Center for Advanced Solar Photophysics, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States.
Acc Chem Res. 2013 Jun 18;46(6):1261-9. doi: 10.1021/ar300228x. Epub 2013 Mar 26.
During carrier multiplication (CM), also known as multiexciton generation (MEG), absorption of a single photon produces multiple electron-hole pairs, or excitons. This process can appreciably increase the efficiency of photoconversion, which is especially beneficial in photocatalysis and photovoltaics. This Account reviews recent progress in understanding the CM process in semiconductor nanocrystals (NCs), motivated by the challenge researchers face to quickly identify candidate nanomaterials with enhanced CM. We present a possible solution to this problem by showing that, using measured biexciton Auger lifetimes and intraband relaxation rates as surrogates for, respectively, CM time constants and non-CM energy-loss rates, we can predict relative changes in CM yields as a function of composition. Indeed, by studying PbS, PbSe, and PbTe NCs of a variety of sizes we determine that the significant difference in CM yields for these compounds comes from the dissimilarities in their non-CM relaxation channels, i.e., the processes that compete with CM. This finding is likely general, as previous observations of a material-independent, "universal" volume-scaling of Auger lifetimes suggest that the timescale of the CM process itself is only weakly affected by NC composition. We further explore the role of nanostructure shape in the CM process. We observe that a moderate elongation (aspect ratio of 6-7) of PbSe NCs can cause up to an approximately two-fold increase in the multiexciton yield compared to spherical nanoparticles. The increased Auger lifetimes and improved charge transport properties generally associated with elongated nanostructures suggest that lead chalcogenide nanorods are a promising system for testing CM concepts in practical photovoltaics. Historically, experimental considerations have been an important factor influencing CM studies. To this end, we discuss the role of NC photocharging in CM measurements. Photocharging can distort multiexciton dynamics, leading to erroneous estimations of the CM yield. Here, we show that in addition to distorting time-resolved CM signals, photocharging also creates spectral signatures that mimic CM. This re-emphasizes the importance of a careful analysis of the potential effect of charged species in both optical and photocurrent-based measurements of this process.
在载流子倍增（CM）过程中，也称为多激子产生（MEG），吸收单个光子会产生多个电子-空穴对或激子。这个过程可以显著提高光转换效率，这在光催化和光伏中特别有益。本综述回顾了近年来在半导体纳米晶体（NCs）中对 CM 过程的理解进展，这是因为研究人员面临着快速识别具有增强 CM 的候选纳米材料的挑战。我们通过展示使用测量的双激子俄歇寿命和带内弛豫率分别作为 CM 时间常数和非 CM 能量损耗率的替代物，可以预测组成变化对 CM 产率的相对变化，为这个问题提供了一个可能的解决方案。事实上，通过研究各种尺寸的 PbS、PbSe 和 PbTe NCs，我们确定这些化合物的 CM 产率的显著差异来自于它们非 CM 弛豫通道的差异，即与 CM 竞争的过程。这一发现可能是普遍的，因为以前观察到的与材料无关的“普遍”俄歇寿命体积缩放表明，CM 过程本身的时间尺度仅受 NC 组成的微弱影响。我们进一步探讨了纳米结构形状在 CM 过程中的作用。我们观察到，与球形纳米颗粒相比，PbSe NCs 的适度伸长（纵横比为 6-7）可以使多激子产率增加约两倍。与伸长的纳米结构相关的增加的俄歇寿命和改善的电荷输运性质表明，铅硫属化物纳米棒是在实际光伏中测试 CM 概念的有前途的系统。从历史上看，实验考虑因素是影响 CM 研究的一个重要因素。为此，我们讨论了 NC 光充电在 CM 测量中的作用。光充电会扭曲多激子动力学，导致对 CM 产率的错误估计。在这里，我们表明，除了扭曲时间分辨 CM 信号外，光充电还会产生模拟 CM 的光谱特征。这再次强调了在对该过程的光电流和光电流测量中仔细分析带电物质的潜在影响的重要性。
