Dye sensitised solar cells (DSSC)

染料敏化太阳能电池（DSSC）是一种很有前途的途径，正在探索收获太阳的能量。自上世纪90年代初Grätzel和O'Regan b[1]首次展示这些光伏设备以来，它们一直是许多研究的主题。图1是一幅展示DSSC工作原理的漫画。未来的改进将来自于衬底/染料/氧化还原对的新配对，二氧化钛纳米颗粒是目前最受欢迎的衬底材料[2]。图1所示。DSSC—染料敏化太阳能电池：通过透明电极传输的光被染料（红色）吸收，该染料覆盖在TiO2纳米颗粒（灰色）上。这个过程形成电子-空穴对（e-/h+）。电子穿过TiO2层到达一个电极，空穴穿过电解质（蓝色）到达另一个电极，从而产生电流。基础研究旨在实现对整个DSSC系统的原子水平理解，已经构建了模型来解释ZnO和TiO2衬底[3]的瞬态吸收光谱。通常会对这两种底物进行比较，因为尽管TiO2具有更高的效率，但就可以合成的纳米结构的多样性而言，ZnO比TiO2具有许多优势。块状材料也具有较高的电子迁移率。2016年的两篇论文描述了常用的N3染料在ZnO和TiO2上的实验和计算研究[4,5]。这些论文对TiO2底物优于ZnO底物的解释不同。在实验研究[4]中，对薄膜的泵浦探针测量结果进行了解释，表明ZnO上形成了界面电子-阳离子配合物，减缓了电子注入。相比之下，计算研究预测TiO2的导带边缘态密度较大是其性能优越的原因[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，我们提出在气相中研究N3，并将其与浸渍在ZnO和TiO2原子表征的单晶衬底上的染料进行比较。我们将使用可变光子能量的双光子光发射（2PPE）和使用可见光泵浦和XUV探针的时间分辨光发射来研究光动力学。这两项测量都将在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2PPE研究N3在气相和ZnO、TiO2上的光动力学，探讨Ti三维带隙态在光激发中的作用。利用时间分辨光发射（TRPES）跟踪染料中LUMO和其他态以及衬底中的带隙和导带边缘态的居群和衰减。图2显示了2PPE和HHG TRPES的原理以及我们之前与TiO2/H2O光催化相关的工作记录的样品光谱。2PPE的工作使我们能够确定除带隙激发外的光激发过程，这在光催化[6]中可能很重要。这涉及到从带隙态到传导带区的羟基局域激发。TRPES光谱指出，用1 eV的光泵送带隙态产生的热电子快速复合（<50 fs）。如果泵浦是带隙光（3.1 eV），也可以观察到较长的分量（约1 ps）。这被解释为VB-CB激子对复合。在这项早期工作中，2PPE测量是在UCL进行的，TRPES测量是在Harwell的Artemis设施进行的。在目前的项目中，这将与伦敦大学学院现有的、但尚未投入使用的HHG设备相结合。

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.