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Abstract: Antimony Selenide, Sb2Se3, is a highly promising solar absorber material with excellent optoelectronic properties; solar cell efficiencies are now poised to exceed 10%, after a rapid rise over the past few years. However, the open-circuit voltage (Voc) of most cells remains low, and such a high Voc deficit, along with defect spectroscopy studies, suggest that recombination via deep trap states may be a limiting factor. A comprehensive study of all the intrinsic defects in Sb2Se3 is warranted -- in this article, we calculate the formation energies and transition levels of these defects using hybrid Density Functional Theory. Our results demonstrate that cation-anion antisite defects have low formation energies, and possess multiple mid-gap transition levels, making them the most likely candidates for previously observed trap states, and possible recombination centres. Suppressing these dominant defects will be crucial for future cell development -- thus we also present potential methods to counteract their detrimental effects and allow further improvement in efficiencies.

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Abstract: Modification of TiO2 to increase its visible light activity and promote higher performance photocatalytic ability has become a key research goal for materials scientists in the past two decades. One of the most popular approaches proposed this as “passivated codoping”, whereby an equal number of donor and acceptor dopants are introduced into the lattice, producing a charge neutral system with a reduced band gap. Using the archetypal codoping pairs of [Nb+N] and [Ta+N] doped anatase, we demonstrate using hybrid density functional theory that compensated codoping is not achievable in TiO2. Our results indicate that the natural defect chemistry of the host system (in this case n-type anatase TiO2) is dominant, and so concentration parity of dopant types is not achievable under any thermodynamic growth conditions. The implications of compensated codoping for band gap manipulation in general are discussed.

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Abstract: Hybrid organic–inorganic halide perovskites are promising materials for thin-film solar cells. However, the toxicity and instability of best-in-class lead–halide perovskite materials make them nonideal. To combat these issues, we replaced lead with bismuth and explored the sensitivity of these new lead-free materials to the valency and bonding of their cationic organic groups. Specifically, we synthesized and characterized the materials properties and photophysical properties of hexane-1,6-diammonium bismuth pentaiodide ((HDA2+)BiI5) and compared them to an analogue containing a more volatile organic group with half the number of carbon and nitrogen atoms in the form of n-propylammonium ((PA+)xBiI3+x, where 1 < x < 3). The full crystallographic structures of (HDA2+)BiI5 and (PA+)xBiI3+x were resolved by single-crystal X-ray diffraction. (HDA2+)BiI5 was shown to be pure-phase and have a one-dimensional structure, whereas (PA+)xBiI3+x was shown to be a mix of one-dimensional and zero-dimensional phases. Structures of the materials were confirmed by synchrotron X-ray diffraction of powders. Both (HDA2+)BiI5 and (PA+)xBiI3+x exhibit steady-state photoluminescence at room temperature. Density functional theory calculations of (HDA2+)BiI5 predict electronic absorption features and a ∼2 eV bandgap that are consistent with those observed experimentally. Structure–property relationships of the materials were examined, and moisture tolerance and film quality were found to be superior for dication-containing (HDA2+)BiI5 in relation to monocation-containing (PA+)xBiI3+x. We hypothesize that these trends are in part due to a molecular bridging effect enabled by the presence of the dicationic hexanediammonium groups in (HDA2+)BiI5. Solar cells fabricated using (HDA2+)BiI5 as the photoactive layer exhibited photovoltaic action while those containing (PA+)xBiI3+x did not, suggesting that organic dicationic groups are beneficial to light-absorber morphology and ultimately solar-cell performance.

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Abstract: A simplistic and low-cost method that dramatically improves the performance of solution-grown hematite photoanodes for solar-driven water splitting through incorporation of nanohybrid metal oxide overlayers was developed. By heating the α-Fe2O3/SnO2–TiO2 electrode in an inert atmosphere, such as argon or nitrogen, the photocurrent increased to over 2 mA/cm2 at 1.23 V versus a reversible hydrogen electrode, which is 10 times higher than that of pure hematite under 1 sun (100 mW/cm2, AM 1.5G) light illumination. For the first time, we found a significant morphological difference between argon and nitrogen gas heat-treated hematite films and discussed the consequences for photoresponse. The origin for the enhancement, probed via theoretical modeling, stems from the facile incorporation of low formation energy dopants into the Fe2O3 layer at the interface of the metal oxide nanohybrid overlayer, which decreases recombination by increasing the electrical conductivity of Fe2O3. These dopants diffuse from the overlayer into the α-Fe2O3 layer readily under inert gas heat treatment. This simple yet effective strategy could be applied to other dopants to increase hematite performance for solar energy conversion applications.

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Abstract: As photovoltaics have grown to become one of the dominant renewable energy generating technologies, attention has fallen upon thin-film materials as a route to lightweight, flexible and portable solar cells. NaSbS2 has recently been proposed as a non-toxic, earth abundant solar absorber for thin-film cells. In this study, we use a combined theoretical and experimental approach to characterize and assess the electronic and optical properties of NaSbS2 as a promising solar absorber. Our results, utilising two theoretical efficiency metrics, demonstrate that NaSbS2 may be limited for use in single-junction cells by a forbidden band gap and slow absorption onset. Other features of its electronic structure, however, indicate that the material may still be promising in thermoelectric applications.