Organic Chromophores Designed for Hole Injection into Wide-Band-Gap Metal Oxides for Solar Fuel Applications

Abstract

The power conversion efficiency of tandem dye-sensitized photoelectrosynthesis cells is limited by the number of p-type metal oxide semiconductors and chromophores available for the photocathode. Here, we introduce a new class of donor–acceptor–donor organic chromophores with deep highest occupied molecular orbital (HOMO) levels to target hole injection into PbTiO3, a wide-band-gap metal oxide alternative to NiO. Electrochemical measurements show that we can tune the range of the chromophores’ electron affinity between ∼4.30 and 3.30 eV and keep the ionization potential at ∼−6.50 eV. The absorption profiles for these chromophores span throughout the visible region. The new chromophore reported here, benzobis­(thiadiazole)­bis­(thiophene-3-carboxylic acid), T2-BBT, adsorbed onto PbTiO3, obtained a maximum absorbed photon-to-current conversion efficiency of 54%. These results highlight the importance of designing chromophores with favorable redox potentials to study promising p-type wide-band-gap metal oxides for photocathodic applications.

Influence of Surface and Structural Variations in Donor–Acceptor–Donor Sensitizers on Photoelectrocatalytic Water Splitting

“…As a reference for BTD2 , the activation energy barrier (Δ E ) required to planarize a BTD ter-heterocycle sharing similar dihedral angles is Δ E = 0.6 kcal mol –1 . For comparison, the energy required to transition BTD1 from the twisted state (Φ = 50°) to the planar conformation (Φ = 180°) is an order of magnitude larger, Δ E = 6.5 kcal mol –1 . Thus, we conclude that the BTD2 structure has a more effective π-conjugation between the thiophene donor and the benzothiadiazole core (induced by planarity) than BTD1 .…”

“…47 For comparison, the energy required to transition BTD1 from the twisted state (Φ = 50°) to the planar conformation (Φ = 180°) is an order of magnitude larger, ΔE = 6.5 kcal mol −1 . 48 Thus, we conclude that the BTD2 structure has a more effective π-conjugation between the thiophene donor and the benzothiadiazole core (induced by planarity) than BTD1. Figure 3A,B shows the UV−visible absorption spectra of BTD1 and BTD2 taken in dichloromethane solution and of the chromophores adsorbed on an electronically inert wide band gap nanostructured Al 2 O 3 films.…”

“…Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a promising approach for artificial photosynthesis by offering a modular design that incorporates various chromophore–catalyst assemblies to drive photoelectrochemical reactions such as water splitting or CO 2 reduction. − While efforts have been made toward developing the tandem DSPEC, challenges in matching light absorptivity and energetic alignment continue to limit the number of viable molecular assemblies available for this system. − Organic chromophores could be a key to overcoming this challenge due to their synthetic accessibility and structural versatility, giving rise to classes of materials exhibiting desirable redox and optical properties for DSPECs. These properties include chromophores with high molar extinction coefficients (ε), broad spectral absorbance, and tunable redox potentials, offering an alternative to metal-based sensitizers, which often are limited by these parameters. − For example, high-throughput computational screenings of diketopyrrolopyrrole derivatives adopting ∼75 000 different building block combinations demonstrated a wide range of optoelectronic properties (various optical band gap, fundamental energy gap, ionization potentials, electron affinities, etc.) that can be achieved using organic molecules .…”

Plasma‐Induced Nanocrystalline Domain Engineering and Surface Passivation in Mesoporous Chalcogenide Semiconductor Thin Films

“…In this regard, optically active materials hold promise for self‐sustainable photon‐to‐electron conversion processes to initiate photo‐assisted electrochemical reactions. Mesoporous semiconductors such as wide‐band gap metal oxides have emerged as potential candidates for optoelectronic and energy conversion applications in the ultraviolet (UV) range [12–14] . Functionalizing semiconductor materials to cover a broader range of the electromagnetic spectrum, including the visible wavelength range, could potentially create new exciting applications [15, 16] …”

Near-Surface Composition, Structure, and Energetics of TiO2 Thin Films: Characterization of Stress-Induced Defect States in Oxides Prepared via Chemical Vapor Deposition versus Solution Deposition from Sol–Gel Precursors

“…TiO 2 thin films are attractive options as electron-selective charge harvesting/injecting interlayers in thin film solar cells (PV) and light-emitting displays because of their energetic alignment with conduction band edges of several device relevant active layers, their high overall chemical stability, and because they can be readily formed by scalable deposition technologies such as chemical vapor deposition (CVD), sol–gel (SG) processing, sputtering, pulsed laser deposition (PLD), and atomic layer deposition (ALD). − Such ultrathin oxide thin films have also seen use as chemically stable “separation layers” between electrolytes and photoactive, possible unstable semiconductors (e.g., Si). The oxide layer is expected to protect the underlying semiconductor from corrosion and provide anchoring sites (and protection) for catalysts that can inject/extract charge from the photoexcited semiconductor through the thin oxide layer, while enhancing rates of fuel-forming electrochemical processes. − In all of these platforms there is a trade-off between chemical stability imparted by the thin oxide, adequate electrical conductivity typically provided by doping via “defects”, and minimized chemical reactivity and surface recombination, often occurring at these same dopant sites. These defect states can be chemically and electrochemically reactive for OPV active layers, for photocatalysts, and especially with the precursors for perovskite PVs. − There remain significant unanswered questions about which macroscopic processing options provide the best electronic properties for oxide-based electron- or hole-selective interlayers, especially where defect states can be formed and removed by small changes in processing conditions. ,,− …”