Improving nanoparticle organic photovoltaic device performance

Recently, the fabrication of organic photovoltaic devices from water-dispersed nanoparticulate materials (solar paint) has attracted increasing interest since it offers the potential of morphological control coupled with device processing in the absence of organic solvents which are toxic to human beings and the environment. However, to date the reported efficiencies of nanoparticulate organic photovoltaic (NP-OPV) devices have been disappointingly low compared to the standard organic photovoltaic devices. This low efficiency reflects a lack of understanding of the structural motif of NP-OPV devices. Furthermore the fabrication conditions used for NP-OPV devices are not well determined as yet. To improve NP-OPV device performance, understanding the structural motive of NP-OPV devices is necessary and OPTIMISED fabrication conditions for NP-OPV need to be determined. By using relevant techniques such as X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning transmission X-ray microscopy (STXM), and differential scanning calorimetry (DSC), we analyse and optimise the conditions for fabricating NP-OPV devices and show a significant improvement in efficiency as compared to previous work. Using calcium and lithium fluoride interface layers between the aluminium and the active layer were also investigated in this project. It is found that proper thickness of the interface also contributes to improving the performance of NP-OPV devices. In this project poly(3-hexylthiophene) (P3HT) is used as the electron donor and phenyl-C61-butyric acid methyl ester (PC61BM), phenyl-C71-butyric acid methyl ester (PC71BM), and indene-C60-bisadduct (ICBA) as the electron acceptors. The highest efficiency of nanoparticle devices fabricated for P3HT:PC61BM is 1.36 %; for P3HT:PC71BM is 2.00 %; and for P3HT:ICBA is 3.29 % which are by far the highest efficiencies reported so far for each system. Moreover an efficiency of 3.29 % is the highest efficiency for NP-OPV devices reported. In this project we also investigate the structural motive of NP-OPV devices and find that for P3HT:PC61BM nanoparticles, the particles are core-shell in nature with a PC61BM-rich core and a P3HT-rich shell. For P3HT:ICBA nanoparticles, the particles are also core-shell in nature with a P3HT-rich shell and ICBA-rich core but upon annealing a highly intermixed P3HT:ICBA is formed, which is driven by the enhanced miscibility of ICBA in crystalline P3HT and this morphological change results in much higher device efficiencies.