Effects of Electron Trapping and Protonation on the Efficiency of Water-Splitting Dye-Sensitized Solar Cells

Abstract

Water-splitting dye-sensitized photoelectrochemical (WS-DSPECs) cells employ molecular sensitizers to absorb light and transport holes across the TiO2 surface to colloidal or molecular water oxidation catalysts. As hole diffusion occurs along the surface, electrons are transported through the mesoporous TiO2 film. In this paper we report the effects of electron trapping and protonation in the TiO2 film on the dynamics of electron and hole transport in WS-DSPECs. When the sensitizer bis(2,2'-bipyridine)(4,4'-diphosphonato-2,2'-bipyridine)ruthenium(II) is adsorbed from aqueous acid instead of from ethanol, there is more rapid hole transfer between photo-oxidized sensitizer molecules that are adsorbed from strong acid. However, the photocurrent and open-circuit photovoltage are dramatically lower with sensitizers adsorbed from acid because intercalated protons charge-compensate electron traps in the TiO2 film. Kinetic modeling of the photocurrent shows that electron trapping is responsible for the rapid electrode polarization that is observed in all WS-DSPECs. Electrochemical impedance spectroscopy suggests that proton intercalation also plays an important role in the slow degradation of WS-DSPECs, which generate protons at the anode as water is oxidized to oxygen.

Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells

“…First, as noted above, irreversibility of porphyrins on TiO 2 has been previously observed (20). Also, we have observed that films of [Ru(bpy) 2 (4,4′-(PO 3 H 2 ) 2 bpy)] with poor hole transport characteristics also exhibit electrochemical irreversibility on TiO 2 (33). Second, in nonaqueous solutions TMP (27) and TTP (39) are reversible, suggesting that the irreversibility we observe is not due to fundamental chemical instability of the sensitizers.…”

“…Faradaic efficiency is calculated as ∼100%, and collection efficiency previously determined is 81%. (33). In the current study, the bulky sensitizers TMP and PAP had the lowest surface coverages (0.45 × 10 −7 mol/cm 2 and 0.42 × 10 −7 mol/cm 2 ); however, TTP exhibited a higher surface coverage, possibly because the tolyl groups can achieve greater planarity, and the lack of ortho-methyl groups would allow closer approach of the macrocycles.…”

“…Using the same method, we found that D app for the porphyrin sensitizers used in this study was too slow to measure (≤10 −11 cm 2 ·s −1 ). By comparison, ruthenium polypyridyl complexes typically have D app values on the order of 10 −9 -10 −11 cm 2 ·s −1 (32,33).…”

“…We previously determined the optimal IrO 2 loading to be 0.5 pmol/cm 2 (19). At this loading, we measured open-circuit photovoltages of 1.07 ± 0.01 V and 1.11 ± 0.03 V with the [Ru(bpy) 2 (4,4′-(PO 3 H 2 ) 2 bpy)] sensitizer (19,33). The photovoltages we observed in this study were generally similar and fell between 1.00 V and 1.10 V ( Table 2).…”

Electron trapping induced electrostatic adsorption of cations: a general factor leading to photoactivity decay of nanostructured TiO2

“…This viewpoint can be supported by recent reports although they did not investigate the mechanism of the proton uptake. 42,43 The existence of surface defect states is inevitable for nanostructured inorganic semiconductors, which is why R-TiO 2 still shows slow photocurrent decay despite of its single crystal nature. Moreover, proton charge compensation uptake is also common for many other metal oxides.…”