• Energy Research

Abstract The effect of (NH4)2S and CS2 chemical etches on surface chemistry and contacting in Sb2Se3 solar cells was investigated via a combination of x-ray photoemission spectroscopy (XPS) and photovoltaic device analysis. Thin film solar cells were produced in superstrate configuration with an absorber layer deposited by close space sublimation. Devices of up to 5.7% efficiency were compared via current–voltage measurements (J–V) and temperature-dependent current–voltage (J–V–T) analysis. XPS analysis demonstrated that both etching processes were successful in removing Sb2O3 contamination, while there was no decrease in free elemental selenium content by either etch, in contrast to prior work. Using J–V–T analysis the removal of Sb2O3 at the back surface in etched samples was found to improve contacting by reducing the potential barrier at the back contact from 0.43 eV to 0.26 eV and lowering the series resistance. However, J–V data showed that due to the decrease in shunt resistance and short-circuit current as a result of etching, the devices show a lower efficiency following both etches, despite a lowering of the series resistance. Further optimisation of the etching process yielded an improved efficiency of 6.6%. This work elucidates the role of surface treatments in Sb2Se3 devices and resolves inconsistencies in previously published works.

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.

Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems.

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.

In this paper, we present single heterojunction p-i-n GaAsSbN/GaAs solar cells grown by low-temperature liquid-phase epitaxy (LPE) – this is of interest as a component of multi-junction solar cell devices. The quaternary absorber layer was characterized by low excitation power photoluminescence to give the temperature dependence of the bandgap. This conformed to the Varshni function at low temperatures to within 10 meV, indicating relatively small alloy potential fluctuations. The absorption properties and the transport of the photogenerated carriers in the heterostructures were investigated using surface photovoltage method. A power conversion efficiency of 4.15% (AM1.5, 1000 W·m−2) was measured for p-i-n GaAsSbN/GaAs solar cells, which is comparable to the efficiency of MOCVD grown devices of this type. This is promising for the first report of LPE grown GaAsSbN/GaAs solar cells since the current record efficiency for the cells based on these compounds grown by MBE stands just at 6%. The long-wavelength photosensitivity of the cells determined from external quantum efficiency and surface photovoltage measurements was shown to be extended to 1040 nm.

We report a high efficiency antimony selenide (Sb2Se3) photovoltaic device structure using a new multi-step close space sublimation deposition process incorporating a Sb2Se3 seed layer; key to achieving higher efficiency devices via close space sublimation. Utilizing a glass|FTO|TiO2|Sb2Se3|PCDTBT|Au structure, a peak efficiency of 6.6% was achieved, which is comparable to the current record devices for this material. Crucially, this device avoids toxic lead in the hole transport material, and cadmium in the window layer. Moreover, the addition of the PCDTBT back contact both maintains peak efficiency of 6.6%, and improves the uniformity of performance, increasing the average efficiency from 4.3% to 6.1%.