Mapping the surface electronic landscape of solution-processed CuIn(S,Se)₂ thin-films as a function of the Cu/In ratio

Abstract

This study explores the influence of the Cu/In ratio, ranging from 0.80 to 1.10, on the surface composition, electronic landscape, and photovoltaic (PV) performance of solution-processed CuIn(S,Se)₂ (CISSe) thin films. X-ray fluorescence (XRF) confirmed that the Cu/In ratio in the CISSe films closely matches that of the precursor solution, while X-ray diffraction (XRD) indicated a consistent Se/(Se + S) ratio of 0.55 across all samples. In contrast, X-ray photoelectron spectroscopy (XPS) revealed that the surface Cu/In ratio is approximately 30% lower than the bulk value. CISSe devices fabricated in a standard substrate configuration (SLG/Mo/CISSe/CdS/i-ZnO/Al : ZnO) exhibited the highest power conversion efficiency (PCE) of 9.1% at a Cu/In ratio of 0.95. As the Cu content increased, PCE dropped sharply, accompanied by a reduction in band tailing (decrease in bulk disorder). For the first time, energy-filtered photoemission electron microscopy was used to reveal a direct correlation between PV performance and the evolution of the surface electronic landscape. The highest PCE corresponded to CISSe absorbers with a mean work function (WF) of 4.9 eV. Increasing the Cu/In ratio beyond 0.95 led to a significant decrease in mean WF. Notably, at a Cu/In ratio of 1.10, nanometer-sized domains with WF values as low as 3.9 eV emerged. These features are discussed in the context of the complex interplay between bulk and surface disorder and their impact on PV performance.

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Broader context

Solution-processing of inorganic thin-film absorbers offers a scalable and cost-effective route for fabricating photovoltaic devices. This method is particularly well-suited for substrates with diverse functionalities, including flexibility, transparency, and complex surface geometries. These approaches involve casting molecular precursors onto a substrate, followed by reactive annealing to form the semiconductor layer. The structure and optoelectronic properties of the compound semiconductors are sensitive to the composition, coordination chemistry, and rheological behavior of the precursor solution. CuIn(S,Se)₂ has garnered significant attention due to the promising power conversion efficiencies achieved in solution-processed devices. Here, we investigate how the Cu/In ratio in the precursor solution influences the structure, morphology, and photovoltaic performance of CuIn(S,Se)₂ devices. Our findings show that device performance markedly declines when the bulk Cu/In ratio exceeds 1.0, despite no significant changes in film structure or morphology. For the first time, we demonstrate—using energy-filtered photoemission electron microscopy—that this performance drop is linked to nanometer-sized Cu_2−xSe surface islands exhibiting work-function values as low as 3.9 eV, which are formed even when the surface Cu/In ratio is 0.8. Our study highlights the intricate nature of the surface electronic landscape in compound semiconductors and its critical impact on photovoltaic performance.

Introduction

Chalcopyrite Cu(In,Ga)(S,Se)₂ (CIGSSe) and CuIn(S,Se)₂ (CISSe) based thin-film solar cells are some of the most promising thin-film photovoltaics (PVs) due to their high absorption coefficients, tunable and direct bandgap, and long-term stability.¹ A state-of-the-art energy conversion efficiency of 23.64% has been achieved for a CIGSSe solar cell deposited using co-evaporation.² In an effort towards decreasing manufacturing costs, solution-based processing methods have been considered as an alternative to vacuum deposition.^3–5 Solution-processed CISSe solar cells have surpassed 14% power conversion efficiency by conditioning the substrate surface and physical properties of the solvent in the precursor solution,^6,7 which is close to the record efficiency obtained by physical vapor deposition methods (15.0%).⁸ As expected for chalcogenides, the bandgap of CISSe can be adjusted in the range of 1.04–1.53 eV by tuning the S and Se ratio,⁹ and some studies suggest that sulfur-containing CISSe can reduce defect densities, thus improving carrier lifetime and device stability.^10,11

High efficiency solution-processed CISSe devices are typically linked to sub-stoichiometric Cu/In ratio of 0.90–0.95,^12,13 which have been rationalized in terms of the role of intrinsic point defects such as vacancies (V_Cu), interstitials (Cu_i) and antisites (In_Cu).^14,15 Furthermore, in Cu-poor chalcopyrite, ordered vacancy compounds (OVC, i.e. CuIn₃Se₅ or CuIn₅Se₈)^16,17 can be unintentionally formed during selenization. Copper selenide (Cu_2−xSe) secondary phases in Cu-rich compositions are often reported, which have a significant impact on device performance.¹⁸ However, it has also been reported that Cu-rich CISe shows improved performance due to its passivated defects and large grain morphology.¹⁹ Our previous analysis on solution processed CISSe, based on variable temperature admittance and photoluminescence spectroscopy, have shown significant contributions of surface recombination, identifying grain growth as a key parameter to control.²⁰ Consequently, there is a delicate balance of morphological and opto-electronic parameters linked to Cu/In ratio in the film which requires further investigation.

In this study, we uncover the impact of Cu/In ratio on the complex surface electronic landscape of solution processed CISSe. We first investigated the device properties, phase formation, and morphology associated with Cu/In ratios between 0.80 and 1.10. The champion device was measured with a Cu/In ratio of 0.95, exhibiting a power conversion efficiency (PCE) of 9.1% and an open-circuit voltage deficit (V_OC,def) of 431 mV. XPS analysis shows that all films are highly Cu-poor, with Cu/In ratios ranging from 0.42 to 0.62. Sub-micron resolution energy-filtered photoemission electron microscopy (EF-PEEM) shows, for the first time, complex spatial variations of the work functions which are linked to surface chemical disorder. As the bulk Cu/In ratio increases above 1, islands with work functions as low as 3.9 eV are generated across the surface, which are assigned to surface confined copper selenide phases. Our analysis concludes that the complex dependence of device performance with Cu/In ratio is the result of delicate balance between a decrease in bulk disorder and an increase in surface disorder with increasing Cu/In ratio.

Results and discussion

The impact of Cu/In ratio on PV metrics of CISSe solar devices are shown in Fig. 1a–d (see also Table S1 in the SI). As described in the experimental methods section of the SI, CISSe were prepared by sequential spin-coating steps of a precursor solution containing thiourea, CuCl, and InCl₃ in a 75 : 25 dimethylformamide/isopropyl alcohol mixture. As shown in our recent study on solution processed CZTSSe, this solvent mixture exhibits the appropriate rheology for deposition of homogeneous precursor films.²¹ Throughout this study, CISSe absorbers will be referred to by their bulk Cu/In ratio, 0.80, 0.95, 1.00 and 1.10 obtained from X-ray fluorescence (XRF). As shown in Table S2, the XRF data follows closely the Cu/In ratio in the precursor solution. The box plots in Fig. 1a–d show that the maximum PCE is achieved in devices with Cu/In ratio of 0.95, which is primarily determined by the open circuit voltage (V_OC) and, to a lesser extent, fill factor (FF). Cu-rich phases also show a significant drop in the short-circuit current (J_sc).

Fig. 1 (a–d) Statistical distribution of the photovoltaic parameters for devices with varying Cu/In ratios. (e) Illuminated and dark J–V and (f) external quantum efficiency (EQE) characteristics of champion devices with varying Cu/In ratios, along with (g) plots of ln(−ln(1 − EQE)) versus E–E_g extracted from EQE.

Fig. 1e display illuminated and dark J–V curves and EQE of the champion cells for each Cu/In composition, as summarized in Table S3. CISSe solar cells with a Cu/In ratio of 0.95 gave a champion PCE of 9.1%, with V_OC of 533 mV, J_sc of 26.4 mA cm⁻² and FF of 64.7%. The PCE decreased for absorbers with Cu/In ratios of 1.00 and 1.10, to 8.9% and 6.2%, respectively. The EQE spectra (Fig. 1f) show that current losses have contributions from the CdS absorption, reflection losses, and, as shown below, some transmission losses may also occur as the films thickness is below 600 nm. The key observation from this data is the V_OC deficit (V_OC,def), decreasing from 486 mV to 431 mV upon increasing the Cu/In ratio from 0.80 to 0.95, and then increasing to 504 mV for Cu/In-1.10 (Table S3), indicating that Cu/In-0.95 exhibits the lowest charge recombination losses of the series.²² The Urbach energy (E_U) was also extracted from EQE, as displayed in Fig. 1g.²³ The E_U decreases from 21.9 to 16.9 meV with increasing Cu/In ratios, suggesting a reduction in band edge fluctuations and bulk disorder with increasing Cu content.²⁴ Similar observations have also been reported for co-evaporated double-graded CIGSSe absorbers.^25–27 Also, Cu-poor devices exhibit higher saturation current density, suggesting a more significant bulk recombination (Table S3). This interesting observation reveals that the overall device performance is not only determined by the bulk opto-electronic properties of the absorber.

Fig. 2a depicts the X-ray diffraction (XRD) patterns of films fabricated by different composition precursors. Regardless of the Cu/In ratio, the (112), (220) and (312) crystallographic planes located at around 27.1°, 46.5°, and 53.5° support the presence of polycrystalline CISSe phase (JCPDS 65-2732 and JCPDS 87-2265). Fig. S1 examines more closely the range of 20° to 40°, confirming that no diffraction features attributed to Cu₂Se and In₂Se₃ secondary phases are observed in this range. Based on the peak position, the estimated Se/(Se + S) of all CISSe absorber is around 0.55, indicating a high replacement of sulfur by selenium and consistent with the band gap values estimated from EQE. Fig. 2b shows the Raman spectra, where the films exhibited characteristic CISe (175 and 220 cm⁻¹) and CIS (295 and 320 cm⁻¹) peaks. The peak at 159 cm⁻¹ is within resonant range of CuIn₃Se₅ OVC.^17,28 Fig. S2 shows the normalized peak ratio of the OVC-related mode to CISSe A¹ mode decreasing with increasing Cu/In ratio, indicating OVCs are more prevalent with Cu-poor films.^29,30 It should be mentioned that Cu₂Se phases are characterized by broad features Raman bands at 260 and 290 cm⁻¹, while CuSe exhibits well defined strong Raman features at 260, 320 and 470 cm⁻¹.^31–37 None of these Raman modes are observed in the spectra shown in Fig. 2b, regardless of the Cu/In ratio.

Fig. 2 (a) XRD patterns and (b) Raman spectra with 488 nm excitation laser of Cu/In-0.80 (red), Cu/In-0.95 (blue), Cu/In-1.00 (green), Cu/In-1.10 films (purple).

The top-down SEM images of the CISSe thin-films with various Cu/In ratios are shown in Fig. 3a–d, exhibiting densely packed polygonal grains for all samples. Although few smaller grains are observed in Cu/In 0.80 films, no systematic changes in the thin-films morphology can be seen with an increasing Cu/In ratio. Cross-sectional SEM images (Fig. 3e–h) exhibit high compact films with a thickness ranging from 550 to 580 nm. An additional contrast in the cross-section analysis can be seen in the boundary between the absorber and the Mo support, which is associated with the Mo(S,Se)_x formed during selenization step. It is important to mention that the addition of SeS₂ to the graphite box regulates the excessive selenization of the Mo layer observed in our previous work, at the expense of S content in the film, which increases the band gap.²⁰

Fig. 3 Top-view SEM images of Cu/In-0.80 (a), Cu/In-0.95 (b), Cu/In-1.00 (c), Cu/In-1.10 absorbers (d). Cross-section SEM images of Cu/In-0.80 (e) Cu/In-0.95 (f), Cu/In-1.00 (g), Cu/In-1.10 absorbers (h).

Fig. 4a and b shows the X-ray photoelectron spectroscopy (XPS) of Cu 2p and In 3d as a function of the bulk Cu/In ratio. The Cu and In peak position, broadening and orbital splitting are similar for all compositions, corresponding to Cu⁺ and In³⁺, respectively.³⁸ Fig. S3a and b show the Se 3d and S 2p/Se 3p XPS spectra, which are comparable for all samples. Interestingly, a low intensity Na 1s peak is observed arising from the diffusion of Na from the SLG during rapid thermal annealing (Fig. S3c).³⁹ As described in Experimental methods (SI), the films were pre-treated using Ar⁺ to remove surface contaminants. Fig. S4 shows the XPS spectra of Se 3d for sample Cu/In-0.80 before and after surface pretreatment, confirming that adventitious oxygen residuals and contaminates are successfully removed. Fig. 4a and b also shows a correlation between the intensity of Cu 2p, In 3d and the bulk Cu/In precursor ratio.³⁹ As displayed in Fig. 4c, the surface Cu/In ratio extracted from the XPS spectra show a significant Cu depletion, with values approximately 30% lower than in the bulk. As shown in Fig. S5, Cu 2p and In 3d spectrum of the Cu/In-1.00 absorber are used to demonstrate the data analysis procedure. Cu surface depletion has been extensively reported in the presence of alkali elements, which can generate surface confined alkali-In–Se.^25,40–47 Alkali-In–Se secondary phases have been linked to a decrease in interfacial recombination rate, thus increasing V_OC and FF.⁴⁸ Fig. S6 illustrates the elemental distribution across the Cu/In-0.95. The data confirms the presence of Na across the film, as well as relatively homogeneous distributions of Cu and In in a ratio consistent with the XRF analysis (Table S2). These observations confirm the diffusion of Na from the substrate upon annealing.

Fig. 4 XPS binding energy profiles of (a) Cu 2p and (b) In 3d for CISSe absorbers of Cu/In-0.80 (red), Cu/In-0.95 (blue), Cu/In-1.00 (green), and Cu/In-1.10 (purple) ratios. (c) Cu/In surface ratio as a function of Cu/In bulk ratio. Error bars in (c) represent statistical errors associated with determining the peak areas.

Fig. 5 shows sub-micron resolution effective work function (WF) maps constructed from energy-filtered photoemission electron microscopy (EF-PEEM) analysis of CISSe absorbers with the various Cu/In ratio. EF-PEEM maps were recorded under monochromatic He I excitation (21.22 eV) with a spatial resolution of 100 nm. The 20 μm × 20 μm field map provides a wealth of information in relation to the surface electronic landscape of CISSe. As shown in Fig. S7 and Table S4, fitting the WF spatial distribution to a Gaussian function enables us to quantify the center of the distribution and the standard deviation which can be linked to spatial inhomogeneities in the surface electronic landscape. Cu/In-0.80 shows a narrow spatial distribution of WF centered at 4.85 eV, which increases to 4.90 eV for Cu/In-0.95. As the Cu content further increases, the center of the WF distribution sharply drops to 4.55 eV. It is interesting to notice that the non-monotonic trend of WF center with Cu/In ratio mirrors the trend of the device V_OC, as discussed further below. The standard deviation also exhibits a complex dependence with the Cu–In ratio, varying between 54 to 102 meV. However, the most striking aspect in this analysis is the long tail towards low WF values observed in the case of Cu/In-1.10. The number of WF counts below 4.3 eV is several orders of magnitude smaller than the WF center, indicating that these are localized nanometer scale domains. The emergence of these domains coincides with the drastic drop in device performance. A variety of parameters can affect the spatial distribution of WF, including topography, local surface potentials and surface confined phases contributions.^21,49–52 However, given that the topographic features of the films are rather similar across the composition range, the low WF hot-spots can be linked to surface confined Cu_2−xSe phases which are characterized by WF values in the range of 3 eV.^53–55 Although it could be expected that the formation of these secondary phases would take place at higher Cu content, it is important to consider that the film surfaces are substantially Cu depleted in comparison to the bulk. As concluded from the Raman analysis (Fig. 2b), there is no evidence of binary Cu chalcogenide phases observed in the bulk. These features are extraordinarily difficult to probe by other techniques just by the virtue of being confined to discreet sub-micron islands at the surface of the absorber.

Fig. 5 3D photoemission WF maps of the Cu/In-0.80 (a), Cu/In-0.95 (b), Cu/In-1.00 (c), Cu/In-1.10 (d) CISSe films.

Fig. 6a contrasts the ultraviolet photoelectron spectra (UPS) of the CISSe films as a function of the Cu/In ratio. Cu/In-0.80 and Cu/In-0.95 exhibits broad features in the valence band (VB) spectra centered at 4.5 and 7.5 eV, which are in good agreement with spectra reported of Cu-poor CISSe.^56–58 These features clearly sharpen in Cu/In-1.00 films, which is consistent with VB spectra reported in CuInSe₂.⁵⁹ The lineshape of the VB spectrum changes significantly for Cu/In-1.10, extending beyond 10 eV in the binding energy scale. These features clearly sharpen in Cu/In-1.00 films, which is consistent with VB spectra reported in CuInSe₂.⁵⁹ The lineshape of the VB spectrum changes significantly for Cu/In-1.10, extending beyond 10 eV in the binding energy scale. This spectral responses exhibit similar features to those reported for Cu₂Se.^60,61 Furthermore, a tail in the onset of electron photoemission is observed in the Cu/In-1.10 spectrum, consistent with tail in WF values. These observations provide further evidence that the regions of low WF observed in the EF-PEEM maps are associated with surface confined sub-micrometer Cu_xSe phases.

Fig. 6 (a) Normalized UPS spectra of the CISSe absorbers of Cu/In-0.80 (red), Cu/In-0.95 (blue), Cu/In-1.00 (green), Cu/In-1.10 ratios (purple). (b) Summarized Urbach Energy, WF center and (c) V_OC,def as a function of Cu/In ratios.

As displayed in Fig. 6b and c, the combination of EF-PEEM data and PV device metrics reveal some important aspects in relation to the composition dependence of CISSe thin films. As Cu/In increases, we observed a close correlation between E_U and WF center, revealing a complex relation between bulk disorder, ordered vacancy compounds (OVC) and the Fermi level. Cu-poor chalcopyrite contains OVCs as well as In_Cu antisites,^19,62 while increasing the Cu content leads to more ordered structures with larger grains.¹⁸ On the other hand, Fig. 6c shows that the V_OC,def sharply increases as the bulk Cu/In ratio increases above 0.95, which is also consistent with previous studies on high efficiency CISSe.⁶³ EF-PEEM reveals a broadening of the spatial distribution of WF values, with evidence of the formation of surface confined binary Cu chalcogenides phases as the surface Cu/In ratio increases just above 80%.⁶³ EF-PEEM reveals a broadening of the spatial distribution of WF values, with evidence of the formation of surface confined binary Cu chalcogenides phases as the surface Cu/In ratio increases just above 80%. This behavior could be the manifestation of the transport rates of Cu⁺vs. Na⁺ under our specific annealing conditions. Consequently, we anticipate that introducing alkali ions in the precursor solution, sodium content in the substrate and annealing temperature can significantly affect the emergence of these low WF surface sites.

Conclusions

This work reveals a complex relationship between bulk and surface disorder in solution-processed CISSe thin films and its impact on optoelectronic properties and photovoltaic performance. The highest PCE was achieved at a Cu/In ratio of 0.95, which also exhibited the highest V_OC in the series. As the Cu/In ratio increased, both V_OC and J_sc declined significantly, despite no observable changes in bulk structural parameters. Interestingly, band tailing, estimated from EQE spectra, systematically decreased with increasing Cu/In ratio.

Bulk composition analysis confirmed a strong 1 : 1 correlation between the Cu/In ratio in the precursor solution and the annealed CISSe films. However, XPS revealed a substantial depletion of Cu at the surface, with surface Cu/In ratios approximately 30% lower than in the bulk. EF-PEEM analysis showed that the mean surface WF peaked at 4.90 eV for a Cu/In ratio of 0.95, then dropped to 4.55 eV at higher Cu content. Most notably, nanoscale-sized domains with WF values as low as 3.9 eV appeared at a Cu/In ratio of 1.10. These domains are attributed to surface-confined Cu_2−xSe islands, which form even when the surface remains Cu-poor. Although these highly confined islands are undetectable by conventional techniques, such as Raman spectroscopy, they exert considerable influence on PV device performance.

Author contributions

Y. Ma wrote the manuscript, fabricated and characterized the CISSe absorbers, data curation and analysis. A. Sheppard and V. Corsetti provided scientific discussions and aided data interpretation, reviewed and edited the manuscript. J. Kenyon, N. Benhaddou and J. W. Bowers contributed to CISSe device fabrication, and JV and EQE measurements. J. Laverock aided in the acquisition and analysis of XPS and EF-PEEM measurements. D. J. Fermin contributed to the manuscript preparation, data analysis and supervised the research project. All authors were involved in the discussions and approved of the manuscript.

Conflicts of interest

The authors declare that there are no conflicts of interest.

Data availability

Data are available at the University of Bristol data repository, data.bris, at https://doi.org/10.5523/bris.3mho5r38ai5k02ro43vzhat07y.

Supplementary information: Experimental methods; Table S1 – average device metrics as a function of the bulk Cu/In ratio; Table S2 – Cu/In ratio in the precursor solution vs. thin-film measured by XRF; Table S3 – best device metrics as a function of the bulk Cu/In ratio; Table S4 – surface Cu/In ratio estimated from XPS and WF center obtained from EF-PEEM; Fig. S1 – XRD patterns in the range of 20 to 40°; Fig. S2 – normalized peak area of OVCs mode to CISe A¹ mode of each Cu/In ratio; Fig. S3 – XPS spectra of (a) Se 3d, (b) S 2p and Se 3p, and (c) Na 1s of the CISSe absorbers; Fig. S4 – XPS spectra of Se 3d for Cu/In-0.80 absorber before and after surface pretreatment; Fig. S5 – extended (raw) spectrum, background based on energy loss theory, and background-subtracted spectrum used for quantification of Cu 2p and In 3d of absorber Cu/In-1.00; Fig. 6 – secondary ion mass spectrometry (SIMS) profile of Cu/In-0.95 absorber; Fig. S7 – the WF histograms of the CISSe absorber as a function of the bulk Cu/In ratio; Raw data are also available at the University of Bristol data repository, data.bris, at https://doi.org/10.5523/bris.3mho5r38ai5k02ro43vzhat07y. See DOI: https://doi.org/10.1039/d5el00116a.

Acknowledgements

Y. Ma is grateful for China Scholarship Council (File No. 202208610015). A. Sheppard acknowledges the financial support of the Engineering and Physical Sciences Research Council (EPSRC, DTA grant EP/T517872/1). V. Corsetti is grateful to the University of Bristol for the Strategic PhD Scholarship 2023. D. J. Fermin acknowledges the support from EPSRC through the SolPV grant EP/V008676/1. N. Benhaddou and J. W. Bowers are grateful through the support via EP/V008692/1. The authors acknowledge use of the University of Bristol NanoESCA II Laboratory.

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