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

Podsumowanie

Here, we present a protocol for the synthesis of CH₃NH₃I and CH₃NH₃Br precursors and the subsequent formation of pinhole-free, continuous CH₃NH₃PbI_3-xBr_x thin films for the application in high efficiency solar cells and other optoelectronic devices.

Streszczenie

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 CH₃NH₃PbI_3-xBr_x by exposing the substrate to vapors of a mixture of CH₃NH₃I and CH₃NH₃Br at reduced pressure and 120 °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 CH₃NH₃PbI_3-xBr_x with 0 ≤ x ≤ 3. Depending on the composition of the vapor phase, the bandgap can be tuned between 1.6 eV ≤ E_g ≤ 2.3 eV. In addition, by varying the composition of the halide precursor and of the vapor phase, we can also obtain CH₃NH₃PbI_3-xCl_x. 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.

Wprowadzenie

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 yield^5,6. The unique combination of these characteristics makes lead halide perovskites very attractive for application in optoelectronic devices, such as single junction^7,8 and multijunction photovoltaics^9,10, lasers^11,12, 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 γ-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, CH₃NH₃PbI_3-xCl_x, CH₃NH₃PbI_3-xBr_x, and CH₃NH₃PbBr₃ can be easily achieved. Specifically, CH₃NH₃PbI_3-xBr_x films over the full composition space can be synthesized with high optoelectronic quality and reproducibility^6,20.

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 ≤ E_g ≤ 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 /CH₃NH₃PbI_3-xBr_x/compact TiO₂/ FTO/glass solar cell architecture.²¹

Protokół

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

1. 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 °C with an ice bath.
2. 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.
3. Allow the reaction to stir for 2 h at 0 ˚C.
4. 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 ˚C for 4 h or until the volatiles are removed.
5. To recrystallize the resulting solid, add warm (~50 °C) ethanol (100 mL) and dissolve the residual material.
6. Slowly add diethyl ether (200 mL) to induce crystallization of a white solid.
7. Vacuum filter the mixture over a coarse 50 mm glass frit filter.
8. 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.
9. 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).
10. 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).
11. Store in the dark and in a desiccator at room temperature in order to minimize decomposition over time.

2. Preparation of Methylammonium Lead Halide (CH₃NH₃PbI_3-xBr_x ) Thin Films^6,20

1. Pre-conditioning of the Schlenk tube
   1. 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 CH₃NH₃PbI_3-xBr_x 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).
   2. 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°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. 
   3. Remove Schlenk tube from the oil bath and leave methylammonium halide under an overpressure of flowing N₂ to avoid moisture intake.
2. Substrate preparation
   1. 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.
   2. Discard the detergent/water by rinsing with ultrapure water (~10 mL) 5 times.
   3. Discard the ultrapure water, add acetone (~3 mL), and sonicate for 15 min at 35 KHz.
   4. Discard the acetone, add isopropanol (~3 mL), and sonicate for 15 min at 35 KHz.
   5. Discard the isopropanol, recover the substrate from the test tube with tweezers, and dry it with a N₂ gun for 15 s.
   6. Deposit TiO₂ compact layer (100 nm) on FTO glass substrates by electron beam evaporation at a substrate temperature of 350 °C, and a deposition rate of 0.5 Å/s using substrate rotation.²¹
3. Preparation of the lead halide precursor solution
   1. For the preparation of MAPbI_3-xBrx (0 < x < 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.
      1. 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.
      2. 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.
   2. Filter the precursor solution with a 0.2 μm polytetrafluorethylene (PTFE) filter.
4. Lead halide deposition
   1. Pre-heat the precursor solution on a hot plate set to 110 °C for 5 min.
   2. With a micropipette, drop 80 μL 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.
   3. In a fumehood, dry precursor film for 15 min at 110 °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.
5. Vapor-assisted annealing
   1. 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.
   2. Immerse the Schlenk tube loaded with the sample in silicone oil bath heated to 120 °C for 2 h.
   3. 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.

Wyniki

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 CH₃NH₃PbI_3-xBr_x films. X-ray diffraction (XRD) ...

Dyskusje

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 solution^2,16,17 and vacuum-based^18,19 methodologies, our process is remarkably amenable to composition tunability of the active layer that can be synthesized over the full CH₃NH₃PbI<...

Materiały

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