光伏材料物理-量子点太阳能电池2.ppt
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1、量子点太阳能电池,量子点的概念 量子点(QuantumDots一QDs)是准零维的纳米材料,由少量的原子所构成。是指颗粒半径(或三个维度的尺寸)小于材料激子玻尔半径的金属或半导体颗粒,其内部电子在各方向上的运动都受到局限,所以量子局限效应特别显著。由于量子局限效应会导致类似原子的不连续电子能级结构,因此量子点又被称为“人造原子”。 如果将某一个维度的尺寸缩到小于一个波长,此时电子只能在另外两个维度所构成的二维空间中自由运动,这样的系统我们称之为量子阱;如果我们再将另一个维度的尺寸缩到小于一个波长,则电子只能在一维方向上运动,我们称之为量子线;当三个维度的尺寸都缩到一个波长以下时,就成为量子点了
2、。,能态密度 对于不同维度的电子体系,许多独特的光学性质来源于它们的态密度。态密度是指单位体积在能量E附近单位能量间隔内的电子态数。每一个量子态可被自旋向上和向下的两个电子所占据。,Quantum well: particle confined by a 1-D potential well, but free in other 2-D, quantum states labeled by n, kx and ky:,Each n represents a branch or subband,Quantum wire: particle confined by 2-D potential we
3、lls, free only in 1-D (1-D free particle), quantum states labeled by n1, n2 and kz:,Quantum dot: particle confined by potential wells in 3-D, quantum states labeled n1, n2 and n3:,All discrete levels, like in atom,量子点的制备方法,自组成法(self-assembly method): 采用分子束外延或化学气相沉积过程,并利用晶格不匹配的原理,使量子点在特定基材表面自聚生长,可大量生
4、产排列规则的量子点。,在GaAs基材上以自组成法生长 InAs量子点的STM影像,量子点的制备方法,微影蚀刻法 以光束或电子束直接在基材上蚀刻制作出所要之图案,由于相当费时因而无法大量生产。,以GaAs基材蚀刻窄圆柱式量子点之SEM影像,水平线条约0.5微米,量子点的制备方法小结,合 成 方 法,Top-down,Bottom-up,晶体表面刻蚀,组成器件,化学制备,生物体系标记,波长范围宽,发射峰尖锐,发射波长可以通过纳米粒子粒径调节,易于自组织,Relaxing Electron Emits Fixed Radiation Electrons relax back to the top e
5、dge of the valence band from the bottom edge of the conduction band. This causes the fixed emission peak of semiconductors.,Bulk Semiconductors - A Fixed Range of Energies,Quantum Dots - Quantum Confinement,If the size of a semiconductor crystal becoes small enough that it approaches the size of the
6、 materials Exciton Bohr Radius. The electron energy levels can no longer be treated as continuous - treated as discrete, meaning that there is a small and finite separation between energy levels. This situation of discrete energy levels is called quantum confinement, and under these conditions,Conce
7、pt of Quantum Confinement Effect,:,Coulomb energy,Particle-in-a-SP quantum localization energy,Bohr exciton radius,R,e,mh*,me*,R,E,r,0,2,2,2,2,4,786,.,1,1,1,2,e,pe,p,-,+,=,D,h,1,1,4,2,0,2,r,B,mh*,me*,e,a,+,=,e,e,p,h,Exciton energy,Eg= Eg.bulk + E,For semiconductors, aB is in a few nm to a few 10-nm,
8、Excitons: electron and hole bound together like a hydrogen atom Hamiltonian of exciton,A Tunable Bandgap According to size of quantum dot semiconductor will measurably alter the bandgap energy a tunable bandgap! This is possible as long as the size of the dot is close to or below the Exciton Bohr Ra
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- 材料 物理 量子 太阳能电池
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