Design and synthesis of optoelectronic materials for organic photovoltaic applications

Since the first report by Tang (1986), Organic photovoltaic solar cell devices have been the subject of intense investigation research. The research started to gain momentum and in the late 1980s to early 90s the major classes of organic photovoltaic solar devices, polymeric solar cells, and titanium dioxide film-based solar cells were termed dye-sensitized solar cells. Both types of organic cells include organic materials, and over time a variety of materials have been used including the use of polymeric entities, metal complexes, and small organic molecules.

The following material properties are vital to the performance of organic solar cells: * Stability of the organic materials used and hence the overall stability of the organic solar cell * Solution processability of organic materials from common organic solvents such as chloroform, chlorobenzene, and acetonitrile. * Light harvesting properties * Narrow energy bandgap

To answer and better understand the above-mentioned material properties, this dissertation presents a systematic design, synthesis, and characterization of a range of novel small molecules with tuned optoelectronic properties. Even though, a large number of small organic molecules have been synthesized before, but still, there is lot of scope for new organic material with improved properties, greater understanding of the material classes currently in use, that may lead to Organic solar cells with higher efficiency, lower cost and improved chemical stability. The investigated new materials are based on donor-¿-bridge-acceptor configurations which have been designed, synthesized, characterized, and implemented in both types of organic photovoltaic devices, bulk-heterojunction solar cells and dyesensitized solar cells.

In my thesis I have presented work on three projects. Firstly, I explored information obtained from literature that these materials indicate strong relationship between the three fragments (donor, central π-spacer and acceptor) of a donor-acceptor design. For example, in my second chapter of the thesis I have presented the use of a strong acceptor, such as cyanopyridone to design and synthesize two small-xxiv molecule electron donors coded as CP2, and CP1. The optical and electrochemical were measured to calculate band gap, HOMO energy values of these molecules. Here CP2 with 4-hexyloxyaniline with greater conjugation than CP1 having 2-ethylhexylamine generated a target (1) with improved optoelectronic properties, (2) having greater miscibility with the acceptor counterpart so as to have favourable blend morphology, and (3) exerting higher hole mobility, thus, leading to devices with enhanced photocurrent density and superior performance, first of its kind reported in the literature.

Thus, demonstrating that cyanopyridone based donor chromophores resulted in an enhanced absorption profile as well as an energy band-gap reduction. The cyanopyridone-functionalized compound was stable and the presence of solubilizing alkyl group on the cyanopyridone acceptor unit delivered a material that can be easily solution processed. Further, we report (chapter IV) a D-A conjugated small molecule donor CP3, containing a barbituric acid as the terminal acceptor functionality capable of hydrogen bonding and a structural analogue CP4, bearing a N-ethylrhodanine terminal acceptor unit, incapable of participating in self-assembly to the same degree. The OPV devices with CP3 and CP4 as donor materials were fabricated, and the device outcome using CP3 distinctly outperformed the devices based on CP4 (CP3 = 6.31% vs CP4 = 3.74%), a result that not only validates the positive impact of self-assembly on the photovoltaic properties of the oligothiophene donor but is amongst the highest and most encouraging efficiencies based on the D-¿-A format. Absorption maxima, as well as extinction coefficients, were increased with increasing acceptor strengths.

Similarly, when r with a higher number of cyclic rings as conjugated π-spacers were used they resulted in the same phenomenon of broader absorption and band-gap reduction. The second project of the thesis deals with the design and synthesis of three molecules coded as T1, T2, T3. It was demonstrated that the intramolecular charge transfer (ICT) transition was enhanced due to acceptor group with increased acceptor strength, which in-turn is responsible for greater light-harvesting and better positioning of energy levels. T1, T2 and T3 the first reported examples in the literature the device outcome reported herein is among the highest efficiencies that has been achieved using a simple device architecture.

The use of rigid thiophenes to generate a series of novel donor-acceptor small organic molecules that contain a common triphenylamine donor and alkylated cyanopyridone acceptor were found to be chemically stable and soluble in common organic solvents such as chloroform, toluene and chlorobenzene. The photovoltaic response was enhanced with the use of stronger acceptors, such as cyanopyridone, when compared with an analogue containing an acceptor of lower strength, such as dicyanovinyl acceptor group. However, the photovoltaic device conversion efficiency remained almost constant when oligothiophenes were compared with rigidified thiophenes.

Finally, the third project of the thesis deals with spiro[fluorene-9,9'-xanthene]-functionalized, 3D acceptor material (coded as SFX1) was designed, synthesized, characterized and utilized for organic photovoltaic applications. The new material SFX1 is a non-planar structure, and exhibited good solubility, thermal stability, and promising optoelectronic properties. The energy levels were complementing those of the commercially available, cheap and two different types of donor polymers namely P3HT and PTB7. SFX1 produced well-intermixed blend surfaces with both donors, though the surface quality of PTB7-based blend was much superior. The device efficiency of PTB7: SFX1 combination (9.42%) is a result that is among the best efficiency numbers in the current literature using 3D acceptors based on non-planar formats and the commercially and cheaply available donor polymers.

Overall, this study provides a systematic examination of the effect of changing the donor, central π-bridge, and acceptor part of the donor-acceptor modular materials for bulk-heterojunction. It is of interest to see how the donor and acceptor modifications in a given donor-acceptor module may affect the performance of benchmark materials used for bulk-heterojunction solar cells.