We fabricated CZTSSe thin films using optimized SLG-Mo/Zn/Cu/Sn (MZCT) as a stacked structure and described the phenomenon of Zn elemental volatilization using the MZCT stacked structure.

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Using an appropriate SeS2/Se weight ratio, band gap grading was realized. By increasing the value of VOC through band gap grading in the depletion region, a record VOC deficit of 0.576 V and an efficiency of 12.3% were obtained.

For kesterite copper zinc tin sulfide/selenide (CZTSSe) solar cells to enter the market, in addition to efficiency improvements, the technological capability to produce flexible and large-area modules with homogeneous properties is necessary. Here, we report a greater than 10% efficiency for a cell area of approximately 0.5 cm 2 and a greater than 8% efficiency for a cell area larger than 2 cm 2 of certified flexible CZTSSe solar cells. By designing a thin and multi-layered precursor structure, the formation of defects and defect clusters, particularly tin-related donor defects, is controlled, and the open circuit voltage value is enhanced. Using statistical analysis, we verify that the cell-to-cell and within-cell uniformity characteristics are improved. This study reports the highest efficiency so far for flexible CZTSSe solar cells with small and large areas. These results also present methods for improving the efficiency and enlarging the cell area.

Although it has been reported that grain boundaries have not to adversely affect solar cell characteristics in CIGS and halide perovskite solar cell, nevertheless, an effective strategy for efficient carrier management in a CZTS layer is to make the grain size not too small. Generally, grain boundary control is a key concern for polycrystalline thin-film solar cells. [5][6][7][8][9][10][11] A light absorber consisting of small grains can degrade the device performance due to the vertical current flow through the multiple grain boundaries. Current and voltage loss can derive from nonradiative recombination of electrons and holes and scattering at grain boundaries. In fact, CZTSSe solar cells with efficiencies above 12% have a grain size over micrometer scale. [12][13][14][15] Generally, as the annealing temperature and time increase, the grain size increases. The efficiency is expected to decrease when the temperature and annealing time are increased due to the decomposition of CZTSSe by Mo [16,17] and the increase in the MoSSe thickness [16] and Sn loss. [18] Therefore, a liquid-assisted grain growth (LGG) method could be a good method for increasing the grain size at low temperatures over a short time while suppressing the Sn loss, growth of MoSSe and CZTSSe decomposition by Mo. The liquid phase that exists during grain growth plays a role as a diffusion path necessary for material movement such that grain growth more effectively occurs. [19] To date, liquid-assisted grain growth (LGG) has been achieved by controlling the partial pressure of chalcogen vapor to form a liquid phase at the grain boundary of CZTSSe nanoparticles. [7,20] Liquid Cu-Se causes LGG in a Cu-rich composition, and the vapor-liquidsolid (VLS) model has been adopted; [21,22] similarly, LGG might occur in the GeSe 2 -Se system. [23][24][25] Additionally, LGG can occur through the eutectic reaction of the Na-Se system; [24][25][26][27] a similar eutectic reaction can occur in other alkali-chalcogen (AX) systems (AX; A = Li, Na, K, Cs, Rb; X = Se, Te). [28] In addition, LGG might occur due to liquid phase generation upon the addition of dopants, such as Sb 2 S 3 , [29] CuSbS 2 , [29] and NaSb 5 S 8 ; [29] similar eutectic reactions can occur in other similar systems (ASb 5 X 8 ; A = Li, Na, K, and Cs, Rb; X = S, Se, and Te). [28] Additionally, LGG can occur with Ag substitution due to the Ag-related alloy. [30] The similarity is the liquid phase formation due to the existence of the eutectic reaction point (Solid A + Solid B → Liquid) or the liquidus line at the process Herein, a liquid-assisted grain growth (LGG) mechanism for a vacuumprocessed Cu 2 ZnSn(S 1−x Se x ) 4 (CZTSSe) absorber that is enabled by the presence of a liquid phase containing predominantly Cu, Sn, and Se (L-CTSe) is suggested to explain the large grain size of up to ≈6 µm obtained at low temperatures, such as 480 °C. In this system, LGG plays a key role in achieving a large grain CZTSSe absorber, but the residual L-CTSe, a key factor in LGG, deteriorates the device performa...

Recently, highly efficient CZTS solar cells using pure metal precursors have been reported, and our group created a cell with 12.6% efficiency, which is equivalent to the long-lasting world record of IBM. In this study, we report a new secondary phase formation mechanism in the back contact interface. Previously, CZTSSe decomposition with Mo has been proposed to explain the secondary phase and void formation in the Mo-back contact region. In our sulfo-selenization system, the formation of voids and secondary phases is well explained by the unique wetting properties of Mo and the liquid metal above the peritectic reaction (η-Cu6Sn5 → ε-Cu3Sn + liquid Sn) temperature. Good wetting between the liquid Sn and the Mo substrate was observed because of strong metallic bonding between the liquid metal and Mo layer. Thus, some ε-Cu3Sn and liquid Sn likely remained on the Mo layer during the sulfo-selenization process, and Cu–SSe and Cu–Sn–SSe phases formed on the Mo side. When bare soda lime glass (SLG) was used as a substrate, nonwetting adhesion was observed because of weak van der Walls interactions between the liquid metal and substrate. The Cu–Sn alloy did not remain on the SLG surface, and Cu–SSe and Cu–Sn–SSe phases were not observed after the final sulfo-selenization process. Additionally, Mo/SLG substrates coated with a thin Al2O3 layer (1–5 nm) were used to control secondary phase formation by changing the wetting properties between Mo and the liquid metal. A 1 nm Al2O3 layer was enough to control secondary phase formation at the CZTSSe/Mo and void/Mo interfaces, and a 2 nm Al2O3 layer was enough to perfectly control secondary phase formation at the Mo interface and Mo–SSe formation.

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When CZTSSe is synthesized using a metal precursor, large voids of nonuniform size form at Mo back contact side. Herein, we demonstrate that the voids and CZTSSe in the lower part of the CZTSSe double layer can be controlled by using an Al2O3-patterned Mo substrate. The CZTSSe in the lower part self-aligns on the Mo-exposed area, while the voids self-align on the Al2O3-coated area. The origin of the self-alignment is expected to be the difference in bonding characteristics between liquid Sn and the metal or oxide surface, e.g., Al2O3. Good wettability generally forms between nonreactive liquid metals and metal surfaces due to the strong metallic bonding. By contrast, poor wettability generally forms between nonreactive liquid metals and oxide surfaces due to the weak van der Waals bonding between the liquid metal and the oxide layer. When the patterning was added, the device efficiency tended to decrease from 8.6% to 10.5%.