Effect of Cu–Sn–Se Liquid Phase on Grain Growth and Efficiency of CZTSSe Solar Cells

Abstract

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...

Effect of Al2O3 Dot Patterning on CZTSSe Solar Cell Characteristics

“…Compared with samples without 5-nm-Al 2 O 3 patterning, the relatively upper surface morphology appears to be flattened. A previous study found that when large voids were formed in the bottom CZTSSe layer, the grain size of the upper CZTSSe formed on the top was small, and the thickness was thin [ 33 ]. When large voids were not formed in the bottom CZTSSe layer, the grain size of the upper CZTSSe formed on the top was large and thick [ 33 ].…”

“…A previous study found that when large voids were formed in the bottom CZTSSe layer, the grain size of the upper CZTSSe formed on the top was small, and the thickness was thin [ 33 ]. When large voids were not formed in the bottom CZTSSe layer, the grain size of the upper CZTSSe formed on the top was large and thick [ 33 ]. Hence, when Sn/Cu/Zn/Mo stacked precursor was used to synthesize the CZTSSe film, the fluctuating morphology of the CZTSSe film was formed.…”

High efficiency Cu2ZnSnS4 solar cells over FTO substrates and their CZTS/CdS interface passivation via thermal evaporation of Al2O3

“…In the rear contact (FTO/Mo/CZTS interfaces) big voids appear. These voids could be formed by liquid phase formation due to compositional nonuniformity during the annealing process, compositional mismatch of the metal precursor itself, and Zn volatilization during the process when using metal precursors [32,33]. In the same way, Fig 3b ) shows the sample BWA.…”

“…This parameter was calculated with respect to the maximum theoretic value represented by the Shockley-Queisser limit [29,30]. Please do not adjust margins Please do not adjust margins cells, using the well stablished Mo as substrate and without passivation layer (501 mV [13], 503 mV [31], 495 mV [32]). Fig.…”

Potential Role of Kesterites in Development of Earth‐Abundant Elements‐Based Next Generation Technology

“…The extensive grain‐sized CZTSSe absorber material was obtained by enabling more incorporation of Se during the selenization process attributed to dense morphology, lower GB recombination, and improved device efficiency. Kim et al [ 102 ] demonstrated the liquid‐assisted grain growth (LGG) mechanism for a CZTSSe absorber using a sputtering method and achieved larger grains (≈5–6 μm) at low temperatures (480 °C) and exhibited a PCE of 11.12%. Karade et al [ 103 ] studied Sn amount's effect and achieved low V oc ‐deficit characteristic by improving carrier separation in sputtered CZTSSe TFSCs, and the fabricated device exhibited a PCE of 11.04%.…”