Hot charge carriers in quantum dot-based electron transfer systems for application in photovoltaics

在过去的几十年里，低成本的太阳能电池作为广泛使用的硅基光伏(PV)器件的替代品引起了人们的极大兴趣。在这一领域，以分子染料为光吸收体的染料敏化太阳能电池(DSSC)在飞秒时间尺度上的电子转移(ET)时间达到了11.9%。量子点太阳能电池(QDSC)是受半导体量子点(QD)的启发而发展起来的一种新型太阳能电池。量子点具有较强的抗光降解能力、较大的近红外消光系数和载流子倍增机制。QDSCs的高转换效率的主要障碍是作为电荷陷阱的亚带隙状态和由于大的表面体积比而导致的非辐射表面复合过程。一种替代半导体量子点的方法是金属量子点(Au，Ag，Cu)，它执行等离子体诱导的Et到TiO2的反应。以前的项目旨在研究QD分子受主组件中的电荷和能量转移过程，重点是电荷分离异质结构和热载流子。此外，还成功地研究了染料/半导体界面的光致电子转移和相干现象。在第一个资助期，我们将研究电荷分离异质结构，并重点研究多激子的分离，特别是壳层效应。此外，空穴(核)和电子(壳)的量子点本征电荷分离动力学将由一个间隔较大的大带隙壳层(洋葱状量子点)来操纵。通过改变壳层厚度，可以改变核壳之间的电子耦合，从而导致不同的电荷分离动力学。我们已经实现了近红外瞬时吸收装置，成功地合成了对光伏应用特别感兴趣的胶体PbS量子点，并开始了光谱表征。我们将测试含有PbS量子点和分子电子受体的体系的电子捕获动力学，并迅速转移到直接在金属氧化物薄膜表面生长PbS量子点的研究。为了避免有毒元素的标准半导体QD，金属QD可以作为替代。我们将研究胶体铜量子点和分子受主组装的表面等离子体诱导的电子轰击，并研究直接生长在金属氧化物薄膜上的铜量子点的光动力学。我们将探索泵浦光子能量对金属量子点/二氧化钛(量子点作为电子给体或接受者)的传输机制的影响。我们计划扩展我们对基于量子点的能量传输系统的研究，并希望包括双光子吸收过程。在理想情况下，大带隙量子点(ZnSe)将起到双光子吸收的作用，并随后传递激发能量以触发某种功能。例如，这一过程可以提高已建立的光触发器的双光子截面。

Low-cost solar cells as an alternative to the widely used silicon-based photovoltaic (PV) devices have attracted immense interest during the past decades. In this field the dye sensitized solar cell (DSSC), with a molecular dye as light absorber, reaches a conversion efficiency of 11.9% with an electron transfer (ET) time on the femtosecond time scale. Inspired by the DSSC the quantum dot solar cell (QDSC) has been developed where the light harvesting is achieved by semiconductor quantum dots (QD). QD exhibit robustness against photodegradation, large extinction coefficients down to the near infrared and the mechanism of carrier multiplication. Main obstacles towards higher conversion efficiencies of QDSCs are sub-band gap states acting as charge traps and nonradiative surface recombination processes because of a large surface-to-volume ratio of QD. An alternative to the semiconductor QD are metal QD (Au, Ag, Cu) performing a plasmon-induced ET to TiO2.The previous project aimed for charge and energy transfer processes in QD-molecular acceptor assemblies with focus on charge separating heterostructures and hot charge carriers. Furthermore, the photoinduced ET and coherent phenomena at the dye/semiconductor interface have been successfully examined. In line with the first funding period we will investigate charge separating heterostructures and focus on the separation of multiexcitons with particular emphasis on the shell effect. Furthermore, the QD intrinsic charge separation dynamics of holes (core) and electrons (shell) will be manipulated by a spacing, large band gap intershell (onion-like QD). By variation of the intershell thickness the electronic coupling between core and shell should be modified leading to different charge separation dynamics. We already implemented a NIR transient absorption set-up, successfully synthesized colloidal PbS QD which are particularly interesting for PV applications and started the spectroscopic characterization. We will test the ET dynamics of systems containing PbS QD and molecular electron acceptors, and quickly move on to the investigation of PbS QD directly grown on the surface of metal oxide thin films. To avoid toxic elements of the standard semiconductor QD, metal QD can serve as an alternative. We will investigate surface plasmon-induced ET in assemblies containing colloidal Cu QD and molecular acceptors and study the photodynamics of Cu QD directly grown on metal oxide films. The effect of pump photon energy on the transfer mechanism in metal QD/TiO2 (QD as electron donor or acceptor) will be explored.We plan to extend our research on QD-based energy transfer systems and want to include the process of two-photon absorption. In an ideal case a large band gap QD (ZnSe) will act as two-photon absorber and subsequently transfer the excitation energy to trigger a certain functionality. This process could e.g. boost the two-photon cross-section of established phototriggers.