Features of KF and NaF Postdeposition Treatments of Cu(In,Ga)Se2

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Chemistry of Materials

Features of KF and NaF post-deposition treatments of Cu(In,Ga)Se2 absorbers for high efficiency thin film solar cells Patrick Reinhard+*, Benjamin Bissig+, Fabian Pianezzi+, Enrico Avancini+, Harald Hagendorfer+, Debora Keller+†, Peter Fuchs+, Max Döbeli‡, Carlos Vigo°, Paolo Crivelli°, Shiro Nishiwaki+, Stephan Buecheler+, and Ayodhya N. Tiwari+*. +

Laboratory for Thin Films and Photovoltaics, Empa – Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland.

†

Electron Microscopy Center, Empa - Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstrasse 129, CH-8600 Duebendorf, Switzerland.

‡

Ion Beam Physics, ETH Zürich – Swiss Federal Institute of Technology, Otto-Stern-Weg 5, CH-8093 Zürich, Switzerland. ° Laboratory of Positron and Positronium Physics, ETH Zürich – Swiss Federal Institute of Technology, Otto-SternWeg 5, CH-8093 Zürich, Switzerland. ABSTRACT: The introduction of a KF post-deposition treatment (KF PDT) of Cu(In,Ga)Se2 (CIGS) thin films has led to the achievement of several consecutive new world record efficiencies up to 21.7% for the CIGS solar cell technology. The beneficial effect of the KF PDT on the photovoltaic parameters was observed by several groups in spite of differing growth methods of the CIGS layer. For CIGS evaporated at lower temperature on alkali-free, flexible plastic substrates, a postdeposition treatment to add Na was already successfully applied. However with the introduction of additional KF under comparable conditions, distinctly different influences on the final absorber alkali content as well as surface properties are observed. In this work we discuss in more details the intrinsically different role of both alkali-treatments by combining several microstructural and compositional analysis methods. The ion exchange of Na by K in the bulk of the absorber is carefully analyzed, and further evidences for the formation of a K-containing layer on the CIGS surface with increased surface reactivity are given. These results shall serve as a basis for the further understanding of the effects of alkali PDT on CIGS and help identifying research needs to achieve even higher efficiencies.

INTRODUCTION Thin film solar cells based on the polycrystalline p-type Cu(In,Ga)Se2 (CIGS) absorber material are a good candidate for manufacturing of low-cost photovoltaics due to their inherent nature of large area coating and achieved efficiency beyond 20% on a laboratory scale.1 The possibility of being deposited onto a flexible substrate is an additional advantage of thin film technology, but faces challenges due to either temperature limitation imposed by the substrate or diffusion of impurities during processing that can limit the achievable efficiency.2 The absorbers used in record efficiency CIGS solar cells are typically grown by co-evaporation of the individual elements onto a heated (>600°C) glass substrate,3-5 or by sputtering of metallic precursors followed by selenization and sulfurization at high temperature.6 Recently we showed that it is also possible to achieve efficiency above 20% on a flexible

plastic substrate even when using a lower temperature (< 450°C) co-evaporation process.7 A key aspect of such improvement is the development of a new potassiumbased treatment of the CIGS layer. The latter allowed to achieve a 20.4% efficient flexible device, which was the highest reported value for the CIGS technology at that time.8 Several other groups were then able to improve this value up to current record of 21.7% by adopting a similar approach for absorbers deposited on glass.9 Addition of alkali elements to the CIGS material is beneficial for the cell performance, mainly due to an increase of the p-type conductivity and passivation of compensating defects.10-12 Generally, Na was believed to be the most beneficial element, and diffuses naturally from the sodalime glass substrate during high-temperature growth.13 In spite of many studies there is still no clear consensus on the exact role and chemical environment of Na in CIGS,

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and whether the presence of the Na atom itself is beneficial (by occupying for example compensating defects lattice sites14) or if the presence of Na catalyzes other reactions such as oxidation of Se vacancies.12 Na amounts of about 0.1 at.% are typically reported for absorbers grown on glass,15 with Na atoms found mainly at grain boundaries regions.16-17 On alkali-free substrates Na can be added after the CIGS growth is finished by diffusing it from evaporated NaF precursor salts in a so-called NaF post-deposition treatment (PDT).18 It triggers similar effects on the solar cell performance as when Na is present from the beginning, but without any influence on the interdiffusion of the CIGS elements during growth. This is especially advantageous for depositions on plastic substrates where interdiffusion is already hindered by the lower substrate temperature. An extension of the NaF PDT by evaporating KF along with Se after the NaF PDT, as schematically shown in figure 1, was recently reported.7 Whereas the addition of K instead of Na before the absorber growth is not as efficient in improving the electronic properties of the CIGS film,19 it was found that NaF PDT and KF PDT lead to distinctly different effects onto the absorber bulk and surface properties.7, 20-21 The contribution by Chirila et al.7 described first aspects of these differences, and how they were used to improve the junction quality, reduce the parasitic absorption losses with a thinner CdS layer, and ultimately facilitated the processing of the 20.4% record efficiency solar cell. Removal of the Na ions from the absorber film was observed after KF PDT, whereas no significant change of crystal structure was seen.9 Moreover a modified absorber surface composition due to the KF PDT was reported, with a Cu and Ga depleted surface region containing significant amounts of K. This modified surface can be combined with a thinner CdS buffer layer without losses in the PV parameters usually observed if no KF PDT is applied. A model based on the inversion of the CIGS surface region due to facilitated Cd in-diffusion during chemical bath deposition was given to explain this finding.7 It was found that the KF PDT reduces recombination at the CdS/CIGS interface much more efficiently than the NaF PDT.20 Additionally, the possibility to nanostructure a surface layer on top of the CIGS via alkali-templated patterning method was recently discussed.21 In spite of the similar alkali nature of NaF and KF both alkali elements show distinct effects on CIGS surface and bulk properties. This observation opens up a new path in the CIGS research, and further efforts in understanding and characterizing these differences are ultimately very likely to lead to further improvements of the cell efficiency well beyond 22%. The KF PDT is not only beneficial for CIGS grown at low-temperature,7 but also for CIGS grown at high-temperature on glass,3,4 or grown by sputtering.22 The efficiency enhancements due to KF PDT are observed on record efficiency devices and allow a wider process window for compositional variations (i. e. higher Ga con-

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tent23). Therefore, the effect of KF PDT at play in all those devices is likely due to the mitigation of common limitations found in all those types of CIGS materials. In this study, we build up and extend on precedent contributions7, 20-21 and discuss more detailed microstructural information comparing the influence of NaF and KF PDT on CIGS thin films grown at low temperature. Several material characterization methods are utilized in order to give an overview of phenomenon observed on the surface and in the bulk of the absorber, as well as to identify further necessary research directions. More specifically, the amount of each alkali elements on the absorber and in the final device are quantified, their distribution within the film is assessed and the resulting influence on the junction formation is discussed.

FIGURE 1. Schematic drawing of the NaF PDT and the NaF&KF PDT applied on low-temperature co-evaporated CIGS thin films.

EXPERIMENTAL SECTION CIGS growth and solar cell processing. CIGS films with a thickness of 2.3-2.5 μm are deposited onto Mocoated polyimide foils by a multi-stage co-evaporation process in high vacuum at a maximum substrate temperature below 450°C.24 Mo serves as quasi-ohmic back contact in the finished solar cell due to the formation of a MoSe2 interface layer during CIGS growth. When the CIGS growth is finished, the substrate temperature is decreased to ca. 350°C and alkali-fluoride are evaporated along with Se onto the thin film. In this study, absorbers were either directly cooled down to room temperature (no PDT), subjected to NaF or KF flux only (NaF PDT or KF PDT), or to NaF followed by KF (NaF&KF PDT). About 15-25 nm of each alkali precursor was added at evaporation rates of 1-2 nm/min. In order to remove the fluorides remnants from the surface and allow characterization of the clean CIGS surface, absorbers are washed for 2 min in a diluted ammonia solution with similar concentration as that used in the CdS chemical bath. Solar cells are finished by depositing a CdS layer according to the recipe given in Ref. 7 for 15 minutes, followed by RF-sputtering of a 50 nm i-ZnO resistive layer and a 200 nm ZnO:Al window layer. Ni/Al/Ni grids (50/4000/30 nm) and a 100 nm MgF2 anti-reflective coating are added. Scanning electron microscopy (SEM). The surface morphology of Pt-coated samples was characterized in a Hitachi S-4800 SEM using in-lens detector, 5 keV acceleration voltage and a working distance of 4 mm.

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X-ray photoelectron spectroscopy (XPS). Measurements were performed in a Quantum 2000 from Physical electronics with a monochromatized Al Kα source (1486.6 eV). Transfer time of sample between deposition chamber and XPS chamber was minimized by processing the samples in subsequent runs immediately before the measurements were made. Spectra were recorded in a fixed analyzer transmission mode, at a base pressure below 5·109 mbar and under an angle of emission of 45°. Pass energy of 117.4 eV and 93.9 eV was used for survey spectra, and detailed peak spectra, respectively. Survey spectra were recorded for 10 min with an energy step size of 0.5 eV/step. Detailed peak spectra were recorded using an energy step size of 0.2 eV. Surface charging was compensated. Inductively coupled plasma mass spectrometry (ICP-MS). For quantitative elemental analysis of the CIGS layers and solar cells, material was detached from 1 to 3 cm2 pieces at the Mo/CIGS interface and immediately dissolved in a solution of 10 ml HNO3 and 1 ml HCl. The solution was diluted to 50 ml with deionized water and analyzed using an Agilent 7500 Ce ICPMS with external calibration using certified metal standards (1000 µg/ml, Merck CertIPUR). Good quality of the measurements was confirmed by analysis of reference materials (NIST SRM 1643e) and spiking experiments with recoveries of 90 to 110%. Elastic recoil detection analysis (ERDA). Samples were characterized by use of elastic recoil detection analysis25 at the PSI/ETH Laboratory for Ion Beam Physics (ETH Zürich, Switzerland). A 13 MeV 127I projectile beam was used under 18° incidence angle and scattered recoils were identified by the combination of a time-of-flight spectrometer with a gas ionization chamber.26 Secondary ion mass spectrometry (SIMS). A time-offlight SIMS5 system from ION-TOF was used to obtain compositional depth profiling data. Primary ions were Bi+ and only positive ions were detected. Oxygen ions (O2+) with 2 keV ion energy were used to sputter a 300 x 300 μm2 area with an ion current of 400 nA. Area of 100 x 100 μm2 was analyzed with Bi+ ions with 25 keV ion energy and with an ion current of 1 pA. Positron annihilation spectroscopy (PAS). PAS measurements were conducted at the ETH slow positron beam (ETH Zürich, Switzerland).27 Positrons from a 22Na source are moderated with a solid noble gas and magnetically guided towards the sample. They are implanted with an energy that can be tuned in the range 0.2-20 keV, corresponding to a mean implantation depth of 40 to 900 nm in CIGS. The annihilation photons are then detected with a high-resolution Germanium detector. Solar cell characterization. Current density-voltage (J-V) characteristics of the solar cells were acquired under simulated standard test conditions (25°C, 1000 W/m2, AM1.5G illumination) in a sun-simulator with a Xe light source and using a four-terminal sensing Keithley 2400

source meter. Peltier cooling was used to control the cell temperature. The reference cell was a monocrystallline Si solar cell from Fraunhofer ISE. RESULTS AND DISCUSSION CIGS film morphology and composition. A direct observation of the differences between NaF and KF PDT is a color change of the surface that is only observed if KF is evaporated. Figure 2a exemplifies this difference in color, showing a photograph of two CIGS layers subjected to NaF&KF PDT or only NaF PDT, along with the corresponding total reflectance measurements. Absorbers without alkali (no PDT) and absorbers with only NaF PDT have the same optical appearance, which is modified as soon as KF is added. This change is not due to a significantly different roughness, as measured by atomic force microscopy (not shown), nor to different oxidation behavior in air, because this difference is already visible in the high vacuum chamber. The presence of alkali remnants on the surface does not account for this difference, because it is still present even after dissolving them from the surface in a diluted ammonia solution, as shown in figure 2b. At this point, the differences in reflection after KF PDT can possibly be attributed to the presence of a surface layer with altered chemical composition,21 therefore altered physical properties.

FIGURE 2. Optical comparison of absorbers subjected to different PDT. (a) Simultaneous photograph of 2 CIGS layers on Mo-coated polyimide foils with NaF&KF PDT and NaF 2 PDT only (Both samples are spanned in 5 x 5 cm Mo frames to keep them flat for easier handling), and the corresponding total reflectance measurement; (b) Total reflectance after washing in diluted ammonia.

Figure 3 shows selected regions from XPS survey spectra corresponding to Na, F, and K peaks on samples with NaF PDT or with KF PDT before and after washing in ammonia. The curves were shifted accordingly for the sake of clarity. It was discussed in ref. 7 that KF PDT only or combined with NaF PDT (NaF&KF) has a similar influence on the CIGS surface properties. For the NaF PDT sample high intensity peak of Na and F are observed,

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whereas all peaks corresponding to the CIGS elements are strongly reduced (not shown). After washing the Na and F peaks completely disappear. In case of the KF PDT sample, K and F peaks have a high intensity, whereas the peaks of the CIGS elements are also very weak, as observed in the case of NaF PDT. After washing the F peak disappears. However not all K is dissolved and a K peak is still distinctly present. Due to the good solubility of NaF (4.13g/(100g H2O)) and KF (102g/(100g H2O)) in aqueous solution28 and the disappearance of the F peak, we can expect that the potassium remaining on the surface is not present as KF and rather forms a compound or binds at an adsorption site during the KF PDT. As reported in ref. 7, a significant Cu and Ga depletion is observed after KF PDT, whereas In and Se peak intensity remains high. Additionally it was also reported that potassium is still present in the interface region even after the CdS deposition, proving that K exists in a form that is not easily soluble in the warm (70°C) ammonia solution used for CdS deposition even after 15 minutes. At the moment it is not possible to clearly conclude on the chemical environment of K in this region by for example XPS peak position analysis. The shape of the K 2p1/2 and K 2p3/2 peaks do change before and after washing, likely due to the overlap with the Se Auger peak after washing. To relate this change with the formation of different K-containing materials on the CIGS surface requires additional analysis. Depthresolved synchrotron-based analysis29 is required to characterize the chemical composition of the CIGS/CdS interface while excluding influence of air exposure or sputtering damage. Scanning electron microscopy (SEM) top view micrographs given in figure 3 support the disappearance of most of the fluoride remnants from the surface. After NaF PDT or KF PDT the surface is covered with a large number of cubic-shaped salt crystals. After washing in diluted ammonia the surface of the NaF PDT sample is very similar to that of a CIGS layer not subjected to any PDT, showing typical facetted CIGS grains with some triangular facets assigned to the 112 crystal orientation.30 On the other hand, the surface of the KF PDT sample after dissolving the salt crystals is conformally covered with a nanostructured layer, as discussed thoroughly in ref. 21.

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uncertainty. Any higher value in the other samples can then be seen as significant amount that is purposely introduced. Na concentration in the final devices is however very close to the detection limit. The NaF PDT sample shows that about 1 at.% Na is deposited, but that more than 90% is removed by the washing step and that no further decrease is measurable after CdS deposition. This means that the majority of the evaporated NaF remains on the surface of an as-deposited absorber and is then dissolved. This agrees well with the presence of alkali salts visible in the SEM micrographs in figure 3. Final Na content in the device is 0.07 (±0.04) at%, which is in the same order of magnitude as the 0.1 at% usually measured in CIGS films deposited on glass at high temperature.15 In the NaF&KF PDT sample, similar amount of Na is present directly after growth as in the case of NaF PDT and about 0.5(±0.01) at.% K is deposited. After washing 0.06 (±0.04) at.% Na concentration is measured, which drops to below the detection limit after CdS deposition, even though similar amounts of NaF was evaporated onto the sample as in the NaF PDT case. This observation is in good agreement with the ion exchange mechanism reported in ref. 7, where it was proposed that Na is removed due to the addition of K. On the other hand, about half the amount of K is removed by the washing step but there remains a higher amount of K (0.23 at.%) compared to Na (0.06 at.%), which is quite similar to what is measured after CdS deposition. This further supports the idea that after deposition part of potassium is present on the surface as KF and that another part is included in a compound which is not soluble in aqueous solution, as also derived from the XPS measurements discussed above. The same observations are valid for films where only KF is evaporated on the surface, showing that there is no significant influence of the presence of Na on the final amount of K in the device.

In order to quantify the amount of Na and K that are typically deposited during PDT and the amount that actually remain in the final solar cell, we performed ICP-MS measurements on absorbers subjected to different PDTs. Results are shown in figure 4a for an absorber without PDT, with NaF PDT and with NaF&KF PDT, respectively, and after 3 processing steps: i) as deposited, ii) after washing in ammonia, and iii) after CdS deposition. The concentration of alkali elements is given in atomic percent compared to CIGS. Without any PDT a Na concentration below 0.05 at% is measured for all steps and gives a good estimate of the measurement error attributed to external contamination of the samples during handling as well as measurement

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FIGURE 3. Selected energy ranges of XPS survey spectra showing the binding energy range of the Na 1s, F 1s and K 2p peaks measured on CIGS layers subjected either to NaF PDT (top) or to KF PDT (bottom). Measurements were made on as deposited absorbers (NaF, KF), and after washing in a diluted ammonia solution (NaF (washed), KF (washed)). Corresponding SEM micrographs show the surface of the CIGS layer before and after washing.

In an experiment to gain further insights on the ion exchange mechanism several films with various combinations in the amount of NaF and KF were deposited. The alkali concentrations after washing are shown in figure 4b. The standard deposition time for our high efficiency solar cells baseline is 20 min NaF followed by 20 min KF, both at a rate of about 1-2 nm/min. When the amount of evaporated NaF is kept constant (20 min) and increasing amounts from 0 to 60 min KF are evaporated (figure 4a, left), it is observed that already after 5 min KF evaporation the final amount of Na present after washing decreases from approximately 0.08 at% to a value that is close or below the detection limit. No further decrease can be observed if higher KF amount (60 min) compared to the standard amount is added due to the Na concentrations below the detection limit. On the other hand, increasing concentrations of K are measured with increasing KF evaporation time. The final amount scales in first approximation almost linearly with the KF evaporation time. This can well be explained by the reactive formation of a K-containing compound in the surface region.7,21 If a diluted HCl etching of a NaF&KF PDT sample is performed to remove the nanostructured surface layer seen in figure 3,21 the K concentration decreases from 0.25 at. % to 0.12 at. %, a value very comparable with the amount of Na found after PDT or diffusion from the substrate. Such an etched absorber does not allow the formation of a good quality junction with a thinner CdS layer as is usually the case for KF PDT samples. This is a similar behavior as observed with absorbers subjected to NaF PDT only. If the amount of KF is kept constant and the NaF evaporation time is increased up to 60 min (figure 4b, right), the final Na concentration does not significantly increase and no influence on the final K concentration is seen.

These ICP-MS results imply that a given amount of Na can be added into the CIGS layer, but all NaF oversupply is washed away during the subsequent chemical bath deposition. However the amount of KF has to be controlled very carefully, because some of the oversupply remains on the surface of the absorber and reacts to form a compound not soluble during the chemical bath deposition. Higher amounts of KF have been shown to lead to a decrease of the final device efficiency (see supplementary material in ref. 7), mainly due to a decreasing Jsc. The ion-exchange mechanism has up to now been reported by several groups and observed mainly through SIMS measurements,7,22,31 or above through elemental analysis. To ensure that those conclusions are not biased by possible SIMS-related artefacts, or by the ICP-MS measurement uncertainty due to sample contamination, additional measurements were necessary. A nondestructive method that can be used to estimate the relative amounts of alkali elements in the CIGS films covered with CdS is elastic recoil detection analysis (ERDA). The details of the method are available elsewhere.25-26 In figure 4c we show such an ERDA spectrum for a CIGS layer treated with NaF&KF PDT. The relationship between energy and time-of-flight yields depth dependent compositional information, as indicated with the arrow in the spectrum. The presence of a significant amount of K in the near surface region is observed. Concentration of Na and K in the NaF&KF PDT sample was compared to the concentration of Na and K in a NaF PDT sample. Relative quantification is possible by summing up the intensity profiles in the CdS and in the CIGS surface region, and normalizing the areas of the peaks of interest with a peak corresponding to an element with expected similar concentration in both samples. Such mass profiles are shown in figure 4d. Sulfur was taken as reference, assuming it to

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be constant in both samples. Here again the observations point towards the ion exchange mechanism, with a significant decrease of the Na concentration in the NaF&KF PDT sample (Na/S < 0.005, K/S = 0.17 ±0.02) compared to the NaF PDT sample (Na/S = 0.04 ±0.02, K/S < 0.003). Measurement error in the sulfur content (different CdS thickness or S in-diffusion) used for normalization could not account for this significant decrease of Na content after KF PDT.

FIGURE 4. (a) Concentration of Na and K, measured by ICP-MS in 3 absorbers subjected to no PDT, only NaF PDT, or NaF&KF PDT after different processing steps: 1) as deposited, 2) after a washing step in a diluted ammonia solution and 3) after CdS deposition. Error bars represent the detection limit; (b) Concentration of Na and K in washed CIGS absorbers for different combinations of NaF and KF evaporation time, as determined by ICP-MS measurements: (left) the NaF evaporation time is constant, and increasing KF evaporation time is used, (right) the KF evaporation time is constant, and increasing NaF evaporation time is used. The same CIGS thin film was used in both plots for the value corresponding to 20 min NaF and 20 min KF evaporation time; (c) ERDA spectrum of a NaF&KF PDT sample covered with a thin CdS layer; (d) the corresponding mass spectrum extracted from the ERDA spectrum in the CdS/CIGS interface region compared to that of a sample subjected only to NaF PDT.

Based on the ICP-MS and ERDA measurements discussed above, the presence of an ion exchange phenomenon is strongly supported and confirms the conclusions drawn from earlier SIMS depth profiles. This phenomenon was also observed on CIGS films grown by other methods than low-temperature co-evaporation,22,31 where K-treated CIGS films always feature a reduced Na content.

No enrichment of Na in the back contact is observed for KF PDT samples (SIMS depth profile measurements, not shown), meaning that Na is probably replaced in the film by K and diffuses towards the surface of the film, before being washed away during the CdS deposition. The additional NaF concentration expected on the surface of asdeposited NaF&KF sample is however too small (below 0.1

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at. %) to be unambiguously differentiated in ICP-MS from the total amount of NaF on the surface. Further experiments are necessary to describe in more details the thermodynamics and kinetics of such an ion exchange reaction. Careful analysis of the diffusion behavior in a CIGS matrix of both elements separately or combined together at different temperature would give first indications on the ion exchange mechanism. Implications for interface formation and solar cell processing. After KF-PDT, the K-containing surface layer discussed above as well as in ref. 21 yields in SIMS depth profile measurements a sharp peak for the K signal at the CdS/CIGS interface.7 In order to achieve highest efficiency with a shorter CdS deposition time than usual, we observe that the presence of such a sharper peak is a necessary condition. This is exemplified in figure 5a, where SIMS elemental depth profiles of two CIGS solar cells with absorbers treated with similar amount of NaF but different amount of KF are shown. Some elements are omitted for clarity. With only 5 min KF PDT (20 NaF & 5 KF), the Na and K depth profiles have a similar shape, with a higher intensity in the CIGS surface region compared to bulk CIGS. The higher peak at the CIGS/Mo interface is typically observed for alkali elements and can be related either to accumulation at this interface or to matrix effects.32 By increasing the KF PDT time to 20 min (20NaF & 20KF), the Na signal intensity decreases over the whole depth, which is in agreement with the ion exchange mechanism discussed above. The K signal intensity is shifted to slightly higher values in the bulk, and a distinct peak at the CdS/CIGS interface appears. This translates into a higher cell efficiency for the cell with the absorber subjected to 20 min KF PDT compared to the cell with the absorber subjected to 5 min KF PDT, mainly due to an increase in Voc and FF (see Table 1). Based on the results presented by Pianezzi et al.20, this can be well explained by reduced interface recombination in spite of a shorter CdS deposition time and reduced absorber carrier concentration. By comparing the intensity profiles of the Cu signal in both cases, no clear difference in the CIGS surface region is observed. The relatively fast sputtering conditions chosen for the measurement do not allow resolving the difference in Cu content at this rough interface. One interpretation explaining the higher efficiency after KF PDT in spite of a shorter deposition time is that increased inversion doping of the CIGS surface is made possible by enhanced formation of CdCu during chemical bath deposition.7,20 This explanation is based on an increased Cd content observed in XPS depth profiles through the CdS/CIGS interface for the NaF&KF PDT sample7 as well as earlier experimental33 and theoretical34 works discussing the defect energy of CdCu. The increased Cd content measured by XPS depth profiling is however susceptible to forward sputtering and roughness induced effects, which were accounted for in

the earlier comparison by showing an unchanged In XPS peak intensity independently of the PDT type. In an attempt to further highlight the enhanced Cd adsorption/facilitated diffusion on CIGS surfaces subjected to a KF PDT, an experiment was conducted where thiourea (the sulphur-providing salt in the chemical bath deposition of CdS) was omitted. Three absorbers subjected to either no PDT, only NaF PDT or NaF&KF PDT were then dipped into this solution for 22 min at room temperature. The surface was characterized by XPS measurements, and results are given in figure 5b for selected peaks corresponding to Cu, In, Ga, Se and Cd energy levels. No significant difference is observable for all elements between the absorber without alkali (no PDT) and subjected only to NaF PDT. However a significantly larger amount of Cd is measured on the NaF&KF PDT absorber, along with a Cu and Ga depletion, whereas In and Se are nearly unmodified. It appears that the KF PDT treatment is very efficient in allowing deposition of a higher Cd content on the surface or in the surface region of CIGS thin films, even from a solution at room temperature. Further detailed measurements and analysis are necessary to assess the chemical environment of the Cd atom in the surface region, but this is in any case a further strong indication supporting the enhanced reactivity of a KFtreated surface. The Ga depletion can be assigned to the formation of a water-soluble Ga-compound whereas the Cu depletion is likely due to the presence of the nanostructured Cu-free surface layer.21 Other interpretations for the possibility to reduce the CdS thickness have however also been proposed, such as enhanced coverage during CdS growth35 or enhanced sulfurization.36 All those aspects are likely intrinsically linked. Whereas we do not find any evidence for enhanced sulfur content in the interface region of the samples presented in ref. 7, the measurements reported in figure 5b would also fit the explanation of a faster and more uniform CdS deposition on surfaces treated with KF PDT. Additionally, significantly increased concentration of Cd can also be explained by a significantly enhanced nanoroughness,21 which offers a higher number of adsorption sites for the Cd atoms. In an attempt to further probe the CIGS/CdS interface in a non-destructive manner, positron annihilation spectroscopy (PAS) was applied on absorbers subjected to different PDTs. PAS is a nondestructive technique, where positrons are implanted in a material at a given depth tuned by adjusting their energy. After thermalization they annihilate with a local electron.37 The Doppler broadening of the annihilation peak is used to assess the electronic momentum distribution and it gives information on the defect density (free volume, vacancy) at a given depth. This feature was already successfully used to study defect concentration (Cu and Se related vacancies) in CIGS.38-39

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FIGURE 5. (a) SIMS depth profiles of finished solar cells with absorbers subjected to 20 min NaF PDT and 5 min KF or 20 min KF; (b) XPS peaks of the surface of 3 absorbers with different PDT subjected to a Cd-dip at room temperature. A much larger Cd content is measurable at the surface of the NaF&KF PDT absorber; (c) S-Parameter derived from positron annihilation spectroscopy measurements as function of positron energy for samples with (blue ■) and without (red ●) KF PDT. For each sample, 6 several measurements were performed, with two measurements per implantation energy. Each measurement consists of 5 x 10 events, 20% of which belong to the annihilation peak.

In figure 5c we present results showing the line shape parameter S, a ratio between the counts in the center of the annihilation peak and the total counts of the peak, for absorbers treated with or without KF PDT. A thin CdS layer was deposited on the CIGS layer, as done in previous experiments on CIGS.38 A larger S-parameter supports the presence of a higher open-volume defect concentration at the implantation region. In our case, a very different behavior is observed in the near surface region of the two samples, with a significantly higher S parameter for the NaF&KF PDT sample up to 400 nm depth compared to the NaF PDT sample. Both samples yield then similar Sparameter values at further depth, which corresponds to the bulk of the absorber. Those results indicate that a more defective crystal structure is present in the interface region for the NaF&KF PDT sample. In earlier studies, a

higher S-value was correlated with a lower Cu/III ratio,38 or the presence of a defective In2Se3 layer near the surface region of a CuInSe2 layer.39 To unambiguously attribute this increase in S-value to modified crystal structure or composition, rather than more free volume (for example due to the nanopatterning of the surface), or modified electronic structure at the junction, further extensive studies are required. Nevertheless, those results are a further proof of the more pronounced impact of the KF PDT on the interface region rather than the bulk.

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TABLE 1. Compositional and average photovoltaic parameters of solar cells with absorbers subjected to different combinations of alkali PDT. NaF PDT

KF PDT

GGI

CGI

Efficiency

VOC

JSC

FF 2

(min)

(min)

(±0.02)

(±0.05)

(%)

(mV)

(mA/cm )

(%)

0

0

0.34

0.79

12.0 ±0.2

541 ±3

34.9 ±0.5

63.6 ±0.9

20

0

0.35

0.82

13.2 ±0.3

624 ±9

34.5 ±0.4

61.5 ±0.6

20

5

0.35

0.79

17.5 ±0.2

673 ±6

34.7 ±0.2

74.9 ±0.4

20

20

0.36

0.80

18.5 ±0.3

695 ±7

35.0 ±1.2

76.0 ±2.1

Finally, the average P-V parameters of 10 cells of representative samples analyzed in this contribution are summarized in table 1. They were all obtained by using a reduced CdS deposition time as explained in ref. 7. The [Ga]/([Ga]+[In]) (GGI) and [Cu]/([Ga]+[In]) (CGI) ratios were determined by XRF measurements and are comparable for every absorber. Without addition of any alkali elements efficiency of 12.0% is achieved, mainly limited by a very low Voc and FF. The current density is however comparable to the other cells with other absorbers, probably due to a much larger space charge region induced by the lower carrier concentration usual for alkali-free absorbers.20 Upon addition of NaF an increase of about 90 mV Voc is observed, but the FF remains low. As soon as KF is added a drastic increase in both Voc and FF is observed, and leads in this study to a highest cell efficiency of 18.5% with 20 min KF evaporation. This set of samples follows well the electronic properties aspects discussed in ref. 20. In order to use a shorter CdS deposition time without any losses in PV parameters, sufficient passivation of the CIGS surface region is necessary in order to reduce interface recombination. The KF PDT is a suitable method to possibly modify the CIGS surface in order to allow sufficient passivation in spite of a shorter CdS chemical bath deposition time. It should be noted that very minor shifts of the Fermi level near the interface (induced by for example bandgap variations or modified doping content) can have a tremendous influence on the extend of the interface recombination. The PV parameters of cells with absorbers without or with insufficient KF amount are therefore very sensitive and subject to significant variations of their P-V parameters when combined with a too short CdS deposition time. Alkali PDT: comparison of NaF and KF. Based on the different aspects discussed above, some features differentiating NaF and KF PDT have been unveiled and are all consistently reproduced by several complimentary methods. Different behavior is seen on i) the bulk and ii) the surface region of the CIGS thin films, but those effects are closely interrelated. The effects discussed in this contribution are limited to CIGS layers grown at low temperature, but are likely applicable on CIGS layers grown by other methods. This is best supported by the fact that efficiency enhancements have also been reported for cells with very different absorber growth methods.3-4,22 With NaF PDT,

higher amounts of NaF than necessary are evaporated onto the surface. Similar amounts of Na as naturally diffusing from glass diffuse into the CIGS layer, whereas the oversupply remains on the surface and is washed away during the chemical bath deposition. When KF is evaporated onto the surface, it first shows a similar behavior as NaF, with K diffusing into the CIGS and leading as well to an ion exchange with Na if NaF PDT was previously done. At the moment, we cannot distinguish between a thermodynamically favored positioning of K over Na in the layer at the given deposition temperature, or a difference in migration kinetics between both elements. At the temperature of deposition, a higher solubility of Na and K in the CIGS layer can be expected, which decreases upon cooling. The ionic radius of potassium being larger than that of sodium, slower out-diffusion of K can lead to higher remaining amount of K. The deposition temperature as well as cooling rate of the sample after end of deposition could have a strong influence on the ion exchange mechanism and should be further investigated. Moreover the presence of Na or K during growth leads to the formation of minority carrier trap and has serious implications for the final device efficiency.40-41 The deposition temperature or the modified Ga-In interdiffusion have a strong influence on the formation of this defect, supporting the idea that temperature is a critical parameter influencing the different behavior of Na and K. At the temperature used for PDT an additional aspect of KF PDT is the formation of a Cu-free, K-containing layer in the CIGS surface region. This layer is not comparable to the Cu-deficient CIGS surface layer usually discussed in literature,42 as it does not contain any Cu.21 It is insoluble in aqueous solution but can be removed by HCl etching.21 The amount of K in the final absorber increases with a higher KF evaporation time, whereas the amount of Na in the final device remains constant even upon increase of the NaF evaporation time. The presence of this K-containing layer was up to now always necessary on our absorbers to allow shortening the CdS deposition time with improved junction quality. The actual mechanism leading to this improved junction quality is debated, being either related to enhanced in-diffusion of Cd or S, a better coverage or due to surface passivation. In all cases, it is clear that higher reactivity of the surface is induced by the KF PDT.

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Because In and Se content seem to be quite unmodified by the KF PDT, the formation of a material containing mainly K, In, and Se is a candidate as K-containing layer. Exact phase or stoichiometry could at the moment however not be determined due to the challenging interpretation of the data for a non-continuous layer of a few nm on a surface with average roughness of several tens of nm. First indications for an increase in bandgap have been reported.43 Other defects such as KCu or KGa are as well possible, and a K-induced Cu and Ga depletion due to the increased presence of the alkali element on the surface and change in electronic properties cannot be excluded. It cannot be excluded at the moment either that K catalyzes the formation of a defective indium selenide compound near the surface. This more pronounced effect of K compared to Na on the surface is likely also at play to some extend in the grain boundaries of the CIGS absorber. Furthermore, it was shown by Jackson et al.23 that the KF PDT allows to process solar cells with a higher absorber band gap without significant losses in the efficiency. Additionally, it was recently shown that insufficient band bending at the CIGS surface44 or at grain boundaries45 can limit the efficiency of CIGS solar cells. In view of the effects of the KF PDT, we can presume that it facilitates the inversion of the surface region and grain boundaries and explain the efficiency enhancements observed in the recent years. Even if many explanations are still possible for the exact role and position of K in and on the layer, it should be reminded that the role of Na and how it is bound in CIGS layers is still not clear, in spite of more than 20 years of research. This shows the difficulty in assessing such small quantities on small length scales and stresses the need for sub-nm resolution characterization methods. Nevertheless, from a solar cell processing point of view many parameters can still be tuned and are likely to lead to further efficiency enhancements.

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shows the presence of a more defective crystal structure in the interface region with the CdS buffer layer. All these compositional modifications come along with an improved junction quality and the possibility to reduce the CdS deposition time without adverse influence on the final cell performance. Hypotheses based on enhanced surface and/or grain boundary inversion/passivation are discussed to explain (at least partly) the enhanced efficiencies due to KF PDT.

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected].

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work at Empa was partly supported by the Swiss National Science Foundation (SNF), the Swiss Federal Office of Energy (BFE) and the European Union (FP7 projects “hipoCIGS” (project number: 241384) and “R2R-CIGS” (project number: 283974)). P. Crivelli and C. Vigo acknowledge financial support through ETH grant 35 14-2. The Laboratory for Electronics/Metrology/Reliability, the Laboratory for Analytical Chemistry, and the laboratory for Nanoscale Materials Science at Empa are acknowledged for SEM, ICP-MS, SIMS and XPS measurements.

CONCLUSION CIGS thin films grown at low temperature on flexible polyimide foils were subjected to combinations of alkali post-deposition treatments with NaF and/or KF. The distinct effects of both treatments on the bulk and surface properties were investigated, and distinct features were revealed based on several material characterization methods. The KF PDT leads to a change in optical appearance of the CIGS surface. SEM analysis combined with XPS studies shows NaF and KF remnants on the surface which are removed in a solution similar to that used for the buffer layer deposition. The removal of Na from the bulk and the formation of a Cu-depleted, K-enriched layer on the surface after KF PDT are confirmed by SIMS, ICP-MS and ERDA measurements. This modified surface shows a pronounced affinity for Cd ions even in a room temperature solution, and positron annihilation spectroscopy

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Features of KF and NaF Postdeposition Treatments of Cu(In,Ga)Se2

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