量子點太陽能電池之開發及應用

Abstract

由光電壓所產生的太陽電能已成為大眾所矚目的一個全球能源產品中具有環境保 護功能的一重要部分. 但是必須投入更多的革新及努力來使其具有比較高的變換效率 結合並降低太陽能生產成本及電費用到傳統的電力能源的水準。本項研究計畫提議從基 礎材料和元件出發研究以新穎量子點太陽能電池作為地面能源的應用的可能性。 我們已經計算過準費密能階會落在QD 電子能態及載子重新結合的強度與QDs周圍 的位能障礙有強烈的關聯. 如果位能障礙被正確地設計及製作，他們在QDs 能壓抑載子 的重新結合。假如能正確設計Ge QD 埋在Si(或GaSb QD 埋在GaAs)太陽能電池裡，與傳 統的Si太陽能電池比較，我們預期雙光子吸收能增加變換效率達25%。我們計劃發展埋 在Si 裡的Ge QDs 及在GaAs 裡埋入GaSb QDs的太陽能電池。我們將有系統的評估埋藏 QD 太陽能光電元件 樣品的特性，包括stoichiometry，表面形態學，光學性能(能隙， 吸收，photoluminescence)和電特性，確QDs 內載子重新結合時間的範圍。最後，我們 希望為能工業生產QD太陽能電池打下基礎。

Photovoltaic solar electricity attracts attention as an important and environmental friendly component of global energy product [1,2]. However essential innovative efforts are needed to combine relatively low-cost production technologies with relatively high conversion efficiencies to bring solar electricity costs down to the level of conventional electricity. This project proposes to conduct basic material and device studies of novel quantum dot (QD) solar cells for terrestrial applications. The high concentrator photovoltaic (HCPV) technology is a one of modern innovative technologies that targets to substitute expensive semiconductor cells with low-cost optics to impact the cost of solar electricity [3]. The ability of concentrators to increase the intensity of illumination is another innovative advantage of HCPV technology that makes possible to put into use energy of otherwise wasted infrared photons to generate additional photocarriers by two-photon excitation of electrons from valence band via intermediate states into conduction band to increase conversion efficiency of solar cells [4-9]. The main problem is that, like the impurity centers, intermediate states usually arrest quasi-Fermi level and increase intensity of recombination. It is the challenging task to overcome this problem by combining those two innovative advantages of HCPV technology in one solar cell. This study targets to develop the necessary scientific and technological understanding to make possible the production of QD solar cells that can operate with high conversion efficiency and reliability. Ground electronic states of QDs are often discussed as intermediate states for solar cell application [4-9]. Compare to impurity centers, QDs offer potentially higher flexibility to design and more alternatives to solar cells. We have already shown that arresting of quasi-Fermi level at QD electronic states and intensity of recombination are strongly dependant on barriers around QDs [10]. If barriers are properly designed, they can suppress recombination of carriers at QDs. It is expected that the two-photon excitation can increase for example by 25% the conversion efficiency of properly designed Ge QD buried in Si solar cells compare to the same quality conventional Si solar cells [10]. Another innovation is that very intensive direct one-photon absorption occurs in Ge QDs when the photon energy is at the edge of indirect bang gap of Si [10]. Though QD buried solar cells promise higher efficiency compare to conventional solar cells [10], the possibilities for even higher performances are significant. Well known that 30-50 nm thick monolayer of undoped ZnO (or CdS) grains deposited between n- and p-doped materials in depletion layer increases conversion efficiency of thin film photovoltaic (TFPV) solar cells, e.g. up to 20% the efficiency of CuInSe/ZnO TFPV solar cells [11,12,13]. Collect necessary knowledge to understand the effect of undoped ZnO grains on efficiency of solar cells to combine the undoped ZnO grains and the buried QDs in one device is one more way to achieve even higher conversion efficiency in Si and GaAs solar cells. The key to this research is realization and experimental proof that properly designed barriers around QDs buried outside the depletion layer of p-n-junction in semiconductor can involve new phenomena in photogeneration and essentially improve photoelectrical characteristics of solar cells. We are going to apply all the knowledge acquired on growth of Ge QDs buried in Si, on growth of GaSb QDs buried in GaAs, and on growth of ZnO layers in Nano Device Laboratory of NCTU and to focused significant resources on developing capabilities for the fabrication of barriers around Ge QD buried in Si solar cells and GaSb QD buried in GaAs solar cells, and 30-50 nm thick monolayer of undoped ZnO grains between n- and p-doped layers in depletion layer of those solar cells, on characterization on these samples and on samples obtained from other sources, on analyzing the results and device performance, and on developing appropriate model for buried QD solar cells. We have a goal to evaluate the properties of the buried QD samples, including stoichiometry, surface morphology, optical properties (modification of energy gaps, absorption, photoluminescence, up-conversion) and electrical properties, to establish absorption spectra for buried QDs and carrier recombination time at buried QDs. Finally, we are going to lay the foundation for making market-value novel QD solar cells for industrial production. Conversion of solar energy is the most important, virtually free of pollution technology for alternative energy resources. Some key technical goals and operational features of our plan are: