High Efficiency Cu₂ZnSn(S,Se)₄ Solar Cells with Shallow Li_Zn Acceptor Defects Enabled by Solution‐Based Li Post‐Deposition Treatment

Abstract

Lithium incorporation in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) materials has been experimentally proven effective in improving electronic quality for application in photovoltaic devices. Herein, a feasible and effective solution‐based lithium post‐deposition treatment (PDT), enabling further efficiency improvement on the high‐performance baseline is reported and the dominant mechanism for this improvement is proposed. In this way, lithium is uniformly incorporated into grain interiors (GIs) without segregation at grain boundaries (GBs), which can occupy the Zn sites with a high solubility in the CZTSSe matrix, producing high density of LiZn antisites with shallower acceptor levels than the intrinsic dominant defect (CuZn antisites). As a result, CZTSSe absorber with better p‐type doping is obtained, leading to a pronounced enhancement in fill factor and a corresponding gain in open‐circuit voltage and short‐circuit current and consequently a significant efficiency boost from 9.3% to 10.7%. This work provides a feasible alternative alkali‐PDT treatment for chalcogenide semiconductors and promotes a better understanding of the mechanism of Li incorporation in kesterite materials.

Band-gap-graded Cu2ZnSn(S,Se)4 drives highly efficient solar cells

“…8,9 However, the similar ion diffusion rates in the hightemperature sulfuration/selenization process and the complexity of defect physics associated with CZTSSe appear to be among the main challenges limiting the further improvement of kesterite device performance, demanding new and efficient step-change improvement strategies. 10,11 Numerous works have reported the passivation of the grain interior and grain boundary defects of CZTSSe, such as (i) the use of isoelectronic cation substitution (Ag, Li for Cu, Cd for Zn, and Ge for Sn) to reduce Cu Zn antisite defects or Sn-related deep energy level defects, [12][13][14][15][16][17][18] (ii) the artificial introduction of passivation layers at the front or back interface, [19][20][21] and (iii) alkali metal doping strategies to passivate grain boundary defects. [22][23][24] However, in the CZTSSe absorber layers, the flat bandgap structure cannot effectively improve the collection of photogenerated electrons and reduce the recombination even when efficient passivation methods are employed, requiring the development of further strategies, such as a graded bandgap.…”

Optimization of the Selenization Pressure Enabling Efficient Cu2ZnSn(S,Se)4 Solar Cells

“…Figure a–c shows the AS spectra taken from 100 Hz to 1 MHz in the temperature range of 120–300 K, and the defect level ( E a ) and the defect density ( N t ) are provided in Figure 4d–i. According to reported first‐principles calculations and experiments, [ 6,45–47 ] the defect levels of E a1 = 162 meV and E a2 = 274 meV for C‐P1 are associated with the Cu Zn and Cu Sn defects. In addition, the defect levels of E a1 = 85 meV, E a2 = 133 meV for C‐P2, and E a = 153 meV for C‐P3 are all associated with the Cu Zn defects.…”

“…[ 25,26 ] According to the theoretical calculations, Cu Zn defects with the lowest formation energy in CZTSSe absorbers grown with copper‐poor and zinc‐rich compositions mainly contribute to p‐type conductivity. [ 45,47,48 ] Here, C‐P1, C‐P2, and C‐P3 all contain Cu Zn acceptor defects, and the defect densities of N t1 = 1.4 × 10 15 cm −3 for C‐P1, N t1 = 2.6 × 10 15 cm −3 , and N t2 = 5.4 × 10 15 cm −3 for C‐P2 and N t = 2.5 × 10 15 cm −3 for C‐P3. Particularly, C‐P2 has the highest density of Cu Zn acceptor, which means that the carrier concentration provided by the Cu Zn acceptor is the highest.…”

Optimizing the Back Contact of Kesterites and Perovskites: Band Edge Design and Defect Engineering in Molybdenum Chalcogenides

“…[1][2][3] At present, the optimizing scheme of back contact of the promising kesterite and perovskite solar cells has not yet reached a consensus compared to their front contact and absorber bulk. [4][5][6][7][8][9][10] For kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) Analogously, molybdenum chalcogenide with the advantages of high mobility, low cost, and thermal stability is promising to replace the commonly used organic spiro-OMeTAD back contact material of perovskite solar cell. [14,15,[32][33][34] Experimentally, monolayer MoS 2 has been tried as the hole transport layer of perovskite CH 3 NH 3 PbI 3 solar cell.…”