Dynamics of Back Electron Transfer in Dye-Sensitized Solar Cells Featuring 4-<i>tert</i>-Butyl-Pyridine and Atomic-Layer-Deposited Alumina as Surface Modifiers

A series of dye-sensitized solar cells (DSCs) was constructed with TiO₂ nanoparticles and N719 dye. The standard I₃^–/I^– redox shuttle and the Co­(1,10-phenanthroline)₃^3+/2+ shuttle were employed. DSCs were modified with atomic-layered-deposited (ALD) coatings of Al₂O₃ and/or with the surface-adsorbing additive 4-<i>tert</i>-butyl-pyridine. Current–voltage data were collected to ascertain the influence of each modification upon the back electron transfer (ET) dynamics of the DSCs. The primary effect of the additives alone or in tandem is to increase the open-circuit voltage. A second is to alter the short-circuit current density, <i>J</i>_SC. With dependence on the specifics of the system examined, any of a myriad of dynamics-related effects were observed to come into play, in both favorable (efficiency boosting) and unfavorable (efficiency damaging) ways. These effects include modulation of (a) charge-injection yields, (b) rates of interception of injected electrons by redox shuttles, and (c) rates of recombination of injected electrons with holes on surface-bound dyes. In turn, these influence charge-collection lengths, charge-collection yields, and onset potentials for undesired dark current. The microscopic origins of the effects appear to be related mainly to changes in driving force and/or electronic coupling for underlying component redox reactions. Perhaps surprisingly, only a minor role for modifier-induced shifts in conduction-band-edge energy was found. The combination of DSC-efficiency-relevant effects engendered by the modifiers was found to vary substantially as a function of the chemical identity of the redox shuttle employed. While types of modifiers are effective, a challenge going forward will be to construct systems in ways in which the benefits of organic and inorganic modifiers can be exploited in fully additive, or even synergistic, fashion

Determining the Conduction Band-Edge Potential of Solar-Cell-Relevant Nb₂O₅ Fabricated by Atomic Layer Deposition

Often key to boosting photovoltages in photoelectrochemical and related solar-energy-conversion devices is the preferential slowing of rates of charge recombinationespecially recombination at semiconductor/solution, semiconductor/polymer, or semiconductor/perovskite interfaces. In devices featuring TiO₂ as the semiconducting component, a common approach to slowing recombination is to install an ultrathin metal oxide barrier layer or trap-passivating layer atop the semiconductor, with the needed layer often being formed via atomic layer deposition (ALD). A particularly promising barrier layer material is Nb₂O₅. Its conduction-band-edge potential <i>E</i>_CB is low enough that charge injection from an adsorbed molecular, polymeric, or solid-state light absorber and into the semiconductor can still occur, but high enough that charge recombination is inhibited. While a few measurements of <i>E</i>_CB have been reported for conventionally synthesized, bulk Nb₂O₅, none have been described for ALD-fabricated versions. Here, we specifically determine the conduction-band-edge energy of ALD-fabricated Nb₂O₅ relative to that of TiO₂. We find that, while the value for ALD-Nb₂O₅ is indeed higher than that for TiO₂, the difference is less than anticipated based on measurements of conventionally synthesized Nb₂O₅ and is dependent on the thermal history of the material. The implications of the findings for optimization of competing interfacial rate processes, and therefore photovoltages, are briefly discussed

Inhibiting Metal Oxide Atomic Layer Deposition: Beyond Zinc Oxide

Atomic layer deposition (ALD) of several metal oxides is selectivity inhibited on alkanethiol self-assembled monolayers (SAMs) on Au, and the eventual nucleation mechanism is investigated. The inhibition ability of the SAM is significantly improved by the in situ H₂-plasma pretreatment of the Au substrate prior to the gas-phase deposition of a long-chain alkanethiol, 1-dodecanethiol (DDT). This more rigorous surface preparation inhibits even aggressive oxide ALD precursors, including trimethylaluminum and water, for at least 20 cycles. We study the effect that the ALD precursor purge times, growth temperature, alkanethiol chain length, alkanethiol deposition time, and plasma treatment time have on Al₂O₃ ALD inhibition. This is the first example of Al₂O₃ ALD inhibition from a vapor-deposited SAM. The inhibitions of Al₂O₃, ZnO, and MnO ALD processes are compared, revealing the versatility of this selective surface treatment. Atomic force microscopy and grazing-incidence X-ray fluorescence further reveal insight into the mechanism by which the well-defined surface chemistry of ALD may eventually be circumvented to allow metal oxide nucleation and growth on SAM-modified surfaces

Engendering Long-Term Air and Light Stability of a TiO₂‑Supported Porphyrinic Dye via Atomic Layer Deposition

Organic and porphyrin-based chromophores are prevalent in liquid-junction photovoltaic and photocatalytic solar-cell chemistry; however, their long-term air and light instability may limit their practicality in real world technologies. Here, we describe the protection of a zinc porphyrin dye, adsorbed on nanoparticulate TiO₂, from air and light degradation by a protective coating of alumina grown with a previously developed post-treatment atomic layer deposition (ALD) technique. The protective Al₂O₃ ALD layer is deposited using dimethylaluminum isopropoxide as an Al source; in contrast to the ubiquitous ALD precursor trimethylaluminum, dimethylaluminum isopropoxide does not degrade the zinc porphyrin dye, as confirmed by UV–vis measurements. The growth of this protective ALD layer around the dye can be monitored by an in-reactor quartz crystal microbalance (QCM). Furthermore, greater than 80% of porphyrin light absorption is retained over ∼1 month of exposure to air and light when the protective coating is present, whereas almost complete loss of porphyrin absorption is observed in less than 2 days in the absence of the ALD protective layer. Applying the Al₂O₃ post-treatment technique to the TiO₂-adsorbed dye allows the dye to remain in electronic contact with both the semiconductor surface and a surrounding electrolyte solution, the combination of which makes this technique promising for numerous other electrochemical photovoltaic and photocatalytic applications, especially those involving the dye-sensitized evolution of oxygen

Distance-Engineered Plasmon-Enhanced Light Harvesting in CdSe Quantum Dots

Improvement of light harvesting in semiconductor quantum dots (QDs) is essential for the development of efficient QD-based solar energy conversion systems. In this study, plasmon-enhanced light absorption in CdSe QDs sensitized on silver (Ag) nanoparticle (NP) films was examined as a function of interparticle (QD to Ag NP) distance. Up to 24-fold plasmonic enhancement of fluorescence from QDs was observed when the particle separation distance was ≥5 nm. The enhancement effect was observed to largely sustain the exciton lifetimes in QDs and to strongly depend on the incident photon wavelength following the plasmon resonant strength of Ag NPs, confirming that the enhanced photoluminescence was mainly due to the enhancement in photoabsorption in CdSe QDs by the plasmon of Ag NPs. This study suggests applications of Ag NPs in QD-based solar energy conversion for significantly improving light harvesting in QDs

Low-Temperature Atomic Layer Deposition of CuSbS₂ for Thin-Film Photovoltaics

Copper antimony sulfide (CuSbS₂) has been gaining traction as an earth-abundant absorber for thin-film photovoltaics given its near ideal band gap for solar energy conversion (∼1.5 eV), large absorption coefficient (>10⁴ cm^–1), and elemental abundance. Through careful in situ analysis of the deposition conditions, a low-temperature route to CuSbS₂ thin films via atomic layer deposition has been developed. After a short (15 min) postprocess anneal at 225 °C, the ALD-grown CuSbS₂ films were crystalline with micron-sized grains, exhibited a band gap of 1.6 eV and an absorption coefficient >10⁴ cm^–1, as well as a hole concentration of 10¹⁵ cm^–3. Finally, the ALD-grown CuSbS₂ films were paired with ALD-grown TiO₂ to form a photovoltaic device. This photovoltaic device architecture represents one of a very limited number of Cd-free CuSbS₂ PV device stacks reported to date, and it is the first to demonstrate an open-circuit voltage on par with CuSbS₂/CdS heterojunction PV devices. While far from optimized, this work demonstrates the potential for ALD-grown CuSbS₂ thin films in environmentally benign photovoltaics

High-Surface-Area Architectures for Improved Charge Transfer Kinetics at the Dark Electrode in Dye-Sensitized Solar Cells

Dye-sensitized solar cell (DSC) redox shuttles other than triiodide/iodide have exhibited significantly higher charge transfer resistances at the dark electrode. This often results in poor fill factor, a severe detriment to device performance. Rather than moving to dark electrodes of untested materials that may have higher catalytic activity for specific shuttles, the surface area of platinum dark electrodes could be increased, improving the catalytic activity by simply presenting more catalyst to the shuttle solution. A new copper-based redox shuttle that experiences extremely high charge-transfer resistance at conventional Pt dark electrodes yields cells having fill-factors of less than 0.3. By replacing the standard Pt dark electrode with an inverse opal Pt electrode fabricated via atomic layer deposition, the dark electrode surface area is boosted by ca. 50-fold. The resulting increase in interfacial electron transfer rate (decrease in charge-transfer resistance) nearly doubles the fill factor and therefore the overall energy conversion efficiency, illustrating the utility of this high-area electrode for DSC

Fast Transporting ZnO–TiO₂ Coaxial Photoanodes for Dye-Sensitized Solar Cells Based on ALD-Modified SiO₂ Aerogel Frameworks

A doubly coaxial photoanode architecture based on templated SiO₂ aerogels was fabricated on transparent conducting oxides for use in dye-sensitized solar cells (DSSCs). These templates were coated with ZnO <i>via</i> atomic layer deposition (ALD) to yield an electronically interconnected, low-density, high-surface-area, semiconductor framework. Addition of a thin conformal layer of a second metal oxide (alumina, zirconia, or titania) <i>via</i> ALD was found to suppress the dissolution of ZnO that otherwise occurs when it is soaked in alcohol solutions containing acidic dyes used for sensitization or in acetonitrile solutions containing a pyridine derivative and the iodide/tri-iodide (I^–/I^–₃) redox shuttle. Electron transport in SiO₂–ZnO–TiO₂ photoelectrodes was found to be nearly 2 orders of magnitude faster than in SiO₂–TiO₂ structures, implying that the interior ZnO sheath serves as the primary electron conduit. In contrast, rates of electron interception by the oxidized form of the redox shuttle were observed to decrease when a TiO₂ shell was added to SiO₂–ZnO, with the decreases becoming more significant as the thickness of the titania shell increases. These effects lead to improvements in efficiency for DSSCs that utilize I^–/I^–₃, but much larger improvements for DSSCs utilizing ferrocene/ferrocenium, a notoriously fast redox shuttle. For the former, overall energy conversion efficiencies maximize at 4.0%. From a variety of experiments, the primary factor limiting aerogel-based DSSC performance is light loss due to scattering. Nevertheless, variants of the doubly coaxial structure may prove useful in devising DSSCs that can achieve excellent energy conversion efficiencies even with fast redox shuttles

Atomic Layer Deposition of the Quaternary Chalcogenide Cu₂ZnSnS₄

Atomic layer deposition (ALD) is a layer-by-layer synthesis method capable of depositing conformal thin films with thickness and compositional control on subnanometer length scales. While many materials have been synthesized by ALD, the technologically important metal sulfides are underexplored, and homogeneous quaternary metal sulfides are absent from the literature. We report an ALD process to synthesize Cu₂ZnSnS₄ (CZTS), a potentially low cost semiconductor being explored for photovoltaic applications. Two strategies are reported: one in which a trilayer stack of binary metal sulfides (i.e., Cu₂S, SnS₂ and ZnS) is deposited and mixed by thermal annealing, as well as a supercycle strategy that is similar to the conventional ALD procedure for forming nanolaminates. Both routes rely on the facile solid state diffusion of chalcogenides for mixing. For this ALD route to the CZTS system, the challenges are nucleation, ion-exchange between the film and the volatile chemical precursors, and phase-stability of binary SnS₂. The thin films were made with no sulfurization step. The X-ray diffraction and Raman spectra were consistent with the formation of CZTS. X-ray fluorescence measurements revealed that the films contained the expected amount of sulfur based on the target oxidation states. Photoelectrochemical measurements under simulated AM1.5 illumination using Eu³⁺ as an electron acceptor demonstrated that the films were photoactive and had an average internal quantum efficiency (IQE) of 12%

Porphyrins as Templates for Site-Selective Atomic Layer Deposition: Vapor Metalation and in Situ Monitoring of Island Growth

Examinations of enzymatic catalysts suggest one key to efficient catalytic activity is discrete size metallo clusters. Mimicking enzymatic cluster systems is synthetically challenging because conventional solution methods are prone to aggregation or require capping of the cluster, thereby limiting its catalytic activity. We introduce site-selective atomic layer deposition (ALD) on porphyrins as an alternative approach to grow isolated metal oxide islands that are spatially separated. Surface-bound tetra-acid free base porphyrins (H₂TCPP) may be metalated with Mn using conventional ALD precursor exposure to induce homogeneous hydroxide synthetic handles which acts as a nucleation point for subsequent ALD MnO island growth. Analytical fitting of in situ QCM mass uptake reveals island growth to be hemispherical with a convergence radius of 1.74 nm. This growth mode is confirmed with synchrotron grazing-incidence small-angle X-ray scattering (GISAXS) measurements. Finally, we extend this approach to other ALD chemistries to demonstrate the generality of this route to discrete metallo island materials