Duy Phong Pham

In and Ga Codoped ZnO Film as a Front Electrode for Thin Film Silicon Solar Cells

Doped ZnO thin films have attracted much attention in the research community as front-contact transparent conducting electrodes in thin film silicon solar cells. The prerequisite in both low resistivity and high transmittance in visible and near-infrared region for hydrogenated microcrystalline or amorphous/microcrystalline tandem thin film silicon solar cells has promoted further improvements of this material. In this work, we propose the combination of major Ga and minor In impurities codoped in ZnO film (IGZO) to improve the film optoelectronic properties. A wide range of Ga and In contents in sputtering targets was explored to find optimum optical and electrical properties of deposited films. The results show that an appropriate combination of In and Ga atoms in ZnO material, followed by in-air thermal annealing process, can enhance the crystallization, conductivity, and transmittance of IGZO thin films, which can be well used as front-contact electrodes in thin film silicon solar cells.

Fabrication of honeycomb textured glass substrate and nanotexturing of zinc oxide front electrode for its application in high efficiency thin film amorphous silicon solar cell

Abstract. A significant part of broad band sunlight remains unabsorbed in a simple structured amorphous silicon solar cell. This absorption can be enhanced by adopting a light-trapping scheme with the help of a textured front surface and back reflector. For this purpose, honeycomb-type texture was fabricated on a glass surface by chemical etching. A 3  μm×3  μm honeycomb patterned optical-mask was used to create an image-pattern of an etch-mask on the glass surfaces. We used 0.5% hydrofluoric acid (HF) as an etchant solution with an optimized etching time ranging between 25 and 28 min. This single textured glass shows an enhancement in diffused transmission of light. In solar cell application, a 630-nm-thick Al-doped ZnO (AZO) layer was deposited over the textured glass surface. An additional random etching was carried out on the AZO with 1% HF acid solution for 10 s. This results to a double textured AZO superstrate on which solar cells were fabricated. The solar cells show higher short circuit current density to 17.2  mA/cm2. Finally, we obtained photovoltaic conversion efficiency of an optimized solar cell as 10.75% (with the corresponding Jsc as 17.03  mA/cm2).

Development of p-type amorphous silicon oxide — Nano crystalline silicon double layer and its application in n-i-p type amorphous silicon solar cell

Single junction n-i-p type amorphous silicon solar cell was investigated with various p-type window layers. A high doped silicon oxide (P1) layer was used to extract holes from the active layer while a highly conducting micro-crystalline silicon p-type layer was used as an electrical contact layer with the transparent conducting oxide front electrode. When electrical conductivity of the second (P2) layer was raised (to 1.1 S.cm-1) the open circuit voltage and short circuit current density of the cells increased. This P2 layer was placed in between P1 and front electrode. Optical band gap of the p-type layers remain close to 2.0 eV. With an optimum fabrication condition of the p-layers, the open circuit voltage and short circuit current density of the cells were found to reach 900 mV and 11 mA/cm2 respectively.

Reduction in Photocurrent Loss and Improvement in Performance of Single Junction Solar Cell Due to Multistep Grading of Hydrogenated Amorphous Silicon Germanium Active Layer

Single junction solar cells were fabricated with intrinsic hydrogenated amorphous silicon germanium (a-SiGe:H) as the active layer, that shows a 10% photovoltaic conversion efficiency. The a-SiGe:H active layer of the solar cells of type-A had constant band gap materials while that of type-B had a four step graded band gap by composition gradient (CG). The cells with composition gradient show an enhancement in fill factor and open circuit voltage (V oc) by 5% and 20 mV respectively, with respect to a cell without the graded band gap active layer. Such an enhanced device performance is attributed to reduction in recombination loss of photo-generated electron hole pairs. The effect of this photo-current loss was investigated in the electrical bias dependent external quantum efficiencies (EQE), illumination dependent current-voltage measurements and dark current-voltage characteristics. The device parameters like reverse saturation current density (J o), series resistance (R s) and diode quality factor (n) were also estimated. In comparison to the cell without the composition gradient, the EQE shows a reduced recombination loss across the whole wavelength range for the cell with the CG. Furthermore, the introduction of the CG results in a significantly increased shunt resistance from 720 to 1200 Ω.cm 2. The estimated n values of the cells under dark operating condition, decreases from 1.8 to 1.7 along with J o from 3 × 10 −7 down to 4.5 × 10 −8 A/cm 2 with CG, while the same parameters decreased from 3.73 to 3.06, 2.96 × 10 −6 to 3.05 × 10 −7 A/cm 2 respectively under AM1.5G insolation.

High Efficiency Inorganic/Inorganic Amorphous Silicon/Heterojunction Silicon Tandem Solar Cells

We investigated high-efficiency two-terminal tandem photovoltaic (PV) devices consisting of a p/i/n thin film silicon top sub-cell (p/i/n-TFS) and a heterojunction with an intrinsic thin-layer (HIT) bottom sub-cell. We used computer simulations and experimentation. The short-circuit current density (Jsc) of the top sub-cell limits the Jsc of the p/i/n-TFS/HIT tandem PV device. In order to improve the Jsc of the top sub-cell, we used a buffer-layer at the p/i and i/n interface and a graded forward-profile (f-p) band gap hydrogenated amorphous silicon germanium active layer, namely i-layer, in the top sub-cell. These two approaches showed a remarkable raise of the top sub-cell’s Jsc, leading to the increase of the Jsc of the PV tandem device. Furthermore, in order to minimize the optical loss, we employed a double-layer anti-reflective coating (DL-ARC) with a magnesium fluoride/indium tin oxide double layer on the front surface. The reduction in broadband reflection on the front surface (with the DL-ARC) and the enhanced optical absorption in the long wavelength region (with the graded f-p band gap) resulted in the high Jsc, which helped achieve the efficiency up to 16.04% for inorganic-inorganic c-Si-based tandem PV devices.

HF etched glass substrates for improved thin-film solar cells

A hemisphere-array textured glass substrate was fabricated for the development of an improved thin-film (TF) silicon solar cell. The HF-H2SO4-etchant system influenced the light path owing to the formation of the strong fluorine-containing HSO3F acid. In particular, the etching system of the various HF concentration with a constant H2SO4 solution is related to make an improvement of optical transmittance and light trapping structure without a uniform pattern. According to the specular transmittance measurements, the haze ratio was maintained for the glass sample etched with 35% HF in the longer-wavelength region. The proposed substrate was implemented in a TF-Si solar cell, and an improved conversion efficiency was observed according to the short-circuit current density owing to the increase in the haze ratio. This morphology, therefore, induces more scattering at the front side of the cell and leads to an improvement of the open circuit voltage gain for the HF 25% cell. It will be helpful to understand the application of thin film solar cell based on the HF-H2SO4 etching system for the readers.

Light Trapping in Silicon Based Tandem Solar Cell: A Brief Review

Among the various types of solar cells, silicon based two terminal tandem solar cell is one of the most popular one. It is designed to split the absorption of incident AM1.5 solar radiation among two of its component cells, thereby widening the wavelength range of external quantum efficiency (EQE) spectra of the device, in comparison to that of a single junction solar cell. In order to improve the EQE spectra further and raise short circuit current density () an optimization of the tradeoff between the top and bottom cell is needed. In an optimized cell structure, the and hence efficiency of the device can further be enhanced with the help of light trapping scheme. This can be achieved by texturing front and back surface as well as a back reflector of the device. In this brief review we highlight the development of light trapping in the silicon based tandem solar cell.