Characterization of Sulfur Bonding in CdS:O Buffer Layers for CdTe-based Thin-Film Solar Cells

Sulfate Speciation Analysis Using Soft X-ray Emission Spectroscopy

The chemical and electronic structures of 15 different sulfates are studied using S L_2,3 soft X-ray emission spectroscopy (XES). Sulfur L_2,3 XES spectra of sulfates are distinctively different from those of other sulfur compounds, which makes XES a powerful technique for sulfate detection. Furthermore, subtle but distinct differences between the spectra of sulfates with different cations are observed, which allow a further differentiation of the specific compound. Most prominently, the position and width of the emission from "S 3s" derived bands systematically vary for different compounds, which can be understood with electronic structure and spectral calculations based on density functional theory.

Semitransparent Sb2S3 thin film solar cells by ultrasonic spray pyrolysis for use in solar windows

The integration of photovoltaic (PV) solar energy in zero-energy buildings requires durable and efficient solar windows composed of lightweight and semitransparent thin film solar cells. Inorganic materials with a high optical absorption coefficient, such as Sb₂S₃ (>10⁵ cm^-1 at 450 nm), offer semitransparency, appreciable efficiency, and long-term durability at low cost. Oxide-free throughout the Sb₂S₃ layer thickness, as confirmed by combined studies of energy dispersive X-ray spectroscopy and synchrotron soft X-ray emission spectroscopy, semitransparent Sb₂S₃ thin films can be rapidly grown in air by the area-scalable ultrasonic spray pyrolysis method. Integrated into a ITO/TiO₂/Sb₂S₃/P3HT/Au solar cell, a power conversion efficiency (PCE) of 5.5% at air mass 1.5 global (AM1.5G) is achieved, which is a record among spray-deposited Sb₂S₃ solar cells. An average visible transparency (AVT) of 26% of the back-contact-less ITO/TiO₂/Sb₂S₃ solar cell stack in the wavelength range of 380-740 nm is attained by tuning the Sb₂S₃ absorber thickness to 100 nm. In scale-up from mm² to cm² areas, the Sb₂S₃ hybrid solar cells show a decrease in efficiency of only 3.2% for an 88 mm² Sb₂S₃ solar cell, which retains 70% relative efficiency after one year of non-encapsulated storage. A cell with a PCE of 3.9% at 1 sun shows a PCE of 7.4% at 0.1 sun, attesting to the applicability of these solar cells for light harvesting under cloud cover.

SnO2-Catalyzed Oxidation in High-Efficiency CdTe Solar Cells

Interfaces at the front of superstrate CdTe-based solar cells are critical to carrier transport, recombination, and device performance, yet determination of the chemical structure of these nanoscale regions has remained elusive. This is partly due to changes that occur at the front interfaces during high temperature growth and substantive changes occurring during postdeposition processing. In addition, these buried interfaces are extremely difficult to access in a way that preserves chemical information. In this work, we use a recently developed thermomechanical cleaving technique paired with X-ray photoelectron spectroscopy to probe oxidation states at the SnO₂ interface of CdTe solar cells. We show that the tin oxide front electrode promotes the formation of nanometer-scale oxides of tellurium and sulfur. Most oxidation occurs during CdCl₂/O₂ activation. Surprisingly, we show that relatively low-temperature anneals (180-260 °C) used to diffuse and activate copper acceptors in a doping/back contact process also cause significant changes in oxidation at the front of the cell, providing a heretofore missing aspect of how back contact processes can modify device transport, recombination, and performance. Device performance is shown to correlate with the extent of tellurium and sulfur oxidation within this nanometer-scale region. Mechanisms responsible for these beneficial effects are proposed.

Rubidium Fluoride Post-Deposition Treatment: Impact on the Chemical Structure of the Cu(In,Ga)Se2 Surface and CdS/Cu(In,Ga)Se2 Interface in Thin-Film Solar Cells

We present a detailed characterization of the chemical structure of the Cu(In,Ga)Se₂ thin-film surface and the CdS/Cu(In,Ga)Se₂ interface, both with and without a RbF post-deposition treatment (RbF-PDT). For this purpose, X-ray photoelectron and Auger electron spectroscopy, as well as synchrotron-based soft X-ray emission spectroscopy have been employed. Although some similarities with the reported impacts of light-element alkali PDT (i.e., NaF- and KF-PDT) are found, we observe some distinct differences, which might be the reason for the further improved conversion efficiency with heavy-element alkali PDT. In particular, we find that the RbF-PDT reduces, but not fully removes, the copper content at the absorber surface and does not induce a significant change in the Ga/(Ga + In) ratio. Additionally, we observe an increased amount of indium and gallium oxides at the surface of the treated absorber. These oxides are partly (in the case of indium) and completely (in the case of gallium) removed from the CdS/Cu(In,Ga)Se₂ interface by the chemical bath deposition of the CdS buffer.

A 1D conical nanotubular TiO2/CdS heterostructure with superior photon-to-electron conversion

Herein, a new strategy to efficiently harvest photons in solar cells is presented. A solar cell heterostructure is put forward, based on a 1D conical TiO2 nanotubular scaffold of high aspect ratio, homogenously coated with a thin few nm layer of CdS light absorber using atomic layer deposition (ALD). For the first time, a large variety of conical nanotube layers with a huge span of aspect ratios was utilized and ALD was used for the preparation of a uniform CdS coating within the entire high surface area of the TiO2 nanotubes. The resulting 1D conical CdS/TiO2 tubular heterostructure acts as a sink for photons. Due to the multiple light scattering and absorption events within this nanotubular sink, a large portion of photons (nearly 80%) is converted into electrons. It is the combination of the scaffold architecture and the light absorber present on the high surface area as a very thin layer, the optimized charge transport and multiple optical effects that make this heterostructure very promising for the next generation of highly performing solar cells.

Two-Dimensional Cadmium Chloride Nanosheets in Cadmium Telluride Solar Cells

In this study we make use of a liquid nitrogen-based thermomechanical cleavage technique and a surface analysis cluster tool to probe in detail the tin oxide/emitter interface at the front of completed CdTe solar cells. We show that this thermomechanical cleavage occurs within a few angstroms of the SnO₂/emitter interface. An unexpectedly high concentration of chlorine at this interface, ∼20%, was determined from a calculation that assumed a uniform chlorine distribution. Angle-resolved X-ray photoelectron spectroscopy was used to further probe the structure of the chlorine-containing layer, revealing that both sides of the cleave location are covered by one-third of a unit cell of pure CdCl₂, a thickness corresponding to about one Cl-Cd-Cl molecular layer. We interpret this result in the context of CdCl₂ being a true layered material similar to transition-metal dichalcogenides. Exposing cleaved surfaces to water shows that this Cl-Cd-Cl trilayer is soluble, raising questions pertinent to cell reliability. Our work provides new and unanticipated details about the structure and chemistry of front surface interfaces and should prove important to improving materials, processes, and reliability of next-generation CdTe-based solar cells.

Stability of Cd1-xZnxOyS1-y Quaternary Alloys Assessed with First-Principles Calculations

One route to decreasing the absorption in CdS buffer layers in Cu(In,Ga)Se₂ and Cu₂ZnSn(S,Se)₄ thin-film photovoltaics is by alloying. Here we use first-principles calculations based on hybrid functionals to assess the energetics and stability of quaternary Cd, Zn, O, and S (Cd_1-xZn_xO_yS_1-y) alloys within a regular solution model. Our results identify that full miscibility of most Cd_1-xZn_xO_yS_1-y compositions and even binaries like Zn(O,S) is outside typical photovoltaic processing conditions. The results suggest that the tendency for phase separation of the oxysulfides may drive the nucleation of other phases such as sulfates that have been increasingly observed in oxygenated CdS and ZnS.

Soft X-ray Spectroscopy of a Complex Heterojunction in High-Efficiency Thin-Film Photovoltaics: Intermixing and Zn Speciation at the Zn(O,S)/Cu(In,Ga)Se2 Interface

The chemical structure of the Zn(O,S)/Cu(In,Ga)Se₂ interface in high-efficiency photovoltaic devices is investigated using X-ray photoelectron and Auger electron spectroscopy, as well as soft X-ray emission spectroscopy. We find that the Ga/(Ga+In) ratio at the absorber surface does not change with the formation of the Zn(O,S)/Cu(In,Ga)Se₂ interface. Furthermore, we find evidence for Zn in multiple bonding environments, including ZnS, ZnO, Zn(OH)₂, and ZnSe. We also observe dehydrogenation of the Zn(O,S) buffer layer after Ar⁺ ion treatment. Similar to high-efficiency CdS/Cu(In,Ga)Se₂ devices, intermixing occurs at the interface, with diffusion of Se into the buffer, and the formation of S-In and/or S-Ga bonds at or close to the interface.

Band Alignment for Rectification and Tunneling Effects in Al2O3 Atomic-Layer-Deposited on Back Contact for CdTe Solar Cell

The present work intends to explain why ultrathin Al₂O₃ atomic-layer-deposited (ALD) on the back contact with rectification and tunneling effects can significantly improve the performance of CdTe solar cells in our previous work [ Liang , J. ; et al. Appl. Phys. Lett. 2015 , 107 , 013907 ]. Herein, we further study the mechanism through establishing the interfacial energy band diagram configuration of the ALD Al₂O₃/Cu_xTe by experiment of X-ray photoelectron spectroscopy and first-principles calculations and conclude to find the band alignment with optimized layer thickness (about 1 nm ALD Al₂O₃) as the key factor for rectification and tunneling effects.