Abstract
Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al2O3. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. Furthermore, surface states significantly affect dye regeneration (with I–/I3– as redox couple) and hole transport in NiO-based p-DSCs. A new dye regeneration mechanism is proposed in which electrons are transferred from reduced dye molecules to intra-bandgap states, and then to I3– species. The intra-bandgap states here act as catalysts to assist I3– reduction. A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO2-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.
Highlights
NiO crystalline mesoporous films have drawn widespread attention in the past decades,[1,2] partly due to NiO being a metal-deficient p-type semiconductor, having valuable electronic,[3−5] magnetic,[6,7] and catalytic properties.[8−10] Another important aspect is that nanocrystalline NiO films combine NiO’s properties into a mesoporous structure which provides a surface area several orders of magnitude larger, compared to the same geometric area in a planar structure.[11]
We applied a modified atomic layer deposition (ALD) procedure to achieve a better control on the surface passivation of mesoporous NiO films by a monolayer of Al2O3
A much lower photocurrent is observed in NiO-A-based dyesensitized solar cells with I−/I3− redox electrolyte
Summary
NiO crystalline mesoporous films have drawn widespread attention in the past decades,[1,2] partly due to NiO being a metal-deficient p-type semiconductor, having valuable electronic,[3−5] magnetic,[6,7] and catalytic properties.[8−10] Another important aspect is that nanocrystalline NiO films combine NiO’s properties into a mesoporous structure which provides a surface area several orders of magnitude larger, compared to the same geometric area in a planar structure.[11]. We experimentally prove that surface states, not the bulk, dominate the conductivity of NiO mesoporous films To our knowledge, this has not been proven in previous studies. When the surface states on NiO are significantly reduced (from 4 per nm[2] to 1 per nm2), the light-independent charge transport can transform into having an exponential dependence on light intensity. The pulse:purge cycle of H2O was run 200 cycles to react with the saturated adsorbed TMA inside the mesoporous NiO films. The detailed experimental procedure for the fstransient absorption measurements has been described elsewhere.[28] Briefly, 800 nm laser pulses from a mode-locked Ti:sapphire laser with temporal widths of roughly 100 fs were amplified by a regenerative amplifier operating at 3 kHz, split, and used to generate pump and probe pulses.
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