretic mobility were measured using a Brookhaven Instruments Nanobrook Omni. Sizing measurements were performed with the Dynamic Light Scattering mode in the backscatter orientation. Electrophoretic mobility was measured in the Phase Analysis Light Scattering mode with alternating voltage of 4 V. Prior to light scattering measurements, CZTS solutions were filtered through 0.2 µm PTFE filters. For DLS under applied voltage, two FTO-coated glass electrodes were inserted into the solution, separated by 2.5 mm-thick rubber spacers. The CZTS NC crystal structure was measured via a Rigaku Smartlab X-Ray Diffractometer using a Cu anode X-ray tube, CuKα = 0.154 nm. Films for XRD from as-synthesized CZTS were prepared via dropcasting.


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Electrophoretic Deposition EPD was performed as bath deposition using two EPD cells, each employing two FTO-coated glass substrates separated by a 3.2 mm insulating rubber spacer, shown in Figure 1. NC concentration ranged between 0.05 mg/mL and 2 mg/mL, with typical deposition performed with 0.7 mg/mL NC in DMF. The first EPD cell, shown in Figure 1a, was designed in such a way to evenly distribute pressure on an O-ring while simultaneously allowing observation of the deposition process from the front. The second EPD cell, shown in Figure 1b, was designed so that nanocrystal deposition could be observed from the side. Prior to EPD, CZTS solutions were passed through a 0.2 µm PTFE filter. Using a sealed O-ring, differences between vertical and horizontal EPD were studied. All EPD was performed in open-air at 4 V DC or 1.7 kV/m. Constant-voltage conditions were used rather than constant-current to avoid the uncontrolled increase in voltage that occurs as the deposition cell becomes more resistive during the process. After EPD, an aliquot of remaining solution was diluted in DMF and its absorbance at 600 nm was measured using a UV-vis spectrophotometer to determine the fraction of NCs deposited. At 4V, deposition is evident within 30 seconds and utilization of the NCs in solution is nearly complete as soon as 5 minutes.


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Figure 1. Bath EPD cells used in this study. In both cells, FTO-coated glass electrodes (blue, transparent) are held apart with a 3.2 mm rubber spacer (red). During deposition, space between electrodes is filled with NC solution. (A) Cell used to observe deposition from the front and prepare ~2 cm diameter films. (B) Cell housed in a glass cuvette, allowing for observation from the side. Rubber is also used to fill void space in the cuvette.

Film Characterization Methods Three primary microscopy techniques were used to study deposited film morphology: scanning electron microscopy (SEM), atomic force microscopy (AFM), and traditional light microscopy. SEM measurements were performed using a Zeiss Supra 50VP. AFM micrographs were taken using a Veeco (Bruker) Metrology, Inc., Multimode Nanoscope IIId scanning Probe Microscope System in contact mode. In addition to micrographs of the film surfaces, AFM was used to measure film thickness by measuring the step height between the film surface and the substrate below, exposed by lightly scratching the film. An Olympus CH-2 with AmScope digital converter attachment was used to collect optical micrographs. Results and Discussion Low Voltage Electrophoretic Deposition Through Surface Chemistry Control In this study, CZTS NCs were prepared using a one-pot, temperature ramp method with 1dodecanethiol- and oleylamine-based ligands from the literature as described above. The resulting particles dispersed well in non-polar solvents such as hexanes. X-ray diffraction (XRD) in Figure S2 shows CZTS NCs as-synthesized were wurzite-phase Cu2ZnSnS4 and remain unchanged after EPD44. UV-vis absorbance (Figure S1) displays the expected onset of band-gap absorption at the band-gap of CZTS (1.4–1.5 eV)19, and dynamic light scattering (DLS) revealed


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CZTS NCs’ hydrodynamic diameter in solution, measured to be 38 nm, consistent with TEM images of the particles (Figure S3). The original NC ligands were removed prior to EPD via solution-exchange of CZTS NCs in hexanes with Na2Se in formamide, such that Se2- ions replaced the original ligands, using a technique modified from the literature41. In addition to the beneficial effects on EPD described below, removing the insulating ligands is anticipated to have a positive impact on the resultant film’s conductivity as deposited, without need to subsequently remove the ligands. For the EPD process, capping the CZTS NCs with a charged ion increases their zeta-potential, enhancing their electrophoretic mobility to (–1.7±0.2)·10-8 m2/V·s as measured by Phase Analysis Light Scattering, an increase of ten-fold over typical literature values of pristine surfactant coated particles18,45. EPD of ligand-exchanged NCs is not only faster, as expected due to the increased particle mobility, it also occurs with a significantly reduced threshold voltage. Prior to ligand exchange, no observable deposit forms in the EPD cell, even up to 20 V; similarly no measurable current is passed through the cell. In contrast, after ligand exchange, films were successfully deposited under voltages as low as 4V DC (1.7 kV/m), with CZTS NCs depositing on the positive electrode, as expected from their negative zeta potential (Figure 2). To the best of our knowledge, this represents at least a three-fold decrease in the minimum voltage for EPD reported thus far for a non-oxide semiconductor. We emphasize that deposition only occurs when a threshold voltage has been overcome, not a threshold electric field strength: at 1V, no deposition occurs, even when the field is 1.2 kV/m, four-fold higher than the lowest field strength (0.3 kV/m) explored for successful 4V deposition, as seen in Figure S4. When well above the threshold voltage, the deposition rate is controlled by electrophoretic transport, which


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is linear with field strength, as expected in either scenario. Previous reports on surfactant capped NCs have indicated a threshold voltage as high as 25V (2.8 kV/m);40 a summary of voltages and field strengths in related literature reports is given in Table S1. Prior to deposition, particles are well dispersed in solution for weeks, easily passing through a 0.2 µm filter. Unlike the few prior examples of intermediate-to-low-voltage EPD,17–21,36,40,46 no ultrasonication is required to create or maintain the dispersion prior to EPD, and no destabilizing agent is required during EPD; thus the present study is a unique example in which a modest voltage alone is sufficient to drive deposit formation.

Figure 2. Current vs time trace during EPD for deposition down (green), horizontal (red), and up (blue). EPD was performed with 4 V bias, 0.7 mg/mL NC concentration in DMF, and 3.2 mm


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electrode spacing. Images above the EPD cell during deposition, with resulting films on the right. Deposition was performed at equal CZTS concentrations; difference in image color between orientations is due lighting differences. Inset: Average current over 5-minute EPD vs CZTS NC concentration (red). Best-fit line (R2 = 0.991). A single point is added representing the average current with only neat DMF (blue square). Additional NC concentrations are shown in Figure S5.

In situ Characterization of Particle Distributions with Correlated Chronoamperometry To observe and quantify the fundamental transport processes occurring during deposition, we designed two custom EPD cells for in situ observation: a face-on (Figure 1a) and a side-on (Figure 1b) cell. Images taken during the EPD process in the face-on observation cell are shown in Figure 2 as a function of time, from left to right, and as a function of the relative orientation of the applied electrical field and gravity from top to bottom. The images are indexed to the measured current density passing through the cell as a function of time (lower panel). In the upper right are photographs of the positively charged electrode after deposition. Gravity clearly exerts a dramatic effect on particle transport. When electrical and gravitational forces are in the same direction, the cell’s appearance does not change as particles deposit from the bulk on to the transparent electrode. However, upon application of a perpendicular electric field, the particles instantly change from gravitationally stable to rapidly settling. In the third case, when gravity directly opposes the electrical force, a highly inhomogeneous distribution of particles is created, with a mottled appearance. It bears repeating that the particles are extremely stable prior to application of the field—for example, they can withstand centrifugation at a relative centrifugal force (RCF) value of 6,000 for several minutes without visibly sedimenting—and the observed


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gravitational instabilities are attributable to the interplay between electrokinetic and gravitational processes. One disprovable hypothesis to account for the non-uniformities that develop in horizontal and upward deposition is voltage-induced particle aggregation in the bulk, followed by sedimentation of the large aggregates. To test for this, we performed in-situ DLS monitoring. The side-on EPD cell (Figure 1b) was used and the DLS signal monitored before and after application of voltage. Under conditions where settling occurs, there was no change in hydrodynamic radius (Figure 3).

Figure 3. Particle size in DMF before and during 4 volt deposition measured by dynamic light scattering. (A) Particle diameter measured before (grey region, red circles) and after (blue circles) voltage is applied. (B) multi-modal particle size distribution before (red) and after (blue) applied voltage.

To quantify particle motion in situ and to understand the mechanism of the gravitational instabilities, the side-on in situ observation cell was used. Figure 4 shows side-view images of the EPD process when depositing perpendicular to gravity (Figure 4a) and against gravity (Figure 4b). As expected from electrophoresis alone, within 10 seconds a NC-poor phase appears


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near the negative electrode as NCs electrophorese towards the positive electrode. In the case of horizontal deposition (gravity perpendicular to electric field, Figure 4a), the NC-rich phase begins to settle downward within 30 seconds. In the case of vertical deposition (gravity opposing electric field, Figure 4b) the interface between the two phases is initially stable and advances at the same rate as horizontal deposition, however, within 30 seconds, instabilities can be clearly seen as the particles separate into plumes reminiscent of a Raleigh-Taylor instability.

Figure 4. Photos of 4 V EPD in DMF in the side-view cell. (A) Side-view of EPD perpendicular to gravity from 0.67 mg/mL NC solution, with positive and negative electrodes on the left and right respectively. (B) Side-view of EPD opposite to gravity, from 0.34 mg/mL NC solution, with positive and negative electrodes on the top and bottom respectively.

As a consequence of the extremely sharp boundary between the NC-rich and the NC-depleted phases it is possible to process the images to measure the average position of this interface and use it as a real-time indicator of the forces on the particles in situ. In an ideal system of constant


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electric field with no forces other than electrophoresis, particles would follow a simple trajectory with a constant speed given by the electrophoretic mobility times the applied voltage, divided by the cell thickness: v = µV/d

Equation (1).

Consequently, the position of the interface within the cell, plotted against time, would also follow a straight-line path. Figure 5, shows the width of the NC-rich phase—which is equal to the position of the interface—from side-view photos taken during depositions such as those shown in Figure 4. The dashed purple and black lines in Figure 5 indicate the prediction based on a constant electric field given by an external voltage of 20 V and 4 V, respectively. In fact, at 20 V (a typical value for EPD in prior reports), this is exactly what is seen: the interface progresses towards the positively charged electrode at the predicted speed (purple line). At 4V (under the conditions common to images in Figures 2 and 4), the interface first moves with the predicted speed (dashed black line) but within 30 seconds it begins to slow (blue line). Similarly, when depositing down (green) the width deviates from the ideal trend within the first minute. The timing and extent of the deviation from the predicted trend both increase with greater particle concentration.


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Figure 5. Position of the interface between the NC-rich and NC-depleted phases vs time, representing the particle mass traversing the reactor. 4 V horizontal deposition with ~0.34 mg/mL CZTS in DMF shown in blue. Downward, high-concentration, and low-concentration depositions shown in green, red, and orange respectively. 20 V horizontal deposition with 0.67 mg/mL CZTS in DMF shown in purple. All deposition was performed horizontally (perpendicular to gravity) except where noted. Prediction for constant electric field (based on Eqn. 1) shown for 4 V and 20 V deposition in dashed black and purple lines respectively.

Dense, crack-free films without sintering. Prior to this report, one significant impediment to using EPD for the absorber layer of a photovoltaic device was the fact that EPD deposits are typically riddled with cracks that form as the deposit dries. Microscopy was used in order to study the present films’ morphology, focusing on the downward deposited films (other deposition geometries are shown in Figure S6). Light microscopy (Figure 6a) reveals a mostly uniform surface on the square-mm scale, with occasional protrusions of high contrast particles (dark spots), presumed to be debris introduced during processing. The surface roughness of these films, measured by AFM (Figure 6a, inset), is 50 nm (RMS). SEM micrographs, such as that shown in Figure 6b, exhibit densely packed nanocrystals. The thickness of a film deposited from a 0.6 mg/mL CZTS in DMF solution was measured to be 420±10 nm by AFM, and the trend of several films from NC solutions of varying concentration is 650 nm/(mg/mL) at 4V for 5 min (Figure S7). Remarkably there is no evidence of cracking in these films.


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Figure 6. Micrographs of EPD film, deposited down. Red boxes indicate the relative magnifications of the panels, but not the regions actually measured. (A) Wide-area light micrograph. Black specs are unknown debris on the film surface. Inset: AFM micrograph showing surface roughness (50 nm). Scale bar: 4 µm. (B) SEM micrograph of film surface, showing scale of film texture. Inset: cross-section of an EPD film on an FTO coated glass substrate.

Discussion For the present case of highly charged, bare CZTS particles, we note three pieces of evidence consistent with an electrochemical rather than electrophysical rate-limiting step for the deposition of particles. First, deposition is governed by a threshold voltage, not a threshold electric field strength (Figure S4). Electrophysical theories of electrodeposition assume that the applied electric field strength is sufficient to overcome the particle-particle electrostatic repulsion. However in the case of successful EPD of highly charged particles, it has been posited previously that the large electrostatic repulsion must instead be negated by complementary charged species26–31, which are generated at the electrode surface when above a threshold voltage


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determined by the redox potential of the species involved. For example, for bare metal oxide particles it is posited that the particle charge is neutralized in the accreting deposit by excess hydronium or hydroxide ions generated at the electrode by electrolysis of water27–31. The second line of evidence is the existence of a significant current beyond that required for electrophoresis alone. For example, the total current passed in a complete deposition run can be compared to the mass-loading of particles in the reactor, shown in Figure 2, lower panel, inset. The slope of this linear trend is 0.025 (mA/cm2)/(mg/mL), corresponding to ~104 charges passed per NC deposited, a value that is much greater than the net charge per particle based on their zeta potential and hydrodynamic radius measured by DLS. The third piece of evidence, elaborated upon in the next section, is that the small magnitude of the charge-screening effect accompanying particle accumulation strongly suggests that the NCs are charge-neutralized at the electrode. Further studies are aimed at identifying the putative electrochemical reactions that are occurring; at this juncture we speculate that a significant portion of the current is consumed in the facile oxidation of the selenide capping agent, both bound and unbound to particle surfaces. The electrolysis of adventitious water naturally present in our system is also a possible contributor as our DMF is neat, but not anhydrous (Figure 2, lower panel inset, blue symbol). In the absence of electrochemical processes, EPD should exhibit a voltage-limited maximum thickness due to the accumulation of charge at the depositing electrode such that the potential drop occurs almost entirely over the deposit, leaving no potential gradient (i.e. electric field) in the bulk to drive electrophoresis. Using the small hydrodynamic radius and high zeta potential determined for these particles by DLS, a rough estimate can be made of the surface charge density of a complete monolayer of particles assembled on the electrode. The voltage drop at such an interface can be estimated from a simple Helmholtz model of this double-layer of


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particles47 to be of order 10 V. In other words, 4V would be insufficient to assemble even one complete monolayer of particles before fully screening the electrode potential. Our in-situ measurements, summarized in Figure 5, probe the magnitude of the electric field in the bulk directly, by determining the velocity of particles at the rear of the advancing particle mass, far from the depositing electrode. Figure 5 provides evidence that at early times or for low particle concentrations, the electric field is nearly constant during low-voltage EPD- the particle speed is exactly that predicted by Equation 1. At later times however, a distinct retardation effect is observed, consistent with double-layer screening effects. For simplicity, we focus on deposition downwards (Figure 5, green curve), for which significant slowing of the electrophoresis is not observed until at least one minute into deposition. By this time, 100’s of nanometers of particles—or equivalently, 10’s of particle layers—have already deposited (Figure S4, 1.2 kV/m). From this, we conclude that particles lose their charge to a significant extent in the process of deposition. With these considerations, it is useful to present an equivalent circuit model describing the depositing electrode and the bulk solution of the EPD cell, shown in Figure 7. The resistance of the bulk solution is given by Rb while that of any hypothetical electrode-electrolyte interfacial charge transfer (Faradaic) processes at the depositing electrode is Rct; the capacitance of the double layer is given by Cd. The total voltage drop from the depositing electrode up to the interface with the counter electrode will be equal to the sum iRct + iRb, where i is the net current in the system. Circuit analysis dictates that the voltage drop over the double-layer, Cd, is equal to iRct in this simple model. If Rct is small compared to Rb, than the majority of this voltage drop will occur in the solution. Hence the gradient in the potential will be roughly equal to the applied voltage divided by the electrode spacing (assuming a facile reaction at the counter electrode).


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This scenario is in good agreement with the observed in situ electrophoretic velocity of particles in the bulk of the reactor at early times or at low concentration, in Figure 5, wherein the displacement of the particle mass across the reactor matches the prediction of Equation 1 (dashed lines). In this case, the slowing of electrophoresis observed at later times could be from an increasing value of Rct as the deposit grows; a large Rct leads to a significant voltage drop over the double layer (greater charge accumulation) and less voltage drop in solution. The increasing charge transfer resistance of a growing EPD deposit is well attested to in the literature on metaloxide particle EPD30,48. Such a system is fundamentally limited by reaction kinetics, rather than transport. In this scenario, the Rayleigh-Taylor-like instability evident in Figures 2 and 4b can be understood: initially, electric fields in the bulk drive the formation of a gravitationally disfavored arrangement of densities; when the field in the bulk is attenuated by the accumulation of charged particles, the arrangement becomes unstable, in analogy to a layer of water above a low density oil.

Figure 7. Equivalent circuit model of an EPD reactor with bulk solution resistance, Rb, interfacial charge-transfer resistance, Rct, and double-layer capacitor, Cd.

Conclusions


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Developing deposition techniques for colloidal, semiconducting NCs has many challenges. In this study, EPD of all-inorganic semiconducting NCs was demonstrated, resulting in significantly reduced voltages—within the electrochemical stability window of the carrier solvent—and deposition times on the order of minutes. As deposition voltage is reduced, the EPD process transitions from being transport- to reaction-limited, which in turn gives rise to material build-up, screening of the electric field due to increased resistance at the charge-transfer interface and, in some cases, settling of the NC-rich phase. At lower NC concentration, it is shown that the EPD process remains transport limited. Although the application of EPD to photovoltaic fabrication is highlighted in this work, we did not characterize the optoelectronic figures of merit (e.g. carrier mobility and lifetime) of the resulting films. We speculate that an optimized thermal processing step, possibly in a sulfur or selenium vapor, would be necessary to consolidate grains and rearrange the dangling bonds of nanocrystal surface atoms before satisfactory performance could be achieved. However, the dense, all-inorganic, uniform films of low surface roughness created by EPD represent a promising starting point for optimization.

Supporting Information. Figures S1-S7 and Table S1 (PDF) Corresponding Author *[email protected] Author Contributions


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A.D., and A.F. conceived and designed the experiments, analyzed the data, and wrote the paper; A.D. and S.M. performed the experiments; A.F. contributed reagents, materials, and analysis tools. Acknowedgements We would like to thank Maureen Tang for helpful discussions of electrochemistry, Swarnendu Chatterjee and Tianshuo Zhao for assistance with SEM and Christopher B. Murray and Weyde Lin from UPenn for sharing the CZTS synthesis protocol. Funding Sources This work was funded by NSF award number CMMI 1463412. XRD and SEM studies were performed at the Centralized Research Facilities at Drexel University. REFERENCES (1) (2)

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Figure 1. Bath EPD cells used in this study. In both cells, FTO-coated glass electrodes (blue, transparent) are held apart with a 3.2 mm rubber spacer (red). During deposition, space between electrodes is filled with NC solution. (A) Cell used to observe deposition from the front and prepare ~2 cm diameter films. (B) Cell housed in a glass cuvette, allowing for observation from the side. Rubber is also used to fill void space in the cuvette. 152x91mm (96 x 96 DPI)


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Figure 2. Current vs time trace during EPD for deposition down (green), horizontal (red), and up (blue). EPD was performed with 4 V bias, 0.7 mg/mL NC concentration, and 3.2 mm electrode spacing. Images above the EPD cell during deposition, with resulting films on the right. Deposition was performed at equal CZTS concentrations; difference in image color between orientations is due lighting differences. Inset: Average current over 5-minute EPD vs CZTS NC concentration (red). Best-fit line (R2 = 0.991). A single point is added representing the average current with only neat DMF (blue square). 338x224mm (96 x 96 DPI)


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Figure 3. Particle size before and during 4 volt deposition measured by dynamic light scattering. (A) Particle diameter measured before (grey region, red circles) and after (blue circles) voltage is applied. (B) multimodal particle size distribution before (red) and after (blue) applied voltage. 160x91mm (96 x 96 DPI)


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Figure 5. Photos of 4 V EPD in the side-view EPD cell. (A) Side-view of EPD perpendicular to gravity from 0.67 mg/mL NC solution, with positive and negative electrodes on the left and right respectively. (B) Sideview of EPD opposite to gravity, from 0.34 mg/mL NC solution, with positive and negative electrodes on the top and bottom respectively. 338x132mm (96 x 96 DPI)


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Figure 6. Position of the interface between the NC-rich and NC-depleted phases vs time, representing the particle mass traversing the reactor. 4 V horizontal deposition with ~0.34 mg/mL CZTS in DMF shown in blue. Downward, high-concentration, and low-concentration depositions shown in green, red, and orange respectively. 20 V horizontal deposition with 0.67 mg/mL CZTS in DMF shown in purple. All deposition was performed horizontally (perpendicular to gravity) except where noted. Prediction for constant electric field (see text) shown for 4 V and 20 V deposition in dashed black and purple lines respectively. 160x106mm (96 x 96 DPI)


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Micrographs of EPD film, deposited down. Blue boxes indicate the relative magnifications of the panels, but not the regions actually measured. (A) Wide-area light micrograph. Black specs are unknown debris on the film surface. Inset: AFM micrograph showing surface roughness (50 nm). Scale bar: 4 µm. (B) SEM micrograph of film surface, showing scale of film texture. Inset: cross-section of an EPD film on an FTO coated glass substrate. 339x125mm (150 x 150 DPI)


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Figure 7. Equivalent circuit model of an EPD reactor with bulk solution resistance, Rb, interfacial chargetransfer resistance, Rct, and double-layer capacitor, Cd. 160x71mm (96 x 96 DPI)


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Influence of Compact, Inorganic Surface Ligands on the

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