4H-Cyclopenta[2,1-b:3,4-b’]dithiophene Donors and Quinoxaline Acceptors with Broadened Absorption Patterns toward Light Harvesting Materials for Organic Photovoltaics

The steady development of new (photoactive) materials for organic solar cell applications has led to continuous improvements in the power conversion efficiencies (PCE) of these devices. Additionally, the use of new solar cell configurations (e.g. tandem devices) has allowed to obtain PCEs that nowadays exceed 10%. Having in mind the relatively low production costs, the development of stable materials and the potential advantages in terms of flexibility, transparency, aesthetics and low-light applications, this technology has a real potential of holding an important share of the future photovoltaics market. The research work described in this PhD thesis was dedicated to the synthesis of two major (electron) donor and (electron) acceptor classes of materials, i.e. 4Hcyclopenta[2,1-b:3,4-b’]dithiophenes (CPDTs) and quinoxalines (Qxs), their integration in low band gap copolymers and evaluation of these derivatives as light harvesting materials for organic photovoltaics. Being among the first low band gap polymers that have been developed for organic solar cells, affording PCEs superior to 5%, PCPDTBT has attracted considerable attention. The one-pot synthetic procedure giving access to the substituted CPDT building block has first been optimized and was then applied to combine CPDT donor units with a 2,1,3-benzothiadiazole acceptor by means of Suzuki polycondensation reactions. The photovoltaic performances of symmetrically substituted PCPDTBT-type polymers, carrying 2-ethyhexyl and dodecyl side chains, have been evaluated in solar cells with bilayer, bulk heterojunction and tandem configurations (Chapter 1). The efficiencies that have been obtained for the 2-ethyhexyl-substituted polymer were close to the reported literature values. The polymer decorated with dodecyl substituents, which was less soluble and therefore more difficult to process, afforded lower PCEs, probably due to a poor film morphology of the active layer blend. On the other hand, this material, being insoluble in chloroform, has allowed the preparation of a bilayer solar cell, for which the presence of a CT (charge transfer) ground state has been detected by the FTPS (Fourier-transform photocurrent spectroscopy) technique. A series of PCPDTBT-type polymers bearing 2-ethylhexyl, octyl or dodecyl side chains, for which the CPDT building block was synthesized using the one-pot procedure and/or a recently developed three step synthetic method, has been subjected to a thermally induced degradation study (Chapter 4). The evolution of the conjugated materials was monitored by UV-Vis/IR spectroscopy and a combined TG-TD-GC/MS technique. The synthesis of the monoalkylated CPDT derivative has afforded a deeper insight into the degradation pathway of this class of materials. Finally, the combined TG-TD-GC/MS technique revealed quasi-identical behavior for all polymers, regardless the synthetic history of the CPDT building block. The synthesis of a new class of small molecules, i.e. 2H-cyclopenta[2,1-b:3,4- b']dithiophene-2,6(4H)-diones, issued from the CPDT family, has been achieved by bromination of 4H-cyclopenta[2,1-b:3,4-b’]dithiophene derivatives in the presence of N-bromosuccinimide (NBS) (Chapter 2). It was proven that the amount of NBS and the solvent play a crucial role in the formation of these products, which were found to be thermodynamically favored (as demonstrated by theoretical calculations). The conversion of 4H-cyclopenta[2,1-b:3,4-b’]dithiophen-4-one to the corresponding 5H-spiro(benzo[1,2-b:6,5-b']dithiophene-4,4'-cyclopenta[2,1-b:3,4-b’]-dithiophen)-5- one was achieved by pinacol rearrangement in the presence of trivalent phosphorus reagents (Chapter 3). The proposed structure and the experimentally observed 13C- and 1H NMR chemical shifts were confirmed by X-ray crystallography and theoretical calculations. The good agreement between the experimental and theoretical data illustrated the power and accuracy of the calculation methods. The synthesis and optical characterization of a series of two dimensionally (2D) conjugated 5,8-dibromoquinoxaline monomers and thiophene-Qx-thiophene triads, bearing alkyl, aryl or heteroaryl side chains separated by ethenyl and/or butadienyl spacers from the electron deficient quinoxaline core, have been performed (Chapter 5). At the monomer stage, the vertical conjugation created by the introduction of the conjugated side chains resulted in the extension of the absorption spectrum toward the visible range and the apparition of a second band as a result of the intramolecular charge transfer from the donor side chains to the electron acceptor Qx core. The synthesis of dibrominated triads, promising coupling units toward the synthesis of conjugated polymers, was achieved by careful design and optimization of the synthetic routes. The synthesis of 2D-conjugated poly(thienoquinoxaline) derivatives was achieved by Stille polycondensation between 2,5-bis(trialkylstannyl)thiophenes and 5,8-dibromoquinoxaline derivatives bearing ethenyl and/or butadienyl spacers between the solubilizing side chains and the quinoxaline core (Chapter 6). The purity of the organostannyl derivative was proven to be crucial for obtaining high degrees of polymerization. These donor polymers were then used, in combination with a fullerene acceptor, in the fabrication of bulk heterojunction solar cells. Additional fractionation of the polymers by means of preparative GPC showed a direct correlation between the number average molecular weight, the optical properties and the photovoltaic performance of the isolated fractions. The combination of benzo[1,2-b:3,4-b]dithiophene (BDT) or 4H-cyclopenta[2,1-b:3,4-b’]dithiophene (CPDT) donors and quinoxaline or bis(thieno)quinoxaline acceptors by Stille polycondensation reactions afforded a series of low band gap polymers (Chapter 7). The lowest onset values, as determined from the absorption spectra, were noticed for PCPDT-Qx (~1.51 eV). Preliminary solar cell results using a PBDTQx2:PC71BM blend showed efficiencies of ~1.0%, superior to the ones reported in literature for similar polymers. Once again, fractionation by preparative GPC was shown beneficial.

Publication year:2012

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