Structural Engineering of Conjugated Small Molecules for Organic Solar Cells

Abstract

Over the past two decades, significant research endeavors have been successful in increasing the power conversion efficiencies (PCE) of photovoltaic device. Most studies in this field have focused on developing new molecular structures and engineering new device architectures in order to optimize light absorption and extract this absorbed energy as efficiently as possible. There are specific molecular structures which has energy harvesting ability responding light such as benzodithiophene (BDT), diketopyrrolopyrrole (DPP), isoindigo (IIG), dithienosilole (DTS), and so on. Among a large number of molecular chromophores reported in the literature, DTS moiety is the silicon (Si) heteroatom implanted at the center of the molecule. Within the organic solar cell community, DTSi analogue, namely dithienogermole (DTGe)-in which the Si atom is replaced with germanium (Ge) atom-has been demonstrated. In accordance with this, in chapter 1, I performed in-depth study about the underlying structureproperty relationships in a relevant class of Ge-containing small semiconductors; how the substitution affects morphology, crystal structure, optical, thermal and material properties compared to the analogous Si-containing small molecules. Furthermore, I characterize the influence of different endcapping groups, bithiophene (Th2) and benzofuran (BFu) coupled with the Ge-fused core backbone. In chapter 2, with DTGe(FBTTh2)2 which described in chapter 1, a thiophene unit is inserted to that small molecule for extending π-conjugated system. A thiophene unit is generally known for having the donating nature which can flow electron from it to the electron deficient unit. In this thesis, by introducing simple thiophene unit with systematic study of two set of molecular structure, we have laid the foundation of how donating bundle influence frontier orbital energy levels, crystalline structure, device performance, and so on.