Extending the Absorption Limit of BiVO4 Photoanodes with Hydrogen Sulfide Treatment

Bismuth vanadate is a promising photoanode material for photoelectrochemical water splitting due to its relative stability, low cost, and nontoxic properties. However, its performance is limited by the large bandgap Eg of 2.4 amp; 8201;eV, and the record photocurrent is already within 90 of its theoretical limit. Further photocurrent enhancement could only be obtained by increasing its optical absorption, for example, by reducing Eg. Herein, sulfur incorporated bismuth vanadate S BiVO4 thin films are synthesized via spray pyrolysis combined with post treatment in hydrogen sulfide environment. Under optimal H2S treatment conditions, sulfur can be incorporated successfully into the BiVO4 lattice, without the formation of any secondary phases. The use of reactive H2S, instead of solid sulfur powders, allows us to decrease the required annealing temperature and increase the kinetics for sulfur incorporation into BiVO4. The Eg of the resulting S BiVO4 films is decreased by gt;200 amp; 8201;meV vs. pristine BiVO4 , which theoretically corresponds to a 20 increase in the theoretical photocurrent limit. Finally, the stability limitation of S BiVO4 is overcome by introducing pulsed laser deposited NiOx protection layers. The modified S BiVO4 NiOx film exhibits higher photocurrent density with no reduction of photocurrent during the 9 amp; 8201;h stability test with AM1.5 illuminatio

High temperature decomposition of Cu 2 BaSnS 4 with Sn loss reveals newly identified compound Cu2Ba3Sn2S8

The earth abundant quaternary compound Cu2BaSnS4 is being currently studied as a candidate for photovoltaics and as a photocathode for water splitting. However, the chemical stability of this phase during synthesis is unclear. The synthesis of other quaternary tin sulphur based absorbers e.g., Cu2ZnSnS4 involves an annealing step at high temperature under sulphur gas atmosphere, which can lead to decomposition into secondary phases involving Sn loss from the sample. As the presence of secondary phases can be detrimental for device performance, it is crucial to identify secondary phase chemical, structural and optoelectronic properties. Here we used a combination of in situ EDXRD XRF and TEM to identify a decomposition pathway for Cu2BaSnS4. Our study reveals that, while Cu2BaSnS4 remains stable at high sulphur partial pressure, the material decomposes at high temperatures into Cu4BaS3 and the hitherto unknown compound Cu2Ba3Sn2S8 if the synthesis is performed under low partial pressure of sulphur. The presence of Cu4BaS3 in devices could be harmful due to its high conductivity and relatively lower band gap compared to Cu2BaSnS4. The analysis of powder diffraction data reveals that the newly identified compound Cu2Ba3Sn2S8 crystallizes in the cubic system space group I 43d with a lattice parameter of a 14.53 1 . A yellow powder of Cu2Ba3Sn2S8 has been synthesized, exhibiting an absorption onset at 2.19 e

Small Atom Doping A Synergistic Strategy to Reduce Sn Zn Recombination Center Concentration in Cu2ZnSnSe4

Kesterite Cu2ZnSnS x Se4 amp; 8722;x CZTSSe is among the most promising inorganic Earth abundant thin film photovoltaic technologies, although currently, the larger voltage deficit compared with more mature chalcogenide technologies is hampering solar to electricity conversion efficiency progress in these materials. Most of the latest reports agree on the CZTSSe defect structure as the main limitation for the open circuit voltage. Small atom doping is suggested as an interesting strategy to reduce the concentration of defects without affecting secondary phase formation. Herein, an innovative approach based on the introduction of LiAlH4 and its further decomposition during the selenization process of CZTSe precursors, as a pathway for hydrogen and lithium alkali transient doping, is explored. This process shows a strong beneficial influence on the crystal growth and solar cell device performance, especially with a significant improvement in V oc and fill factor. A reduction of nonradiative recombination and a remarkable fourfold increase in the carrier lifetime correlating with the reduction of the open circuit voltage V oc deficit below 330 amp; 8201;mV is demonstrated. A mechanism on how small atoms Li and H interact to reduce the concentration of SnZn recombination centers while keeping doping relatively unchanged is proposed, opening fundamental perspectives for the simple and universal transient doping of thin film chalcogenide compound

Structure Flexibility of the Cu(2)ZnSnS(4) Absorber in Low-Cost Photovoltaic Cells: From the Stoichiometric to the Copper-Poor Compounds

International audienceHere we present for the very first time a single-crystal investigation of the Cu-poor Zn-rich derivative of Cu(2)ZnSnS(4). Nowadays, this composition is considered as the one that delivers the best photovoltaic performances in the specific domain of Cu(2)ZnSnS(4)-based thin-film solar cells. The existence of this nonstoichiometric phase is definitely demonstrated here in an explicit and unequivocal manner on the basis of powder and single-crystal X-ray diffraction analyses coupled with electron microprobe analyses. Crystals are tetragonal, space group I ̅4, Z = 2, with a = 5.43440(15) Å and c = 10.8382(6) Å for Cu(2)ZnSnS(4) and a = 5.43006(5) Å and c = 10.8222(2) Å for Cu(1.71)Zn(1.18)Sn(0.99)S(4)

Radiative recombination properties of near stoichiometric CuInSe2 thin films

The properties of electronic defects and their relation to structural defects are of high relevance for CuInSe2 photovoltaic absorbers. Here, we use Raman scattering and steady state photoluminescence to study the intrinsic optoelectronic properties of near stoichiometric CuInSe2 samples with a lateral composition gradient around the Cu saturation point. Apart from a well known shallow defect band at 0.97 eV, we also observe a deep defect band at 0.8 eV, which is not discernable in photoluminescence spectra at lower temperatures. The preparation of a laterally graded sample with a very precise relative composition range by in situ process control allows for a measurement of a significant decrease of the photoluminescence emission yield at the Cu poor Cu rich transition on a very narrow composition scale. Possible assignments of the bands to structural point defects are discusse

Influence of Copper Composition on Cu2BaSn S,Se 4 Solution Deposited Films and Photovoltaic Devices with Over 5 Efficiency

Cu2BaSn S,Se 4 is currently in the spotlight for prospective environmentally friendly, stable, thin film solar cell application, with demonstrated device power conversion efficiency PCE exceeding 5 for vacuum deposited absorbers. As suggested by first principles calculations, experimental studies involving related Cu2ZnSn S,Se 4 and Cu In,Ga S,Se 2 absorbers prove that the detailed chemical composition typically plays a sensitive role in altering defects and electronic properties of these complicated compound semiconductors. Herein, the copper composition of Cu2BaSn S,Se 4 has been systematically modified, employing a solution based deposition approach, to provide a more complete picture of the phase stability and optoelectronic property sensitivity for this material. X ray diffraction and scanning electron microscopy show that phase purity is preserved over a film Cu content range of nominally 0.94 amp; 8804; [Cu] [Ba Sn] amp; 8804; 1.01. Terahertz spectroscopy and Hall effect measurements reveal that the majority carrier hole density of amp; 8764;1013 cm 3 and mobility amp; 8764;5 cm2 V s , as well as the minority carrier lifetime a bulk lifetime of 180 ps and a surface recombination velocity gt;106 cm s , are nominally independent of Cu content. The champion PCEs exceed 4.7 for all copper compositions in the phase pure region, with a record value of 5.1 , similar to the reported values for record vacuum deposited devices. These results suggest that Cu2BaSn S,Se 4 films and solar cells at the current performance level may be less sensitive to Cu stoichiometry compared to kesterite materials and therefore may provide a more stable material platform to prepare thin film solar cell

Water Adsorption Enhances Electrical Conductivity in Transparent P Type CuI

CuI has been recently rediscovered as a p type transparent conductor with a high figure of merit. Even though many metal iodides are hygroscopic, the effect of moisture on the electrical properties of CuI has not been clarified. In this work, we observe a 2 fold increase in the conductivity of CuI after exposure to ambient humidity for 5 h, followed by slight long term degradation. Simultaneously, the work function of CuI decreases by almost 1 eV, which can explain the large spread in the previously reported work function values. The conductivity increase is partially reversible and is maximized at intermediate humidity levels. On the basis of the large intragrain mobility measured by THz spectroscopy, we suggest that hydration of grain boundaries may be beneficial for the overall hole mobilit

Claude Roger : [affiche] / A. Choubrac ; L. Weyl lith.

Affich

Electronic Structure of the CdS Cu In,Ga Se2 Interface of KF and RbF Treated Samples by Kelvin Probe and Photoelectron Yield Spectroscopy

Ambient pressure Kelvin probe and photoelectron yield spectroscopy methods were employed to investigate the impact of the KF and RbF postdeposition treatments KF PDT, RbF PDT on the electronic features of Cu In,Ga Se2 CIGSe thin films and the CdS CIGSe interface in a CdS thickness series that has been sequentially prepared during the chemical bath deposition CBD process depending on the deposition time. We observe distinct features correlated to the CBD CdS growth stages. In particular, we find that after an initial CBD etching stage, the valence band maximum VBM of the CIGSe surface is significantly shifted by 180 amp; 8722;620 mV toward the Fermi level. However, VBM positions at the surface of the CIGSe are still much below the VBM of the CIGSe bulk. The CIGSe surface band gap is found to depend on the type of postdeposition treatment, showing values between 1.46 and 1.58 eV, characteristic for a copper poor CIGSe surface composition. At the CdS CIGSe interface, the lowest VBM discontinuity is observed for the RbF PDT sample. At this interface, a thin layer with a graded band gap is found. We also find that K and Rb act as compensating acceptors in the CdS layer. Detailed energy band diagrams of the CdS CIGSe heterostructures are propose

Investigation of near stoichiometric polycrystalline CuInSe2 thin films by photoreflectance spectroscopy

The bandgap of CuInSe2 thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near stoichiometric polycrystalline CuInSe2 CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe2 absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 amp; 8201;eV notably decreases by 20 30 amp; 8201;meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 amp; 8201;eV does not show a redshift, which requires further theoretical explanatio