Back Electron Transfer Research Articles

Complementary properties of silver nanoparticles on the photovoltaic performance of titania nanospheres based photoanode in dye-sensitized solar cells

In this study, the fabrication of photoanode of dye-sensitized solar cell using silver@titania nanospheres and N719 dye, and its enhanced photovoltaic performance in the dye-sensitized solar cell is described. Silver@titania with different mol. % of silver (1, 3 and 5 %) are synthesized using facile sol-gel and photochemical reduction method and are characterized by suitable analytical techniques. The synthesized nanocomposite materials showed nanospheres like morphology. Moreover, the photoluminescence studies revealed that incorporation of silver nanoparticles on the titania nanospheres surface suppressed the charge recombination process and which is more beneficial for dye-sensitized solar cells. Silver@titania nanospheres are successfully employed as a photoanode and demonstrated with an enhanced solar-to-electrical energy conversion efficiency of 5.24 %, under full sun illumination (100 mW cm−2, AM 1.5G) which is 30.67 % enhancement than that of bare titania nanospheres (4.01 %). This enhanced efficiency is attributed to the rapid interfacial charge transfer process and plasmonic effect offered by silver nanoparticles present in the silver@titania nanospheres. The improved charge transfer process led to minimize the back electron transfer reaction in the device. With complementary properties of silver nanoparticles, the high performance demonstrated by the silver@titania nanospheres could be an excellent candidate for the light energy harvesting applications.

Synthesis of Quaternary Carbon-Centered Benzoindolizidinones via Novel Photoredox-Catalyzed Alkene Aminoarylation: Facile Access to Tylophorine and Analogues

Photoredox-catalyzed aminoarylation and thioamination of unactivated alkenes have been developed, providing novel synthetic routes to access synthetically challenging quaternary carbon-centered ben...

Decoding Charge Recombination through Charge Generation in Organic Solar Cells

The in‐depth understanding of charge carrier photogeneration and recombination mechanisms in organic solar cells is still an ongoing effort. In donor:acceptor (bulk) heterojunction organic solar cells, charge photogeneration and recombination are inter‐related via the kinetics of charge transfer states—being singlet or triplet states. Although high‐charge‐photogeneration quantum yields are achieved in many donor:acceptor systems, only very few systems show significantly reduced bimolecular recombination relative to the rate of free carrier encounters, in low‐mobility systems. This is a serious limitation for the industrialization of organic solar cells, in particular when aiming at thick active layers. Herein, a meta‐analysis of the device performance of numerous bulk heterojunction organic solar cells is presented for which field‐dependent photogeneration, charge carrier mobility, and fill factor are determined. Herein, a “spin‐related factor” that is dependent on the ratio of back electron transfer of the triplet charge transfer (CT) states to the decay rate of the singlet CT states is introduced. It is shown that this factor links the recombination reduction factor to charge‐generation efficiency. As a consequence, it is only in the systems with very efficient charge generation and very fast CT dissociation that free carrier recombination is strongly suppressed, regardless of the spin‐related factor.

Delayed fluorescence as a new screening method of plant species for urban greening: an experimental study using four bryophytes

Selecting suitable plant species is important in urban greening because urban environments often provide unfavorable conditions for plant growth. We focused on delayed fluorescence (DF), i.e., light emitted from plants immediately following light exposure as a result of back electron transfer in photosynthesis, as an indicator of photosynthetic activity. Moss greening, urban greening using bryophytes, has attracted attention as a new domain of greening that does not require soil. Therefore, tolerance to drying, which is a characteristic environmental factor in cities, of four bryophytes—Bryum argenteum Hedw., Racomitrium japonicum Dozy et Molk., Hypnum plumaeforme Wilson, and Polytrichum commune Hedw.—was evaluated in the laboratory using DF. Assuming an urban environment, experiments were performed to measure the transition of DF following the cessation of irrigation and re-watering. The recovery of accumulated DF values (0–200 s) by rehydration 8 days after irrigation cessation was increased in the order H. plumaeforme ≈ P. commune < R. japonicum < B. argenteum. B. argenteum presented an increase in the accumulated amount of DF 8 h after irrigation cessation, and presented a significantly higher DF when compared with the other three species at 16 h. Together with results of previous studies, these findings of desiccation tolerance in the studied species indicate that DF is a useful method for characterizing desiccation tolerance in plants and selecting potentially useful plants for urban greening.

Photodetection efficiency enhancement of IR sensitive organic photodetectors via triplet energy transfer

Development of the low optical gap organic semiconductors that can emit and sense light in the near to mid IR is an important research area. Photodetection in the IR region comes with its unique challenges. As the optical gap is lowered, often a back electron transfer event from the charge-transfer state of triplet character (3CT) to the molecular triplet state (T1) of photoexcited donor works as a terminal loss channel. Here, we propose a new mechanism for enhancing free charge generation efficiency by attaining a desired control over the spin-dependent recombination pathways in low optical gap polymer-fullerene systems. In carefully designed organic cascades of Pentacene/C60:PCPDTBT: 70-PCBM, we show that 3CT mediated Dexter type triplet energy transfer (TET) process from PCPDTBT to pentacene is instrumental in enhancing external quantum efficiency (EQE) in the IR region of PCPDTBT. Our results show that EQE in the IR region of this multilayer cascade benefits from triplet exciton recycling by at least ˜20%. The proposed TET process is generic in allowing manipulation of spin-dependent recombination pathways in favor of efficient photocurrent generation in low optical gap organic diodes.

Mechanism of the photorelease of alcohols from the 9-phenyl-9-tritylone protecting group.

The 9-phenyl-9-tritylone photoremovable protecting group was examined using both photoproduct analysis and laser flash photolysis in order to determine the details of its mechanism of alcohol release. It is shown that formation of the tritylone anion radical is required for alcohol release. Attempts to trigger release via intramolecular photoinduced electron transfer were unsuccessful due to rapid back electron transfer reactions of the triplet diradical anion.

Superior ZnS deposition for augmenting the photostability and photovoltaic performance of PbS quantum-dot sensitized solar cells

Simply by elevating the successive ionic layer adsorption and reaction temperature for ZnS deposition, we successfully demonstrated improved photostability of PbS-based quantum dot-sensitized solar cells (QDSCs) which was credited to efficient surface coverage of ZnS over PbS QDs and TiO2. Using open-circuit voltage decay and cyclic voltammetry techniques distinctly, it was affirmed that the back electron transfer at TiO2/PbS/electrolyte interface was reduced. Superior deposition of ZnS in terms of surface coverage not only exhibited increase in photocurrent density (from 20.7 mA/cm2 to 22.3 mA/cm2) and power conversion efficiency (from 3.1% to 3.4%), it also stabilized the normalized current significantly.

Electrochemical modulation of plasmon-induced charge separation behaviour at Au-TiO2 photocathodes.

Plasmon-induced charge separation (PICS) at the interface between a plasmonic nanoparticle and a semiconductor becomes less efficient as the plasmon resonance wavelength increases, because the energy of a photon may not be sufficiently higher than the interfacial Schottky barrier height. In this study, we developed PICS photocathodes by coating Au nanoparticles of different sizes on an ITO electrode with a thin TiO2 layer, and applied negative potentials to those photocathodes so as to suppress back electron transfer and improve the PICS photocurrent responses. The photocurrent enhancement factor was increased as the particle size was decreased, and enhancement of about two orders of magnitude was observed for small Au nanoparticles when bias voltage of 0.5 V was applied. In some cases the photocurrent enhancement was accompanied by a slight redshift of the photocurrent peak, which was caused by a lowered barrier. This technique would be useful for tuning the photocurrents when it is applied to devices such as electrochemical LSPR sensors and photodetectors.

Effect of the Interfacial Energy Landscape on Photoinduced Charge Generation at the ZnPc/MoS2 Interface.

Monolayer transition-metal dichalcogenide crystals (TMDC) can be combined with other functional materials, such as organic molecules, to form a wide range of heterostructures with tailorable properties. Although a number of works have shown that ultrafast charge transfer (CT) can occur at organic/TMDC interfaces, conditions that would facilitate the separation of interfacial CT excitons into free carriers remain unclear. Here, time-resolved and steady-state photoemission spectroscopy are used to study the potential energy landscape, charge transfer, and exciton dynamics at the zinc phthalocyanine (ZnPc)/monolayer (ML) MoS2 and ZnPc/bulk MoS2 interfaces. Surprisingly, although both interfaces have a type-II band alignment and exhibit sub-100 fs CT, the CT excitons formed at the two interfaces show drastically different evolution dynamics. The ZnPc/ML-MoS2 behaves like typical donor-acceptor interfaces in which CT excitons dissociate into electron-hole pairs. On the contrary, back electron transfer occur at ZnPc/bulk-MoS2, which results in the formation of triplet excitons in ZnPc. The difference can be explained by the different amount of band bending found in the ZnPc film deposited on ML-MoS2 and bulk-MoS2. Our work illustrates that the potential energy landscape near the interface plays an important role in the charge separation behavior. Therefore, considering the energy level alignment at the interface alone is not enough for predicting whether free charges can be generated effectively from an interface.

A Silicon-Based Heterojunction Integrated with a Molecular Excited State in a Water-Splitting Tandem Cell.

Semiconductor-based photocathodes with high light-absorption capability are of interest in the production of solar fuels, but many of them are limited by low efficiencies due to rapid interfacial back electron transfer. We demonstrate here that a nanowire-structured p-type Si (p-Si) electrode, surface-modified with a perylene-diimide derivative (PDI'), can undergo photoreduction of a surface-bound, water reduction catalyst toward efficient H2 evolution under a low applied bias. At the electrode interface, the PDI' layer converts green light into high-energy holes at its excited state for extraction of photogenerated electrons at the photoexcited p-Si. The photogenerated electrons at the reduced PDI' are subsequently transferred to the molecular H2-evolution catalyst. Involvement of the photoexcited PDI' enables effective redox separation between the electrons at the reduced catalyst and the holes at the valence band of p-Si. The heterojunction photocathode was used in a tandem cell by coupling with a dye-sensitized photoanode for solar-driven water splitting into H2 and O2.

Photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in ionic liquids: Effect of cation alkyl-chain length

Transient absorption spectroscopy has been applied to study the photo-excitation dynamics of N, N-dimethyl-p-nitroaniline in twelve imidazolium/phosphonium cation-based ionic liquids (ILs) with alkyl chains of different lengths. The transient absorption spectra showed an initial bleach signal around the absorption center (400 nm) and a stimulated emission band around 500 nm. The bleach signal was recovered mostly within a few picoseconds, and a hot band absorption appeared around 450 nm. The time profiles of the band integrals for the bleach and absorption spectral regions were simulated by a sum of four exponential decays. The internal conversion rate corresponding to the back electron transfer (b-ET) rate from the charge-separated excited state, was determined by the time profile of the band integral of the bleach region. The vibrational energy relaxation (VER) rate in the ground state was determined from the time profile of the hot band absorption decay band integral. The b-ET rate decreased with increasing number of alkyl carbons in the cation (#C). The solvent dependence was discussed in terms of the free energy change at the Marcus inverted region, which was indicated by the blue-shift of the ground state absorption band. The change of the free energy is ascribed to the change of the charge concentration of ILs. The VER rate decreased slightly with increasing #C. The reason was discussed in terms of the interaction of DMpNA with the polar region of ILs. The intersystem crossing yield also increased with increasing #C, reflecting an increase in the non-polar region of the ILs.

Poly(3,4-ethylenedioxypyrrole) coating and poly(4-styrenesulfonate) polyanions enhance solar cell performance

Co-sensitized photoanode with nitrogen doped graphene particles (NGPs) and CdS capable of harvesting light efficiently in the visible region is combined with a poly(3,4-ethylenedioxypyrrole) (PEDOP)@carbon(C)-cloth based counter electrode (CE) for the first time. The conduction band of NGPs is wedged between that of CdS and TiO2, and charge separation is improved in TiO2/NGPs/CdS compared to TiO2/CdS, thus resulting in a power conversion efficiency (PCE), greater by 37% and 305% in comparison to TiO2/CdS and TiO2/NGPs cells. The electrolyte is a polysulfide/silica gel modified with a polyanionic additive of sodium poly(4-styrenesulfonate) (NaPSS). NaPSS polymer chains serve as a passivating layer at the photoanode/electrolyte interface and reduce back electron transfer, while simultaneously allowing efficient hole transfer to the electrolyte. An optimal NaPSS content is ascertained from the highest PCE, conductivity and longest photovoltage decay time achieved for the electrolyte with 5 wt% NaPSS, compared to electrolytes with lower and higher NaPSS contents. The efficacy of PEDOP@C-cloth as a CE for the solar cell is confirmed from a lower charge transfer resistance and a higher diffusion current density for polysulfide reduction at the CE/electrolyte interface compared to sole C-cloth. This amplified solar cell performance is attained owing to the synergy of all the three components: NGPs at the photoanode, NaPSS in the electrolyte and PEDOP at the CE thus manifesting in a PCE of 7.7% for the TiO2/NGPs/CdS/polysulfide/silica/NaPSS gel/PEDOP@C-cloth cell relative to 3.5% attained for the TiO2/CdS/polysulfide/silica gel/C-cloth cell.

Efficiency Enhancement in Dye-Sensitized Solar Cells with ZnO and TiO2 Blocking Layers

Dye-sensitized solar cells (DSSCs) have great potential for solar generation owing to their low cost and ease of fabrication compared with silicon-based photovoltaic devices. One of the major issues with TiO2-based DSSCs is the recombination loss at the substrate–electrolyte interface due to the mesoporous nature of the TiO2 film. It was proposed earlier that introduction of a blocking layer at the substrate–TiO2 interface could reduce this recombination loss by preventing direct contact of the substrate and electrolyte. In this study, ZnO blocking layers (ZBLs) with different thicknesses were spin-coated onto fluorine-doped tin oxide (FTO) glass and the influence studied in comparison with a conventional TiO2 blocking layer (TBL) prepared using the same technique. The ZBL functions as an energy barrier at the FTO–electrolyte interface to prevent backtransfer of electrons to the electrolyte from the FTO. The blocking effect of the ZBL was verified by an enhancement of the fill factor (FF) and open-circuit photovoltage (Voc) of the DSSC, leading to an improvement in the power conversion efficiency (PCE) from 3.86% to 4.34% for the ZBL with optimum thickness of 120 nm. The suppression of the dark current density revealed a reduction of charge recombination with increasing ZBL thickness. Further increase in the ZBL thickness resulted in reduced cell performance due to a drastic decrease of the short-circuit current density (Jsc). The PCE was slightly improved to 4.36% by replacing the ZBL with a 100-nm TBL. These results suggest that the facile and low-cost sol–gel spin-coating technique is a feasible method for formation of a ZBL or TBL to reduce the recombination loss in DSSCs.

Superior light harnessing and charge injection kinetics utilizing mirror-like nano cuboidal ceria coupled with reduced graphene oxide in zinc oxide nanoparticle based photovoltaics

Efficiency in nanoparticle based photovoltaics is limited by optical transparency, light absorption as well as detrimental back transfer of electron at the hetero-interfaces. Three dimensional (3D) micro/nanostructures with excellent light scattering properties play pivotal role in light harvesting efficiency in DSSCs. Present study deals with the design and development of ternary hybrid photoanode utilizing high quality mirror like nano-cuboidal ceria (CeO2 NC) and 2D- reduced graphene oxide (RGO) sheets in conjunction with ZnO nanoparticle. A ∼6% power conversion efficiency has been achieved for photoanode with optimized CeO2 NC loaded with 1 wt% RGO into ZnO NP. CeO2 NC owing to its size and high quality mirror like facets provides a better light harvesting by multiple interactions of incident photon with the absorber as revealed by UV–Vis diffused reflectance and IPCE analysis. 2D- RGO is proposed to act as an electron sink and provides faster electron transport pathway. Inclusion of 2D- RGO sheets yields a better charge injection kinetics (keinj ∼ 2.3 × 108 s−1 for ternary, 1.1 × 108 s−1 for reference device) and collection at FTO as well as elevated recombination resistance (Rrec) and photo-induced electron life time (τe), unveiled by Electrochemical Impedance Spectroscopic (EIS) analysis corroborates a reduced reverse tunneling of photo-injected electron at ZnO/sensitizer/redox couple interface.

Tuning Chemical Interface Damping: Interfacial Electronic Effects of Adsorbate Molecules and Sharp Tips of Single Gold Bipyramids.

The optimization of the localized surface plasmon resonance (LSPR)-decaying channels of hot-electrons is essential for efficient optical and photochemical processes. Understanding and having the ability to control chemical interface damping (CID) channel contributions will bring about new possibilities for tuning the efficiency of plasmonic hot-electron energy transfer in artificial devices. In this scanning electron microscopy-correlated dark-field scattering study, the CID was controlled by focusing on the electronic nature of disubstituted benzene rings acting as adsorbates, as well as the effects of sharp tips on gold bipyramids (AuBPs) with similar aspect ratios to those of gold nanorods. The results showed that the sharp tips on single AuBPs, as well as the electronic effects of the adsorbate molecules, increase the interfacial contact between the nanoparticles and adsorbate molecules. Electron withdrawing groups (EWGs) on the adsorbates induce larger homogeneous LSPR line widths compared to those of electron donating groups (EDGs). Depending on the location (ortho, meta, and para) of the EDG, the effect of benzene rings with an EDG, which was considered to be induced by sulfur atoms bound to the nanoparticle surface, is weakened by the back transfer of electrons facilitated by the difference in the availability of the electrons of the EDG. Therefore, this study reports that the CID in the LSPR total decay channels can be tuned by controlling the electron withdrawing and electron donating features of adsorbate molecules with the surface topology of metal.

Bimolecular photoinduced electron transfer between 7-methylbenzo[a]pyrene and aromatic amine donors in stationary and static regimes

Determination of primary/intrinsic electron transfer rate (ket = 1/τet, s−1) in bimolecular photoinduced electron transfer reactions (PET) in solution phase monitoring fluorescence quenching is a controversial issue, predominantly due to constraint imposed by diffusion, and measurement of rate of formation of transient intermediates, therefore primary electron transfer rate, was suggested (J. Phys. Chem. B 2015, 119, 11253 − 11261). Herein, we report electron transfer dynamic of excited 7-methylbenzo[a]pyrene (MBP) in acetonitrile (ACN) in presence of different aromatic amine donors from very low concentration to finally in neat donors to access intrinsic, diffusion free, rate constants of bimolecular PET by using steady state and femtosecond time resolved fluorescence and femtosecond transient absorption (fsTA) spectroscopy. It appears that in lower concentration of donors, diffusion control or stationary regime of ET, observed dynamics by time resolved fluorescence as well as transient absorption spectroscopy replicate to each other leading to determine the bimolecular quenching rates (kd, M−1 s−1) instead of primary or intrinsic ET rates. In very higher donor concentration or in neat donors, non-diffusive or intrinsic regime of ET, amalgamation of femtosecond time-resolved emission and fsTA studies along with global and target analysis determines the primary/intrinsic ET rates (ket) between excited MBP and studied donors which are one to two orders of magnitude larger than corresponding bimolecular quenching rate constants (kd) in ACN. Intrinsic electron transfer occurs with very high rate of ket (∼1011 s−1) and the process lies in Marcus normal region and back electron transfer occur very slow kbet (∼108 s−1), the process lies in Marcus inverted region.

Improve the performance of the quantum dot sensitized ZnO nanotube solar cells with inserting ZnS-MnS composites layers

Interface passivation layer deposited onto photoanodes has been proven to be an available way to suppress charge recombination and enhance the power conversion efficiency in quantum dot-sensitized solar cells. In this work, the ZnS-MnS composites layers inserted in ZnO/CdS nanotubes photoanodes interface is firstly fabricated via ion-exchange and chemical bath deposition methods, simultaneously. By inserting the ZnS-MnS composites, electron back transfer is efficiently hindered. In addition, the uniform ZnS-MnS layer effectively blocks the direct contact between the polysulfide electrolyte and ZnO nanotubes, which inhabits the recombination of electron-hole pairs occurred at the interface. Besides, the MnS layer is in favour of the CdS quantum dots in higher load, profiting the light absorption intensity in the visible light region. Thus ZnO/ZnS-MnS/CdS nanotubes quantum dot-sensitized solar cells have efficiency of 3.62%, which is much higher than that of the solar cell with ZnS passivation layer (2.33%). This work demonstrates that constructing ZnS-MnS interface passivation layer with a proper band gap structure is a promising approach to enhance the performance of quantum dot-sensitized solar cells.

Hole Hopping Across a Protein-Protein Interface.

We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCuI, where two adjacent tryptophan residues (W124 and W122) are inserted between the CuI center and a Re photosensitizer coordinated to a H126 imidazole (Re = ReI(H126)(CO)3(dmp)+, dmp = 4,7-dimethyl-1,10-phenanthroline). Optical excitation of this mutant in aqueous media (≤40 μM) triggers 70 ns electron transport over 23 Å, yielding a long-lived (120 μs) ReI(H126)(CO)3(dmp•-)WWCuII product. The Re126FWCuI mutant (F124, W122) is not redox-active under these conditions. Upon increasing the concentration to 0.2-2 mM, {Re126WWCuI}2 and {Re126FWCuI}2 are formed with the dmp ligand of the Re photooxidant of one molecule in close contact (3.8 Å) with the W122' indole on the neighboring chain. In addition, {Re126WWCuI}2 contains an interfacial tryptophan quadruplex of four indoles (3.3-3.7 Å apart). In both mutants, dimerization opens an intermolecular W122' → //*Re ET channel (// denotes the protein interface, *Re is the optically excited sensitizer). Excited-state relaxation and ET occur together in two steps (time constants of ∼600 ps and ∼8 ns) that lead to a charge-separated state containing a Re(H126)(CO)3(dmp•-)//(W122•+)' unit; then (CuI)' is oxidized intramolecularly (60-90 ns) by (W122•+)', forming ReI(H126)(CO)3(dmp•-)WWCuI//(CuII)'. The photocycle is closed by ∼1.6 μs ReI(H126)(CO)3(dmp•-) → //(CuII)' back ET that occurs over 12 Å, in contrast to the 23 Å, 120 μs step in Re126WWCuI. Importantly, dimerization makes Re126FWCuI photoreactive and, as in the case of {Re126WWCuI}2, channels the photoproduced "hole" to the molecule that was not initially photoexcited, thereby shortening the lifetime of ReI(H126)(CO)3(dmp•-)//CuII. Although two adjacent W124 and W122 indoles dramatically enhance CuI → *Re intramolecular multistep ET, the tryptophan quadruplex in {Re126WWCuI}2 does not accelerate intermolecular electron transport; instead, it acts as a hole storage and crossover unit between inter- and intramolecular ET pathways. Irradiation of {Re126WWCuII}2 or {Re126FWCuII}2 also triggers intermolecular W122' → //*Re ET, and the Re(H126)(CO)3(dmp•-)//(W122•+)' charge-separated state decays to the ground state by ∼50 ns ReI(H126)(CO)3(dmp•-)+ → //(W122•+)' intermolecular charge recombination. Our findings shed light on the factors that control interfacial hole/electron hopping in protein complexes and on the role of aromatic amino acids in accelerating long-range electron transport.

Computational study on the removal of photolabile protecting groups by photochemical reactions

The application and versatility of photolabile protecting groups (PPGs) have continued to expand since their inception almost 50 years ago. In the present work we employ density functional theory (DFT) to characterize ground and excited state properties of several compounds with PPGs and to explore possible, controllable photodissociation reactions that remove these PPGs. The reaction mechanism is examined by analyzing the DFT results as well as dynamical simulations of nonadiabatic electron transfer reactions using multilayer multiconfiguration time-dependent Hartree theory. The photoexcited compounds appear to undergo ultrafast back electron transfer to the ground electronic state via internal conversion. For some compounds, the electronic-nuclear couplings facilitate sufficient photo energy transfer to the nuclear degrees of freedom that dominate the reaction coordinate, leading to the dissociation reaction that removes the PPGs.

Light-driven water oxidation by a dye-sensitized photoanode with a chromophore/catalyst assembly on a mesoporous double-shell electrode.

A mesoporous atomic layer deposition (ALD) double-shell electrode, Al2O3 (insulating core)//ALD ZnO|ALD TiO2, on a fluorine-doped tin oxide (FTO) conducting substrate was explored for a photoanode assembly, FTO//Al2O3 (insulating core)//ALD ZnO|ALD TiO2|-chromophore-catalyst, for light-driven water oxidation. Photocurrent densities at photoanodes based on mesoporous ALD double-shell (ALD ZnO|ALD TiO2|) and ALD single-shell (ALD ZnO|, ALD TiO2|) electrodes were investigated for O2 evaluation by a generator-collector dual working electrode configuration. The high photocurrent densities obtained based on the mesoporous ALD ZnO|ALD TiO2 photoanode for O2 evolution arise from a significant barrier to back electron transfer (BET) by the optimized tunneling barrier in the structure with the built-in electric field at the ALD ZnO|ALD TiO2 interface. The charge recombination is thus largely decreased. In the films, BET following injection has been investigated through kinetic nanosecond transient absorption spectra, and the results of energy band analysis are used to derive insight into the internal electronic structure of the electrodes.