Kinetic Competition in a Coumarin Dye-Sensitized Solar Cell: Injection and Recombination Limitations upon Device Performance

There is great interest in synthesizing organic dyes to replace organometallic dyes as sensitizers in nanocrystalline TiO2 solar cells. We present a detailed comparison of interfacial electron transfer kinetics in dye-sensitized solar cells containing the coumarin based organic dye (NKX2677) against those observed for the ruthenium based organometallic dye, N719, including analysis of excited state lifetimes, injection kinetics, regeneration kinetics, and recombination to both oxidized dyes and electrolyte. We find three primary differences in behavior between these two dyes which limit the performance of NKX2677-sensitized solar cells: a shorter lifetime of the electron injecting state for NKX2677 versus N719 (primarily due to it being a singlet rather than triplet sensitizer); a faster rate constant for recombination to the electrolyte (RCE) for NKX2677-sensitized solar cells; and the greater tendency for NKX2677 to aggregate, reducing the electron injection efficiency. The shorter excited state lifetime results in relatively fast injection kinetics being required for efficient electron injection, with transient photoluminescence measurements indicating <60 ps injection halftime for NKX2677 compared with similar to 350 ps injection halftimes for N719. This faster injection is achieved with NKX2677 by virtue of its relatively more negative excited state reduction potential, but is associated with a greater free energy loss driving electron injection. The faster recombination to the electrolyte is assigned to "catalysis" of this two-electron reaction by the sensitizer dye, most probably resulting from a local increase in the concentration of oxidized redox couple at the dye-sensitized interface, and provides a further limitation on the open circuit voltage achieved with NKX2677-sensitized solar cells. We conclude by discussing the extent to which these results are likely to reflect relatively generic differences between organic and rurthenium based organometallic sensitizer dyes and thus the implications for the development of efficient organic dye-sensitized solar cells.