Department of Energy EngineeringOrganic solar cells (OSCs), a kind of promising photovoltaics in next generation due to lots of advantages such as mechanical flexibility, light weight, easy bandgap tuning, easy fabrication and so on, is direct conversion system from sunlight to electricity through the absorption of photons in active materials. Since light consist of radiant particles (photons) and electromagnetic field (waves) is necessary to work solar cells, there is promising route to enhance the performance of OSCs via combination of the inherent potential in organic solar cells and optical engineering. Furthermore, the studies using light manipulation are expected not only to lead to commercialization of OSCs, but also to use in other optoelectronics engineering as strategies. The main research is as follows.
Firstly, the localized surface plasmon resonance (LSPR) phenomenon generated by light is used in organic solar cells, which arise from two different electron transport layers (ETLs) incorporated with silver (Ag) nanoparticles. According to Mie theory for spherical particles, the LSPR frequency changes depending on the refractive index of surroundings, which shows the different influence and improvement of performance on organic solar cells. In addition, for this study, we used block copolymer micellar lithography (BCML) for Ag dot arrays, optical calculation considering quantum size effect, nonlocal response and plasmon coupling effects.
Secondly, to broaden photon absorption without the light waste, hetero organic tandem solar cells (HOTSCs) were fabricated by accounting into limited factors of tandem structure system as well as requirements of each layers. In this study, two new polymers of wide band gap were used in bottom solar sub-cells, while relatively narrower bandgap polymer was used in top solar sub-cells. Thus, the tandem solar cells in series shows high performance with increased open-circuit voltage (Vocs).
Thirdly, semitransparent organic solar cells were studied with transparent electrodes based on multilayers of metal and metal oxide. Since organic photoactive layers are so colorful that OSCs are suitable for building-integrated photovoltaics (BIPV), transparent top electrodes were key for semitransparent device. Using Sb2O3 layers in dielectric-metal-dielectric (DMD) structure, not only colorful semitransparent solar cells with over around 30% of average visible transmittance (AVT: 380 nm ??? 780 nm) were fabricated, but also roles of each layers of DMD electrode were studied.
Further, as forth topic, colorant-based electrodes are also interesting approach for colorful semitransparent OSCs. The structure of color filter is reverse to DMD transparent electrode, which has configuration of metal-metal oxide-metal. Incorporating DMD electrode and color filter electrode, high performance and selective clear-colorful semitransparent OSCs were implemented. These optic engineering of metal and metal oxide layers and light manipulation will provide breakthroughs for innovative OSCs.
In addition, as sub-research (Chapter 6-10), I also studied controlling morphologies and molecular packing structures of photoactive films with various conjugated donor polymers, which were modified side chains on the backbones, and processing method including thermal-, additives-, solvent treatment and so on. Finally, I suggest the potential of flexible OSCs fabricated by a roll-to-roll (R2R) process without performance loss through dual thermal treatment of slot die coating for optimal film morphology and molecular orientation.
These strategic approaches from light manipulation and trying film optimization of various polymers and processing methodology may draw realization of aesthetic OSCs commercial market.clos
