Victor Rejón

A comparative study of CdS:F and CdS:O thin films deposited by reactive RF-sputtering technique for window layer application in solar cells

In this work, we report our results on the study of CdS thin films doped with fluorine (CdS:F) and oxygen (CdS:O) prepared by the RF-sputtering technique with CHF3 and O2 as reactive gases, respectively. XPS and XRD measurements showed that CdS:F is composed of CdS and CdF2 compounds, while CdS:O is composed of CdS, CdSO4, CdSO3 and CdO2. Both CdS:F and CdS:O samples are highly oriented along the (0 0 2) plane, but with less crystallinity than CdS film. Scanning electron microscopy images showed that the morphology is changed due to the presence of reactive gases. The optical transmittance in the short wavelength range (λ < 500 nm) is improved in doped films, as higher values were achieved in CdS:O samples. By increasing the reactive gas concentration during the sputtering, the bandgap of CdS:O films is increased, while for CdS:F films it slightly decreases; in both kinds of samples, the refractive index is decreased and all doped films have higher electrical resistivity than CdS.

Thermal behavior of ITO/TiO2/CdS/CdTe solar cells

In this work, a TiO2 thin film was deposited as a buffer layer onto ITO film at front contact of CdS/CdTe solar cell. The TiO2 thin film was prepared by reactive rf-sputtering from a Ti target. The CdTe film was grown by conventional CSS technique. The cells were activated by annealing at 400 °C in Argon-Oxygen-CHClF2 atmosphere. From DRX analysis of TiO2 thin film was determinate that it has Rutile phase. The solar cell was characterized by J-V curve and Efficiency vs Temperature, the latter was nonlinear. The solar cell efficiency was around 9.5% at room temperature.

Refractive index properties of oxygenated amorphous CdS thin film deposited by rf-sputtering

The refractive index of 1.21 to 2.38 of oxygenated amorphous CdS thin films can be fabricated by using reactive RF-Sputtering from CdS target of 4N of purity. The oxygenated amorphous CdS with 2.3 of refractive index has a band gap of 2.5 e V. These properties are optically appropriate for matching the ITO and CdTe thin films in amorphous-CdS/CdTe solar cells application. For 10-20% of Oxygen/Argon and 50-70 Watts of the RF-Sputtering power, all thin film shows high transmittance, low refractive index of 1.21 and 3.7 eV of band gap. XRD spectra shows amorphous evidence. This thin film could be used as an antireflective layer on glass substrate.

The ZnO-reflectance effect on the heterojunction ITO/ZnO/CdS/CdTe

The reported structure in this work was fabricated using layers ITO/ZnO/CdS/CdTe grown onto borosilicate commercial glass. The thin films of ITO and CdS were deposited using rf-sputtering in argon atmosphere from ITO and CdS targets. The ZnO layer also was deposited by rf-sputtering from metallic Zn target. All targets were of 5N of purity. The CdTe film was about 8 μm of thickness, was deposited onto CdS using a conventional closed space sublimation technique. By using basic calculus for optical antireflective coatings, was demonstrated that film ZnO has the nearest refraction index as needing for optical matching for ITO and CdS layers. The reflectance of the structure glass/ITO/ZnO/CdS/CdTe when the ZnO thickness is varied from 100-400 nm it is reported. In this work also the reflectance of the intermediate structures such as glass/ITO, glass/ITO/ZnO and glass/ITO/ZnO/CdS are reported. We found a minimum reflectance of 9% of the complete structure between 500-850 nm of wavelength. By taking the optimal thickness, a solar cell was fabricated, was activated by using CHClF2 gas mixed with Ar at 400 °C for 5 minutes. As a back contact 15 nm of Cu and 500 nm of Mo were deposited by rf-sputtering in Ar atmosphere. The efficiency was about 10.1% for a cell of 1.01 cm2. After two months, the solar cell preserves its characteristics.

Discrete-time dynamic feedback linearization of a VTOL using observed states

Abstract This paper addresses the trajectory tracking problem for a remotely controlled quad-rotor vertical take off and landing aircraft (VTOL), under the restriction that only the inertial coordinates are available for measurement. The problem is solved in two steps: first, a discrete-time local exponential observer is designed which allows estimating the roll and pitching angles as well as all the velocities of the VTOL; Second, a discrete-time dynamic linearizing controller is proposed and the VTOL actual states variables are replaced by their corresponding estimates. It is shown that a kind of separation principle holds, in the sense that exponential convergence to the prescribed trajectory is preserved. Real-time experiments show that the proposed observer-controller scheme exhibits good performance.

The CdS/CdTe solar cells with reactively sputtered a-MoOx/Mo back contact

In this work amorphous MoOx thin film was used as buffer layer at back contact of CdS/CdTe solar cell. The a-MoOx thin film was made by reactive rf-sputtering from a Mo target. From DRX analysis of MoOx/glass thin film, both α-MoO3 and β-MoO3 phases were identified. The CdTe film was grown by conventional CSS technique. The cells were activated by annealing at 400 °C in Argon-Oxygen-CHClF2 atmosphere. The curves of J-V and its efficiency vs. temperature of the solar cells were obtained. The solar cell efficiency at room temperature was around 11.2%. The temperature coefficient of the efficiency was 0.014%/°C in range of 40-65 °C.

Selecting CdS:F or CdS:O for window layer application in CdTe-based solar cells

In this work CdS:F and CdS:O thin films were studied. Films were prepared by the RF-sputtering technique in an atmosphere containing Ar mixed with CHF₃ or O₂ reactive gases. XPS measurements showed that CdS:F films are composed by CdS and CdF₂ while CdS:O comprises the CdS, CdSO₄, CdSO₃ and CdO₂ compounds. Modulation of optical properties was achieved by changing the reactive gas concentration during deposition. According to the transmittance, bandgap and refractive index values, it was concluded that CdS:O thin films are more suitable for window layer application.

Study of CdS/CdTe solar cells activated with N-O 2 -CHClF 2 gas mixture

The activation process is an important step for fabricating CdTe based solar cells. In this work, it was made by using Argon-CHClF2, Argon-Oxygen-CHClF2 and Nitrogen-Oxygen-CHClF2 gas mixtures. Solar cells with 12.0% of efficiency for 1 cm2 were obtained when it is activated by using Nitrogen-Oxygen-CHClF2 gas mixture. We show that Nitrogen also plays an important role for improving the solar cell efficiency. The solar cell reported in this work were fabricated using ZnO/CdS/CdTe layers grown onto commercial ITO used as TCO. Both ZnO and CdS thin films were deposited using rf-sputtering. The first one is working as a buffer layer. The CdS is deposited in Argon atmosphere and it is not intentionally treated before depositing CdTe thin film with 8 μm thickness by the conventional CSS technique. The cell uses a simple sputtered Cu (5nm)/Mo (750nm) back contact. Illuminated and darkness J-V curves, HR-SEM images before and after the activation process are presented. The effect of the gas mixtures on the parameters of the CdS/CdTe solar cell is discussed.

The CdS/CdTe solar cell protected by ITO/Mo bilayer at improved back contact

In this work CdS/CdTe solar cell that uses a Mo thin film as a back contact on last layer was protected against MoOx formation and scratching by using an ITO thin film. The back contact was fabricated by using Cu/Mo and Cu/Mo/ITO. The Cu layer was used to form CuxTe onto CdTe film. The films Cu, Mo and ITO were deposited by rf-sputtering. The cell efficiency is degraded when it is annealed at 400 °C in air. We show evidences that the p-n junction remains good and the oxidation of Mo is the main cause of the cell efficiency degradation. Also we show that the ITO thin film deposited onto Mo avoid the MoOx formation when the solar cell is annealed at same conditions. All solar cell characteristics are preserved. The solar cells were fabricated in the superstrate configuration by using Glass/ITO/ZnO/CdS/CdTe/back contact layers. The CdTe film was grown by conventional CSS technique. The cells were activated by using CHClF2-argon-oxygen gases. DRX spectra and HR-SEM were made for Mo/Glass and ITO/Mo/CdTe before and after annealing at 400 °C in air.

Improved Cu/Mo back contact for CdS/CdTe solar cells

The simple back contact Cu (10 nm)/Mo (500 nm) gives low series resistance when the CdTe surface is both smooth and grain size is up to 5 μm, for this case the solar cell efficiency is typically around 8-12%. There are cases where the CdTe surface have an opposite characteristics, in these cases the series resistance is too high, it produces poor fill factor and efficiency. It can be drastically improved at 11.0% by using multilayer Cu/Mo/Cu/Mo without degrading the Jsc and Voc values. The improved efficiency is due to an important change in the fill factor via lowering the series resistance. The solar cells were fabricated using ITO/ZnO/CdS/CdTe layers. The ZnO, CdS, Cu and Mo films were deposited by rf-sputtering. The CdTe film was grown by conventional CSS technique. The cells were activated by using CHClF2-argon-oxygen gases.