Roll-to-roll printing of organic photovaltaic cells and modules: Dissertation

Organic photovoltaics (OPV), one of the emerging thin-film photovoltaic technologies, has gained considerable interest being flexible, light weight and transparent. OPVs can be processed by using roll-to-roll (R2R) printing and coating methods which can lead to significant manufacturing cost reduction. Gravure printing brings the advantage of layer patterning directly in the printing process. This differentiates gravure printing from coating technologies. Importantly, this increases product design freedom by enabling large-area arbitrary shape and size structures. This opens up possibility to use OPVs also as decorative elements on the surfaces of interior and exterior building spaces. Secondly, gravure printing enables high repeatability and accuracy in thin-film deposition. In this Thesis, the applicability of gravure printing was demonstrated firstly in the laboratory by using a standard OPV device configuration. The layer properties (layer thickness, uniformity) of the gravure-printed hole transport layer and photoactive layer were optimized by engineering the printability using printing master parameters, ink formulations and printing parameters. The electrical functionality of these printed layers was studied in organic solar cells using a standard measurement method. The cells were further connected to modules. After this, the outlined processing conditions for OPV modules (active area 15 cm2) were transferred to R2R pilot production environment. Small pinholes were found to form readily in the photoactive layer in R2R gravure printing with a standard cell configuration. However, by using an ultrathin evaporated insulating interlayer electrical short-circuit could be inhibited, leading to considerably improved performance with a maximum efficiency of 1.9%. In addition, a R2R printing process for inverted OPV configuration modules was developed and demonstrated. The device structure consisted of five layers, which were either gravure or screen printed. Few hundred fully R2R printed modules with the active area ranging from 14-97 cm2 were fabricated with excellent yield. With the 97 cm2 sized modules an average output power of 0.17W was generated (power conversion efficiency of 1.8±0.1 %). The main achievements of this thesis are i) gravure printing based R2R thin-film deposition technology for OPV, ii) printed standard and inverted device structures, iii) R2R manufactured large-area flexible solar modules, iv) OPV process upscaling to R2R pilot level and v) investigation of characterization methods for OPV modules.