Introduction
Preparation and stability of CZTS inks
Preparation methods
True solutions
Tab.1 Constituents of solutions in papers |
Colloidal suspensions
Tab.2 Factors influencing products of hydrothermally prepared CZTS NCs |
factor | ||||
---|---|---|---|---|
influence | additive [26] | sulfur source [28] | reaction duration [24] | reaction temperature [24] |
what | phase | phase | purity | purity |
how | ethylenediamine (EN) increases orthorhombic CZTS | different sulfur sources produce different phases | longer time promotes the formation of pure CZTS | higher purity at higher reaction temperature |
why | EN reduces the surface energy of CZTS crystals | reaction rate of Zn2+ and sulfur sources determines CZTS crystal structure | complete the reaction | higher temperature provides more energy |
Strategies for stabilization
Physical method
Carbon-chain ligands capping
Self-stabilization
Fig.5 CZTS nanoinks preparation and characterization. (a) Photos of CZTS nanoinks processed by one-pot mixing of aqueous Sn-MCC and Cu/Zn sources; (b) Raman spectrum of aqueous Sn-MCC solution; (c) FTIR spectra of CZTS inks vacuum-dried (dried ink) and the precipitation from centrifugation (centri-powder); (d) TEM morphology of the dispersed NCs with the SAD pattern; (e) high-resolution TEM image of a few NCs with measured lattice distance of 0.31nm corresponding to the (111) lattice distance of Cu/ZnS; (f) EDS analysis of as-made CZTS NCs. A Mo grid with carbon support film was used to manifest that the Cu signal is from NCs; (g) DLS characterization of the CZTS nanoink; (h) Zeta-potential curve of aqueous nanoink associated with MCC capping [13] |
Fabrication of CZTS films
Deposition techniques
Spin-coating
Spray deposition
Electrodeposition
SILAR
Tab.3 Comparison of different deposition techniques with highest efficiencies |
techniques | time-consuming | simplicity | large-scale production | solution/ suspension | quality of film | highest efficiency |
---|---|---|---|---|---|---|
spincoating [11] | no | yes | no | both | general | 6.62% |
spray [16] | no | yes | yes | both | general | 8.60% |
electrodeposition [41] | no | no | yes | solution | good | 3.40% |
SILAR [46] | yes | yes | yes | solution | general | 1.85% |
Annealing
Tab.4 Comparison of annealing conditions for CZTS film deposited using different techniques |
components | solution/ suspension | deposition technique | temperature/°C | time/min | atmosphere | reaction |
---|---|---|---|---|---|---|
Cu2S+ metal elements [14] | suspension | spin-coating | 400-530 | 30 | N2 + H2S (5%) | liquid phase sintering |
metal sulfides [52] | solution | SILAR | 200-500 | 120 | N2 + S | solid state reaction |
metal oxides [18] | suspension | doctor-blading | 250-600 | 30 | S | sulfuration |
metastable CZTS [28] | suspension | doctor-blading | 550-600 | 30 | S | phase transformation (to kesterite) |
kesterite CZTS [25] | suspension | spin-coating | 450 | 60 | N2 | growth |
metal sulfides [13] | suspension | spin-coating | 540-600 | 10-15 | N2 + Se | solid state reaction |
Fig.9 Characterization of CZTSSe film. (a) Raman curves of the CZTSSe film produced by annealing the CZTS film in an atmosphere containing different amounts of Se and S, indicating a fully tunable composition and consequently a band gap; (b) XRD pattern of the CZTSSe absorber film. Peaks indexed to Mo and MoSe2 (from substrate) as well as the standard diffraction patterns for CZTS (JSPDS 26-0575) and CZTSe (JSPDS 52-0868) are included for reference; (c) cross-sectional and (d) top-view SEM images of the CZTSSe absorber film [13] |
Optimization
Optimal compositional ratios
Tab.5 Compositional ratios of the precursor elements for as-deposited, annealed, and annealed and chemically treated samples. The as-deposited samples were deposited from stoichiometry and nonstoichiometry solutions [7] |
samples | Cu/(Zn+ Sn) | Zn/Sn |
---|---|---|
As-deposited (from stoichiometry solution) | 0.96 | 0.79 |
annealing with S (550°C) (from stoichiometry solution) | 1.01 | 0.98 |
annealing with S (550°C) + KCN (from stoichiometry solution) | 0.58 | 0.90 |
annealing with S+ Sn (550°C) (from stoichiometry solution) | 0.99 | 0.92 |
annealing with S+ Sn (550°C) + KCN (from stoichiometry solution) | 0.52 | 0.94 |
As-deposited (from nonstoichiometry solution) (-20% Cu and+20% Zn) | 1.00 | 1.13 |
annealing with S+ Sn (550°C) (from nonstoichiometry solution) (-20% Cu and+20% Zn) | 0.80 | 1.37 |
Doping
Fig.11 (a) SEM image of typical solar cell based on CZTS:Na nanocrystals. Electrical characterization of the CZTS:Na- and CZTS- based devices; (b) current-voltage (J -V) characteristics under air mass 1.5 illumination, 100 mW/cm2; (c) EQE spectrum of the device without any applied bias; (d) TRPL of the device under low injection; (e) capacitance-voltage measurement, with the measurement frequency of 11 kHz, the DC bias ranging from 0 to - 0.5 V, and the temperature at 300 K [68] |
Surface treatments
Fig.12 Electronic characterization for CZTSSe devices S2 and S3. Admittance spectroscopy (AS) of S2 (a) and S3 (b) with temperature range of 180 to 300 K; (c) the trap conductance spectra (Gm-Gd)/w; and equivalent circuit model; (d) Arrhenius plots of S3 and S2 derived from AS patterns. The estimated energetic depths of the defect (Ea) for S3 and S2 are 101 and 156 meV, respectively [71] |