Lili Wu

Controlling CH3NH3PbI3–xClx Film Morphology with Two-Step Annealing Method for Efficient Hybrid Perovskite Solar Cells

The methylammonium lead halide perovskite solar cells have become very attractive because they can be prepared with low-cost solution-processable technology and their power conversion efficiency have been increasing from 3.9% to 20% in recent years. However, the high performance of perovskite photovoltaic devices are dependent on the complicated process to prepare compact perovskite films with large grain size. Herein, a new method is developed to achieve excellent CH3NH3PbI3-xClx film with fine morphology and crystallization based on one step deposition and two-step annealing process. This method include the spin coating deposition of the perovskite films with the precursor solution of PbI2, PbCl2, and CH3NH3I at the molar ratio 1:1:4 in dimethylformamide (DMF) and the post two-step annealing (TSA). The first annealing is achieved by solvent-induced process in DMF to promote migration and interdiffusion of the solvent-assisted precursor ions and molecules and realize large size grain growth. The second annealing is conducted by thermal-induced process to further improve morphology and crystallization of films. The compact perovskite films are successfully prepared with grain size up to 1.1 μm according to SEM observation. The PL decay lifetime, and the optic energy gap for the film with two-step annealing are 460 ns and 1.575 eV, respectively, while they are 307 and 327 ns and 1.577 and 1.582 eV for the films annealed in one-step thermal and one-step solvent process. On the basis of the TSA process, the photovoltaic devices exhibit the best efficiency of 14% under AM 1.5G irradiation (100 mW·cm(-2)).

Preparation of Cu2Te Thin Films and Back-Contact Formation of CdTe Solar Cells

Cu2Te thin films were prepared by a coevaporation method. The structural, optical, and electronic properties of Cu2Te thin films were investigated using X-ray diffraction, UV–visible–IR transmittance and reflectance spectra, and Hall measurements. The results show that single-phase Cu2Te thin films can be obtained after annealing at 170 °C, and that annealing temperatures higher than 200 °C induce the Cu2Te coexisting phase. Subsequently, CdTe solar cells with a Cu2Te layer were fabricated and annealed at various temperatures. CdTe solar cells with a single-phase hexagonal Cu2Te layer annealed at a temperature of 180 °C show a good ohmic-contact behavior.

Preparation and Properties of SnO2 Film Deposited by Magnetron Sputtering

Tin oxide SnO2 films were prepared by RF magnetron sputtering. The effects of oxygen partial pressure percentage on the SnO2 property have been investigated to obtain relatively high-resistivity SnO2 films which could be used as buffer layers to optimize the performance of CdTe/CdS solar cells. The oxygen partial pressure percentage varied in the range of 1%~10%. The results show that the introduction of oxygen would suppress the deposition and growth of SnO2 films. Electrical measurement suggests that the film resistivity decreases with the increase of oxygen pressure. The SnO2 films with resistivity of 232 Ω cm were obtained in pure Ar atmosphere. All SnO2 films fabricated with different oxygen partial pressure percentage have almost the same optical band gap.

Preparation of novel CdS/ZnS composite window layer for CdTe thin film solar cell

In this work, a composite window layer of the sandwich structure containing CdS/ZnS multilayer for CdTe thin film solar cell was designed. Two composite windows of this kind with 5 cycles and 8 cycles CdS/ZnS respectively were prepared by pulsed laser deposition, and finally were fabricated into CdTe-based thin film solar cell. SEM images show that the pulsed laser deposited composite window layers have good crystallinity. CdS and ZnS are layer-by-layer stacking together as the design. In comparison with CdS single-layer window, the composite windows have a blue-shift transmittance edge due to introduction of wide-bandgap ZnS, and show higher transmittance in the short-wavelength region. CdTe solar cells with the composite window obtain further improved performance, including higher short-circuit current (JSC) and energy conversion efficiency. Also, enhanced QE of CdTe solar cells with the composite window is observed in both short-wavelength and visible region. Meanwhile, some differences exist in the J–V and QE curves between the two composite window solar cells, indicating the volume proportion of ZnS may be an important factor that affects the performance of CdTe solar cell with the CdS/ZnS composite window.

Band diagrams and performance of CdTe solar cells with a Sb2Te3 back contact buffer layer

Sb2Te3 thin films were prepared by vacuum co-evaporation and the crystallinity of the films was greatly improved after annealing at 573 K in N2 ambient. Then they were deposited on the CdTe thick films. Band diagrams of the as-deposited and annealed CdTe/Sb2Te3 interfaces were constructed. Consequently, Sb2Te3 was used as a back contact layer for CdTe thin film solar cells and the cell performance was investigated. It was found that the Sb impurities accumulated in the CdTe grain boundaries diffuse deeply in the CdTe layer, and more photogenerated electrons and holes are separated by the segregated SbCd+ donors into the GBs. What is more, the doping concentration in the vicinity of the CdTe/CdS heterojunction increases for the formation of substitutional SbTe- acceptors under the Cd-rich conditions. For the introduction of the p-type Sb2Te3 layers as the back contact to the CdTe thin film solar cells, the performance of CdTe thin film solar cells has been greatly improved and an efficiency of 13.1% (FF=62...

Preparation and Characterization of Thin Films by Coevaporation

Deposition of thin films on soda-lime glass substrates by coevaporation of Sb and Te is described in this paper. thin films were characterized by x-ray diffraction (XRD), x-ray fluorescence (XRF), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), electrical conductivity measurements, and Hall measurements. The abnormal electrical transport behavior occurred from in situ electrical conductivity measurements. The results indicate that as-grown thin films are amorphous and undergo an amorphous-crystalline transition after annealing, and the posttreatment can effectively promote the formation of Sb-Te bond and prevent oxidation of thin film surface.

AlSb THIN FILMS PREPARED BY DC MAGNETRON SPUTTERING AND ANNEALING

Aluminum antimonide (AlSb) is thought to be a potential material for high efficiency solar cells. In this paper, AlSb thin films have been fabricated by DC magnetron sputtering on glass substrates. The sputtering target consists of aluminum and antimony, and the area ratio of Al to Sb is 7:3, which is derived from research into the relationship between the deposition rates of both the metals and sputtering power. XRD and AFM measurements show that the as-deposited films are amorphous, but become polycrystalline with an average grain size of about 20 nm after annealing in an argon atmosphere. From optical absorption measurements of annealed AlSb films, a band gap of 1.56 eV has been demonstrated. Hall measurements show that the films are p-type semiconductors. The temperature dependence of dark conductivity tested in vacuum displays a linear lnσ to 1/T curve, which indicates a conductivity activation energy of around 0.61 eV.

Interface Study of ITO/ZnO and ITO/SnO2 Complex Transparent Conductive Layers and Their Effect on CdTe Solar Cells

Transparent ITO/ZnO and ITO/SnO2 complex conductive layers were prepared by DC- and RF-magnetron sputtering. Their structure and optical and electronic performances were studied by XRD, UV/Vis Spectroscopy, and four-probe technology. The interface characteristic and band offset of the ITO/ZnO, ITO/SnO2, and ITO/CdS were investigated by Ultraviolet Photoelectron Spectroscopy (UPS) and X-ray Photoelectron Spectroscopy (XPS), and the energy band diagrams have also been determined. The results show that ITO/ZnO and ITO/SnO2 films have good optical and electrical properties. The energy barrier those at the interface of ITO/ZnO and ITO/SnO2 layers are almost 0.4 and 0.44 eV, which are lower than in ITO/CdS heterojunctions (0.9 eV), which is beneficial for the transfer and collection of electrons in CdTe solar cells and reduces the minority carrier recombination at the interface, compared to CdS/ITO. The effects of their use in CdTe solar cells were studied by AMPS-1D software simulation using experiment values obtained from ZnO, ITO, and SnO2. From the simulation, we confirmed the increase of , FF, , and by the introduction of ITO/ZnO and ITO/SnO2 layers in CdTe solar cells.

Cd2SnO4 transparent conductive oxide: a promising alternative candidate for highly efficient hybrid halide perovskite solar cells

Organic–inorganic hybrid perovskite solar cells have attracted significant research attention in terms of perovskite materials, fabrication, device architecture, and interfacial engineering to increase their power conversion efficiency (PCE). However, the state-of-the-art front electrode of perovskite solar cells is mainly focused on indium tin oxide (ITO) and fluorine-doped tin oxide (FTO). To further improve the optical characteristics of front electrodes for perovskite devices, it is necessary to explore a new and suitable transparent conductive oxide material. Herein, we introduce a Cd2SnO4 film for constructing a perovskite device with a novel structure. The as-prepared Cd2SnO4 film shows higher optical transmission in the visible region compared to the FTO substrate. The matching energy band alignment can ensure efficient carrier transport and collection between the TiO2 layer and Cd2SnO4 electrode. The higher PCEs with an average of 15.58% under AM 1.5 irradiation for Cd2SnO4-based perovskite solar cells were obtained compared to those of the FTO-based devices. This renders the Cd2SnO4 film a promising transparent conductive oxide candidate for highly efficient perovskite solar cells. However, the toxicity of lead and cadmium components still remain a major concern for its commercial applications.

Synthesis and Characterization of CZTS Thin Films by Sol-Gel Method without Sulfurization

One process of layer-by-layer sol-gel deposition without sulfurization was developed. The CZTS films with 1.2 μm and the sulfur ratio of ~48% were prepared and their characteristics were measured. The as-deposited and annealed films are of Kesterite structure. The as-deposited films do not present obvious electric conduction type. However, the annealed 9-LAY-ANN film is p-type conduction and has sheet resistance of 4.08 kΩ/□ and resistivity of 4.896 × 10−1 Ω·cm. The optic energy gap is 1.50 eV for as-deposited films and is 1.46 eV after being annealed. The region deposited by using Lo-Con solution is more compact than that by the Hi-Con solution from SEM morphology images.