Performance Enhancement of an a-Si:H/μc-Si:H Heterojunction p-i-n Solar Cell by Tuning the Device Parameters

In this work, the solar cell design parameters like- layer thickness, bandgap, donor and acceptor concentrations are varied to find optimum structure of a hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (μc-Si:H) heterojunction p-i-n solar cell. A thin a-Si:H p-layer of 1 to 5 nm followed by a thick a-Si:H i-layer of thickness 1400 to 1600 nm and then thin n-layer of thickness 1 to 5 nm with acceptor concentration of 102 cm−3 and donor concentration of 1020 cm−3 and the bandgaps of p-, i-, and n- layers with higher bandgaps closer to 2.2 eV for a-Si:H p-layer, 1.85 eV for a-Si:H i-layer, and 1.2 eV for μc-Si:H n-layer have showed better performances. The optimum cell has a JSC of 18.93 mA/cm2, VOC of 1095 mV, Fill factor of 0.7124, and efficiency of 14.77%. The overall external quantum efficiency of the numerically designed cell also remained very high from 85-95 % for wavelengths of 300-650 nm range. This indicates that the device will perform its best under both high and low frequency i.e. ultra-violet, near visible and visible light wavelengths. Dhaka Univ. J. Sci. 69(2): 88-95, 2021 (July)

Comparative Study on the Quality of Microcrystalline and Epitaxial Silicon Films Produced by PECVD Using Identical SiF4 Based Process Conditions

Hydrogenated microcrystalline silicon (µc-Si:H) and epitaxial silicon (epi-Si) films have been produced from SiF4, H2 and Ar mixtures by plasma enhanced chemical vapor deposition (PECVD) at 200 °C. Here, both films were produced using identical deposition conditions, to determine if the conditions for producing µc-Si with the largest crystalline fraction (XC), will also result in epi-Si films that encompass the best quality and largest crystalline silicon (c-Si) fraction. Both characteristics are of importance for the development of thin film transistors (TFTs), thin film solar cells and novel 3D devices since epi-Si films can be grown or etched in a selective manner. Therefore, we have distinguished that the H2/SiF4 ratio affects the XC of µc-Si, the c-Si fraction in epi-Si films, and the structure of the epi-Si/c-Si interface. Raman and UV-Vis ellipsometry were used to evaluate the crystalline volume fraction (Xc) and composition of the deposited layers, while the structure of the films were inspected by high resolution transmission electron microscopy (HRTEM). Notably, the conditions for producing µc-Si with the largest XC are different in comparison to the fabrication conditions of epi-Si films with the best quality and largest c-Si fraction.

Biogas/Biomethane Quality and Requirements for Combined Heat and Power (CHP) Units/Gas Grids with a Special Focus on Siloxanes-a Short Review

The presence of siloxanes in biogas and biomethane is a major barrier to use them as renewable energy sources in Combined Heat and Power (CHP) units and national grids systems. Siloxanes in the shape of methyl siloxanes (incl. L2, L3, L4, D3, D4, D5, D6), Trimethylsilanol (TMSOH), as well as other contaminants such as H2S, NH3, relative Humidity (rH), halogenated compounds (including organic chlorine and fluorine), and Volatile Organic Compounds (VOCs) presented in biogas upgraded to biomethane quality are detrimental to engines, turbines and gas grids, therefore it is necessary to remove them before its high-value utilization. Under the oxidation, process siloxanes are converted into microcrystalline silicon dioxide (SiO2) deposits that can shorten the lifetime of the engine and affect the gas grids. The review presents the actual requirements of biogas and biomethane quality in context to their utilization in CHP units and national gas grids. Moreover, the methods of siloxanes removal based on adsorption, absorption, cryogenic condensation, membranes, and biofiltration are described.

Optical Absorption in Si:H Thin Films: Revisiting the Role of the Refractive Index and the Absorption Coefficient

This paper reports on absorption properties of thin films of hydrogenated amorphous and microcrystalline silicon considered for absorption-based applications, such as solar cell, photodetectors, filters, sensors, etc. A series of four amorphous and four microcrystalline samples PECVD deposited under varied hydrogen dilution was under consideration. Various absorption metrics, based separately on the absorption coefficient and the refractive index (single pass absorption, optical path length, classical light trapping limit) or direct absorptance calculated by the Yablonovitch concept based on a mutual role of them were examined and compared. Differences in absorption abilities are related to the evolving thin film microstructure.

Investigation of Microcrystalline Silicon Thin Film Fabricated by Magnetron Sputtering and Copper-Induced Crystallization for Photovoltaic Applications

Microcrystalline silicon, which is widely used in the microelectronics industry, is usually fabricated by chemical vapor deposition techniques. In recent years, magnetron sputtering has been considered as an alternative because it is a simpler, cheaper and more eco-friendly technique. The big drawback of this technique, however, is the need to recrystallize the as-deposited amorphous silicon, which can be done by metal-induced crystallization. Among the different suitable metals, copper has not been extensively investigated for this purpose. Furthermore, the applicability of the microcrystalline film prepared by this method has not been evaluated for photovoltaic device fabrication. Therefore, this paper reports the fabrication of p-type microcrystalline silicon thin film by magnetron sputtering and copper-induced crystallization techniques, and evaluates its appropriateness for solar cell fabrication. In the first step, 60 nm of silicon followed by 10 nm of copper were deposited on n-type silicon wafer and glass substrates, both by the magnetron sputtering technique. Then, the as-deposited samples were annealed at temperatures from 450 °C to 950 °C. The crystal properties of the resulting films were characterized by Raman and X-ray diffraction spectroscopies and optical and secondary emission microscopies, while their electrical characteristics were determined by Hall-effect, J-V curve and external quantum efficiency measurements. These characterizations confirmed the formation of a layer of microcrystalline silicon mostly in the <111> direction with a crystallization ratio of 93% and a largest grain size of 20 nm. The hole concentration and mobility of the fabricated p-type microcrystalline silicon layer were about 1017~1019 cm−3 and 8 cm2/V.s, respectively. By using the fabricated film as the emitter layer of a p-n junction solar cell, a good rectification ratio of 4100 and reverse saturation current density of 85 nA.cm−2 were measured under dark conditions. The highest photovoltaic conversion efficiency, i.e., 2.6%, with an open-circuit voltage of 440 mV and short-circuit current density of 16.7 mA/cm2, were measured under AM1.5G irradiance. These results indicate that microcrystalline silicon created by magnetron sputtering and copper-induced crystallization has considerable potential for photovoltaic device fabrication.

Validation of MEMS Based New MicroLPCVD Technique for Thin Films Deposition

In this work we present and optimized a new microCVD technique to produce thin films for microelectronics devices. The technique was first presented in a recent work and here we successfully utilize to produce microcrystalline Silicon films at 1 Torr and temperatures of 740 oC, utilizing SiH4 as precursor gas. The technique is based on a MEMS micro- heater and presents 3 main advantages: (1) the heating region is very localized, (2) allows to attain easily temperatures as high as 1000 oC) and (3) present very short heating and cooling time. In addition to the fabrication process of MEMS micro-heater, we also present a multiphysics computer simulation study to understand its thermal response of MESM micro-heater and to predicting the final temperatures attainable with the technique.