The fabrication of the CZTSSe solar cell using the developed solution route includes four procedures as depicted in Figure 2.1. The whole approach includes four steps: precursor preparation, film deposition and solvent evaporation, high-temperature selenization, and device fabrication. The details for each step are described as follows.
Figure 2.1: A schematic of solution deposition of thin-film solar cells.
2.3.1 Molecular Precursor Preparation
In the present work, “molecular precursor” or “precursor solution” refers to a pure solution in which the elements are homogeneously mixed at a molecular level. The CZTSSe molecular precursors were prepared by dissolving the cation sources and elemental chalcogens in a solvent mixture of a primary amine and a monothiol. The general preparation procedures for CZTSSe precursor solutions are: 1) dissolving the cation sources in a mixture of primary amine/s and monothiol/s to prepare solution A; 2) dissolving the anion sources in a mixture of primary amine/s and monothiol/s to prepare solution B. Here, the cation sources refer to elemental metals (Zn, Cu, etc.), metal salts (Cu(OAc)2, CuCl, SnCl2, etc.), organometallic complexes (Cu(acac)2, Sn(acac)2Cl2, etc.), and metal oxides (ZnO, Cu2O, SnO, etc.). The anion sources refer to elemental sulfur (S) and selenium (Se). The primary amine can be butylamine, hexylamine, etc. and the monothiol can be ethanethiol, propanethiol, etc. The solvent mixture can be with one or more types of primary amine and monothiol; 3) mixing cation and anion precursors at a specific volume ratio to prepare CZTSSe precursor solution. Solution conditions are optimized in order to obtain high-efficiency solar cells. Herein, the solution condition refers to the types of dissolved cation/anion sources, the concentrations of Cu and Zn with respect to Sn, the absolute concentrations of Sn, the types of primary amine/s and monothiol/s in the solvent mixture, and the amine/thiol volume ratios.
2.3.2 Film Deposition and Solvent Evaporation
The CZTSSe precursor solution was spin coated on a one-by-one inch molybdenum-sputtered (~800 nm) soda lime glass (SLG) substrate. After spin coating a layer, the coated layer was annealed inside a heating chamber with an argon atmosphere at 250oC for 5 mins in order to evaporate the solvents. This coating step was repeated until the desired film thickness was obtained.
The film deposition and solvent evaporation procedure is performed inside a nitrogen glovebox
with < 0.1 ppm water and oxygen. Figure 2.2 shows the designed heating chamber, as well as the spin coating and solvent evaporation setup in the glovebox.
Figure 2.2: A schematic drawing of spin coating and house-made graphite heating chamber in the glovebox. The heating chamber is designed for annealing the as-coated films under Ar/N2/vacuum condition and preventing the deposition of Se/S inside the glovebox. Note that
Se/S vapor is highly toxic to humans.
2.3.3 Selenization
The thin film was finally annealed at 500oC in a selenium atmosphere for 30 mins to achieve a higher crystallinity and promote grain growth. A temperature controlled tube furnace was used for the selenization purpose, as shown in Figure 2.3. The samples and selenium pellets were put into a sealed graphite box (inset in Figure 2.3). The sample box was parked at the cold zone of the quartz tube while the temperature inside the tube furnace was ramping. After the temperature at the center of tube became stable at 500oC, the graphite box was pushed into the center of the tube furnace by
a push-pull rod. The melted selenium pellets created a saturated selenium atmosphere around the samples. The amount of selenium pellets was well-controlled to create a saturated atmosphere.
Before pre-heating, the tube furnace also was purged three times using Ar to ensure the purity of the gas atmosphere. After the desired time passed, the furnace was quickly cooled to the room temperature. A constant flow of Ar was applied into the tube in order to facilitate the cooling of the samples.
Figure 2.3: Picture of tube furnace for selenization. The inset is the layout of selenium pellets and samples in a graphite box.
2.3.4 Device Fabrication
Solar cells were fabricated from the above-described CZTSSe films by creating the following additional layers: chemical bath deposition of ~50 nm cadmium sulfide (CdS), sputtering of ~80 nm intrinsic zinc oxide (ZnO) and ~220 nm tin-doped indium oxide (ITO). On the top of the device, Ni/Al metal contacts were deposited by electron-beam deposition. For some devices, a ~100 nm antireflective magnesium fluoride (MgF2) coating was applied on the top to enhance the light absorption. The top-view of a solar cell is shown in Figure 2.4a and the cross-section schematic
drawing is shown in Figure 2.4b. There are six cells with a total area of 0.47 cm2 on a one-by-one inch substrates. The active area of the solar cells is normally ~ 0.45-0.46 cm2.
Figure 2.4: a) Top-view image of the fabricated solar cell. There are six cells on a one-by-one inch substrate. b) The schematic drawing of the solar cell cross-section.
2.3.5 Characterization
The material phases, film morphology, and compositions were characterized using X-ray diffraction (XRD) (Rigaku), Raman spectroscopy with an excitation wavelength of 633 nm (HORIBA HR800 system), and scanning electron microscopy (SEM) equipped with energy X-ray dispersive spectroscopy (EDX) (FEI Quanta) equipped with an Oxford EDX silicon drift detector (SDD, X-MaxN 80 mm2).
The J-V characteristics were measured with a four-point probe station using a Keithley 2400 series sourcemeter and a Newport Oriel simulator with AM 1.5 illumination. The solar simulator was calibrated to 100 mW/cm2 using a Si reference cell certified by NIST. External quantum efficiency (EQE) was studied under 0 V and -1 V bias in order to gain more insight into the solar cell performance.
A linear superposition of Gaussian and Lorentzian distributions is used to fit the Raman spectra in this study. Equation 2.1 shows the fitting function used in Labspec software.
where A is the peak amplitude, ω is the peak FWHM, xc is the location of the peak maximum, and g is the weighting factor for the Gaussian contribution.