Advancing spray deposition for low-cost solar cell production

Abstract

Polymeric solar cells may be the least expensive means to convert solar irradiation to electricity.1 This cost reduction is based on soluble organic active layers coupled with ultralow-cost deposition technologies. However, profitable production and sales of these devices require improvements in device stability, efficiency, and processing.Material and processing developments of organic photovoltaics (OPVs) in the past few years have resulted in certified small-area (<1cm2) device efficiencies of approximately 6% with multiple-material systems.2 The ability to approach competitive power-conversion efficiency has thus heightened commercial interest in product development. Together with improved device performance, it is important to develop process technology matched to the needs of organic devices. To succeed as a useful manufacturing technique, the specific deposition approach must be highly scalable while still producing films with the quality of laboratory deposition methods, e.g., spin coating. Some scalable organic thin-film deposition techniques that show promise are inkjet printing, airbrush spray, ultrasonic spray, evaporative spray, slot coating, and screen printing. Among these, spray deposition is historically scalable to large areas, and may also be applicable to a range of organic electronic devices such as tailored photodiodes and LEDs. Independent studies of low-cost spray depositions using a handheld airbrush to deposit the active layer in an OPV device show power-conversion efficiencies of 2.83 and 2.35%.3, 4 The active layer in these studies was composed of poly(3-hexylthiophene) (P3HT) and [6,6]phenyl-C61 butyricmethyl ester (PCBM), which typically result in 4% efficient OPV devices when spin coating is used. We have evaluated ultrasonic spray deposition for reducing the cost of functional thin-film fabrication (see Figure 1). Figure 1. Ultrasonic spray deposition. Solution is delivered to the ultrasonic nozzle, creating a spray with uniform-diameter droplets. The spray is focused onto the substrate using nitrogen as the carrier gas. Thick films for this study were built up layer by layer. P3HT:PCBM: Poly(3hexylthiophene):[6,6]phenyl-C61 butyric-methyl ester. PEDOT:PSS: poly(3,4-ethylenedioxythiophene):polystyrene sulfonate. ITO: Indiumtin oxide.