Low-Cost, Efficient, and Durable H2 Production by Photoelectrochemical Water Splitting with CuGa3Se5 Photocathodes.

Photoelectrochemical (PEC) water splitting is an elegant method of converting sunlight and water into H2 fuel. To be commercially advantageous, PEC devices must become cheaper, more efficient, and much more durable. This work examines low-cost polycrystalline chalcopyrite films, which are successful as photovoltaic absorbers, for application as PEC absorbers. In particular, Cu-Ga-Se films with wide band gaps can be employed as top cell photocathodes in tandem devices as a realistic route to high efficiencies. In this report, we demonstrate that decreasing Cu/Ga composition from 0.66 to 0.31 in Cu-Ga-Se films increased the band gap from 1.67 to 1.86 eV and decreased saturated photocurrent density from 18 to 8 mA/cm2 as measured by chopped-light current-voltage (CLIV) measurements in a 0.5 M sulfuric acid electrolyte. Buffer and catalyst surface treatments were not applied to the Cu-Ga-Se films, and they exhibited promising stability, evidenced by unchanged CLIV after 9 months of storage in air. Finally, films with Cu/Ga = 0.36 (approximately stoichiometric CuGa3Se5) and 1.86 eV band gaps had exceptional durability and continuously split water for 17 days (∼12 mA/cm2 at -1 V vs RHE). This is equivalent to ∼17 200 C/cm2, which is a world record for any polycrystalline PEC absorber. These results indicate that CuGa3Se5 films are prime candidates for cheaply achieving efficient and durable PEC water splitting.

Soft X-ray Spectroscopy of a Complex Heterojunction in High-Efficiency Thin-Film Photovoltaics: Intermixing and Zn Speciation at the Zn(O,S)/Cu(In,Ga)Se2 Interface.

The chemical structure of the Zn(O,S)/Cu(In,Ga)Se2 interface in high-efficiency photovoltaic devices is investigated using X-ray photoelectron and Auger electron spectroscopy, as well as soft X-ray emission spectroscopy. We find that the Ga/(Ga+In) ratio at the absorber surface does not change with the formation of the Zn(O,S)/Cu(In,Ga)Se2 interface. Furthermore, we find evidence for Zn in multiple bonding environments, including ZnS, ZnO, Zn(OH)2, and ZnSe. We also observe dehydrogenation of the Zn(O,S) buffer layer after Ar+ ion treatment. Similar to high-efficiency CdS/Cu(In,Ga)Se2 devices, intermixing occurs at the interface, with diffusion of Se into the buffer, and the formation of S-In and/or S-Ga bonds at or close to the interface.

Characterization of Sulfur Bonding in CdS:O Buffer Layers for CdTe-based Thin-Film Solar Cells.

On the basis of a combination of X-ray photoelectron spectroscopy and synchrotron-based X-ray emission spectroscopy, we present a detailed characterization of the chemical structure of CdS:O thin films that can be employed as a substitute for CdS layers in thin-film solar cells. It is possible to analyze the local chemical environment of the probed elements, in particular sulfur, hence allowing insights into the species-specific composition of the films and their surfaces. A detailed quantification of the observed sulfur environments (i.e., sulfide, sulfate, and an intermediate oxide) as a function of oxygen content is presented, allowing a deliberate optimization of CdS:O thin films for their use as alternative buffer layers in thin-film photovoltaic devices.

Facile synthesis of highly photoactive α-Fe2O3-based films for water oxidation.

This work reports a facile method for preparing highly photoactive α-Fe(2)O(3) films as well as their implementation as photoanodes for water oxidation. Transparent α-Fe(2)O(3) films were prepared by a new deposition-annealing (DA) process using nontoxic iron(III) chloride as the Fe precursor, followed by annealing at 550 °C in air. Ti-doped α-Fe(2)O(3) films were prepared by the same method, with titanium butoxide added as the Ti precursor. Impedance measurements show that the Ti-dopant serves as an electron donor and increases the donor density by 2 orders of magnitude. The photoelectrochemical performance of undoped and Ti-doped α-Fe(2)O(3) photoanodes was characterized and optimized through controlled variation of the Fe and Ti precursor concentration, annealing conditions, and the number of DA cycles. Compared to the undoped sample, the photocurrent onset potential of Ti-doped α-Fe(2)O(3) is shifted about 0.1-0.2 V to lower potential, thus improving the photocurrent and incident photon to current conversion efficiency (IPCE) at lower bias voltages. Significantly, the optimized Ti-doped α-Fe(2)O(3) film achieved the highest photocurrent density (1.83 mA/cm(2)) and IPCE values at 1.02 V vs RHE for α-Fe(2)O(3) photoanode. The enhanced photocurrent is attributed to the improved donor density and reduced electron-hole recombination at the time scale beyond a few picoseconds, as a result of Ti doping.

Does the silver moss Bryum argenteum exhibit sex-specific patterns in vegetative growth rate, asexual fitness or prezygotic reproductive investment?

BACKGROUND AND AIMS: Expected life history trade-offs associated with sex differences in reproductive investment are often undetected in seed plants, with the difficulty arising from logistical issues of conducting controlled experiments. By controlling genotype, age and resource status of individuals, a bryophyte was assessed for sex-specific and location-specific patterns of vegetative, asexual and sexual growth/reproduction across a regional scale. METHODS: Twelve genotypes (six male, six female) of the dioecious bryophyte Bryum argenteum were subcultured to remove environmental effects, regenerated asexually to replicate each genotype 16 times, and grown over a period of 92 d. Plants were assessed for growth rates, asexual and sexual reproductive traits, and allocation to above- and below-ground regenerative biomass. KEY RESULTS: The degree of sexual versus asexual reproductive investment appears to be under genetic control, with three distinct ecotypes found in this study. Protonemal growth rate was positively correlated with asexual reproduction and sexual reproduction, whereas asexual reproduction was negatively correlated (appeared to trade-off) with vegetative growth (shoot production). No sex-specific trade-offs were detected. Female sex-expressing shoots were longer than males, but the sexes did not differ in growth traits, asexual traits, sexual induction times, or above- and below-ground biomass. Males, however, had much higher rates of inflorescence production than females, which translated into a significantly higher (24x) prezygotic investment for males relative to females. CONCLUSIONS: Evidence for three distinct ecotypes is presented for a bryophyte based on regeneration traits. Prior to zygote production, the sexes of this bryophyte did not differ in vegetative growth traits but significantly differed in reproductive investment, with the latter differences potentially implicated in the strongly biased female sex ratio. The disparity between males and females for prezygotic reproductive investment is the highest known for bryophytes.