Chalcopyrite Thin Film Solar Cells Research Articles

Nitrogen doped single layer graphene for CZTS-based thin film solar cells

CdS thin films are commonly utilized as a buffer layer in chalcopyrite thin-film solar cells. However, due to the toxic nature of Cadmium (Cd), ongoing efforts are being directed towards exploring alternative options. In this contribution, doped graphene film with remarkable optical and electrical properties has been introduced for the first time as an alternative buffer layer, replacing CdS in CZTS thin-film solar cell application. In this study, nitrogen-doped graphene (N-doped graphene) film was utilized as a substitute buffer layer in the CZTS thin-film solar cell structure, replacing the conventional CdS thin film. For comparative analysis, CZTS/N-doped graphene and CZTS/CdS traditional solar cell structures were fabricated and separately characterized. The CZTS thin films produced were examined through EDX, XRD, SEM, Raman, optical transmission and PL spectroscopy measurements. According to performed analyses, the Cu-poor and Zn-rich kesterite CZTS thin films exhibited a uniform and dense polycrystalline microstructure as observed in surface and cross-sectional SEM images. XRD spectra of the kesterite CZTS thin film displayed predominant peaks corresponding to the (112), (220/204), and (312/116) diffraction planes of the kesterite CZTS phase. Raman spectra showed a dominant peak at ∼336 cm−1 associated with the kesterite CZTS phase. PL emission spectra indicated transitions from the conduction band to defect levels. The CVD-grown doped graphene film exhibited a 3.43 I2D/IG ratio and a 25 cm−1 FWHM of the 2D peak, indicating a single-layer graphene according to Raman analysis. Permanent nitrogen doping with a 2% atomic concentration was confirmed by XPS measurement. The optical transmission measurement of the single layer doped graphene film showed a 95% transmittance value. Nitrogen doping was contributed to decrease the sheet resistance of the graphene film. The Glass/Mo/CZTS/N-doped graphene/i-ZnO/ITO/Al solar cell displayed a VOC of 0.267 V, JSC of 24.6 mA/cm2, FF of 36.46, and η of 2.37%, showing higher FF and Jsc values but lower conversion efficiency compared to the Glass/Mo/CZTS/CdS/i-ZnO/ITO/Al traditional solar cell structure. Hence, the superior working function and transparency properties position the N-doped graphene film as a competitive buffer layer for use in CZTS-based thin-film solar cells.

Effect of Gallium Content on Diode Characteristics and Solar Cell Parameters of Cu(In1-xGax)(Se0.98Te0.02)2 Thin Film Solar Cells Produced by Three-stage Co-evaporation at Low Temperature

In this study, Cu(In1-xGax)(Se0.98Te0.02)2 thin film solar cells with x values of 0.17, 0.20, 0.23, and 0.26 were successfully produced by three-stage co-evaporation technique at low temperatures. The diode characteristics and solar cell parameters of thin film chalcopyrite solar cells with the structure of SLG/Mo/Cu(In1-xGax)(Se0.98Te0.02)2/CdS/ZnO/ITO/Ni-Al-Ni were investigated by current-voltage measurements. The ideality factor, series resistance, and barrier height were obtained by the Cheung-Cheung method using the current-voltage results measured in the dark at room temperature. Open-circuit voltage, short-circuit current density, fill factor, and the power conversion efficiency of the thin film solar cells were derived from the current-voltage measurements realized by a four-point measurement setup under AM1.5G standards at room temperature. It was found that the increase in the gallium content first decreased the ideality factor, however, it increased again after exceeding the x value of 0.23. While the amount of gallium was increasing, fluctuations were observed in the series resistance values. The barrier height first increased with the increasing amount of gallium and decreased after exceeding the x value of 0.23. The solar cell parameters increased by increasing the x value up to 0.23 and decreased after exceeding this point. It was found that the diode parameters have an effect on each other but the most effective diode parameter was the ideality factor. The efficiency of the Cu(In1-xGax)(Se0.98Te0.02)2 thin film solar cells was increased from 3.7% to 6.3% by increasing the x value from 0.17 to 0.23.

Back Contact Plasma Treatment Enables 14.5% Efficient Solution‐Processed CuIn(S,Se)2 Solar Cells

AbstractSolution‐processed chalcopyrite thin film solar cells have made great progress but still lag behind the vacuum‐based ones due to inferior absorber quality and non‐ideal interfaces. Here, plasma treatment of the molybdenum (Mo) substrate is reported to modify the back interface and improve the efficiency of solution‐processed copper indium sulfoselenide (CuIn(S,Se)2, CISSe) solar cells. The results show plasma treatment oxides the surface Mo/MoO2 to high oxidation oxides MoO3‐x (0 ≤ x < 1). The higher surface polarity greatly improves the wettability of the substrate to the precursor solution and results in formation of more compact and uniform precursor film, which grows to void‐free larger grains absorber film with better adhesion to the substrate. In addition, the much higher chemical stability of MoO3‐x than Mo significantly reduces the thickness of high resistive MoSe2 layer. The improved absorber quality and back interface property decreases charge carrier recombination in the absorber and back contact interface, resulting in 14.53% efficiency solution‐processed CISSe solar cell, which is improved by 13% compared to device without plasma treatment.

Impact of tellurium as an anion dopant on the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 thin-film solar cells with rubidium fluoride post-deposition treatment

The development of wide-bandgap Cu(In,Ga)Se2 thin films is crucial in order to reach the theoretical Shockley–Queisser limit values in single-crystal solar cells. However, the performance of solar cells based on wide-bandgap thin film absorbers has lagged significantly compared to that of their narrow-bandgap counterparts. Herein, we develop a feasible strategy to improve the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 chalcopyrite thin-film solar cells by simultaneously doping with both RbF PDT and Te2− anions as dopants in the absorber layer during the three-stage co-evaporation process. Besides inducing significant change in the GGI gradient, the synergistic effect of the Te2− anion dopant is rather beneficial in terms of controlling grain size, defects in grain boundaries, and charge carrier lifetime for encouraging charge separation and extraction, which contributes to simultaneously boosting short-circuit current density and fill factor. Te-poor devices afford an impressive efficiency of 9.58%, compared to 6.43% for control devices. More importantly, the efficiency and Voc values obtained for wide-bandgap-based thin-film solar cells containing Te anions were the highest compared to their counterparts as reported in the literature. These results demonstrate the role of Te2− anions in wide-bandgap absorber thin films on the photovoltaic performance of thin-film solar cells and the potential of this approach for use in reasonable and effective design of highly efficient wide-bandgap thin-film solar cells.

Hierarchically structured sub-bands in chalcopyrite thin-film solar cell devices

The study utilizes the inherent physiochemical properties of vanadium-incorporated copper gallium sulfide telluride (CuGa1−xVx(S,Te)2) thin films deposited via a chemical spray pyrolysis route and evokes how the art of intermediate band structuring favours the solar cell efficiency.

Surface Passivation and Detrimental Heat-Induced Diffusion Effects in RbF-Treated Cu(In,Ga)Se2 Solar Cell Absorbers.

Alkali postdeposition treatments of Cu(In,Ga)Se2 absorbers with KF, RbF, and CsF have led to remarkable efficiency improvements for chalcopyrite thin film solar cells. However, the effect of such treatments on the electronic properties and defect physics of the chalcopyrite absorber surfaces are not yet fully understood. In this work, we use scanning tunneling spectroscopy and X-ray photoelectron spectroscopy to compare the surface defect electronic properties and chemical composition of RbF-treated and nontreated absorbers. We find that the RbF treatment is effective in passivating electronic defect levels at the surface by preventing surface oxidation. Our X-ray photoelectron spectroscopy (XPS) data points to the presence of chemisorbed Rb on the surface with a bonding configuration similar to that of a RbInSe2 bulk compound. Yet, a quantitative analysis indicates Rb coverage in the submonolayer regime, which is likely causing the surface passivation. Furthermore, ab initio calculations confirm that RbF-treated surfaces are less prone to oxidation (in the form of Ga, In, and Se oxides) than bare chalcopyrite surfaces. In addition, elemental diffusion of Rb along with Na, Cu, and Ga is found to occur when the samples are annealed under ultrahigh vacuum conditions. Magnetic sector secondary ion mass spectrometry measurements indicate that there is a homogeneous spatial distribution of Rb on the surface both before and after annealing, albeit with an increased concentration at the surface after heat treatment. Depth-resolved magnetic sector secondary ion mass spectrometry measurements show that Rb diffusion within the bulk occurs predominantly along grain boundaries. Scanning tunneling and XPS measurements after subsequent annealing steps demonstrate that the Rb accumulation at the surface leads to the formation of metallic Rb phases, involving a significant increase of electronic defect levels and/or surface dipole formation. These results strongly suggest a deterioration of the absorber-window interface because of increased recombination losses after the heat-induced diffusion of Rb toward the interface.

Mechanistic study of the gas-phase chemistry during the spray deposition of Zn(O,S) films by mass spectrometry

The mass spectrometer, is a powerful tool to identify species and investigate reactions in the gas phase. In this work, the mechanism of aerosol assisted chemical vapor deposition (AACVD) of Zn(O,S) films prepared from H2S and zinc acetylacetonate (Zn(acac)2) precursor solutions is elucidated by mass spectrometry. The thermochemical behavior of Zn(acac)2 is investigated by characterizing the influence of the solvent (H2O or ethanol), the pH value of the precursor solution and the effect of the reactant H2S, and by tracking gaseous intermediate products using mass spectrometry. Based on these results, a proton-promoted thermolysis mechanism for the AACVD Zn(O,S) film formation is then proposed, which is initiated by the hydrolysis with H2O as the first stage, followed either by the rearrangement with an intramolecular proton or by the reaction with an extramolecular proton to produce ZnO or Zn(O,S). A real time mass tracking of the AACVD process reveals that only an adequate amount of H2S promotes the chemical gas-phase decomposition and sulfurization process, while an excess of H2S depletes the gaseous Zn(acac)2 concentration and consequently reduces the film growth rate. The knowledge of the thermal decomposition process helps to optimize synthesis conditions and to adjust film properties to meet the requirement of the application in chalcopyrite or kesterite thin film solar cells.

High Efficiency Solution-Processed Aluminum-Alloyed Chalcopyrite Thin Film Solar Cells

CuInGaSe2 (CIGSe) chalcopyrite is one most efficient materials for light conversion to electricity with a record efficiency surpassing all other thin film photovoltaic technologies, achieving 23.4% efficiency. Record efficiencies of CIGSe are typically achieved using 1.16 eV absorbers even though Shockley-Queisser (SQL) limit predicts an optimum bandgap of around 1.4 eV for a single junction solar cell. In theory, higher efficiencies can be expected from wider gap chalcopyrite, however, all attempts to increase efficiency with wide bandgap Ga-rich CIGSe have led to defective materials. Another alternative to increase the band gap of chalcopyrites is by partial replacement of In with other group-V atoms, such as aluminum (Al). The band gap energy of CuInAlSe2 (CIASe) can be tuned from 1.0 eV (CISe) to 2.7 eV (CASe). In addition, Al is also very appealing to reduce the raw materials' cost since it is much more abundant in the earth’s crust than Ga (Al: 82,000 ppm, Ga: 19 ppm) and significantly cheaper (Al: ~$2/kg, Ga: ~$200/kg).In an attempt to fabricate high-efficiency CIASe devices, we demonstrated an alternative and low cost method by fabricating bandgap tunable Cu(In,Al)(S,Se)2 (CIASSe) utilizing molecular ink consisted of Cu, In, Al, and sulfur containing salts dissolved in methanol. In this study, Cu-In-Al-S containing ink was spin coated into a molybdenum substrate and followed by selenization at high temperature to form selenium-rich CIASSe. We are able to achieve a device with maximum efficiency of 11.6% using CIASSe having relatively low-content of Al (Al/(Al+In)~0.1), significantly higher efficiency than CISSe (no Al) fabricated with the same process (8.3%-efficient). To our understanding, this study is the first demonstration of a relatively high-efficiency Aluminum-alloyed chalcopyrite solar cells through solution-based process. Details of the fabrication process and materials characterization will be discussed.

Correlating facet orientation, defect-level density and dipole layer formation at the surface of polycrystalline CuInSe2 thin films

Individual grains of chalcopyrite solar cell absorbers can facet in different crystallographic directions at their surfaces. To gain a deeper understanding of the junction formation in these devices, we correlate variations in the surface facet orientation with the defect electronic properties. We use a combined analytical approach based on scanning tunneling spectroscopy (STS), scanning electron microscopy, and electron back scatter diffraction (EBSD), where we perform these experiments on identical surface areas as small as 2 × 2 µm2 with a lateral resolution well below 50 nm. The topography of the absorber surfaces indicates two main morphological features: micro-faceted, long basalt-like columns and their short nano-faceted terminations. Our STS results reveal that the long columns exhibit spectral signatures typical for the presence of pronounced oxidation-induced surface dipoles in conjunction with an increased density of electronic defect levels. In contrast, the nano-faceted terminations of the basalt-like columns are largely passivated in terms of electronic defect levels within the band gap region. Corresponding crystallographic data based on EBSD experiments show that the surface of the basalt-like columns can be assigned to intrinsically polar facet orientations, while the passivated terminations are assigned to non-polar planes. Ab-initio calculations suggest that the polar surfaces are more prone to oxidation and resulting O-induced defects, in comparison to non-polar planes. Our results emphasize the correlation between morphology, surface facet orientations and surface electronic properties. Furthermore, this work aids in gaining a fundamental understanding of oxidation induced lateral inhomogeneities in view of the p-n junction formation in chalcopyrite thin-film solar cells.

ALD-ZnMgO and absorber surface modifications to substitute CdS buffer layers in co-evaporated CIGSe solar cells

High efficiency chalcopyrite thin film solar cells generally use chemical bath deposited CdS as buffer layer. The transition to Cd-free buffer layers, ideally by dry deposition methods is required to decrease Cd waste, enable all vacuum processing and circumvent optical parasitic absorption losses. In this study, Zn1−xMgxO thin films were deposited by atomic layer deposition (ALD) as buffer layers on co-evaporated Cu(In,Ga)Se2(CIGS) absorbers. A specific composition range was identified for a suitable conduction band alignment with the absorber surface. We elucidate the critical role of the CIGS surface preparation prior to the dry ALD process. Wet chemical surface treatments with potassium cyanide, ammonium hydroxide and thiourea prior to buffer layer deposition improved the device performances. Additional in-situ surface reducing treatments conducted immediately prior to Zn1−xMgxO deposition improved device performance and reproducibility. Devices were characterised by (temperature dependant) current-voltage and quantum efficiency measurements with and without light soaking treatment. The highest efficiency was measured to be 18%.

Selenization of CuInS2 by rapid thermal processing – an alternative approach to induce a band gap grading in chalcopyrite thin-film solar cell absorbers?

Characterization of selenium-treated CuInS2 reveals a band gap grading and a surface Cu enrichment, opening new chalcopyrite absorber tailoring opportunities.

β-In2S3 for photovoltaic devices: investigation of the native point defects with ab initio first-principle calculations

Herein, we report a theoretical investigation based on DFT calculations devoted to the nature of charge carriers in the β-In2S3 material used as buffer in chalcopyrite thin-film solar cells. Our simulations led to unambiguous results concerning the incapability for this material to be a p-type semiconductor. Furthermore, it is demonstrated that the insertion of indium into a Td interstitial site seems to be the driving force leading to a natural n-type conductivity. Our calculations took into account different atmospheres in order to be directly comparable with experimental data. In particular, the competitive InS structure was considered. It is shown that the S-poor condition strongly lesser the defect formation energy and should be privileged to achieve high free electron concentrations for potential applications.

Restraining the Band Fluctuation of CBD‐Zn(O,S) Layer: Modifying the Hetero‐Junction Interface for High Performance Cu2ZnSnSe4 Solar Cells With Cd‐Free Buffer Layer

Zn(O,S) film is a promising low‐cost and environment‐friendly Cd‐free buffer layer for chalcopyrite and kesterite thin film solar cells. However, the devices with Zn(O,S) buffer layer usually suffer from poor interface performance, resulting in a much lower efficiency, especially for kesterite solar cells. Here, the band fluctuation caused by ZnO secondary phase in Zn(O,S) layer is identified as the main reason deteriorating the device performance. By a concentrated ammonium etching and subsequent soft annealing treatment, the detrimental ZnO and Zn(OH)2 secondary phases are eliminated from the Zn(O,S) layer and the hetero‐junction performance is improved significantly. Consequently, the power conversion efficiency of the Zn(O,S)/CZTSe solar cells was improved from 1.17% to a favorable value of 7.2%. Temperature dependent J–V properties reveal a defect level assisted charge carrier transport mechanism across the Zn(O,S)/CZTSe interface. These encouraging results imply that Zn(O,S) buffer layer is a promising substitution for toxic CdS in future manufacturing of high performance thin film solar cells.

Theoretical band alignment in an intermediate band chalcopyrite based material

Band alignment is key to enhance the performance of heterojunction for chalcopyrite thin film solar cells. In this paper we report ab initio calculations of the electronic structures of CuGaS2:Cr with various Cr compositions, CuAlSe2 and ZnSe and the band alignment between their interfaces. We use density functional theory and the more accurate self-consistent GW scheme to obtain improved bulk band-gaps and band offsets. Band alignments of the interfacial region for CuGaS2:Cr/CuAlSe2 and CuGaS2:Cr/ZnSe systems were aligned with respect of an average electrostatic potential. Our results are in good agreement with experimental values for the bulk band-gaps. These theoretical band alignments show a characteristic staggered band alignment for the design of heterojunction devices in photovoltaic applications.

Sudden stress relaxation in compound semiconductor thin films triggered by secondary phase segregation

In polycrystalline compound semiconductor thin films, structural defects such as grain boundaries as well as lateral stress can form during film growth, which may deteriorate their electronic performance and mechanical stability. In Cu-based chalcogenide semiconductors such as $\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{Se}}_{2}$ or ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$, temporary Cu excess during film growth leads to improved microstructure such as a reduced grain boundary density, a strategy that has been used for decades for high-efficiency chalcopyrite thin film solar cells. However, the mechanisms responsible for the beneficial effect of Cu excess are yet not fully clarified. Here, we investigate the evolution of lateral stress, grain growth, and Cu-Se segregation during Cu-Se deposition onto Cu-poor ${\mathrm{CuInSe}}_{2}$. Real-time x-ray diffraction and fluorescence analysis with a double-detector setup reveals that sudden stress relaxation occurs shortly prior to Cu-Se segregation at the surface and precisely coincides with domain growth and change of texture. Numerical reaction-diffusion modeling provides an explanation for the observed delay of Cu-Se segregation. Our results show that partial recrystallization of the film can be already reached without the necessity of an overall Cu-rich film composition and thus suggest a new synthesis route for the fabrication of high-quality chalcopyrite absorber films.

Beta Irradiation Effect on Cu(In, Ga)Se<sub>2</sub> Thin-Films

One of the most promising absorber materials for thin-film solar modules are polycrystalline chalcopyrite thin-film solar cells based on Cu (In, Ga)Se2 (CIGS). By having the direct band gap and high absorption coefficient, CIGS compounds have high solar to electricity conversion efficiency, reliability, and stability. The changes of physical properties of sol-gel derived CIGS thin-films were investigated after the beta irradiation. The effect of ionization radiation on the optical materials is promising in the radiation science and air and space science. The ionized radiation causes changes of physical and chemical properties by exciting the free carriers and forming electron-hole pairs. The irradiaton effect on the CIGS thin-films is evaluated by determining the optical band gap of the films exposed to the beta radiation source by using Sr-90 radioisotope. The variations in structural and optical properties were considered with respect to the absorbed dose level to investigate the characteristic properties of CIGS thin-films.

Correlating the Local Defect-Level Density with the Macroscopic Composition and Energetics of Chalcopyrite Thin-Film Surfaces.

The unusual defect chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin films is a main issue for a profound understanding of recombination losses in chalcopyrite thin-film solar cells. Especially, impurity-driven passivation of electronic levels due to point defects segregating at the surface and at grain boundaries is extensively debated. By combining current imaging tunneling spectroscopy with photoelectron spectroscopy, the local defect-level density and unusual optoelectronic grain-boundary properties of this material are correlated with the macroscopic energy levels and surface composition. Vacuum annealing of different CIGSe materials provides evidence that Na diffusion from the glass substrate does not affect the surface defect passivation or grain-boundary properties of standard Cu-poor materials. Furthermore, we find no major impact on the observed thermally activated dipole compensation or the accompanying change in surface band bending (up to 0.6 eV) due to Na. In contrast, Cu-rich CIGSe shows an opposing surface defect chemistry with only minor heat-induced band bending. Our results lead to a comprehensive picture, where the highly desirable type inversion at the p/n interface in standard chalcopyrite thin-film solar cells is dominated by band bending within the CIGSe absorber rather than the result of Na impurities or an n-type defect phase segregating at the interface. This is in accordance with recent studies suggesting a surface reconstruction as the origin for Cu depletion and band-gap widening at the surface of chalcopyrite thin films.

Study of Zn(O,S) films grown by aerosol assisted chemical vapour deposition and their application as buffer layers in Cu(In,Ga)(S,Se)2 solar cells

Manufacturing chalcopyrite thin film solar cells from cost-efficient and environmentally friendly materials is of high priority. Therefore, the materials used in buffer layer production nowadays (such as indium and cadmium) need to be substituted. Zn(O,S) is considered to be a potential buffer layer material when deposited with a fast and inexpensive method. Zn(O,S) layers have been prepared by aerosol assisted chemical vapour deposition (AACVD) technique. AACVD technique is a simple non-vacuum process where the thin film deposition temperatures do not exceed 250°C. 10mM spray solution was made by dissolving zinc(II)acetylacetonate monohydrate in ethanol. The films were grown on Mo substrate at 225°C (film growth temperature). The effect of deposition parameters (spray solution concentration, N2 flow rate, H2S flow rate) on Zn(O,S) thin film properties were studied with SEM and XRD. Thereupon optimising the deposition parameters, homogeneous and compact Zn(O,S) thin films were obtained and the films were employed in the chalcopyrite thin film solar cell structure by growing films on Cu(In,Ga)(S,Se)2 substrates industrially produced by BOSCH Solar CISTech GmbH. The resulting cells were studied using current–voltage and quantum efficiency analysis and compared with solar cell references that include In2S3 and CdS as buffer layer deposited by ion layer gas reaction and chemical bath deposition, respectively. The best output of the solar cell containing Zn(O,S) as buffer layer and without intrinsic ZnO under standard test conditions (AM 1.5G, 100mW/cm2, 25°C) is: Voc=573mV, Jsc=39.2mA/cm2, FF=68.4% and efficiency of 15.4% being slightly better than the In2S3 or CdS containing solar cell references.

Interfaces and Extended Structural Defects in Chalcopyrite Thin-Film Solar Cells Studied by Transmission Electron Microscopy

Electronic defects at interfaces between different materials or at extended structural defects such as grain boundaries (GB) or dislocations can deteriorate the performance of a semiconductor device considerably. Cu(In,Ga)Se2 (CIGS) thin-film solar cells consist of several interfaces between individual layers of different materials. In addition, the main light-absorbing layer, the CIGS absorber, exhibits a high density of GBs (i.e., the average grain size is typically smaller than the layer thickness) as well as of dislocations (up to 10 10 cm -2 in high-efficiency solar cells [1]). Still, CIGS solar cells exhibit highest power-conversion efficiencies of up to 20.8 % [2]. Today, the roles of interfaces and extended structural defects in the solar-cell device are not yet fully understood, although extensive research efforts have been made. We applied various methods in transmission electron microscopy (TEM), such as inline electron holography, electron energy-loss spectroscopy (EELS), as well as energy-dispersive X-ray spectroscopy (EDS) in order to obtain more information about these features.

Interface engineering and characterization at the atomic-scale of pure and mixed ion layer gas reaction buffer layers in chalcopyrite thin-film solar cells

In this work, we investigate the p–n junction region for two different buffer/Cu(In,Ga)(Se,S)2 (CIGSSe) samples having different conversion efficiencies (the cell with pure In2S3 buffer shows a lower efficiency than the nano-ZnS/In2S3 buffered one). To explain the better efficiency of the sample with nano-ZnS/In2S3 buffer layer, combined transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies were performed. In the pure In2S3 buffered sample, a CuIn3Se5 ordered-defect compound is observed at the CIGSSe surface, whereas in the nano-ZnS/In2S3 buffered sample no such compound is detected. The absence of an ordered-defect compound in the latter sample is explained either by the presence of the ZnS nanodots, which may act as a barrier layer against Cu diffusion in CIGSSe hindering the formation of CuIn3Se5, or by the presence of Zn at the CIGSSe surface, which may disturb the formation of this ordered-defect compound. In the nano-ZnS/In2S3 sample, Zn was found in the first monolayers of the absorber layer, which may lead to a downward band bending at the surface. This configuration is very stable (Fermi level pinning at the conduction band, as observed for Cd in Cu(In,Ga)Se2) and reduces the recombination rate at the interface. This effect may explain why the sample with ZnS nanodots possesses a higher efficiency. This work demonstrates the capability of correlative transmission electron microscopy, atom probe tomography, and X-ray photoelectron spectroscopy studies in investigating buried interfaces. The study provides essential information for understanding and modeling the p–n junction at the nanoscale in CIGSSe solar cells. Copyright © 2014 John Wiley & Sons, Ltd.