Thermodynamic Oxidation and Reduction Potentials of Photocatalytic Semiconductors in Aqueous Solution

An approach is introduced to calculate the thermodynamic oxidation and reduction potentials of semiconductors in aqueous solution. By combining a newly developed ab initio calculation method for compound formation energy and band alignment with electrochemistry experimental data, this approach can be used to predict the stability of almost any compound semiconductor in aqueous solution. Thirty photocatalytic semiconductors have been studied, and a graph (a simplified Pourbaix diagram) showing their valence/conduction band edges and oxidation/reduction potentials relative to the water redox potentials is produced. On the basis of this graph, the thermodynamic stabilities and trends against the oxidative and reductive photocorrosion for compound semiconductors are analyzed, which shows the following: (i) some metal oxides can be resistant against the oxidation by the photogenerated holes when used as the n-type photoanodes; (ii) all the nonoxide semiconductors are susceptible to oxidation, but they are resistant to the reduction by the photogenerated electrons and thus can be used as the p-type photocathodes if protected from the oxidation; (iii) doping or alloying the metal oxide with less electronegative anions can decrease the band gap but also degrade the stability against oxidation

Influence of Defects and Synthesis Conditions on the Photovoltaic Performance of Perovskite Semiconductor CsSnI₃

CsSnI₃ is a prototype inorganic halide perovskite that has recently been proposed as a strong candidate for photovoltaic applications because of its unique semiconductor properties. Through first-principle calculations, we show that the concentration control of intrinsic defects is critical for optimizing the photovoltaic properties of CsSnI₃. Under a Sn-poor condition, a high concentration of acceptor defects, such as Sn or Cs vacancies, can form easily and produce a high p-type conductivity and deep-level defects that can become electron–hole recombination centers, all with high energy. This condition is optimal for growing CsSnI₃ as hole-transport material in solar cells. In contrast, when Sn becomes richer, the concentration of acceptor defects decreases; therefore, the p-type conductivity may drop to a moderate level, which can increase the shunt resistance and, thus, the efficiency of the solar cells with CsSnI₃ as the light absorber material (LAM). However, under the Sn-rich condition, the concentration of a deep-level donor defect Sn_I will increase, causing electron traping and non-radiative electron–hole recombination. Therefore, we propose that a moderately Sn-rich condition is optimal when CsSnI₃ is used as the LAM. The defect properties of CsSnI₃ are general, and the underlying chemistry is expected to be applicable to other halide perovskite semiconductors

Indolo[3,2,1-jk]carbazole Derivatives-Sensitized Solar Cells: Effect of π‑Bridges on the Performance of Cells

Four organic dyes based on indolo­[3,2,1-jk]­carbazole (IC-1, IC-2, IC-3, and IC-4) with different π-bridges (benzene ring and thiophene ring) are used for dye-sensitized solar cells (DSSCs) to investigate the effect of π-bridge on their photovoltaic performance. The introduction of thiophene ring as π-bridge (the dye IC-2) greatly improves the cell performance compared to benzene ring. The increasing conjugation length of the molecules decreases the performance of DSSCs. The best performance of DSSC based on IC-2 is obtained with a <i>V</i>_oc of 0.66 V and a conversion efficiency of 3.68%. The poor performance of DSSCs based on IC-1 and IC-3 which contains only benzene ring as the π-bridge can be attributed to poor spectral coverage and higher electron charge transfer resistance as evaluated from EIS studies

Improved Carrier Lifetimes of CdSe Thin Film via Te Doping for Photovoltaic Application

Cadmium selenide (CdSe) solar cells have proven to be a remarkable potential top cell for a silicon-based tandem application. However, the defects and short carrier lifetimes of CdSe thin films greatly limit the solar cell performance. In this work, a Te-doped strategy is proposed to passivate the Se vacancy defects and increase the carrier lifetime of the CdSe thin film. The theoretical calculation helps to reveal the mechanism of nonradiative recombination of the CdSe thin film in depth. After Te-doping, the calculated capture coefficient of CdSe can be reduced from 4.61 × 10–8 cm3 s–1 to 2.32 × 10–9 cm3 s–1. Meanwhile, the carrier lifetime of CdSe thin film is increased nearly 3-fold from 0.53 to 1.43 ns. Finally, the efficiency of the Cd(Se,Te) solar cell is improved to 4.11%, about a relative 36.5% improvement compared to the pure CdSe solar cell. Both theoretical calculations and experiments prove that Te can effectively passivate bulk defects and improve the carrier lifetime of CdSe thin films, deserving further exploration to improve solar cell performance

CuSbS₂ as a Promising Earth-Abundant Photovoltaic Absorber Material: A Combined Theoretical and Experimental Study

Recently, CuSbS₂ has been proposed as an alternative earth-abundant absorber material for thin film solar cells. However, no systematic study on the chemical, optical, and electrical properties of CuSbS₂ has been reported. Using density functional theory (DFT) calculations, we showed that CuSbS₂ has superior defect physics with extremely low concentration of recombination-center defects within the forbidden gap, espeically under the S rich condition. It has intrinsically p-type conductivity, which is determined by the dominant Cu vacancy (<i>V</i>_Cu) defects with the a shallow ionization level and the lowest formation energy. Using a hydrazine based solution process, phase-pure, highly crystalline CuSbS₂ film with large grain size was successfully obtained. Optical absorption investigation revealed that our CuSbS₂ has a direct band gap of 1.4 eV. Ultraviolet photoelectron spectroscopy (UPS) study showed that the conduction band and valence band are located at 3.85 eV and −5.25 eV relative to the vacuum level, respectively. As the calculations predicted, a p-type conductivity is observed in the Hall effect measurements with a hole concentration of ∼10¹⁸ cm^–3 and hole mobility of 49 cm²/(V s). Finally, we have built a prototype FTO/CuSbS₂/CdS/ZnO/ZnO:Al/Au solar cell and achieved 0.50% solar conversion efficiency. Our theoretical and experimental investigation confirmed that CuSbS₂ is indeed a very promising absorber material for solar cell application

Composition- and Band-Gap-Tunable Synthesis of Wurtzite-Derived Cu₂ZnSn(S_1–<i>x</i>Se_<i>x</i>)₄ Nanocrystals: Theoretical and Experimental Insights

The wurtzite-derived Cu₂ZnSn(S_1–<i>x</i>Se_<i>x</i>)₄ alloys are studied for the first time through combining theoretical calculations and experimental characterizations. <i>Ab initio</i> calculations predict that wurtzite-derived Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ are highly miscible, and the band gaps of the mixed-anion alloys can be linearly tuned from 1.0 to 1.5 eV through changing the composition parameter <i>x</i> from 0 to 1. A synthetic procedure for the wurtzite-derived Cu₂ZnSn(S_1–<i>x</i>Se_<i>x</i>)₄ alloy nanocrystals with tunable compositions has been developed. A linear tunable band-gap range of 0.5 eV is observed in the synthesized alloy nanocrystals, which shows good agreement with the <i>ab initio</i> calculations

Deciphering Halogen Competition in Organometallic Halide Perovskite Growth

Organometallic halide perovskites (OHPs) hold great promise for next-generation, low-cost optoelectronic devices. During the chemical synthesis and crystallization of OHP thin films, a major unresolved question is the competition between multiple halide species (e.g., I^–, Cl^–, Br^–) in the formation of the mixed-halide perovskite crystals. Whether Cl^– ions are successfully incorporated into the perovskite crystal structure or, alternatively, where they are located is not yet fully understood. Here, in situ X-ray diffraction measurements of crystallization dynamics are combined with ex situ TOF-SIMS chemical analysis to reveal that Br^– or Cl^– ions can promote crystal growth, yet reactive I^– ions prevent them from incorporating into the lattice of the final perovskite crystal structure. The Cl^– ions are located in the grain boundaries of the perovskite films. These findings significantly advance our understanding of the role of halogens during synthesis of hybrid perovskites and provide an insightful guidance to the engineering of high-quality perovskite films, essential for exploring superior-performing and cost-effective optoelectronic devices

A One-Dimensional Organic Lead Chloride Hybrid with Excitation-Dependent Broadband Emissions

Organic–inorganic metal halide hybrids have emerged as a new class of materials with fascinating optical and electronic properties. The exceptional structure tunability has enabled the development of materials with various dimensionalities at the molecular level, from three-dimensional (3D) to 2D, 1D, and 0D. Here, we report a new 1D lead chloride hybrid, C₄N₂H₁₄PbCl₄, which exhibits unusual inverse excitation-dependent broadband emission from bluish-green to yellow. Density functional theory calculations were performed to better understand the mechanism of this excitation-dependent broadband emission. This 1D hybrid material is found to have two emission centers, corresponding to the self-trapped excitons (STEs) and vacancy-bound excitons. The excitation-dependent emission is due to different populations of these two types of excitons generated at different excitation wavelengths. This work shows the rich chemistry and physics of organic–inorganic metal halide hybrids and paves the way to achieving novel light emitters with excitation-dependent broadband emissions at room temperature

A One-Dimensional Organic Lead Chloride Hybrid with Excitation-Dependent Broadband Emissions

Organic–inorganic metal halide hybrids have emerged as a new class of materials with fascinating optical and electronic properties. The exceptional structure tunability has enabled the development of materials with various dimensionalities at the molecular level, from three-dimensional (3D) to 2D, 1D, and 0D. Here, we report a new 1D lead chloride hybrid, C₄N₂H₁₄PbCl₄, which exhibits unusual inverse excitation-dependent broadband emission from bluish-green to yellow. Density functional theory calculations were performed to better understand the mechanism of this excitation-dependent broadband emission. This 1D hybrid material is found to have two emission centers, corresponding to the self-trapped excitons (STEs) and vacancy-bound excitons. The excitation-dependent emission is due to different populations of these two types of excitons generated at different excitation wavelengths. This work shows the rich chemistry and physics of organic–inorganic metal halide hybrids and paves the way to achieving novel light emitters with excitation-dependent broadband emissions at room temperature

Pt-Mediated Reversible Reduction and Expansion of CeO₂ in Pt Nanoparticle/Mesoporous CeO₂ Catalyst: In Situ X‑ray Spectroscopy and Diffraction Studies under Redox (H₂ and O₂) Atmospheres

Here, we report the Pt nanoparticle mediated reduction (oxidation) and lattice expansion (contraction) of mesoporous CeO₂ under H₂ (O₂) atmospheres and in the temperature range of 50–350 °C. We found that CeO₂ in the Pt/CeO₂ catalyst was partially reduced in H₂ (and fully oxidized back in O₂) as demonstrated by several in situ techniques: APXPS spectra (4d core levels) for the topmost surface, NEXAFS total electron yield spectra (at the M_5,4 edges) in the near surface regions, and (N)­EXAFS fluorescence spectra (at the L₃ edge) in the bulk. Moreover, XRD and EXAFS showed the reversible expansion and contraction of the CeO₂ unit cell in H₂ and O₂ environments, respectively. The expansion of the CeO₂ cell was mainly associated with the formation of oxygen vacancies as a result of the Pt-mediated reduction of Ce⁴⁺ to Ce³⁺. We also found that pure mesoporous CeO₂ can not be reduced in H₂ under identical conditions but can be partially reduced at above 450 °C as revealed by APXPS. The role of Pt in H₂ was identified as a catalytic one that reduces the activation barrier for the reduction of CeO₂ via hydrogen spillover