This paper employs photoinduced absorption and electrochemical techniques to analyze the charge carrier dynamics that drive photoelectrochemical water oxidation on bismuth vanadate (BiVO₄), both with and without cobalt phosphate (CoPi) co-catalyst. These results are correlated with spectroelectrochemical measurements of Co^II oxidation to Co^III in a CoPi/FTO (fluorine doped tin oxide) electrode during dark electrocatalytic water oxidation. Electrocatalytic water oxidation exhibits a non-linear dependence on Co^III density, with a sharp onset at 1 × 10¹⁷ Co^III cm⁻². These results are compared quantitatively with the degree of CoPi oxidation observed under conditions of photoinduced water oxidation on CoPi–BiVO₄ photoanodes. For the CoPi–BiVO₄ photoanodes studied herein, ≤5% of water oxidation proceeds from CoPi sites, making the BiVO₄ surface the predominant water oxidation site. This study highlights two key factors that limit the ability of CoPi to improve the catalytic performance of BiVO₄: 1) the kinetics of hole transfer from the BiVO₄ to the CoPi layer are too slow to effectively compete with direct water oxidation from BiVO₄; 2) the slow water oxidation kinetics of CoPi result in a large accumulation of Co^III states, causing an increase in recombination. Addressing these factors will be essential for improving the performance of CoPi on photoanodes for solar-driven water oxidation.

This paper is concerned with the photophysics of triplet excitons in conjugated donor polymers, and their quenching by molecular oxygen. These photophysics are assayed by transient absorption spectroscopy, and correlated with X-ray diffraction measurements of relative material crystallinity. Eleven different donor polymers are considered, including representatives from several classes of donor polymers recently developed for organic solar cell applications. Triplet lifetimes in an inert (nitrogen) environment range from <100 ns to 5 μs. A remarkably quantitative correlation is observed between these triplet lifetimes and polymer XRD strength, with more crystalline polymers exhibiting shorter triplet lifetimes. Given the broad range of polymers considered, this correlation indicates that material crystallinity is the dominant factor determining triplet lifetime for the polymers studied herein. The rate constant for oxygen quenching of these triplet states, determined from a comparison of transient absorption data under inert and oxygen environments, also show a correlation with material crystallinity. Overall these dependencies result in the yield of oxygen quenching of polymer triplet states increasing strongly as the crystallinity of the polymer is reduced. These photophysical data are compared with photochemical stability of these donor polymers, assayed by photobleaching studies of polymer films under continuous light exposure in an oxygen environment. A partial correlation is observed, with the most stable polymers being the most crystalline, exhibiting negligible oxygen quenching yields. These results are discussed in terms of the likely origins of the correlations between material crystallinity and photophysics, and in terms of their implications for the environmental stability of such donor polymers in optoelectronic devices.

Photoinduced charge separation in bulk heterojunction solar cells is studied using a series of thiazolo-thiazole donor polymers that differ in their side groups (and bridging atoms) blended with two acceptor fullerenes, phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) and a fullerene indene-C60 bisadduct (ICBA). Transient absorption spectroscopy is used to determine the yields and lifetimes of photogenerated charge carriers, complimented by cyclic voltammetry studies of materials energetics, wide angle X-ray diffraction and transmission electron microscopy studies of neat and blend film crystallinity and photoluminescence quenching studies of polymer/fullerene phase segregation, and the correlation of these measurements with device photocurrents. Good correlation between the initial polaron yield and the energetic driving force driving charge separation, ΔE_CS is observed. All blend films exhibit a power law transient absorption decay phase assigned to non-geminate recombination of dissociated charges; the amplitude of this power law decay phase shows excellent correlation with photocurrent density in the devices. Furthermore, for films of one (relatively amorphous) donor polymer blended with ICBA, we observe an additional 100 ns geminate recombination phase. The implications of the observations reported are discussed in terms of the role of materials' crystallinity in influencing charge dissociation in such devices, and thus materials design requirements for efficient solar cell function.

A visible-light driven H₂ evolution system comprising of a Ru^II dye (RuP) and Co^III proton reduction catalysts (CoP) immobilised on TiO₂ nanoparticles and mesoporous films is presented. The heterogeneous system evolves H₂ efficiently during visible-light irradiation in a pH-neutral aqueous solution at 25 °C in the presence of a hole scavenger. Photodegradation of the self-assembled system occurs at the ligand framework of CoP, which can be readily repaired by addition of fresh ligand, resulting in turnover numbers above 300 mol H₂ (mol CoP)⁻¹ and above 200,000 mol H₂ (mol TiO₂ nanoparticles)⁻¹ in water. Our studies support that a molecular Co species, rather than metallic Co or a Co-oxide precipitate, is responsible for H₂ formation on TiO₂. Electron transfer in this system was studied by transient absorption spectroscopy and time-correlated single photon counting techniques. Essentially quantitative electron injection takes place from RuP into TiO₂ in approximately 180 ps. Thereby, upon dye regeneration by the sacrificial electron donor, a long-lived TiO₂ conduction band electron is formed with a half-lifetime of approximately 0.8 s. Electron transfer from the TiO₂ conduction band to the CoP catalysts occurs quantitatively on a 10 μs timescale and is about a hundred times faster than charge-recombination with the oxidised RuP. This study provides a benchmark for future investigations in photocatalytic fuel generation with molecular catalysts integrated in semiconductors.

The physical origin of the open-circuit voltage in bulk heterojunction solar cells is still not well understood. While significant evidence exists to indicate that the open-circuit voltage is limited by the molecular orbital energies of the heterojunction components, it is clear that this picture is not sufficient to explain the significant variations which often occur between cells fabricated from the same heterojunction components. We present here an analysis of the variation in open-circuit voltage between 0.4–0.65 V observed for a range of P3HT/PCBM solar cells where device deposition conditions, electrode structure, active-layer thickness and device polarity are varied. The analysis quantifies non-geminate recombination losses of dissociated carriers in these cells, measured under device operating conditions. It is found that at open-circuit, losses due to non-geminate recombination are sufficiently large that other loss pathways may effectively be neglected. Variations in open-circuit voltage between different devices are shown to arise from differences in the rate coefficient for non-geminate recombination, and from differences in the charge densities in the photoactive layer of the device. The origin of these differences is discussed, particularly with regard to variations in film microstructure. By separately quantifying these differences in rate coefficient and charge density, and by application of a simple physical model based upon the assumption that open-circuit is reached when the flux of charge photogeneration is matched by the flux of non-geminate recombination, we are able to calculate correctly the open-circuit voltage for all the cells studied to within an accuracy of ±5 mV.

Here, a new methodology for analyzing the charge-density dependence of carrier mobility in organic semiconductors, applicable to the low-charge-density regime (10¹⁴–10¹⁷ cm⁻³) corresponding to the operation conditions of many organic optoelectronic devices, is reported. For the P3HT/PCBM blend photovoltaic devices studied herein, the hole mobility µ is found to depend on charge density n according to a power law µ(n) ∝ n^δ, where δ = 0.35. This dependence is shown to be consistent with an energetic disorder model based upon an exponential tail of localized intra-band states.

The function of organic solar cells is based upon charge photogeneration at donor/acceptor heterojunctions. In this paper, the origin of the improvement in short circuit current of poly(3-hexylthiophene)/6,6-phenyl C₆₁-butyric acid methyl ester (P3HT/PCBM) solar cells with thermal annealing is examined. Transient absorption spectroscopy is employed to demonstrate that thermal annealing results in an approximate two-fold increase in the yield of dissociated charges. The enhanced charge generation is correlated with a decrease in P3HT's ionization potential upon thermal annealing. These observations are in excellent quantitative agreement with a model in which efficient dissociation of the bound radical pair into free charges is dependent upon the bound radical state being thermally hot when initially generated, enabling it to overcome its coulombic binding energy. These observations provide strong evidence that the lowest unoccupied molecular orbital (LUMO) level offset of annealed P3HT/PCBM blends may be only just sufficient to drive efficient charge generation in polythiophene-based solar cells. This has important implications for current strategies to optimize organic photovoltaic device performance based upon the development of smaller optical bandgap polymers.