Integrating a Semitransparent, Fullerene-Free Organic Solar Cell in Tandem with a BiVO4 Photoanode for Unassisted Solar Water Splitting.

We report an unassisted solar water splitting system powered by a diketopyrrolopyrrole (DPP)-containing semitransparent organic solar cell. Two major merits of this fullerene-free solar cell enable its integration with a BiVO4 photoanode. First is the high open circuit voltage and high fill factor displayed by this single junction solar cell, which yields sufficient power to effect water splitting when serially connected to an appropriate electrode/catalyst. Second, the wavelength-resolved photoaction spectrum of the DPP-based solar cell has minimal overlap with that of the BiVO4 photoanode, thus ensuring that light collection across these two components can be optimized. The latter feature enables a new water splitting device configuration wherein the solar cell is placed first in the path of incident light, before the BiVO4 photoanode, although BiVO4 has a wider bandgap. This configuration is accessed by replacing the reflective top electrode of the standard DPP-based solar cell with a thin metal film and an antireflection layer, thus rendering the solar cell semitransparent. In this configuration, incident light does not travel through the aqueous electrolyte to reach the solar cell or photoanode, and therefore, photon losses due to the scattering of water are reduced. Moreover, this new configuration allows the BiVO4 photoanode to be back-illuminated, i.e., through the BiVO4/back contact interface, which leads to higher photocurrents compared to front illumination. The combination of a semitransparent single-junction solar cell and a BiVO4 photoanode coated with oxygen evolution catalysts in a new device configuration yielded an unassisted solar water splitting system with a solar-to-hydrogen conversion efficiency of 2.2% in water.