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.

Sb2Se3 has attracted much attention in recent years for its potential as an absorber layer in photovoltaics (PV) and photocatalysis. It has experienced a rapid rise in PV performance from 2% to nearly 10% in only a few years.1,2 The material has a very high absorption coefficient and a bandgap of 1.18 eV, making it a good candidate for use in PV.3,4 Furthermore, Sb and Se are Earth-abundant and low-cost,5 and Sb2Se3 can be fabricated via a wide variety of scalable methods.6–10 Sb2Se3 also attracts great interest due to its unusual 1D nanoribbon structure and Sb 5s2 lone pair of electrons.3,11,12 This structure means that strong, covalently bonded 1D nanoribbons are bound by weaker van der Waals interactions in two dimensions.

Several studies have evidenced the formation of an Sb2O3 contaminant layer on the surface of Sb2Se3 thin films and crystals alike.12–14 This has attracted interest from a PV perspective due to the implications of an intermediate layer between Sb2Se3 and a back contact layer for charge transport/extraction in superstrate devices13,15 (and could also have implications for heterojunction formation in substrate devices). This relies heavily on both the conductivity of the Sb2O3 interlayer and the band alignments between Sb2Se3, Sb2O3, and the metal contact. However, a full understanding of this oxide is difficult due its contaminant nature. Band alignment estimates from natural band positions with respect to the vacuum level are avoided here as they do not account for any charge transfer at the real interface, and natural alignment measurements using photoemission are difficult for very wide bandgap materials. Furthermore, there is significant variation in the reported ionization potential of Sb2O3 in the literature.16,17 Previously, attempts have been made to reproduce the effects of native oxide through deposition of an ultra-thin Sb2O3 film at the back surface;15 however, a clearer analysis is possible using bulk crystals.

The aforementioned crystal structure of Sb2Se3 means that, in two spatial directions, highly oriented bulk crystals can be easily cleaved or exfoliated to expose a pristine surface. In this study, bulk crystals with natively grown oxide were cleaved in situ to allow photoemission measurements of the valence band both with and without this surface contamination. A valence band subtraction method was used to measure the valence band offset (VBO) between these two layers without having to destructively remove the contamination or attempt to reproduce it with another method. This subtraction method was used previously by Fleck et al., but no quantitative analysis was carried out.15 Additionally, thermally evaporated thin films are used to measure the band alignment between Sb2O3 and close space sublimation (CSS) grown Sb2Se3 via the Kraut method (a common technique used to determine valence band offsets between materials) for comparison.18 

The Sb2Se3 bulk crystals were fabricated via the Bridgman melt-growth technique using a single-zone vertical furnace. A sealed ampoule containing manually ground Sb2Se3 granules (5N purity, Alfa Aesar) was placed with the bottom tip in line with the peak of the temperature profile in the furnace and heated to 620 °C to melt the source material. It was then held for around 6 h to allow full melting and homogenization of the powder. The ampoule was then lowered through the natural temperature gradient of the furnace at 0.6 °C/mm toward the lower, open end of the furnace (at room temperature) at a rate of 1.15 mm/h for 7 days. The ampoule was rotated slowly throughout to ensure homogeneous heating. A more detailed description of the process and characterization of the crystals is provided elsewhere.12,19–22

Three thin film samples were used in this work to carry out the Kraut method. A thick film of each material (Sb2Se3 and Sb2O3) was required to measure core level and valence band maximum (VBM) binding energy as well as an “interfacial” sample—a thin layer of Sb2O3 on Sb2Se3 to measure core level binding energies. An 2μm thick Sb2Se3 film was deposited via CSS onto solution-processed TiO2 films on fluorine-doped SnO2 (FTO)-coated glass substrates (matching the usual solar cell structure6,19). A two-stage process was used—2 min with a source temperature of 390 °C, substrate heating at 360 °C, and a pressure of 0.05 Torr followed by 15 min with a source temperature of 470 °C at a pressure of 10 Torr. An 75 nm thick film of Sb2O3 was deposited via thermal evaporation onto an FTO-coated glass substrate. A thin Sb2O3 film was deposited onto Sb2Se3 for the interface measurement. Sb2O3 thickness was limited to 22 nm, as determined using atomic force microscopy, and the Sb2Se3 layer was identical to the thick Sb2Se3 sample. More detailed descriptions of the process and characterization of the films have been carried out elsewhere.6,23,24

Hard x-ray photoemission spectroscopy (HAXPES) was carried out at the I09 beamline at the Diamond Light Source facility. An x-ray energy of 5.921 keV was selected using a double-crystal Si(111) monochromator followed by a Si(004) channel-cut crystal. The energy resolution was 250 meV, determined by fitting a Gaussian-broadened Fermi–Dirac distribution to the Fermi edge of a gold reference sample. A Scienta Omicron EW4000 high-energy analyzer was used to acquire the data, with an acceptance angle of ±28°. All spectra were calibrated using the Fermi level of a Au reference sample. Following the initial, as-received measurement, the crystals were cleaved in situ to expose a pristine (010) surface (in the Pbnm space group setting25), as demonstrated by Hobson et al. and Don et al. in previous works.12,19 All peak positions from curve fitting are reported with an error of ±0.05 eV. HAXPES core-level spectra were curve fitted using CASAXPS software with Voigt line shapes after subtracting a Shirley background.26 

The valence band spectra of a highly oriented bulk crystal material can provide a wealth of information about its electronic properties. Figure 1(a) shows the valence band spectra of an Sb2Se3 bulk crystal both prior to and after in situ cleaving to remove any surface contamination. Straight line fits are used to determine the energy position of the leading edge—this energy is representative of separation between the VBM and the Fermi level, and henceforth, any reference to VBM is a reference to energy separation between the valence band and the Fermi level. The results of this fit are a valence band to Fermi level separation of 0.88 and 0.97 eV for the uncleaved and cleaved Sb2Se3 crystals, respectively. This difference has significant implications for PV technology, as has been discussed in other work.15 

FIG. 1.

(a) Valence band spectra of (red) cleaved Sb2Se3 crystal and (blue) uncleaved Sb2Se3 crystal and (b) valence band spectra of (green) Sb2O3 thin film and (black) valence band subtracted Sb2O3. All spectra are shown with linear fits and measured VBM energies. Data for part (a) are reproduced with permission from Don et al., J. Mater. Chem. C 8, 12615 (2020). Copyright 2015 Author(s), licensed under a Creative Commons Attribution 4.0 License.

FIG. 1.

(a) Valence band spectra of (red) cleaved Sb2Se3 crystal and (blue) uncleaved Sb2Se3 crystal and (b) valence band spectra of (green) Sb2O3 thin film and (black) valence band subtracted Sb2O3. All spectra are shown with linear fits and measured VBM energies. Data for part (a) are reproduced with permission from Don et al., J. Mater. Chem. C 8, 12615 (2020). Copyright 2015 Author(s), licensed under a Creative Commons Attribution 4.0 License.

Close modal

Figure 1(a) shows the valence band spectra of the Sb2Se3 bulk crystal before and after in situ cleaving. The spectra show some difference in intensity at binding energies above 3 eV, and this difference is particularly pronounced between 3 and 4 eV. As previously reported by Don et al.,12 this intensity difference is attributed to a surface layer of Sb2O3 that is known to grow on the surface of Sb2Se3 upon exposure to air. The two spectra have been normalized to be matching in intensity at 1.7 eV.27 This point was chosen for normalization for two reasons—first, the shapes of the valence bands are very similar up to 2 eV, with the shape of the spectrum from the uncleaved sample beginning to differ towards higher binding energy. By normalizing to this point, there is no negative intensity in the difference spectrum,27 with the contribution from Sb2O3 beginning to show from 2 eV. This point also corresponds to binding energy at which the Sb 5s orbital has a significant contribution in the Sb2Se3 density of states.12 It therefore corresponds to a peak in the valence band density of states (rather than background intensity or a peak shoulder/edge), making it more reliable. Second, at this point, no contribution from Sb2O3 would be expected (due to its wide bandgap and n-type conductivity), meaning that the intensity at this point originates solely from Sb2Se3 in both spectra. No energy shift was necessary to align the two samples, which lent weight to the assumption that the valence band edge position was representative of Sb2Se3’s VBM-EF separation in both the oxidized and pristine states. By subtracting the cleaved spectrum from the uncleaved, the remaining intensity, called the difference spectrum, should originate from this Sb2O3 layer.27 This is shown by the black circular data points in Fig. 1(b) and will henceforth be referred to as “Sb2O3-sub.”

To ascertain whether this method was successful, the Sb2O3-sub-difference spectrum was compared to the valence band spectrum of a thin film of Sb2O3 (henceforth referred to as “Sb2O3-film”). This is a thermally evaporated thin film deposited onto Sb2Se3 but with a thickness of 75 nm. These data are represented by the green square data points in Fig. 1(b) and shows good agreement with the Sb2O3-sub data. Both exhibit a peak in intensity at the edge of the valence band, with another broader feature at 5–6 eV below the valence band edge. However, the two spectra do not line up in energy, with the Sb2O3-film valence band lying roughly 0.5 eV to higher binding energy than Sb2O3-sub. There is also a peak in the Sb2O3-sub-spectrum at 1 eV, corresponding exactly to the edge of the Sb2Se3 data. This spike is attributed to a difference in mid-gap states between the air exposed crystal and the in situ cleaved crystal.28,29

Again using linear fits, the VBM energies were determined to be 3.07 and 2.60 eV, respectively, for the Sb2O3-film and Sb2O3-sub. This highlights a difference between the deposited film and native oxide, although whether this is due to a difference in thickness or has a different origin is unclear (as discussed below). Using the valence band edge of the Sb2O3-sub, it is possible to determine the band alignment between the valence band of Sb2Se3 and Sb2O3 contamination.

The bandgap of Sb2Se3 is well known to be 1.18 eV,4 while there is a range of reported bandgap values for Sb2O3 from 3.6 to 4 eV.30–32 For this study, we have used a DFT-calculated bandgap of approximately 4 eV.33 This was calculated using the HSE06 functional that is known to achieve accurate bandgap values and is in line with many values reported in the literature.30–32 This is toward the upper limit of the range reported in the literature, but, as will be shown later, any smaller values within the reported range would have no bearing on the conclusions of this work—the nature of the conduction band offset is the same for all reasonable values of the Sb2O3 bandgap. Using the valence band subtraction method, the VBO between Sb2Se3 and Sb2O3 can be measured as the difference in the two valence band edge energies of the uncleaved crystal and native oxide from the Sb2O3-sub-spectrum. Figure 2(a) shows the resulting experimentally determined band alignment between Sb2Se3 and its native oxide. The VBO of 1.72 eV shown in Fig. 2(a) implies a straddling alignment (where both the CBM and VBM of one material lie within the bandgap of the other), as would be expected with two materials of such different bandgaps. An uncertainty of 0.14 eV was determined for the linear fitting procedure, which is not sufficient to change the straddling nature of the alignment.

FIG. 2.

Band alignment between (a) Sb2Se3 bulk crystal and its native Sb2O3 determined by valence band subtraction and (b) close space sublimation-deposited Sb2Se3 and a thermally evaporated Sb2O3 thin film determined using the Kraut method. Conduction band energies have been determined by adding the known bandgap values for Sb2Se3 and Sb2O3 of 1.18 and 4 eV.

FIG. 2.

Band alignment between (a) Sb2Se3 bulk crystal and its native Sb2O3 determined by valence band subtraction and (b) close space sublimation-deposited Sb2Se3 and a thermally evaporated Sb2O3 thin film determined using the Kraut method. Conduction band energies have been determined by adding the known bandgap values for Sb2Se3 and Sb2O3 of 1.18 and 4 eV.

Close modal

In order to verify the nature of the alignment between Sb2Se3 and Sb2O3, the VBO between thin films of Sb2Se3 and Sb2O3 was measured via the Kraut method. This method is widely used to directly measure the offset between the valence bands of one material deposited on another using Eq. (1). ECLX denotes the core-level binding energy of material X, EVX is the binding energy of the VBM of material X, and ΔECL represents the binding energy separation of two core levels from different materials in the interfacial sample,

ΔEV=(ECLBEVB)(ECLAEVA)+ΔECL.
(1)

By taking advantage of the fact that the band energy shift that occurs upon interface formation is consistent for the valence band, conduction band, and, importantly, core levels, this method allows the true offset to be determined as long as the top layer is thin enough to enable the photoelectrons from the substrate layer to pass through the top layer.18 The use of HAXPES allows for depths of 30 nm to be probed, much greater than XPS (10 nm) or UPS (2 nm).23 For this measurement, an Sb2O3 layer was deposited to a thickness of 22 nm onto a 1.5 μm thick layer of Sb2Se3. Separate, thicker layers of Sb2Se3 (2 μm) and Sb2O3 (75 nm) were also used for the measurement. In Eq. (1), the energy separation between core levels and the valence band edge, (ECLEV), is measured for a thick film of each material, and then the energy separation between core levels from each material, ΔECL, is measured for the thin interfacial sample.

Figure 3 shows the HAXPES core level spectra of Sb 3d and O 1s regions for Sb2Se3, Sb2O3, and Sb2O3/Sb2Se3 interface thin films. The spectrum from the Sb2Se3 film [Fig. 3(a)] is, as expected, dominated by Sb bonded to Se, with a very small contribution from Sb bonded to O. This is a result of oxidation of the surface when the sample was briefly exposed to air. The signal is so small that it does not indicate a complete layer of Sb2O3, such as the one removed by in situ cleaving.12 The spectrum from the Sb2O3 film [Fig. 3(b)] is dominated by Sb bonded to O and the overlapping O 1s peak. This sample also shows some trace contamination from metallic Sb at 528.79 eV, likely a by-product of the deposition process or some slight oxygen deficiency in the source material. The interfacial sample [Fig. 3(c)] shows signals from both Sb2Se3 and Sb2O3 with the oxide signal dominating as expected for the uppermost layer. Other core levels used for the Kraut method calculations and valence band spectra for thin films are included in Figs. S1–S4 in the supplementary material.

FIG. 3.

Core level spectra of Sb 3d and O 1s regions for (a) the Sb2Se3 thin film, (b) the Sb2O3 thin film, and (c) the Sb2O3/Sb2Se3 interfacial sample.

FIG. 3.

Core level spectra of Sb 3d and O 1s regions for (a) the Sb2Se3 thin film, (b) the Sb2O3 thin film, and (c) the Sb2O3/Sb2Se3 interfacial sample.

Close modal

Figure 2(b) shows the VBO as measured by the Kraut method. The measured VBO was 1.90 eV, taken as an average of the different valence band offsets calculated using different combinations of core-level peaks (see Table S2 in the supplementary material). The error on the measurement was taken as the standard deviation of the different calculated values and was determined to be 0.13 eV. The offset of 1.90±0.13 eV is consistent with that determined by the subtraction method and would also signify a straddling alignment. For detailed breakdown of measured peak positions and valence band offsets, see Tables S1 and S2 in the supplementary material.

While the two VBO values determined for native oxide on the Sb2Se3 crystal and the evaporated Sb2O3 on Sb2Se3 film are consistent with each other, there are several possible reasons for any small differences. These include how the native oxide forms on the surface, which occurs under very different conditions from a thermally evaporated film, and has been shown to be accompanied by elemental selenium at the surface [Eq. (2)].15 Additionally, the crystal surface studied is made up of only one crystal orientation, whereas the polycrystalline film’s surface includes multiple different orientations.6,19,34 Ultimately, however, the two results can be considered consistent with each other taking into account the uncertainty on the values, which was taken as the standard deviation in the calculated values (0.13 eV) for the Kraut method results and as 0.14 eV for the subtraction method (determined from uncertainty in the linear extrapolation of the valence band onsets),

2Sb2Se3+3O22Sb2O3+3Se2.
(2)

These results have implications for Sb2Se3’s use as a PV material due to great difficulty in avoiding Sb2O3 formation at the back surface, as reported by Fleck et al.15 The presence of Sb2O3 between the Sb2Se3 layer and the back contact had a significant effect on the degree of “rollover” (a feature in J–V curves indicative of a back contact potential barrier) in Sb2Se3 solar cells and the formation of Ohmic contacts. Attempts to remove this Sb2O3 contamination have also been shown to have mixed effects on the performance of the PV devices.13,14 Fleck et al. also attempted to replicate the effects of Sb2O3 contamination by depositing ultra-thin films of Sb2O3 onto the back contact via thermal evaporation. They reported that a thin layer of Sb2O3 could suppress recombination at the interface between Sb2Se3 and Au provided the holes could tunnel through the Sb2O3 layer. Thicker layers of Sb2O3, even up to 5 nm, were found to be detrimental to device performance. This is supported by the conclusions of this work, which shows that any significant thickness of Sb2O3 would provide a significant barrier to hole transport through the back contact due to the magnitude of the negative VBO.

The significant offset between the two valence bands can be explained by looking in greater detail at the valence orbital density of states for the two materials. Antimony chalcogenides are known to exhibit strong cation s–anion p orbital mixing, leading to the existence of a stereochemically active lone pair of electrons at the valence band edge.12,36 The mixing of the cation s states with the anion p states results in the formation of bonding and anti-bonding states. The anti-bonding states in turn hybridize with the cation p orbitals to form bonding and anti-bonding states that make up the edges of the valence and conduction bands, respectively. The configuration energies of the anion p-orbitals therefore play a significant role in determining the position of the valence band maximum on an absolute energy scale, and this has been shown to result in a lower ionization potential in materials containing these lone pairs compared to those without. Both Sb2Se3 and Sb2O3 are predicted to undergo this lone pair formation (experimentally evidenced for Sb2Se3 by Don et al.,12) but, as shown in Fig. 4, there is a significant “jump” in the orbital energies when going from sulfur to oxygen in the chalcogenide series, with the selenium orbital energies being similar to those of sulfur.35 The valence band of Sb2O3 would therefore be expected to sit significantly lower (or have a higher ionization potential) than that of Sb2Se3, as seen experimentally in this work.

FIG. 4.

Orbital energies of Sb and the chalcogenide series. The energy values used for this are taken from Ref. 35.

FIG. 4.

Orbital energies of Sb and the chalcogenide series. The energy values used for this are taken from Ref. 35.

Close modal

The band alignment between Sb2O3 and Sb2Se3 was measured via two methods. The first method found the difference between the valence band spectra of a native oxide contaminated and an in situ cleaved bulk Sb2Se3 crystal, while the second utilized the Kraut method for a thermally evaporated Sb2O3 film on a polycrystalline Sb2Se3 grown by close space sublimation. A VBO of 1.72±0.14 eV was measured via the valence band subtraction method between an Sb2Se3 crystal and the contaminant native Sb2O3 layer, leading to a straddling alignment. This is supported by a similar result obtained by the Kraut method, which yielded a VBO of 1.90±0.13 eV between a Sb2O3 thin film grown on Sb2Se3. The small difference in the two valence band offsets, however, may be due to differences in the electronic properties of the thermally evaporated film and the native oxide contamination as well as the different Sb2Se3 surfaces. The magnitude of the VBO can be explained by examining the orbital energies of the chalcogenide series. Due to the presence of a stereochemically active lone pair at the valence band edge, the chalcogenide p orbital plays a major role in determining the band edge position of the series. As such, there is a significant drop in p orbital energy and an increase in ionization potential moving up the chalcogenide series from selenium and sulfur to oxygen. These results further the understanding of the crucial interface at oxidized Sb2Se3 surfaces, and this novel band alignment information can be used to further the Sb2Se3 device performance.

See the supplementary material for core-level and valence band photoemission spectra from the Sb2Se3 and Sb2O3 thin films and core-level photoemission spectra from the Sb2O3/Sb2Se3 interface sample; table of binding energies of core levels and VBMs used in Kraut method calculations; and table of Kraut method-determined VBOs using different combinations of core levels.

The Engineering and Physical Sciences Research Council (EPSRC) is acknowledged for the funding of H.S. (Grant No. EP/N509693/1); T.D.C.H. and K.D. (Grant No. EP/T006188/1); O.S.H. (Grant No. EP/M024768/1); L.A.H.J. (Grant No. EP/R513271/1); J.E.N.S., T.J.F., and M.J.S. (Grant No. EP/L01551X/1); J.D.M. (Grant No. EP/N014057/1); and T.D.V. (Grant No. EP/N015800/1). Paul Warren of NSG Group is thanked for discussions, funding of H.S., and for supplying coated glass substrates. Diamond Light Source is acknowledged for I09 beam time under Proposal No. SI23160-1.

The data that support the findings of this study are available within the article and its supplementary material.

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