Table S1 The deposition conditions of the Zn_1-xMg_xO and Zn_1-xMg_xO:Al films by the co-sputtering process under room temperature (25°C).

Table S2 Simulation parameters for each layer in the flexible and Cd-free CIGSSe solar cells with a structure in Figure S1 consisting of CIGSSe absorber/buffer/TCO using SCAPS.

Table S3 Numerical photovoltaic performances of the flexible and Cd-free CIGSSe solar cells with a structure in Figure S1 based on input parameters in Table S2, where the degenerate TCO with F_n of TCO > E_C of TCO as well as the TCO with F_n of TCO = E_C of TCO are applied in the device. Here, in a case of the degenerate TCO with F_n of TCO > E_C of TCO, the ΔE_C-TA is defined by F_n of TCO – E_C of CIGSSe absorber. The E_C of CIGSSe absorber is −3.94 eV. In a case of the TCO with F_n of TCO = E_C of TCO, the ΔE_C-TA is defined by E_C of TCO – E_C of CIGSSe absorber. Their corresponding energy band diagrams in Table S3 are depicted in Figure S6. Here, the CBO is fixed at +0.11 eV. The E_V, E_C, F_n, electronic E_g, and optical E_g are shown in Table S3.

Table S4 Simulation parameters for each layer in the flexible and Cd-free CIGSSe solar cells with a structure in Figure S1 consisting of CIGSSe absorber/buffer/TCO using SCAPS.

Table S5 The CBO, ΔE_C-TA, ΔE_C-TB, and numerical photovoltaic performances at positions I, II, and III in Figure S14. The CBO is kept at 0 eV, leading to the ΔE_C-TA equal to the ΔE_C-TB.

Figure S1. Schematic layers of the flexible and Cd-free CIGSSe solar cells fabricated by all-dry process with a structure of SUS/glass (SiO₂)/Mo/CIGSSe absorber/buffer/TCO. In this experiment, the Zn_1-xMg_xO and Zn_1-xMg_xO:Al were applied as buffers and TCO layers, respectively. The Mg content ([Mg]/([Mg] + [Zn])) of Zn_1-xMg_xO was varied from 0 (pure ZnO) to 0.41, whereas that of the Zn_1-xMg_xO:Al was changed from 0 (pure ZnO:Al) to 0.19, respectively.

Figure S2. Conduction and valence bands of the CIGSSe solar cells with a structure in Figure S1 for the determinations of CBO, ΔE_C-TA, and ΔE_C-TB values. The CBO is determined by E_C of buffer – E_C of absorber, and ΔE_C-TA is defined by E_C of TCO – E_C of absorber. The ΔE_C-TB is calculated by E_C of TCO – E_C of buffer. The corresponding conduction bands in the case of CBO, ΔE_C-TA, and ΔE_C-TB values of over 0 (spike-like formation) are shown in Figure S3(a). The corresponding conduction bands in the case of CBO, ΔE_C-TA, and ΔE_C-TB values of below 0 (cliff-like formation) are presented in Figure S3(b).

Figure S3. Conduction bands in the CIGSSe device in Figure S1 in the case of (a) CBO, ΔE_C-TA, and ΔE_C-TB values of over 0 (spike-like formation) as well as (b) CBO, ΔE_C-TA, and ΔE_C-TB values of below 0 (cliff-like formation). Zn_1-xMg_xO and Zn_1-xMg_xO:Al are denoted by ZMO and AZMO, respectively.

Figure S4 Examples of (a) transmittance (T) and reflectance (R) of ZnO and Zn_0.84Mg_0.16O films on SLG substrates, as well as (b) their plots of (αhν)¹ as a function of photon energy (hν).

Figure S5 Examples of PYS spectra of ZnO and Zn_0.84Mg_0.16O films on SLG substrates.

Figure S6 Zoom-in numerical energy band diagrams of the corresponding solar cells in Table S3 in cases of the degenerate TCO with F_n of TCO > E_C of TCO and the TCO with F_n of TCO = E_C of TCO with the ΔE_C-TA values of (a) -0.082 eV, (b) -0.002 eV, (c) + 0.248 eV and (d) + 0.288 eV, respectively. F_n and F_p denote the quasi-Fermi levels of electrons and holes, respectively. It is noted that the CBO is constant at +0.11 eV. The simulation was conducted under the bias of 0 V.

Figure S7 Numerical conversion efficiency (η) of the flexible and Cd-free CIGSSe solar cells with a structure in Figure S1 based on input parameters in Table S2, where the degenerate TCO with F_n of TCO > E_C of TCO as well as the TCO with F_n of TCO = E_C of TCO are applied. Here, in a case of the degenerate TCO with F_n of TCO > E_C of TCO, the ΔE_C-TA is defined by F_n of TCO – E_C of CIGSSe absorber. In a case of the TCO with F_n of TCO = E_C of TCO, the ΔE_C-TA is defined by E_C of TCO – E_C of CIGSSe absorber. It is noted that the CBO is constant at +0.11 eV.

Figure S8 The corresponding 3D mapping images of (a) experimental η, (b) J_SC, (c) FF, and (d) V_OC values as functions of CBO and ΔE_C-TA in Figure 5 for the flexible, Cd-free, and all-dry process CIGSSe solar cell on SUS substrates with a structure in Figure S1. The experimental CBO and ΔE_C-TA values are based on Figure 1.

Figure S9 The experimental J-V characteristics of the flexible, Cd-free, and all-dry process CIGSSe solar cell on SUS substrates with a structure in Figure S1 at positions A, B, and C in Figure 5, and Figure 6, as well as Table 2. The J-V characteristics of the solar cells were measured in the temperature range from 200 K to 350 K under an equivalent AM 1.5G illumination using a cryostat cooled with liquid-N₂ and heated by temperature controller (Model 9,700, Scientific Instruments). Temperature is denoted by T.

Figure S10 (a) J_SC and (b) V_OC as a function of temperature for the real Cd-free, flexible, and all-dry process CIGSSe solar cells in Figure S1 at positions A, B, and C in Figure 5, and Figure 6, as well as Table 2. The graphs were made based on Figure S9.

Figure S11 Suns-V_OC characteristics of the flexible, Cd-free, and all-dry process CIGSSe solar cell on SUS substrates with a structure in Figure S1 at positions A, B, and C in Figure 5, and Figure 6, as well as Table 2. The Suns-V_OC characteristics were measured by WCT − 120 (Sinton Instruments).

Figure S12 Mott-Schottky plots (1//C²-V) of the flexible, Cd-free, and all-dry process CIGSSe solar cell on SUS substrates with a structure in Figure S1 at positions A, B, and C in Figure 5, and Figure 6, as well as Table 2. The measurements were conducted using LCR meter (Hewlett-Packard 4284A) under a frequency of 10 kHz. The V_bi is built-in potential, and the N_A are carrier density.

Figure S13 The J-V characteristic of the flexible, Cd-free, and all-dry process CIGSSe solar cell on SUS substrates with a structure in Figure S1 at position B (CBO of +0.11 eV and ΔE_C-TA of +0.16 eV) in Figure 5, 6 and 7, which was fabricated by all-dry process (Dry). And the J-V characteristic of the reference flexible CIGSSe solar cell with a structure of SUS/glass/Mo/CIGSSe/CBD-Zn(O,S,OH)_x buffer/ZnO:B/Ni-Al grid, where the CBD-Zn(O,S,OH)_x buffer was deposited by wet-process chemical bath deposition process (Wet). Heat-light soaking process at 110°C for 4 hours under AM1.5G illumination was conducted on both solar cells to further increase the photovoltaic performances.²

Figure S14 Contour maps of (a) numerical η values as functions of CBO and ΔE_C-TA and (b) numerical η values as functions of CBO and ΔE_C-TB for the CIGSSe solar cells with a structure in Figure S1. The simulation was conducted using SCAPS under the variations of CBO, ΔE_C-TA, and ΔE_C-TB values based on Table S4. The E_V values in Table S4 are mainly obtained from the PYS measurement. N/A denotes not available data. The CBO at positions I, II, and III is constant at 0 eV, leading to the ΔE_C-TA equal to the ΔE_C-TB.