Slot-Die Coating to Make Efficient Wide-Bandgap Perovskite Layers

By Samudrapom DamReviewed by Susha Cheriyedath, M.Sc.Oct 4 2022

A group of researchers recently published a paper in the journal ACS Energy Letters that demonstrated the feasibility of using the slot-die coating method to fabricate triple halide perovskite layers for scalable and efficient manufacturing of perovskite/silicon tandem solar cells.

Study: Slot-Die Coated Triple-Halide Perovskites for Efficient and Scalable Perovskite/Silicon Tandem Solar Cells. Image Credit: Fotografie - Schmidt/Shutterstock.com

Background

The efficiency of tandem solar cells, combining a silicon bottom cell and a tunable halide perovskite top cell with a 1.68 eV wide bandgap, has surpassed the efficiency of Auger recombination limited single-junction silicon solar cells and recently attained a 29.8% power conversion efficiency (PCE) on one cm².

Both perovskite/silicon tandem solar cells and perovskite single-junction solar cells with 25.7% PCE on 0.1 cm² are synthesized by spin coating perovskite layers. However, achieving thin film homogeneity on a large scale using the spin-coating technique is a significant challenge. Specifically, controlling the film thickness from the center towards the edge of the substrate is most difficult with an expanding active area.

Additionally, 90% of the antisolvents and halide perovskites precursor are spun off during the spin coating method, which makes the method environmentally and economically unviable for large-scale perovskite/silicon tandem and perovskite single-junction solar cell production.

Moreover, transferring the high PCEs to commercial, mass-produced, large-area silicon bottom cells is another significant challenge for the mass production of perovskite/silicon tandem solar cells. Spin-coating halide perovskites on 140 μm or less than 140 μm thick and rough silicon wafers is considerably difficult.

Multiple printing techniques, such as slot-die coating, inkjet printing, and blade coating, can be used as alternatives to spin coating for upscaling. However, both inkjet printing and blade coating have several disadvantages, such as slow deposition speed and nozzle clogging.

The slot-die coating process possesses several advantages, including easy cleaning, accurate thickness, and easy operation. Slot-die coater platforms are equipped with a continuous ink supply syringe and an automatic coating table.

Nitrogen gas flows from nitrogen knives are used extensively to induce film formation for controlling the coated wet film drying process. Slot-die coating with nitrogen gas quenching can be a suitable method for low-cost and scalable deposition of halide perovskites as this method can be upscaled to achieve roll-to-roll production.

Identifying an ink composition of a wide bandgap halide perovskite that leads to high PCE, crystal quality, and thermal stability, and no phase segregation is crucial for the halide perovskite coating processes.

Common ink compositions used extensively for perovskite/silicon tandem applications include double-cation perovskites comprising formamidinium (FA⁺) and cesium (Cs⁺) and triple-cation perovskites containing FA⁺, Cs⁺, and methylammonium (MA⁺).

Dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) are typically used as a solvent system for most spin-coating ink compositions due to its high solubility. However, DMSO is not suitable for scaling to nitrogen-quenching assisted large area halide perovskite deposition without antisolvents.

High MA concentration, phase stability, and bandgap are the major drawbacks of perovskite/silicon tandem solar cells prepared using printing techniques until now. Ink systems with wide bandgap compositions and low MA content are crucial for scaling the production of tandem solar cells. 

The Study

In this study, researchers developed a suitable solvent and ink system to fabricate a wide bandgap, efficient triple-halide perovskite layer containing (Cs_0.22 FA_0.78) lead (Pb)-(iodine (I)_0.85 bromine (Br)_0.15)_3, designated as Cs22Br15, + five mol % methylammonium lead chloride (MAPbCl₃) using the slot-die coating method to scale up the production of perovskite/silicon tandem solar cells. The slot-die coated wet halide perovskite was dried by a “nitrogen knife”.

Researchers investigated the formation of halide perovskite films and assessed the influence of the annealing temperature and the drying method on the perovskite film formation. A triple-halide perovskite with antisolvent treatment was prepared using the spin coating method and used as a reference due to its exceptional performance.

The ink system/salt was prepared by adding 0.05 mol of MAPbCl₃ to every mol of Cs22Br15. One mol of total salt was then diluted in 664 µl DMF and 50 µl N-methyl-2-pyrrolidone (NMP) at room temperature.

Perovskite single-junction solar cells with a indium tin oxide (ITO)/hole transport layer (HTL)/perovskite/lithium fluoride (LiF)/buckminsterfullerene (C₆₀)/(bathocuproine (BCP), tin(IV) oxide (SnO₂)/(silver, copper) configuration were prepared.

The ITO-covered glass was used to deposit the inverted planar structure perovskite layer. The HTL layer in the perovskite single-junction solar cells was ([2-(9H-carbazol-9-yl)ethyl]phosphonic acid) (2PACz).

The spin coating method was used to deposit the 2PACz, while the perovskite solution Cs22Br15 + five mol % MAPbCl₃ was deposited using the slot-die coating method under a nitrogen atmosphere. The wet perovskite film was then annealed at different temperatures.

Additionally, silicon heterojunction cells with rear side p/n junction made from 120 μm thick Czochralski thin wafers with no extra mechanical or chemical surface polishing and perovskite/silicon tandem solar cells were prepared in this study.

The silicon bottom cells were annealed for 10 min at 210 ^oC before the deposition of the perovskite top cell to fabricate the tandem perovskite/silicon solar cells with a saw damage etched n-type Czochralski silicon/HTL/transparent conductive oxide (TCO)/perovskites/LiF/C₆₀/SnO₂/indium zinc oxide (IZO) configuration.

The coating methods and process, steps of HTL, perovskites, LiF, C₆₀, and SnO₂ used in the perovskite single-junction cell preparation were also deposited on silicon bottom cells. Subsequently, IZO was deposited using the sputtering method.

Researchers performed time-dependent steady-state absolute photoluminescence (PL) measurements, intensity-dependent quasi-Fermi level splitting (QFLS) and absolute and transient PL measurements, single junction solar cell characterization, tandem solar cell characterization, reflection and transmission measurements, grazing incidence X-ray diffraction measurements, and X-ray diffraction patterns (XRD) measurements. 

Observations

Researchers successfully fabricated a triple-halide perovskite film with top cell optimized bandgaps, high PL quantum yield (PLQY), and improved film quality using the slot-die coating method. They have also efficiently integrated halide perovskites with industrial silicon bottom cells in a tandem architecture, demonstrating the potential of fabricating industrially relevant and scalable perovskite solar cells.

The chloride in triple-halide perovskites reduced the amount of bromide required for top cell optimized bandgaps, improved surface morphology, and induced surface passivation. The addition of five mol % MAPbCl₃ into the double-cation Cs22Br15 perovskites increased their bandgap from 1.63 eV to 1.68 eV, which resulted in exceptional optoelectronic properties and no phase segregation.

The optimization of the annealing and drying conditions from 100 to 170 ^oC for 20 min improved the transient and absolute PL in charge carrier lifetimes and QFLS. Optimized crystallization and annealing conditions enabled large gain sizes, which reduced the charge collection losses, leading to higher current density in the perovskite/silicon tandem solar cells.

An optimized trade-off was identified between the detrimental large lead iodide (PbI₂) aggregate formation on the film’s top surface and sample crystallinity when the film was annealed at 150 ^oC. A stabilized power output of 19.4% was achieved due to improved cell performance and stability of halide perovskites single-junction cells. The power output was highest for halide perovskites with top cell optimized bandgaps.

A two-terminal monolithic perovskite/tandem solar cell with 25.2% PCE on one cm² active area was fabricated by integrating the optimized perovskite absorber layers with silicon bottom cells.

Taken together, the findings of this study demonstrated the scalable fabrication of perovskite/silicon tandem solar cells with optimized bandgap using the slot-die coating method.

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