Development of Printing Technique for Organic Solar Cells

Abstract

Organic semiconductors are a promising future electronic material owing to their outstanding characteristics such as solution-phase processability, high flexibility, low weight, semi-transparency, and low-cost manufacturing. Of immense recent interest is the process of printing semiconductors from OS solutions; this method facilitates the production of large-area and flexible organic electronic devices. Organic solar cells (OSCs), a particular class of devices using organic semiconductors, have been continuously improved, and currently, their power-conversion efficiencies (PCEs) exceed 16%. However, typical fabrications of large-area flexible photovoltaic devices still yield a significantly reduced PCE; this reduced PCE may be less than 50% that of small-sized laboratory cells. These observed reductions in efficiency are typically caused by the printing processes introducing complicated control factors not present in the lab-scale spin-coating process used to generate flexible and transparent electrodes (FTEs). When fabricating high-performance, large-area, and flexible OSCs, appropriate printing techniques are essential to form uniform multilayers that feature high quality, and FTEs with low sheet resistance, high transmittance in the visible range, and excellent mechanical flexibility. In this thesis, we present simplified tandem OSCs fabricated through a versatile self-organization printing method, as well as semi-transparent flexible OSCs using FTEs comprising ultra-thin metal and chemically modified conducting polymers. Chapter 1 introduces a general overview of the field of large-area and flexible organic electronics. First, the origin of organic semiconductor properties is described. Secondly, the fundamental operating mechanism of the OSCs is presented. Thirdly, the various printing technologies for the printable photovoltaic cells are introduced. Finally, the flexible and transparent electrodes (FTEs) for fabricating large-area and flexible organic electronic devices. Chapter 2 presents a new self-assembly printing method to achieve the simple and efficient tandem OSCs by understanding a clear mechanism on the vertical self-assembly of the organic nanocomposite. The vertical self-organization of the organic nanocomposite originates from a ‘spontaneous thermodynamic mechanism’ induced by the significantly different surface energies of these materials and strongly depends on entanglements between the polymer chains within the organic nanocomposite. Through doctor blade and slot-die printing processes using organic nanocomposite, we achieved the most simplified tandem structure ever fabricated, with only four component layers, and efficient tandem operation with the high PCE of 9.1%. Chapter 3 describes a novel window film organic photovoltaic (WFOPV) using high-performance FTEs for effectively compensating the building energy consumption. Despite the highly beneficial functions of semi-transparent and flexible OPVs, previous studies have not sufficiently explored these devices due to the specific and complex characteristics of FTEs which are desired electrical, optical, and surface properties. By carefully devising the interfacial engineering to overcome poor adhesion problems between component layers (e.g., photoactive layer and interfacial layer) and FTEs, we successfully create ultra-thin metal electrodes as a bottom FTE on flexible substrates and chemically modified conducting polymers transferred onto the photoactive layer as a top FTE. Further, our proposed device exhibits excellent device performances as the semi-transparent and flexible OPVs and shows a great potential as energy saving window films due to its outstanding transparency, flexibility, and photovoltaic properties. Chapter 4 provides an overall summary of the work and the conclusions drawn in this thesis.