George Mason University , Virginia 22030, United States.
Acc Chem Res. 2013 Jun 18;46(6):1242-51. doi: 10.1021/ar300283j. Epub 2013 Mar 5.
Solar energy production, one of the world's most important unsolved problems, has the potential to be a source of clean, renewable energy if scientists can find a way of generating cheap and efficient solar cells. Generation of multiple excitons from single photons is one way to increase the efficiency of solar energy collection, but the process suffers from low efficiency in bulk materials. An increase of multiexciton generation efficiency in nanocrystals was proposed by Nozik in 2002 and demonstrated by Schaller and Klimov in 2004 in PbSe nanocrystals. Since then, scientists have observed efficient multiexciton generation in nanostructures made of many semiconductors using various measurement techniques. Although the experimental evidence of efficient carrier multiplication is overwhelming, there is no complete theory of this phenomenon. Researchers cannot develop such a theory without a self-consistent description of the Coulomb interaction and a knowledge of mechanisms of electron and hole thermalization in nanostructures. The full theoretical description requires the strength of the Coulomb interaction between exciton and multiexciton states and the thermalization rates, which both vary with the dimensionality of the confining potential. As a result, the efficiency of multiexciton generation depends strongly on the material and the shape of the nanostructure. In this Account, we discuss the theoretical aspects of efficient carrier multiplication in nanostructures. The Coulomb interaction couples single excitons with multiexciton states. Phenomenological many-electron calculations of the evolution of single-photon excitations have shown that efficient multiexciton generation can exist only if the rate of the Coulomb mixing between photo-created exciton and biexciton states is significantly faster than the rate of exciton relaxation. Therefore, to increase multiexciton generation efficiency, we need to either increase the exciton-biexciton mixing rate or suppress the exciton relaxation rate. Following this simple recipe, we show that multiexciton generation efficiency should be higher in semiconductor nanorods and nanoplatelets, which have stronger exciton-biexciton coupling due to the enhancement of the Coulomb interaction through the surrounding medium.
太阳能的产生是世界上最重要的未解决问题之一，如果科学家能够找到一种方法来制造廉价高效的太阳能电池，那么太阳能就有可能成为一种清洁可再生的能源。从单个光子中产生多个激子是提高太阳能收集效率的一种方法，但该过程在体材料中效率较低。Nozik 于 2002 年提出了在纳米晶中增加多激子产生效率的方法，Schaller 和 Klimov 于 2004 年在 PbSe 纳米晶中证明了这一点。自那时以来，科学家们使用各种测量技术在由多种半导体制成的纳米结构中观察到了高效的多激子产生。尽管实验证据表明有效的载流子倍增，但这种现象并没有完整的理论。如果没有对库仑相互作用的自洽描述以及对纳米结构中电子和空穴热化机制的了解，研究人员就无法开发出这样的理论。完整的理论描述需要激子和多激子态之间的库仑相互作用的强度和热化率，这两者都随限制势的维度而变化。因此，多激子产生的效率强烈依赖于材料和纳米结构的形状。在本报告中，我们讨论了纳米结构中高效载流子倍增的理论方面。库仑相互作用将单激子与多激子态耦合。单光子激发的演化的唯象多电子计算表明，只有光生激子和双激子态之间的库仑混合速率明显快于激子弛豫速率时，才能存在有效的多激子产生。因此，为了提高多激子产生效率，我们需要要么增加激子-双激子混合速率，要么抑制激子弛豫速率。遵循这个简单的配方，我们表明，由于周围介质增强了库仑相互作用，半导体纳米棒和纳米板中的多激子产生效率应该更高，因为激子-双激子耦合更强。
