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

低成本太阳能电池作为广泛使用的硅基光伏（PV）器件的替代品，在过去的几十年中吸引了巨大的兴趣。在这一领域，染料敏化太阳能电池（DSSC），与分子染料作为光吸收剂，达到11.9%的转换效率与电子转移（ET）的时间在飞秒的时间尺度。受DSSC的启发，已经开发了量子点太阳能电池（QDSC），其中通过半导体量子点（QD）实现光捕获。量子点表现出对光降解的鲁棒性，大的消光系数下降到近红外和载流子倍增的机制。由于量子点具有较大的表面积与体积比，导致量子点的子带隙态作为电荷陷阱和非辐射表面复合过程，这是量子点实现更高转换效率的主要障碍。金属量子点（Au，Ag，Cu）作为半导体量子点的替代材料，可以通过等离子体激元诱导电子转移到二氧化钛上。此外，在染料/半导体界面的光诱导ET和相干现象已被成功地检查。在第一个资助期内，我们将调查电荷分离异质结构，并专注于分离的多激子，特别强调壳效应。此外，量子点固有的电荷分离动力学的空穴（核）和电子（壳）将操纵的间隔，大带隙间壳（洋葱状量子点）。通过改变壳层间厚度，核与壳之间的电子耦合应被修改，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态吸收设置，成功合成了胶体PbS QD，这对PV应用特别感兴趣，并开始了光谱表征。我们将测试包含PbS量子点和分子电子受体的系统的ET动力学，并迅速转移到直接生长在金属氧化物薄膜表面上的PbS量子点的研究。为了避免标准半导体QD的有毒元素，金属QD可以作为替代品。我们将研究表面等离子体诱导的ET在组装含有胶体铜量子点和分子受体，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QD/TiO_2（QD作为电子给体或受体）中能量转移机制的影响，并计划将我们的研究扩展到基于QD的能量转移系统，并希望包括双光子吸收过程。在理想情况下，大带隙QD（ZnSe）将充当双光子吸收体，并且随后转移激发能量以触发特定功能。该过程可以例如提高已建立的光触发器的双光子横截面。

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.