Interface structure and band alignment of CZTS/CdS heterojunction: An experimental and first-principles DFT investigation

We report a phase-pure kesterite Cu2ZnSnS4 (CZTS) thin films, synthesized using radio frequency (RF) sputtering followed by low-temperature H2S annealing and confirmed by XRD, Raman spectroscopy and XPS measurements. Subsequently, the band offsets at the interface of the CZTS/CdS heterojunction were systematically investigated by combining experiments and first-principles density functional theory (DFT) calculations, which provide atomic-level insights into the nature of atomic ordering and stability of the CZTS/CdS interface. A staggered type II band alignment between the valence and conduction bands at the CZTS/CdS interface was determined from Cyclic Voltammetry (CV) measurements and the DFT calculations. The conduction and valence band offsets were estimated at 0.10 and 1.21 eV, respectively, from CV measurements and 0.28 and 1.15 from DFT prediction. Based on the small conduction band offset and the predicted higher positions of the VBmax and CBmin for CZTS than CdS, it is suggested photogenerated charge carriers will be efficient separated across the interface, where electrons will flow from CZTS to the CdS and and vice versa for photo-generated valence holes. Our results help to explain the separation of photo-excited charge carriers across the CZTS/CdS interface and it should open new avenues for developing more efficient CZTS-based solar cells

Structural and optical properties of CdTe thin films deposited using RF Magnetron sputtering

In this work, we have studied the influence of RF power on structural and optical properties of CdTe thin films deposited by indigenously designed locally fabricated RF magnetron sputtering. Films were analyzed by using variety of techniques such as low angle X- ray diffraction, UV-Visible spectroscopy, Raman spectroscopy etc. to study its structural and optical properties. Low angle XRD analysis showed that CdTe films are polycrystalline and has cubic structure with preferred orientation in (111) direction. Raman scattering studies revealed the presence of CdTephase over the entire range of RF power studied. The UV-Visible spectroscopy analysis showed that the band gap decreases with increase in RF power. However, CdTe films deposited at higher RF power has optimum band gap values (1.44-1.60 eV). Such optimum band gap CdTe can be use as absorber material in CdS/CdTe and ZnO/CdTe solar cells

Electrochemical synthesis of core-shell ZnO/CdS nanostructure for photocatalytic water splitting application

We have successfully synthesized ZnO NRs and ZnO/CdS core-shell structures by a facile two step chemical routes viz. electrodeposition and chemical bath deposition. Plane ZnO nanorods films were deposited by using three electrode electrodeposition on FTO glass substrates. The ZnO/CdS core-shell structures were deposited by immersing plane ZnO nanorod films into a bath containing precursor solution of CdS in chemical bath deposition. Formation of ZnO NRs and ZnO/CdS core-shell structures has been confirmed by UV-Visible absorption, Raman spectroscopy and scanning electron microscopy. The synthesized ZnO NRs and ZnO/CdS core-shell structures has been also characterized for photoelectrochemical (PEC) properties, Mott-Schottky analysis, electrochemical impedance spectroscopy (EIS) and efficiency measurements of PEC system. It has been found that the photocurrent conversion efficiency in water splitting is higher for ZnO/CdS core-shell photoanode than ZnO NRs photoanode. These results suggest that addition of CdS with ZnO NRs is beneficial in increasing the visible light absorption and to enhance the photocurrent conversion efficiency in water splitting. Thus, ZnO/CdS core-shell configuration can be a prospective candidate for efficient PEC splitting of water

Growth of hydrogenated nano-crystalline silicon (nc-Si:H) films by plasma enhanced chemical vapor deposition (PE-CVD)

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were prepared by home-made PE-CVD systemfromgas mixture of pure SiH4 and H2 at various deposition pressures. Obtained results exhibited that deposition rate increases with increase in deposition pressure. Raman spectroscopy analysis revealed that deposition pressure in PE-CVD is a critical process parameter to induce nanocrystallization in Si:H films. The FTIR spectroscopy analysis results indicate that with increase in deposition pressure hydrogen bonding in films shifts from Si-H to Si-H2 and (Si-H2)n bonded species bonded species. The bonded hydrogen content didn’t show particular trend with optical band gap with change in deposition pressure. The obtained results indicates that 400 mTorr is an optimized deposition pressure of our PE-CVD unit to synthesize nc-Si:H films. At this optimized deposition pressure nc-Si:H films with crystallite size ∼ 5.43 nm having good degree of crystallinity (∼77%) and high band gap (ETauc∼ 1.85 eV) were obtained with a low hydrogen content (4.28 at. %) at moderately high deposition rate (0.75 nm/s). The ease of the present work is to optimize deposition pressure to obtain device quality intrinsicnc-Si:H layer in view of its used in p-i-n solar cells

Effect of calcination temperature on the properties of CZTS absorber layer prepared by RF sputtering for solar cell applications

In present work, we report synthesis of nanocrystalline Kesterite copper zinc tin sulfide (CZTS) films by RF magnetron sputtering method. Influence of calcination temperature on structural, morphology, optical, and electrical properties has been investigated. Formation of CZTS has been confirmed by XPS, whereas formation of Kesterite-CZTS films has been confirmed by XRD, TEM, and Raman spectroscopy. It has been observed that crystallinity and average grain size increase with increase in calcination temperature and CZTS crystallites have preferred orientation in (112) direction. NC-AFM analysis revealed the formation of uniform, densely packed, and highly interconnected network of grains of CZTS over the large area. Furthermore, surface roughness of CZTS films increases with increase in calcination temperature. Optical bandgap estimated using UV–Visible spectroscopy decreases from 1.91 eV for as-deposited CZTS film to 1.59 eV for the film calcinated at 400 °C which is quite close to optimum value of bandgap for energy conversion in visible region. The photo response shows a significant improvement with increase in calcinations temperature. The employment these films in solar cells can improve the conversion efficiency by reducing recombination rate of photo-generated charge carriers due to larger grain size. However, further detail study is needed before its realization in the solar cells

Experimental and theoretical study into interface structure and band alignment of the Cu2Zn1–xCdxSnS4 heterointerface for photovoltaic applications

To improve the constraints of kesterite Cu2ZnSnS4 (CZTS) solar cell, such as undesirable band alignment at p–n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu2Zn1–xCdxSnS4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental–theoretical approach was employed to characterize and assess the optoelectronic properties of Cu2Zn1–xCdxSnS4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu2Zn1–xCdxSnS4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu2Zn1–xCdxSnS4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu2CdSnS4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p–n junction in the ultrafast time scale and highlight a route to improve device performances

Optical, structural and morphological study of CdS nanoparticles: Role of sulphur source

Cadmium sulfide (CdS) nanoparticles were synthesized by simple and low cost homemade hot injection method at low process temperature using different sulphur sources. The effects of sulphur concentration on the structural, morphological, and optoelectronic properties of synthesized CdS films were studied using a range of characterization techniques: X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM) and UV-Visible spectroscopy. The XRD studies revealed the formation of hexagonal type CdS nanoparticles. The varying morphology dependence on the sulphur source was ascertained from FESEM analysis. The longitudinal optical (LO) phonon vibrational modes of CdS were assigned in Raman spectra at 300 and 600 cm−1. The band gap of the CdS particles was estimated to be 2.30 eV from Tauc’s plots. Consistent with the experimental results, our first-principles DFT calculations predict the band gap of CdS nanoparticles to increase with decreasing S concentration: Cd52S52 (2.38 eV) Cd52S51 (2.52 eV) and Cd52S50 (2.65 eV), with both the valence and conduction band edges demonstrated to be dominated by S-p states

Synthesis and characterization of chemical spray [yrolysed CZTS thin films for solar cell applications

In present work, thin films of CZTS have been prepared by chemical spray pyrolysis (CSP) by spraying precursor solution directly onto the soda lime glass (SLG) substrates by varying sulphur molar concentration. Copper chloride [CuCl2.2H2O], zinc chloride [ZnCl2.2H2O], tin chloride [SnCl4.5H2O] and thiourea [(NH2)2CS] were used as precursor materials to deposit CZTS thin films by using home-built chemical spray pyrolysis system. Influence of sulphur variation on structural, optical, morphology and electrical properties of CZTS films have been investigated by using variety techniques such as low angle x-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), UV-Visible spectroscopy, four probe method, etc. The formation of CZTS has been confirmed by low angle XRD and Raman spectroscopy. The structural analysis reveals formation of kesterite tetragonal phase with preferential orientation along (112) direction. The band gap values of CZTS thin films have been calculated and found in the range 2 - 2.25 eV over the entire range of sulphur variation studied. The change in band gap may be due to quantum confinement effects at nanoscale. The morphological studies show formation of islands of nanoscale particulate clusters which constitute the films in most of the samples. The films exhibit higher resistivity values (in KΩ) which may be due to presence of the strain in the films

Photoelectrochemical investigation on the cadmium sulfide (CdS) thin films prepared using spin coating technique

Photoelectrochemical cell technology is one of the simplest technologies, which converts light energy directly into electricity. The synthesis of cadmium sulfide (CdS) nanocrystals (NCs) was performed by the facile hot injection method. The NCs were characterized by different techniques such as XRD, Raman, UV-Vis, FESEM, and XPS. The XRD pattern confirms the phase pure hexagonal CdS NCs. The band gap of NCs calculated from the UV-Visible spectrum is at 2.40 eV, indicating good absorption in the visible spectrum. XPS analysis confirmed the presence of individual elements in CdS NCs. The CdS thin-films having different thicknesses were prepared on FTO substrates using the spin coating technique. Photoelectrochemical (PEC) investigation of CdS NCs thin-films photoelectrodes was performed by varying its thickness. The increase in the thickness of thin-films increased photocurrent density

Single Crystal, High Band Gap CdS Thin Films Grown by RF Magnetron Sputtering in Argon Atmosphere for Solar Cell Applications

Single crystal thin films of CdS were grown onto glass substrates by RF magnetron sputtering at var ious substrate temperatures. Structural, optical and morphology properties of these films were investigated through low angle XRD, Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive x-ray (EDX) spectroscopy, UV-Visible spectroscopy etc. Formation of single crystal CdS films has been confirmed by low angle XRD and Raman spectroscopy analysis. Low angle XRD showed that CdS films has preferred orientation in (111) direction. Improvement of crystallinity and increase in average grain size of CdS crystallites has been observed with increase in substrate temperature. Surface morphology investigated using SEM showed that CdS films deposited over entire range of substrate temperature are highly smooth, dense, homogeneous, and free of flaws and cracks. The EDX data revealed the formation of high-quality nearly stoichiometric CdS films by RF magnetron sputtering. Furthermore, the CdS films deposited at low substrate temperatures (< 200 0C) are slightly S rich while deposited at higher substrate temperatures (> 200 0C) are slightly Cd rich. The UV-Visible spectroscopy analysis showed that an average transmission ~ 80-90 % in the visible range of the spectrum having band gap ~ 2.28 -2.38 eV, which is quite close to the optimum value of band gap for a buffer layer in CdTe/CdS, Cu2S/CdS hetero-junction solar cells