Nanostructured p-Type Semiconductor Electrodes and Photoelectrochemistry of Their Reduction Processes

Abstract

This review reports the properties of p-type semiconductors with nanostructured features employed as photocathodes in photoelectrochemical cells (PECs). Light absorption is crucial for the activation of the reduction processes occurring at the p-type electrode either in the pristine or in a modified/sensitized state. Beside thermodynamics, the kinetics of the electron transfer (ET) process from photocathode to a redox shuttle in the oxidized form are also crucial since the flow of electrons will take place correctly if the ET rate will overcome that one of recombination and trapping events which impede the charge separation produced by the absorption of light. Depending on the nature of the chromophore, i.e., if the semiconductor itself or the chemisorbed dye-sensitizer, different energy levels will be involved in the cathodic ET process. An analysis of the general properties and requirements of electrodic materials of p-type for being efficient photoelectrocatalysts of reduction processes in dye-sensitized solar cells (DSC) will be given. The working principle of p-type DSCs will be described and extended to other p-type PECs conceived and developed for the conversion of the solar radiation into chemical products of energetic/chemical interest like non fossil fuels or derivatives of carbon dioxide.

Effect of Alkyl Chain Length on the Sensitizing Action of Substituted Non‐Symmetric Squaraines for p‐Type Dye‐Sensitized Solar Cells

“…[7,8] Dyesensitizers in the excited state can behave either as electron donors towards the supporting semiconductor with occurrence of electron injection from the SOMO of the excited dye to the conduction band (CB) of the semiconductor (of n-type), [9] or as electron acceptors with occurrence of hole injection in the valence band (VB) of the supporting semiconductor (of p-type) and resulting electron transfer to an empty or half-empty level of the electronically excited dye-sensitizer. [10] The dual role the dye-sensitizer can play in DSCs allows the classification of these devices into n-type (n-DSC) and p-type (p-DSC) with the n-type displaying the largest conversion efficiencies (above 14 %). [11] Insofar, the p-type counterparts haven't surpassed the efficiency limit of 3 %.…”

First Evidence of Electrode Reconstruction in Mesoporous NiO After Operation as Photocathode of Dye‐Sensitized Solar Cells

“…Therefore, dye‐sensitizers in the excited state can behave either as electron donors (D in Figure ) towards the supporting semiconductor with occurrence of electron injection from the SOMO of the excited dye to the conduction band (CB) of the semiconductor of n ‐type (Figure ), or, alternatively, as electron acceptors (A in Figure ) with occurrence of hole injection in the valence band (VB) of the supporting semiconductor of p ‐type (Figure ). The latter process results in the electron transfer from the semiconductor to an empty or half‐empty level of the electronically excited dye‐sensitizer . The electron transfer process between the excited sensitizer and the supporting substrate is usually described in terms of the matching of the frontier energy levels between dye and semiconductor and in terms of the physical properties of the optically excited state of the sensitizer when this is in the immobilized state.…”

“…The latter process results in the electron transfer from the semiconductor to an empty or half-empty level of the electronically excited dye-sensitizer. [28] The electron transfer process between the excited sensitizer and the supporting substrate is usually described in terms of the matching of the frontier energy levels between dye and semiconductor and in terms of the physical properties of the optically excited state of the sensitizer when this is in the immobilized state. Within the context of DSC the most important excited state properties of the sensitizer include its lifetime, the charge distribution, the geometric conformation and its optical absorption when the dye is in the surface immobilized state.…”

Effect of Sodium Hydroxide Pretreatment of NiOx Cathodes on the Performance of Squaraine‐Sensitized p‐Type Dye‐Sensitized Solar Cells

“…The structure of the sensitizer, along with the widening of the absorption band from the visible to the NIR region, plays a crucial role to maximize the photocurrent and the overall efficiency in DSSCs. To our knowledge, few NIR absorbing dyes have been proposed, so far, for the sensitization of nickel oxide photocathodes . Squaraines are well known for their sharp and intense absorption in the NIR region of the spectrum .…”