The application of advanced nano and micro fabrication techniques on encapsulated layers to enhance the efficiency of thin film solar cells

薄膜太陽能電池相對於傳統單晶矽太陽能電池具有節省材料、可製作於可撓曲基板和可調變吸收波段等優勢，近年來被廣泛的研究與討論，但受限於較低的載子遷移率與吸收光徑的不足，其效率仍有很大的進步空間，如何能降低薄膜厚度並有效地增加吸收光徑成為改善薄膜太陽能電池效率的關鍵之一。 本論文中，主要將結構設計在封裝層上，此設計能夠避免結構對於元件電性的影響，以及避免結構的尺寸受到薄膜厚度的限制而無法最佳化，結構的設計分為非週期性的粒子散射以及週期性的光柵繞射，研究結果顯示，粒子的折射率越大，尺寸越大，將具有較佳的散射效果，我們將介電粒子旋鍍在元件封裝層外，直徑為722nm的聚苯乙烯粒子相較於直徑為1160nm、676nm以及462nm的二氧化矽粒子有較佳的散射能力，可使元件效率有5.2%的增益，若在粒子表面濺鍍上一層高折射率的二氧化鈦薄膜，將具有更優異的散射效果，元件效率增加更為巨大。此外，若結構尺寸遠大於波長，其行為將由幾何光學來探討，我們利用奈米壓印技術在PC板上製作出微米等級倒金字塔結構，此結構除了有良好的抗反射能力外，100%的光會因反射與折射而偏折，其霧度可達到92%以上，將其應用在元件上可使效率由2.80%上升至3.04%，其電流密度的增益更達到15%。而相較於利用介電粒子的散射，週期性光柵的繞射能產生較大角度的繞射光，在週期小於波長的情況下，一階繞射的角度將能超過空氣與光柵介面的臨界角，而使光被侷限在主動層中，我們最佳化的結構為週期600nm深度900nm的三角形PDMS光柵上披覆上250nm高折射率的二氧化鈦，此結構因漸變折射率而具有良好的抗反射效果，且能使一階繞射效率提升至76%，根據理論計算，在吸收不足的近紅外光波段，主動層的吸收增益能達到106%。Thin film solar cells are widely studied and discussed because of their several advatages, such as low cost of materials, flexibility and tunable absorption band. However, the performance of thin film solar cells is limited by poor carrier mobility and insufficient absorption. Therefore one of the most important issues is increasing the effective absorption length and decreasing the thickness of thin film solar cells. In this study, the harvest enhancement structures wewe designed on encapsulated layers of thin film solar cells. This design can avoid damaging the electronic properties of devices. Futhermore, the size of the structures would not be limited by the thickness of active layer. The light harvest enhancement structures are classified into two types of scattering by dielectric particles and diffraction gratings. This study reveals that the dielectric particles with larger refractive index and size possess better scattering effects. In the experiment, several kinds of dielectric particles are spun on an encapsulated layer. The polystyrene (PS) particles have better scattering ability than the SiO2 particles. The efficiency of solar cells was increased from 2.12% to 2.23% and reached the enhancement of 5.2% by coating the PS particles with a diameter of 722nm. Moreover, adding a high refractive index of TiO2 layer on the textured structures, the scattering ability increased and caused the efficiency increasing larger. Besides, we fabricated inverted pyramidial structures on a Polycarbonate (PC) substrate by the nanoimprint technique. The size of the inverted pyramidial structure is far more than the wavelength of incident light, so the behavior of light will be pridicted by geometric optics. The inverted pyramidial structure which possesses the ability of antireflection and 92% haze will increase the efficiency from 2.80% to 3.04% and the enhancement of current density reach 15%. Comparing to the scattering by nanoparticles, diffraction grating could lead to a large diffractive angle. When the period is smaller than the wavelength of incident light, the first-order diffractive angle will exceed the critical angle between air and the grating. And the light will be trapped in the active layer of a solar cell. The optimal structure is the triangle grating of polydimethylsiloxane (PDMS) which is coated by a high refractive index layer. Because this structure possesses excellent properties of antireflection and high diffraction efficiency, the absorption enhancement of an active layer will reach 106% at the wavelength of 700nm