Amorphous Organic Molecular Glasses for Photovoltaic Applications
Significant progress has been achieved recently in the development of solution-processed organic photovoltaic devices, and they are now in the early stages of commercialization. However, achieving a smooth and homogenous film morphology without disturbing the optical and electronic properties of the active layer remains a crucial challenge to overcome for developing small-molecule materials. Strong molecular forces between conjugated small molecules can result in the nucleation of molecular crystals in solution, thereby resulting in an incomplete film coverage with high surface roughness. In addition, most small-molecule donor and acceptor materials tend to crystallize rapidly during solvent and thermal annealing to yield films of poor quality with polycrystalline small domains separated by grain boundaries, which act as electron traps and limit the long-range transport of electrons, and thusly impacting negatively device performance. To improve film quality with enhanced efficiency, several efforts are focused on molecular design which is based on high solubility, high optical absorption, high charge carrier mobility and a high interfacial gap between donor and acceptor. The approach studied herein consists in improving the film quality of PV devices by synthesizing amorphous glass-forming donors and acceptors with improved optical and electronic properties. Organic molecular glasses are a class of glass-forming materials that usually show outstanding glass-forming ability, and high kinetic stability towards crystallization. Organic materials functionalized with glass-forming mexylaminotriazine derivatives can remain indefinitely in the amorphous state at working temperatures, thereby preserving their optical and electronic properties. In this thesis, we developed molecular glasses functionalized with n- and p-type organic small molecules and studied their optoelectronic and photovoltaic properties. This thesis mainly focuses on the optical, electronic and photovoltaic properties of target mexylaminotriazine-functionalized amorphous donor and acceptor materials which show outstanding PV performance compared to their crystalline analogues. In addition, the structural and morphological properties of their respective thin films studied by XRD, AFM and SEM are also discussed.