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CCBY Antimony selenide (Sb$_2$Se$_3$) is an emerging chalcogenide photovoltaic absorber material that has been the subject of increasing interest in recent years, demonstrating rapid efficiency increases with a material that is simple, abundant, and stable. This paper examines the material from both a theoretical and practical standpoint. The theoretical viability of Sb$_2$Se$_3$ as a solar photovoltaic material is assessed and the maximum spectroscopically limited performance is estimated, with a 200 nm film expected to be capable of achieving a photon conversion efficiency of up to 28.2%. By adapting an existing CdTe close-spaced sublimation (CSS) process, Sb$_2$Se$_3$ material with large rhubarb-like grains is produced and solar cells are fabricated. We show that the established CdS window layer is unsuitable for use with CSS, due to intermixing during higher temperature processing. Substituting CdS with the more stable TiO$_2$, a power conversion efficiency of 5.5% and an open-circuit voltage V$_oc$ of 0.45 V are achieved; the voltage exceeding current champion devices. This paper demonstrates the potential of CSS for scalable Sb$_2$Se$_3$ deposition and highlights the promise of Sb$_2$Se$_3$ as an abundant and low-toxicity material for solar applications.

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.

We investigated the formation of photovoltaic (PV) devices using direct n-Si/MAPI (methylammonium lead tri-iodide) two-sided heterojunctions for the first time (as a possible alternative to two-terminal tandem devices) in which charge might be generated and collected from both the Si and MAPI. Test structures were used to establish that the n-Si/MAPI junction was photoactive and that spiro-OMeTAD acted as a “pinhole blocking” layer in n-Si/MAPI devices. Two-terminal “substrate” geometry devices comprising Al/n-Si/MAPI/spiro-OMeTAD/Au were fabricated and the effects of changing the thickness of the semitransparent gold electrode and the silicon resistivity were investigated. External quantum efficiency and capacitance–voltage measurements determined that the junction was one-sided in the silicon—and that the majority of the photocurrent was generated in the silicon, with there being a sharp cutoff in photoresponse above the MAPI bandgap. Construction of band diagrams indicated the presence of an upward valence band spike of up to 0.5 eV at the n-Si/MAPI interface that could impede carrier flow. Evidence for hole accumulation at this feature was seen in both Kelvin-probe transients and from unusual features in both current–voltage and capacitance–voltage measurements. The devices achieved a hysteresis-free best power conversion efficiency of 2.08%, V OC 0.46 V, J SC 11.77 mA/cm2, and FF 38.4%, demonstrating for the first time that it is possible to create a heterojunction PV device directly between the MAPI and n-Si. Further prospects for two-sided n-Si/MAPI heterojunctions are also discussed.

Incorporation of CdSe layers into CdTe thin film solar cells has recently emerged as a route to improve cell performance. It has been suggested that the formation of lower band gap CdTe(1-x)Se(x) phases following Se diffusion induces bandgap grading which may increase the carrier lifetime and thereby open circuit voltage. In this study we investigate the impact of CdSe incorporation on CdTe solar cell performance. We demonstrate that the standard CdS/CdTe device architecture is incompatible with Se incorporation, owing to large optical losses. An alternative cell structure with an oxide partner layer replacing the CdS with SnO2/CdSe/CdTe is developed, leading to cell efficiencies of > 13.5%. The differences in processing required for effective selenium incorporation are investigated with performance improvements resulting from additional post-growth annealing. Finally, other oxides such as TiO2, ZnO and FTO are demonstrated to be unsuitable partner layers but highlight that the choice of partner layer is key to further improving the performance.

In this manuscript, the inorganic perovskite CsPbI2Br is investigated as a photovoltaic material that offers higher stability than the organic-inorganic hybrid perovskite materials. It is demonstrated that CsPbI2Br does not irreversibly degrade to its component salts as in the case of methylammonium lead iodide but instead is induced (by water vapor) to transform from its metastable brown cubic (1.92 eV band gap) phase to a yellow phase having a higher band gap (2.85 eV). This is easily reversed by heating to 350 °C in a dry environment. Similarly, exposure of unencapsulated photovoltaic devices to water vapor causes current (JSC) loss as the absorber transforms to its more transparent (yellow) form, but this is also reversible by moderate heating, with over 100% recovery of the original device performance. NMR and thermal analysis show that the high band gap yellow phase does not contain detectable levels of water, implying that water induces the transformation but is not incorporated as a major component. Performances of devices with best efficiencies of 9.08% (VOC = 1.05 V, JSC = 12.7 mA cm-2 and FF = 68.4%) using a device structure comprising glass/ITO/c-TiO2/CsPbI2Br/Spiro-OMeTAD/Au are presented, and further results demonstrating the dependence of the performance on the preparation temperature of the solution processed CsPbI2Br films are shown. We conclude that encapsulation of CsPbI2Br to exclude water vapor should be sufficient to stabilize the cubic brown phase, making the material of interest for use in practical PV devices.