Perovskite process development, thin film synthesis, structural and morphological characterization were performed at the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. C.M.S.-F. acknowledges financial support from the Swiss National Science Foundation (P2EZP2_155586).

Caution: Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in these syntheses are acutely toxic, carcinogenic, and toxic to reproduction. Implosion and explosion risks are associated with the use of a Schlenk line. Please make sure to check the integrity of the glass apparatus before performing the procedure. Incorrect use of the Schlenk line in association with a liquid nitrogen cold trap may result in condensation of liquid oxygen (pale blue) that can become explosive. Please ensure to receive appropriate on the job training by experts before using vacuum-systems, Schlenk lines, and cryogenic liquids. Please use all appropriate safety practices when performing the synthesis including the use of engineering controls (fume hood) and personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes). All of the following procedures described below are performed in a fume hood in air, unless stated differently.
1. Preparation of Methylammonium Halide
<ol>
	<li>To a 250 mL round bottom flask equipped with a stir bar, add ethanol (100 mL) and methylamine (190 mmol, 16.5 mL, 40%wt in H2O), and cool the flask to 0 &#176;C with an ice bath.</li>
	<li>While the methylamine solution is stirring (for about 5 min at 600 revolution per min (rpm)), add HI (76 mmol, 10 mL, 57%wt in H2O) or HBr (76 mmol, 8.6 mL, 48%wt in H2O) dropwise, and seal the flask with a septum.</li>
	<li>Allow the reaction to stir for 2 h at 0 &#730;C.</li>
	<li>Remove the reaction flask from the ice bath and evaporate the solvent and unreacted volatile components at reduced pressure (~50 Torr) with a rotary evaporator equipped with a water bath at 60 &#730;C for 4 h or until the volatiles are removed.</li>
	<li>To recrystallize the resulting solid, add warm (~50 &#176;C) ethanol (100 mL) and dissolve the residual material.</li>
	<li>Slowly add diethyl ether (200 mL) to induce crystallization of a white solid.</li>
	<li>Vacuum filter the mixture over a coarse 50 mm glass frit filter.</li>
	<li>Recover the supernatant and add diethyl ether (200 mL) to induce additional crystallization of white solid. Vacuum filter the mixture over a second coarse 50 mm glass frit filter.</li>
	<li>Combine the white solids on a coarse 50 mm glass frit filter and, while vacuum filtering, wash the resulting powder with diethyl ether three times (~30 mL each time).</li>
	<li>Dry the white solid under vacuum. This procedure yields (58.9 mmol, 9.360 g, 77%) of methylammonium iodide (CH3NH3I) and (55.5 mmol, 6.229 g, 73%) of methylammonium bromide (CH3NH3Br).</li>
	<li>Store in the dark and in a desiccator at room temperature in order to minimize decomposition over time.</li>
</ol>
2. Preparation of Methylammonium Lead Halide (CH3NH3PbI3-xBrx ) Thin Films6,20
<ol>
	<li>Pre-conditioning of the Schlenk tube
	<ol>
		<li>Load a 50 mL Schlenk tube (diameter 2.5 cm) with 0.1 g of methylammonium halide. To prevent the chemicals sticking to the walls of the test tube, use a weighing paper cylinder to transfer the methylammonium halide into the tube. 
		NOTE: The final ratio of I/(I+Br) in CH3NH3PbI3-xBrx is determined by the methylammonium halide composition in the test tube. For example, to achieve 30% I content, the Schlenk tube is loaded with 0.03 g CH3NH3I and 0.07 g CH3NH3Br. Actual obtained compositions may vary with experimental setup, so calibration of the synthesis conditions to yield specific target compositions is necessary. In the present case, this was accomplished by measuring halide content in the synthesized films via energy dispersive X-ray spectroscopy (EDX).</li>
		<li>Use a Schlenk line equipped with a rotary pump to connect and evacuate the tube. Adjust the pressure to 0.185 Torr. Then, immerse the test tube in a silicone oil bath pre-heated to 120&#176;C, with a magnetic stirrer (600 rpm) for 2 h (pre-conditioning of the Schlenk tube). 
		NOTE: This step allows for sublimation of the methylammonium precursor along the sides of the Schlenk tube. It is important to ensure the sublimation of the methylammonium precursor during the two hours of pre-conditioning. A thin layer of methylammonium precursor will condense along the sides of the Schlenk tube to cover the lower half of the tube. If sublimation of the methylammonium precursor is not observed or is happening too fast, check if the pressure of the Schlenk line and the temperature of the oil bath are correct, or try to use fresh methylammonium halide precursor.&#160;</li>
		<li>Remove Schlenk tube from the oil bath and leave methylammonium halide under an overpressure of flowing N2 to avoid moisture intake.</li>
	</ol>
	</li>
	<li>Substrate preparation
	<ol>
		<li>Sonicate one substrate (glass or FTO coated glass, 14 x 16 mm2) with water (~3 mL) containing detergent for 15 min in a test tube (diameter 1.5 cm and height 15 cm) at 35 KHz.</li>
		<li>Discard the detergent/water by rinsing with ultrapure water (~10 mL) 5 times.</li>
		<li>Discard the ultrapure water, add acetone (~3 mL), and sonicate for 15 min at 35 KHz.</li>
		<li>Discard the acetone, add isopropanol (~3 mL), and sonicate for 15 min at 35 KHz.</li>
		<li>Discard the isopropanol, recover the substrate from the test tube with tweezers, and dry it with a N2 gun for 15 s.</li>
		<li>Deposit TiO2 compact layer (100 nm) on FTO glass substrates by electron beam evaporation at a substrate temperature of 350 &#176;C, and a deposition rate of 0.5 &#197;/s using substrate rotation.21</li>
	</ol>
	</li>
	<li>Preparation of the lead halide precursor solution
	<ol>
		<li>For the preparation of MAPbI3-xBrx (0 &#60; x &#60; 3), dissolve PbI2 (0.8 mmol, 0.369 g) and PbBr2 (0.2 mmol, 0.073 g) in 1 mL DMF to achieve a final concentration of 0.8 M of PbI2 and 0.2 M of PbBr2. Sonicate for 5 min at 35 KHz to fully dissolve the precursor.
		<ol>
			<li>For the preparation of pure iodine or bromine films, dissolve PbI2 (1 mmol, 0.461 g) or PbBr2 (0.8 mmol, 0.294 g) in 1 mL DMF, to achieve a final concentration of 1 M and 0.8 M, respectively. Sonicate for 5 min at 35 KHz to fully dissolve the precursor.</li>
			<li>For the preparation of chlorine-doped methylammonium lead iodide perovskite films, dissolve PbI2 (0.369 g) and PbCl2 (0.056 g) in 1 mL DMF, to achieve a final concentration of 0.8 M of PbI2 and 0.2 M of PbCl2. Sonicate for 5 min at 35 KHz to fully dissolve the precursor.</li>
		</ol>
		</li>
		<li>Filter the precursor solution with a 0.2 &#956;m polytetrafluorethylene (PTFE) filter.</li>
	</ol>
	</li>
	<li>Lead halide deposition
	<ol>
		<li>Pre-heat the precursor solution on a hot plate set to 110 &#176;C for 5 min.</li>
		<li>With a micropipette, drop 80 &#956;L of the pre-heated lead halide precursor solution onto the non-rotating substrate (glass or TiO2 deposited on FTO coated glass; 14 x 16 mm2 size). Spin at 500 rpm for 5 s with an acceleration rate of 500 rpm s-1, and 1500 rpm for 3 min with an acceleration rate of 1,500 rpm s-1.</li>
		<li>In a fumehood, dry precursor film for 15 min at 110 &#176;C on a hot plate under flowing N2. 
		NOTE: A crystalizing dish is used and placed over the substrate to allow the precursor to dry in a N2 atmosphere. To vary the thickness of the resulting perovskite film, the speed of the second spin-coating step can be varied from 1,200 to 12,000 rpm to achieve film thickness in the range of 500 to 100 nm. To further decrease the film thickness, diluted precursor solution can be used.</li>
	</ol>
	</li>
	<li>Vapor-assisted annealing
	<ol>
		<li>Load sample into the Schlenk tube (prepared according to instructions in section 2.1.2). Adjust the pressure to 0.185 Torr. 
		NOTE: The sample sits in the test tube above the methylammonium halide without being in direct contact with it. To slow down incorporation of methylammonium, the lead halide surface is oriented to face away from the methylammonium halide.</li>
		<li>Immerse the Schlenk tube loaded with the sample in silicone oil bath heated to 120 &#176;C for 2 h.</li>
		<li>Take out the sample and quickly rinse it by dipping it in a beaker containing isopropyl alcohol. Immediately dry the rinsed sample with a N2 gun. 
		NOTE: To prepare pure CH3NH3PbI3 use PbI2 as the halide precursor and pure methylammonium iodide in the vapor-assisted annealing step. To prepare CH3NH3PbBr3 use PbBr2 as the halide precursor and pure methylammonium bromide in the vapor-assisted annealing step.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

In order to fabricate highly efficient organo-lead planar perovskite heterojunctions, the homogeneity of the active layer is a key requirement. With respect to existing solution2,16,17 and vacuum-based18,19 methodologies, our process is remarkably amenable to composition tunability of the active layer that can be synthesized over the full CH3NH3PbI<...

Hybrid organic-inorganic lead halide perovskites (CH3NH3PbX3, X = I, Br, Cl) are a new class of semiconductors that has emerged rapidly within the last few years. This material class shows excellent semiconductor properties, such as high absorption coefficient1, tunable bandgap2, long charge carrier diffusion length3, high defect tolerance4, and high photoluminescence quantum yield5,6. The unique combination of these characteristics makes lead halide perovskites very attractive for application in optoelectronic devices, such as single junction7,8 and multijunction photovoltaics9,10, lasers11,12, and LEDs13.
CH3NH3PbX3 films can be fabricated by a variety of synthetic methods14, which aim at improving the efficiency of this semiconducting material for energy applications15. However, optimization of photovoltaic devices relies on the quality of the halide perovskite active layer, as well as its interfaces with charge selective contacts (i.e. electron and hole transport layers), which facilitate photocarrier collection in these devices. Specifically, continuous, pinhole-free active layers are necessary to minimize shunt resistance, thereby improving device performance.
Among the most widespread methods for fabricating organo-lead halide perovskite thin films are solution-based and vacuum-based processes. The most common solution process uses equimolar ratios of lead halide and methylammonium halide dissolved in dimethylformamide (DMF), dimethylsulfoxide (DMSO), or &#947;-butyrolactone (GBL), or mixtures of these solvents.2,16,17 Precursor molarity and solvent type, as well as annealing temperature, time and atmosphere, must be precisely controlled to obtain continuous and pinhole-free films.16 For example, to improve surface coverage, a solvent-engineering technique was demonstrated to yield dense and extremely uniform films.17 In this technique, a non-solvent (toluene) is dripped onto the perovskite layer during the spinning of the perovskite solution.17 These approaches are usually well suited for mesoscopic heterojunctions, which employ mesoporous TiO2 as an electron selective contact with increased contact area and reduced carrier transport length.
However, planar heterojunctions, which use selective contacts based on thin (usually TiO2) films, are more desirable because they provide a simple and scalable configuration that can be more easily adopted in solar cell technology. Therefore, the development of organo-lead halide perovskite active layers that show high efficiency and stability under operation for planar heterojunctions may lead to technological advancements in this field. However, one of the main challenges to fabricate planar heterojunctions is still represented by the homogeneity of the active layer. A few attempts, based on vacuum processes, have been made to prepare uniform layers on thin TiO2 films. For example, Snaith and collaborators have demonstrated a dual evaporation process, which yield highly homogenous perovskite layers with high power conversion efficiencies for photovoltaic applications.18 While this work represents a significant advancement in the field, the use of high vacuum systems and the lack of tunability of the composition of the active layer limit the applicability of this method. Interestingly, extremely high uniformity has been achieved with the vapor-assisted solution process (VASP)19 and modified low pressure VASP (LP-VASP)6,20. While the VASP, proposed by Yang and collaborators19, requires higher temperatures and the use of a glove box, the LP-VASP is based on the annealing of a lead halide precursor layer in the presence of methylammonium halide vapor, at reduce pressure and relatively low temperature in a fumehood. These specific conditions enable access mixed perovskite compositions, and fabrication of pure CH3NH3PbI3, CH3NH3PbI3-xClx, CH3NH3PbI3-xBrx, and CH3NH3PbBr3 can be easily achieved. Specifically, CH3NH3PbI3-xBrx films over the full composition space can be synthesized with high optoelectronic quality and reproducibility6,20.
Herein, we provide a detailed description of the protocol for the synthesis of organic-inorganic lead halide perovskite layers via LP-VASP, including the procedure for synthesizing the methylammonium halide precursors. Once the precursors are synthesized, formation of CH3NH3PbX3 films consists of a two-step procedure that comprises i) the spin-coating of the PbI2/PbBr2 (PbI2, or PbI2/PbCl2) precursor on glass substrate or fluorine-doped tin oxide (FTO) coated glass substrate with planar TiO2, as electron transport layer, and ii) the low pressure vapor-assisted annealing in mixtures of CH3NH3I and CH3NH3Br that can be finely adjusted depending on the desired optical bandgap (1.6 eV&#160;&#8804; Eg &#8804; 2.3 eV). Under these conditions, the methylammonium halide molecules present in the vapor phase slowly diffuse into the lead halide thin film yielding continuous, pinhole-free halide perovskite films. This process yields a two-fold volume expansion from the starting lead halide precursor layer to the completed organic-inorganic lead halide perovskite. The standard thickness of the perovskite film is about 400 nm. It is possible to vary this thickness between 100-500 nm by changing the speed of the second spin coating step. The presented technique results in films of high optoelectronic quality, which translates to photovoltaic devices with power conversion efficiencies of up to 19% using a Au/spiro-OMeTAD /CH3NH3PbI3-xBrx/compact TiO2/ FTO/glass solar cell architecture.21

<table><tbody><tr><td>Lead (II) bromide, 99.999%</td><td>Sigma-Aldrich</td><td>398853</td><td>Acute toxicity, Carcinogenicity</td></tr><tr><td>Lead (II) Iodide, 99.9985%</td><td>Alfa Aesar</td><td>12724</td><td>Acute toxicity, light sensitive</td></tr><tr><td>N, N-Dimethylformamide, &gt; 99.9%</td><td>Sigma-Aldrich</td><td>270547</td><td>Acute toxicity, flamable; store in well ventilated place</td></tr><tr><td>Isopropyl alcohol, 99.5%</td><td>BDH</td><td>BDH1133-4LP</td><td>Flamable</td></tr><tr><td>Methylamine ca. 40% in water</td><td>TCI</td><td>M0137</td><td>Acute toxicity, flamable; Corrosive</td></tr><tr><td>Hydrobromic acid 48 wt. % in H2O, &amp;ge;99.99%</td><td>Sigma-Aldrich</td><td>339245</td><td>Acute toxicity, Corrosive; air and light sensitive; store in well ventilated place</td></tr><tr><td>Hydroiodic acid 57 wt. % in H2O, distilled, stabilized, 99.95%</td><td>Sigma-Aldrich</td><td>210021</td><td>Corrosive; air and light sensitive; store in well ventilated place&lt;br/&gt; Recommended storage temperature 2/8 &amp;deg;C; air and light sensitiv</td></tr><tr><td>Ethyl Ether Anhydrous BHT Stabilized/Certified ACS</td><td>Fisher Chemicals</td><td>E 138-4</td><td>Acute toxicity, flamable</td></tr><tr><td>Ethanol Denatured (Reagent Alcohol), ACS</td><td>BDH</td><td>BDH1156-4LP</td><td>Flamable</td></tr><tr><td>Alconoxdetergent</td><td>Sigma-Aldrich</td><td>242985</td><td>Soap utilized for substrate cleaning</td></tr><tr><td>Milli-QIntegral 3 Water Purification System</td><td>EMD Millipore</td><td>ZRXQ003WW</td><td>Dispenser of ultrapure water</td></tr><tr><td>Fluorine-doped Thin Oxide (FTO) coated glass</td><td>Thin Film Devices</td><td>Custom</td><td>Glass: dimensions 13.8mm x 15.8mm &amp;plusmn; 0.2mm, thickness 2.3mm &amp;plusmn; 0.1mm; FTO: dimensions 3000&amp;Aring; &amp;plusmn; 100&amp;Aring;, resistivity 7-10 ohms/sq, transmission 82% @ 550nm)</td></tr><tr><td>Glass substrates</td><td>C &amp;amp; A Scientific - Premiere</td><td>9101-E</td><td>Plain. Length: 75 mm, Width: 25 mm, Thickness: 1 mm</td></tr><tr><td>Ultrasonic Cleaner with Digital Timer and Heater</td><td>VWR</td><td>97043-992</td><td>2.8 L (0.7 gal.)24L x 14W x 10D cm (97/16x 51/2x 315/16&quot;)</td></tr><tr><td>Nuclear Magnetic Resonance Advance 500</td><td>Bruker</td><td>Z115311</td><td></td></tr><tr><td>Quanta 250 FEG Scanning Electron Microscope</td><td>FEI</td><td>743202032141</td><td>Equipped with a Bruker Xflash 5030 Energy-dispersive X-ray detector</td></tr><tr><td>SmartLab X-ray diffractometer</td><td>Rigaku</td><td>2080B411</td><td>Using Cu K&amp;alpha; radiation at 40 kV and 40 mA</td></tr></tbody></table>

low pressure vapor-assisted solution process for tunable band gap pinhole-free methylammonium lead halide perovskite films

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, we present a protocol for the fabrication of this material via the low-pressure vapor assisted solution process (LP-VASP) method, which yields ~19% power conversion efficiency in planar heterojunction perovskite solar cells. First, we report the synthesis of methylammonium iodide (CH3NH3I) and methylammonium bromide (CH3NH3Br) from methylamine and the corresponding halide acid (HI or HBr). Then, we describe the fabrication of pinhole-free, continuous methylammonium-lead halide perovskite (CH3NH3PbX3 with X = I, Br, Cl and their mixture) films with the LP-VASP. This process is based on two steps: i) spin-coating of a homogenous layer of lead halide precursor onto a substrate, and ii) conversion of this layer to CH3NH3PbI3-xBrx by exposing the substrate to vapors of a mixture of CH3NH3I and CH3NH3Br at reduced pressure and 120 &#176;C. Through slow diffusion of the methylammonium halide vapor into the lead halide precursor, we achieve slow and controlled growth of a continuous, pinhole-free perovskite film. The LP-VASP allows synthetic access to the full halide composition space in CH3NH3PbI3-xBrx with 0&#160;&#8804; x&#160;&#8804; 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV&#160;&#8804; Eg&#160;&#8804; 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH3NH3PbI3-xClx. Films obtained from the LP-VASP are reproducible, phase pure as confirmed by X-ray diffraction measurements, and show high photoluminescence quantum yield. The process does not require the use of a glovebox.

Proton nuclear magnetic resonance (NMR) spectra were taken after the methylammonium halide synthesis to verify the molecule purity (Figure 1). Scanning electron microscopy (SEM) images were acquired before and after vapor annealing (Figure 2) to characterize the morphology and the homogeneity of both the mixed lead halide precursor and the CH3NH3PbI3-xBrx films. X-ray diffraction (XRD) ...

Watch this Scientific Journal Video about Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films at JoVE.com

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Here, we present a protocol for the synthesis of CH3NH3I and CH3NH3Br precursors and the subsequent formation of pinhole-free, continuous CH3NH3PbI3-xBrx thin films for the application in high efficiency solar cells and other optoelectronic devices.

Organo-lead halide perovskites have recently attracted great interest for potential applications in thin-film photovoltaics and optoelectronics. Herein, ...

low-pressure-vapor-assisted-solution-process-for-tunable-band-gap

Research

JoVE Journal

Chemistry

9.5K Views.  Lawrence Berkeley National Laboratory. Hier presenteren we een protocol voor de synthese van33NH CH I en CH3NH3Br precursoren en de daaropvolgende vorming van pinhole-vrije, continue CH3NH3PbI3-xBrx dunne lagen voor de toepassing in hoge efficiëntie zonnecellen en andere opto-elektronische apparaten.

Video: Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Template Directed Synthesis of Plasmonic Gold Nanotubes with Tunable IR Absorbance

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) membranes have been under development since the 1990's and have become a common method to template the synthesis of high aspect ratio nanostructures, mostly by electrochemical growth or pore-wetting. Recently, these membranes have become commercially available in a wide range of pore sizes and densities, leading to an extensive library of functional nanostructures being synthesized from AAO membranes. These include composite nanorods, nanowires and nanotubes made of metals, inorganic materials or polymers 3-10. Nanoporous membranes have been used to synthesize nanoparticle and nanotube arrays that perform well as refractive index sensors, plasmonic biosensors, or surface enhanced Raman spectroscopy (SERS) substrates 11-16, as well as a wide range of other fields such as photo-thermal heating 17, permselective transport 18, 19, catalysis 20, microfluidics 21, and electrochemical sensing 22, 23. Here, we report a novel procedure to prepare gold nanotubes in AAO membranes. Hollow nanostructures have potential application in plasmonic and SERS sensing, and we anticipate these gold nanotubes will allow for high sensitivity and strong plasmon signals, arising from decreased material dampening 15.

Solution-suspendable gold nanotubes with controlled dimensions can be synthesized by electrochemical deposition in porous anodic aluminum oxide (AAO) membranes using a hydrophobic polymer core. Gold nanotubes and nanotube arrays hold promise for applications in plasmonic biosensing, surface-enhanced Raman spectroscopy, photo-thermal heating, ionic and molecular transport, microfluidics, catalysis and electrochemical sensing.

A nearly parallel array of pores can be produced by anodizing aluminum foils in acidic environments1, 2. Applications of anodic aluminum oxide (AAO) ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Engineering

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Flash Infrared Annealing for Perovskite Solar Cell Processing

Organic&#8211;inorganic perovskites have an impressive potential for the design of next generation solar cells and are currently considered for upscaling and commercialization. Currently, perovskite solar cells rely on spin-coating which is neither practical for large areas nor environmentally friendly. Indeed, one of the conventional and most effective lab-scale methods to induce perovskite crystallization, the antisolvent method, requires an amount of toxic solvent that is difficult to apply on larger surfaces. To solve this problem, an antisolvent-free and rapid thermal annealing process called flash infrared annealing (FIRA) can be used to produce highly crystalline perovskite films. The FIRA oven is composed of an array of near-infrared halogen lamps with an illumination power of 3,000 kW/m2. A hollow aluminum body enables an effective water-cooling system. The FIRA method allows the synthesis of perovskite films in less than 2 s, achieving efficiencies &#62;20%. FIRA has a unique potential for the industry because it can be adapted to continuous processing, is antisolvent-free, and does not require lengthy, hour-long annealing steps.

We describe a flash infrared annealing method used for the synthesis of perovskite and mesoscopic-TiO2 films. Annealing parameters are varied and optimized for processing on fluorine-doped tin oxide (FTO) glass and indium tin oxide-coated polyethylene terephthalate (ITO PET), subsequently giving devices power conversion efficiencies &#62;20%.

Organic&#8211;inorganic perovskites have an impressive potential for the design of next generation solar cells and are currently considered for upscaling ...

Environment

A Modular Microfluidic Technology for Systematic Studies of Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light emitting diodes (LEDs) and photovoltaics (PVs). Among this material group, inorganic/organic perovskites have demonstrated significant improvement and potential towards high-efficiency, low-cost PV fabrication due to their high charge carrier mobilities and lifetimes. Despite the opportunities for perovskite QDs in large-scale PV and LED applications, the lack of fundamental and comprehensive understanding of their growth pathways has inhibited their adaptation within continuous nanomanufacturing strategies. Traditional flask-based screening approaches are generally expensive, labor-intensive, and imprecise for effectively characterizing the broad parameter space and synthesis variety relevant to colloidal QD reactions. In this work, a fully autonomous microfluidic platform is developed to systematically study the large parameter space associated with the colloidal synthesis of nanocrystals in a continuous flow format. Through the application of a novel translating three-port flow cell and modular reactor extension units, the system may rapidly collect fluorescence and absorption spectra across reactor lengths ranging 3 - 196 cm. The adjustable reactor length not only decouples the residence time from the velocity-dependent mass transfer, it also substantially improves the sampling rates and chemical consumption due to the characterization of 40 unique spectra within a single equilibrated system. Sample rates may reach up to 30,000 unique spectra per day, and the conditions cover 4 orders of magnitude in residence times ranging 100 ms - 17 min. Further applications of this system would substantially improve the rate and precision of the material discovery and screening in future studies. Detailed within this report are the system materials and assembly protocols with a general description of the automated sampling software and offline data processing.

Detailed herein are the operation and assembly protocols of a modular microfluidic screening platform for the systematic characterization of colloidal semiconductor nanocrystal syntheses. Through fully adjustable system arrangements, highly efficient spectra collection may be carried out across 4 orders of magnitude reaction time scales within a mass transfer-controlled sampling space.

Colloidal semiconductor nanocrystals, known as quantum dots (QDs), are a rapidly growing class of materials in commercial electronics, such as light ...

Solution-Processed "Silver-Bismuth-Iodine" Ternary Thin Films for Lead-Free Photovoltaic Absorbers

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. However, poor surface morphologies and relatively high bandgap energies have limited their potential. Silver-bismuth-iodine (Ag-Bi-I) is a promising semiconductor for optoelectronic devices. Therefore, we demonstrate the fabrication of Ag-Bi-I ternary thin films using material solution processing. The resulting thin films exhibit controlled surface morphologies and optical bandgaps according to their thermal annealing temperatures. In addition, it has been reported that Ag-Bi-I ternary systems crystallize to AgBi2I7, Ag2BiI5, etc. according to the ratio of the precursor chemicals. The solution-processed AgBi2I7 thin films exhibit a cubic-phase crystal structure, dense, pinhole-free surface morphologies with grains ranging in size from 200 to 800 nm, and an indirect bandgap of 1.87 eV. The resultant AgBi2I7 thin films show good air stability and energy band diagrams, as well as surface morphologies and optical bandgaps suitable for lead-free and air-stable single-junction solar cells. Very recently, a solar cell with 4.3% power conversion efficiency was obtained by optimizing the Ag-Bi-I crystal compositions and solar cell device architectures.

Herein, we present detailed protocols for solution-processed silver-bismuth-iodine (Ag-Bi-I) ternary semiconductor thin films fabricated on TiO2-coated transparent electrodes and their potential application as air-stable and lead-free optoelectronic devices.

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. ...

Inkjet Printing All Inorganic Halide Perovskite Inks for Photovoltaic Applications

A method for synthesizing photoactive inorganic&#160;perovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are demonstrated. The ink synthesis is based on a simple wet chemical reaction and the inkjet printing protocol is a facile step by step method. The inkjet printed thin films have been characterized by X-ray diffraction, optical absorption spectroscopy, photoluminescent spectroscopy, and electronic transport measurements. X-ray diffraction of the printed quantum dot films indicates a crystal structure consistent with an orthorhombic room temperature phase with (001) orientation. In conjunction with other characterization methods, the X-ray diffraction&#160;measurements show high quality films can be obtained through the inkjet printing method.

A protocol for synthesizing inorganic-lead-halide hybrid perovskite quantum dot inks for inkjet printing and the protocol for preparing and printing the quantum dot inks in an inkjet printer with post characterization techniques are presented.

A method for synthesizing photoactive inorganic&#160;perovskite quantum dot inks and an inkjet printer deposition method, using the synthesized inks, are ...

Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation

In this work, we demonstrate a facile method for colloidal lead halide perovskite nanoplatelet synthesis (Chemical formula: L2[ABX3]n-1BX4, L: butylammonium and octylammonium, A: methylammonium or formamidinium, B: lead, X: bromide and iodide, n: number of [BX6]4- octahedral layers in the direction of nanoplatelet thickness) via ligand-assisted reprecipitation. Individual perovskite precursor solutions are prepared by dissolving each nanoplatelet constituent salt in N,N-dimethylformamide (DMF), which is a polar organic solvent, and then mixing in specific ratios for targeted nanoplatelet thickness and composition. Once the mixed precursor solution is dropped into nonpolar toluene, the abrupt change in the solubility induces the instantaneous crystallization of nanoplatelets with surface-bound alkylammonium halide ligands providing colloidal stability. Photoluminescence and absorption spectra reveal emissive and strongly quantum-confined features. X-ray diffraction and transmission electron microscopy confirm the two-dimensional structure of the nanoplatelets. Furthermore, we demonstrate that the band gap of perovskite nanoplatelets can be continuously tuned in the visible range by varying the stoichiometry of the halide ion(s). Lastly, we demonstrate the flexibility of the ligand-assisted reprecipitation method by introducing multiple species as surface-capping ligands. This methodology represents a simple procedure for preparing dispersions of emissive 2D colloidal semiconductors.

This work demonstrates facile room-temperature synthesis of colloidal quantum-confined lead halide perovskite nanoplatelets by ligand-assisted reprecipitation method. Synthesized nanoplatelets show spectrally narrow optical features and continuous spectral tunability throughout the visible range by varying the composition and thicknesses.

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

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