Understanding the Role of Surface States on Mesoporous NiO Films

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 Al 2 O 3 . 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 – /I 3 – 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 I 3 – species. The intra-bandgap states here act as catalysts to assist I 3 – 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 TiO 2 -based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.

NiO/ZrO2 nanocomposites as photocathodes of tandem DSCs with higher photoconversion efficiency with respect to parent single-photoelectrode p-DSCs

“…This indicates that low levels of ZrO2 do not alter the reactivity of NiO towards a given sensitizer, i.e. under these conditions ZrO2 does not practically dilute the surface concentration of defective Ni(III) sites 77 in correspondence of which dye-anchoring is supposed to occur. 33,58 The further increase of Zr content in NZNC films (when Zr/Ni > 0.05) leads to the formation of ZrO2 macrostructures in association with the decrease of dyeloading capability (vide infra Table 2, first column on the right).…”

Printable and Homogeneous NiOx Hole Transport Layers Prepared by a Polymer‐Network Gel Method for Large‐Area and Flexible Perovskite Solar Cells

“…In addition, due to the differentiated particle surface state, the smaller size particles are more likely to be adsorbed to the surface of large particles during the dispersion process, thereby forming agglomeration. [29][30][31] The grains with large size differences usually result in a rougher film morphology, which in turn leads to poor interface contact. [32][33][34] Therefore, the uniformity of NiO x film fabricated based on low-temperature solutions is usually a big challenge, especially under flexible or large-area conditions.…”

Unraveling the Mechanisms of Beneficial Cu-Doping of NiO-Based Photocathodes

“…In addition to the dye and catalyst design, the properties of NiO and, in particular, surface defects are likely to play an important role in the fast charge recombination often observed. − Unlike the dye and catalyst, modification of NiO has however gained less attention so far, , although Li + , , Mg 2+ , , and Co 2+ doping have been investigated by a number of groups and were observed to reduce charge recombination between NiO and dye. The band gap of NiO of 3.4–4.0 eV is assigned to the O 2p → Ni 3d electronic transition. − The intrinsic p-type character of NiO is generally accepted to originate from the presence of Ni vacancies where holes could localize .…”