Dye sensitised solar cells (DSSC)

背景染料敏化太阳能电池（DSSC）是一个有前途的途径，正在探索收获太阳能。自20世纪90年代初Grätzel和O 'Regan首次展示这些光伏器件以来，它们一直是许多研究的主题。一幅展示DSSC如何工作的漫画如图1所示。未来的改进将来自于底物/染料/氧化还原对的新配对，其中TiO 2纳米颗粒是目前受欢迎的底物材料[2]。DSSC-染料敏化太阳能电池：透明电极透射的光被染料（红色）吸收，染料覆盖在TiO 2纳米颗粒（灰色）上。该过程形成电子-空穴对（e-/h+）。电子穿过TiO 2层到达一个电极，空穴穿过电解质（蓝色）到达另一个电极，产生电流。旨在实现整个DSSC系统原子水平理解的基础研究已经构建了模型来解释ZnO和TiO 2衬底的瞬态吸收光谱[3]。通常对这两种基材进行比较，因为虽然TiO 2具有更高的效率，但ZnO在可以合成的纳米结构的种类方面比TiO 2具有许多优势。体材料还具有更高的电子迁移率。2016年的两篇论文描述了ZnO和TiO 2上常用的N3染料的实验和计算研究[4，5]。这些文件不同的解释，二氧化钛基板的优越性超过氧化锌。在实验研究[4]中，对薄膜的泵浦探针测量被解释为表明在ZnO上形成了界面电子-阳离子复合物，其减缓了电子注入。相比之下，计算研究预测，TiO 2导带边缘的态密度较大是其上级性能的原因[5]。这种差异指出了研究这些复杂系统所面临的挑战。然而，很明显，光动力学的测量是理解DSSC中电荷转移和能量耗散的关键。在这里，我们建议调查N3在气相中，并将其与染料浸涂到原子特征的ZnO和TiO 2单晶基板上进行比较。我们将使用可变光子能量的双光子光电发射（2 PPE）和使用可见光泵浦和XUV探针的时间分辨光电发射来研究光动力学。这两个测量都将在fs时间尺度上进行。这是一个具有挑战性和雄心勃勃的项目，将提供的结果，将改变我们对DSSC功能的理解。利用2 PPE研究了N3在气相中以及在ZnO和TiO 2上的光动力学行为，探讨了Ti 3d带隙态在光激发中的作用.使用时间分辨光电子能谱（TRPES），以遵循人口和衰减的LUMO和其他国家的染料，以及带隙和导带边缘状态在substrate.MethodologyFigure 2显示的原则，2 PPE和HHG TRPES沿着与样品的光谱记录与我们以前的工作有关的TiO 2/H2O纳米粒子。2 PPE的工作使我们能够确定除了带隙激发之外的光激发过程，这在量子化学中可能是重要的[6]。这涉及从带隙状态到导带区域中的状态的羟基局部激发。TRPES光谱指向通过用1 eV光泵浦带隙态产生的热电子的快速复合（<50 fs）。一个更长的组件（CA。如果泵浦是带隙光（3.1eV），则也观察到（3.1ps）。这被解释为VB-CB激子对复合。在这项早期的工作中，2 PPE测量在UCL进行，TRPES测量在Harwell的Artemis设施进行。在当前项目中，这将通过UCL的现有但尚未投入使用的HHG装置进行增强。

BackgroundDye sensitised solar cells (DSSC) are one of the promising avenues that are being explored to harvest the sun's energy. They have been the subject of much research since these photovoltaic devices were first demonstrated by Grätzel & O'Regan [1] in the early 1990's. A cartoon showing how DSSC's work is shown in Fig. 1. Future improvements will come from new pairings of substrate/dye/redox couples, with TiO2 nanoparticles being the currently favoured substrate material [2].Fig 1. DSSC--Dye-sensitized solar cells: Light transmitted by the transparent electrode is absorbed by a dye (red), which coats TiO2 nanoparticles (grey). The process forms electron-hole pairs (e-/h+). Electrons travel through the TiO2 layer to one electrode as holes travel through an electrolyte (blue) to the other electrode, generating electric current.Fundamental studies aimed at achieving an atomic level understanding of the overall DSSC system have constructed models to explain transient absorption spectra for ZnO and TiO2 substrates [3]. A comparison of these two substrates is commonly made because although TiO2 has a higher efficiency, ZnO has many advantages over TiO2 in terms of the variety of nanostructures that can be synthesised. The bulk material also has higher electron mobility. Two 2016 papers describe experimental and computational studies of the commonly employed N3 dye on ZnO and TiO2 [4,5]. These papers differ in their interpretation of the superiority of TiO2 substrates over that of ZnO. In the experimental study [4], a pump probe measurement on thin films was interpreted to indicate that an interfacial electron-cation complex is formed on ZnO that slows electron injection. In contrast, the computational study predicted that the larger density of states at the conduction band edge of TiO2 is the reason for its superior performance [5]. This discrepancy points to the challenges faced in studies of these complex systems. Nevertheless, it is clear that measurements of the photodynamics hold the key to understanding charge transfer and energy dissipation in DSSC. Here we propose to investigate N3 in the gas phase and compare it to the dye dip-coated onto atomically characterised single crystal substrates of ZnO and TiO2. We will study the photodynamics using two-photon photoemission (2PPE) with variable photon energy, and with time resolved photoemission using a visible light pump and XUV probe. Both these measurements will be carried out on the fs timescale. This is a challenging and ambitious project that will provide results that will transform our understanding of DSSC functionality.Objectives1. Use 2PPE to explore the photodynamics of N3 in the gas phase and on ZnO and TiO2 and explore the role of Ti 3d band gap states in the photoexcitation.2. Use time resolved photoemission (TRPES) to follow the population and decay of the LUMO and other states in the dye as well as band gap and conduction band edge states in the substrate.MethodologyFigure 2 shows the principles of 2PPE and HHG TRPES along with sample spectra recorded in connection with our previous work related to TiO2/H2O photocatalysis. The 2PPE work allowed us to identify a photoexcitation process in addition to band gap excitation that could be important in photocatalysis [6]. This involves hydroxyl-localised excitation from band gap states to states in the conduction band region. TRPES spectra point to the rapid recombination (<50 fs) of hot electrons created by pumping the band gap states with 1 eV light. A longer component (ca. 1 ps) is also observed if the pump is band gap light (3.1 eV). This is interpreted as VB-CB exciton pair recombination. In this earlier work, 2PPE measurements were carried out at UCL, with TRPES measurements at the Artemis facility at Harwell. In the current project this would be augmented with an existing, but as yet uncommissioned HHG apparatus at UCL.