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Letter
Impact of a RbF Post-Deposition Treatment on the Electronic Structure of the CdS/Cu(In,Ga)Se Heterojunction in High-Efficiency Thin-Film Solar Cells 2
Dirk Hauschild, Dagmar Kreikemeyer-Lorenzo, Philip Jackson, Theresa Friedlmeier, Dimitrios Hariskos, Friedrich Reinert, Michael Powalla, Clemens Heske, and Lothar Weinhardt ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00720 • Publication Date (Web): 12 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017
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ACS Energy Letters
Impact of a RbF Post-Deposition Treatment on the Electronic Structure of the CdS/Cu(In,Ga)Se2 Heterojunction in High-Efficiency Thin-Film Solar Cells
D. Hauschild1,2,3,*, D. Kreikemeyer-Lorenzo1, P. Jackson4, T. Magorian Friedlmeier4, D. Hariskos4, F. Reinert3, M. Powalla4, C. Heske1,2,5, and L. Weinhardt1,2,5,*
1
Institute for Photon Science and Synchrotron Radiation (IPS), Karlsruhe Institute of Technology (KIT), Hermann-v.-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany 2
Institute for Chemical Technology and Polymer Chemistry (ITCP), Karlsruhe Institute of Technology (KIT), Engesserstr. 18/20, 76128 Karlsruhe, Germany
3
Experimental Physics VII, University of Würzburg, Am Hubland, 97074 Würzburg, Germany 4
Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg (ZSW), Meitnerstraße 1, 70563 Stuttgart, Germany
5
Department of Chemistry and Biochemistry, University of Nevada, Las Vegas (UNLV), 4505 Maryland Parkway, Las Vegas, NV 89154-4003, USA
*Authors
to
whom
correspondence
should
be
addressed:
[email protected],
[email protected] 1 ACS Paragon Plus Environment
ACS Energy Letters
Abstract Recently, a world-record efficiency of 22.6 % was achieved by applying a RbF post-deposition treatment (PDT) on a Cu(In,Ga)Se2 (CIGSe) thin-film solar cell absorber surface. Here, we study the impact of this RbF-PDT on the electronic structure of the CIGSe surface and the CdS/CIGSe interface using ultraviolet and x-ray photoelectron spectroscopy (UPS and XPS), as well as inverse photoemission spectroscopy (IPES). After RbF-PDT, we find a small downward shift of the band edges, while the surface band gap value itself is not affected. In addition, a further downward band bending in the CIGSe absorber is observed upon formation of the RbF-PDT CdS/CIGSe interface. We derive a flat conduction band alignment between the RbF-PDT CIGSe absorber and the CdS buffer, commensurate with the high efficiencies of solar cell devices prepared with RbF-PDT.
Table of Contents Image
CBO: 0.03 ± 0.16
1.0 0.5 Energy rel. EF (eV)
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CBM 0.64
0.52
0.50 EF
0.0
-0.5
0.82
0.99
2.10
-1.0 VBO: -1.06
-1.5 -2.0 -2.5
no - PDT CIGSe surface
RbF - PDT ± 0.14 CIGSe surface interface
CdS surface
VBM
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Using a heavy alkali fluoride (i.e., RbF or CsF) post-deposition treatment (PDT) of Cu(In,Ga)Se2 (CIGSe) surfaces, solar-cell efficiencies exceeding 22% have recently been achieved1, marking another significant improvement after the original introduction of the KF-PDT2–4. Numerous efforts were undertaken to control, improve, and understand the impact of the KFPDT treatment2,5–15. In comparison with CIGSe absorbers that did not undergo PDT, several studies found a more pronounced copper depletion at the KF-PDT CIGSe surface,2,9,11,15. Furthermore, Pianezzi et al. attribute the beneficial role of KF-PDT to a stronger electronically inverted CIGSe surface region10. Pistor et al.9 used x-ray photoelectron spectroscopy (XPS) to investigate the influence of the KFPDT on the CIGSe surface, reported a valence band maximum shift away from the Fermi level (EF), and suggested a band-gap widening as a result. A similar conclusion was reached in a later study using a combination of ultraviolet and inverse photoelectron spectroscopy (UPS/IPES)11. It was speculated that an In2Se3 or a K-In-Se passivation layer is formed at the CIGSe surface13. In contrast, a study using hard x-ray photoelectron spectroscopy (HAXPES) reported an upward shift of the VBM of ~0.2 eV (i.e., towards EF) after KF-PDT of a CIGSe sample12. In this paper, we report on the impact of the RbF-PDT on the CIGSe absorber surface, as well as on the electronic structure of the CdS/CIGSe heterojunction, and find that the band gap of the absorber surface is not affected, while the interface energetics significantly change upon RbF-PDT. Two sets of identical CIGSe absorbers were prepared by co-evaporation of Cu, In, Ga, and Se onto a Mo back contact at the Zentrum für Sonnenenergie- und Wasserstoff-Forschung (ZSW) (details of this process can be found elsewhere 5,16). Subsequently, one set of samples underwent a RbF post-deposition treatment (similar to the reported KF treatment2), while the other set remained untreated. Afterwards, some of the absorbers in the two sets (RbF-PDT and no-PDT) were coated with CdS with increasing thicknesses (nominally 0 to 10 nm) after immersing them for 2, 90, and 180 sec in a CdS chemical bath (CBD). The 2 sec process corresponds to the adsorption of a very thin “growth start layer” on the CIGSe surface (note that the growth start is very complex, see, e.g.,
17–19
). The 90 sec process aimed at depositing a CdS layer with a
thickness in the range of 1 to 3 nm, and the 180 sec process corresponds to the formation of a closed CdS layer. Note that the thickness and coverage of the CdS layer at its early growth stage is furthermore influenced by the CIGSe grain orientation20, the oxidation state of the CIGSe 3 ACS Paragon Plus Environment
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surface21, and the modification of the CIGSe surface region after PDT22. Hence, we observed that the 180 sec CBD-CdS film deposited on the RbF-PDT CIGSe absorber represents a closed layer, while the CdS layers on no-PDT CIGSe absorbers are less homogeneous and likely not closed22. “Twin” devices of the here-studied RbF-PDT samples, completed in the standard ZSW process, routinely show average efficiencies of about 22 %, peaking at the certified world-record efficiency of 22.6 %1. The no-PDT “twin“ devices have reached a maximum efficiency of 20.3 %. For further details (e.g., I-V curves), the reader is referred to Ref. [1]. The two sample sets were briefly exposed to air (less than 5 min), sealed under nitrogen atmosphere, and shipped to the University of Würzburg for characterization. In Würzburg, the samples were unpacked and loaded into the UHV system (base pressure below 2 × 10-10 mbar) without additional air exposure. The measurements were carried out with a nonmonochromatized Mg/Al Kα x-ray source (for XPS), a He discharge lamp (for He I and He II UPS), and a VG CLAM 4 electron analyzer. The XPS survey and detail spectra were recorded with pass energies of 50 and 20 eV, respectively. No charging was observed. While the absorberrelated UPS spectra were recorded with He II, the buffer-related spectra were recorded with He I in order to avoid an overlap of the strong He II-excited satellites of the Cd 4d level with the valence band maximum region. For IPES, a Staib NEK150 electron source and a custom-built photon detector with a Hamamatsu R6834 photomultiplier and a Semrock Hg01-245 mercury line filter were employed (central photon energy of 4.88 eV)23. A total energy resolution of 100 and 375 meV was achieved for UPS and IPES, respectively, as derived from the Fermi edge of a sputter-cleaned copper foil that was also used for calibration. In order to remove adsorbates and oxides from the sample surfaces24 and therefore to “uncover” the absorber surface, a low-energy argon ion treatment was performed25. An ion energy of 50 eV was chosen (at a sample current of 50 - 100 nA/cm²) to prevent the surface damage observed at higher energies (e.g., 500 eV)26. Prior to the Ar+-treatment and after each treatment step, UPS/IPES as well as XPS measurements were performed to monitor the decrease of surface adsorbates and to rule out the formation of unwanted metallic phases. The CIGSe absorbers were cleaned in 30 min steps, and a clear decrease of the O1s signal and a reduction of oxide species (e.g., InOx) were already observed after the first treatment step. The ion treatment steps were continued until the spectral shape of the valence and conduction bands did not further change 4 ACS Paragon Plus Environment
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between two ion treatment steps (i.e., 90 min for the CIGSe surface). XPS spectra of the RbFtreated absorber (not shown) indicate clear Rb-related signals (as expected) and a slightly different surface composition (e.g., less copper at the surface) than the non-treated CIGSe absorber surface (unpublished). The surface of the 180 sec CBD CdS layer was cleaned in two steps of 10 minutes each. Note that, upon ion treatment, a small fluorine signal (2-3 %) was detected on both absorber types – with and without PDT. Furthermore, for the 180 sec CdS/Cu(In,Ga)Se2 sample, we detected a small amount of zinc (< 1 %) at its surface, which is tentatively assigned to residual amounts of zinc in the Cd source of the chemical bath.
In Fig. 1, the UPS and IPES spectra of the RbF-PDT absorber and the no-PDT absorber (bottom), as well as the 180 sec CBD-CdS/RbF-PDT CIGSe layer (top) are presented. All sample surfaces were cleaned with a low-energy Ar+-treatment (90 min for the absorbers and 40 min for the CdS buffer, respectively). The UPS spectra of the PDT and no-PDT CIGSe differ significantly. As expected, the no-PDT sample exhibits several distinct features, e.g., the feature at about 3 eV, which is associated with the Cu 3d-derived band27. In contrast, the UPS spectrum of the PDT CIGSe absorber shows significantly reduced spectral intensity in this range. The IPES spectra also differ: after RbF-PDT, the main edge is shifted by ~ 0.05 eV towards EF, some additional intensity is found close to the conduction band minimum (CBM), and the peak at approx. 2.5 eV is less pronounced after treatment. In order to determine the valence band maximum (VBM) and the CBM at both surfaces, we use a linear extrapolation of the leading edges28 of the UPS and IPES spectra, respectively (see inset of Fig. 1). The resulting VBM and CBM values of the RbF-PDT CIGSe and no-PDT CIGSe absorber, as well as the resulting surface band gaps, are listed for increasing Ar+ ion treatment times in Tab. 1.
After 60 and 90 min of Ar+ treatment, we find a downward shift of the CBM of the RbF-PDT CIGSe absorber surface; the effect for no-PDT CIGSe is less pronounced. The CBM of the clean CIGSe surface is 0.12 ± 0.07 eV lower for RbF-PDT than for the no-PDT case. This result is in contrast to a reported upward shift of the CBM (of 0.39 eV) for KF-PDT CIGSe absorbers11. For 5 ACS Paragon Plus Environment
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the valence band maximum, we also find a downward shift (of 0.17 ± 0.05 eV) for the RbFtreated surface when compared to the no-PDT sample. Similar downward shifts of the VBM were also observed after KF-post-deposition treatment9,11. For the no-PDT absorber, we find an electronic surface band gap of 1.46 ± 0.11 eV. This value is in agreement with other surfacesensitive measurements (e.g., 1.4 ± 0.15 for CuInSe2 26 and 1.4 ± 0.2 for CuIn(S,Se)227,29) and can be explained with a copper-poor absorber surface composition26,30 (while the expected bulk band gap is much smaller). For the Rb-PDT sample, we determine an electronic surface band gap of 1.51 ± 0.11 eV. Hence, the electronic surface band gaps for the no-PDT and Rb-PDT samples differ by less than the error in the surface band gap determination. This is in contrast to the reported strong increase of the surface band gap for after KF-PDT, which was explained by the formation of a K-In-Se compound11,13. For the CdS buffer layer, we determine an electronic surface band gap of (2.60 ± 0.14) eV, slightly higher than the values previously reported for CBD-CdS surfaces25,26,31. We tentatively ascribe this to the presence of additional Cd-species (e.g., Cd(OH)2 or CdSO4)32 at the CdS buffer layer surface. (Nevertheless, we refer to the buffer layer as CdS in this paper). In addition, the detected zinc atoms at the CdS surface and a possible contribution of S-Zn bonds (Egap of ZnS: ~3.6 eV33) could also contribute to an increase of the measured surface band gap. With the corresponding CBM and VBM values (as derived by the linear extrapolations shown in Fig. 1) and the changes in band bending induced by the interface formation (as derived by XPS core level shifts), we can determine the offsets at the absorber/buffer interface. For the latter, we used a “thin” buffer layer sample (i.e., 90 sec CdS-CBD), where absorber-related XPS peaks are still visible. The interface-induced band bending can be derived by comparing the relative shift of the pure absorber surface core levels (i.e., Se 3d5/2, In 3d5/2, Cu 2p3/2, and Ga 2p3/2) to those with a thin buffer layer. The corresponding relative shifts are presented in Tab. 2.
On average, the absorber-related core levels show a negligible shift (-0.01 ± 0.07 eV) for the noPDT, while a significant shift (-0.15 ± 0.10 eV) is found for the RbF-PDT absorber. This result suggests that the Fermi level is pinned for the no-PDT interface, while the RbF-PDT causes an additional downward band bending, presumably by passivating the defects that would otherwise lead to a Fermi level pinning. Such an additional downward band bending is expected to be 6 ACS Paragon Plus Environment
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ACS Energy Letters
beneficial since it decreases interface recombination34,35, and was also suggested by Pianezzi et al. for the KF-PDT CdS/CIGSe interface (based on temperature-dependent J–V measurements and solar-cell device simulations)10. In order to determine a potential band bending in the buffer layer, we evaluate the Cd 3d5/2 and S 2p3/2 core levels of the RbF-PDT CdS/CIGSe samples and find a relative shift of -0.10 ± 0.01 eV. Together with the VBM and CBM energies derived for the RbF-PDT absorber and buffer layer surface, this shift can be used to derive the full band diagram of the RbF-PDT CdS/CIGSe interface, as illustrated in Fig. 2. We find a flat conduction band alignment of (0.03 ± 0.16) eV and a negative valence band offset of (-1.06 ± 0.14) eV for the RbF-PDT CdS/CIGSe interface. Note that the 180 sec CBD CdS film for the no-PDT sample series22 did not appear to be completely closed, and thus it is not possible to reliably determine a potential band bending in the CdS layer for this sample series.
The flat band alignment and the additional downward band bending in the absorber surface are commensurate with the superior performance of the RbF-treated device: A flat band alignment is usually reported for high-efficiency copper-poor absorber surfaces14,26,36, as it allows for an unimpeded minority charge carrier transfer with minimal interface recombination34,35. In summary, we have investigated the RbF post-deposition-treated CIGSe surface, the nontreated CIGSe surface, and the corresponding CdS/CIGSe interfaces by means of x-ray and UV photoelectron spectroscopy, as well as inverse photoemission spectroscopy. Our results show that the absorber band edges shift downward upon RbF-PDT, while the surface band gap remains constant within the error bar. In addition, our results indicate that the RbF-PDT leads to an additional downward band bending in the CIGSe absorber upon interface formation with the CdS buffer. This result is in agreement with the reduced interface recombination reported for postdeposition-treated chalcopyrite surfaces. Taking the interface-induced band bending into account, we derive a flat conduction band alignment (0.03 ± 0.16) eV at the RbF-PDT CdS/CIGSe interface.
Acknowledgement 7 ACS Paragon Plus Environment
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The ZSW acknowledges the CIS-ProTec project (FKZ 0325715A) for funding.
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UPS He I
IPES
CdS/Cu(In,Ga)Se2
Intensity (arb. u.)
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ACS Energy Letters
He II Cu(In,Ga)Se2
2.60 eV
1.51 eV 1.46 eV -4 -2 0
2
∆ = 0.05 eV
RbF-PDT No-PDT
-8
-6
-4 -2 0 2 Energy rel. EF (eV)
4
6
Fig. 1 UPS and IPES spectra of the Cu(In,Ga)Se2 absorber with (red) and without (black) RbFPDT (bottom), as well as the CBD-CdS/Cu(In,Ga)Se2 interface after RbF-PDT (top, 180 s CBDCdS). All samples were treated with 50 eV Ar+ ions at a sample current of 50-100 nA/cm² (absorbers: 90 min, CBD-CdS: 20 min). The spectra are plotted on a common energy axis relative to EF. IPES spectra are shown as measured (dots) and after smoothing with a Savitzky9 ACS Paragon Plus Environment
ACS Energy Letters
Golay filter (thin solid line). The inset shows the region of the band extrema of the absorbers with a linear extrapolation (blue) of the leading edge. The derived surface band gaps are also shown.
CBO: 0.03
1.0 0.5 Energy rel. EF (eV)
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0.0
-0.5
± 0.16
0.52
0.50
± 0.10
± 0.10
0.99
2.10
± 0.05
± 0.05
EF
-1.0 VBO: -1.06
-1.5
± 0.14
-2.0 -2.5
CIGSe surface
interface
CdS surface
Fig. 2 Illustration of the band alignment at the Rb-PDT CdS/Cu(In,Ga)Se2 interface. The determined band edges at the CIGSe and CdS surfaces are shown on the left and right, respectively. In the center, the conduction and valence band offsets (CBO and VBO, respectively) at the interface are shown, taking into account the interface-induced band bending.
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Table 1. Conduction band minimum, valence band maximum, and electronic surface band gap energies of the no-PDT and RbF-PDT CIGSe absorbers after different Ar+ ion treatment steps. The values in parentheses are still influenced by the presence of adsorbates and are thus not used for the determination of the band alignment. CIGSe Ar+
No-PDT CBM (eV)
RbF-PDT
VBM (eV)
Egap (eV)
CBM (eV)
VBM (eV)
Egap (eV)
treatment time (min) 0
(0.62±0.15) (0.75±0.05) (1.37±0.16) (0.70±0.10) (1.08±0.05) (1.78±0.11)
30
(0.73±0.15) (0.82±0.05) (1.55±0.16) (0.79±0.10) (1.11±0.05) (1.90±0.11)
60
0.64±0.10
0.80±0.05
1.44±0.11
0.57±0.10
0.97±0.05
1.54±0.11
90
0.64±0.10
0.82±0.05
1.46±0.11
0.52±0.10
0.99±0.05
1.51±0.11
Table 2. Core-level binding energies of the RbF-PDT CIGSe absorber, the no-PDT CIGSe absorber, and the respective 90 sec CBD-CdS/CIGSe samples. The relative shifts, as well as their average, are shown to determine the additional band bending at the absorber surface upon interface formation. No-PDT Core Level
Se 3d5/2
In 3d5/2 (eV)
Cu 2p3/2 (eV)
Ga 2p3/2 (eV)
(eV)
average shift (eV)
CIGSe
54.60
445.22
932.66
1118.34
90s CdS/CIGSe
54.59
445.25
932.77
1118.24
Shift
0.01
-0.03
-0.11
0.10
-(0.01 ± 0.07)
RbF-PDT CIGSe
54.49
445.14
932.56
1118.25
90s CdS/CIGSe
54.51
445.24
932.87
1118.40
Shift
-0.03
-0.10
-0.31
-0.15
-(0.15 ± 0.10)
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References:
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
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