Organic photovoltaics (OPV) is an emergent renewable technology that could enable humanity to cleanly generate abundant amounts of electricity from the Sun. Whilst promising many potential advantages such as reduced manufacturing costs, current OPVs suffer from drawbacks of relatively low efficiencies and poor device lifetimes. To address these issues, much research attention has been directed at improving the organic semiconductors featured at the core of these systems. In particular, recent developments have centred on using the ‘low band-gap’ strategy to produce new donor components for the donor-acceptor bulk-heterojunction (BHJ) blends. This thesis investigates two alternative approaches to donor material design to assess their potential for use in the OPV application. In the first research topic, the dendronised polymer (denpol) architecture was employed to explore how rigidifying the polymer backbone affects its photovoltaic properties; for the second topic, an iodinated polymer was examined to determine its effectiveness as a triplet state donor material. The studies were performed by functionalising a conjugated polymer backbone with dendrons/iodine; the polymer employed was the archetypal poly[1,4-phenylenevinylene] (PPV) system.
In the denpol investigation, a series of three PPVs featuring pendent, rigid, π-conjugated, ‘biphenyl’ dendrons (two generations: G1-PPV and G2-PPV), and carbazole based dendrons (second generation: G2K-PPV) was studied. The denpols were prepared via a novel synthetic route that incorporated the macromonomer approach, the Gilch polymerisation technique, and the convergent dendron synthesis methodology. A range of chemical and thermal measurements, including gel permeation chromatography, 1H nuclear magnetic resonance, microanalysis, infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry, were performed to characterise the new polymers. The structural and optoelectronic properties of the denpols were elucidated from their steady-state absorption and emission spectra in both the solution and solid state. It was found that: i) whilst the G1 and G2 dendrons did not cause twisting of the polymer backbone, the bulkiest G2K dendron reduced the delocalisation length of the π-conjugated PPV backbone; ii) the increase of the dendron size caused a systematic decrease in the backbone absorbance; and iii) only the two second generation dendrons were large enough to rigidify the normally flexible PPV backbone. This third observation was further confirmed by a solvatochromism study. BHJ solar cells fabricated using G1-PPV and a fullerene acceptor at various blend compositions showed that a 1 : 3 ratio of G1-PPV : fullerene gave devices with the highest efficiency of 0.44%, which is a thirty-fold improvement over the only other literature report for denpol based solar cell. However, only preliminary results were obtained for the G2-PPV and G2K-PPV materials; the best performing BHJ device from the latter denpol was ~0.1% efficient.
In the iodinated polymer investigation, a novel iodine containing PPV (I-polymer) and its non-iodinated counterpart (H-polymer = G1-PPV) were compared. The target polymer was prepared via a method analogous to that used for the denpol synthesis; and the characterised I-polymer sample was revealed to be largely similar to the H-polymer in terms of molecular weight and purity. Contrasting the optoelectronic properties of the two systems, it was found that whilst the optical absorption and ionisation potential of the PPV backbone were mostly unaffected by the presence/absence of the iodo functional groups, the photoluminescence quantum yield of the I-polymer was significantly lower than that of the H-polymer. A transient electron paramagnetic resonance study demonstrated that triplet state excitons formed by iodine-mediated intersystem crossing from the singlet state was the cause behind the fluorescence quenching observation. Charge mobility determination using the field effect transistor technique showed the hole transport in the I-polymer was 4.5 times better than the H-polymer; with a value of 1 × 10-3 cm2/Vs, the I-polymer also possess the highest hole mobility ever reported for a PPV derivative. Bilayer heterojunction solar cells fabricated using the two polymers as the donor and C60 as the acceptor showed efficiencies of ~0.35% in the I-polymer devices compared to the best efficiency of ~0.1% achieved by the H-polymer. As the polymer layer thicknesses in the bilayer cells were increased, the efficiencies of those devices featuring the iodinated polymer did not decrease in contrast to the protonated polymer, suggesting that triplet excitons with enhanced diffusion lengths (up to ~45 nm) were present in the I-polymer solar cells.