Optoelectronic materials for bulk-heterojunction and dye-sensitized solar cells

Since the first report by Tang (1986), organic photovoltaic devices have been the subject of intense investigation. The research started to gain momentum in the late 1980s to early 90s and the two major classes of organic photovoltaic devices, polymeric solar cells and titanium dioxide film based solar cells, evolved. Polymer solar cells were termed as bulk-heterojunction solar cells from the mid 90s and the titanium dioxide film based cells were termed as dye-sensitized solar cells. Both types of organic cells include organic materials and over time, a variety of materials has been used. Some examples include the use of polymeric entities, metal complexes and small organic molecules. As organic materials and solvents are used in such type of solar cells, the following material properties are vital to the performance of organic solar cells: • Stability of organic materials 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 band-gap For investigating the above mentioned material properties, this dissertation presents a systematic study of newly designed materials with tuned optoelectronic properties. The investigated new materials are based on a donor-π-bridge-acceptor module where they have been designed, synthesized, characterized and applied in both types of organic photovoltaic devices, bulk-heterojunction solar cells and dye-sensitized solar cells. The materials which were designed, synthesized, characterized and tested for bulk-heterojunction solar cells indicated that there is a strong relationship between the three fragments (donor, central π-spacer and acceptor) of a donor-acceptor design. For example, the use of a strong acceptor, such as cyanopyridone, resulted in an enhanced absorption profile as well as an energy band-gap reduction. The cyanopyridone-functionalised compound was stable and the presence of solubilising alkyl group on the cyanopyridone acceptor unit delivers a material that can be easily solution processed. Further variation of the acceptors allows tuning of the electronic properties that contained a common triphenylamine donor and alkylated tetrathiophene π-bridge. Absorption maxima as well as extinction coefficients were increased with increasing acceptor strengths. The use of an alkylated tetrathiophene unit not only enhanced the solubility of the final material, but gave an additional absorption maximum at the higher energy to help maximise absorption of the visible spectrum. Greatest absorption of the visible spectrum was obtained when the tetrathiophene π-bridge was combined with the strongest acceptor, 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile. Similar observations were found when oligothiophenes were replaced with rigidified thiophenes where the use of the latter with a higher number of cyclic rings as conjugated π-spacers resulted in the same phenomenon of broader absorption and band-gap reduction. The use of rigidified 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. Direct amine substitution of the oligothiophene helped to achieve materials with enhanced solubility, significant spectral red-shift and reduced optical band-gaps. The photovoltaic performance highlights the superior performance of these direct nitrogen substituted oligothiophenes over conventional triaryl amino-oligothiophenes for dye-sensitized solar cells applications but not for bulk-heterojunction devices. New materials were also investigated for their potential applications in dye-sensitized solar cells. Within the scope of this thesis, designed materials were based on the donor-π-bridge-acceptor module and a study of donor, central π-block and an acceptor/anchoring unit is presented to address the above mentioned material properties. The introduction of a new acceptor group, 4-(cyanomethyl)benzoic acid, helped to enhance the dye’s absorption profile as well as photovoltaic response was improved. The enhancement of such parameters was observed when the new acceptor group was studied in-conjunction with either oligothiophenes or bridged thiophenes as central π-spacers. The dye-sensitized solar cell performance of new acceptor group was studied by comparing it with more widely used cyanoacrylic acid acceptor unit in a pair of analogous dyes. The dyes featuring the new acceptor unit outperformed the model sensitizers comprising the cyanoacrylic acid group in terms of its photovoltaic performance, which is mainly attributed to the superior charge injection properties. It was further observed that when the new acceptor unit was used in-conjunction with the replaced bridging para-phenyl group with a thiophene group, material properties such as solubility, absorption profile and energy conversion efficiency were enhanced. Further evaluation of the donor-π-acceptor module helped to investigate dithienopyrrole building block as a central π-bridge and its performance was compared with the more widely used cyclopentadithiophene linker unit in a pair of two analogous dyes. The dyes were tested with standard iodine/iodide as well as metal-complex electrolytes and it was observed that the dithienopyrrole π-bridge performed better when compared with cyclopentadithiophene group in terms of its photovoltaic performance. Increased electron lifetimes were further observed with the use of dithienopyrrole π-bridge for both the electrolyte systems. Dithienopyrrole π-bridge was also used to generate a set of novel dyes where it was used in-conjunction with thienothiophene and 3-hexylthiophenes to extend the central π-bridge. This was done in practice to enhance the absorption profile, solubility of the new sensitizers and to reduce the energy band gap. 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/dyes for both bulk-heterojunction and dye-sensitized solar cells. 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. Similarly, efficiency enhancement in the dye-sensitized solar cells with the use of a new acceptor unit suggests that (1) it’s advisable to synthesize structural analogues of currently known high efficiency sensitizers, implementing the design rules examined in this thesis and (2) to tailor dye-structures in conjunction with novel redox mediators such as cobalt polypyridyls and ferrocenes