| Summary: | 博士 === 國立臺灣師範大學 === 光電科技研究所 === 103 === Recently, both scientific and industrial communities are dedicated to exploring and searching alternative ways of renewable energy due to the inevitable shortage of natural resource. Among them, the solar light was longtime considered as a permanent energy, and that leads to a prompt and intensive development associated with the solar energy technology. In this thesis, we apply several novel structures mainly on the solar cells composed of different based materials, and validate its feasibility in term of the enhanced power conversion efficiency of the devices. First, we propose a brand new structure of antireflection coating (ARC) which combines a low-reflectivity 2-dimensional (2D) Si-based nanorod array, and the slanted indium-tin-oxide (ITO) film simultaneously with excellent electrical (conductive) and lossless optical (transparent) features. Second, as the demand of one single device exhibiting a full-solar-spectrum response is increased, we numerically evaluate the III-nitride solar cells with high indium contents by the grading of indium compositions scheme. Finally, we demonstrate a general strategy by simply casting cadmium selenide (CdSe) quantum dots (QDs) upon InGaP/GaAs/Ge tandem solar cells to tailor the incident solar spectrum, and to achieve current matching between every sub-cells. The highlight of our scientific achievement is briefly described as follows.
1. Use of Si-based nanorods/nanowires solar cells with slanted ITO films to enhance optical absorption for photovoltaic applications
The Si-nanorods/nanowires offer a promising architecture that has been widely recognized as attractive devices for photovoltaic applications. We adopt a slanted ITO film as an intermediate layer by using oblique-angle sputtering deposition to further reduce the Fresnel reflection of the device. Besides, the slanted ITO film exhibits the resistivity of 1.07x10^-3 Ω-cm underwent RTA treatment of T=450°C, and the doping concentration and the carrier mobility by Hall measurement amount to 3.7x10^20 cm-3 and 15.8 cm2/V-s, respectively. It is acceptable to perform as a transparent conductive film for photovoltaic applications. Theoretically, the proposed structures exhibit high optical absorption over a broad range of wavelengths and incident angles and an improvement of power conversion efficiency () approximately 42% over that of its bare Si counterpart. Yet the real device of proposed schme shows a low value of =0.26%, which is mainly attributed to the mis-aligning doped polarity at p-Si/n-ITO interface and the high aspect ratio of Si-nanowires, resulting in large series resistance and small shunt resistance, and excerbating the surface recombination process accompanied with high reverse current characteristics.
2. Polarization-induced doping III-nitride n-i-p solar Cells
We numerically evaluate a new type of III-nitride n-i-p solar cells by the so-called polarization-induced doping, which is induced by the graded InxGa1-xN layers of linearly increasing (from x=0% to 30%) and decreasing (from x=30% to 0%) the indium composition to construct the conductive p- and n-type regions, respectively. As the conductive n- and p-type regions are formed by electrostatic field ionization but not by the thermal activation, the concentration of field-induced carriers is independent of thermal freezeout effects, and the device can provide stable power conversion efficiency even operated at low temperatures.
3. Current matching using CdSe QDs to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells
We explore a promising strategy using CdSe QDs to enhance the photocurrent of the limited subcell to match with those of the other subcells and to enhance the power conversion efficiency of InGaP/GaAs/Ge tandem solar cells. The underlying mechanism is mainly attributed to the photon conversion of the QDs that tailors the incident spectrum of solar light; the enhanced efficiency of the device is therefore strongly dependent on the QD’s dimensions. By appropriately selecting and spreading CdSe QDs upon the InGaP/GaAs/Ge solar cell, the power conversion efficiency shows an enhancement of 10.39% compared to the conventional devices.
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