Jui-Fu Yang

Effects of RF power on the structural, optical and electrical properties of Al-doped zinc oxide films

Abstract In this study, we discussed the effects of growth parameters on the structural and optical properties of Al-doped zinc oxide (AZO) deposited at room temperature by radio-frequency magnetron sputtering. The AZO films have been characterized in detail using X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, Hall-effect measurement system and UV–visible spectrophotometer. It was found that the morphological, structural, electrical and optical properties of AZO films are greatly dependent on sputtering power. Collision between sputter species and surface morphology play important roles in optoelectrical properties of AZO films. According to our experimental results, the AZO films can be used in versatile devices to meet various requirements.

Enhanced broadband and omnidirectional performance of Cu(In,Ga)Se2 solar cells with ZnO functional nanotree arrays

An effective approach is demonstrated for enhancing photoelectric conversion of Cu(In,Ga)Se2 (CIGS) solar cells with three-dimensional ZnO nanotree arrays. Under a simulated one-sun condition, cells with ZnO nanotree arrays enhance the short-circuit current density by 10.62%. The omnidirectional anti-reflection of CIGS solar cells with various ZnO nanostructures is also investigated. The solar-spectrum weighted reflectance is approximately less than 5% for incident angles of up to 60° and for the wavelengths primarily from 400 nm to 1000 nm. This enhancement in light harvesting is attributable to the gradual refractive index profile between the ZnO nanostructures and air.

Dandelion-shaped nanostructures for enhancing omnidirectional photovoltaic performance

Broadband and omnidirectional light harvesting is important in photovoltaic technology because of its wide spectral range of radiation and the suns movement. This study reports the fabrication and characterization of zinc oxide (ZnO) dandelions on Cu(In,Ga)Se2 (CIGS) solar cells. The fabrication of dandelions involves the combination of self-assembled polystyrene (PS) nanospheres and the hydrothermal method, which is one of the simplest and cheapest methods of fabricating a three-dimensional, closely packed periodic structure. This study also investigates the optimization on dimension of the PS nanospheres using the rigorous coupled-wave analysis (RCWA) method. This study uses an angle-resolved reflectance spectroscope and a homemade rotatable photo I-V measurement to investigate the omnidirectional and broadband antireflections of the proposed dandelion structure. Under a simulated one-sun condition and a light incident angle of up to 60°, cells with ZnO dandelions arrays enhanced the short-circuit current density by 31.87%. Consequently, ZnO dandelions are suitable for creating an omnidirectionally antireflective coating for photovoltaic devices.

High quality sustainable Cu2ZnSnSe4 (CZTSe) absorber layers in highly efficient CZTSe solar cells

Polycrystalline Cu2ZnSnSe4 (CZTSe) thin films were directly deposited on Mo-coated glass substrates by evaporation and following selenization. Single-phase CZTSe films were formed in the temperature range of 480–540 °C, with a selenization step of 30 min. X-ray diffraction and Raman spectroscopy revealed that these thin films exhibited high crystallinity and strong preferential orientation along the (112) direction, confirming the presence of the kesterite CZTSe phase. The films prepared at temperatures above 520 °C showed many voids at the bottom of the CZTSe absorber layer, due to Sn loss during high-temperature growth, as confirmed by scanning electron microscopy and energy dispersive X-ray spectroscopy analyses. The band gaps (Eg) of the CZTSe thin films, which were obtained by photoluminescence spectroscopy, varied from 0.88 to 0.93 eV, depending on the SnSex loss during selenization. The solar cell fabricated with the CZTSe film grown at 500 °C showed the best conversion efficiency of 7.18%, with an open-circuit voltage of 0.38 V, a short-circuit current density of 42.34 mA cm−2, and a fill factor of 44%. Further improvements in the microstructure and Sn loss of CZTSe films may increase the efficiency of the CZTSe solar cells.

Improved dye-sensitized solar cell with a ZnO nanotree photoanode by hydrothermal method

This study investigated the influence of ZnO nanostructures on dye adsorption to increase the photovoltaic conversion efficiency of solar cells. ZnO nanostructures were grown in both tree-like and nanorod (NR) arrays on an AZO/FTO film structure by using a hydrothermal method. The results were observed in detail using X-ray diffraction, field-emission scanning electron microscopy (FE-SEM), UV-visible spectrophotometry, electrochemical impedance spectroscopy, and solar simulation. The selective growth of tree-like ZnO was found to exhibit higher dye adsorption loading and conversion efficiency than ZnO NRs. The multiple ‘branches’ of ‘tree-like nanostructures’ increases the surface area for higher light harvesting and dye loading while reducing charge recombination. These improvements result in a 15% enhancement in power conversion. The objective of this study is to facilitate the development of a ZnO-based dye-sensitized solar cell.

Enhanced omnidirectional and weatherability of Cu 2 ZnSnSe 4 solar cells with ZnO functional nanorod arrays

This paper presents the use of nanorods of different sizes, deposited from a chemical solution, as an antireflection layer in copper–zinc–tin selenide (CZTSe) solar cells. With the aid of the nanorods, the surface reflection of the CZTSe solar cells was reduced from 7.76% to 2.97%, and a cell efficiency of 14% was obtained as a result. Omni-directional anti-reflection was verified by the angle-dependent reflection measurements. The nanorod arrays also provided the CZTSe solar cells with a hydrophobic surface, allowing it to exhibit high resistance against humidity during weatherability tests. This shows that the surface passivation brought by the nanorod layer at the surface could effectively extend the lifetime of the CZTSe solar cells. The rate of efficiency decay of the CZTSe solar cells was reduced by 46.85% from that of the device without a nanorod array at the surface, indicating that this surface layer not only provided effective resistance against reflection at the device surface, but also served as a passivation layer and humidity-resistant surface-protection layer.

Cu2ZnSnSe4 Thin Film Solar Cell with Depth Gradient Composition Prepared by Selenization of Sputtered Novel Precursors

In this study, we proposed a new method for the synthesis of the target material used in a two stage process for preparation of a high quality CZTSe thin film. The target material consisting of a mixture of CuxSe and ZnxSn1-x alloy was synthesized, providing a quality CZTSe precursor layer for highly efficient CZTSe thin film solar cells. The CZTSe thin film can be obtained by annealing the precursor layers through a 30 min selenization process under a selenium atmosphere at 550 °C. The CZTSe thin films prepared by using the new precursor thin film were investigated and characterized using X-ray diffraction, Raman scattering, and photoluminescence spectroscopy. It was found that diffusion of Sn occurred and formed the CTSe phase and CuxSe phase in the resultant CZTSe thin film. By selective area electron diffraction transmission electron microscopy images, the crystallinity of the CZTSe thin film was verified to be single crystal. By secondary ion mass spectroscopy measurements, it was confirmed that a double-gradient band gap profile across the CZTSe absorber layer was successfully achieved. The CZTSe solar cell with the CZTSe absorber layer consisting of the precursor stack exhibited a high efficiency of 5.46%, high short circuit current (JSC) of 37.47 mA/cm2, open circuit voltage (VOC) of 0.31 V, and fill factor (F.F.) of 47%, at a device area of 0.28 cm2. No crossover of the light and dark current-voltage (I-V) curves of the CZTSe solar cell was observed, and also, no red kink was observed under red light illumination, indicating a low defect concentration in the CZTSe absorber layer. Shunt leakage current with a characteristic metal/CZTSe/metal leakage current model was observed by temperature-dependent I-V curves, which led to the discovery of metal incursion through the CdS buffer layer on the CZTSe absorber layer. This leakage current, also known as space charge-limited current, grew larger as the measurement temperature increased and completely overwhelmed the diode current at a measurement temperature of 200 °C. This is due to interlayer diffusion of metal that increases the shunt leakage current and decreases the efficiency of the CZTSe thin film solar cells.

Influence of annealing condition on the photovoltaic performance of Cu 2 ZnSnSe 4 thin film solar cells

The synthesis of highly crystalline Cu2ZnSnSe4 (CZTSe) absorber films through two-stage process of the selenization of a sputtered metallic CZT precursor layers on Mo substrate followed by annealing under selenium vapor was a promising approach for fabricating high efficiency solar cells. In this study, CZTSe films have been characterized in detail using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), Raman spectroscopy, and solar simulator. It was found that the structural and optical properties of CZTSe films. The SEM and Raman scattering analysis showed the formation of the CZTSe compound and crystalline quality of the CZTSe can be controlled through varying by the heating rate. Increase of the heating rate can increase the grain size of CZTSe absorber films, and lead to effect the CZTSe compound. Solar cells with the Al/Ni/AZO/i-ZnO/CdS/CZTSe/Mo showed the conversion efficiency of 0.2 % to 1.78 % for 0.13 cm2 with Voc = 0.06 V to 0.21 V, Jsc = 11.65 mA/cm2 to 31.81 mA/cm2, and FF = 0.28 to 0.31.

Growth of Cu 2 ZnSnSe 4 thin films by selenization of magnetron sputtered precursors for solar cells

Precursors of the Cu2ZnSnSe4 (CZTSe) absorber were deposition on Mo/glass substrate by radio-frequency (RF) magnetron sputtering at room temperature. The precursors were converted into CZTSe absorber by annealing in the selenium vapors at the substrate temperature of 550°C. CZTSe films have been characterized in detail using X-ray diffraction (XRD), Raman spectroscopy, photo luminescence (PL), energy dispersive spectrometer (EDS), and solar simulator. It was found that the structural and optical properties of CZTSe films. The p-type CZTSe absorber shows a peak below 0.9 eV at room temperature. Solar cells with the AZO/ZnO/CdS/CZTSe/Mo showed the best conversion efficiency of 1.78% for 0.13 cm2 with Voc= 0.21 V, Jsc= 27.1 mA/cm2, and FF= 31.3%.