Evidence of Enhanced Carrier Collection in Cu(In,Ga)Se2 Grain

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Energy, Environmental, and Catalysis Applications 2

Evidence of Enhanced Carrier Collection in Cu(In,Ga)Se Grain Boundaries: Correlation with Microstructure

Mohit Raghuwanshi, Bo Thöner, Purvesh Soni, Matthias Wuttig, Roland Wuerz, and Oana Cojocaru-Miredin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02328 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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ACS Applied Materials & Interfaces

Evidence of Enhanced Carrier Collection in Cu(In,Ga)Se2 Grain Boundaries: Correlation with Microstructure Mohit Raghuwanshi*†, Bo Thöner†, Purvesh Soni†, Matthias Wuttig†, Roland Wuerz║, Oana Cojocaru-Mirédin† † RWTH Aachen, I. Physikalisches Institut IA; Sommerfeldstrasse 14, 52074 Aachen, Germany ║ Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstraße 1, 70563 Stuttgart, Germany

KEYWORDS Cu(In,Ga)Se2, Electron Beam Induced Current, Electron back scattered Diffraction, Grain boundaries, Solar Cells

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ABSTRACT

Solar cells containing a polycrystalline Cu(In,Ga)Se2 absorber outperform the ones containing a monocrystalline absorber showing a record efficiency of 22.9%. However, the grain boundaries (GBs) are very often considered to be partly responsible for the enhanced recombination activity in the cell, and thus cannot explain the registered record efficiency. Therefore, in the present work, we resolve this conundrum by performing correlative electron beam induced current– electron backscatter diffraction investigations on more than 700 grain boundaries and demonstrating that 58% of the grain boundaries exhibit an enhanced carrier collection compared to the grain interior. Enhanced carrier collection thus indicates GBs are beneficial for the device performance. Moreover, 27% of the grain boundaries are neutral and 15% are recombination active. Correlation with microstructure shows that most of the Σ3 GBs are neutral, whereas the random high-angle grain boundaries are either beneficial or detrimental. Enhanced carrier collection observed for a big fraction of high-angle grain boundaries supports the “typeinversion” model and hence the downward band bending at GBs. The decrease in current collection observed at one of the high-angle grain boundaries is explained by Cu being enriched at this GB, and hence by the upward shift of the valence band maximum.

1. INTRODUCTION Latest advances in preparation of Cu(In,Ga)Se2 (CIGS) based thin film solar cells has led to a record efficiency of 22.9% in the thin film category 1. However, this value is still low compared to the maximum theoretical limit of 33.7% predicted by Shockley and Queisser 2 for a material with a bandgap of 1.2-1.3 eV. This strong discrepancy of more than 10% between the


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experimental and theoretical efficiency has been attributed mainly to the charge recombination phenomenon taking place in the solar cells 3. For example, in a typical multicrystalline Si solar cells characterized by a record efficiency of 22.3% 4, origin of this efficiency loss is mainly ascribed to the recombination phenomenon mainly taking place at the grain boundaries (GBs) and heterojunction. More specifically, the presence of asymmetrical and highly incoherent GBs in Si absorbers degrades the cell efficiency due to point defects and impurities 5,6 accumulated at the GBs which induces non-radiative recombination. Hence, one would expect the presence of the GBs in CIGS should be the main limiting factor for the cell efficiency due to its small grain size (~2µm). However, over the years the maximum efficiency of CIGS was always observed from its polycrystalline form rather than its monocrystalline counterpart

7–9

, suggesting that the GBs might not be so detrimental for the

device performance. Three existent models explain the beneficial properties of GBs in CIGS. First, the “structural barrier model”

10,11

states that Cu is depleted at GBs (induced by a Cu vacancy surface

reconstruction) leading thus to the formation of a hole barrier at GBs

12

. This configuration

drastically reduces the recombination activity at GBs. Moreover, Na accumulation at GBs enhances this quality due to reduced repulsion between Se-p orbitals and Cu-d orbitals

12

.

Second, the “electronic barrier model” states that GBs in CIGS are positively charged and possess a built-in potential at GBs as observed by Yan et al.

13

using scanning Kelvin probe

microscopy. They apparently cause a downward band bending in the conduction and valence band at the GB

14–16

which leads to the formation of a potential barrier at the GB and thus

reduces the recombination by attracting electrons and repelling holes. Third, the “type-inversion” model affirms that type inversion from p to n occurs at GBs as observed by scanning capacitance


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microscopy 17. In such a case, electrons (minority carriers here) could channel through a secured pathway via GBs towards the n-type layer thus avoiding repulsion with the holes present in the CIGS bulk. But this observation was in contradiction with previous (electron beam induced current) EBIC results

17

. Carrier collection properties of GBs in CIGS were unfortunately only

studied little to date mainly in cross-section

18

and at the back-surface (close to the Mo/CIGS

interface) configurations 19 by EBIC and cathodoluminescence 20,21. Moreover, these studies 19–21 had rather highlighted the lowered carrier collection at the GBs (approximately 6 to 8% decrease in EBIC current was registered at the position of random high-angle GBs) which is not in line with the proposed models described before. It is worthwhile to highlight here that understanding the charge carrier collection behavior at the GBs originating from the space charge region (SCR) of the CIGS film, i.e. the first 400 nm of the CIGS absorbers 22, is important since the majority of the photons are absorbed by this top part region. Therefore, in this work we focus on understanding the electrical and structural properties of GBs originating from the top and middle parts of the CIGS film by performing EBIC - EBSD (electron beam induced current) correlative studies. Enhancement or descent in EBIC current at GBs were recorded and successfully correlated with the GB type. Various configurations and regions in the thin films were investigated independently to establish appropriate conclusions. Moreover, these correlative studies help us to validate or nullify the hypothesis that GBs present in the CIGS absorber are the ones which limit the efficiency of the CIGS thin film solar cells. In the end, a correlative EBIC-EBSD-APT (atom probe tomography) study is shown as a proof of concept. The obtained results are compared with our recent findings on GB structure and composition

23,24

. These reported preliminary results demonstrate the feasibility in reaching our


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ultimate goal, which is understanding the interdependence between the electrical activity, structure, and composition at various GB types.

2. MATERIALS AND METHODS 2 µm CIGS (p-type) absorbers deposited on a Mo-coated soda lime glass substrate were produced by ZSW (Zentrum für Sonnenenergie und Wasserstoff-Forschung) using a multi-stage inline co-evaporation process

25

. The CdS (n-type) buffer layer was deposited using chemical

bath deposition, whereas the intrinsic ZnO layer and the front window layer ZnO:Al (n-type) were deposited using magnetron sputtering followed by deposition of Ni/Al/Ni grids for current collection. Solar cells studied in this work were Cu and Ga poor ([Cu]/([In]+[Ga]) =0.86 and [Ga]/([Ga]+[In]) =0.28) with a power conversion efficiency of 16.4%, open circuit voltage 674 mV, short circuit current of 15.9 mA and fill factor of 76.4%. A Ga-grading was also performed as reported in ref.

22

with the aim to improve the carrier collection. A typical stack of the

completed solar cell used in this work is shown in Figure 1(a). The main configuration employed in this work to perform the quantitative EBIC investigations was the edge scan configuration (described with more details in the experimental section). The results obtained were compared with the ones obtained in cross section configuration. The investigation is important in both the configuration to obtain a better perspective of origins of recombination in the thin film. A short schematization of these two configurations is shown in Figure 1(b). In the edge scan configuration, the top two layers, i.e. ZnO and CdS, are removed by focused ion beam (FIB) by positioning the surface of the sample parallel to the ion beam. The CdS- and ZnO-free surface obtained was then investigated by both, EBIC and EBSD, revealing the charge collection and


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structure of the GBs in the top part of the film, i.e. in the CIGS space charge region. To investigate the middle part of the thin film or the bottom of the space charge region, 1 µm CIGS thin film was polished using FIB until reaching the middle part of the thin film, i.e. ~1 µm from the Mo layer.

3. RESULTS AND DISCUSSIONS 3.1. Enhanced current collection at CIGS grain boundaries: correlation with grain boundary geometry Correlative EBIC-EBSD measurements performed on the middle part of the thin films are shown in Figure 2. EBIC results in Figure 2 clearly show that a high proportion of GBs (58%) in CIGS are bright which means that the electron hole separation is enhanced in the vicinity of GBs as compared to the grain interior and hence are beneficial for the device. As the p-n junction is removed from the surface, the electron-hole separation during EBIC measurements is effectuated by the GBs, which are mainly type inverted (n-type) compared to the p-type CIGS bulk (discussed later in this section). Hence, the excitons created are separated by this built-in potential at the GBs. The electrons then laterally have to channel through the GB network towards the ZnO layer to be collected as EBIC current. The bright contrast observed in the EBIC map indicates low recombination activity in vicinity of GBs as compared to the grain interior. Figure 2 also shows the presence of some pronounced dark and bright grains which are most probably due to differences in carrier concentration levels between different grains 18. The SEM image shows a fine polished and cleaned region of interest (ROI) with presence of few curtailing lines. Figure 2(b) shows the EBSD map obtained from the ROI. Figure 2(c) shows


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the corresponding quantitative EBIC image which correlates well with the GB map in Figure 2(d), a guide to the eye for grain correlation is indicated by yellow regions. The EBIC current within the grains is uniform and shows existence of mostly bright GBs accompanied with some dark ones. Comparing Figures 2(c) and (d) it is evident that most of the random high-angle grain boundary (RHAGB) are either bright or dark, whereas most of the twin boundaries were neutral. Presence of both dark and bright grains were also detected as can be seen from Figure 2(c) which can be due to differences in their local carrier concentrations. EBIC analysis performed here was quantitative and hence the increase/decrease of EBIC current at GBs and inside grains was recorded and correlated with the type of GB. From the figures 2(c) and 2(d) some notable differences in size and shape of GBs are evident. This arises mainly due to the difference in electron penetration between EBIC and EBSD calculated using the CASINO

26

software (refer to supplementary information for more details).

5 kV (for EBIC) and 20 kV (for EBSD) acceleration voltages were used, resulting in ~10 times bigger electron penetration depth for the EBSD measurement. Moreover, typical acquisition time for an EBIC map is 15 seconds whereas for an EBSD map it is 2 hours, which are long enough for small drifts of the sample causing changes in shape and size. Thus due to both factors (penetration depth and drifts in EBSD), the size and shape of grains do not perfectly match in EBSD and EBIC. However, EBIC and EBSD maps can be compared to correlate the current change at a GB with its geometry. A typical example is shown in Figure 2(d) where the yellow regions highlight the same grain in EBIC and EBSD. Thus, the current profile of a GB is obtained from the quantitative EBIC image and is correlated to its respective type, RHAGB or Σ3 GB.


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Figure 3 describes the EBIC-EBSD correlative studies performed on the top part of the film (just below the heterojunction). Comparing these studies with the ones performed in the middle region (Figure 2), no significant difference in GBs structure and current collection is noticed. To improve the statistics, at least five other distinct regions were investigated for both regions (top and middle). η=IEBIC/IBeam profiles were extracted from all the identified GBs and were correlated with their respective type (RHAGB and Σ3). There were some GBs (in EBSD or in EBIC) which could not be precisely correlated. These GBs were not accounted for in our analysis. To compare the electrical behavior of various types of GBs, η profiles were extracted from individual GBs and the relative change in η at a GB was obtained and tabulated as ∆η, given by: ∆ η = (η GB - η Grain) / η Grain

(1)

Hence, positive values of ∆η correspond to bright GBs indicating a lower recombination activity and thus a better separation of charged carriers at a GB than in the grain interior and vice versa. Similarly, zero value of ∆η means no change in η at the GB. Hence, sufficient quantitative statistics were obtained from both regions and are compared in Figure 4, leading to following observations: 1)

For both regions, very few (2.5 %) RHAGBs were found to have ∆η=0 implying that

most of the random GBs are either beneficial or detrimental in terms of device performance. Existence of both bright and dark GBs must be due to difference in their respective local concentration and needs to be further investigated. 2)

78% of the total RHAGB were found electrically active with enhanced carrier collection.


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3)

Comparing ∆η for RHAGB for the top and middle region of the thin film, a shift towards

higher ∆η is evident in the top region indicating better electrical properties of GBs in the SCR as compared to the middle part of the thin films. 4)

For Σ3 twin boundaries with 60° and 71° misorientation angles most (62%) of the GBs

are found electrically neutral, indicating that most of the twin boundaries are electrically neutral. 5)

46% of the Σ3 twin boundaries with 60° misorientation angle were not found electrically

neutral. This result is in contrast with previous observations

18,19

where no difference was

observed at Σ3 GB, probably due to less statistics. 6)

Another noticeable feature is a higher percentage (81%) of neutral GBs for Σ3 GBs

twinned at 71°. Our observations here show that GBs in CIGS were mostly bright (beneficial) being characterized by an efficient charge carrier separation and thus leading to a better device performance. Possible reasons for the existence of beneficial GBs are associated to: 1)

Local built-in potential at GB/Hole barrier properties. An energy barrier is formed at the

GB which attracts electrons and repels holes due to the positively charged GB, thus assisting electron hole separation

12

. This is a consequence of downward band bending of valence and

conduction band at the GB as shown in Figure 5(a). In this case, holes are repelled from the GB and electrons are attracted. Hole barrier properties may also arise from Cu depletion at GBs thus reducing p-d repulsion 12 and avoiding recombination at GBs. Hence, due to the columnar nature of GBs in CIGS, electrons are channeled via GBs to the n-side and hence are effectively collected. As electrons are minority carriers in CIGS, their efficient collection improves the overall device performance.


9


2)

Type inversion at GB. Previous works

27,28

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reported systematic change in the elemental

composition at GBs explaining their type inversion behavior

17

using scanning capacitance

microscopy (SCM). Hence, local p-n junctions are formed at GBs as shown in Figure 5(b) along with downward band bending. As GBs are columnar, electrons can again similarly move through a recombination free path towards the n-side. Similar results were also predicted for CdTe thin film solar cells

29,30

and hence an increase in fraction of columnar GBs (or transversal GBs)

would result in better efficiency of the cells. This also explains why RHAGB are either bright or dark but not neutral and why most of the Σ3 twin boundaries are charge neutral (they are rather horizontal boundaries and thus do not collect the electrons in the cell). Moreover, Σ3 twin boundaries are highly symmetric and segregation of various elements forming an n-type interface is much less probable. 3)

Upward band bending at GB. Opposite to the first case, there can be an upward band

bending at the GB which would result in repulsion of electrons and attraction of holes at the GB 18,31

. This reduces recombination with safe channeling of holes towards the back contact but may

not largely affect the efficiency as electrons are the minority carriers. Hence, we do not support the concept/idea of upward band bending. Also ∆η at GBs were found to be up to 100% which means carrier separation at GBs is sometimes even twice as strong as that of the grain.

3.2 Composition of an active/dark RHAGB: Correlative EBIC-EBSD-APT measurement In this section, we detail one of the obtained correlative EBIC-EBSD-APT measurement as a proof of concept showing thus our ultimate goal which is to correlate the electrical and structural properties of grain boundaries obtained by EBIC and EBSD with the grain boundary


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composition by APT. However, the scope of the present work is not to show a quantitative study of the performed EBSD-EBIC-APT correlative investigations. Figure 6 shows an example of such a correlative EBIC-EBSD-APT study as a proof of concept. Beyond the GBs showing typically an increased current collection (bright areas, which are recombination-inactive GBs), there are GBs showing a decreased current collection (dark areas, which is a signature for a recombination-active GB). One of these GBs is highlighted in the EBIC image (Figure 6(a)) and shows decrease in EBIC contrast at a GB. An APT needle shaped specimen was fabricated with this specific dark GB situated within the first 400 nm from the apex using the procedure described in ref. 23. The goal here is to determine the composition of this recombination-active GB by APT investigations and to finally compare it with the composition of the inactive GBs. Moreover, Figure 6(b) shows the transmission EBSD map of the APT specimen, which provides valuable information not only on the exact location of the GB, but also its precise structural properties (mainly the disorientation angle and thus the GB type). A disorientation angle of 24° was measured for this GB referring to a RHAGB. Figure 6(c) shows the obtained 3D atomic volume. Surprisingly, no Na was found to be segregated at this active/dark RHAGB. However, the change in Cu density and concentration were enough to clearly identify the position of the GB in the 3D atomic volume. Figure 6(d) shows the composition of the elements in the vicinity of the dark RHAGB. It is interesting to note that the GB is Cu-enriched and In-depleted which agrees well with the work reported in ref.

27

, where

Cu-rich GBs are predicted to be detrimental for device performance. Additionally, the corresponding grains contained unexpectedly no Ga. These observations corroborated with recently reported results

27

indicate clearly the strong variation in composition between GBs in

CIGS absorber. Moreover, it seems that the composition and structure is strongly linked with the


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electrical properties. Understanding the interdependence between the structure, composition, and properties is the goal of our running research project and will be treated elsewhere. It is apparent to note that the tilt angle of GB appears to be different in the 3D APT image than in EBIC and EBSD map where the GB is perpendicular to the tip axis. This is mainly because the GB is tilted at some angle (~45° in this case) from normal direction. This is the main reason for diffused contrast of GBs in EBIC image and also explains the presence of un-indexed points near GBs in EBSD, which result from overlap of signals from two grains and a GB. Reasons behind recombination-active GBs can hence be attributed to undesirable segregation of elements at GBs. Cu-rich GBs are hence confirmed to be undesirable, due to their enhanced carrier recombination implying thus the absence of either type inversion or of downward band bending of conduction and valence band at the GB. In addition, Na was found to be absent at this specific dark/active GB which is a potential reason for recombination active GB due to its ability to passivate GBs

12

. Moreover, the inability to form NaInSe2 compound at GB might also be a

potential reason for the recombination active GB. This compound is indeed reported to be beneficial for the device performance by increasing the effective hole densities and by eliminating InCu defects 37. Complementary to our study, Schwarz et al.

23

recently demonstrated that alkali elements in

CIGS are only present in RHAGB and were absent in Σ3 GBs. Moreover, no significant change in elemental compositions was observed for Σ3 GBs. Comparing these important findings with our results (where Σ3 GBs are found neutral), we can clearly emphasize that the alkali impurities and the elemental concentration at GBs play a key role for the device efficiency. Their correlation with the structural and electrical properties provides valuable information to further optimize the cell efficiency via defect engineering.


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4. CONCLUSIONS In the present study, we clearly demonstrate that more than half of the GBs (58%) show an enhanced minority carrier collection (here electrons) compared with the CIGS bulk. This alone explains why the CIGS solar cells containing a polycrystalline absorber exhibit a higher efficiency than their monocrystalline counterparts. Moreover, 15% of the GB are recombination active and hence detrimental for the cell performance. It follows naturally from this study that by decreasing further this fraction to much lower values (5 % or lower) the record cell efficiency can be further enhanced approaching thus the maximum theoretical limit of 33.7% predicted by Shockley and Queisser. Moreover, the EBIC results corroborated with the EBSD results clearly prove that most of the bright/dark GBs corresponded to RHAGBs, whereas the neutral GBs correspond to Σ3 twin boundaries. However, some of the Σ3 GBs twinned at 60° were found bright, being thus the first time when such behavior was observed. Various mechanisms explaining existence of bright GBs in EBIC are proposed among which we strongly support the “type inversion” mechanism characterized by a downward band bending at the GBs. This not only explains the superior performance of polycrystalline CIGS but also explains the formation of bright GB contrast in EBIC. One correlative APT example is also demonstrated which show accumulation of Cu and absence of Na at a dark GB. This hence rules out the possibility of GB passivation by Na at this dark GB.

5. EXPERIMENTAL SECTION


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In this work, the probed area of the thin film was approximately 12 µm × 20 µm for the edge scan configuration as shown in Figure 1(b). In the cross section configuration, the thin films were placed normal to the ion gun and the cross section of the CIGS thin-film was first milled to obtain a flat surface and then cleaned using a low acceleration voltage and (5 kV) current (0.16 nA). The latter one, i.e. the low-kV cleaning step, is very important to minimize ion beam damage and amorphization on the surface. This way, we avoid the formation of topological artifacts on EBIC maps (for more details refer to Figure S1 in supplementary information) and also improve the data quality in EBSD. EBIC measurements were conducted with a SmartEBIC, Gatan Inc. setup installed in a dual beam Helios Nanolab 650, FEI system. The post-processing of EBIC data was performed using the DigitalMicrographTM software by Gatan. For both configurations (edge-scan and crosssection), the front and back EBIC contacts were made on ZnO and Mo layers, respectively. Thus, the electron-hole pairs, generated in CIGS by the electron beam, will be separated by the p-n junction so that the electrons will be collected to the n-side and the holes to the p-side. All the EBIC measurements were performed under zero bias with a resolution of 512×1024 pixels and a dwell time of 40 µs. The gain of EBIC was optimized to be 5×105 under zero bias conditions to obtain the best quality EBIC maps. The scanning area during acquisition was also kept constant for all measurements. Please refer to supporting information for more details on optimization of parameters for EBIC and for EBIC on passivated and non-passivated CIGS surface. Electron Back Scattered Diffraction (EBSD) Kikuchi patterns were detected with the EDAXHikari camera installed in the dual-beam Helios Nanolab 650, FEI. EBSD measurements were


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performed using 20 kV acceleration voltage and 1.6 nA probe current in SEM. 4 × 4 binning mode with step size of 50 nm was used for data acquisition. Atom Probe Tomography (APT) measurements were conducted on a local electrode atom probe (LEAPTM 4000 Si, Cameca Instruments). During measurements, the specimen was maintained at 50K temperature and was field evaporated using laser pulses with an energy of 0.3 nanojoules. A green laser (wavelength 532 nm) with 200 kHz repetition rate was used for the experiment maintaining a constant detection rate of 0.3 %. FIGURES

Figure 1. (a) Cross section SEM image (false color) of CIGS solar cell stacks on a cleaved cross section, different layers are indicated by different colors. (b) SEM image (false color) of a polished and cleaned surface prepared site-specifically using FIB. Schematic EBIC set-up circuit diagram is shown. Direction of electron beam impact in edge scan and cross section configuration is shown schematically.


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Figure 2. (a) FIB-polished and cleaned surface of CIGS solar cell illustrating the SEM image of the middle part of the thin films. Rectangular ROI indicates the region where correlative measurements were performed. (b) EBSD inverse pole figure map obtained from the same segment, color legend shows the orientation of the grains along (001) direction. (c) Quantitative EBIC image of the same segment, legend represents value of η. In this EBIC map, blue arrows highlight two of the detected bright GBs, whereas red arrows highlight two of the detected dark GBs. (d) GB mapping obtained from EBSD data, Color legend shows the type of GB, red color represents RHAGB and green and blue represents Σ3 GB with misorientation angles of 60° and 71°, respectively. Correlation between some grains are shown using yellow colored regions. The two bright GBs (blue arrows) and two dark GBs (red arrows) shown in Figure 2c are also indicated in the EBIC map to assist the readers.


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Figure 3. (a) EBIC and (b) EBSD GB map obtained from a segment from the top region of the CIGS absorber after fine polishing the surface. The yellow regions indicate guide for the eye to correlate the two images. Color legend shows the type of GB, red color represents RHAGB and green and blue represents Σ3 GB with misorientation angles of 60° and 71°, respectively.


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70

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

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-50

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Figure 4. Change in η at GBs (∆η) with respect to η in corresponding grains for RHAGB, twin boundary at 60° (T60) and twin boundary at 71° (T70). Y-axis represents counts of ∆η for their respective range. SCR represents space charge region which is up to 300 nm from heterojunction and middle region correspond to 1µm to 1.5 µm from the heterojunction.

Figure 5. Schematic description of the band bending near the GB for (a) downward band bending at a GB, showing a 3D depiction, (b) downward band bending including type inversion at a GB, (c) upward band bending at a GB leading to accumulation of holes at the GB.


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Figure 6. Correlative EBIC-EBSD-APT measurement on a recombination-active (dark) GB. (a) EBIC map shows the targeted (dark) GB and location of APT specimen. (b) Transmission EBSD map on prepared specimen. (c) 3D APT volume showing distribution of Cu atoms. (d) Composition profile of elements across the highlighted GB.

ASSOCIATED CONTENT Supporting Information is available from the website. Supporting information includes the optimization of cleaning parameters in FIB, simulation of penetration depth of electrons in CIGS, the effects of Al deposition on Kikuchi pattern quality and EBIC measurements on back side of CIGS.


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AUTHOR INFORMATION Corresponding Author [email protected]

Funding Sources This work was funded by the Federal Ministry of Education and Research in Germany (BMBF 03X5522A).

ACKNOWLEDGMENT The authors are thankful to Dr. Daniel Abou-Ras for fruitful discussions. This work was funded by the Federal Ministry of Education and Research in Germany (BMBF 03X5522A) which is gratefully acknowledged.

REFERENCES (1)

https://www.pv-magazine.com/2017/12/20/solar-Frontier-Hits-New-Thin-Film-Cell-

Efficiency-Record/. (2)

Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of P‐n Junction Solar

Cells. Journal of Applied Physics 1961, 32 (3), 510–519.


20

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3)

Engelhardt, F.; Schmidt, M.; Meyer, T.; Seifert, O.; Parisi, J.; Rau, U. Metastable

Electrical Transport in Cu(In,Ga)Se2 Thin Films and ZnO/CdS/Cu(In,Ga)Se2 Heterostructures. Physics Letters A 1998, 245 (5), 489–493. (4)

J. Benick, R. Müller, F. Schindler, A. Richter, H. Hauser, F. Feldmann, P. Krenckel, S.

Riepe, M. C. Schubert,M. Hermle, S. W. Glunz. APPROACHING 22% EFFICIENCY WITH MULTICRYSTALLINE N-TYPE SILICON SOLAR CELLS; Amsterdam, The Netherlands, 2017. (5)

Di Sabatino, M.; Stokkan, G. Defect Generation, Advanced Crystallization, and

Characterization Methods for High-Quality Solar-Cell Silicon. Phys. Status Solidi A 2013, 210 (4), 641–648. (6)

Grain Boundaries in Semiconductors: Proceedings of the Materials Research Society

Annual Meeting, November 1981, Boston Park Plaza Hotel, Boston, Massachusetts, U.S.A.; Leamy, H. J., Pike, G. E., Seager, C. H., Materials Research Society, Annual Meeting, Grain Boundaries in Semiconductors Symposium, Eds.; North-Holland: New York, 1982. (7)

Yip, L. S.; Shih, I. Photovoltaic Cells with Efficiency Exceeding 10% Using

Monocrystalline CuInSe2 Substrates. In Proceedings of 1994 IEEE 1st World Conference on Photovoltaic Energy Conversion - WCPEC (A Joint Conference of PVSC, PVSEC and PSEC); 1994; Vol. 1, pp 210–213 vol.1. (8)

Du, H.; Champness, C. H.; Shih, I. Results on Monocrystalline CuInSe2 Solar Cells. Thin

Solid Films 2005, 480–481, 37–41.


21


(9)

Page 22 of 26

Jackson, P.; Wuerz, R.; Hariskos, D.; Lotter, E.; Witte, W.; Powalla, M. Effects of Heavy

Alkali Elements in Cu(In,Ga)Se2 Solar Cells with Efficiencies up to 22.6%. Phys. Status Solidi RRL 2016, 10 (8), 583–586. (10) Persson, C.; Zunger, A. Compositionally Induced Valence-Band Offset at the Grain Boundary of Polycrystalline Chalcopyrites Creates a Hole Barrier. Applied Physics Letters 2005, 87 (21), 211904. (11) Siebentritt, S.; Sadewasser, S.; Wimmer, M.; Leendertz, C.; Eisenbarth, T.; Lux-Steiner, M. C. Evidence for a Neutral Grain-Boundary Barrier in Chalcopyrites. Phys. Rev. Lett. 2006, 97 (14), 146601. (12) Persson, C.; Zunger, A. Anomalous Grain Boundary Physics in Polycrystalline CuInSe2: The Existence of a Hole Barrier. Phys. Rev. Lett. 2003, 91 (26), 266401. (13) Yan, Y.; Jiang, C.-S.; Noufi, R.; Wei, S.-H.; Moutinho, H. R.; Al-Jassim, M. M. Electrically Benign Behavior of Grain Boundaries in Polycrystalline CuInSe_{2} Films. Phys. Rev. Lett. 2007, 99 (23), 235504. (14) Jiang, C.-S.; Noufi, R.; Ramanathan, K.; AbuShama, J. A.; Moutinho, H. R.; Al-Jassim, M. M. Does the Local Built-in Potential on Grain Boundaries of Cu(In,Ga)Se2 Thin Films Benefit Photovoltaic Performance of the Device? Applied Physics Letters 2004, 85 (13), 2625– 2627. (15) Jiang, C.-S.; Noufi, R.; AbuShama, J. A.; Ramanathan, K.; Moutinho, H. R.; Pankow, J.; Al-Jassim, M. M. Local Built-in Potential on Grain Boundary of Cu(In,Ga)Se2 Thin Films. Applied Physics Letters 2004, 84 (18), 3477–3479.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(16) Rau, U.; Taretto, K.; Siebentritt, S. Grain Boundaries in Cu(In, Ga)(Se, S)2 Thin-Film Solar Cells. Appl. Phys. A 2009, 96 (1), 221–234. (17) Sadewasser, S.; Abou-Ras, D.; Azulay, D.; Baier, R.; Balberg, I.; Cahen, D.; Cohen, S.; Gartsman, K.; Ganesan, K.; Kavalakkatt, J.; Li, W.; Millo, O.; Rissom, TH.; Rosenwaks, Y.; Schock, H.-W.; Schwarzman, A.; Unold, T. Nanometer-Scale Electronic and Microstructural Properties of Grain Boundaries in Cu(In,Ga)Se2. Thin Solid Films 2011, 519 (21), 7341–7346. (18) Masahiro Kawamura; Tomoyuki Yamada; Naoki Suyama; Akira Yamada; Makoto Konagai. Grain Boundary Evaluation of Cu(In1-xGax)Se2 Solar Cells. Japanese Journal of Applied Physics 2010, 49, 062301–062301. (19) Kavalakkatt, J.; Abou-Ras, D.; Haarstrich, J.; Ronning, C.; Nichterwitz, M.; Caballero, R.; Rissom, T.; Unold, T.; Scheer, R.; Schock, H. W. Electron-Beam-Induced Current at Absorber Back Surfaces of Cu(In,Ga)Se2 Thin-Film Solar Cells. Journal of Applied Physics 2014, 115 (1), 14504. (20) Abou-Ras, D. Correlative Microscopy Analyses of Thin-Film Solar Cells at Multiple Scales. Materials Science in Semiconductor Processing. (21) Abou-Ras, D.; Schäfer, N.; Rissom, T.; Kelly, M. N.; Haarstrich, J.; Ronning, C.; Rohrer, G. S.; Rollett, A. D. Grain-Boundary Character Distribution and Correlations with Electrical and Optoelectronic Properties of CuInSe2 Thin Films. Acta Materialia 2016, 118 (Supplement C), 244–252.


23


Page 24 of 26

(22) Wuerz, R.; Eicke, A.; Kessler, F.; Paetel, S.; Efimenko, S.; Schlegel, C. CIGS Thin-Film Solar Cells and Modules on Enamelled Steel Substrates. Solar Energy Materials and Solar Cells 2012, 100, 132–137. (23) Schwarz, T.; G. Stechmann, B. Gault, O. Cojocaru-Mirédin, R. Wuerz, and D. Raabe. Correlative Transmission Kikuchi Diffraction and Atom Probe Tomography Study of Cu(In,Ga)Se2 Grain Boundaries. Progress in Photovoltaics: Research and Applications (accepted). (24) Cojocaru-Mirédin, O.; Schwarz, T.; Abou-Ras, D. Assessment of Elemental Distributions at Line and Planar Defects in Cu(In,Ga)Se2 Thin Films by Atom Probe Tomography. Scripta Materialia 2018, 148, 106–114. (25) Voorwinden, G.; Kniese, R.; Jackson, P.; Powalla, M. In-Line Cu(In,Ga)Se2 CoEvaporation Process on 30 Cm × 30 Cm Substrates with Multiple Deposition Stages. Proceedings of the 22nd European Photovoltaic Solar Energy Conference 2007, 2115–2118. (26) Drouin, D.; Couture, A. R.; Joly, D.; Tastet, X.; Aimez, V.; Gauvin, R. CASINO V2.42—A Fast and Easy-to-Use Modeling Tool for Scanning Electron Microscopy and Microanalysis Users. Scanning 2007, 29 (3), 92–101. (27) Raghuwanshi, M.; Cadel, E.; Pareige, P.; Duguay, S.; Couzinie-Devy, F.; Arzel, L.; Barreau, N. Influence of Grain Boundary Modification on Limited Performance of Wide Bandgap Cu(In,Ga)Se2 Solar Cells. Applied Physics Letters 2014, 105 (1), 13902.


24

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Raghuwanshi, M.; Cadel, E.; Duguay, S.; Arzel, L.; Barreau, N.; Pareige, P. Influence of Na on Grain Boundary and Properties of Cu(In,Ga)Se2 Solar Cells. Prog. Photovolt: Res. Appl. 2017, 25 (5), 367–375. (29) Li, C.; Wu, Y.; Poplawsky, J.; Pennycook, T. J.; Paudel, N.; Yin, W.; Haigh, S. J.; Oxley, M. P.; Lupini, A. R.; Al-Jassim, M.; Pennycook, J.; Yan, Y. Grain-Boundary-Enhanced Carrier Collection in CdTe Solar Cells. Phys. Rev. Lett. 2014, 112 (15), 156103. (30) Poplawsky, J. D.; Paudel, N. R.; Li, C.; Parish, C. M.; Leonard, D.; Yan, Y.; Pennycook, S. J. Direct Imaging of Cl‐ and Cu‐Induced Short‐Circuit Efficiency Changes in CdTe Solar Cells. Advanced Energy Materials 2014, 4 (15) 1400454.. (31) Nicoara, N.; Lepetit, T.; Arzel, L.; Harel, S.; Barreau, N.; Sadewasser, S. Effect of the KF Post-Deposition Treatment on Grain Boundary Properties in Cu(In, Ga)Se2 Thin Films. Sci Rep 2017, 7. (32) Melanie Nichterwitz. Charge Carrier Transport in Cu(In,Ga)Se2 Thin Fillm Solar-Cells Studied by Electron Beam Induced Current and Temperature and Illumination Dependent Current Voltage Analyses, University of Berlin: Berlin, 2012. (33) Haney, P. M.; Yoon, H. P.; Gaury, B.; Zhitenev, N. B. Depletion Region Surface Effects in Electron Beam Induced Current Measurements. J Appl Phys 2016, 120 (9), 95702. (34) Heise, S. J.; Gerliz, V.; Hammer, M. S.; Ohland, J.; Keller, J.; Hammer-Riedel, I. LightInduced Changes in the Minority Carrier Diffusion Length of Cu(In,Ga)Se2 Absorber Material. Solar Energy Materials and Solar Cells 2017, 163, 270–276.


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Page 26 of 26

(35) Contreras, M. A.; Mansfield, L. M.; Egaas, B.; Li, J.; Romero, M.; Noufi, R.; RudigerVoigt, E.; Mannstadt, W. Wide Bandgap Cu(In,Ga)Se2 Solar Cells with Improved Energy Conversion Efficiency. Prog. Photovolt: Res. Appl. 2012, 20 (7), 843–850. (36) Abou-Ras, D.; Dietrich, J.; Kavalakkatt, J.; Nichterwitz, M.; Schmidt, S. S.; Koch, C. T.; Caballero, R.; Klaer, J.; Rissom, T. Analysis of Cu(In,Ga)(S,Se)2 Thin-Film Solar Cells by Means of Electron Microscopy. Solar Energy Materials and Solar Cells 2011, 95 (6), 1452– 1462. (37) Wei, S.-H.; Zhang, S. B.; Zunger, A. Effects of Na on the Electrical and Structural Properties of CuInSe2. Journal of Applied Physics 1999, 85 (10), 7214–7218.

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Evidence of Enhanced Carrier Collection in Cu(In,Ga)Se2 Grain

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