Atomic Layer Grown Zinc–Tin Oxide as an Alternative Buffer Layer for Cu₂ZnSnS₄-Based Thin Film Solar Cells: Influence of Absorber Surface Treatment on Buffer Layer Growth

Abstract

Zn1–x Sn x O y (ZTO) deposited by atomic layer deposition has shown promising results as a buffer layer material for kesterite Cu2ZnSnS4 (CZTS) thin film solar cells. Increased performance was observed when a ZTO buffer layer was used as compared to the traditional CdS buffer, and the performance was further increased after an air annealing treatment of the absorber. In this work, we study how CZTS absorber surface treatments may influence the chemical and electronic properties at the ZTO/CZTS interface and the reactions that may occur at the absorber surface prior to atomic layer deposition of the buffer layer. For this, we have used a combination of microscopy and synchrotron-based spectroscopies with variable information depths (X-ray photoelectron spectroscopy, high-energy X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy), allowing for an in-depth analysis of the CZTS near-surface regions and bulk material properties. No significant ZTO buffer thickness variation is observed for the differently treated CZTS absorbers, and no differences are observed when comparing the bulk properties of the samples. However, the formation of SnO x and compositional changes observed toward the CZTS surface upon an air annealing treatment may be linked to the modified buffer layer growth. Further, the results indicate that the initial N2 annealing step integrated in the buffer layer growth by atomic layer deposition, which removes Na–CO x species from the CZTS surface, may be useful for the ZTO/CZTS device performance.

Cadmium‐Free Kesterite Thin‐Film Solar Cells with High Efficiency Approaching 12%

“…In CZTSSe solar cells, several types of eco‐friendly and Cd‐free buffer layers including In 2 S 3 , ZnS(O,OH), (Zn Mg)O, Zn(O, S), and (Zn, Sn)O have been used. [ 14–30,31–35 ] Among them, (Zn, Sn)O commonly known as “ZTO” is a propitious substitute due to its non‐toxicity and promising results that have been reported previously by the ALD method. ALD technique is a highly favorable method since it is well‐suited with other vacuum‐based strategies used in CZTS/CZTSSe thin film solar cells.…”

“…[1][2][3] Therefore, CZTSSe-thin S), and (Zn, Sn)O have been used. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] Among them, (Zn, Sn)O commonly known as "ZTO" is a propitious substitute due to its non-toxicity and promising results that have been reported previously by the ALD method. ALD technique is a highly favorable method since it is well-suited with other vacuum-based strategies used in CZTS/CZTSSe thin film solar cells.…”

“…[30] Besides, ALD allows fastidious control of stoichiometry, atomic scale uniformity, and surface homogeneity across a large area and has demonstrated its effectiveness as a buffer and passivation layer in solar cells. [31][32] For example Li et al [33] deposited ZTO (50 nm thick) by ALD on the CZTSSe thin film and demonstrated that the bandgap of ZTO and conduction band offset (CBO) can be adjusted by Zinc:Tin (Zn:Sn) ratio during the ALD processing. The resulting device with ZTO exhibited a PCE of 8.6%, which was higher than the control device (8.1%) with CdS buffer layer.…”

“…In addition, the wider bandgap of ZTO facilitates to transfer greater number of photons to the CZTSSe absorber, and more photocarriers are generated in the photo-absorber in that spectral range, thus increasing Jsc (Figure 1b). Meanwhile, the ZTO buffer layer can optimize the band alignment of the [38] CZTS ALD-ZTO 0.630 18.7 49.0 5.7 Tajima et al [18] CZTS ALD-ZTO 0.653 16.1 61.9 6.5 Martinet al [32] CZTS ALD-ZTO 0.682 17.9 0.60 7.4 Platzer et al [17] CZTSSe ALD-ZTO 0.414 34.1 60.8 8.6 Li et al [33] CZTS ALD-ZTO 0.679 21.6 61.4 9.0 Ericson et al [30] CZTS ALD-ZTO 0.720 20.5 63.5 9.3 Cui et al [34] CZTS ALD-ZTO 0.746 19.1 68.0 9.7 Larsen et al [35] CZTS ALD-ZTO 0.736 22.0 0.66 10.2 Cui et al [39] CZTSSe Supttered-ZTO 0.445 36.3 69.0 11.2 Lee et al [19] CZTS Photochemical-ZTO 0.516 16.8 35.0 3.0 Zhang et al [26] CZTSSe ALD-Zn(O,S) 0.313 29.4 33.0 3.1 Hong et al [23] CZTS ALD-Zn(O,S) 0.482 17.2 0.56 4. 6 Ericson et al [25] CZTSSe ALD-Zn(O,S) 0.327 26.6 51.1 4.7 Hong et al [24] CZTSSe CBD-Zn(O,S) 0.336 25.0 51.0 5.0 Steirer et al [28] CZTSe Sulfurization-ZnO-Zn(O,S) 0.610 21.1 40.0 5.1 Yang et al [27] CZTSSe CBD-ZnS(O,OH) 0.389 29.0 52.0 5.8 Grenet et al [20] CZTSSe CBD-Zn(O,S) 0.708 19.2 54.0 7.2 Huang et al [22] CZTSSe CBD-Zn(O,S) 0.358 33.5 60.0 7.2 Li et al [21] CZTSSe ALD-Zn(O,S) 0.496 35.6 56.0 9.8 Jeong et al [29] Ag-CZTSSe ZTO 0.498 36.28 66.…”

Depth-Dependent Atomic-Scale Structural Changes in (Ag,Cu)(In,Ga)Se2 Absorbers Relevant for Thin-Film Solar Cells

“…The extended X-ray absorption fine structure (EXAFS) region of XAS enables investigation of the atomic-scale structure and gives information about the element-specific local structural parameters such as coordination numbers, bond lengths, and degree of disorder. Typically, XAS is used to characterize average bulk properties, but depth profiling and surface measurements can also be realized using angle-resolved XAS (changing the incidence angle of an incoming X-ray beam). − To the best of our knowledge, the use of angle-resolved XAS to study the atomic structure in depth of Ag-alloyed CIGS solar cells with compositional gradients has not yet been employed.…”