The design, synthesis, and characterization of a series of diketopyrrolopyrrole-based copolymers with different chalcogenophene comonomers (thiophene, selenophene, and tellurophene) for use in field-effect transistors and organic photovoltaic devices are reported. The effect of the heteroatom substitution on the optical, electrochemical, and photovoltaic properties and charge carrier mobilities of these polymers is discussed. The results indicate that by increasing the size of the chalcogen atom (S < Se < Te), polymer band gaps are narrowed mainly due to LUMO energy level stabilization. In addition, the larger heteroatomic size also increases intermolecular heteroatom-heteroatom interactions facilitating the formation of polymer aggregates leading to enhanced field-effect mobilities of 1.6 cm(2)/(V s). Bulk heterojunction solar cells based on the chalcogenophene polymer series blended with fullerene derivatives show good photovoltaic properties, with power conversion efficiencies ranging from 7.1-8.8%. A high photoresponse in the near-infrared (NIR) region with excellent photocurrents above 20 mA cm(-2) was achieved for all polymers, making these highly efficient low band gap polymers promising candidates for use in tandem solar cells.

We have synthesized and characterized a series of triphenylamine-based hole-transport materials (HTMs), and studied their function in solid-state dye sensitized solar cells (ss-DSSCs). By increasing the electron-donating strength of functional groups (-H <-Me <-SMe <-OMe) we have systematically shifted the oxidation potential and ensuing photocurrent generation and open-circuit voltage of the solar cells. Correlating the electronic properties of the HTM to the device operation highlights a significant energy offset required between the Dye-HTM highest occupied molecular orbital (HOMO) energy levels. From this study, it is apparent that precise control and tuning of the oxidation potential is a necessity, and usually not achieved with most HTMs developed to date for ss-DSSCs. To significantly increase the efficiency of solid-state DSSCs understanding these properties, and implementing dye-HTM combinations to minimize the required HOMO offset is of central importance.

Two new D-p-A type organic sensitizers, MP124 and MP-I-50, were synthesized and their electrochemical and spectroscopic properties studied. Efficiencies of DSSC devices utilizing these dyes were also investigated, where sensitization solvent, sensitization time and additive concentration were all varied. Under standard AM 1.5G simulated solar radiation, optimized MP124 devices show an efficiency of 7.45% (V oc ¼ 0.73 V; J sc ¼ 14.44 mA cm À2 ; FF ¼ 70%) while optimized MP-I-50 devices show an efficiency of 5.66% (V oc ¼ 0.68 V; J sc ¼ 12.06 mA cm À2 ; FF ¼ 69%). Transient absorption spectroscopy studies show that regeneration of dye cations by the red-ox electrolyte was more efficient in MP124 cells which is attributed to its higher HOMO energy leading to greater driving force for the regeneration reaction. Transient photovoltage studies showed that electron lifetimes were longer lived in MP124 explaining the higher V oc for these cells compared to MP-I-50 cells. DFT and MP2 calculations indicate that this is due to the greater tendency of MP-I-50 to form charge-transfer complexes with the I 2 species in the electrolyte, due to the presence of an additional EDOT in its structure compared to MP124. This work highlights the effect that small changes to the sensitizer structure can have on the interfacial charge transfer reactions and ultimately on the device efficiency.

The molecular structure of the catalyst strongly influences the kinetics of charge separation and recombination.