Star-shaped molecules for organic photovoltaics: Synthesis and structure-property relationships.

摘要

In this thesis, the development and structure-property relationships of a novel class of two-dimensional star-shaped molecules synthesized for organic photovoltaics are presented. A promising approach towards low cost photovoltaics for power generation is fabrication of solar cells based on organic semiconductors. In addition to being a potentially reliable and environmentally friendly energy source, organic materials offer unique advantages such as low cost, lightweight, flexibility and high form factor. Though this field has witnessed tremendous progress over the last decade, power conversion efficiencies of the existing solar cells are still poor, prohibiting their wide spread commercial use. The available organic materials mainly suffer from low charge carrier mobilities and inefficient absorption in the bulk of the solar spectrum leading to low photocurrent generation. Charge transport in conjugated polymers is generally one-dimensional, while three-dimensional mobility is limited by the necessity for chain-to-chain activated hopping mechanism. Self-organization observed in conjugated crystalline small molecules and polymers favors strong intermolecular interactions in the pi-pi stacking distance and offers potential to improve charge carrier mobilities in organic materials. This motivated us to design and synthesize five different solution processible two-dimensional star-shaped molecules CN-X, THX-L, THX- S, THX-S=O and THX-D/A that offer interesting possibilities for improvement of charge mobilities and photophysical properties. The developed molecules self-organize in the form of pi-stacks and contain a tetra-substituted central phenyl ring with four phenylenevinylene or thiophene arms with conjugation through the arms and central phenyl core. The designed synthetic route is versatile, which enables facile incorporation of different electron rich and deficient moieties and solubilizing groups, thus facilitating systematic engineering of material properties like energy levels, bandgap, charge transport and solubility. The two-dimensional structural architecture with different functional groups led to molecules with small optical bandgaps in the range of 1.8-2.2 eV, absorbing in the bulk of the solar spectrum with band edges extending up to 700 nm and exhibiting strong intermolecular interactions in the pi-pi stacking distance of 3.5 A. A representative molecule, THX- L, from a set of four different oligothiophenes has shown field effect mobility of as high as 0.02 cm2V-1s-1 , which is amongst the best mobilities reported thus far for solution processible organics. In this thesis, the designed synthetic strategy, the fundamental structure-property relationships that govern the performance of the star molecules and their potential application mainly in solar cells and to some extent in light emitting diodes and field-effect transistors will be discussed. One relationship we have also investigated is whether the star architecture offers any fundamental advantages over linear molecules by comparing the properties of CN-X with its linear analog CN-L, which will be presented in the thesis.