Potassium Fluoride Post-deposition Treatment Research Articles

Potassium versus Sodium in Cu(In,Ga)Se2—Similarities and Differences in the Electrical Characteristics of Solar Cells and Thin Films after NaF or KF Postdeposition Treatment

The improvement of CIGSe solar cells efficiency due to potassium fluoride postdeposition treatment (KF‐PDT) is often interpreted as being caused by changes in the interface region. The impact of KF‐PDT on bulk properties of the CIGSe absorbers is investigated by measuring hole densities in the solar cells and conductivities of corresponding thin films and is then analyzed together with similar data for NaF‐treated absorbers and thin films. It is found that both alkali metals change the electrical properties of the whole CIGSe layer and not only the absorber surface. By measuring electrical conductivity and doping level in thermal equilibrium (relaxed) and metastable state (after light soaking) as well as their evolution with the amount of Na and K after postdeposition treatment (PDT) at various temperatures, one can observe effects that can only be explained by the influence of alkali metals on grain boundary properties. In contrast to Na, low concentration of potassium leads to a decrease in conductivity. Persistent photoconductivity exhibits a linear relationship with the relaxed conductivity in both NaF‐ and KF‐treated samples. These effects directly show that electrical transport in CIGSe is limited by potential barriers at the grain boundaries, which are impacted by the alkali treatment.

Analyzing the causes of limited performance improvement in cigs devices after potassium fluoride post-deposition treatment using SCAPS

Recently, potassium fluoride post-deposition treatment (KF-PDT) attracts more attention for which has beneficial effect on open circuit voltage (Voc) with the CIGS solar cell conversion efficiency increased from 20.3 % to 22.9 %. However, the short circuit current density (Jsc) and fill factor (FF) are not always increased after KF-PDT. In this study, we investigate the absorber characteristics after KF-PDT and analyze the major factors which limit the improvement of Jsc and FF with SCAPS simulation. According to the simulation results, we find that the declining of Jsc and FF are due to the high electronic barrier formed at the interface of absorbe1(KInSe2)/absorber2 after KF-PDT. However, the high electronic barrier can be lowered by increasing of ZnCu or CdCu concentration near absorber surface and by reducing the defects concentration on the buffer/absorber interface, thereby the improvement of Jsc and FF. In addition, the device performance of conduction band offset ΔAB≥0 eV at buffer/absorber interface is enhanced obviously after potassium post treatment. Therefore, increasing the ZnCu or CdCu concentration and maintaining the ΔAB≥0 eV are key points to further improve the device performance after KF-PDT.

Investigations of potassium-induced surface treatment of Cu(In,Ga)Se2 (CIGSe) thin film solar cells prepared by two-stage process using elemental selenium

The influences of alkaline doping of CIGS absorbers by a potassium fluoride post-deposition treatment (KF-PDT) were investigated systematically. Three factors that influence the CIGS solar cell performances were investigated: (i) deposition temperature, (ii) post-deposition annealing, and (iii) the thickness of the potassium fluoride (KF) layer. It is deduced that Cu vacancies were more effectively occupied by K instead of In, which prevents the formation of InCu donor defects and facilitates the subsequent Cd doping into the CIGS region. We confirmed large densities of deep defects induced by KF-PDT, leading to significant FF losses of CIGS solar cells. The potassium diffusion and redistribution during post-annealing is critical to micro-structure and crystallinity of CIGS absorber, as well as the defect properties. We pointed out the effects of surface treatment extend to the bulk of CIGS, dependent on the chemical compositions. With modified post-annealing process towards homogeneous potassium distribution and high-quality CGIS crystals, room temperature KF-PDT remains an effective strategy to enhance the performances of CIGS solar cells prepared by two-stage process.

An investigation on performance enhancement for KF post deposition treated CIGS solar cells fabricated by sputtering CIGS quaternary targets

Copper Indium Gallium Selenide (Cu(In,Ga)Se2, CIGS) absorbers were fabricated by sputtering from quaternary CIGS targets and post-selenization. Potassium (K) was doped into absorbers by potassium fluoride post deposition treatment (KF-PDT). The performances of cells and the concentration distribution of alkali elements were investigated. Recombination of CIGS cells was also detected. The introduction of K led to an increase in efficiency from 11.1% for K-free cells to 14.9% for K-doped cells, with higher open-circuit voltage (Voc). For depth profile of alkali elements in absorbers, the content of Na decreased and the content of K increased after KF-PDT. The decrease of interface recombination at CIGS/CdS interfaces is considered to be the reason for the enhanced efficiency.

Effects of combined additional indium deposition and potassium fluoride post-deposition treatments on Cu(In,Ga)Se2 thin film solar cells

The effect of additional indium on copper indium gallium selenide (CIGS) thin films and solar cells was investigated with respect to potassium fluoride post-deposition treatment (KF-PDT) using current-voltage, external quantum efficiency, scanning electron microscopy, X-ray photoelectron spectroscopy, time-resolved photoluminescence and capacitance-voltage measurements. The cell performance, particularly open-circuit voltage (Voc) improved drastically by the combined treatments of additional indium deposition after CIGS growth and subsequent KF-PDT. A Cu deficient layer at the CIGS surface increased after both treatments rather than only KF-PDT. Photoluminescence intensity, lifetime and net carrier concentration of KF-untreated CIGS solar cells did not change significantly by only additional indium deposition. However, they improved because of the combined treatments. Copyright © 2017 John Wiley & Sons, Ltd.

Potassium Fluoride Ex Situ Treatment on Both Cu-Rich and Cu-Poor CuInSe2 Thin Film Solar Cells

In this paper, we show that CuInSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIS) absorbers grown under Cu-excess have better collection efficiencies compared to Cu-poor ones. We also show that an ex situ potassium fluoride postdeposition treatment leads to an improvement in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> for CIS absorbers grown under both Cu-excess and Cu-poor conditions. Additionally, for absorbers grown under Cu-excess, the junction breakdown, which is observed in reverse bias of untreated cells, is removed. This improvement is based mainly on improving the interface of the CIS absorber grown under Cu-excess to the cadmium sulphide buffer layer through moving the dominant recombination from the interface to the bulk. In contrast to observations in the literature, the treated surface is not completely Cu-free.

In Situ and Ex Situ Investigations of KF Postdeposition Treatment Effects on CIGS Solar Cells

In situ and ex situ characterization were performed to assess the effects of potassium fluoride postdeposition treatment (KF PDT) on Cu(In,Ga)Se2 (CIGS) films and solar cells. Real-time spectroscopic ellipsometry enabled the in situ observation of the modification of the CIGS dielectric function and surface roughness thickness during KF PDT. In addition, deep-level transient spectroscopies (DLTS) and deep-level optical spectroscopies and scanning-DLTS enabled the identification of the nature and location of trap levels. Two main traps were present in both films near Ev + 0.56/0.61 eV and Ev + 0.99 eV, but with clearly lower concentrations for the latter in the KF PDT film. Finally, devices were fabricated, and higher efficiencies were found for the KF PDT devices. SCAPS modeling indicates that the traps identified by deep-level spectroscopies are in part responsible for these changes.

Effect of the KF post-deposition treatment on grain boundary properties in Cu(In, Ga)Se2 thin films

Significant power conversion efficiency improvements have recently been achieved for thin-film solar cells based on a variety of polycrystalline absorbers, including perovskites, CdTe, and Cu(In,Ga)Se2 (CIGS). The passivation of grain boundaries (GBs) through (post-deposition) treatments is a crucial step for this success. For the case of CIGS, the introduction of a potassium fluoride post-deposition treatment (KF-PDT) has boosted their power conversion efficiency to the best performance of all polycrystalline solar cells. Direct and indirect effects of potassium at the interface and interface-near region in the CIGS layer are thought to be responsible for this improvement. Here, we show that also the electronic properties of the GBs are beneficially modified by the KF-PDT. We used Kelvin probe force microscopy to study the effect of the KF-PDT on the CIGS surface by spatially resolved imaging of the surface potential. We find a clear difference for the GB electronic properties: the KF-PDT increases the band bending at GBs by about 70% and results in a narrower distribution of work function values at the GBs. This effect of the KF-PDT on the GB electronic properties is expected to contribute to the improved efficiency values observed for CIGS thin-film solar cells with KF-PDT.

Coevaporated KInSe2: A Fast Alternative to KF Postdeposition Treatment in High-Efficiency Cu(In,Ga)Se2Thin Film Solar Cells

Cu(In,Ga)Se2-based thin film solar cells have reached 22.3% energy conversion efficiency. This outstanding level of performance has been made possible by the use of a so-called potassium fluoride postdeposition treatment (KF-PDT) after the absorber synthesis. Such a treatment, consisting of evaporating KF under Se atmosphere, has been suggested to enhance the formation of a KInSe2 surface layer. In this paper, we propose an alternative to the standard KF-PDT, in which we simultaneously evaporate KF and In under Se atmosphere. We compare by X-ray photoemission spectroscopy the modified absorber surfaces in both cases and discuss the advantages of this alternative in terms of robustness and rapidity of the process.

Effect of potassium fluoride post-deposition treatment on Cu(In,Ga)Se2 thin films and solar cells fabricated onto sodalime glass substrates

In this study, we investigated the effect of potassium fluoride post-deposition treatment (KF-PDT) on Cu(In1-xGax)Se2 (CIGS) thin films and solar cells fabricated onto sodalime glass (SLG) substrates. Secondary Ion Mass Spectrometery (SIMS) analysis showed that the K increased in the entire CIGS layer after KF-PDT, however Na seemed no change because of a large amount of K and Na diffused from SLG substrates. X-ray photoelectron spectroscopy (XPS) analysis revealed that the KF-PDT depleted Cu, whereas increased K and Ga at the near surface CIGS layer, before the rinsing process. Removal of native oxides from KF-treated CIGS thin film was observed based on rinsing solutions. Ammonia aqueous solution removed Ga compounds more efficiently from KF-treated CIGS thin film than that of distilled water. Time-resolved photoluminescence (TRPL) revealed that the carrier lifetime and PL intensity greatly increased after KF-PDT, resulting in the reduction of photo-generated carrier recombination. The Voc loss (Eg/q-Voc) decreased from 0.52 to around 0.44V for KF-treated CIGS solar cells with band gap (Eg) of 1.06–1.23eV. Electron beam-induced current (EBIC) analysis indicated that the effective carrier collection of the CIGS solar cells widened toward the Mo back-contact side after KF-PDT. This phenomenon could be attributed to the increase in carrier lifetime. C-V measurements suggested that KF-PDT not only increased hole concentration but also passivated bulk defects.

Point contacts at the copper-indium-gallium-selenide interface—A theoretical outlook

For a long time, it has been assumed that recombination in the space-charge region of copper-indium-gallium-selenide (CIGS) is dominant, at least in high efficiency solar cells with low band gap. The recent developments like potassium fluoride post deposition treatment and point-contact junction may call this into question. In this work, a theoretical outlook is made using three-dimensional simulations to investigate the effect of point-contact openings through a passivation layer on CIGS solar cell performance. A large set of solar cells is modeled under different scenarios for the charged defect levels and density, radius of the openings, interface quality, and conduction band offset. The positive surface charge created by the passivation layer induces band bending and this influences the contact (CdS) properties, making it beneficial for the open circuit voltage and efficiency, and the effect is even more pronounced when coverage area is more than 95%, and also makes a positive impact on the device performance, even in the presence of a spike at CIGS/CdS heterojunction.

Investigation of the potassium fluoride post deposition treatment on the CIGSe/CdS interface using hard X-ray photoemission spectroscopy – a comparative study

The impact of the potassium fluoride post deposition treatment on CIGSe chalcopyrite absorbers is investigated by means of depth resolved hard X-ray photoemission spectroscopy of the near surface region. Two similar, slightly Cu-poor CIGSe absorbers were used with one being treated by potassium fluoride prior to the chemical bath deposition of an ultrathin CdS layer. The thickness of the CdS layer was chosen to be in the range of about 10 nm in order to allow the investigation of the CIGSe/CdS interface by the application of hard X-rays, increasing the information depth up to 30 nm. Besides strong intermixing on both samples, an increased Cu depletion of the KF treated absorber was observed in combination with an increased accumulation of Cd and S. In addition, a general shift of about 0.15 eV to higher binding energies of the CIGSe valence band at the absorber surface as well as the CIGSe and CdS related core levels was measured on the KF treated sample. This phenomenon is attributed to the impact of additional cadmium which acts as donor and releases further electrons into the conduction band of the absorber. Finally, the electrons accumulate at the CdS surface after having passed the interface region. This additional surface charge leads to a pronounced shift in the photoemission spectra as observed on the KF treated CIGSe absorber compared to the non-treated absorber.