RbF Postdeposition Treatment Research Articles

Grain boundaries are not the source of Urbach tails in Cu(In,Ga)Se2 absorbers

The presence of Urbach tails in Cu(In,Ga)Se2 (CIGSe) absorbers has been identified as a limiting factor for the performance of the CIGSe solar cells. The tail states contribute to both radiative and non-radiative recombination processes, ultimately leading to a reduction in the open-circuit voltage and, consequently, decreasing the overall efficiency of CIGSe devices. Urbach tails result from structural and thermal disorders. The Urbach tails can be characterized by the Urbach energy, which is associated with the magnitude of the tail states. Within polycrystalline CIGSe absorbers, grain boundaries can be considered as structural disorder and, therefore, can potentially contribute to the Urbach tails. In fact, it has been proposed that the band bending at grain boundaries contribute significantly to the tail states. This study focuses on examining the correlation between Urbach tails and the band bending at the grain boundaries. The Urbach energies of the CIGSe samples are extracted from photoluminescence (PL) measurements, which reveal that the introduction of Sodium (Na) into the material can lead to a reduction in the Urbach energy, and an even further decrease can be achieved through the RbF post-deposition treatment. The band bending at the grain boundaries is investigated by Kelvin probe force microscopy measurements. A thorough statistical analysis of more than 340 grain boundaries does not show any correlation between Urbach tails and grain boundaries. We measure small band bending values at the grain boundaries, in the range of the thermal energy (26 meV at room temperature). Furthermore, our intensity dependent PL measurements indicate that Urbach tails are, at least in part, a result of electrostatic potential fluctuations. This supports the model that the introduction of alkali elements mainly decreases the magnitude of electrostatic potential fluctuations, resulting in a subsequent reduction in the Urbach energy.

High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency

Chalcopyrite-based solar cells have reached an efficiency of 23.35%, yet further improvements have been challenging. Here we present a 23.64% certified efficiency for a (Ag,Cu)(In,Ga)Se2 solar cell, achieved through the implementation of a series of strategies. We introduce a relatively high amount of silver ([Ag]/([Ag] + [Cu]) = 0.19) into the absorber and implement a ‘hockey stick’-like gallium profile with a high concentration of Ga close to the molybdenum back contact and a lower, constant concentration in the region closer to the CdS buffer layer. This kind of elemental profile minimizes lateral and in-depth bandgap fluctuations, reducing losses in open-circuit voltage. In addition, the resulting bandgap energy is close to the local optimum of 1.15 eV. We apply a RbF post-deposition treatment that leads to the formation of a Rb–In–Se phase, probably RbInSe2, passivating the absorber surface. Finally, we discuss future research directions to reach 25% efficiency.

Impact of additives on the chemical bath deposition of Zn(O,S) used as buffer layer in high-efficiency Cu(In,Ga)Se2-based solar cells

Cu(In,Ga)Se2-based solar cells achieve highest efficiencies up to 23.4% by using solution-grown Zn(O,S) as the buffer layer. State-of-the-art procedures for Zn(O,S) deposition based on the ammonia-thiourea approach are still characterized by a low growth rate and a high material consumption of reactants. One of the approaches to improve the Zn(O,S) deposition rate is the implementation of compounds leading to a much faster thiourea decomposition and thus to a higher availability of sulfide anions. In this work, we test the impact of oxidative and reductive additives, namely sodium peroxydisulfate Na2S2O8 and hydroxylamine NH2OH, respectively. The Zn(O,S) growth rate could be significantly increased and the consumption of reactants drastically reduced without compromising the performance of fabricated solar cell devices. The composition of the layers was quantified by time-of-flight secondary ion mass spectrometry measurements and the [S]/([S]+[O]) molar ratio was found to vary between 0.65 and 0.85 depending on the thiourea and additive concentration. For cell preparation, in-line co-evaporated Cu(In,Ga)Se2 with RbF post-deposition treatment was used and conversion efficiencies up to 19%, comparable to reference cells with a CdS buffer, could be realized. We discuss the reaction mechanism and conclude for both additives that the resulting sulfide concentration increase evolves via formation of N-hydroxyguanidine intermediates.

Setting boundaries on the recipe for a successful RbF post-deposition treatment of CIGS

RbF post-deposition treatments have been explored in the literature for increasing the open-circuit voltage, fill factor, and hence the efficiency of Cu(In,Ga)Se 2 solar cells. However, given the few papers documenting the experimental steps, it was difficult to quickly reproduce the results. This contribution describes some of the optimization steps that led to a successful RbF PDT based on device performance. We present results that put boundaries on the temperatures of the RbF cell and the lamp (for sample heating) setpoint. The best recipe for our specific growth process is documented in detail so that others may copy the procedure and possibly arrive at a successful RbF PDT in a reasonable time. • RbF post-deposition treatment of CIGS was more difficult to optimize than KF post-deposition treatment. • The importance of RbF PDT parameters from most to least is RbF cell temperature, substrate temperature, and Se flux. • Successful RbF PDTs were achieved with RbF cell temperature from 505 °C through 555 °C with the best at 540 °C. • Lamp setpoint temperature from 400 °C through 540 °C work for RbF PDTs. • Lists detailed recipe for NREL’s best RbF post-deposition treatment.

Rubidium Fluoride Absorber Treatment for Wide‐Gap (Ag,Cu)(In,Ga)Se2Solar Cells

This contribution studies the potential of an RbF postdeposition treatment (RbF‐PDT) of wide‐gap (Ag,Cu)(In,Ga)Se2(ACIGS) absorbers to improve the corresponding solar cell performance. While a higher open‐circuit voltage (VOC) and short‐circuit current density are achieved, a lower fill factor (FF) is observed for most of the devices subjected to an RbF‐PDT. However, the drop in FF can be avoided for some close‐stoichiometric samples, leading to maximum efficiencies beyond 16% (without antireflection coating) at a bandgap energy (Eg) of 1.43 eV. For off‐stoichiometric ACIGS, a recordVOCvalue of 926 mV atEg = 1.44 eV is reached. LowerVOCdeficits likely require enhanced bulk quality of wide‐gap chalcopyrite absorbers. Extensive material analysis shows that the heavy alkali PDT of ACIGS with high Ag and Ga contents leads to similar absorber modifications as commonly observed for low‐gap Cu(In,Ga)Se2(CIGS). Rubidium is continuously distributed at “internal” (grain boundaries) and “external” (buffer and back contact) absorber interfaces. The results indicate that Rb diffusion into the absorber bulk (including 1:1:2 and 1:3:5 compounds) is restricted. Furthermore, the formation of a very thin RbInSe2surface layer is suggested. It remains open, which effects alter the device characteristics after RbF‐PDT.

Detrimental Impact of Na Upon Rb Postdeposition Treatments of Cu(In,Ga)Se2 Absorber Layers

Postdeposition Treatment In article number 2100390, Jessica de Wild and co-workers performed a RbF postdeposition treatment on AlOx back-passivated Cu(In,Ga)Se2 solar cells. It is found that Na present at the AlOx/Mo back contact inhibits Rb diffusion into the absorber layer. A detrimental redistribution of the alkali follows, leading to an injection barrier at the front and reduction of the acceptor concentration in the bulk.

Impact of RbF post deposition treatment on CdS/CIGSe and Zn(O,S)/CIGSe interfaces – A comparative HAXPES study

In conventional Cu(In,Ga)Se2 (CIGSe) solar cells a chemical bath deposited CdS thin film is used as a buffer layer. However, it is desired to replace CdS due to the toxicity of cadmium and the rather narrow bandgap energy of CdS. Zn(O,S) is considered to be one of the most attractive candidates as an alternative, non-toxic buffer layer with a larger bandgap. This paper aims to compare the properties of the CdS/CIGSe and the Zn(O,S)/CIGSe interfaces depending on the absorber composition and the application of an RbF post deposition treatment (PDT). Synchrotron-based hard X-ray photoelectron spectroscopy revealed a strong correlation of Cd diffusion and concentration of VCu in CIGSe before the PDT. Additionally, the RbF-PDT enhanced the Cd diffusion into the CIGSe. On the other hand, it was found that Zn atoms are not as easily incorporated into the CIGSe as Cd atoms. As a result, we consider the formation of donor-like ZnCu+ defects at the interface to be less likely than the formation of CdCu+ defects. Moreover, we observed that the Zn(O,S)/CIGSe interface is less sensitive to changes of the CIGSe composition and to the RbF-PDT compared to the CdS/CIGSe interface.

Detrimental Impact of Na Upon Rb Postdeposition Treatments of Cu(In,Ga)Se2 Absorber Layers

Passivation of the Cu(In,Ga)Se2 (CIGS)/Mo back contact using AlO x is studied to reduce the recombination at this interface. Herein, RbF postdeposition treatment (RbF‐PDT), a well‐established method to improve absorber and front interface properties is used on back‐passivated solar cells. It is found that this combination deteriorates the performance due to formation of an injection barrier at the front and reduced acceptor concentration. Photoluminescence yield and decay times show no indication of increased defect recombination, as both are improved. With time‐of‐flight secondary ion mass spectroscopy, in‐depth and lateral alkali profiles are measured. It is shown that the Na concentration is higher at the AlO x /Mo back contact and that Rb accumulates at the CdS/CIGS interface. It is hypothesized that Na at the back is released during the RbF‐PDT and inhibits Rb diffusion into the CIGS layer. Rb remains at the front and acceptor concentration is reduced. Modeling of dark and light current–voltage characteristics shows that the injection barrier and low doping are responsible for the reduced V oc and fill factor (FF). It is suggested that the commonly observed FF losses upon heavier alkali PDT can be eliminated by adapting the initial Na amount.

Effects of Sputtered InxSy Buffer on CIGS with RbF Post-Deposition Treatment

Indium sulfide (InxSy) is an attractive candidate to be used as a buffer layer for Cu(In,Ga)Se2 (CIGS) thin-film solar cells and modules. InxSy sputtering would be the preferred deposition method of choice as it allows for much higher growth rates compared to all other current deposition methods. A key feature of CIGS absorber processing, established during the last years, is the use of a post-deposition treatment (PDT) with alkali compounds. In this work, we apply rf-magnetron sputtered InxSy buffers from ceramic targets, optionally doped with NaF, on industrially relevant CIGS absorbers with or without RbF-PDT prepared in an in-line coater. We analyze the influence of wet chemical treatments of the CIGS surface after RbF-PDT and alkali accumulation at the InxSy/CIGS interface on the solar cell performance. Good efficiencies could be achieved with sputtered InxSy on CIGS with RbF in an all-dry process. An InxSy deposition temperature around 200 °C and/or a post-annealing step of the complete cell in air emerged as a key feature for decent efficiencies. This is also due for sputtering from a NaF-doped In2S3 target. Our best cell with sputtered InxSy on CIGS with RbF-PDT shows an efficiency of 18.0% with a high open-circuit voltage of 701 mV.

Depth-resolved analysis of the effect of RbF post deposition treatment on CIGSe with two different Cu concentrations

Photovoltaic devices based on Cu(In,Ga)Se2 (CIGSe) absorbers are among the most attractive non-Si alternatives. The key to their steadily increasing efficiency is a post-deposition treatment (PDT) with alkali salts. For co-evaporated CIGSe, a RbF-PDT was demonstrated as the most efficient, however, the mechanism of the RbF influence on the CIGSe absorber is not completely understood. Here, we focus on the impact of RbF on the surface of co-evaporated CIGSe absorbers in dependence on their bulk composition. Surface-as well as bulk-sensitive methods with overlapping information depths are used to examine an overall depth profile of RbF-free and RbF-treated CIGSe samples with different Cu contents. We show a gradual depletion of copper towards the surface for the as-deposited CIGSe absorber layers. The following RbF-PDT sharpens this effect especially for the sample grown with the overall lower Cu content. Under the Cu-depleted surface layer, the composition of the CIGSe gradually changes until it reaches the respective bulk composition. As a result of the RbF-PDT, Ga diffuses towards the surface and Rb gets incorporated at the surface where GaF3 and RbInSe2 secondary phases are formed, respectively. A higher Cu content leads to less surface oriented Ga diffusion, less Rb incorporation and to a thinner RbInSe2 layer. A thinner RbInSe2 barrier layer, in turn, maintains the gain in open-circuit voltage and prevents the fill factor loss of the CIGSe device.

Cross-sectional Kelvin probe force microscopy on Cu(In,Ga)Se2 solar cells: Influence of RbF and KF post-deposition treatment on the surface potential of the absorber layer

Post-deposition treatment (PDT) of the absorber layer with alkali fluorides has led to a significant increase in the efficiency of Cu(In,Ga)Se2 (CIGS) thin-film solar cells. In this contribution, we investigate the influence of alkali PDTs on the absorber's surface potential by means of Kelvin probe force microscopy (KPFM). To this end, we perform KPFM on cross sections of complete CIGS solar cells. To improve the reliability of the measurement procedure, we deposit a gold layer on top of the solar cell as a reference layer. Using this approach, we study the influence of RbF and KF PDT on the absorber's surface potential for CIGS solar cells with different absorber and buffer compositions. In all cases, an increased surface potential of the cross section of the absorber layer is measured for the cells with PDT.

Failure and Recovery Modes of Submicron Cu(In,Ga)Se2 Solar Cells with High Cu Content.

As an emerging route to further lower the production cost, reducing the thickness of Cu(In,Ga)Se2 (CIGS) absorber has drawn substantial attention and has been intensively studied in recent years. However, thickness-induced change still limits the device performance of thin CIGS solar cells. Herein, by examining a series of submicron CIGS solar cells with varied Cu content through their photovoltaic (PV) performance, the optimal Cu content in these submicron CIGS devices is found to be lower than that of the normal thickness CIGS devices. Electrical and compositional characterizations reveal that reduced thickness makes absorber vulnerable to the shunt paths formed with high Cu content. By intentionally lowering the Cu content in submicron CIGS, shunt resistance (Rsh) of the devices is significantly improved and therefore a high fill factor (FF) is achieved. Moreover, RbF postdeposition treatment (PDT) can passivate the shunt paths in the high Cu content samples to a very good extent, manifesting by the significantly improved FF.

Electrostatic potential fluctuations and light‐soaking effects in Cu(In,Ga)Se2 solar cells

AbstractIn Cu(In,Ga)Se2 (CIGS) thin‐film solar cells, laterally inhomogeneous distributions of point defects may induce electrostatic potential fluctuations and thus reduce the open‐circuit voltage (Voc). In the present work, we investigate possible origins of fluctuating potentials and estimate the amplitude of fluctuations and Voc losses in solar cells with various [Ga] in the CIGS absorber, with different buffer layers and with different durations of an RbF postdeposition treatment (PDT). Electron‐beam‐induced current measurements were employed to study the local difference in the width of the space‐charge region (wSCR). It is shown that the amplitude of fluctuations in the wSCR depends significantly on the choice of buffer system and on the duration of the RbF PDT. In addition, energy‐dispersive X‐ray spectroscopy and cathodoluminescence measurements reveal that band‐gap fluctuations do not have substantial impact on the device performance. Finally, some of the investigated cells were exposed to light soaking, which was found to be a means to reduce the detected electrostatic potential fluctuations and also to increase the effective electron diffusion length in the CIGS absorber for a part of the investigated cells.

Effectiveness of an RbF Post Deposition Treatment of CIGS Solar Cells in Dependence on the Cu Content of the Absorber Layer

In this contribution, the effectiveness of an RbF post deposition treatment (PDT) is evaluated in dependence on the Cu content of the absorber layer of Cu(In,Ga)Se $_{2}$ solar cells. It is shown that the PDT only acts beneficially on the open-circuit voltage and fill factor ( FF ) on samples with rather high Cu content, while it deteriorates all parameters of the solar cells in samples with low Cu content. In order to clarify the behavior of the open-circuit voltage, the well-known exchange mechanism of Rb and Na during the PDT is analyzed as a function of the Cu content of the absorber layers and discussed in regard to theoretical publications. Furthermore, a model explaining the observed effect on the FF based on the formation of an RbInSe $_{2}$ (RIS) layer during the RbF-PDT is proposed. The model supposes that the RIS layer acts as a barrier for the photocurrent and therefore lowers the FF . It is evaluated theoretically in dependence of the properties of the RIS layer using one-dimensional solar cell capacitance simulator (SCAPS) simulations. Finally, the proposed model is also tested and confirmed experimentally by directly depositing RIS onto untreated Cu(In,Ga)Se $_{2}$ layers.

Efficiency Improvement of Near‐Stoichiometric CuInSe2 Solar Cells for Application in Tandem Devices

AbstractState‐of‐the‐art Cu(In,Ga)Se2 (CIGS) solar cells are grown with considerably substoichiometric Cu concentrations. The resulting defects, as well as potential improvements through increasing the Cu concentration, have been known in the field for many years. However, so far, cells with high Cu concentrations show decreased photovoltaic parameters. In this work, it is shown that RbF postdeposition treatment of CuInSe2 solar cells allows for capturing the benefits from the improved absorber quality with increasing Cu content. A reduced defect density and an increased doping level for cells with high Cu concentrations close to stoichiometry are demonstrated. Implementing a high mobility front transparent conductive oxide (TCO), the improved absorbers with 1.00 eV bandgap yield a solar cell efficiency of 19.2%, and combined with a perovskite top cell a 4‐terminal tandem efficiency of 25.0% are demonstrated, surpassing the record efficiency of both subcell technologies.

Diffusion of Alkali Metals in Polycrystalline CuInSe2 and Their Role in the Passivation of Grain Boundaries.

The behavior of alkali atom point defects in polycrystalline CuInSe2 is studied. In this work, three grain boundary models, one coherent twin boundary and two twin boundaries with dislocation cores, are considered. Total energy calculations show that all alkali metals tend to segregate at the grain boundaries. In addition, the segregation of alkali atoms is more pronounced at the grain boundaries with the dislocation cores. The diffusion of alkali metals along and near grain boundaries is studied as well. The results show that the diffusion of alkali atoms in the grain boundary models is faster than within the bulk. In addition, the ion exchange between Na and Rb atoms at the grain boundaries leads to the Rb enrichment at the grain boundaries and the increase of the Na concentration in the bulk. While the effects of Na and Rb point defects on the electronic structure of the grain boundary with the anion-core dislocation are similar, Rb atoms passivate the grain boundary with the cation-core dislocation more effectively than Na. This can explain the further improvement of the solar cell performance after the RbF-postdeposition treatment.

RbF post deposition treatment for narrow bandgap Cu(In,Ga)Se2 solar cells

Multi-junction solar cells are known to have a considerably increased efficiency potential over their typical single junction counterparts. In order to produce low cost and lightweight multi-junction devices, the availability of suitable narrow (<1.1 eV) bandgap bottom cells is paramount. A possible absorber for such a bottom cell is the Cu(In,Ga)Se2 (CIGS) compound semiconductor, one of the most efficient thin film materials to date.In this contribution we report on the RbF post deposition treatment of narrow bandgap CIGS absorbers grown with a single bandgap grading approach. We discuss the necessary deposition conditions and the observed improvements on solar cells performance. A certified record efficiency of 18.0% for an absorber with 1.00 eV optoelectronic bandgap is presented and its suitability for perovskite/CIGS tandem devices is shown.

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.

Direct insights into RbInSe2 formation at Cu(In,Ga)Se2 thin film surface with RbF postdeposition treatment

Scanning transmission electron microscopy-energy dispersive X-ray spectroscopy measurements performed on Cu(In,Ga)Se2 (CIGS) thin-film devices grown with RbF postdeposition treatment (RbF-PDT) revealed the formation of a RbInSe2 compound at the p-CIGS/n-CdS heterointerface. However, this type of Rb-compound formation was not observed with elemental In-free ternary CuGaSe2 (CGS) devices. The film surface of CIGS grown with RbF-PDT was found to be Ga-depleted; thus, the practical interface structure turned out to be CIGS/(CuInSe2, RbInSe2)/CdS. In contrast to the significant improvements observed in In-containing CIGS photovoltaic device performance with RbF-PDT, no significant improvements have been observed in In-free CGS devices thus far. These results suggest that the presence of elemental In plays a key role in obtaining beneficial alkali Rb effects for enhancing device performance as well as surface modification with RbF-PDT.

Effects of RbF postdeposition treatment and heat-light soaking on the metastable acceptor activation of CuInSe2 thin film photovoltaic devices

Granular particles formed on a CuInSe2 (CIS) thin film surface with alkali-halide RbF post-deposition treatment (RbF-PDT) are found to concentrate preferably on other than smooth (112)-planes present on the CIS film surface. As a consequence, a large number of distinctive pores are selectively formed at relatively rough facets. Although the open circuit voltage and fill factor values of CIS solar cells improve with either RbF-PDT or heat-light soaking (HLS) treatment, the effects of these treatments are distinguishable. The ratio of carrier recombination rates at the interface (Ri) to that in the bulk (Rb), namely, Ri/Rb, decreases with RbF-PDT. HLS treatment performed on RbF-PDT CIS devices leads to a further decrease in the value of Ri/Rb, whereas the value of Ri/Rb of CIS devices fabricated without RbF-PDT increases with HLS treatment. Ternary CIS solar cells grown with RbF-PDT and HLS treatment reproducibly demonstrate enhanced photovoltaic efficiencies.