Characterization of electronic structure of Cu2ZnSn(S Se1−)4 absorber layer and CdS/Cu2ZnSn(S Se1−)4 interfaces by in-situ photoemission and inverse photoemission spectroscopies

Terada, Norio; Yoshimoto, Sho; Chochi, Kosuke; Fukuyama, Takayuki; Mitsunaga, Masahiro; Tampo, Hitoshi; Shibata, Hajime; Matsubara, Koji; Niki, Shigeru; Sakai, Nobuo; Kato, Takuya; Sugimoto, H.

doi:10.1016/j.tsf.2014.09.037

Cited by 32 publications

(36 citation statements)

References 15 publications

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“…Besides, all samples show cross‐over behavior between dark and illuminated curves, which could, for example, come from the electron barrier at the CdS/CZTS junction or the back contact barrier . The latter is more likely for our case, as the CdS/CZTS interface is widely reported with a negative band offset . The device F5 shows higher series resistance than other devices seen as a slight bend down behavior of its J–V curve in the first quadrant, which might be due to the large amount of ZnS phase segregated at the back contact interface.…”

Section: Resultsmentioning

confidence: 82%

Influence of the Cu₂ZnSnS₄ absorber thickness on thin film solar cells

Ren

Scragg

Frisk

et al. 2015

Physica Status Solidi (a)

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In this study, we investigate the influence of absorber thickness on Cu 2 ZnSnS 4 (CZTS) solar cells, ranging from 500 to 2000 nm, with nearly constant metallic composition. Despite the observed ZnS and SnS phases on the surface and backside of all absorber films, scanning electron microscopy, Raman scattering, and X-ray diffraction show no large variations in material quality for the different thicknesses. The open-circuit voltage (V oc ), short-circuit current and overall power conversion efficiency of the fabricated devices show an initial improvement as the absorber thickness increases but saturate when the thickness exceeds 750 nm. External quantum efficiency (EQE) measurements suggest that the current is mainly limited by collection losses. This can result from nonoptimal bulk quality of the CZTS absorber (including the presence of secondary phases), which is apparently further reduced for the thinnest devices. The observed saturation of V oc agrees with the expected influence from strong interface recombination. Finally, an effective collection depth of 750-1000 nm for the minority carriers generated in the absorber can be estimated from EQE, indicating that the proper absorber thickness for our device process is approximately 1000 nm. Performance could be improved for thicker films, if the collection depth can be increased.

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Section: Resultsmentioning

confidence: 82%

Influence of the Cu₂ZnSnS₄ absorber thickness on thin film solar cells

Ren

Scragg

Frisk

et al. 2015

Physica Status Solidi (a)

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“…Another explanation could be that the pump-probe approach might result in an overestimation of CBO if the surface/interface is particularly defect-rich causing a Fermi level pinning [132]. Indeed, while the interface-induced band bending (iibb, see [128]) is found to be>+0.4 eV [122,124] for the CZTSe/CdS heterojunction, iibb seems to be limited to (0.0±0.1) eV [120] for the CZTS/ CdS interface. This indicates that, while the Fermi level can to some extent move freely upon the deposition of CdS at the interface to the S-free kesterite absorber, it seems to be pinned at the CZTS/CdS interface.…”

Section: Window Layermentioning

confidence: 99%

“…[S]/([S]+[Se]) absorber ratio of 0.28[122]. Without going into detail of why the determined CBO and VBO values vary somewhat in the cases where more than one data point is available (most of it can certainly be ascribed to the variations in absorber composition across laboratories and employed preparation routes), we find it possible to describe the[S]/([S]+[Se]) driven CBO and VBO evolution with confidence envelopes (depicted as gray hatched areas in figure 4) that have an uncertainty of approximately ±0.25 eV.…”

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confidence: 99%

“…Just as the optical band gap, i.e. the separation between valence band maximum (E V ) and CB minimum (E C ), can easily be tuned by the [S]/([S]+[Se]) ratio of the absorber, also the offsets between the E V and E C energy levels of the kesterite and the CdS buffer layer can be expected to change.Figure 4shows reported VB and CB offsets (VBO: circles and CBO: triangles) between CdS and CZTSSe as a function of the [S]/([S]+[Se]) ratio of the absorber[93,[120][121][122][123][124][125][126][127]. The black symbols indicate results from direct measurements of the VBO by means of UV (UPS) or x-ray (XPS) photoelectron spectroscopy and CBO by means of inverse photoemission spectroscopy from[120][121][122][123][124] (see…”

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confidence: 99%

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Back and front contacts in kesterite solar cells: state-of-the-art and open questions

et al. 2019

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We review the present state-of-the-art within back and front contacts in kesterite thin film solar cells, as well as the current challenges. At the back contact, molybdenum (Mo) is generally used, and thick Mo(S, Se) 2 films of up to several hundred nanometers are seen in record devices, in particular for selenium-rich kesterite. The electrical properties of Mo(S, Se) 2 can vary strongly depending on orientation and indiffusion of elements from the device stack, and there are indications that the back contact properties are less ideal in the sulfide as compared to the selenide case. However, the electronic interface structure of this contact is generally not well-studied and thus poorly understood, and more measurements are needed for a conclusive statement. Transparent back contacts is a relatively new topic attracting attention as crucial component in bifacial and multijunction solar cells. Front illuminated efficiencies of up to 6% have so far been achieved by adding interlayers that are not always fully transparent. For the front contact, a favorable energy level alignment at the kesterite/CdS interface can be confirmed for kesterite absorbers with an intermediate [S]/([S]+[Se]) composition. This agrees with the fact that kesterite absorbers of this composition reach highest efficiencies when CdS buffer layers are employed, while alternative buffer materials with larger band gap, such as Cd 1−x Zn x S or Zn 1−x Sn x O y , result in higher efficiencies than devices with CdS buffers when sulfurrich kesterite absorbers are used. Etching of the kesterite absorber surface, and annealing in air or inert atmosphere before or after buffer layer deposition, has shown strong impact on device performance. Heterojunction annealing to promote interdiffusion was used for the highest performing sulfide kesterite device and air-annealing was reported important for selenium-rich record solar cells. Cu 2 ZnSnS 4 (CZTS) absorbers for solar cell applications was first reported by Ito and Nakazawa [1]. Early results on CZTS device performance were reported by Friedlmeier et al [2], Seol et al [3], and Katagiri et al [4]. Later a group at IBM reached record performances with power conversion efficiencies (PCEs) above 10% for CZTSSe devices in 2010 [5] and the current record PCE of 12.6% in 2013 [6]. In 2018, researchers from DGIST reached the same performance, 12.6%, but for a larger device area [7]. The device structure used for kesterite solar cells was originally copied from that of Cu(In, Ga)Se 2 , (CIGSe) TFSCs, and is formed by sequential deposition, upon a soda lime glass substrate, of a Mo back contact, the absorber, a CdS buffer layer, and a ZnO/ZnO:Al bi-layer window (i.e. transparent top contact). This structure, schematically shown for CZTSSe in figure 1, is not necessarily ideal for kesterite solar cells, and extensive work has been invested into studies of alternative backand front contacts and related deposition processes. In this paper, the state-of-the-art and current open questions related to the back and front c...

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“…The evaluation of the band offset of the buffer layer/CZTSSe has been investigated . Tearada et al reported that the CBO of the CdS buffer layer/CZTS interface is –0.1 eV and that of the CdS buffer layer/CZTSSe interface is 0.3 eV when the S/(S + Se) ratio is 0.28 . Therefore, it is necessary to develop the buffer layer for optimization of the CBO of the buffer layer/CZTSSe interface.…”

Section: Introductionmentioning

confidence: 99%

Annealing effect on Cu₂ZnSn(S,Se)₄ solar cell with Zn_1–xMg_xO buffer layer

Hironiwa

Matsuo

Chantana

et al. 2015

Physica Status Solidi (a)

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(Zn,Mg)O buffer layer has attracted attention with the potential to control the conduction-band offset (CBO) at the interface between the buffer layer and the Cu 2 ZnSn(S,Se) 4 (CZTSSe) absorber, where the (Zn,Mg)O buffer layer is deposited by sputtering. The bandgap energy (E g ) of (Zn,Mg)O layers is tunable from 3.3 to 7.7 eV by changing the Mg/(Zn þ Mg) ratio. Thus, short-wavelength response of solar cells with a (Zn,Mg)O buffer layer is higher than that with a CdS buffer layer. Nevertheless, the solar-cell performances with a (Zn,Mg)O buffer layer is significantly lower than that with a CdS buffer layer. The low performances are attributed to the sputtering damage on the CZTSSe absorber. To recover sputtering damage, the annealing treatment was conducted on CZTSSe solar cells. However, after the annealing treatment, the performances of CZTSSe solar cells decreased. Photoluminescence (PL) spectra of the CZTSSe absorbers without and with the annealing treatment were measured to evaluate the (Zn,Mg)O/CZTSSe junction quality. From the results, the PL intensity presents a slight variation, while the E g of the CZTSSe absorbers decreases. Thus, the defects near the space-charge region activate. Consequently, the decrease of the sputtering damage was not confirmed by the annealing treatment. Therefore, a damage-free method for the deposition of (Zn,Mg)O layer is necessary.

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Characterization of electronic structure of Cu2ZnSn(S Se1−)4 absorber layer and CdS/Cu2ZnSn(S Se1−)4 interfaces by in-situ photoemission and inverse photoemission spectroscopies

Cited by 32 publications

References 15 publications

Influence of the Cu₂ZnSnS₄ absorber thickness on thin film solar cells

Influence of the Cu₂ZnSnS₄ absorber thickness on thin film solar cells

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

Annealing effect on Cu₂ZnSn(S,Se)₄ solar cell with Zn_1–xMg_xO buffer layer

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Characterization of electronic structure of Cu2ZnSn(S Se1−)4 absorber layer and CdS/Cu2ZnSn(S Se1−)4 interfaces by in-situ photoemission and inverse photoemission spectroscopies

Cited by 32 publications

References 15 publications

Influence of the Cu2ZnSnS4 absorber thickness on thin film solar cells

Influence of the Cu2ZnSnS4 absorber thickness on thin film solar cells

Back and front contacts in kesterite solar cells: state-of-the-art and open questions

Annealing effect on Cu2ZnSn(S,Se)4 solar cell with Zn1–xMgxO buffer layer

Contact Info

Product

Resources

About

Influence of the Cu₂ZnSnS₄ absorber thickness on thin film solar cells

Influence of the Cu₂ZnSnS₄ absorber thickness on thin film solar cells

Annealing effect on Cu₂ZnSn(S,Se)₄ solar cell with Zn_1–xMg_xO buffer layer