Solar Cell Coating

The literature demonstrates an ultrasonic nozzle at nearly every position in a solar-cell stack. Organic photovoltaic studies sprayed the P3HT:PCBM donor-acceptor photoactive blend. Perovskite studies deposited single-cation, mixed-cation, triple-cation, and lead-free double-perovskite (Cs₂AgBiBr₆) absorbers, while related work used the same platform for TiO₂ and SnO₂ electron-transport layers and spiro-OMeTAD hole-transport layers. Kesterite research sprayed CZTS as either nanocrystal dispersions or molecular precursor inks. One fully spray-coated study deposited the SnO₂ transport layer, the triple-cation absorber, and the hole-transport layer all by spray, reporting 19.4% reverse-scan efficiency in small-area cells.

A photovoltaic ink must stay chemically stable while also atomizing, wetting, drying, and converting into the intended electronic material. The critical properties are solids concentration, viscosity, surface tension, density, solvent volatility, particle size, dispersion stability, and resistance to precipitation at the feed line or nozzle. Perovskite inks carry lead or silver-bismuth halides, ammonium halides, and cesium salts in polar solvents; organic inks use aromatic solvents; oxide transport layers use nanoparticle dispersions. Filtration, inert tubing, chemically compatible seals, and low-dead-volume plumbing are essential parts of the coating system. Several studies also demonstrate concurrent pumping — metering separate precursor solutions independently and combining them near the atomizing zone — so donor-to-acceptor ratio, halide composition, or solids loading can be changed without preparing a new bulk ink for each condition.

Typical substrates include fluorine-doped tin oxide (FTO) glass, indium tin oxide (ITO) glass, molybdenum-coated soda-lime glass, and ITO-coated flexible polymer films. Degreasing and particulate removal are normally followed by UV-ozone or oxygen-plasma surface activation when the substrate and underlying layers permit it. Poor cleaning or insufficient activation produces dewetting, pinholes, local thickness loss, and electrical shunts; excessive wetting encourages edge accumulation and uncontrolled lateral flow. The objective is a reproducible contact angle that allows neighboring droplets to merge without flooding the surface.

The main adjustable variables are liquid flow rate, ultrasonic frequency and power, nozzle-to-substrate distance, shaping-gas flow, traverse speed, raster spacing, substrate temperature, and pass count — and they interact. A key finding: ultrasonic power must be high enough for stable atomization but should not be used as the primary thickness control. Thickness is more reliably set by calibrated liquid delivery, motion speed, raster overlap, and number of passes. Multiple light passes beat one heavily flooded pass, because incremental deposition reduces runoff and allows partial solvent removal between layers — but each pass must still overlap the previous track while wet enough to level, or banding and rough boundaries result.

A multilayer device needs more than independently optimized coatings. Each new solvent must be compatible with the layer below — a later pass must not dissolve, swell, delaminate, or chemically reduce the previously deposited film. Successful stacks use orthogonal solvent systems, controlled drying between layers, and surface treatments that preserve electronic interfaces. Charge-selective layers must be continuous enough to prevent direct absorber-electrode contact yet thin enough to avoid excess series resistance, and the absorber must give complete pinhole-free coverage.

Process qualification spans wet-film behavior and finished-device properties: viscosity and surface tension, flow-rate calibration, spray-width mapping, film-thickness uniformity, surface roughness, crystal structure, pinhole density, sheet resistance — then open-circuit voltage, short-circuit current density, fill factor, power-conversion efficiency, and device-to-device variation. Statistical design-of-experiments methods outperform one-factor-at-a-time adjustment because temperature, speed, pass count, flow, and solvent composition frequently interact. Scale-up means holding stable mass-per-area delivery over larger width and longer runs; the literature shows ultrasonic spray coating flexible sheets, centimeter-scale active areas, and curved surfaces, though efficiency commonly decreases as area increases — underscoring the need for thickness mapping and uniform thermal management. Lead-containing perovskite processing additionally requires enclosed spraying, filtered exhaust, and validated waste handling.

A 2009 study from the National Renewable Energy Laboratory and the Colorado School of Mines shows the method step by step. The P3HT:PCBM absorber was sprayed inside a nitrogen glovebox with a 120 kHz ultrasonic nozzle, a mass-flow-regulated carrier gas and a metering pump: 2 mg/mL ink at 0.33 mL/min, 7 L/min of carrier gas, the nozzle 5 cm above a substrate held at 25 °C. Fifty passes of about 11 nm each built a 0.55 µm active layer, and two final passes at double the flow rate smoothed the surface. Single droplets dried as rings, so coverage came from overlap, and coffee-stain texture was still visible after fifty passes. Solvent choice decided the outcome. Chlorobenzene ink gave cells of 3.0% after annealing and 3.2% with the smoothing passes, close to the spin-coated cells of the period; p-xylene ink formed large crystallites and reached 0.1% at room temperature, or 1.2% with the substrate heated to 78 °C. Annealing improved every device, mainly through higher current.