Templated Growth of Fullerene C60 Crystals by Triptycene in Polymer Blend Films
Molecular semiconductors such as fullerene C60 have become ubiquitous components of organic electronic devices, owing to their electronic structure and favourable material processing properties. In most conjugated polymer-fullerene films that form the active layer in bulk heterojunction (BHJ) organic solar cells, organisation of the fullerene phases to the correct nanoscale dimensions for exciton charge separation and transportation to the device electrodes is driven by excess fullerene addition. While this approach can deliver acceptable film morphology for a BHJ solar cell, it is not optimal as the photoactive polymer component of the film becomes diluted by C60 thereby reducing device efficiency. This motivates a supramolecular approach as an alternative method to control fullerene assembly and give morphological control of conjugated polymer films. Triptycene (TPC) is a readily available molecule whose rigid paddle wheel structure and hydrophobicity present three excellent C60 binding cavities. Triptycene has the potential to template the macroscopic assembly of fullerene molecules within a polymer-fullerene blend film, thereby controlling phase separation without excess fullerene addition. In this project, the ability of TPC to template the assembly of C60 was investigated in single crystals, polymer films, and in functional electronic devices. Blue-shifted fluorescence from TPC·C60 co-crystals was used as a spectroscopic signature to probe the molecular environment of C60 dispersed through an optically transparent polystyrene polymer film, and confirm that TPC hosts C60 molecules within the polymer matrix. Ultraviolet-visible (UV-Vis) spectroscopy of the polystyrene:C60:TPC films confirmed a reduction in the orbital overlap between adjacent C60 molecules providing further evidence that TPC had spatially separated C60 molecules upon templating the macroscopic assembly. When TPC was added to conjugated polymer poly[2-methoxy-5-(2-ethyhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) and MEHPPV: C60 films as a blend additive, fluorescence spectroscopy identified two unique effects: (1) the suppression of excimer states when TPC spatially separated the conjugated polymer chains, and (2) the assembly of C60 into larger domains to drive polymer and C60 phase separation, giving morphological control of the polymer film. The fabrication of polystyrene:C60:TPC sandwich devices showed the electronic conduction of C60 was unaltered by spatial separation and reduction in electronic coupling between neighbouring C60 molecules caused by TPC templation. MEHPPV: C60 BHJ solar cells suffered a loss in photocurent when TPC was added to the active layer when compared to fabricated devices that used excess fullerene addition to control film morphology. However, due to time constraints, only one polymer film composition was able to be tested. Since the polymer film morphology was shown to be sensitive to the molar ratios of C60 and TPC, there is immense potential to further investigate TPC as a blend additive in conjugated polymer films and optimise the film composition to obtain desirable morphology for a BHJ solar cell. The functionalisation of TPC could provide a method to further enhance interactions between TPC and C60 and provide greater control over C60 self-assembly within a polymer film.