Proton-Induced Trap States, Injection and Recombination Dynamics in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Abstract

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize a sensitized metal oxide and a water oxidation catalyst in order to generate hydrogen and oxygen from water. Although the Faradaic efficiency of water splitting is close to unity, the recombination of photogenerated electrons with oxidized dye molecules causes the quantum efficiency of these devices to be low. It is therefore important to understand recombination mechanisms in order to develop strategies to minimize them. In this paper, we discuss the role of proton intercalation in the formation of recombination centers. Proton intercalation forms nonmobile surface trap states that persist on time scales that are orders of magnitude longer than the electron lifetime in TiO2. As a result of electron trapping, recombination with surface-bound oxidized dye molecules occurs. We report a method for effectively removing the surface trap states by mildly heating the electrodes under vacuum, which appears to primarily improve the injection kinetics without affecting bulk trapping dynamics, further stressing the importance of proton control in WS-DSPECs.

Dynamics of Electron Injection in SnO2/TiO2 Core/Shell Electrodes for Water-Splitting Dye-Sensitized Photoelectrochemical Cells

“…These nonmobile states are generally ascribed to uncoordinated metal centers and can act as acceptors states for charge injection and, as a result, will not be observed with TRTS. The process of injection into nonmobile surface states in aqueous electrolyte has been observed by others using sensitized ZnO 2 , as well as in our previous work focused on proton-induced surface trap states, which also detailed the inability of TRTS to observe these electrons . The passivation of these states is further supported by the lack of slow trapping in any of the ALD coated samples when compared to the bare SnO 2 .…”

“…The process of injection into nonmobile surface states in aqueous electrolyte has been observed by others using sensitized ZnO 2 34,35 as well as in our previous work focused on protoninduced surface trap states, which also detailed the inability of TRTS to observe these electrons. 36 The passivation of these states is further supported by the lack of slow trapping in any of the ALD coated samples when compared to the bare SnO 2 . The slow decay in the TRTS trace for bare SnO 2 is suggestive of trapping into surface states, as none of the TiO 2 -coated samples exhibit any long time, slow loss of THz amplitude associated with trapping.…”

Dye-sensitized photoelectrochemical water oxidation through a buried junction

“…After cooling to room temperature, a mesoporous TiO 2 film was doctorbladed onto the compact layer by using a 20-nm TiO 2 nanoparticle paste prepared by a previously reported method (20). The electrode films were then sintered at 300°C for 20 min, 350°C for 10 min, and 500°C for 30 min.…”

Linear correlation models for the redox potential of organic molecules in aqueous solutions

“…Energy technologies such as organic solar cells [1,2], light-emitting diodes [3], dye-sensitized solar cells [4][5][6][7][8][9][10][11], and artificial photosynthetic systems [12][13][14][15][16][17][18][19][20][21][22][23] rely on the reduction and oxidation (redox) of different organic molecules. The calculation of the redox potential of organic molecules in solutions is crucial for the rational design of such technologies.…”