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 explos...

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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 border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<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, &#62; 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, &#8805;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 
			Recommended storage temperature 2/8 &#176;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 &#177; 0.2mm, thickness 2.3mm &#177; 0.1mm; FTO: dimensions 3000&#197; &#177; 100&#197;, resistivity 7-10 ohms/sq, transmission 82% @ 550nm)</td>
		</tr>
		<tr>
			<td>Glass substrates</td>
			<td>C &#38; 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&#34;)</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&#945; 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, ...

the overall goal of this process is to synthesize homogenous pinhole-free and high quality methylammonium lead halide perovskite films over the full halide composition space for application in optoelectronics such as solar cells, lasers and LEDs. This synthetic protocol is designed such that it can be adapted to different laboratories and it provides a synthesis route to make faster and reproducible lead halide perovskites over the full highlight composition space. The main advantage of this technique is the use of low processing temperature and standard equipment available in most laboratories such as, fume hoods and schlenk lines.Demonstrating the procedure will be Aya Buckley and Phuong Ngo, students from UC Berkeley. To begin this procedure, add 100 milliliters of ethanol to a 250 milliliter round bottom flask equipped with the stir bar. Add 16.5 milliliters of 40%methylamine in water.Using an ice bath, cool the flask to zero degree celsius. Add 10 milliliters of 76 millimolar hydroiodic acid drop wise, while stirring. Then, seal the flask with a septum.Stir the reaction for two hours at zero degree celsius. Then, remove the flask from the ice bath using a rotary evaporator equipped with a water bath, evaporate the solvent in unreacted volatile components at 50 Torr and 60 degree celsius for four hours or until the volatiles are removed. Once the solvent has evaporated, add 100 milliliters of warm ethanol to the flask to dissolve the residual material.Slowly add 200 milliliters of diethyl ether to induce the crystallization of a white solid. After this, vacuum filter the mixture over a 50 millimeter glass frit filter. Recover the supernatant and add 200 milliliters of diethyl ether to induce additional crystallization.Vacuum filter the mixture again over the 50 millimeter glass frit filter. While vacuum filtering, wash the solid three times with diethyl ether. Dry the washed solid under vacuum.When cleaning the apparatus, the quality of starting material is also important. Therefore, careful recrystallization of the material volume precursor is particularly important. Store the dried solid in the desiccator in the dark at room temperature, to minimize decomposition.To begin, preheat a silicone oil bath equipped with a magnetic stir bar to 120 degree celsius. Next, use a weighing paper cylinder to transfer 0.1 grams of methylammonium halide to a 50 milliliter schlenk tube. To precondition the methylammonium halide, attach the tube to a schlenk line equipped with the rotary pump and evacuate the tube.Adjust the pressure to 0.185 Torr. Then, immerse the tube in the silicone bath for two hours while stirring the bath at 600 rpm. After this, remove the schlenk tube from the oil bath.Leave the tube under an overpressure of flowing nitrogen gas, to prevent moisture intake. Then make the precursor solutions by dissolving 0.146 grams lead bromide in one milliliter DMF, and then 0.369 grams of lead iodide and 0.073 grams of lead bromide in one milliliter of DMF. Sonicate these for approximately five minutes at 35 kilohertz to fully dissolve the precursor.Using a 0.2 micron PTFE filter, filter the precursor solution. Heat the filtered solution to 110 degree celsius for five minutes. Using a micropipette, deposit 80 microliters of heated lead halide precursor solution onto the substrate.Then, spin the substrate at 500 rpm for five seconds with an acceleration rate of 500 rpm/s and at 1500 rpm, for three minutes with an acceleration rate 1500 rpm.s. Next, transfer the substrate onto a hot plate in a fume hood. Dry the precursor film at 110 degree celsius for 15 minutes under flowing nitrogen gas.Load the dried substrate into the prepared schlenk tube orienting the lead halide surface away from the methylammonium halide in the tube. Adjust the pressure in the schlenk tube to 0.185 Torr. Then, immerse the tube in the silicone oil bath heated to 120 degree celsius for two hours.After this, remove the tube from the oil bath. Remove the sample from the schlenk tube and quickly dip it into a beaker containing isopropyl alcohol. Use an N2 gun to immediately dry the rinsed sample.In this study, pinhole-free methylammonium lead halide thin films are synthesized by LP vasp and subsequently characterized. NMR analysis of the precursors shows methyl group shifts at delta 2.35 pbm and delta 2.37 pbm. An ammonium shift centered at delta 7.65 pbm and delta 7.45 pbm, for methylammonium bromide and methylammonium iodide respectively, confirming their identity and purity.Scanning electron microscopy is then used to analyze mixed flat halide films and kneeled in 100%50%and 30%methylammonium iodide. Each of the faceted films is seen to be pinhole free with grain size up to 700 nanometers. While the standard thickness of the perovskite films is about 400 nanometers, the thickness can be changed by varying the spin coating rotational speed with higher speeds yielding thinner films and vice versa.X ray diffraction patterns are then collected to confirm phase purity and conversion of the precursors to methylammonium lead halide perovskite films. Residual or not converted lead iodide phase is seen to show a peak at approximately 12.7 degrees. The methylammonium lead halide films are seen films to exhibit a shift to higher angles due to a gradual replacement of the larger iodine atoms by smaller bromide atoms.While attempting this procedure, please keep in mind that the use of a schlenk line includes explosion and implosion risks. So, before you perform this experiment, please check the integrity of the apparatus. The development of this technique paves the way for researchers in the field of thin film solar cells, as well as, optoelectronics to synthesize high quality organometal highlight perovskite thin films.With this process, you can make thin film solar cells in planar geometry, reaching efficiencies of up to 19%and they show enhanced long term stability. After watching this video, you should have a good understanding of how to synthesize homogenous, phenofree, high optoelectronic quality, material volume lead halide film by LP vasp.

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Research

JoVE Journal

Chemistry

9.4K Views.  Lawrence Berkeley National Laboratory. the overall goal of this process is to synthesize homogenous pinhole-free and high quality methylammonium lead halide perovskite films over the full halide composition space for application in optoelectronics such as solar cells, lasers and LEDs. This synthetic protocol is designed such that it can be adapted to different laboratories and it provides a synthesis route to make faster and reproducible lead halide perovskites over the full highlight composition space. The main advantage of this ...