No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

This thesis presents a universal transfer printing method to introduce a thin layer of interlayer nanoparticle material in the cathode-organic layer interface in organic device. The use of reverse micelles for making nanoparticles restricts the nanoparticles to be directly synthesized on the organic active layer , therefore a transfer printing method using graphene was derived and a characterization method was needed to detect the transfer of nanoparticles in the whole device system. Raman spectroscopy was found to be the best candidate in studying these organic systems. The oxidation behavior and interaction of CVD graphene on Cu with oxygen plasma and mild annealing was monitored closely by a detailed Raman trilogy studies. Raman results also show evidence of graphene oxide successfully transferred to the target organic layer. Raman spectroscopy was further explored to understand all material in the transferred system including the micelles, type of nanoparticles and the organic layer, which then provides valuable insights to the evolution of the different phases of nanoparticle material formed by the reverse micelles technique. Raman was also used to confirm the first-reported formation of the hot-topic perovskites materials in reverse micelles. An extended Raman technique, the unconventional inverted-TERS, was used to detect a monolayer of micelles which was otherwise impossible for a normal Raman setting. The underlying mechanisms of this technique with high-resolution were also proposed. In order to understand and explore the tunability of reverse micelles on nanoparticle synthesis, a study with the pervovskite material was performed. There were evidence of precursors interacting with the pyridine group in the micelles core, which affects nanoparticle formation. The size of nanoparticles is also found to be tunable by using micelles of different block lengths and different solvents. All these findings contribute to future optimization on the nanoparticles to be transfer printed into devices interlayer and ultimately to benefit on the improvement on organic photovoltaics. / Thesis / Doctor of Philosophy (PhD)

In this thesis, the interface between the electron donor polymer and the electron acceptor fullerene in organic photovoltaic devices is studied. Starting from a bilayer system of donor and acceptor materials, the proximity of polymer and fullerene throughout the bulk of the devices is improved by inducing an interdiffusion of the two materials by heating the devices in the vicinity of the glass transition temperature of the polymer. In this manner, a concentration gradient of polymer and fullerene throughout the bulk is created. The proximity of a fullerene within 10 nm of any photoexcitation in the polymer ensures that the efficient charge separation occurs. Measurements of the absorption, photoluminescence, and photocurrent spectra as well as I-V characteristics are used to study the interdiffusion and its influence on the efficiency of the photovoltaic devices. In addition, the film morphology is studied on a microscopic level with transmission electron microscopy and with Auger spectroscopy combined with ion beam milling to create a depth profile of the polymer concentration in the film. Initial studies to induce an interdiffusion were done on poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) as the electron donor polymer and the buckminsterfullerene C60 as the electron acceptor. Interdiffused devices show an order of magnitude photoluminescence quenching with concomitant increase in the photocurrents by an order of magnitude. Variation of the polymer layer thickness shows that the photocurrents increase with decreasing thickness down to 70 nm due to charge transport limitation. The choice of layer thickness in organic photovoltaic devices is critical for optimization of the efficiency. The interdiffusion process is also monitored in situ and a permanent increase in photocurrents is observed during the heat treatment. Transmission electron microscopy (TEM) studies on cross sections of the film reveal that C60 interdiffuses into the MEH-PPV bulk in the form of >10 nm clusters. This clustering of C60 is a result of its tendency to crystallize and the low miscibility of C60 in MEH-PPV, leading to strong phase separation. To improve the interdiffusion process, the donor polymer is replaced by poly(3-octylthiophene-2,5-diyl) (P3OT), which has a better miscibility with C60. Again, the photocurrents of the interdiffused devices are improved significantly. A monochromatic power conversion efficiency of 1.5 % is obtained for illumination of 3.8 mW/cm2 at 470 nm. The polymer concentration in unheated and interdiffused films is studied with Auger spectroscopy in combination with ion beam milling. The concentration profile shows a distinct interface between P3OT and C60 in unheated films and a slow rise of the P3OT concentration throughout a large cross-section of the interdiffused film. TEM studies on P3OT/C60 films show that C60 still has some tendency to form clusters. The results of this thesis demonstrate that thermally-controlled interdiffusion is a viable approach for fabrication of efficient photovoltaic devices through nanoscale control of composition and morphology. These results are also used to draw conclusions about the influence of film morphology on the photovoltaic device efficiency and to identify important issues related to materials choice for the interdiffusion process. Prospective variations in materials choice are suggested to achieve better film morphologies. / Ph. D.