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Recently, a record open-circuit voltage (1.1 V) is obtained for wide-bandgap Cu2ZnGeS4 (CZGS) thin-film solar cells with a Zn1-xSnxOy (ZTO) buffer grown by atomic layer deposition (ALD) (N. Saini et al. Sol. RRL 2021, 2100837). However, a low short-circuit current and a small variation in device performance with ZTO properties are observed, suggesting similar interface properties. Since the CZGS absorber cannot be etched due to peeling, it is suggested that the presence of an oxide interlayer can influence the device performance. Here, the chemical and electronic properties of the near-surface region of CZGS absorber and its interface with ZTO buffers with varying properties (bandgap and thickness) are studied nondestructively by employing excitation-dependent X-ray photoelectron spectroscopy. The formation of Ge oxide species is indicated at the absorber surface during device fabrication, which is preserved during ALD. Further, it is shown that the ZTO/CZGS interface properties are similar and not influenced by variations in ZTO properties, which can explain the small variation in device performance. Thus, attention shall be given to possible absorber surface cleaning treatment typically applied before the buffer deposition for Ge-containing CZGS, and an optimization of the ALD ZTO buffer layer growth shall be considered for a non-etched absorber.

Two inorganic electron-selective layers (ESLs), Sn1-xGexOy (TGO) and Zn1-xGexOy (ZGO), were developed by using atomic layer deposition (ALD) with in situ quartz crystal monitoring. To ensure (Ag,Cu)(In,Ga)Se-2 (ACIGS) solar cell compatibility, a 120 degrees C ALD process was developed for GeOy using Ge(N(CH3)(2))(4) and H2O as precursors. In the ALD supercycle approach, the GeOy ALD cycle was interchanged with either ZnO or SnOy cycles to deposit TGO and ZGO with varying conduction band positions (Ec), respectively. The material properties were experimentally verified using X-ray photoelectron spectroscopy and optical absorption and by employing these films as ESLs in ACIGS solar cells. There, the open-circuit voltage initially increased as the Ge content of the TGO and ZGO films increased due to the ESL E-c simultaneously shifting up from the low position in ZnO or SnOy to match the ACIGS E-c. As the Ge content increased further, the fill factor (FF) of these devices decreased since the ESL E-c became positioned significantly above the ACIGS E-c, forming an energy barrier as seen from ACIGS. As a result, the efficiency of the ACIGS solar cell peaked for an intermediate Ge content for both TGO and ZGO. Using good TGO and ZGO compositions in ACIGS solar cells gave efficiencies of up to 14.8 and 17.0%, respectively, which were lower than the reference best cell efficiencies of up to 19.5% for CdS and 18.2% for Zn1-xSnxOy (ZTO). ZGO was, however, able to shift its Ec further up than ZTO, making it a potent ESL for high-band-gap absorbers. Based on the results, we listed a few key properties that are required for a good ACIGS solar cell ESL and gave a few suggestions on how they are linked to the previous success of ZTO.

Understanding crystallization is crucial to enable improved design approaches for inorganic materials. Despite this importance, a deepened understanding of the mechanisms controlling this phase transition is still lacking. Herein, we employ in situ heating in a transmission electron microscope (TEM) to unravel crystallization phenomena in an Al–Ti–O system. Specifically, annealing of amorphous chemical vapor deposited (CVD) Al2TiO5-based coatings has been carried out in the temperature interval 700–900 °C. Crystallization typically occurs through a two-step process, where numerous smaller (<10 nm) crystals are initially formed, followed by rapid crystal growth. Examinations of the initial crystallization stages indicate that the amorphous-to-crystalline transformation is polymorphic and that several phases may nucleate simultaneously, including Al6Ti2O13 and Al16Ti5O34. A transient (time-dependent) nucleation rate is discovered based on the displayed nucleation behavior. Subsequent evaluations of the growth stages also reveal a preferential growth into larger crystals, and the coherent findings of this study indicate that crystallization occurs through a diffusionless (displacive) mechanism.

Oxygen is a commonly overlooked element influencing the properties of many metal oxides. By combining several analytical in situ techniques and theoretical calculations, we demonstrate that oxygen plays a vital part in the phase formation and crystallization of Al2TiO5-based chemical vapor-deposited coatings. Rutherford backscattering spectrometry (RBS) corroborates a polymorphic transformation during crystallization. Subsequent hard X-ray photoelectron spectroscopy (HAXPES) shows that crystallization occurs through a displacive (diffusionless) mechanism. Coupled with theoretical calculations, the crystallization and co-formation of Al2TiO5, Al6Ti2O13, and Al16Ti5O34 are suggested to be driven by the migration of oxygen ions and their corresponding vacancies.

The use of Cu(In,Ga)S2 as a top cell in tandem solar cell, despite having suitable properties for such an application, is hampered by a high open-circuit voltage (VOC) deficit. The deficit arises from a poor optoelectronic quality of the absorbers - engendered by phase separation - and the inadequate translation of the optoelectronic quality of the absorber into device VOC. In this work, we report the role of first stage substrate temperature in the mitigation of phase separation and optimized Cu-excess during growth in Cu(In,Ga)S2, which leads to reduced VOC deficit, resulting in a device with 15.6 % PCE with a VOC of ∼ 981 mV when completed with atomic layer deposited (Zn,Sn)O and Al:ZnMgO transparent conductive oxide.

This study evaluates the effect of silver alloying, stoichiometry, and deposition temperature of wide-gap (Ag,Cu)GaSe2 (ACGS) absorber films for solar cell applications. Devices using a standard CdS buffer exhibit a strong anticorrelation between the open-circuit voltage (V-OC) and short-circuit current density (J(SC)), with V-OC decreasing and J(SC) increasing toward stoichiometric absorber composition. Increasing the ACGS deposition temperature leads to larger grains and improved J(SC), while V-OC is not affected. By adding more silver to the absorber (maximum tested [Ag]/([Ag]+[Cu]) [AAC] = 0.4), the widening of the space charge region (SCR) significantly enhances carrier collection. Experimental quantum efficiency spectra can be accurately simulated when assuming a very low diffusion length and perfect collection in the SCR. The highest efficiency of 8.3% (without antireflection coating [ARC]) is reached for an absorber with AAC = 0.4 grown at 600 degrees C. Replacing CdS by a (Zn,Sn)O buffer with lower electron affinity strongly mitigates interface recombination. Moreover, the V-OC-J(SC) anticorrelation is not evident anymore and the highest efficiency of 11.2% (11.6% w/ARC, V-OC = 985 mV, J(SC) = 18.6 mA cm(-2), fill factor = 61.0%) is reached for a close-stoichiometric ACGS solar cell with AAC = 0.4 processed at 650 degrees C.

Ag alloying and the introduction of alkali elements through a postdeposition treatment are two approaches to improve the performance of Cu(In,Ga)Se2 (CIGS) thin film solar cells. In particular, a postdeposition treatment of an alkali metal fluoride of the absorber has shown a beneficial effect on the solar cells performance due to an increase in the open circuit voltage (VOC) for both (Ag,Cu)(In,Ga)Se2 (ACIGS) and CIGS based solar cells. Several reasons have been suggested for the improved VOC in CIGS solar cells including absorber surface and interface effects. Less works investigated how the applied postdeposition treatment influences the ACIGS absorber surface and interface properties and the subsequent buffer layer growth. In this work we employed hard X-ray photoelectron spectroscopy to study the chemical and electronic properties at the real functional interface between a CdS buffer and ACIGS absorbers that have been exposed to different alkali metal fluoride treatments during preparation. All samples show an enhanced Ag content at the CdS/ACIGS interface as compared to ACIGS bulk-like composition, and it is also shown that this enhanced Ag content anticorrelates with Ga content. The results indicate that the absorber composition at the near-surface region changes depending on the applied alkali postdeposition treatment. The Cu and Ga decrease and the Ag increase are stronger for the RbF treatment as compared to the CsF treatment, which correlates with the observed device characteristics. This suggests that a selective alkali postdeposition treatment could change the ACIGS absorber surface composition, which can influence the solar cell behavior., Title in Web of Science: Surface/Interface Effects by Alkali Postdeposition Treatments of (Ag,Cu)(In,Ga)Se-2 Thin Film Solar Cells

Designing heterostructure materials at the nanoscale is a well-known method to enhance gas sensing performance. In this study, a mixed solution of zinc chloride and tin (II) chloride dihydrate, dissolved in ethanol solvent, was used as the initial precursor for depositing the sensing layer on alumina substrates using the ultrasonic spray pyrolysis (USP) method. Several ZnO/SnO2 heterostructures were grown by applying different ratios in the initial precursors. These heterostructures were used as active materials for the sensing of H2S gas molecules. The results revealed that an increase in the zinc chloride in the USP precursor alters the H2S sensitivity of the sensor. The optimal working temperature was found to be 450 degrees C. The sensor, containing 5:1 (ZnCl2: SnCl2 center dot 2H(2)O) ratio in the USP precursor, demonstrates a higher response than the pure SnO2 (similar to 95 times) sample and other heterostructures. Later, the selectivity of the ZnO/SnO2 heterostructures toward 5 ppm NO2, 200 ppm methanol, and 100 ppm of CH4, acetone, and ethanol was also examined. The gas sensing mechanism of the ZnO/SnO2 was analyzed and the remarkably enhanced gas-sensing performance was mainly attributed to the heterostructure formation between ZnO and SnO2. The synthesized materials were also analyzed by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray, transmission electron microscopy, and X-ray photoelectron spectra to investigate the material distribution, grain size, and material quality of ZnO/SnO2 heterostructures.

Herein, a new route of sulfur grading in CuInSe2 (CISe) thin-film solar absorbers by introducing an ultrathin (<50 nm) sacrificial sputtered CuInS2 (CIS) layer on top of the CISe. Different CIS top layer compositions (Cu-poor to Cu-rich) are analyzed, before and after a high-temperature treatment in selenium (Se)- or selenium+sulfur (SeS)-rich atmospheres. An [S]/([S] + [Se]) grading from the surface into the bulk of the Se- and SeS-treated samples is observed, and evidence of the formation of a mixed CuIn(S,Se)2 phase by Raman analysis and X-ray diffraction is provided. The optical bandgap from quantum efficiency measurements of solar cells is increased from 1.00 eV for the CISe reference to 1.14 and 1.30 eV for the Se- and SeS-treated bilayer samples, respectively. A ≈150 mV higher VOC is observed for the SeS-treated bilayer sample, but the cell exhibits blocking characteristics resulting in lower efficiency as compared with the CISe reference. This blocking is attributed to an internal electron barrier at the interface to the sulfur-rich surface layer. The difference in reaction routes and possible ways to improve the developed sulfurization process are discussed.

The Cu2ZnGe X Sn1-X S4 (CZGTS) thin-film solar cells have a limited open-circuit voltage (V OC) due to bulk and interface recombination. Since the standard CdS buffer layer gives a significant cliff-like conduction band offset to CZGTS, alternative buffer layers are needed to reduce the interface recombination. This work compares the performance of wide bandgap Cu2ZnGeS4 (CZGS) solar cells fabricated with nontoxic Zn x Sn1–x O y (ZTO) buffer layers grown by atomic layer deposition under different conditions. The V OC of the CZGS solar cell improved significantly to over 1 V by substituting CdS with ZTO. However, V OC is relatively insensitive to ZTO bandgap variations. The short-circuit current is generally low but is improved with KCN etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for nonetched devices., Title in thesis list of papers: Record 1.1 V open circuit voltage for Cu2ZnGeS4 based thin-film solar cells using atomic layer deposition Zn1-xSnxOy buffer layers

Zn1-xSnxOy (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnOx and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na-COx species from the CZTS surface, may be useful for the ZTO/CZTS device performance.

Cu(In,Ga)S-2 holds the potential to become a prime candidate for use as the top cell in tandem solar cells owing to its tunable bandgap from 1.55 eV (CuInS2) to 2.50 eV (CuGaS2) and favorable electronic properties. Devices above 14% power conversion efficiency (PCE) can be achieved by replacing the CdS buffer layer with a (Zn,Mg)O or Zn(O,S) buffer layer. However, the maximum achievable PCE of these devices is limited by the necessary high heating temperatures during or after buffer deposition, as this leads to a drop in the quasi-Fermi level splitting (qFLs) and therefore the maximum achievable open-circuit voltage (V-OC). In this work, a low-temperature atomic layer deposited (Zn,Sn)O thin film is explored as a buffer layer to mitigate the drop in the qFLs. The devices made with (Zn,Sn)O buffer layers are characterized by calibrated photoluminescence and current-voltage measurements to analyze the optoelectronic and electrical characteristics. An improvement in the qFLs after buffer deposition is observed for devices prepared with the (Zn,Sn)O buffer deposited at 120 degrees C. Consequently, a device with a V-OC value above 1 V was achieved. A 14% PCE is externally measured and certified for the best solar cell. The results show the necessity of developing a low-temperature buffer deposition process to maintain and translate absorber qFLs to device V-OC.

Negative thermal expansion is an elusive property found among certain materials, whose potential applications have remained limited due to the many challenges faced in their synthesis. Herein, we report the successful formation of aluminium titanate-based coatings (Al2TiO5), a material renowned for its low-to-negative thermal expansion, by the co-deposition of aluminium-isopropoxide and titanium-isopropoxide in a hot-wall chemical vapour deposition instrument. While coatings grown at 450 °C were amorphous as-deposited, a short-range order into the Al2TiO5-phase was found and analysed by using Raman spectroscopy. Upon subsequent annealing at 700 °C for 3 hours, crystalline coatings were achieved without forming any binary phases. The selective synthesis of the Al2TiO5 phase is ascribed to the precursors’ inherent chemical similarities, resulting in a kinetic targeting of this phase and a short-range homogeneity, entailing its preferred crystallisation. The role of local coordination is expressed by demonstrating the formation of intergrowth phases ascribed to lower coordinating interstices in the compound. Both the formation and crystallisation temperatures reported herein, as well as the timescales needed for the synthesises, are considerably lower than any conventional adopted solid-state techniques used so far to attain the Al2TiO5 phase.

The Cu2ZnGe X Sn1-X S4 (CZGTS) thin-film solar cells have a limited open-circuit voltage (V OC) due to bulk and interface recombination. Since the standard CdS buffer layer gives a significant cliff-like conduction band offset to CZGTS, alternative buffer layers are needed to reduce the interface recombination. This work compares the performance of wide bandgap Cu2ZnGeS4 (CZGS) solar cells fabricated with nontoxic Zn x Sn1–x O y (ZTO) buffer layers grown by atomic layer deposition under different conditions. The V OC of the CZGS solar cell improved significantly to over 1 V by substituting CdS with ZTO. However, V OC is relatively insensitive to ZTO bandgap variations. The short-circuit current is generally low but is improved with KCN etching of the CZGS absorber before deposition of the ZTO buffer layer. A possible explanation for the device behavior is the presence of an oxide interlayer for nonetched devices., Title in thesis list of papers: Record 1.1 V open circuit voltage for Cu2ZnGeS4 based thin-film solar cells using atomic layer deposition Zn1-xSnxOy buffer layers

High-end organic–inorganic lead halide perovskite semitransparent p–i–n solar cells for tandem applications use a phenyl-C61-butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnOx electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I)3 and (Cs,FA)Pb(Br,I)3 and in this study on Cs0.05FA0.79MA0.16PbBr0.51I2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnOx exposure was therefore suggested to form a nonideal perovskite/SnOx interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnOx growth on the perovskite, the mass dynamics of monitor crystals coated by partial p–i–n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20–50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnOx on the perovskite. Although measurements on the perovskite bulk below and the SnOx film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.

This contribution concerns the effect of the Ag content in wide‐gap Agw Cu1‐w In1‐x Gax Se2 (ACIGS) absorber films and its impact on solar cell performance. First‐principles calculations are conducted, predicting trends in absorber band gap energy (E g) and band structure across the entire compositional range (w and x ). It is revealed that a detrimental negative conduction band offset (CBO) with a CdS buffer can be avoided for all possible absorber band gap values (E g = 1.0–1.8 eV) by adjusting the Ag alloying level. This opens a new path to reduce interface recombination in wide‐gap chalcopyrite solar cells. Indeed, corresponding samples show a clear increase in open‐circuit voltage (V OC) if a positive CBO is created by sufficient Ag addition. A further extension of the beneficial compositional range (positive CBO at buffer/ACIGS interface) is possible when exchanging CdS with Zn1‐y Sny Oz , because of its lower electron affinity (χ). Nevertheless, the experimental results strongly suggest that at present, residual interface recombination still limits the performance of solar cells with optimized CBO, which show an efficiency of up to 15.1% for an absorber band gap of E g = 1.45 eV.

Amorphous tin-gallium oxide (a-SGO) grown with atomic layer deposition was evaluated as a buffer layer in (Ag,Cu)(In,Ga)Se2 thin-film solar cells in search for a new material that is compatible with a variety of absorber band gaps. Hard and soft X-ray photoelectron spectroscopy on absorber/a-SGO stacks combined with J–V characterization of solar cells that were fabricated, showed that the conduction band alignment at the absorber/a-SGO interface can be tuned by varying the cation composition and/or growth temperature. Here, the surface band gap was 1.1 eV for the absorber. However, optical band gap data for a-SGO indicate that a suitable conduction band alignment can most likely be achieved even for wider absorber band gaps relevant for tandem top cells. A best efficiency of 17.0% was achieved for (Ag,Cu)(In,Ga)Se2/a-SGO devices, compared to η = 18.6% for the best corresponding CdS reference. Lower fill factor and open-circuit voltage values were responsible for lower cell efficiencies. The reduced fill factor is explained by a larger series resistance, seemingly related to interface properties, which are yet to be optimized. Some layer constellations resulted in degradation in fill factor during light soaking as well. This may partly be explained by light-induced changes in the electrical properties of a-SGO, according to analysis of Al/SGO/n-Si metal-oxide-semiconductor capacitors that were fabricated and characterized with J–V and C–V. Moreover, the introduction of a 1 nm thick Ga2O3 interlayer between the absorber and a-SGO improved the open-circuit voltage, which further indicates that the absorber/a-SGO interface can be improved.

Herein, sulfurization of CuInSe(2)and CuGaSe2(CGSe) absorber layers is compared to improve the understanding of sulfur incorporation into Cu(In,Ga)Se(2)films by annealing in a sulfur atmosphere. It is found for Cu-poor CuInSe(2)that for an annealing temperature of 430 degrees C, sulfur is incorporated into the surface of the absorber and forms an inhomogeneous CuIn(S,Se)(2)layer. In addition, at 530 degrees C, a surface layer of CuInS(2)is formed. In contrast, for Cu-poor CuGaSe(2)samples, S can only be introduced at 530 degrees C, mainly forming an alloy of CuGa(S,Se)(2), where no closed CuGaS(2)layer is found. In Cu-rich CuGaSe(2)samples, however, selenium is substituted by S already at 330 degrees C, which can be explained by a rapid phase transformation of Cu2 - xSe into Cu2 - x(S,Se). This transformation facilitates S in-diffusion and catalyzes CuGa(S,Se)(2)formation, likewise that previously reported to occur in CuInSe2. Finally, the Cu-poor CuInSe(2)solar cell performance is improved by the sulfurization step at 430 degrees C, whereas for the 530 degrees C sample, a decreasing fill factor and short-circuit current density are observed, indicating lower diffusion length accompanied by possible formation of an electron transport barrier. In contrast, the electrical characteristics deteriorate for all sulfurized Cu-poor CuGaSe(2)cells.

Surface treatments of Cu2ZnSnS4 have shown a beneficial effect on the solar cells performance due to a reduction in the open-circuit voltage (VOC) deficit. Several reasons have been suggested for the VOC deficit, including an unfavorable band alignment at the buffer/Cu2ZnSnS4 interface. Herein, the influence of Cu2ZnSnS4 surface treatment (air exposure and air anneal) on the electronic and chemical properties of Cu2ZnSnS4 and CdS/Cu2ZnSnS4 interfaces is investigated. Using hard X-ray photoelectron spectroscopy, it is shown that the band alignment at the CdS/Cu2ZnSnS4 interface is not significantly altered by the applied surface treatment. The device enhancement is instead connected to interface passivation for the surface-treated Cu2ZnSnS4 samples due to the formation of SnOx, which is shown to not be fully removed upon KCN etching prior to the buffer layer deposition. In addition, a surface treatment of the Cu2ZnSnS4 absorber prior to buffer layer deposition influences the growth of CdS buffer, as a thicker CdS-overlayer is observed to grow on a surface-treated Cu2ZnSnS4 sample as compared with a nontreated sample. This suggests that a reoptimization of the CdS thickness for a given Cu2ZnSnS4 surface treatment is required.