CsPbBr3 Solar Cells: Controlled Film Growth through Layer-by-Layer

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Article Cite This: Chem. Mater. 2017, 29, 9767-9774

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CsPbBr3 Solar Cells: Controlled Film Growth through Layer-by-Layer Quantum Dot Deposition Jacob B. Hoffman,†,‡ Gary Zaiats,† Isaac Wappes,†,‡ and Prashant V. Kamat*,†,‡,§ †

Radiation Laboratory, ‡Department of Chemistry & Biochemistry, and §Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: All inorganic cesium lead bromide (CsPbBr3) perovskite is a more stable alternative to methylammonium lead bromide (MAPbBr3) for designing high open-circuit voltage solar cells and display devices. Poor solubility of CsBr in organic solvents makes typical solution deposition methods difficult to adapt for constructing CsPbBr3 devices. Our layer-by-layer methodology, which makes use of CsPbBr3 quantum dot (QD) deposition followed by annealing, provides a convenient way to cast stable films of desired thickness. The transformation from QDs into bulk during thermal annealing arises from the resumption of nanoparticle growth and not from sintering as generally assumed. Additionally, a large loss of organic material during the annealing process is mainly from 1-octadecene left during the QD synthesis. Utilizing this deposition approach for perovskite photovoltaics is examined using typical planar architecture devices. Devices optimized to both QD spin-casting concentration and overall CsPbBr3 thickness produce champion devices that reach power conversion efficiencies of 5.5% with a Voc value of 1.4 V. The layered QD deposition demonstrates a controlled perovskite film architecture for developing efficient, high open-circuit photovoltaic devices.

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been utilized to make perovskite films for photovoltaic applications. While the overall PCE is limited by a relatively larger bandgap,36 CsPbBr3 has been targeted as a potential material for stable high-voltage solar cells.27−29 Specifically, Akkerman and co-workers have synthesized semisuspended CsPbBr3 nanocrystal “inks” that feature an orthorhombic perovskite structure (γ phase).27 Deposition through this route results in good surface coverage and devices possessing high open-circuit voltages. Recently, we have developed a layer-by-layer deposition procedure for bulk CsPbBr3 from fully suspended CsPbBr3 QDs that provides controllable thickness and a cubic perovskite crystal structure.37 In this report, the growth process of deposited CsPbBr3 QDs during film formation is more closely examined through electron microscopy and thermogravimetric analysis (TGA). The mechanism for the QD to bulk transformation and the performance of photovoltaic devices constructed though the layer-by-layer deposition method are discussed.

INTRODUCTION Lead halide (APbX3, X = Cl−, Br−, I−) perovskite semiconductors have emerged as an attractive material for lightharvesting applications,1−7 possessing superior properties for charge separation coupled with simple solution processability.8−17 Recent interest in this class of materials for photovoltaics has led to a rapid rise in the performance of laboratoryassembled devices, with a highest reported power conversion efficiency of 22.1% for methylammonium (MA) lead iodide.6 The significant interest in perovskite-structure semiconductors has also led to the development of perovskite-structure nanomaterials. Such materials can be easily spin- or drop-cast on substrates, forming well-packed, ordered films.18−20 Once deposited, APbX3 nanomaterials have been shown to have high PL quantum yields and low thresholds for amplified spontaneous emission, making such films promising candidates for lighting and lasing applications.18−24 Films of perovskite-structured nanomaterials have also been under investigation for photovoltaic applications in CsPbX3 perovskites.25−27 The solution-based deposition methods typically used for MAPbX3 are difficult to directly adapt for CsPbX3, due to the lower solubility of cesium precursors in commonly used deposition solvents.28−30 Additionally, the cesium perovskite possessing the smallest band gap, CsPbI3, is not structurally stable as a bulk crystal.31−34 CsPbI3 quantum dots (QDs) have been utilized to overcome both difficulties through direct deposition of CsPbI3 QDs by spin-casting followed with stabilization of the perovskite structure through postdeposition chemical treatment.26,35 CsPbBr3 QDs have also © 2017 American Chemical Society

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EXPERIMENTAL SECTION

Materials. Acetone (HPLC grade, Fischer Scientific), ethanol (200 proof, anhydrous, Koptec), n-heptane (99%, spectrophotometric grade, Sigma-Aldrich), n-hexane (95%, anhydrous, Sigma-Aldrich), cesium carbonate (Cs2CO3, 99.9%, metals basis, Alfa Aesar), lead(II) bromide (PbBr2, 98%, Sigma-Aldrich), 1-octadecene (1-ODE) (95%, Received: September 4, 2017 Revised: October 24, 2017 Published: October 25, 2017 9767

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Chemistry of Materials Sigma-Aldrich), oleic acid (OAc) (technical grade, 90%, SigmaAldrich), oleylamine (OAm) (technical grade, 70%, Sigma-Aldrich), chlorobenzene (Alfa Aesar, 99.5%), FK 102 Co(III) TFSI salt (Co[PyPz]3[TFSI]3, Dyesol), lithium bistrifluoromethanesulfonimide (LiTFSI, Sigma-Aldrich, 99.95%), titanium diisopropoxide bis(acetylacetonate) (TAA, Sigma-Aldrich, 75 wt % in IPA), and fluorine-doped tin oxide conducting glass (FTO, TEC 7, Pilkington Glass) were used without further purification. Synthesis and Cleaning of CsPbBr3 QDs. The synthesis of CsPbBr3 QDs followed a procedure published by Protesescu et al. with alterations to scale up the reaction.21 A precursor solution of cesium oleate (Cs-oleate) was prepared in a 25 mL three-neck flask by suspending Cs2CO3 (0.154 g) in OAc (1.5 mL) and 1-ODE (1.4 mL). The suspension was heated to 100 °C and placed under vacuum for 1 h. During this time, Cs2CO3 fully dissolved, forming Cs-oleate. After degassing, the solution was put under nitrogen and heated to 150 °C. In a second 25 mL three-neck flask, PbBr2 (0.414 g) was suspended in OAc (3 mL) and 1-ODE (3 mL), and then, the reaction mixture was degassed under vacuum at 100 °C for 1 h. The reaction mixture was put under nitrogen atmosphere and heated to 170 °C where OAm (3 mL) was added. After PbBr2 was fully dissolved and the solution had recovered to 170 °C, 2 mL of 150 °C Cs-oleate precursor was injected. CsPbBr3 QDs immediately formed and precipitated. Then, the reaction was cooled down to 90 °C using an ice−water bath. After cooling, the product was divided among four centrifuge tubes with 40 mL of 1-ODE split between tubes to prevent OAc and OAm from freezing. The precipitated QDs were separated from the reaction mixture via centrifugation at 7800 rpm for 10 min. Excess ligand was removed by rinsing each centrifuge tube with 5 mL of 1-ODE. Excess 1-ODE was discarded followed by each tube being rinsed with acetone and dried under compressed air. QDs were then resuspended in a solution of 9:1 (by volume) hexane/heptane, and the mixture was centrifuged again to remove any nonsuspended material. Deposition of CsPbBr3 Films. Prior to deposition, FTOconducting glass was cleaned through sonication in ethanol for 5 min followed by oxygen plasma cleaning for 5 min. Films of bulk CsPbBr3 were deposited using a modified layer-by-layer deposition procedure previously developed by our group.37 Per cycle, 15 μL of CsPbBr3 QDs were deposited through dynamic spin-casting at 5000 rpm for 30 s on FTO using a Hamilton syringe. The concentration of the deposition solution was standardized to produce the desired deposition thickness per cycle. After QD deposition, films were annealed in a fume hood for 3 min on a hot plate set to 225 °C. At high deposition solution concentrations, vapor was observed to rise from the film during the annealing stage. Directly following annealing, the films were removed from the hot plate and allowed to cool for 1 min. After cooling, the films were dipped into hexane in an attempt to remove residual 1-ODE and capping ligands. The procedure was then repeated to achieve the desired film thickness. It is important to use personal protective equipment while carrying out the above-mentioned procedures. All our CsPbBr3 film deposition was conducted in a fume hood with controlled ventilation and exhaust to exercise safety in handling lead compounds and volatile organics. Microscopy Experiments. In situ temperature-controlled transmission electron microscopy (TEM) images of annealed CsPbBr3 QDs were obtained with an FEI Titan 80-300 microscope operated at 300 keV and equipped with the DENS Solutions Inc. “wildfire system”. QDs were rinsed with acetonitrile prior to the experiments to remove excess organics. Scanning electron microscopy (SEM) images were captured using an FEI-Magellan 400 scanning electron microscope with a Schottky field emitter source mounted on the gun module. TGA and Differential Scanning Calorimetry Experiments. TGA and differential scanning calorimetry (DSC) experiments were performed using a Mettler-Toledo TGA/DSC 1 STARe system. TGA and DSC were run simultaneously at a scan rate of 10 °C/min under both air and nitrogen atmospheres. An aluminum crucible was used for the various organics tested with a temperature range from 28 to 500 °C. QD samples were dried on a hot plate set to 60 °C to remove

hexane prior to the run. An alumina crucible was used in the QD experiment to scan a greater temperature range (28−900 °C). Device Assembly and Characterization. A FTO substrate was masked, and the exposed area was etched by reacting zinc powder with 2 M hydrochloric acid for 5 min. After thorough rinsing of the etched area with deionized water, the substrates were cleaned through sonication in an ethanol bath for 15 min, followed by an oxygen plasma treatment for 5 min. A TiO2 layer was deposited through spincasting a 0.3 M solution of titanium diisopropoxide bis(acetylacetonate) in anhydrous 1-butanol at 3000 rpm for 30 s. The spin-cast films were dried on a hot plate at 120 °C for 10 min before annealing at 500 °C for 30 min in a high-temperature oven. The CsPbBr3 perovskite layer was deposited to the desired film thickness using the procedure described in the above section. Spiro-OMeTAD was deposited as a hole transport layer via spincasting at 3000 rpm in a nitrogen glovebox with water content less than 5 ppm. The solution was composed of 72.3 mg/mL spirooMeTAD, 28.8 μL/mL 4-tert-butylpyridine, 17.5 μL/mL of a bis(trifluoromethane) sulfonimide lithium salt solution, and 10 mol % (relative to spiro-oMeTAD) of FK102 Co(II) PF6 salt in anhydrous chlorobenzene. The bis(trifluoromethane) sulfonimide lithium and FK102 Co(II) PF6 salts were added through solutions consisting of 520 and 300 mg/mL of each respective material in anhydrous acetonitrile. The electrodes were kept overnight in a dry air desiccator before a 100 nm thick gold counter electrode was deposited through thermal evaporation using a shadow mask at 10−6 torr at a 10 Å/s deposition rate. Finally, indium contacts were soldered on the cells, and the finished devices were stored in a dry air desiccator until testing. Photocurrent voltage curves were recorded using a Princeton Applied Research 2273 (PARstat) potentiostat. As a light source, a simulated solar spectrum was generated from a 300 W xenon arc lamp equipped with an AM 1.5G filter. The power density of the light source was 100 mW/cm2, measured using a Thorlabs S302C thermal power sensor. Voltage was scanned at a rate of 10 mV/s in both forward (Jsc to Voc) and reverse (Voc to Jsc) directions to minimize impacts from hysteresis. Prior to measurement, the cells were allowed to equilibrate for 30 s at Jsc and Voc for forward and reverse scan directions, respectively.

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QUANTUM DOT GROWTH OBSERVED THROUGH MICROSCOPY The use of thick QD films in photovoltaics requires replacement or removal of the long alkyl chained ligands typically used in the synthesis of colloidal QDs, so that the nanocrystals have close contact.38−40 For chalcogenide QDs, this is accomplished through either ligand exchange with short polar ligands or partial QD fusion from thermal annealing.41−48 The deposition method we developed involves a similar thermal annealing of spin-cast CsPbBr3 QDs (Scheme 1).37 This step triggers a transformation of QDs into a layer of bulk crystallites, after which the procedure can be repeated to create a film of desired thickness. Our previous report showed QD Scheme 1. Schematic of QD Layer-by-Layer Depositiona

a

QDs are spin-cast onto a substrate followed by thermal annealing for 1 min at 225 °C to induce transformation of QDs into bulk. The process can then be repeated to create a bulk perovskite film of desired thickness. 9768

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observation of larger crystal formation suggests that QD growth is simply resuming at the annealing temperatures. Top down SEM images were also obtained at lower magnifications (5000×) in order to examine morphological changes on a wider scale. Before annealing (B), residual organic material was clearly present on the film surface evidenced by a significantly darker image from the insulating material. After 1 min of annealing time (D), some organic material was removed from the film surface, indicated by the presence of both dark and light surface regions. At 3 min of annealing time (F), the darker areas attributed to organic residue are no longer observable on the film surface. The loss of organics is interesting as the annealing temperatures used are well below the ambient boiling points of the organic species present in the QD synthesis. Real-time monitoring of CsPbBr3 growth was accomplished through in situ temperature-controlled TEM images at 250 °C, with snapshots of the sample taken every 15 s for 30 min. Figure 2 shows the progress of the growth at 0 (A), 13 (B), and

growth through a loss of quantum confinement in UV−visible absorbance, as well as in SEM images postannealing that show a polycrystalline bulk film.37 While these experiments showed that a transformation to bulk occurred, it did not further our understanding of the mechanistic details of the process. To investigate the transformation more closely, top down SEM images of CsPbBr3 QDs were taken preannealing as well as at 1 and 3 min of annealing at 225 °C (Figure 1). When

Figure 1. Top down SEM images of spincast QDs on an FTO substrate at two different magnifications for (A, B) 0, (C, D) 1, and (E, F) 3 minute annealing times. At high magnification (A, C, E), as the annealing time increased, individual QDs disappeared forming larger crystals and revealing the underlying FTO substrate. At lower magnifications (B, D, F), dark nonconducting organic material deposited with the QDs gradually disappeared. Magnification was 150 000× for A, 100 000× for C and E, and 5000× for B, D, and F. Insets in A, C, and E are histograms of the CsPbBr3 crystal size.

Figure 2. In situ temperature-controlled TEM images of CsPbBr3 QDs at (A) 0, (B) 13, and (C) 30 minutes (at 250 °C). The longer growth time frame compared to the SEM experiment is attributed to electron beam carbonization of the organic capping ligands. Larger bulk crystals are grown as QDs are consumed.

(C) 30 min, but a compiled animation of the complete experiment is included in the Supporting Information. Prior to annealing, the sample showed significant aggregation on the TEM grid (A), triggered by rinsing the grid with acetonitrile. This was done to remove the excess ligand from the grid, avoiding carbonization from prolonged exposure to the instrument electron beam. After 13 min of annealing, a significant number of the QDs disappear and a large crystallite of CsPbBr3 appears at the edge of the aggregate (B). By 30 min, growth has ended with the majority of the QDs disappearing, the bulk crystal enlarging, and some smaller crystals remaining within the aggregate area. The growth is similar to what was observed in the SEM experiments with two major differences: the time scale of the TEM experiment was significantly longer, and the majority of growth occurred at the aggregate edge. Both of these effects were attributed to an amorphous residue remaining at the initial QD positions, hindering the growth process by trapping CsPbBr3 (Figure S2). This residual organic skeleton likely arose from carbonization of the remaining ligands bound to the QD

examined at 150 000× (A) and 100 000× (C, E) magnification, top down SEM showed the transformation from QDs to bulk crystallites of CsPbBr3 perovskite. Preannealing (A), the QDs are clearly visible, self-aggregating into clusters as has been observed in the literature for this material.19,21 The average QD size was 16 nm (A, inset), found through TEM (Figure S1). Between clusters of QDs, darker areas of nonconducting material can be observed suggesting the presence of excess high-boiling-point organic material present in the spin-casting solution. After 1 min of annealing (C), CsPbBr3 QD aggregates have disappeared, forming larger crystallites with a 50 nm average size (C, inset). Additionally, areas of the underlying FTO substrate become visible. By 3 min (E), the CsPbBr3 crystals grow larger with an average size of 85 nm (E, inset), eliminating the smaller bulk crystallites observed at 1 min. Our prior report involving QD deposition initially attributed this transformation to QD sintering, similar to what has been employed for chalcogenide QDs in QDSCs. However, the 9769

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Figure 3. (A) TGA and (B) DSC experiments at a scan rate of 10 °C/min under air for (a) CsPbBr3 QDs after solvent removal, (b) 1-ODE, (c) OAm, and (d) OAc. In TGA, the main QD mass loss at the annealing temperature used (225 °C) is attributed to excess 1-ODE from the QD reaction. In DSC, the main QD transition matched a transition for 1-ODE at 200−290 °C attributed to 1-ODE oxidation.

Figure 4. (A) Cross-sectional SEM of CsPbBr3 devices (350 nm thickness) made with varying QD solution concentrations. Devices that use less layers (higher QD concentration) contain more morphological defects than those with more layers (lower QD concentration). (B) Device performance as a function of CsPbBr3 deposition cycles to reach a total thickness of 350 nm for (i) PCE, (ii) Voc, (iii) Jsc, and (iv) FF. Current density−voltage curves were recorded at a scan rate of 10 mV/s in forward and reverse directions. Each point represents 12−15 contacts from the same QD synthesis.

organics present as well as with the QDs. Figure 3A shows the relative mass loss versus temperature for the QDs (a), 1-ODE (b), OAm (c), and OAc (d). In the QD sample, the hexane and heptane used to suspend the QDs were dried prior to the run to better simulate the conditions of a spin-cast film. As the temperature increased, a major mass loss was observed from 200 to 300 °C within the annealing temperature of 225 °C. When compared to the organics used in the CsPbBr3 QD reaction, this first and most significant mass loss closely reflects the TGA of 1-ODE (b), indicating that 1-ODE is likely the major residual organic evaporating during annealing. This result is consistent with the QD synthesis and cleaning procedure, as 1-ODE is in excess during the reaction and is used to clean the QDs prior to resuspension. Above the annealing temperature, a lesser secondary mass loss occurs (300−400 °C), arising from a combination of a residual tail observed in the TGA of 1-ODE (b) and the higher boiling point organic capping ligands used to stabilize the QDs (c, d). It is important to note that the organics removed during this secondary mass loss are unlikely to leave the film during the annealing stage because 300−400 °C is significantly higher than the annealing temperature. XPS supports this conclusion as a significant amount of carbon remains on the film postannealing, and elements in OAc and OAm functional groups (oxygen and nitrogen) increase in relative percentage (Table S1). A previous report by Kulbak and co-workers shows

surface as other areas not directly observed show similar growth as seen by SEM. The observed real-time loss of smaller QDs as larger bulk crystallites enlarge confirms that thermal annealing triggers resumption of QD growth. In the absence of reaction precursors, material migrates from smaller to larger QDs in a defocusing process analogous to Ostwald ripening observed normally in hot-injection reactions.49 A similar process has been observed in films of PbS QDs when annealed and at similar temperatures.47

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TRACKING ORGANIC MATERIAL LOSS The loss of a significant amount of organic material through the annealing process is interesting due to the high boiling points of 1-ODE and the organic capping ligands relative to the annealing temperature. The removal of organics is necessary to achieve good performing devices as residual carbon can hamper device performance through increased resistance. During the annealing process, the loss of organic material can be observed by the naked eye as a vapor rising from the film surface. Additionally, elemental analysis through X-ray photoelectron spectroscopy (XPS) before and after annealing shows that the relative percentage of carbon drops from 79% to 50%, while relative amounts of oxygen, nitrogen, and CsPbBr3 increase (Table S1). To investigate the loss of organic material, TGA was employed under air atmosphere conditions for all individual 9770

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Figure 5. (A) PCE (i), Voc (ii), Jsc (iii), and FF (iv) for CsPbBr3 solar cells as a function of CsPbBr3 thickness. (B) Current density−voltage curves of our champion device. Current density−voltage curves were recorded at a scan rate of 10 mV/s in the forward and reverse directions. Each point represents 8−15 contacts from separate batches of devices.

that CsPbBr3 is stable up to 600 °C, suggesting a higher annealing temperature can be used to remove the remaining organics without damage to the material.28 However, in practice, the normally yellow material quickly becomes white at temperatures over 300 °C suggesting degradation. This degradation is supported by TGA coupled with DSC of CsPbBr3 at higher temperatures (Figure S3). DSC (Figure 3B) of the high-temperature organics used in the QD reaction revealed additional information about the annealing process. Endothermic transitions corresponding to the expected ambient pressure boiling points of 1-ODE (315 °C) (b), OAm (350 °C) (c), and OAc (364 °C) (d) are observed in each respective DSC run. An additional feature is observed for both 1-ODE (b, 200−290 °C) and OAc (c, 225− 322 °C), exhibiting a small exothermic transition followed by a larger endothermic transition. We attribute these features to be connected to the oxidation of each respective species as the features are not present when the experiment is ran under a nitrogen atmosphere (Figure S4). It is interesting to note the 1ODE tail observed in TGA is not present under nitrogen atmosphere, suggesting the tail is the result of 1-ODE oxidation products. The features attributed to oxidation are present in the DSC scan for the QDs (a), indicating that organic oxidation is a present process occurring during film annealing.

investigated how this parameter impacts device performance using 350 nm CsPbBr3 active layers deposited using QD solution concentrations of 4.0 × 10−6, 8.0 × 10−6, and 1.2 × 10−5 M, which were found through a recently published molar extinction coefficient.50 The average efficiencies for each QD concentration are shown in Figure 4B, with each concentration represented as the number of cycles required to complete the deposition. Between solar cells with CsPbBr3 layers of 6 and 24 deposition cycles, the average PCE (i) of the devices improved from 1.35% to 2.25% and 1.15% to 2.13% for the forward and reverse scan directions, respectively. The improvement in PCE came from increases in open-circuit voltage (Voc) (ii), short circuit current (Jsc) (iii), and fill factor (FF) (iv), with the most significant gains seen in Jsc. We attribute the difference in performance to a more gradual deposition rate resulting in a more uniform CsPbBr3 layer. This morphological difference is shown in cross-sectional SEM where CsPbBr3 layers with 6 deposition cycles contain significantly more cracks and pinholes compared to samples with 24 deposition cycles (Figure 4A). Next, we investigated the effect of varied CsPbBr 3 thicknesses on device performance. Performance parameters for devices as a function of CsPbBr3 thickness are shown in Figure 5A for both forward and reverse scan directions. The general PCE (i) of the solar cells increases with CsPbBr3 thickness until 250 nm where it levels off with an average cell performance of approximately 2%. The majority of the difference in performance comes from changes in Voc (ii), which drastically increases from 0.2 V at 40 nm thickness to 1.2 V at 300 nm thickness. Cross-sectional SEM images show areas without CsPbBr3 coverage in thinner devices (Figure S5). Areas without full coverage have electron and hole transport layers in direct contact, causing cell shorting and lowering measured photovoltage. In addition to changes in Voc, Jsc (iii) gradually increased with CsPbBr3 thickness, which is attributed to thicker CsPbBr3 layers absorbing more light. This correlation is shown by direct comparison of Jsc to the light absorbed by CsPbBr3 (Figure S6). Values for FF (iv) remain consistent around 0.4, suggesting active layer thickness plays a minimal role in that parameter. When compared to other devices with similar architectures, our devices have comparable values for Voc and Jsc. However,

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QUANTUM DOT GROWTH AND SOLID-STATE PHOTOVOLTAICS Our CsPbBr3 deposition method provides a simple pathway to achieve films of controllable thickness and growth rate. These features provide a potential advantage over solution-based deposition methods for cesium-based perovskites, which suffer from poor solubility of cesium halide salts in typical solvents used for perovskite deposition. To test the light-harvesting applications of our deposition method, we constructed devices with a planar architecture using a 30 nm thick TiO2 compact layer. The benchmark materials of Spiro-oMeTAD and gold were used to complete the solar cell assembly (Figure 4A). The concentration of the QD spin-casting solution impacts the number of deposition cycles required to achieve an active layer of a targeted thickness, with a lower spin-casting solution concentration requiring more deposition cycles. We first 9771

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Chemistry of Materials there is a difference in FF with our devices possessing lower values, leading to a lower average PCE. While the average performance of our devices is lower than those using traditional deposition methods, our champion device achieves comparable efficiencies up to 5.5% with values of 1.4 V, 7 mA/cm2, and 0.55 for Voc, Jsc, and FF, respectively (Figure 5B). In all devices, hysteresis exists as is common in perovskite-type solar cells, but hysteresis is significantly diminished in our champion device. Despite comparable overall performance to typical deposition methods in champion devices, the values for FF remained low. One possible explanation for low FF is the high percentage of residual carbon (50% as estimated from XPS) from oxidation products and native capping ligands on the film surface postannealing, leading to increased series resistance. The comparable voltages and currents achieved with only partial organic material removal show that perovskite deposition through QD annealing is a promising platform for device assembly. Our deposition method is similar to the efforts by Akkerman and co-workers featuring semisuspended CsPbBr3 nanocrystal inks synthesized at room temperature using low boiling point organics.27 Devices assembled this way have significantly lower relative carbon content and possess superior FF as all organics used in the synthesis can evaporate at low temperatures. However, the room-temperature reaction keeps CsPbBr3 in the orthorhombic perovskite phase. Additionally, nanocrystals appear to be only semisuspended in organic solvent, while when passivated with typical longchained capping ligands, QDs are fully suspended in nonpolar solvent for months after synthesis. This provides the potential advantage of long-term storage prior to deposition. In the future, the best parts of both approaches could be achieved by adjusting the QD synthesis to use lower-boiling-point ligands that do not boil at the reaction temperature (170 °C) but evaporate within the annealing temperature similar to 1-ODE.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Prashant V. Kamat: 0000-0002-2465-6819 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. This article is contribution number NDRL no. 5185 from the Notre Dame Radiation Laboratory. During the temperature-controlled TEM experiments, the authors would like to acknowledge the NDIIF TEM core facility for free use of the Titan transmission electron microscope, DENS Solutions Inc. for providing the DENS wildfire heating holder and nanochip, and Dr. Sergei Rouvimov for assistance during the experiment.

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REFERENCES

(1) Sum, T. C.; Mathews, N. Advancements in Perovskite Solar Cells: Photophysics behind the Photovoltaics. Energy Environ. Sci. 2014, 7 (8), 2518−2534. (2) Lewis, N. S. Research Opportunities to Advance Solar Energy Utilization. Science 2016, 351 (6271), 353−363. (3) Hodes, G. Perovskite-Based Solar Cells. Science 2013, 342 (6156), 317−318. (4) Heo, J. H.; Im, S. H. Highly Reproducible, Efficient HysteresisLess CH3NH3PbI3-xClx Planar Hybrid Solar Cells without Requiring Heat-Treatment. Nanoscale 2016, 8 (5), 2554−2560. (5) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131 (17), 6050−6051. (6) NREL. Best Research-Cell Efficiencies. http://www.nrel.gov/ ncpv/images/efficiency_chart.jpg. (7) Manser, J. S.; Christians, J. A.; Kamat, P. V. Intriguing Optoelectronic Properties of Metal Halide Perovskites. Chem. Rev. 2016, 116 (21), 12956−13008. (8) Ponseca, C. S.; Savenije, T. J.; Abdellah, M.; Zheng, K.; Yartsev, A.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; et al. Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-Long Balanced Mobilities, and Slow Recombination. J. Am. Chem. Soc. 2014, 136 (14), 5189−5192. (9) Yin, W.-J. W.; Yang, J.-H. J.; Kang, J.; Yan, Y.; Wei, S.-H. Halide Perovskite Materials for Solar Cells: A Theoretical Review. J. Mater. Chem. A 2015, 3 (17), 8926−8942. (10) Yin, W. J.; Shi, T.; Yan, Y. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Adv. Mater. 2014, 26 (27), 4653−4658. (11) Feng, J.; Xiao, B. Crystal Structures, Optical Properties, and Effective Mass Tensors of CH3NH3PbX3 (X = I and Br) Phases Predicted from HSE06. J. Phys. Chem. Lett. 2014, 5 (7), 1278−1282.

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CONCLUSION In summary, an alternative method for CsPbBr3 perovskite deposition that relies on the layer-by-layer deposition and subsequent annealing of CsPbBr3 QDs was further explored as a potential way to assemble CsPbBr3 solar cells. The mechanism of the transformation from QDs to bulk was found through SEM and in situ TEM experiments. As opposed to QD sintering that was initially assumed, QD growth was found to resume at the annealing temperature with defocusing processes dominating. A loss of organic material during the film annealing was determined to be primarily from evaporation of 1-ODE, with the native capping ligands and oxidation products of 1-ODE remaining postannealing. Planar CsPbBr3 solar cells made using this deposition method were optimized by varying the QD solution concentration as well as active layer thickness; champion devices had comparable PCE to similar devices made via typical deposition routes. Further optimization may be achieved by tuning the QD reaction organics to evaporate during annealing.

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and postannealing, an extended TGA-DSC temperature range of CsPbBr3 QDs, TGA-DSC of organics under nitrogen atmosphere, cross-sectional SEM of devices using different CsPbBr3 QD concentrations as well as varied CsPbBr3 thickness, and light absorbance compared to average current density at varied device thickness (PDF) Sequential temperature-controlled TEM images depicting QD growth (AVI)

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03751. The organic skeleton of the TEM experiment, a table of relative elemental composition from XPS of films pre9772

DOI: 10.1021/acs.chemmater.7b03751 Chem. Mater. 2017, 29, 9767−9774

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DOI: 10.1021/acs.chemmater.7b03751 Chem. Mater. 2017, 29, 9767−9774