Considerable efforts are dedicated to increasing the open-circuit voltage (Voc) of dye-sensitized solar cells (DSSCs) by slowing charge recombination dynamics using atomic layer deposition, alkyl-substituted dyes, coadsorbents, and other strategies. In this report, we introduce metal-ion coordination to a metal oxide bound dye as an alternative means of increasing Voc. Metal-ion coordination has minimal influence on the photophysical and electrochemical properties of the N3 dye, but presumably because of increased steric hindrance at the interface, it slows charge recombination kinetics and increases Voc by upwards of 130 mV relative to the parent N3 DSSC. With respect to the nature of the metal ion, the trend in decreasing short-circuit current (Jsc) and increasing Voc correlates with the charge of the coordinated metal ion (MIV → MIII → MII). We attribute this trend to electrostatic interactions between the metal cation and I– or I3–, with the more highly charged cations maintaining a higher concentration of mediator anions in proximity to the surface and, as a result, increasing the regeneration and recombination rates.

Self-assembled bilayers offer a promising strategy to directly harness photon upconversion via triplet–triplet annihilation (TTA-UC) and increase maximum theoretical solar cell efficiencies from 33% to >43%. Here we demonstrate that the choice of redox mediator in these solar cells has a profound influence on both the light harvesting and TTA-UC efficiency. Devices with CoII/III(phen)3 as the redox mediator produced the highest photocurrent yet generated from TTA-UC (0.158 mA cm–2) under 1 sun. Despite generating less photocurrent, CoII/III(pz-py-pz)2 devices achieved maximum TTA-UC efficiency at excitation intensities well below solar irradiance (0.8 mW cm–2), which is on par with the lowest value yet reported for any TTA-UC system. The large variation in performance with respect to mediator is attributed to triplet excited-state quenching via (1) energy transfer or paramagnetic quenching by the CoII species and (2) excited-state electron transfer to CoIII species.

Photon upconversion via triplet–triplet annihilation (TTA-UC) is an intriguing strategy to potentially increase maximum solar cell efficiencies from 33% to greater than 43%. TTA-UC has been incorporated into a dye-sensitized solar cell (DSSC) but the low solar energy conversion efficiency can be attributed, at least in part, to the relatively narrow absorption features of the sensitizer molecule. Here we incorporate multiple sensitizers into a TTA-UC DSSC using bilayer and trilayer self-assembly via metal ion linkages. The sensitizers' complementary absorption features increase broad-band light absorption and work cooperatively to achieve peak TTA-UC efficiency at sub-solar irradiance (<1 sun or <100 mW cm−2). The trilayer DSSC, with high sensitizer density and directional energy transfer cascade towards the charge seperation interface, exhibited the highest efficiency yet reported for harvesting low energy light in an integrated TTA-UC solar cell (1.2 × 10−3%).

We demonstrate the photocatalytic protonation of a silylenol ether using 7-bromo-2-naphthol as an ESPT catalyst with phenol as the sacrificial proton source. Greater than 95% conversion is achieved with 1 mol% catalyst. The reaction cycle is dependent on the significantly increased acidity of the catalyst in the excited state as well as the long lifetime for the triplet excited state of 7-bromo-2-naphthol. The reaction does not occur in the absence of light (367 nm) and can readily be controlled by light intensity modulation. We also demonstrate that a 72% reaction yield can be obtained with unsubstituted naphthol as the catalyst by coupling triplet energy transfer, via a visible light absorbing (445 nm) sensitizer, into the catalytic cycle. These results open the door to an entirely new class of sensitized photocatalytic reactions that harness the excited state acidity of ESPT dyes.

Enantiopure excited state proton transfer (ESPT) dyes were used for the asymmetric protonation of silyl enol ether. Under 365 nm irradiation, with 3,3′-dibromo-VANOL as the ESPT dye, up to 49% enantioselectivity with a 68% yield of product was observed at room temperature. The reaction is effective with a range of silyl enol ethers and can also be achieved with visible light upon the addition of triplet sensitizer. The relatively low ee of the protonated product is due to the racemization/decomposition of the ESPT dye in the excited state as indicated by circular dichroism, HPLC, and UV–vis spectroscopy.

Current high efficiency dye-sensitized solar cells (DSSCs) rely on the incorporation of multiple chromophores, via either codeposition or preformed assemblies, as a means of increasing broad band light absorption. These strategies have some inherent limitations including decreased total light absorption by each of the dyes, low surface loadings, and complex synthetic procedures. In this report, we introduce an alternative strategy, self-assembled bilayers, as a simple, stepwise method of incorporating two complementary chromophores into a DSSC. The bilayer devices exhibit a 10% increase in Jsc, Voc, and η over the monolayer devices due to increased incident photon-to-electron conversion efficiency across the entire visible spectrum and slowed recombination losses at the interface. Directional energy and electron transfer toward the metal oxide surface are key steps in the bilayer photon-to-current generation process. These results are important as they open the door to a new architecture for harnessing broadband light in dye-sensitized devices.Keywords: bilayer; DSSC; electron transfer; energy transfer; self-assembly

The rate and efficiency of electron transfer events at the semiconductor–dye–electrolyte interface is of critical importance to the overall performance of dye-sensitized solar cells (DSSCs) and dye-sensitized photoelectrosynthesis cells. In this work, we introduce self-assembled bilayers composed of a metal oxide electrode, bridging molecules, linking ions, and dye as an effective strategy to manipulate interfacial electron transfer events at the photoanode of DSSCs. Spectroelectrochemical measurements including current–voltage, incident photon-to-current efficiency, and electrochemical impedance spectroscopy are used to quantify interfacial electron transfer and transport events with respect to the length of the bridging molecules. The general trend in increased lifetime and diffusion length in TiO2 as well as an increase in open circuit voltage with bridge length indicate that the bilayer is an effective strategy in inhibiting the TiO2(e–) to redox mediator recombination events. However, the increased separation between the dye and the semiconductor also reduces the electron injection rate resulting in a decrease in photocurrent as the bridge length increases. The observed enhancement in open circuit voltages are far outweighed by the significant decrease in photocurrent and thus overall device performance decreases with increasing bridge length.Keywords: bilayer; dye-sensitized; open circuit voltage; recombination; self-assembled

Molecular photon upconversion via triplet–triplet annihilation (TTA-UC), combining two or more low energy photons to generate a higher energy excited state, is an intriguing strategy to surpass the maximum efficiency for a single junction solar cell (<34%). Here, we introduce self-assembled bilayers on metal oxide surfaces as a strategy to facilitate TTA-UC emission and demonstrate direct charge separation of the upconverted state. A 3-fold enhancement in transient photocurrent is achieved at light intensities as low as two equivalent suns. This strategy is simple, modular and offers unprecedented geometric and spatial control of the donor–acceptor interactions at an interface. These results are a key stepping stone toward the realization of an efficient TTA-UC solar cell that can circumvent the Shockley–Queisser limit.

Forward and back electron transfer at dye–semiconductor interfaces are pivotal events in dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. Here we introduce self-assembled bilayers as a strategy for manipulating electron transfer dynamics at these interfaces. The bilayer films are achieved by stepwise layering of bridging molecules, linking ions, and dye molecules on the metal oxide surface. The formation of the proposed architecture is supported by ATR-IR and UV–vis spectroscopy. By using time-resolved emission and transient absorption, we establish that the films exhibit an exponential decrease in electron transfer rate with increasing bridge length. The findings indicate that self-assembled bilayers offer a simple, straightforward, and modular method for manipulating electron transfer dynamics at dye–semiconductor interfaces.

•Research into molecularly sensitized energy systems is disproportionately weighted towards optimizing device performance rather than increasing lifetimes.•Extending the lifetime of these systems in aqueous or humid environments is advantageous as it could reduce costs and enable new devices.•A new atomic layer deposition treatment shows great promise for extending the lifetime of molecular attachment to inorganic surfaces under aqueous conditions.The recent transition of dye-sensitized solar cells (DSCs) from the lab to small-scale production validates the commercial viability of energy technologies that combine molecular functionality with nanomaterials. Commercialization of DSC technology is the result of advances in both device performance and lifetime. While academic researchers often focus on the former (performance), the latter (lifetime) is essential for commercial success. Increasing the lifetime of molecularly sensitized devices in either humid or aqueous environments will reduce device costs and enable a range of new device opportunities.

Photon upconversion via triplet–triplet annihilation (TTA-UC) is an intriguing strategy to potentially increase maximum solar cell efficiencies from 33% to greater than 43%. TTA-UC has been incorporated into a dye-sensitized solar cell (DSSC) but the low solar energy conversion efficiency can be attributed, at least in part, to the relatively narrow absorption features of the sensitizer molecule. Here we incorporate multiple sensitizers into a TTA-UC DSSC using bilayer and trilayer self-assembly via metal ion linkages. The sensitizers' complementary absorption features increase broad-band light absorption and work cooperatively to achieve peak TTA-UC efficiency at sub-solar irradiance (<1 sun or <100 mW cm−2). The trilayer DSSC, with high sensitizer density and directional energy transfer cascade towards the charge seperation interface, exhibited the highest efficiency yet reported for harvesting low energy light in an integrated TTA-UC solar cell (1.2 × 10−3%).

We demonstrate the photocatalytic protonation of a silylenol ether using 7-bromo-2-naphthol as an ESPT catalyst with phenol as the sacrificial proton source. Greater than 95% conversion is achieved with 1 mol% catalyst. The reaction cycle is dependent on the significantly increased acidity of the catalyst in the excited state as well as the long lifetime for the triplet excited state of 7-bromo-2-naphthol. The reaction does not occur in the absence of light (367 nm) and can readily be controlled by light intensity modulation. We also demonstrate that a 72% reaction yield can be obtained with unsubstituted naphthol as the catalyst by coupling triplet energy transfer, via a visible light absorbing (445 nm) sensitizer, into the catalytic cycle. These results open the door to an entirely new class of sensitized photocatalytic reactions that harness the excited state acidity of ESPT dyes.