Department of Chemistry, University of Rochester , Rochester, New York 14627, United States.
Acc Chem Res. 2013 Jun 18;46(6):1280-9. doi: 10.1021/ar3002365. Epub 2013 Mar 4.
Third-generation photovolatics require demanding cost and power conversion efficiency standards, which may be achieved through efficient exciton multiplication. Therefore, generating more than one electron-hole pair from the absorption of a single photon has vast ramifications on solar power conversion technology. Unlike their bulk counterparts, irradiated semiconductor quantum dots exhibit efficient exciton multiplication, due to confinement-enhanced Coulomb interactions and slower nonradiative losses. The exact characterization of the complicated photoexcited processes within quantum-dot photovoltaics is a work in progress. In this Account, we focus on the photophysics of nanocrystals and investigate three constituent processes of exciton multiplication, including photoexcitation, phonon-induced dephasing, and impact ionization. We quantify the role of each process in exciton multiplication through ab initio computation and analysis of many-electron wave functions. The probability of observing a multiple exciton in a photoexcited state is proportional to the magnitude of electron correlation, where correlated electrons can be simultaneously promoted across the band gap. Energies of multiple excitons are determined directly from the excited state wave functions, defining the threshold for multiple exciton generation. This threshold is strongly perturbed in the presence of surface defects, dopants, and ionization. Within a few femtoseconds following photoexcitation, the quantum state loses coherence through interactions with the vibrating atomic lattice. The phase relationship between single excitons and multiple excitons dissipates first, followed by multiple exciton fission. Single excitons are coupled to multiple excitons through Coulomb and electron-phonon interactions, and as a consequence, single excitons convert to multiple excitons and vice versa. Here, exciton multiplication depends on the initial energy and coupling magnitude and competes with electron-phonon energy relaxation. Multiple excitons are generated through impact ionization within picoseconds. The basis of exciton multiplication in quantum dots is the collective result of photoexcitation, dephasing, and nonadiabatic evolution. Each process is characterized by a distinct time-scale, and the overall multiple exciton generation dynamics is complete by about 10 ps. Without relying on semiempirical parameters, we computed quantum mechanical probabilities of multiple excitons for small model systems. Because exciton correlations and coherences are microscopic, quantum properties, results for small model systems can be extrapolated to larger, realistic quantum dots.
第三代光电需要苛刻的成本和功率转换效率标准，这可以通过高效的激子倍增来实现。因此，从单个光子的吸收中产生一个以上的电子-空穴对，对太阳能转换技术具有广泛的影响。与体材料相比，受激半导体量子点由于限制增强的库仑相互作用和较慢的非辐射损耗，表现出高效的激子倍增。对量子点光伏中复杂光激发过程的确切特征描述仍在进行中。在本述评中，我们专注于纳米晶体的光物理，并研究了激子倍增的三个组成过程，包括光激发、声子诱导退相和碰撞离化。我们通过从头计算和分析多电子波函数来量化每个过程在激子倍增中的作用。在光激发态中观察到多激子的概率与电子相关的大小成正比，其中相关电子可以同时跨越能带隙被同时激发。多激子的能量直接由激发态波函数确定，定义了多激子产生的阈值。在存在表面缺陷、掺杂剂和离化的情况下，该阈值会受到强烈的干扰。在光激发后几飞秒内，量子态通过与振动原子晶格的相互作用失去相干性。单激子和多激子之间的相位关系首先消散，然后多激子裂变。单激子通过库仑和电子-声子相互作用与多激子耦合，因此单激子转化为多激子，反之亦然。在这里，激子倍增取决于初始能量和耦合强度，并与电子-声子能量弛豫竞争。多激子在皮秒内通过碰撞离化产生。量子点中激子倍增的基础是光激发、退相和非绝热演化的集体结果。每个过程都具有独特的时间尺度，并且整个多激子产生动力学在大约 10 ps 内完成。我们没有依赖半经验参数，而是为小模型系统计算了多激子的量子力学概率。由于激子相关和相干是微观量子性质，因此小模型系统的结果可以外推到更大的、实际的量子点。
