Routes to increase performance for antimony selenide solar cells using inorganic hole transport layers

1 Introduction

Antimony selenide (Sb₂Se₃), as a simple and low-cost compound with a direct energy band gap (∼1.18 eV), high absorption coefficient ($>10^5$ cm⁻¹) and high carrier mobility (∼10 cm²/Vs, is a promising emergent light absorber for photovoltaic (PV) applications (Chen et al., 2015; Chen et al., 2017; Birkett et al., 2018). As a material, Sb₂Se₃ is mainly composed of (Sb₄Se₆)_n as 1-D ribbon structures, where the ribbons are strongly coupled by covalent bonds running along the c-axis with weaker Van der Waals (VdW) interactions between the ribbons. Thus, stacking of the ribbons occurs due to the weaker VdW bonds (Deringer et al., 2015). Hole mobility is enhanced in the c-axis and can reach 45 cm²/Vs along the ribbons (Black et al., 1957).

A number of studies have reported that Sb₂Se₃ thin films with preferred crystallographic orientation along the (hk1) direction, particularly (221), resulted in devices with higher efficiencies (Leng et al., 2014; Yuan et al., 2016; Li et al., 2017). The improved performance is often attributed to increased charge transport through the (hk1)-oriented ribbons perpendicular to the substrate and benign grain boundaries in this material (Chen et al., 2017; Williams et al., 2020). Wang and co-workers demonstrated the dependence of Sb₂Se₃ PV device performance on the preferred crystal orientation of the absorber (Wang et al., 2017). In that work, by optimising growth conditions, Sb₂Se₃ solar cells with preferred (211) and (221)-orientations on CdS and ZnO achieved higher efficiencies (5.6% and 6.0%, respectively) than those with (020) and (120)-orientations (3.2% and 4.8%, respectively). For planar Sb₂Se₃ solar cells in substrate orientation, a record efficiency of 6.5% has been reported with the Cd_0.75Zn_0.25S buffer layer being used as an alternative to CdS (Figure 1A shows standard substrate device). Meanwhile, Sb₂Se₃ devices with this buffer layer but in a superstrate structure (Figure 1B) have achieved an efficiency of 7.6% (Wen et al., 2018). Recently, a record substrate device efficiency of 9.2% was obtained by growing (001)-oriented Sb₂Se₃ nanorod arrays on sputtered molybdenum layers (Li et al., 2019). A conformal interfacial TiO₂ layer was used to mitigate the migration of elemental antimony (Sb) into the CdS buffer layer, as interdiffusion has been shown to create a detrimental CdSe interlayer (Phillips et al., 2019).

FIGURE 1. Standard planar (A) substrate and (B) superstrate configuration Sb₂Se₃ solar cells.

In this work, thin transition metal oxides, NiO and MoO_x, are applied as HTLs in substrate Sb₂Se₃ devices to improve carrier selectivity at the back electrode by controlling inter-diffusion and formation of secondary phase materials (such as MoSe₂) at the interface. Additionally, NiO and MoO_x HTLs are deposited on superstrate Sb₂Se₃ films before making Au back contacts to alter the energy band alignments at the back contact effectively producing an electron reflector, and minimising carrier recombination.

In the first part of this study, Sb₂Se₃ substrate/superstrate device simulations using solar cell capacitance simulator (SCAPS) are conducted in order to interpret the effect of HTLs on Sb₂Se₃ device characteristics (Burgelman et al., 2000). We then characterise the material properties of MoO_x and NiO thin films deposited at room temperature by electron beam evaporation. At this temperature it was found that NiO formed a crystalline film, unlike MoO_x which was amorphous. Sb₂Se₃ absorber films were then fabricated by close-space sublimation (CSS) and thermal evaporation (TE) techniques and incorporated into superstrate and substrate solar cell configurations. HTLs were inserted at the metal electrode/Sb₂Se₃ absorber interface and their effect on Sb₂Se₃ crystallinity and solar cell performance is comprehensively studied.

2 Experimental section

2.1 Device fabrication

The basic structure of substrate Sb₂Se₃ solar cells was as follows: Soda lime glass (SLG)/Mo/Sb₂Se₃/CdS/ZnO/ITO/Ni-Al. Mo coated soda lime glass (SLG) substrates measuring 7.5 cm² × 2.5 cm² were used in this study. NiO or MoO_x HTLs were deposited between the Mo electrode and Sb₂Se₃. Thin HTL films of 15 nm thickness were deposited using e-beam evaporation. 500 nm thick Sb₂Se₃ layers were prepared by TE of crystalline/powder Sb₂Se₃ source material (Alfa Aesar, 99.99%) at a deposition rate of ∼15 Å/s. The substrates were maintained at a temperature of 300°C throughout the deposition. The Sb₂Se₃ films were subsequently subjected to a heat treatment at 300°C for 30 min in Ar atmosphere in a tube furnace to promote recrystallisation. For the CSS Sb₂Se₃ films, a compact seed layer was grown at 0.05 mbar N₂ for 5 min with a source temperature of 350°C, followed by a 30 min growth step at 13 mbar and a source temperature of 450°C to produce a compact and highly orientated grain structure. The substrate was then rapidly cooled with N₂. An n-type CdS buffer layer (∼60 nm) was deposited by chemical bath deposition followed by DC-pulsed sputtering deposition of an i-ZnO (∼35 nm) layer plus a transparent conductive window layer ITO (∼200 nm). Front contact grids comprising Ni (∼50 nm) and Al (∼1,000 nm) were deposited through a shadow mask by e-beam evaporation. Finally, 0.16 cm² cells were defined by mechanical scribing on each substrate.

Superstrate Sb₂Se₃ solar cells have the following configuration: SLG/ITO/CdS/Sb₂Se₃/Au with NiO or MoO_x HTLs deposited between the metal contact and Sb₂Se₃ absorber. The ITO layer was deposited by DC-pulsed sputtering and Sb₂Se₃ layers were grown by TE and CSS as detailed above. Finally, Au back contacts with an area of 0.07 cm² were deposited through a shadow mask by e-beam evaporation.

2.2 Material and device characterisation

The crystal structures of Sb₂Se₃ were characterised by X-ray diffraction (XRD) with Cu Kα1 (1.54056 Å) radiation (Rigaku SmartLab SE). The surface morphology and cross-sectional images of Sb₂Se₃ films were taken by scanning electron microscopy (SEM, Tescan Mira 3 FEG-SEM). Optical spectroscopy measurements were performed using a Shimadzu UV-2600 spectrophotometer fitted with an integrating sphere. Kelvin probe force microscopy (KFPM) measurements were done using a KP Technology KP020 single point kelvin probe system fitted with a standard 2 mm Au tip.

Current-density vs. voltage (J-V) measurements of Sb₂Se₃ thin film solar cells were performed using an Abet Technologies solar simulator at 1-sun (100 mW/cm²) illumination equivalent to air mass 1.5 global spectrum with light power density calibrated using a Si reference cell.

2.3 Device simulation

Device simulation was carried out for both substrate and superstrate configuration Sb₂Se₃ solar cell using Solar Cell Capacitance Simulator (SCAPS 1-D), which is based on the solutions to Poisson’s equation and continuity equation for electrons and holes in the vertical heterostructure of multilayer thin film PV device (Burgelman et al., 2000). The input parameters of the solar cells were defined with the Sb₂Se₃, HTL and electron transport layer (ETL) semiconducting properties, including experimentally determined bandgaps, electron affinity, density of states (Zeng et al., 2016), mobility of charge carriers (Chen et al., 2017), acceptor/donor concentrations (Wang et al., 2015), and defect state density (Leijtens et al., 2016). Defects were introduced at the Sb₂Se₃/CdS interface to simulate realistic device performance.

3 Results and discussion

3.1 Simulated Sb₂Se₃ devices

Simulation analysis using SCAPS software was implemented to evaluate the performance of reference substrate and superstrate Sb₂Se₃ solar cells and those incorporating MoO_x and NiO as HTLs, subsequently referred to as samples Ref, MoO_x and NiO, respectively (see Table 1 for film properties). Figure 2 shows the J-V curves and corresponding box plots of J-V parameters of both Sb₂Se₃ device configurations with incorporated HTLs. Regarding the substrate devices, all device parameters are improved, with the exception of V_oc which shows a slight decrease for devices with a HTL (down from 0.423 V for the reference device to 0.408 and 0.411 V for MoO_x and NiO devices, respectively). However, devices with MoO_x HTL show evidence of roll-over behaviour. The roll-over phenomenon, which occurs near the V_oc in a light J-V curve, is due to Schottky energy barrier formed at the absorber/metal interface at a solar cell back contact (Eisenbarth et al., 2011; Hädrich et al., 2011). It acts as a reverse biased diode when the main junction is forward biased, blocking carrier transport for increasing forward bias, resulting in roll-over behaviour in light J-V characteristics. The baseline J_sc in the reference device was 29.9 mA/cm², rising to 31.2 and 31.3 mA/cm² in MoO_x and NiO devices, respectively. Addition of HTL films to the reference device demonstrated a notable increase in FF for substrate devices. The FF in the reference device was 47.0%, rising to a maximum of 55.6% and 56.0% in the MoO_x and NiO devices, respectively. The increase in J_sc and FF of devices with integrated HTL materials directly translates into improvements in power conversion efficiency, PCE [η = 5.9% (Ref), 6.7% (MoO_x) and 7.2% (NiO)]. The current-blocking energy barrier at the back contact of the MoO_x substrate device could explain the lower PCE in comparison to the device with a NiO HTL. It is important to note that the results shown are not representative of the maximum conversion efficiencies that may be achieved with Sb₂Se₃, as we are focusing solely on the effect of the HTL, while using currently available materials parameters.

FIGURE 2. (A) J-V curves and (B) J-V parameters of simulated Sb₂Se₃ solar cells with different HTL materials. Roll-over behaviour is observed in the J-V curve of substrate devices with MoO_x HTL.

For superstrate Sb₂Se₃ solar cells, devices with an incorporated HTL showed an increase in J_sc of around 8% from 31.5 mA/cm² observed in the reference device to 33.5 and 34.0 mA/cm² in the devices with a MoO_x and NiO HTL, respectively. As a result of the improvement in J_sc, the PCE of solar cells with a HTL increased to 7.5% (MoO_x) and 8.0% (NiO) from the reference value of 7.3%. Interestingly, no roll-over was seen in the J-V curve for the MoO_x device which could be related to the use of Au as metallic back contact rather than Mo in the substrate devices. The work function (WF) of a metal employed as a rear contact on a PV device plays an important role in facilitating hole extraction at the contact (Fleck et al., 2020). Typically, Au is reported to have a WF of 5.10 eV (Michaelson, 1977) and Mo has WFs ranging from 4.50–4.95 eV, depending on the preferred crystal orientation of the metal (Green, 1969; Michaelson, 1977; Hölzl and Schulte, 1979). To illustrate the effect of back contact metal WF on substrate/superstrate Sb₂Se₃ device performance, Figure 3 shows the dependence of J-V parameters on the WF of Mo and Au metals. It is apparent that the J-V parameters of all substrate devices are sensitive to variations in the value of Mo WF. In the Ref and MoO_x substrate devices, V_oc decreases monotonically with Mo WF where a significant drop is observed from 0.432 V to 0.422 V at WF 4.95 eV to 0.036 V and 0.093 V at WF 4.50 eV for Ref and MoO_x devices, respectively. This is a clear indication of an increasing back contact barrier with decreasing Mo WF. This phenomenon has been observed experimentally in Sb₂Se₃ solar cells previously (Liu et al., 2014; Li et al., 2017). The V_oc in the NiO device is less affected by the Mo WF, reducing from 0.422 V at WF 4.95 eV to 0.319 V at WF 4.50 eV. A similar trend is seen in J_sc, FF and η parameters for the substrate devices. However, a low Mo WF of 4.50 eV causes a notable decrease in FF of the MoO_x device (12.3%), compared to the Ref and NiO devices (26.0% and 32.4%).

FIGURE 3. J-V parameters of simulated Sb₂Se₃ substrate devices with Mo back contact (varying Mo WF between 4.50–4.95 eV) and simulated Sb₂Se₃ superstrate devices with Au back contact (WF at 5.1 eV).

In order to understand the improvement of the device performance with the introduction of HTLs, it is necessary to consider the energy band alignment at the interfaces at the back of the PV devices. Figure 4 shows the simulated energy band diagrams of substrate and superstrate Sb₂Se₃ devices incorporating NiO and MoO_x HTLs. Due to a small electron affinity (EA = 1.46 eV (NiO), 2.05 eV (MoO_x)) and large band gaps (E_g ∼3.80 eV (NiO), 3.50 eV MoO_xx)) in both HTL materials, a large potential energy barrier is formed at the back contact, reflecting electrons. This barrier minimises carrier recombination at the back interfaces with Sb₂Se₃ and improves conductivity at the back electrode. However, it is apparent that a non-negligible hole barrier of 0.26 and 0.29 eV is formed at the MoO_x/Sb₂Se₃ interface of the substrate and superstrate devices, respectively, which can manifest as J-V roll-over behaviour seen in the simulated MoO_x substrate device. Thus the SCAPS simulations indicate the incorporation of a MoO_x or NiO HTL into substrate and superstrate configuration Sb₂Se₃ solar cells increases device performance compared to a standard solar cell by the introduction of a high energy barrier to electrons at the rear absorber/metal interface.

FIGURE 4. Energy level alignment for the devices in substrate (A) and superstrate (B) orientati1ons. Devices without a hole transport layer (top), with a MoO_x layer (middle) and a NiO layer (bottom) are shown.

3.2 Fabricated Sb₂Se₃ devices

100 nm thick films of MoO_x and NiO were deposited on SLG at room temperature to facilitate characterisation of the HTLs. Figure 5 shows surface morphology SEM images of the respective HTLs. The MoO_x film exhibits an amorphous, flake-like structure in comparison to a compact crystalline morphology observed in the NiO film. XRD patterns in Figure 6 confirm the amorphous and crystalline nature of the MoO_x and NiO films, respectively. All the diffraction peaks in the NiO thin film were identified and indexed to cubic NiO (JCPDS number 04-0835) and no diffraction peaks of other impurity phases were observed.

FIGURE 5. Top-down SEM image of a 100 nm (A) MoO_x film and (B) NiO films on glass. Inset: Higher magnification image of the NiO film, showing the nanostructure.

FIGURE 6. XRD pattern of 100 nm films of (A) MoOx and (B) NiO on soda lime glass (SLG). Reference XRD data for MoO₂, MoO₃ and NiO are shown underneath the XRD with JPDCS card ID 65-5787, 35-0609 and 04-0,835 respectively.

Supplementary Materials S1A shows the spectral transmittance and reflectance of the NiO and MoO_x films on SLG. Both HTLs are highly transparent in the visible and near-infrared wavelength region and their transmittance falls sharply at ultraviolet wavelengths. However, the amorphous MoO_x film has slightly lower transmittance/higher reflectance in the sub-600 nm wavelength region compared to the crystalline NiO film. The bandgap energy (E_g) of the HTL films was calculated by extrapolation of the linear region of the Tauc plot to the x-axis, according to the relation (Tauc et al., 1966):

where α is the absorption coefficient of the semiconductor material, h is Planck’s constant, ν is the frequency of the electromagnetic radiation and A is a constant of proportionality. The estimated E_g values of NiO and MoO_x films are 3.95 and 3.85 eV, respectively (see Supplementary Materials S1B). A HTL film thickness of 15 nm was incorporated into the superstrate/substrate device to ensure a conformal coating of the HTL. A HTL requires a thickness sufficient to preserve the desired material properties and not impede charge transport considerably which would detrimentally increase series resistance in the finished devices.

3.3 Superstrate devices

TE and CSS deposition techniques were employed for Sb₂Se₃ film growth on SLG/ITO/CdS superstrates. For TE, the SLG/ITO/CdS superstrates were heated to 300°C prior to Sb₂Se₃ deposition in order to promote the growth of preferred (hk1) crystal orientations while minimising (hk0) orientations (Zhou et al., 2015) (hk0) planes, specifically (120), have been found to be detrimental to carrier transport (Guo et al., 2018; Wen et al., 2018; Li et al., 2019). The (hk0)-oriented Sb₂Se₃ nanoribbons are stacked parallel to the ITO/SLG superstrate where conductivity is inhibited by electrically insulating VdW bonds between the stacked nanoribbons. A seed layer is used in Sb₂Se₃ films deposited via CSS. This seed layer has a high density of nucleation points for the second stage of growth during the CSS process, which improves uniformity, raising the average efficiency of devices (Hutter et al., 2018a). Transmittance and reflectance data for a representative TE Sb₂Se₃ film was used to determine the E_g from a Tauc plot, which gave a E_g value of 1.17 eV in good agreement with (Birkett et al., 2018), see Supplementary Materials S2A,B. XRD patterns for Sb₂Se₃ films deposited by TE and CSS are shown in Figure 7A. The peaks in both XRD patterns are sharp and well resolved indicating the polycrystalline nature of the Sb₂Se₃ thin films. The lattice planes are cross-referenced to JCPDS card no. 15-0861 confirming the formation of orthorhombic Sb₂Se₃ with space group Pbnm. Both XRD patterns show similar characteristics, exhibiting strong (211) and (221) peaks with minimal contributions from (hk0) planes. Figures 7B–E shows the top and cross-sectional SEM images of Sb₂Se₃ thin films deposited by TE and CSS. The different growth techniques result in contrasting Sb₂Se₃ film morphologies. TE produces Sb₂Se₃ films of uniform thickness of ∼500 nm and densely packed grains, confirming the good crystallinity of the films, consistent with the XRD results (Figures 7B,D). However, this deposition method did not form a conformal coating of the Sb₂Se₃ film across the entire superstrate with the presence of pinholes observed, see Supplementary Materials S3A.

FIGURE 7. (A) XRD patterns of Sb₂Se₃ layers deposited by TE and CSS on ITO/CdS superstrates with standard diffraction pattern for Sb₂Se₃ (JCPDS15-0861) included for reference and SEM images of corresponding TE (B,D) and CSS (C,E) Sb₂Se₃ samples.

Conversely, CSS-grown Sb₂Se₃ films have a rough surface morphology with exceptionally large grains in comparison to the TE films and the grains extend the full depth of the layer. Larger grains are a prerequisite for better device performance as charge mobility is faster along the Sb₂Se₃ ribbons than hopping between the ribbons (see Figures 7C,E). The CSS films also showed a degree of porosity but not to the extent observed in the TE films, Supplementary Materials S3B. The presence of pinholes in the Sb₂Se₃ films is detrimental to device performance as shunting pathways may be formed upon subsequent deposition of the Au back contact (Hutter et al., 2018b).

J-V measurements under 1-sun illumination (100 mW/cm²) were performed on Sb₂Se₃ devices in the standard superstrate configuration and devices incorporating MoO_x and NiO HTLs. The light J-V curves were fitted using a single diode model to extract the values of series (R_s) and shunt (R_sh) resistances. Figure 8 compares the statistical distribution of the key PV parameters for these devices, where a minimum of 10 cells of each device type were measured. On average, there was a slight increase in V_oc when a NiO HTL was incorporated into the CSS device structure. Using a NiO HTL layer increased V_oc to 0.226 V from values of 0.201 and 0.186 V for Ref and MoO_x devices, respectively. The mean J_sc of NiO cells was also enhanced to 15.94 mA/cm² compared to Ref (11.34 mA/cm²) and MoO_x (10.54 mA/cm²) cells despite a slightly lower average FF in the NiO devices. This translates into a higher mean NiO CSS device efficiency of 1.01% with Ref and MoO_x devices achieving efficiencies of 0.71 and 0.59% respectively. Notwithstanding the higher average R_s (2.6 Ωcm²) and lower R_sh (74 Ωcm²) values for NiO CSS solar cells compared to Ref (R_s = 2.9 Ωcm², R_sh = 119 Ωcm²) and MoO_x (R_s = 1.3 Ωcm², R_sh = 167 Ωcm²) cells, using NiO as a HTL increases performance by boosting J_sc in CSS Sb₂Se₃ superstrate devices compared to the standard and MoO_x based devices.

FIGURE 8. J-V parameters of superstrate TE and CSS Sb₂Se₃ devices with incorporated MoO_x and NiO HTLs. □ is the average value and × is the minimum and maximum position. The three horizontal lines of each box stand for the 25%, 50%, and 75% of the reading distribution. The whisker range is determined by the standard deviation of the sampled devices. IQR is the inter-quartile range.

The average J-V parameters of TE Sb₂Se₃ superstrate devices followed a similar trend to those observed in the CSS devices [V_oc: 0.209 V (Ref) → 0.214 V (MoO_x) → 0.288 V (NiO), J_sc: 1.94 mA/cm² (MoO_x) → 8.05 mA/cm² (Ref) → 13.48 mA/cm² (NiO) ⇒ η: 0.10% (MoO_x → 0.72% (Ref) → 1.38% (NiO)]. It is worth noting that the mean FF of the Ref TE cells (38.0%) was higher in relation to the cells with a HTL (27.2% MoO_x, 34.6% NiO). This correlates to an increase in R_sh of 382 Ωcm² in Ref samples from R_sh values of 349 Ωcm² and 154 Ωcm² measured in MoO_x and NiO cells, respectively. In TE superstrate device configuration, the thin MoO_x film appears to form a more resistive layer compared to Ref and NiO devices (R_s: 10.8 Ωcm² MoO_x, 4.1 Ωcm² Ref and 6.6 Ωcm² NiO). Thus, overall device performance in MoO_x based solar cells is negatively impacted by low J_sc and high R_s which could be related to the amorphous nature of the MoO_x thin film and the presence of a current-blocking barrier at the back contact highlighted in device simulations. Despite lower FF in NiO based solar cells, device efficiencies exceed those of Ref and MoO_x TE devices due to improvements in V_oc and J_sc showing the benefit of using NiO as a HTL in superstrate Sb₂Se₃ solar cells.

3.4 Substrate devices

Figure 9 shows the XRD patterns of substrate Sb₂Se₃ thin films deposited via TE and CSS. All diffraction peaks are in good agreement with the orthorhombic Sb₂Se₃ (JCPDS 15-0861), which presents in the form of (hk0), (hk1) or (hk2). No diffraction peaks of other impurity phases were observed. TE Sb₂Se₃ films on Mo and Mo/MoO_x substrates show (020) and (120) peaks compared to all other Sb₂Se₃ films. The presence of (020) and (120) crystal orientations in thin Sb₂Se₃ films adversely affects PV device performance (Leng et al., 2014; Yuan et al., 2016; Li et al., 2017). However, when using a NiO HTL in TE Sb₂Se₃ films, it can be observed that the intensity of the diffraction peaks of Sb₂Se₃ is dominated by (221) and (211) crystal plane orientations. Furthermore, when using the Mo/NiO substrate, Sb₂Se₃ film shows an increased peak intensity for the (002) orientation. Since h and k miller indices have a zero value, it indicates that the (Sb₄Se₆)_n ribbons grow perpendicular to the substrate surface (Li et al., 2019). For CSS Sb₂Se₃ films, Ref and MoO_x samples demonstrate a higher (002) peak intensity than NiO.

FIGURE 9. XRD patterns of Sb₂Se₃ films deposited by TE or CSS on top of NiO, MoO_x and Mo-coated SLG.

Figures 10, 11 show SEM images of Sb₂Se₃ films on Mo-coated SLG deposited by TE and CSS methods, respectively. The top-down SEM images of the TE films (Figures 10A–C) show a difference in morphology depending on the presence of the underlying HTL. The MoO_x sample exhibits larger Sb₂Se₃ grains than the Ref sample and the presence of pinholes in both samples is patently obvious. On the other hand, the Sb₂Se₃ grains in the NiO sample appear more angular in nature although pinholes are still present in the film. The dissimilarity in morphology is emphasised in SEM cross-section images of the TE Sb₂Se₃ films (Figures 10D–F). Voids at the absorber/Mo interface are apparent in the Ref TE sample whereas the MoO_x sample shows a homogenous film with large grains. For the NiO sample, the Sb₂Se₃ grains appear column-like with no voids at the Mo interface. The top-down SEM image of all types of CSS Sb₂Se₃ thin films (Figures 11A–C) show significantly larger grains compared to the TE films. However, Sb₂Se₃ film in the Ref sample is on average thicker (∼1,000 nm) than the MoO_x (∼550 nm) and NiO (∼700 nm), see Figures 11D–F. The NiO sample also has a smoother surface topography.

FIGURE 10. Top-down and cross-sectional SEM images of reference substrate (A,D), MoO_x (B,E) and NiO (C,F) of Sb₂Se₃ films deposited by thermal evaporation.

FIGURE 11. Top-down and cross-sectional SEM images of reference substrate (A,D), MoO_x (B,E) and NiO (C,F) of Sb₂Se₃ films deposited by close-space sublimation.

To quantify the difference in orientations between the substrate Sb₂Se₃ thin films, the texture coefficient (TC) of diffraction peaks of the samples was calculated based on the following equation (Zoppi et al., 2006):

where I_(hkl) is the measured peak intensity of (hkl) plane and I₀ (hkl) the intensity in the standard XRD pattern. N is the total number of reflections considered for the calculation. A diffraction peak with a relatively large TC value ($>$1) indicates a preferred orientation of the grain along this direction. Figure 12 shows the TC for Sb₂Se₃ thin films with HTLs deposited by (A) TE and (B) CSS. It is apparent from Figure 12 that NiO HTL plays a critical role in eliminating the detrimental (hk0) planes in the TE samples and at the same time, significantly increases absorber growth in planes, i.e., (211), (221) that are perpendicular to the substrate surface. This further supports the enhanced device performance in solar cells when NiO is used as the HTL. In CSS samples, this templating effect of HTLs is not observed as no (hk0) planes are grown in the Ref and MoO_x samples. MoO_x increases the growth of favoured crystal planes including (211), (221), and (002) compared to the Ref substrate sample whereas NiO appears to inhibit the growth of the preferential planes, which may be attributed to rendering the seed layer ineffective but further study will be required to fully understand the reason.

FIGURE 12. Texture coefficient analysis from XRD patterns of Sb₂Se₃ films deposited via TE (A) and CSS (B) with different hole transport layers in substrate configuration. A diffraction peak with a relatively large TC value ($>$1) indicates a preferred orientation of the grain along this direction.

Figure 13 shows the variation in J-V parameters measured for a minimum batch size of 10 Sb₂Se₃ solar cells in substrate configuration deposited by TE and CSS incorporating HTLs. The use of MoO_x/NiO HTLs adversely affects all device parameters in CSS-based solar cells. This can be explained by lower average R_sh values of 55 Ωcm² and 47 Ωcm² determined for MoO_x and NiO device types, respectively, compared to 172 Ωcm² in the Ref devices. The reason for the reduction in R_sh of the substrate devices with a HTL is not obvious. Only working TE devices were achieved by incorporating a NiO HTL, which can be attributed to the templating effect of the NiO film which eliminated the deleterious (hk0) crystal planes and promoted the growth of preferred (211) and (221) planes. As highlighted in device simulations, the performance of substrate Sb₂Se₃ solar cells can be dependent on the WF of Mo back contact (see Figure 3). Mo metal typically has a WF in the range of 4.5–4.95 eV. KPFM measurements on Mo coated SLG prior to Sb₂Se₃ deposition determined the Mo WF to be 4.6 eV. According to simulations, device performance of Ref and MoO_x substrate devices is severely impacted at the observed Mo WF. Simulated NiO device performance is affected to a lesser degree.

FIGURE 13. J-V parameters of substrate TE and CSS Sb₂Se₃ devices with incorporated MoO_x and NiO HTLs. □ is the average value and × is the minimum and maximum position. The three horizontal lines of each box stand for the 25%, 50% and 75% of the reading distribution. The whisker range is determined by the standard deviation of the sampled devices. IQR is the inter-quartile range.

R_s values for both TE and CSS substrate Sb₂Se₃ devices were significantly higher than their superstrate counterparts and had a detrimental effect on overall substrate device performance. This could be related to a non-optimal sputtered ITO layer in the substrate devices with a typical sheet resistance of ∼ 35 Ω/□ (Qu et al., 2016) compared to commercially available ITO-coated glass slides used in superstrate devices with sheet resistances of 8–12 Ω/□ (Sigma Aldrich).

3.5 Simulated and fabricated device comparison

Experimentally determined device parameters, such as R_s and R_sh and apparent doping density (N_A) of the Sb₂Se₃ absorber, were incorporated into SCAPS simulations of TE/CSS Sb₂Se₃ devices in superstrate/substrate configurations in order to replicate the observed behaviour of the fabricated devices. For an accurate representation of the fabricated cells, the N_A value for the Sb₂Se₃ absorber in the CSS devices was set to a value previously determined for the same CSS deposition process used in this study with a Sb₂Se₃ absorber thickness of 1 μm (Phillips et al., 2019). An experimentally determined N_A value for a typical 500 nm thick TE Sb₂Se₃ absorber was used in TE device simulations (see Table 1 for TE/CSS Sb₂Se₃ film properties). Figure 14 shows device performance of the simulated TE/CSS Sb₂Se₃ devices with experimentally determined R_s, R_sh and N_A values. Similar trends are observed for all device parameters of the simulated and fabricated solar cells in both device configurations indicating the simulated devices are a reasonable representation of actual Sb₂Se₃ solar cells (see Figures 8, 13). However, in superstrate configuration, simulations overestimate all J-V parameters, indicating factors other than R_s, R_sh and N_A are influencing device performance. Material properties such as carrier lifetimes, defects and band tails states have been cited as having a detrimental effect on overall device performance (Chen and Tang, 2020). In that work, a number of bulk defects in Sb₂Se₃ were identified with energy levels within the Sb₂Se₃ bandgap ranging from 0.18–0.94 eV above the valence band maximum. For simulation purposes, a mid-gap donor defect (0.62 eV) was introduced for the Sb₂Se₃ bulk to reproduce realistic device performance (Wen et al., 2018; Ma et al., 2020). Chen and Tang (2020) also highlighted significant recombination occurring at the n-p interface which severely impacts both V_oc and J_sc. The presence of additional Sb₂Se₃ bulk defects and increased absorber/buffer interface defect concentration could account for the differences observed between the simulated and fabricated devices studied here.

TABLE 1. Device simulation parameters, d: layer thickness, E_g: bandgap, χ: electron affinity, ɛ/ɛ₀: dielectric constant, N_C/V: effective density of states C: conduction band (CB) V: valence band (VB), μ_e,h: carrier mobility, N_A/D: apparent doping density D: donor A: acceptor, σ_e,h: capture cross section, N_int: interface defect concentration, E_t: defect energy level relative to CB/VB and N_bulk: bulk defect concentration. Subscripts e and h are electron and hole, respectively.

FIGURE 14. Comparison of J-V parameters of simulated TE and CSS Sb₂Se₃ solar cells with different HTL materials in substrate and superstrate device configurations.

In addition, it is worth noting actual superstrate devices which incorporate a MoO_x HTL under-perform in relation to standard simulated superstrate devices (see Figure 8). This decrease in performance is not observed in the fabricated substrate Sb₂Se₃ solar cells with a MoO_x HTL. This discrepancy can be accounted for by different processing conditions applied during deposition of substrate and superstrate devices. During deposition of Sb₂Se₃ layer on SLG/Mo/HTL substrate, the substrate temperature is maintained at 300°C which is sufficient to crystallise the MoO_x film, see Supplementary Materials S4. The crystallised MoO_x film consists of a mixture of MoO₂, MoO₃ and intermediate reduced oxide phases. The phase composition affects the electronic and optical properties of the MoO_x film, with MoO₂ content lowering the resistivity, transmittance and bandgap (Inzani et al., 2017). Simulations also show a roll-over in the J-V curves for superstrate Sb₂Se₃ devices in both configurations (see Figure 15), indicating the presence of a barrier to carrier transport at the back contact seen in simulated energy band alignments as previously discussed (Figure 4).

FIGURE 15. J-V curves for simulated TE and CSS Sb₂Se₃ devices with HTLs in superstrate configuration. Roll-over behaviour is evident in both TE and CSS devices with MoO_x HTL indicating a carrier transport barrier at the back contact.

4 Conclusion

Numerical simulations of standard planar superstrate and substrate Sb₂Se₃ solar cells along with the effect of incorporating MoO_x and NiO HTLs, demonstrated an increase in device efficiency for cells with a HTL which was achieved by an increase in J_sc for both substrate and superstrate device configurations. Both HTLs have high bandgaps and low electron affinities compared to Sb₂Se₃ absorber which manifests as a large barrier for electrons at the metallic back electrode and facilitates hole extraction. However, a roll-over effect was seen in the simulated J-V curve of the substrate device with MoO_x HTL, suggesting a current-blocking barrier at the back contact caused by non-optimal energy band alignment. Material characterisation of the HTL materials deposited by E-beam evaporation at room temperature revealed MoO_x formed an amorphous layer while NiO crystallised in cubic crystal orientation. 15 nm thick HTLs were incorporated into superstrate/substrate solar cells with Sb₂Se₃ absorbers deposited by thermal evaporation and close-space sublimation. For CSS superstrate solar cells with NiO HTL, device efficiency was enhanced by a 40% increase in J_sc compared to reference and MoO_x based devices. TE superstrate cells incorporating NiO as HTL also demonstrated improved efficiencies achieved by higher V_oc and J_sc. In the superstrate TE cells with MoO_x HTL, J_sc was severely inhibited which is attributed to MoO_x forming a more resistive layer due to its amorphous nature. Conversely, the presence of a MoO_x or NiO HTL in substrate CSS-deposited Sb₂Se₃ solar cells reduced device performance which is linked to lower average R_sh observed in these cells. Optimisation of HTL thickness and/or re-optimisation of the absorber deposition could potentially alleviate this issue. Simulations reveal a connection between the WF of the Mo metal back contact and substrate device performance. For an experimentally determined Mo WF of 4.6 eV, all device J-V characteristics are significantly reduced, whereas substrate devices with NiO HTL are only marginally affected. In addition, XRD analysis of TE Sb₂Se₃ films with NiO HTL revealed a templating effect on Sb₂Se₃ crystal orientation where detrimental (020)/(120) crystal planes were eliminated and preferred (211)/(221) planes increased in intensity which resulted in increased device performance of substrate Sb₂Se₃ solar cells. NiO shows more promise as a HTL in Sb₂Se₃ PV devices, and crucially can act as a templating layer when the Sb₂Se₃ deposition method does not already impart the desired structure, as is often the case with TE devices.

Data availability statement

Data is available via this link: https://figshare.com/projects/Routes_to_Increase_Performance_for_Antimony_Selenide_Solar_Cells_using_Inorganic_Hole_Transport_Layers/140140.

Author contributions

SC, RV, LP, and JM fabricated absorbers and solar cells, SC conducted material and device characterisation, data analysis and device simulations. SC wrote the manuscript with contribution from OH. OH, YQ, VB, NB, and GZ discussed results and contributed to manuscript mofification and finalisation. VB and YQ supervised the project.

Acknowledgments

The authors acknowledge the EPSRC Centre for Doctoral Training in Renewable Energy Northeast Universities (ReNU) for funding through grant EP/S023836/1. The authors also appreciate the support from North East Centre for Energy Materials (NECEM) (EP/R021503/1) and British Council Newton Fund Institutional Links Grant in Ultralight Absorber for Remote Energy Source (U-AREs, No. 623804307). Discussions regarding NiO thin film deposition with Prof. Elizabeth Gibson, School of Natural and Environmental Sciences, Newcastle University is acknowledged.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchem.2022.954588/full#supplementary-material

References