新穎高效率混合型之量子點光電元件-子計畫六：光電元件中膠體奈米粒子光吸收與能量轉移率的理論研究

Abstract

近年來運用膠體奈米粒子(colloidal nanoparticles)來提升光電及太陽能電池元件 的特性有許多重大突破。利用奈米粒子的尺寸量子化效應（quantum size effect）除 可用來調整元件的吸收光波長外並可能大幅提高其光-電的轉換效率。其中利用所謂核 -殼型量子點(core-shell quantum dots)更解決了長期以來膠體粒子光電性質不穩定 的問題(blinking problem) 。此外透過奈米粒子間的非輻射能量轉移(energy transfer)，可使得元件整體的吸收光譜更廣，效率更高。其中最具代表性的能量轉移 機制即所謂福斯特共振能量轉移現象（Förster resonance energy transfer, FRET）， 是量子點之間，透過長程的偶極-偶極耦合的能量轉移。 在本計畫中，我們將藉由解析方法與大型的數值計算程式，建構一套有效的數值模 擬方法－來研究太陽能電池中膠體量子點吸收光譜與量子點間的能量轉移率。透過緊 密束縛(Tight-Binding) 方法我們計算單一量子點及耦合量子點的電子結構及激子能 譜。由於核-殼型量子點間材料的不晶格匹配，我們的方法將結合valence force field (VFF)模型計算核-殼型量子點中的應力分佈；接著以緊密束縛法所計算的電子結構及 吸收光譜為基礎，利用 Configuration interaction (CI) method 考慮激子間的庫倫 交互作用包含偶極-偶極耦合，再利用黃金費米定則(Fermi's golden rule)精確的計 算量子點的吸收光譜以及耦合量子點間的能量轉移率。透過本計劃發展的數值計算工 具我們將提供大量的數值資料足以用來描繪量子點在實驗系統中複雜的光電行為，此 理論將會與實驗比對並解釋其結果進而提供設計奈米粒子光電元件有用的物理依據。

Semiconductor quantum dots (QDs) are known as promising nano-materials whose usefulness have been evidenced in a variety of advanced applications. As compared with another widely known epitaxial quantum dots, colloidal nanocrystals are especially advantageous in the wide-range engineering of the energy quantization and used as excellent wave-length tunable light sources by means of delicate size control. Especially, core-shell nanoparticles have shown their excellence in the optical properties, with the removal of the so-called blinking problem. In the solar cell application, colloidal nanocrystals have been evidenced their usefulness in the photon absorption and energy transfer rates of the devices. In this project, we attempt to develop the computation techniques and analysis methods for studying the electronic structures, optical absorption spectra, and energy transfer dynamics of colloidal QDs embedded in hybrid voltaic solar-cell systems. For core-shell nanocrystals, the valence force field (VFF) model is employed to calculate the strain and piezoelectricity potential. The theory and computation techniques will be built up in the framework of sp3d5s* tight-binding (TB) theory and configuration interaction (CI) method which allow for precise treatment of single particle spectra and inter-particle Coulomb interactions, respectively. The combination of the both sophisticated methods are used to seek for the “numerically exact” solutions of correlated exciton wave function and spectrum, based on which the rates of optical absorption in and energy transfer between QDs can be quantitatively studied. The numerical computation and physical analysis will be compared with existing experiments and are conducted to provide useful guideline for engineering the energy flow in QD-hybrid solar cell devices.