Rubidium Fluoride Research Articles

Organic Molecule and Inorganic Salt Synergistic‐Modified SnO2 for Efficient Perovskite Solar Cells

Element doping and interface modification strategy are effective methods to regulate the electrical properties of SnO2 electron transport material, SnO2/perovskite (PVK) interface, and PVK crystal growth. Herein, rubidium fluoride (RbF) is introduced into SnO2 colloidal dispersion, and then an ultra‐thin layer of 4‐carboxy‐3‐fluorobenzoboric acid (FBCA) is applied to the SnO2 layer surface. This synergistic modification strategy can improve the electrical conductivity of the electron transport layer, increase the chemical connection of the buried interface, improve the crystallization and grain growth of PVK, and thus promote the performance and stability of devices. The results show that the PVK solar cells (PSCs) with the synergistic‐modified SnO2 electron transport material (M‐SnO2) obtain an optimum power conversion efficiency of 21.92% and the unencapsulated PSCs sustain 91% and 87% of the original value, which stored in a nitrogen atmosphere and ambient atmosphere (25 ± 5 °C, 30–50% relative humidity) more than 1000 h, respectively.

Simultaneous Defect Passivation and Electric Level Regulation with Rubidium Fluoride for High-Efficiency CsPbI2Br Perovskite Solar Cells.

Due to the good balance of efficiency and stability, CsPbI2Br perovskite solar cells (PSCs) recently have attracted widespread attention. However, the improvement in photovoltaic performance for CsPbI2Br PSCs was mainly limited by massive defects and unmatched energy levels. Surface modification is the most convenient and effective strategy to decrease defect densities of perovskite films. Herein, we deposited rubidium fluoride (RbF) onto the surface of CsPbI2Br perovskite films by spin-coating. The numerous defects could be significantly passivated by RbF, resulting in suppressed nonradiative recombination. Furthermore, the CsPbI2Br perovskite film after RbF treatment exhibits a deeper Fermi level, and an additional built-in electric field forms to promote charge transport. Consequently, the champion device achieves a high efficiency of 10.82% with an improved VOC of 1.14 V, and it also exhibits excellent stability after long-term storage. This work offers a simple and effective approach to enhance the photovoltaic performance and stability of PSCs for broader applications in the future.

Rubidium fluoride additive for high-efficiency and low-hysteresis all-inorganic CsPbI3 perovskite solar cells

All-inorganic CsPbI3 perovskite solar cells (PSCs) technology is gradually maturing because of its excellent photoelectric characteristics. However, the hysteresis phenomenon induced by ion migration in the perovskite film not only seriously affects the performance of the device, but also accelerates the degradation of the film, which limits the further improvement of power conversion efficiency (PCE) for CsPbI3 PSCs. Herein, in this paper, a new inorganic fluorine-containing additive rubidium fluoride (RbF) was introduced as a precursor additive. The incorporation of RbF effectively improved the crystallization kinetics of CsPbI3 perovskite film and effectively suppressed the occurrence of hysteresis. The defects on the CsPbI3 perovskite film are remarkably inhibited and the carrier dynamics process is greatly promoted with the incorporation of 0.03 mol% RbF. In addition, the non-radiative recombination is significantly suppressed, and the device stability is substantially improved. In particular, by doping 0.03 mol% RbF into the CsPbI3, the hysteresis index of PSCs decreases to 0.003. The introduction of RbF effectively improves the device performance, and the highest efficiency has reached to 17.21%. The environmental stability has also been significantly enhanced with the RbF doping.

Effect of Rubidium Fluoride on Grain Sintering and Optoelectronic Properties of Nanostructured CuInSe2 Thin Films Obtained by Solution Processing

Chalcopyrite CuInSe2 (CISe) and Cu(In, Ga)(S, Se)2 (CIGS) absorber layers, have emerged as promising alternatives in the solar cell field due to their unique properties such as power conversion efficiencies (PCEs) above 20 %, direct bandgap, and high absorption coefficient. This enables the making of high-quality PV devices with absorbers from 2 μm thick, significantly reducing the use of raw materials. Additionally, the CISe absorber layer is a desirable material for Perovskite/CIS tandem configuration with a narrow band gap at the bottom that has demonstrated PCEs close to 25 %, and potential applications in lightweight and/or flexible substrates. Recently, the addition of alkali elements such as sodium, potassium, rubidium, and cesium via post-deposition techniques (PDTs) has demonstrated an improvement in CIGS-based solar cells’ performance. In this study, 10, 20, and 30 nm thick layers of rubidium fluoride were post-deposited on CISe-films made by solution processing techniques and then selenized under a selenium-argon atmosphere to improve the CISe photoelectronic properties such as the number of charge carriers collected and grain growth, critical characteristics to ensure useful photovoltaic devices. Thus, the effect of rubidium fluorine on CISe-based solar cells was analyzed using several characterization techniques. According to the results, thin films made by an amine-thiol mixture with uniform atomic composition were obtained. The crystallinity and grain growth improved with an increase in rubidium fluoride addition. Moreover, with 10 nm of rubidium fluoride, an improvement in the lifetime of the charge carrier, photoluminescence intensity, and the number of carriers collected by the solar cells was obtained.

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.

Evaluating Recombination Mechanisms in RbF Treated Cu(In${}_\mathrm{x}$Ga$_\mathrm{1-x}$)Se$_{2}$ Solar Cells

Rubidium fluoride (RbF) postdeposition treatment (PDT) has been shown to improve the performance of Cu(In <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${}_\mathrm{x}$</tex-math></inline-formula> Ga <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_\mathrm{1-x}$</tex-math></inline-formula> )Se <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula> (CIGS) photovoltaic devices. In this study, temperature-dependent current voltage (JVT) and time-resolved photoluminescence (TRPL) experiments were combined with modeling using the solar cell capacitance simulator (SCAPS) computer code to investigate the effect of the RbF PDT. Two devices, one as-deposited and one with RbF PDT, were deposited by a three stage coevaporation process. JVT measurements suggest the dominant recombination mechanism may be tunneling-enhanced recombination via bandtail states, but that defect states in the bandgap can also be important. RbF PDT is shown to decrease the characteristic energy of the bandtails. TRPL data show an increase in the minority carrier lifetime after RbF PDT, leading to an improved open-circuit voltage. SCAPS modeling indicates that the dominant recombination mechanism is dependent on the specific defect makeup of a device, suggesting that small changes in processing conditions can impact device behavior. This explains the observation that, for some devices, defect states in the gap dominate while others, as is the case here, appear to be dominated by bandtails.

How small changes make a difference: Influence of low silver contents on the effect of RbF‐PDT in CIGS solar cells

AbstractDespite the efficiencies above 20% achieved with (Ag,Cu)(In,Ga)Se2 (ACIGS) solar cells, further efficiency improvements are necessary. One possibility is the implementation of a postdeposition treatment (PDT) process. The aim of this study is therefore to investigate the effect of rubidium‐fluoride (RbF)‐PDT on the performance and physical properties of the ACIGS absorber. For this purpose, the RbF source temperature of the PDT process of ACIGS films with Ag/(Ag+Cu) (AAC) ratios of 5% from a multistage co‐evaporation inline process was systematically varied. It was shown that the efficiency of the devices is reduced by the PDT process, unlike observed for Cu(In,Ga)Se2 (CIGS) absorber, and is strongly influenced by the amount of rubidium. The behavior can be attributed to a strong reduction of the doping, which results from a changed doping mechanism. Furthermore, evidence for the formation of an additional layer was found. In addition, deep level transient spectroscopy (DLTS) measurements were performed on the samples showing a strong signal at low temperatures. This minority trap signal is strongly influenced by the amount of Rb and shows a systematically changing energetic position towards the middle of the band gap and an increasing density. Based on pulse variation measurements, the associated defect could be identified as an extended defect, indicating a location of the defect at grain boundaries.

DLTS investigations on CIGS solar cells from an inline co-evaporation system with RbF post-deposition treatment

In this study, Deep Level Transient Spectroscopy (DLTS) measurements have been performed on Cu(In,Ga)Se2 (CIGS) solar cells from an inline co-evaporation system. The focus of this investigation is directed on the effect of rubidium-fluoride (RbF)-post-deposition treatment (PDT) on the defects in the CIGS absorber layer. Different traps can be identified and their properties are calculated. Herein, different methods of evaluations have been used to verify the results. Specifically, one minority trap around 400 meV was found to show a significant reduction of the trap density due to the alkali treatment. In contrast, a majority trap at approximately 600 meV is unaffected.

Synergy of mesoporous SnO2 and RbF modification for high-efficiency and stable perovskite solar cells

Electron transport layer (ETL) is very critical to the performance of perovskite solar cells (PSCs), and optimization work on ETL has received extensive attentions especially on tin oxide (SnO2) since it is an excellent ETL material widely applied in high-efficiency PSCs. Thereinto, introducing mesoporous structure and surface modification are two important approaches which are commonly applied. Herein, based on the previous work in low-temperature fabrication process of mesoporous SnO2 (m-SnO2), we introduced a modification process with rubidium fluoride (RbF) to the m-SnO2 ETL, and successfully achieved a synergy of the m-SnO2 and RbF modification: not only the shortcoming of the m-SnO2 in interfacial traps was overcome, but also the carrier collection efficiency was further improved. The PSCs based on the m-SnO2 ETL with RbF modification demonstrated outstanding performances: a champion power conversion efficiency (PCE) of 22.72% and a stability performance of maintaining 90% of the initial PCE after 300 h of MPP tracking were obtained without surface passivation of perovskite film. Hence, utilizing the abovementioned synergy is a cost-effective and feasible strategy for fabricating high-efficiency and stable PSCs since the fabrication process of the m-SnO2 ETL is a kind of low temperature process and RbF is cheap.

Study of defect properties and recombination mechanism in rubidium treated Cu(In, Ga)Se2 solar cells

Heavy alkali-metal treatment of Cu(In,Ga)Se2 (CIGSe) absorbers has been emerging as a key process for achieving over 23% high conversion efficiencies in CIGSe solar cells. Here, we investigate the effect of rubidium fluoride post-deposition treatment (RbF-PDT) on the electronic and carrier recombination properties of narrow bandgap (narrowgap) gap and wide bandgap (widegap) CIGSe solar cells using thermal admittance spectroscopy (TAS), transient photocapacitance spectroscopy (TPC), as well as time-resolved photoluminescence (TRPL). We find that the activation energy of the main capacitance step in TAS spectra of narrowgap and widegap CIGSe solar cells reduces after RbF-PDT. On the other hand, capacitance–voltage (C–V) and temperature-dependent current–voltage (IVT) measurements demonstrate that the built-in potential, as well as the activation energy Ea, increases upon RbF-PDT both for narrowgap and widegap samples, pointing to reduced interface recombination. TPC revealed an appreciable reduction of the optical response of bulk defects in the narrowgap and widegap CIGSe, suggesting improvement of bulk properties after RbF treatment. TRPL confirmed that RbF-PDT significantly reduces carrier recombination in the bulk of narrowgap and widegap CIGSe absorbers and at the surface, leading to extended carrier lifetimes. Analysis of open-circuit voltage (VOC) losses due to nonradiative recombination in the bulk of the CIGSe showed a strong correlation between enhanced carrier lifetime and improved VOC for narrow gap CIGSe cells. In contrast, although we observed a substantial decrease of VOC losses in widegap CIGSe bulk, the analysis indicated that the key to photovoltaic performance enhancement is improved interface quality.

Rubidium Fluoride Modified SnO2 for Planar n‐i‐p Perovskite Solar Cells

AbstractRegulating the electron transport layer (ETL) has been an effective way to promote the power conversion efficiency (PCE) of perovskite solar cells (PSCs) as well as suppress their hysteresis. Herein, the SnO2 ETL using a cost‐effective modification material rubidium fluoride (RbF) is modified in two methods: 1) adding RbF into SnO2 colloidal dispersion, F and Sn have a strong interaction, confirmed via X‐ray photoelectron spectra and density functional theory results, contributing to the improved electron mobility of SnO2; 2) depositing RbF at the SnO2/perovskite interface, Rb+ cations actively escape into the interstitial sites of the perovskite lattice to inhibit ions migration and reduce non‐radiative recombination, which dedicates to the improved open‐circuit voltage (Voc) for the PSCs with suppressed hysteresis. In addition, double‐sided passivated PSCs, RbF on the SnO2 surface, and p‐methoxyphenethylammonium iodide on the perovskite surface, produces an outstanding PCE of 23.38% with a Voc of 1.213 V, corresponding to an extremely small Voc deficit of 0.347 V.

Fabrication of high-efficiency Cu2(Zn,Cd)SnS4 solar cells by a rubidium fluoride assisted co-evaporation/annealing method

CCZTS solar cells with high efficiency over 10.6% are fabricated by a Rb-assisted thermal co-evaporation method.

RbF modified FTO electrode enable energy-level matching for efficient electron transport layer-free perovskite solar cells

The development of highly efficient electron transport layer free perovskite solar cells (ETL-free PSCs) with simplified and economical device configurations can significantly motivate the commercialization of PSCs. However, the performance of ETL-free PSCs has been hampered by the sluggish charge extraction and severe charge carrier recombination due to the energy-level mismatch at the interface of the perovskite and the transparent conductive electrode FTO (fluorine doped tin-oxide). In this study, this issue is well solved by modifying the FTO surface with a simple, low-cost and non-toxic rubidium fluoride (RbF) interlayer. An interfacial dipole layer is formed on the FTO surface by inserting a RbF layer, which tunes the work function of FTO, eliminates the electron transport barrier and optimizes the energy-level alignment at the FTO/perovskite interface, thereby enhancing the charge transfer and suppressing the carrier recombination. Consequently, the rigid ETL-free PSCs with RbF layer yield high efficiencies of up to 18.79%, higher than that of ETL-free devices on bare FTO (16.03%). By virtue of the low-temperature processability, a superior PCE of 15.7% has been achieved by flexible ETL-free PSCs fabricated on RbF modified plastic substrate. This study provides a simple, efficient and environmentally friendly approach to modify the FTO electrode for fabricating ETL-free PSCs, which contribute to promote the design of advanced interface materials for simplified and high-performance perovskite photovoltaics.

Partition of the Quaternary Reciprocal System Na,Rb||F,I,CrO4 and Investigation of the Stable Tetrahedron NaF–RbI–RbF–Rb2CrO4

The quaternary reciprocal system Na,Rb||F,I,CrO4 was studied, low-melting mixtures based on which are promising as fusible electrolytes for chemical current sources, heat-storage materials, and media for growing single crystals. The system was partitioned into simplexes using graph theory, and the tree of phases of the system was constructed. The tree of phases is linear and contains four stable tetrahedra sharing stable cutting triangles. The combined stable tetrahedron NaF–RbI–RbF–Rb2CrO4 was investigated by differential thermal analysis. The crystallization surface of this stable tetrahedron is represented by the volumes of sodium fluoride, rubidium iodide, rubidium fluoride, rubidium chromate, and the compound Rb3CrO4F. Monovariant equilibrium lines meet at two quaternary invariant points: eutectic E◻491 and peritectic P◻508.

RbF as a Dendrite-Inhibiting Additive in Lithium Metal Batteries.

Lithium metal is considered to be one of the most potential anode materials on account of its high theoretical specific capacity and lowermost electrochemical potential. Nevertheless, the critical challenges of dendrite growth and low Coulombic efficiency (CE) of the lithium metal anode during cycling prevent the commercial application of the lithium metal battery. Herein, rubidium fluoride (RbF) as additives is utilized to inhibit the lithium dendrite growth. Benefiting from the stronger electropositive property of Rb+, it can adsorb on the surface of protuberances during the lithium deposition process, form an electrostatic shielding field of the surface, and guide the uniform deposition of Li+. The CE can maintain above 90% with the additives of RbF in the carbonate electrolyte. Besides that, RbF improves the cycle life and decreases the polarization, a stable cycling performance of more than 1000 h is obtained, and the overvoltage is controlled below 100 mV in the cycling test of Li∥Li symmetrical cells at 0.5 mA cm-2. The addition of RbF offers a feasible way for the development of lithium metal batteries (LMBs) in the future.

Influence of RbF post deposition treatment on heterojunction and grain boundaries in high efficient (21.1%) Cu(In,Ga)Se2 solar cells

Post deposition treatments (PDT) by alkali fluorides applied to chalcopyrite-based absorbers have produced record efficiencies in thin-film solar devices in the past few years and recently the efficiency of 22.6% was achieved with Cu(In,Ga)Se2 (CIGS) using rubidium fluoride (RbF) PDT. However, the effects of RbF-PDT towards changes in its interfacial and grain boundary (GB) properties are still not fully understood. In this work, cells with efficiency higher than 21% are investigated by combination of atom probe tomography (APT) and transmission electron microscopy (TEM) to show how changes in GB and interface chemistry may facilitate high efficiencies. APT studies, carried out at the interface between CIGS absorber and solution-grown CdS buffer layer, show In enrichment and Cu depletion along with traces of Rb. Our APT studies reveal higher amounts of Rb (1.5 at. %) and lower amounts of Na and K (<0.5 at. %) at GBs as compared with previous studies (on non-PDT samples) thus indicating substitution of Na and K by Rb. However, concentration of all alkali elements inside the grain bulk is below detection limit of APT. The concentration of Rb at the GBs in CIGS is measured depth-dependent using both APT and TEM, which consistently shows the increase in Rb towards the Mo back contact. In addition, a pronounced Cu depletion is observed at the GBs which might enhance hole-barrier properties of the GBs, thus improving charge carrier collection and hence the overall efficiency of the device. Thus, understanding effects of RbF-PDT at the atomic scale provides new insights concerning the further improvement of CIGS absorber and interfaces.

Analysis of Recombination Mechanisms in RbF-Treated CIGS Solar Cells

In this paper, we studied the effect of rubidium fluoride (RbF) post-deposition treatment (PDT) on the properties of Cu(In,Ga)Se2 (CIGS) solar cells. Specifically, the recombination mechanisms were analyzed by a series of characterizations including thermal and optical defect spectroscopies, temperature dependent current density–voltage measurements, and time resolved photoluminescence. It was found that the main effect of RbF PDT on the solar cell was an increase of the open circuit-voltage, $V_{{\text{oc}}}$ , by 30 mV due to a decrease of the values of the diode quality factor and reverse saturation current. Recombination mechanisms were identified as being in the CIGS space charge region, likely at the grain boundaries and near the CIGS surface. Breakdown of contributions to the $V_{{\text{oc}}}$ increase showed that part of it is due to an increase of the majority carrier concentration (16 mV) and another to the increase in the minority carrier lifetime (1 mV). The latest is mostly due to a reduction in the EV+0.99 eV deep-level trap density. An additional CIGS surface modification (contributing 13 mV), observed by the secondary ion mass spectrometry, is essential to explain the full change in $V_{{\text{oc}}}$ .

Diffusion of Rb in polycrystalline Cu(In,Ga)Se2 layers and effect of Rb on solar cell parameters of Cu(In,Ga)Se2 thin-film solar cells

The diffusion of the heavy alkali element rubidium (Rb) in Cu(In,Ga)Se2 (CIGS) layers was investigated over a temperature range from 148 °C to 311 °C by outdiffusion from a rubidium fluoride layer. The diffusion profiles were measured by secondary ion mass spectrometry. By using CIGS layers with different grain sizes, diffusion along grain boundaries could be distinguished from diffusion into the grain interior. Rb was found to diffuse from the CIGS surface along grain boundaries but also within the grain bulk. Based on these data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation DV (Rb) = 3.8·10−8 exp(−0.44 eV/kBT) cm2 s−1 and the fast diffusion along the grain boundaries by DGB (Rb) = 5.7·10−9 exp(−0.29 eV/kBT) cm2 s−1. Further, the effect of Na on Rb diffusion was investigated by comparing Rb diffusion into a Na-containing CIGS layer in contrast to Rb diffusion into an alkali-free CIGS layer. This comparison revealed some aspects of the ion exchange mechanism. Finally, the effect of Rb on the solar cell parameters of CIGS thin-film solar cells was investigated. Rb was found to enhance the open-circuit voltage, the fill factor, and charge carrier density in a similar manner as observed for potassium and sodium.

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.

In vacuo XPS investigation of Cu(In,Ga)Se2 surface after RbF post-deposition treatment

Latest record efficiencies of Cu(In,Ga)Se2 (CIGSe) solar cells were achieved by means of a rubidium fluoride (RbF) post-deposition treatment (PDT). To understand the effect of the RbF-PDT on the surface chemistry of CIGSe and its interaction with sodium that is generally present in the CIGSe absorber, we performed an X-ray photoelectron spectroscopy (XPS) study on CIGSe thin films as-deposited by a three-stage co-evaporation process and after the consecutive RbF-PDT. The sample transfer from the deposition to the XPS analysis chamber was performed via an ultra-high vacuum transfer system. This allows to minimize air exposure, avoiding oxide formation on the CIGSe surface, especially for alkali-treated absorbers. Beside an expected reduction of Cu- and Ga-content at the surface of RbF-treated CIGSe films, we find that Rb penetrates the CIGSe and, contrary to fluorine, it is not completely removed by subsequent ammonia etching. The remaining Rb contribution at 110.0 eV binding energy, which appears after the RbF-PDT is similar to the one detected on a co-evaporated RbInSe2 reference sample and together with a new Se 3d contribution may hence belong to an Rb-In-Se secondary phase on the CIGSe surface. In addition, Na is driven towards the surface of the CIGSe absorber as a direct result of the RbF-PDT. This proves the ion-exchange mechanism in the absence of moisture and air/oxygen between heavy Rb atoms incorporated via PDT and lighter Na atoms supplied by the glass substrate. A remaining XPS signal of Na 1 s is observed after etching the vacuum transferred RbF-CIGSe sample, indicating that Rb and/or F are not as much a driving force for Na as oxygen usually is.