Earth abundant kesterite copper-zinc-tin-sulfide–selenide (CZTS–Se) is considered as cost-effective material for next generation solar cells. However, current CZTS–Se solar cells have much lower efficiency than CIGS solar cells. Rapid progress in achieving the target efficiency in CZTS–Se solar cells is hindered by the narrow phase stability of the quaternary phase, Cu2ZnSn(SxSe1−x)4, and the existence of other competitive and complex secondary phases and defects. This resulted in structural inhomogeneity, local fluctuation of open circuit voltage and high carrier recombination that finally lead to poor device performance and repeatability issues. The higher performance of off-stoichiometric CZTS materials, copper-poor and zinc-rich, and their inherent association with secondary phases and defects force the scientific community to investigate them together. This work aims to provide a comprehensive review for optimum growth conditions to achieve efficient kesterite CZTS–Se material under different conditions, complementary characterization techniques to detect unwanted phases, defects and defect-complexes and various approaches to reduce the secondary phases, defects and defect-complexes for higher performance in CZTS–Se solar cells. Understanding and addressing the structural inhomogeneity, control growth and material characterization are expected to yield closer performance parity between CZTS–Se and CIGS solar cells.

Strategic review of secondary phases, defects and defect-complexes in kesterite CZTS–Se solar cells

Although the substitution of Cu by Ag to suppress CuZn defects offers several advantages in overcoming the large open-circuit voltage (Voc) deficit for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, an excellent performance has not been achieved to date primarily due to the Fermi level pinning at the CdS/absorber interface and large recombination at the absorber/Mo interface. Herein, we developed a composition grading strategy to achieve a V-shaped Ag-graded structure with a higher Ag content on both the back and front surfaces of the (Cu,Ag)2ZnSn(S,Se)4 (CAZTSSe) layer. The key advantages of this Ag-graded structure are as follows: the higher content towards the CdS/absorber interface can create weak n-type donor defects and retard Fermi level pinning, whereas the lower content at the interlayer maintains the conductivity and light absorption; moreover, the other higher content towards Mo back contact can effectively suppress the recombination and improve the utilization of long-wave incident light. By appropriately adjusting the Ag gradient, we demonstrated a significant increase in Voc, and an unexpected conversion efficiency of 11.2% was achieved. This is the highest efficiency achieved to date for Ag-substituted CZTSSe solar cells, and the result supports a new aspect that synthesis of a composition-graded CAZTSSe absorber has great potential for future research.

Engineering of interface band bending and defects elimination via a Ag-graded active layer for efficient (Cu,Ag)2ZnSn(S,Se)4 solar cells

Layered gallium telluride (GaTe) has attracted much attention recently, due to its extremely high photoresponsivity, short response time, and promising thermoelectric performance. Different from most commonly studied two-dimensional (2D) materials, GaTe has in-plane anisotropy and a low symmetry with the C2h3 space group. Investigating the in-plane optical anisotropy, including the electron–photon and electron–phonon interactions of GaTe is essential in realizing its applications in optoelectronics and thermoelectrics. In this work, the anisotropic light-matter interactions in the low-symmetry material GaTe are studied using anisotropic optical extinction and Raman spectroscopies as probes. Our polarized optical extinction spectroscopy reveals the weak anisotropy in optical extinction spectra for visible light of multilayer GaTe. Polarized Raman spectroscopy proves to be sensitive to the crystalline orientation of GaTe, and shows the intricate dependences of Raman anisotropy on flake thickness, photon and...

In-Plane Optical Anisotropy of Layered Gallium Telluride

Rare earth ions doped phosphors for improving efficiencies of solar cells

Highly sensitive phototransistors based on two-dimensional (2D) GaTe nanosheet have been demonstrated. The performance (photoresponsivity, detectivity) of the GaTe nanosheet phototransistor can be efficiently adjusted by using the applied gate voltage. The devices exhibit an ultrahigh photoresponsivity of 274.3 AW−1. The detectivity of 2D GaTe devices is ∼1012 Jones, which surpasses that of currently-exploited InGaAs photodetectors (1011−1012 Jones). To reveal the origin of the enhanced photocurrent in GaTe nanosheets, theoretical modeling of the electronic structures was performed to show that GaTe nanosheets also have a direct bandgap structure, which contributes to the promotion of photon absorption and generation of excitons. This work shows that GaTe nanosheets are promising materials for high performance photodetectors.

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Highly sensitive phototransistors based on two-dimensional GaTe nanosheets with direct bandgap

Radiation detectors have been fabricated on 8 mm × 8 mm substrates, ~390 μm in thickness, diced from a (0001) 4H-SiC semi-insulating (SI) wafer (≥ 1012 Ohm-cm). The crystals used for detector fabrication have been characterized by x-ray diffraction (XRD), electron beam induced current (EBIC), chemical etching, cross-polarized imaging, and Raman spectroscopy. Current-voltage (I-V) characteristics showed very low leakage current (≤ 50 pA at -800 V) and the capability of detector's operation ≥ 470 K. EBIC investigations revealed that the screw dislocations produce dark EBIC contrast indicating high leakage current in the defective regions. Thermally stimulated current (TSC) measurements and high temperature resistivity measurements revealed deep level centers with activation energies 1.1-1.2 eV, and 1.56 eV. The TSC peak at ~460 K associated with the ~1.2 eV center was much stronger than the other high temperature peaks (e.g., 370 K due to vanadium impurity, 0.95 eV below of conduction band edge), indicating that this level along with the 1.56 eV level should dominate in controlling the resistivity and carrier lifetime in the studied 4H-SiC. Based on the literature data, we associate these centers with intrinsic defects and/or V-related complex. Nuclear detection measurements on the single-element SiC detectors with 241Am X-γ ray source clearly detected 59.6 keV and other low energy x-rays.

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Defect levels in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have been investigated by current-mode deep level transient spectroscopy. Experiments were carried out on two CZTSSe cells with photoconversion efficiencies of 4.1% and 7.1% measured under AM 1.5 illumination. The absorber layer of the 4.1% efficiency cell was prepared by annealing evaporated ZnS/Cu/Sn stacked precursor under S/Se vapor, while the absorber of the 7.1% efficiency cell was prepared by co-evaporation of the constituent elements. The 4.1% efficiency CZTSSe cell with a S/(S + Se) ratio of 0.58 exhibited two dominant deep acceptor levels at Ev + 0.12 eV, and Ev + 0.32 eV identified as CuZn(-/0) and CuSn(2-/-) antisite defects, respectively. The 7.1% efficiency cell with purely Se composition S/(S + Se) = 0 showed only one shallow level at Ev + 0.03 eV corresponding to Cu-vacancy (VCu). Our results revealed that VCu is the primary defect center in the high-efficiency kesterite solar cell in contrast to the detrimental CuZn and CuSn a...

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Defect levels in Cu2ZnSn(SxSe1−x)4 solar cells probed by current-mode deep level transient spectroscopy

In recent years, copper (Cu)-based quaternary kesterite compounds—Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4(CZTSe), and mixed chalcogenide Cu2ZnSn(SxSe1−x)4 (CZTSSe) have emerged as the potential alternative to the existing CIGS and CdTe absorbers in thin film solar cells. Comprising of earth-abundant, inexpensive, and environmentally benign consituent elements CZTS(Se) is an excellent choice for thin film photovoltaic absorber with tunable direct bandgap (1.0–1.5 eV), large optical absorption coefficient (>104 cm−1), and p-type conductivity. This chapter presents an overview of the various vacuum and non-vacuum based techniques employed for the preparation of CZTS(Se) absorber layer and discuss the physical properties of the material, defect physics, and photovoltaic performances of high-efficiency solar cells achieved following various absorber preparation routes.

Earth-Abundant Cu2ZnSn(S,Se)4 (CZTSSe) Solar Cells

Optical downconversion in rare earth (Tb3+ and Yb3+) doped CdS nanocrystals

In this work we investigated a new method of growing detector grade large GaTe layered chalcogenide single crystals. GaTe ingots (2" diameter) were grown by a novel method using graphite crucible by slow crystallization from a melt of high purity (7 N) Ga and zone refined (ZR) Te precursors in an argon atmosphere. GaTe samples from the monocrystalline area of the ingot have been cleaved mechanically and characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis by x-rays (EDX), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), transfer length method (TLM), resistivity measurements using van der Pauw technique, Hall effect, and capacitance-voltage (C-V) measurements. Our investigations reveal a novel method of growing superior quality GaTe crystals for large volume inexpensive nuclear radiation detectors.

Sandip Das

Papers

Characterization of Semi-Insulating 4H Silicon Carbide for Radiation Detectors

Defect levels in Cu2ZnSn(SxSe1−x)4 solar cells probed by current-mode deep level transient spectroscopy

Earth-Abundant Cu2ZnSn(S,Se)4 (CZTSSe) Solar Cells

Optical downconversion in rare earth (Tb3+ and Yb3+) doped CdS nanocrystals

Layered GaTe Crystals for Radiation Detectors