Molecular design for improved self-organization of conjugated molecules for optoelectronics applications
Lam, Kwan Hang
Date of Issue2014
School of Materials Science and Engineering
Institute of Materials Research and Engineering
Organic solar cells offer interesting benefits like flexibility, solution processablility and good power conversion efficiency in low lighting conditions. Among these desirable qualities, solution processability is of particular interest for many applications as it provides the possibility of low cost and large scale fabrication. In general, solution processable organic solar cells are often categorized according to the nature of the donors in the active blend - polymeric or small molecule solar cells. In polymer solar cells, the active ink will mostly be comprised of a polymeric donor and a small molecule acceptor. Although all-polymer solar cells have been reported, they tend to have relatively lower efficiency. While polymers offer good film-forming properties, generally good solubility and high charge carrier mobility, there are disadvantages like polydispersity, poor batch to batch reproducibility and difficulty in purification. On the other hand, in solution processable small molecule solar cells both the donor and acceptor are small molecules and these molecules offer the advantages of monodispersity, ease of purification and good batch to batch reproducibility. However, in order to impart solubility, alkyl chains are usually substituted onto the conjugated core. These solubilising chains often disrupt the stacking and crystallinity of the molecules thereby resulting in lower charge carrier mobility in solution processed small molecule devices. In order to circumvent this problem, hydrogen-bonding is proposed as a viable approach where hydrogen-bond induced interaction is postulated to enhance both the intermolecular connectivity and crystallinity in solution processed small molecule films. Hence small molecule functionalised with two carboxylic acids and alkoxy solubilising groups, abbreviated as QT-DA, was synthesized and its thermal, optical and structural properties were investigated and the impact of some of these properties on the device performance were also explored to understand the influence of molecular design. A diester analogue, abbreviated as QT-ES, was also synthesized and used as a comparison to study the effects of hydrogen-bonding on device performances. Increase in mobility for both the in-plane and out-of-plane direction was observed for the acid-functionalised molecule as compared to the diester analogue. Organic solar cells were fabricated with QT-DA/ QT-ES as the donor molecules and PC61BM as the acceptor. A two-fold increase in power conversion efficiency was observed for the QT-DA blend as compared to QT-ES. The main contributing factor was found to be due to an increase in the short circuit current density of the QT-DA:PC61BM device. This was attributed to an enhanced crystallinity in the as-cast QT-DA blend films and thus a higher hole mobility in the films. Formation of nano-fibers in QT-DA was observed in the as-cast blend films which were due to the presence of hydrogen-bond induced stacking in the molecule. The effect of the position of the functional groups on the molecular orientation and device performance was also investigated. A positional isomer of QT-DA, abbreviated as isoQT-DA, was synthesized and compared with respect to QT-DA. It was found that the crystallinity and device performance was lower in isoQT-DA as compared to QT-DA. Despite so, enhanced crystallinity was obtained in the as-cast blend film of isoQT-DA as compared to QT-ES which correlates well with the proposed concept. Furthermore, an increase in solubility was observed for isoQT-DA indicating that solubilising chains substituted at the α- and ω-position of a molecule maybe a possible design strategy for synthesis of molecules with high solubility.