Overview Of Spray Coating Technique

Spray coating is a touch-free, fast, vacuum-free, scalable and R2R compatible casting technique, in which the ink is forced through a nozzle, forming a fine aerosol. A carrier gas and electrostatic charging are often involved in directing the aerosol towards the coating substrate. Even though this coating process is R2R compatible that enables a reasonably detailed pattern formation, reaching up to millimeter scale, since the aerosol droplets have to level off after reaching the substrate, which is not necessarily granted, obtaining smooth surfaces is always a challenge. This method, however, is capable of depositing every layer of a PSC, as long as it is solution processed. A wide viscosity range for the ink solutions is acceptable here and it is also possible to prepare films of varying thickness.

Some processing parameters such as the distance between sample and airbrush, spray duration, flow rate, substrate temperature, pressure, the concentration of the blend solution, co-solvent mixture and number of times the substrate is sprayed have also been extensively studied. Distance between nozzle and substrate has been identified as one of the key processing parameters that directly affect the morphology of the deposited layer. There are many examples where researchers have tried to analyze and optimize the nozzle to substrate distance for the active layer deposition. Incidentally, three regions have been identified by Vak et al. between airbrush nozzle and the substrates, termed as ‘wet’, ‘intermediate’, and ‘dry’, where they declared the ‘intermediate zone’ as the region to achieve the best linear control over thickness as a function of spraying time. Similarly, Susanna et al. revealed the position of the ‘intermediate zone’ to be at 17 cm (distance between sample and airbrush) as the deposed material exhibited good uniformity with a maximum PCE of 4.1%, whereas the films remained wet below 15 cm while produced dry and powdery films over 20 cm from the substrate. Likewise, Saitoh et al. obtained uniform thickness at 20 cm distance, as compared to a distance of 10 cm and 30 cm which lead to irregular and thick, and less uniform and thin films, respectively.

Different types of solvents have been used by various researchers to study the effects of solvent on PSC properties and performance fabricated by spray coating process,[128-130] as the choice of solvents directly affects the nozzle–substrate distance for thickness optimization and film morphology parameters. Susanna et al. used a mixed solvent system along with a 40°C heated substrate to achieve a PCE of 4.1% with a cell area of 2.5 cm2. A good solvent must be fast enough to dry the solvent to prevent droplets from re-dissolving the sublayers, but not too fast as to allow the formation of a homogenous and pin-hole free film.

Recently, optimization of the established systems, as well as deposition of electrodes via spray coating method are under consideration. In this view, Krantz et al. by spray-coating silver nanowires as the top electrode layer was able to develop a system that demonstrated comparable performance with the system where the silver-top electrode was thermally evaporated. Similarly, the spray-on methodology was also employed to coat PEDOT: PSS transparent electrodes with stretchable and mechanically robust properties. Instances of depositing the ZnO and TiO2 electron transporting layers on hot substrates by spray pyrolysis have been reported.

Even though obtaining a uniform and pin-hole free film via spray coating method is still a major obstacle, Wang et al. was able to effectively spray-coat the active layer of a fullerene-based system by employing thermal annealing at 80°C in air and achieved a high PCE of 5%. Kang et al. further fabricated a complete spray-coated semi-transparent PSC, achieving a PCE of 2.35%, demonstrating that all-solution-processed PSCs, exhibiting reasonable PCEs can indeed be fabricated. Steps have also been taken to ensure layer homogeneity by employing methods such as imposed ultrasonic vibration on the substrate, further improving the wetting and performance of spray-on films. This method can increase the electrical conductivity of spray-on PEDOT:PSS films up to four times, and thus is considered as a viable technique for the fabrication of other solution-processed layers. Besides PSCs, this method has already been employed in a number of industrial applications, such as in automotive industry and fabrication of thermal barrier coatings.