Growth and Characterization of Sb2Se3 Single Crystals for Fundamental Studies

Three methods of growing bulk crystalline samples of Sb2 Se3 to provide material for basic studies have been investigated and preliminary racterization is reported. These growth methods were: A) melt-growth, similar to vertical Bridgman, b) dynamic vapor transport over a temperature gradient (Piper-Polich method) and c) a static vapor method in which the source material is transported in nearly iso-thermal conditions. The melt-growth method produced the largest single crystals (up to 4 mm diameter), while the vapor methods both yielded polycrystalline boules with mm-sized grains. Powder XRD confirmed the boules to comprise orthorhombic Sb2 Se3, having lattice parameters a = 11.7808 Å b = 3.9767 Å and c = 11.6311 Å. Cleavage facets were parallel to (100). Raman peaks at 191 (A g and 211 cm -1 were excited anisotropically. FTIR reflectance features showed some sensitivity to s- and ppolarization

Vegard Relation and Raman Band Reference Data Generated from Bulk Crystals of Kesterite-Phase Composition Series Cu2ZnSnS4xSe4–4x (CZTSSe, 0 ≤ x ≤ 1)

Solid solutions in the series Cu2ZnSnS4xSe4–4x (CZTSSe) are of interest for PV applications. The purpose of this work was to grow bulk crystalline samples over the entire composition range to allow the Vegard relation (lattice parameter variation with composition) and the systematic behavior of Raman bands to be defined to generate reference data. Samples with 0 ≤ x ≤ 1 were synthesized from the elements and grown into crystalline form from solution in either KCl/NaCl eutectic or elemental Sn. Details of the crystal growth outcomes, including the use of a quartz seed plate to make thick film samples, are described. Ordered kesterite-type material was formed upon crystallization, and X-ray diffraction demonstrated linear Vegard relationships, with the lattice parameters varying with composition as a (Å) = −0.268(3)x + 5.6949(17) and c (Å) = −0.516(6)x + 11.345(3). Raman spectroscopy yielded two dominant peaks, these being kesterite A modes associated with the Se and S modes in CZTSe and CZTS. These varied in wavenumber linearly as ωCZTSe (cm–1) = (44.6 ± 1.6)x + (194.6 ± 0.8) and ωCZTS (cm–1) = (7.1 ± 1.3)x + (329.0 ± 0.8). Crystallization was also shown to promote ordering. The variation of lattice parameters with composition exhibited significant differences from those observed in previous studies. Also, while the Raman S mode behavior differed from previous reports, the Se modes were similar. These differences are discussed

Band alignment of Sb2O3 and Sb2Se3

Antimony selenide (Sb2Se3) possesses great potential in the field of photovoltaics (PV) due to its suitable properties for use as a solar absorber and good prospects for scalability. Previous studies have reported the growth of a native antimony oxide (Sb2O3) layer at the surface of Sb2Se3 thin films during deposition and exposure to air, which can affect the contact between Sb2Se3 and subsequent layers. In this study, photoemission techniques were utilized on both Sb2Se3 bulk crystals and thin films to investigate the band alignment between Sb2Se3 and the Sb2O3 layer. By subtracting the valence band spectrum of an in situ cleaved Sb2Se3 bulk crystal from that of the atmospherically contaminated bulk crystal, a valence band offset (VBO) of −1.72 eV is measured between Sb2Se3 and Sb2O3. This result is supported by a −1.90 eV VBO measured between Sb2O3 and Sb2Se3 thin films via the Kraut method. Both results indicate a straddling alignment that would oppose carrier extraction through the back contact of superstrate PV devices. This work yields greater insight into the band alignment of Sb2O3 at the surface of Sb2Se3 films, which is crucial for improving the performance of these PV devices

Defect properties of Sb2Se3 thin film solar cells and bulk crystals

As an absorber in photovoltaic devices, Sb2Se3 has rapidly achieved impressive power conversion efficiencies despite the lack of fundamental knowledge about its electronic defects. Here, we present a deep level transient spectroscopy (DLTS) study of deep level defects in both bulk crystal and thin film device material. DLTS study of Bridgman-grown n-type bulk crystals revealed traps at 358, 447, 505, and 685 meV below the conduction band edge. Of these, the energetically close pair at 447 and 505 meV could only be resolved using the isothermal transient spectroscopy (rate window variation) method. A completed Sb2Se3 thin film solar cell displayed similar trap spectra with traps identified at 378, 460, and 690 meV. The comparable nature of defects in thin film and bulk crystal material implies that there is minimal impact of polycrystallinity in Sb2Se3 supporting the concept of benign grain boundaries. We acknowledge the engineering and physical sciences research council for funding via Grant Nos. EP/N01457/1, EP/L01551X/1, and EP/M024768/1

GeSe photovoltaics: doping, interfacial layer and devices

Germanium selenide (GeSe) bulk crystals, thin films and solar cells are investigated with a focus on acceptor-doping with silver (Ag) and the use of an Sb2Se3 interfacial layer. The Ag-doping...</jats:p

n-type CdTe:In for photovoltaics: in situ doping, type verification and compensation effects

Abstract We explored the in-situ doping of cadmium telluride thin films with indium to produce n-type absorbers as an alternative to the near-universal choice of p-type for photovoltaic devices. The films were grown by close space sublimation from melt-synthesised feedstock. Transfer of the indium during film growth was limited to 0.0014%–0.014%—unless reducing conditions were used which yielded 14%–28% efficient transport. While chunks of bulk feedstock were verified as n-type by the hot probe method, carrier type of thin film material was only able to be verified by using hard x-ray photoelectron spectroscopy to determine the Fermi level position within the band gap. The assignment of n-type conductivity was consistent with the rectification behaviour of a p-InP/CdTe:In junction. However, chloride treatment had the effect of compensating n-CdTe:In to near-intrinsic levels. Without chloride, the highest dopant activation was 20% of the chemical concentration of indium, this being for a film having a carrier concentration of n = 2 × 1015 cm−3. However, the activation was often much lower, and compensation due to over-doping with indium and native defects (stoichiometry) are discussed. Results from preliminary bifacial devices comprising Au/P3HT/ZnTe/CdTe:In/CdS/FTO/glass are presented.</jats:p

Vacancy-Ordered Double Perovskite Cs2TeI6 Thin Films for Optoelectronics.

Alternatives to lead- and tin-based perovskites for photovoltaics and optoelectronics are sought that do not suffer from the disadvantages of toxicity and low device efficiency of present-day materials. Here we report a study of the double perovskite Cs2TeI6, which we have synthesized in the thin film form for the first time. Exhaustive trials concluded that spin coating CsI and TeI4 using an antisolvent method produced uniform films, confirmed as Cs2TeI6 by XRD with Rietveld analysis. They were stable up to 250 °C and had an optical band gap of ∼1.5 eV, absorption coefficients of ∼6 × 104 cm-1, carrier lifetimes of ∼2.6 ns (unpassivated 200 nm film), a work function of 4.95 eV, and a p-type surface conductivity. Vibrational modes probed by Raman and FTIR spectroscopy showed resonances qualitatively consistent with DFT Phonopy-calculated spectra, offering another route for phase confirmation. It was concluded that the material is a candidate for further study as a potential optoelectronic or photovoltaic material

Sb 5s2 lone pairs and band alignment of Sb2Se3: a photoemission and density functional theory study

Lone pair Sb 5s orbitals are identified at the valence band maximum of Sb2Se3 bulk crystals using photoemission and density functional theory. The resulting band alignments are determined and implications for solar cell applications are discussed.</p