Identification of the Missing Component in the Mitochondrial Benzamidoxime Prodrug-converting System as a Novel Molybdenum Enzyme

Amidoximes can be used as prodrugs for amidines and related functional groups to enhance their intestinal absorption. These prodrugs are reduced to their active amidines. Other N-hydroxylated structures are mutagenic or responsible for toxic effects of drugs and are detoxified by reduction. In this study, a N-reductive enzyme system of pig liver mitochondria using benzamidoxime as a model substrate was identified. A protein fraction free from cytochrome b5 and cytochrome b5 reductase was purified, enhancing 250-fold the minor benzamidoxime-reductase activity catalyzed by the membrane-bound cytochrome b5/NADH cytochrome b5 reductase system. This fraction contained a 35-kDa protein with homologies to the C-terminal domain of the human molybdenum cofactor sulfurase. Here it was demonstrated that this 35-kDa protein contains molybdenum cofactor and forms the hitherto ill defined third component of the N-reductive complex in the outer mitochondrial membrane. Thus, the 35-kDa protein represents a novel group of molybdenum proteins in eukaryotes as it forms the catalytic part of a three-component enzyme complex consisting of separate proteins. Supporting these findings, recombinant C-terminal domain of the human molybdenum cofactor sulfurase exhibited N-reductive activity in vitro, which was strictly dependent on molybdenum cofactor.

SARS-CoV-2: Viral Loads of Exhaled Breath and Oronasopharyngeal Specimens in Hospitalized Patients with COVID-19

Background

SARS-CoV-2 seems mainly transmissible via respiratory droplets. We compared the time-dependent SARS-CoV-2 viral load in serial pharyngeal swab with exhaled breath (EB) samples of hospitalized COVID-19 patients.

Methods

In this prospective proof of concept study, we examined hospitalized patients who initially tested positive for SARS-CoV-2. Paired oronasopharyngeal swab and EB specimens were taken at different days of hospitalization. EB collection was performed through a simple, noninvasive method using an electret air filter-based device. SARS-CoV-2 RNA detection was determined with real-time quantitative reverse transcription polymerase chain reaction.

Results

Of 187 serial samples from 15 hospitalized patients, 87/87 oronasopharyngeal swabs and 70/100 EB specimens tested positive. Comparing the number of SARS-CoV-2 copies, the viral load of the oronasopharyngeal swabs was significantly higher (CI 99%, P<<0,001) than for EB samples. The mean viral load per swab was 7.97 × 10⁶ (1.65 × 10²-1.4 × 10⁸), whereas EB samples showed 2.47 × 10³ (7.19 × 10¹-2.94 × 10⁴) copies per 20 times exhaling. Viral loads of paired oronasopharyngeal swab and EB samples showed no correlation.

Conclusions

Assessing the infectiousness of COVID-19 patients merely through pharyngeal swabs might not be accurate. Exhaled breath could represent a more suitable matrix for evaluating infectiousness and might allow screening for superspreader individuals and widespread variants such as Delta.

Hepatic microsomal N-hydroxylation of adenine to 6-N-hydroxylaminopurine

The enzymatic N-hydroxylation of the purine base adenine to the genotoxic and mutagenic compound 6-N-hydroxylaminopurine is reported for the first time. Adenine was N-oxygenated in vitro by aerobic incubations with 3-methylcholanthrene or isosafrole induced microsomal fractions of rat liver homogenates and NADPH. The formation of 6-N-hydroxylaminopurine in the incubation mixtures under widely differing conditions was assayed using newly-developed, high-performance liquid- and thin-layer chromatographic methods. Optimal reaction conditions and kinetic parameters were determined. Neither superoxide anion nor hydrogen peroxide was directly involved in the N-hydroxylation reaction. Oxidases like xanthine oxidase and peroxidase (in the presence of hydrogen peroxide) did not catalyse this N-hydroxylation. The involvement of cytochrome P-450 isoenzymes in this reaction is supported by the observation that the N-hydroxylation is only observed after pretreatment of the rats with 3-methylcholanthrene or isosafrole. Other inducers (phenobarbital, ethanol, 5-pregnen-3 beta ol-20-one-16 alpha-carbonitrile) were without effect. This is the first example of the microsomal transformation of an endogenous substance to a toxic derivative by usually foreign substances (xenobiotics) metabolizing cytochrome P-450 isoenzymes. The significance for the in vivo situation is discussed on the basis of the data obtained in this study.

Activated coagulation factors in human malignant effusions and their contribution to cancer cell metastasis and therapy

We have shown that the thrombin G-protein coupled receptors (GPCR) designated as protease-activated receptors (PAR-1) are expressed in primary cancer cells isolated from peritoneal and pleural malignant effusions. Here, our main goal was to evaluate several coagulation and thrombin activation effectors and markers in a series of 136 malignant effusions from cancer patients with gastrointestinal, lung and mammary carcinomas. All these patients present a highly activated coagulation system in blood and their malignant effusions, as indicated by high levels of prothrombin F1.2 fragments and D-dimers. Notably, we detected in the effusions all the coagulation factors of the tissue factor pathway inducing thrombin activation, namely factorsVII, V, X and II, as well as high VEGF levels and IGF-II in mature and precursor forms. Fibrin clot formation also correlated with higher levels of free ionized calcium (iCa), suggesting that iCa and its binding protein albumin are regulatory factors for fibrinogenesis in effusions. Consequently, thrombin,VEGF and IGFII appear to converge in the promotion of survival and invasivity of the metastatic cancer cells from blood to the malignant effusions. Thus, we add new insights on the interconnections between blood coagulation disorders in cancer patients and thrombin activation in malignant effusions, including their functional interaction with PAR in metastatic cancer cells. Based on these data we propose to counteract the metastatic cascades by targeted invalidation of key effectors of the coagulation system. Therefore, potential therapeutic approaches include the application of thrombin protease inhibitors, VEGF-blocking antibodies, and drugs targeting the VEGF and thrombin signaling pathways, such as tyrosine kinase or GPCR inhibitors.

Mammary fibroblasts regulate morphogenesis of normal and tumorigenic breast epithelial cells by mechanical and paracrine signals

Stromal factors play a critical role in the development of the mammary gland. Using a three dimensional-coculture model we demonstrate a significant role for stromal fibroblasts in the regulation of normal mammary epithelial morphogenesis and the control of tumor growth. Both soluble factors secreted by fibroblasts and fibroblast-derived modifications of the matrix compliance contribute to the regulation of epithelial cell morphogenesis. Readjustment of matrix tension by fibroblasts can even induce a phenotypic reversion of breast carcinoma cells. These data offer a basis to develop new strategies for the normalization of the tumor stroma as an innovative target in cancer therapy.

The pivotal role of the mitochondrial amidoxime reducing component 2 in protecting human cells against apoptotic effects of the base analog N6-hydroxylaminopurine

N-Hydroxylated nucleobases and nucleosides as N-hydroxylaminopurine (HAP) or N-hydroxyadenosine (HAPR) may be generated endogenously in the course of cell metabolism by cytochrome P450, by oxidative stress or by a deviating nucleotide biosynthesis. These compounds have shown to be toxic and mutagenic for procaryotic and eucaryotic cells. For DNA replication fidelity it is therefore of great importance that organisms exhibit effective mechanisms to remove such non-canonical base analogs from DNA precursor pools. In vitro, the molybdoenzymes mitochondrial amidoxime reducing component 1 and 2 (mARC1 and mARC2) have shown to be capable of reducing N-hydroxylated base analogs and nucleoside analogs to the corresponding canonical nucleobases and nucleosides upon reconstitution with the electron transport proteins cytochrome b5 and NADH-cytochrome b5 reductase. By RNAi-mediated down-regulation of mARC in human cell lines the mARC-dependent N-reductive detoxication of HAP in cell metabolism could be demonstrated. For HAPR, on the other hand, the reduction to adenosine seems to be of less significance in the detoxication pathway of human cells as HAPR is primarily metabolized to inosine by direct dehydroxylamination catalyzed by adenosine deaminase. Furthermore, the effect of mARC knockdown on sensitivity of human cells to HAP was examined by flow cytometric quantification of apoptotic cell death and detection of poly (ADP-ribose) polymerase (PARP) cleavage. mARC2 was shown to protect HeLa cells against the apoptotic effects of the base analog, whereas the involvement of mARC1 in reductive detoxication of HAP does not seem to be pivotal.

Phosphono analogs of glutathione: inhibition of glutathione transferases, metabolic stability, and uptake by cancer cells

Glutathione transferases (GSTs) have been shown to play an important role in multiple drug resistance in cancer chemotherapy. The inactivation of GST isoforms could lead to an enhanced activity of cytotoxic drugs. Thus, we have developed glutathione phosphono analogs [(S)-gamma-glutamyl-(2RS)-(+/-)-2-amino-(dialkoxyphosphinyl)-ac etylgl ycines], which were previously shown to be inhibitors of GSTP1-1. In the present study, the inhibition characteristics of these analogs, including isoenzyme specificities, type of inhibition, and determination of K(i) values, were determined. The inhibition of class alpha GSTs was competitive towards GSH. A mixed-type, non-competitive inhibition of class mu and pi GSTs was observed. The K(i) values varied between 880 +/- 210 and 0.45 +/- 0.1 microM. The inhibitors were most effective towards class mu GSTs. In order to investigate the potential use of these GST inhibitors in intact cellular systems, two additional approaches were examined. Firstly, the metabolic stability was tested with purified gamma-glutamyl transpeptidase and cell homogenates as well as during incubation of cell lines. No appreciable degradation was observed in any of the tested systems. Secondly, to facilitate cellular uptake, three derivatives were synthesized in which the glycine carboxylic group was esterified. Uptake and a possible intracellular cleavage to the corresponding free acids were monitored by HPLC analysis. The esters were effectively transported into HT29 (colon cancer) and EPG85-257P (gastric cancer) cells, respectively, and readily converted into the more active free acids. In conclusion, the tested inhibitors may be regarded as model compounds for the development of modulating agents in cancer chemotherapy.

The reduction of 6-N-hydroxylaminopurine to adenine by xanthine oxidase

The genotoxic and mutagenic compound 6-N-hydroxylaminopurine (HAP) can be detoxified in vitro by enzymatic N-reduction to adenine. This reaction is catalysed by both rat and rabbit liver cytosolic fractions. The formation of adenine was monitored using HPLC. Subcellular distribution of the activity, kinetic parameters and the influence of various cofactors and inhibitors were determined. The N-reduction required NADH or hypoxanthine or xanthine and was strongly inhibited by allopurinol. These observations suggested that the N-reductase activity is due to xanthine oxidase (EC 1.2.3.2). Moreover, the involvement of xanthine oxidase is supported by the observation that purified cow milk xanthine oxidase also catalysed this reaction. The N-reduction of HAP was inhibited only weakly by oxygen. In addition, the formation of adenine is catalysed by either the oxidase or dehydrogenase form of xanthine oxidase. Thus, this reaction should be significant for the in vivo detoxification of HAP.

Impact of SnF2 Addition on the Chemical and Electronic Surface Structure of CsSnBr3

We report on the chemical and electronic structure of cesium tin bromide (CsSnBr₃) and how it is impacted by the addition of 20 mol % tin fluoride (SnF₂) to the precursor solution, using both surface-sensitive lab-based soft X-ray photoelectron spectroscopy (XPS) and near-surface bulk-sensitive synchrotron-based hard XPS (HAXPES). To determine the reproducibility and reliability of conclusions, several (nominally identically prepared) sample sets were investigated. The effects of deposition reproducibility, handling, and transport are found to cause significant changes in the measured properties of the films. Variations in the HAXPES-derived compositions between individual sample sets were observed, but in general, they confirm that the addition of 20 mol % SnF₂ improves coverage of the titanium dioxide substrate by CsSnBr₃ and decreases the oxidation of Sn^II to Sn^IV while also suppressing formation of secondary Br and Cs species. Furthermore, the (surface) composition is found to be Cs-deficient and Sn-rich compared to the nominal stoichiometry. The valence band (VB) shows a SnF₂-induced redistribution of Sn 5s-derived density of states, reflecting the changing Sn^II/Sn^IV ratio. Notwithstanding some variability in the data, we conclude that SnF₂ addition decreases the energy difference between the VB maximum of CsSnBr₃ and the Fermi level, which we explain by defect chemistry considerations.

Drug Metabolism by the Mitochondrial Amidoxime Reducing Component (mARC): Rapid Assay and Identification of New Substrates

For the development of new drugs, the investigation of their metabolism is of central importance. In the past, the focus was mostly on the consideration of established enzymes leading to oxidations such as cytochrome P450. However, reductive metabolism by the mARC enzyme system can play an important role in particular for nitrogen containing functional groups. A rapid test was established to give developers of new drugs in the preclinical stage the opportunity to test the metabolism by mARC. To demonstrate the relevance and validity of the new test system, known and potential substrates were applied to this new assay. All known substrates could be detected by the system. Furthermore, several new substrates were found including long-established drugs such as hydroxyurea and new compounds in development such as epacdadostat.

Functional characterization of protein variants encoded by nonsynonymous single nucleotide polymorphisms in MARC1 and MARC2 in healthy Caucasians

Human molybdenum-containing enzyme mitochondrial amidoxime reducing component (mARC), cytochrome b5 type B, and NADH cytochrome b5 reductase form an N-reductive enzyme system that is capable of reducing N-hydroxylated compounds. Genetic variations are known, but their functional relevance is unclear. Our study aimed to investigate the incidence of nonsynonymous single nucleotide polymorphisms (SNPs) in the mARC genes in healthy Caucasian volunteers, to determine saturation of the protein variants with molybdenum cofactor (Moco), and to characterize the kinetic behavior of the protein variants by in vitro biotransformation studies. Genotype frequencies of six SNPs in the mARC genes (c.493A>G, c.560T>A, c.736T>A, and c.739G>C in MARC1; c.730G>A and c.735T>G in MARC2) were determined by pyrosequencing in a cohort of 340 healthy Caucasians. Protein variants were expressed in Escherichia coli. Saturation with Moco was determined by measurement of molybdenum by inductively coupled mass spectrometry. Steady state assays were performed with benzamidoxime. The six variants were of low frequency in this Caucasian population. Only one homozygous variant (c.493A; MARC1) was detected. All protein variants were able to bind Moco. Steady state assays showed statistically significant decreases of catalytic efficiency values for the mARC-2 wild type compared with the mARC-1 wild type (P < 0.05) and for two mARC-2 variants compared with the mARC-2 wild type (G244S, P < 0.05; C245W, P < 0.05). After simultaneous substitution of more than two amino acids in the mARC-1 protein, N-reductive activity was decreased 5-fold. One homozygous variant of MARC1 was detected in our sample. The encoded protein variant (A165T) showed no different kinetic parameters in the N-reduction of benzamidoxime.

The antibiotic resistome and microbiota landscape of refugees from Syria, Iraq and Afghanistan in Germany

BACKGROUND

Multidrug-resistant bacteria represent a substantial global burden for human health, potentially fuelled by migration waves: in 2015, 476,649 refugees applied for asylum in Germany mostly as a result of the Syrian crisis. In Arabic countries, multiresistant bacteria cause significant problems for healthcare systems. Currently, no data exist describing antibiotic resistances in healthy refugees. Here, we assess the microbial landscape and presence of antibiotic resistance genes (ARGs) in refugees and German controls. To achieve this, a systematic study was conducted in 500 consecutive refugees, mainly from Syria, Iraq, and Afghanistan and 100 German controls. Stool samples were subjected to PCR-based quantification of 42 most relevant ARGs, 16S ribosomal RNA gene sequencing-based microbiota analysis, and culture-based validation of multidrug-resistant microorganisms.

RESULTS

The fecal microbiota of refugees is substantially different from that of resident Germans. Three categories of resistance profiles were found: (i) ARGs independent of geographic origin of individuals comprising BIL/LAT/CMA, ErmB, and mefE; (ii) vanB with a high prevalence in Germany; and (iii) ARGs showing substantially increased prevalences in refugees comprising CTX-M group 1, SHV, vanC1, OXA-1, and QnrB. The majority of refugees carried five or more ARGs while the majority of German controls carried three or less ARGs, although the observed ARGs occurred independent of signatures of potential pathogens.

CONCLUSIONS

Our results, for the first time, assess antibiotic resistance genes in refugees and demonstrate a substantially increased prevalence for most resistances compared to German controls. The antibiotic resistome in refugees may thus require particular attention in the healthcare system of host countries.

Physicochemical compatibility and emulsion stability of propofol with commonly used analgesics and sedatives in an intensive care unit

Objectives

The purpose of this study was the determination of the physicochemical compatibility and emulsion stability of propofol with other sedatives and analgesics (clonidine hydrochloride, dexmedetomidine, 4-hydroxybutyric acid, (S)-ketamine, lormetazepam, midazolam hydrochloride, piritramide, remifentanil hydrochloride and sufentanil citrate) that are frequently administered together intravenously.

Methods

Drugs were mixed with propofol and stored without light protection at room temperature. Samples were taken at 10 points of time over 7 days. The physical stability and emulsion stability in particular were analysed by visual and microscopical inspection and by measurement of the pH value, zeta potential and globule size distribution. In addition, high-performance liquid chromatography and mass spectrometry were used to identify chemical incompatibilities.

Results

4-Hydroxybutyric acid, midazolam hydrochloride, piritramide and remifentanil hydrochloride are physically incompatible when mixed with propofol. The reason for this is the development of an increased fraction of oil droplets >5 µm leading to a higher risk of emboli. Moreover, propofol is chemically incompatible with remifentanil. The sorption of propofol to the rubber stopper of the syringe was another detectable incompatibility.

Conclusions

Propofol should not be administered with 4-hydroxybutyric acid, remifentanil hydrochloride, midazolam hydrochloride and piritramide through the same intravenous line. Based on the risk of sorption to the rubber material, propofol should be used with caution. A drug loss might occur that leads to an underdosing of the patient requiring a dose adjustment to avoid any adverse consequences. As a result of this study, the drug safety in intensive care units could be optimised.

Purification and characterization of class alpha and Mu glutathione S-transferases from porcine liver

Six cytosolic GSTs from porcine liver were purified by a combination of glutathione affinity chromatography and ion-exchange HPLC. The isoenzymes were characterized by SDS-PAGE, gel filtration, isoelectric focusing, immunoblotting analysis and determination of substrate specificities and inhibition characteristics. The purified GSTs belong to the alpha and mu classes, respectively. No class pi isoenzyme was isolated or detected. The class alpha GST pA1-1* exists as a homodimer (M(r) = 25.3 kDa), whereas GST pA2-3* consists of two subunits with different M(r) values (27.0 and 25.3 kDa). The estimated pI values were 9.5 and 8.8, respectively. Furthermore, four class mu porcine GSTs, pM1-1*, pM1-2*, pM3-?* and pM4-?*, were isolated. The isoenzyme pM1-1* possesses a relative molecular mass of 27.2 kDa and a pI value of 6.2. Additional pM1 isoenzymes hybridize with the subunit pM2* (M(r) = 25.2) to furnish a heterodimer, which shows a pI value of 5.8. The other class mu isoenzymes are heterodimers with pI values of 5.45 and 5.05. Substrate specificities and inhibition characteristics correlate very well with those of the corresponding human isoenzymes. The results are discussed with regard to the usefulness of porcine GSTs as an in vitro testing model.

Phosphono analogues of glutathione as new inhibitors of glutathione S-transferases

Phosphono-analogues of glutathione containing the O = P(OR)2 moiety in place of the cysteinyl residue CH2SH 1a-1d were prepared by solution phase peptide synthesis. Benzyl, benzyloxy-carbonyl, and tert-butyl protecting groups were used to mask the individual amino acid functional groups. The formation of peptide bonds was achieved by the usual peptide synthesis via activation of carboxylic functions with cyclohexylcarbodiimide and subsequent reaction with free amino groups. The thus obtained, fully-protected peptides were each purified by normal phase column chromatography. Deprotection was accomplished by hydrogenolysis and by treatment with HBr/acetic acid yielding the desired phosphonic acid diester 1a-1d. The inhibition of the glutathione conjugation of 1-chloro-2,4-dinitrobenzene by human placental glutathione S-transferase was studied by determining the IC50 values of the new glutathione analogues. The IC50 values were 291 microM, 139 microM, 64 microM, and 21 microM for the dimethyl, diethyl, diisopropyl, and di-n-butyl esters, respectively. The results clearly show that the formal substitution of the glutathione thiol function by phosphonic acid esters leads to a new class of glutathione S-transferase inhibitors. Further investigations directed at the question of whether or not these glutathione analogues are suitable for a modulation in chemotherapy are in progress.

In vitro oxygenation of N,N'-diphenylguanidines

1. The enzymic C-oxygenation of N,N'-diphenylguanidine (DPG) to N-(4-hydroxyphenyl)-N'-phenylguanidine (4HPG) and the N-oxygenation of N,N'-bis-(pentafluorophenyl)-guanidine (BPG) to N"-hydroxy-N,N"-bis-(pentafluorophenyl)-guanidine (HBPG) is reported. 2. The metabolites were identified by t.l.c. and mass spectral analysis using synthetic reference compounds. 3. Rat and rabbit liver homogenates (9000 g supernatant and microsomes) were used as enzyme source. 4. The enzymic oxygenations were both O2 and NADPH dependent. NADPH could not be replaced by hydrogen peroxide. 5. 15N-n.m.r. spectroscopy was used to elucidate structure and tautomerism of BPG and HBPG.

Reduction of Nomega-hydroxy-L-arginine to L-arginine by pig liver microsomes, mitochondria, and human liver microsomes

Nomega-Hydroxy-L-arginine, the intermediate in nitric oxide formation from L-arginine catalyzed by NO synthase, can be released into the extracellular space. It has been suggested that it can circulate and exert paracrine effects. Since it cannot only be used as substrate by NO synthases, but can also be oxidized by cytochrome P450 and other hemoproteins in a superoxide-dependent manner, it has been proposed that it can serve as NO donor. In the present study, the in vitro reduction of Nomega-hydroxy-L-arginine was examined. Pig and human liver microsomes as well as pig liver mitochondria were capable of reducing Nomega-hydroxy-L-arginine to L-arginine in an oxygen-insensitive enzymatic reaction. These results demonstrate that this metabolic pathway has to be considered when suggesting Nomega-hydroxy-L-arginine as NO-precursor. The reconstituted liver microsomal system of a pig liver CYP2D enzyme, the benzamidoxime reductase, was unable to replace microsomes to produce L-arginine from Nomega-hydroxy-L-arginine.

Microsomal N-oxygenation of adenine to adenine 1-N-oxide

During investigations on the N-oxygenation of adenine (1) the enzymatic formation of adenine 1-N-oxide 3 was demonstrated for the first time. The identity of this metabolite was confirmed by its chromatographic behaviour and UV-spectrum recorded after HPLC separation. Adenine 1-N-oxide (3) and similar oxygenated derivatives of adenine were synthesized as reference substances. The enzymatic formation of 3 exhibits the typical characteristics of a reaction catalysed by microsomal mono-oxygenases. In induction experiments, an increase in the rate of formation of 3 after pretreatment with phenobarbital was observed. A participation of those isoenzymes of the cytochrome P-450 enzyme system which can be induced by phenobarbital is assumed.

Activation of the coagulation system in cancerogenesis and metastasation

The activation of the coagulation system in cancer patients is a well-known phenomenon responsible for recurrent clinical problems. A number of fascinating molecular mechanisms have been recognized showing that the tumor not only activates the coagulation system, but vice versa, activated coagulation proteins are able to induce molecular effects in tumor cells. The molecular basis is the expression of defined membrane receptors by tumor cells that are activated, for example, by thrombin. As the liberation of thrombin from prothrombin is one of the key events in coagulation, it's impact upon biological processes, such as cancerogenesis and metastasation, seems to be a regular pathophysiological consequence. These perceptions are not only interesting for the comprehension of cancerogenesis, metastasation, and clinical phenomena, but they also have a high impact upon modern strategies of tumor therapy. Especially, the development of clinically useful coagulation inhibitors, such as modern low molecular weight heparins or melagatran, created the possibility of therapies that combine cell biological approaches with apoptosis-inducing principals such as chemotherapy. Several clinical studies that demonstrate the implication of these strategies have already been published recently. In this article the cell biological basics for these approaches are reviewed.

Formation of a K-In-Se Surface Species by NaF/KF Postdeposition Treatment of Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers

A NaF/KF postdeposition treatment (PDT) has recently been employed to achieve new record efficiencies of Cu(In,Ga)Se₂ (CIGSe) thin film solar cells. We have used a combination of depth-dependent soft and hard X-ray photoelectron spectroscopy as well as soft X-ray absorption and emission spectroscopy to gain detailed insight into the chemical structure of the CIGSe surface and how it is changed by different PDTs. Alkali-free CIGSe, NaF-PDT CIGSe, and NaF/KF-PDT CIGSe absorbers grown by low-temperature coevaporation have been interrogated. We find that the alkali-free and NaF-PDT CIGSe surfaces both display the well-known Cu-poor CIGSe chemical surface structure. The NaF/KF-PDT, however, leads to the formation of bilayer structure in which a K-In-Se species covers the CIGSe compound that in composition is identical to the chalcopyrite structure of the alkali-free and NaF-PDT absorber.

Evidence for Chemical and Electronic Nonuniformities in the Formation of the Interface of RbF-Treated Cu(In,Ga)Se2 with CdS

We report on the initial stages of CdS buffer layer formation on Cu(In,Ga)Se₂ (CIGSe) thin-film solar cell absorbers subjected to rubidium fluoride (RbF) postdeposition treatment (PDT). A detailed characterization of the CIGSe/CdS interface for different chemical bath deposition (CBD) times of the CdS layer is obtained from spatially resolved atomic and Kelvin probe force microscopy and laterally integrating X-ray spectroscopies. The observed spatial inhomogeneity in the interface's structural, chemical, and electronic properties of samples undergoing up to 3 min of CBD treatments is indicative of a complex interface formation including an incomplete coverage and/or nonuniform composition of the buffer layer. It is expected that this result impacts solar cell performance, in particular when reducing the CdS layer thickness (e.g., in an attempt to increase the collection in the ultraviolet wavelength region). Our work provides important findings on the absorber/buffer interface formation and reveals the underlying mechanism for limitations in the reduction of the CdS thickness, even when an alkali PDT is applied to the CIGSe absorber.