Kinetic Evidence of Two Pathways for Charge Recombination in NiO-Based Dye-Sensitized Solar Cells

Mesoporous nickel oxide has been used as electrode material for p-type dye-sensitized solar cells (DSCs) for many years but no high efficiency cells have yet been obtained. One of the main issues that lowers the efficiency is the poor fill factor, for which a clear reason is still missing. In this paper we present the first evidence for a relation between applied potential and the charge recombination rate of the NiO electrode. In particular, we find biphasic recombination kinetics: a fast (15 ns) pathway attributed to the reaction with the holes in the valence band and a slow (1 ms) pathway assigned to the holes in the trap states. The fast component is the most relevant at positive potentials, while the slow component becomes more important at negative potentials. This means that at the working condition of the cell, the fast recombination is the most important. This could explain the low fill factor of NiO-based DSCs

Injection and Ultrafast Regeneration in Dye-Sensitized Solar Cells

Injection of an electron from the excited dye molecule to the semiconductor is the initial charge separation step in dye-sensitized solar cells (DSC’s). Though the dynamics of the forward injection process has been widely studied, the results reported so far are controversial, especially for complete DSC’s. In this work, the electron injection in titanium dioxide (TiO₂) films sensitized with ruthenium bipyridyl dyes N3 and N719 was studied both in neat solvent and in a typical iodide/triiodide (I^–/I₃^–) DSC electrolyte. Transient absorption (TA) spectroscopy was used to monitor both the formation of the oxidized dye and the arrival of injected electrons to the conduction band of TiO₂. Emission lifetime of the dye-sensitized films was recorded with time-correlated single photon counting to reveal nanosecond time scales of injection. It was found that the injection dynamics of the N3 and N719 dyes are similar. In solvent the injection from both dyes occurs in the femto- to picosecond time scale while in the I^–/I₃^– electrolyte, it slows down significantly, extending to the nanosecond time domain. The presence of the electrolyte was found to increase the excited state lifetime of the dyes, implying that injection efficiency remains high despite the slower kinetics of injection compared to neat solvent. A remarkable new finding was that the prominent absorption signal of the oxidized dye observed in neat solvent vanished almost completely in the presence of the electrolyte, while the arrival of electrons to the conduction band of TiO₂ was practically unaltered, only slowed down. The observed disappearance of the oxidized dye population in the I^–/I₃^– electrolyte is most likely related to the reduction of the oxidized dye by iodide I^–, which is the first step of the dye regeneration process. To the best of our knowledge, this is the first time initial dye regeneration has been shown to occur in a few picoseconds after injection

Ultrafast Electron Transfer Between Dye and Catalyst on a Mesoporous NiO Surface

The combination of molecular dyes and catalysts with semiconductors into dye-sensitized solar fuel devices (DSSFDs) requires control of efficient interfacial and surface charge transfer between the components. The present study reports on the light-induced electron transfer processes of p-type NiO films cosensitized with coumarin C343 and a bioinspired proton reduction catalyst, [FeFe]­(mcbdt)­(CO)₆ (mcbdt = 3-carboxybenzene-1,2-dithiolate). By transient optical spectroscopy we find that ultrafast interfacial electron transfer (τ ≈ 200 fs) from NiO to the excited C343 (“hole injection”) is followed by rapid (<i>t</i>_1/2 ≈ 10 ps) and efficient surface electron transfer from C343^– to the coadsorbed [FeFe]­(mcbdt)­(CO)₆. The reduced catalyst has a clear spectroscopic signature that persists for several tens of microseconds, before charge recombination with NiO holes occurs. The demonstration of rapid surface electron transfer from dye to catalyst on NiO, and the relatively long lifetime of the resulting charge separated state, suggests the possibility to use these systems for photocathodes on DSSFDs

Time-Resolved IR Spectroscopy Reveals a Mechanism with TiO₂ as a Reversible Electron Acceptor in a TiO₂–Re Catalyst System for CO₂ Photoreduction

Attaching the phosphonated molecular catalyst [Re^IBr­(bpy)­(CO)₃]⁰ to the wide-bandgap semiconductor TiO₂ strongly enhances the rate of visible-light-driven reduction of CO₂ to CO in dimethylformamide with triethanolamine (TEOA) as sacrificial electron donor. Herein, we show by transient mid-IR spectroscopy that the mechanism of catalyst photoreduction is initiated by ultrafast electron injection into TiO₂, followed by rapid (ps-ns) and sequential two-electron oxidation of TEOA that is coordinated to the Re center. The injected electrons can be stored in the conduction band of TiO₂ on an ms-s time scale, and we propose that they lead to further reduction of the Re catalyst and completion of the catalytic cycle. Thus, the excited Re catalyst gives away one electron and would eventually get three electrons back. The function of an electron reservoir would represent a role for TiO₂ in photocatalytic CO₂ reduction that has previously not been considered. We propose that the increase in photocatalytic activity upon heterogenization of the catalyst to TiO₂ is due to the slow charge recombination and the high oxidative power of the Re^II species after electron injection as compared to the excited MLCT state of the unbound Re catalyst or when immobilized on ZrO₂, which results in a more efficient reaction with TEOA

Suppression of Forward Electron Injection from Ru(dcbpy)₂(NCS)₂ to Nanocrystalline TiO₂ Film As a Result of an Interfacial Al₂O₃ Barrier Layer Prepared with Atomic Layer Deposition

Subnanometer-thick Al₂O₃ barrier layers on nanocrystalline TiO₂ film were prepared with atomic layer deposition (ALD). The method allowed variation of barrier thicknesses at atomic resolution also deep in nanoporous structures, which makes it a superior method as compared to, e.g., sol−gel techniques. In this letter we present results on the effect of Al₂O₃ barriers of various thicknesses on forward electron injection in dye-sensitized solar cells. A decrease in the amplitude of the oxidized Ru(dcbpy)₂(NCS)₂ dye absorption signal due to singlet injection was observed already after one deposition cycle that produces a discontinuous layer with nominal thickness of 1 Å. More than two layer coatings also slowed down the triplet injection. The findings indicate suppression of total electron injection, which is probably due to the Al₂O₃-induced weakening of electronic coupling between the dye and TiO₂ as well as modification of the TiO₂ electronic structure