Optimisation of charge carrier mobility in nanoporous metal oxide films

通过烧结金属氧化物纳米颗粒形成的高表面积纳米多孔膜在工业规模上是高度稳定、无毒且廉价的。他们在气体传感和催化领域找到了广泛的应用，其中高表面积对于最大化分子与薄膜的相互作用至关重要。它们还在第三代太阳能电池中用作电荷传输层，例如染料或钙钛矿敏化电池，其中通过用光吸收材料涂覆纳米多孔膜来确保电子和空穴的有效光注入。对于太阳能电池，以及对于纳米多孔膜的其他重要应用，例如燃料电池和光电化学电池中的电极，良好的电荷载流子迁移率也是基本要求。不幸的是，尽管纳米多孔氧化物膜具有许多优点，但其电子迁移率通常非常差。例如，纳米多孔TiO 2、ZnO和SnO 2薄膜的迁移率已被证明比相应单晶的迁移率小两到四个数量级。这种低迁移率是限制（光）电化学和光伏应用的效率的关键因素，通常归因于在表面和纳米粒子之间的界面处增加的电荷载流子捕获，因为电荷捕获与表面附近的离子有关，我们假设，在烧结成膜之前，通过对纳米粒子表面进行适当的化学修饰，应该可以消除这些陷阱。这种方法将保留纳米多孔膜在高表面积、无毒和可加工性方面的优点，同时提高流动性。这种改性以前已经尝试过，但由于缺乏对电荷捕获的起源或表面改性的影响的理解，成功受到限制。在这里，我们建议联合收割机的预测能力的第一原理理论建模与结构，光谱和非物理材料表征，以量化的因素负责在纳米多孔氧化物薄膜的表面和界面的电荷捕获在原子水平。一旦在未改性的薄膜上验证和改进，理论方法将用于评估减少电荷捕获的改性策略。特别是，我们将考虑在氧化物纳米颗粒表面附近引入/取代阴离子和阳离子，以消除有问题的捕获位点。理论上筛选各种可能的修饰途径（即不同的阳离子和阴离子）的能力是我们提出的方法的关键优势。这种策略的应用，测试和优化可能会提供一个新的范例，知识为主导的设计太阳能氧化物materials.We的目标是证明我们的方法的有效性，通过增加纳米结构的TiO 2和ZrO 2的流动性，以提高钙钛矿敏化太阳能电池的效率，这是新兴的有吸引力的第三代光伏技术。预计到2022年，第三代光伏市场的规模将增长到380亿美元，使其成为具有重大经济影响潜力的领域。提高纳米多孔氧化物的迁移率可以使这些器件的效率从其当前水平（约20%）接近约30%的理论最大值。将整体效率从20%提高到23%，将使总功率输出增加15%，再加上较低的制造成本，这将使该技术非常有吸引力。我们将与领先的纳米TiO 2（Cristal）和钙钛矿敏化太阳能电池（Dyesol Limited）制造商合作，测试我们改性薄膜的性能。更一般地说，通过控制改性来定制纳米多孔膜中界面的电子性质的能力应该在包括传感、催化和电子学在内的其他技术中找到应用。

High surface area nanoporous films formed by sintering metal oxide nanoparticles are highly stable, non-toxic and inexpensive to produce on an industrial scale. They find a wide range of applications in gas sensing and catalysis where high surface area is essential to maximise the interaction of molecules with the film. They also find applications as charge transport layers in third generation solar cells, e.g. dye- or perovskite-sensitised cells, where efficient photoinjection of electrons and holes is ensured by coating nanoporous films with a light absorbing material. For solar cells, as well as for other important applications of nanoporous films such as electrodes in fuel cells and photoelectrochemical cells, good charge carrier mobility is also an essential requirement. Unfortunately, despite their numerous advantages, the electronic mobility of nanoporous oxide films is in general very poor. For example, the mobilities of nanoporous TiO2, ZnO and SnO2 films have been shown to be between two and four orders of magnitude smaller than those of corresponding single crystals. This low mobility is a key factor limiting the efficiency of (photo-)electrochemical and photovoltaic applications and is usually attributed to increased charge carrier trapping at surfaces and at interfaces between nanoparticles.Since charge trapping is associated with ions near surfaces we hypothesise that it should be possible to eliminate these traps by suitable chemical modification of the surfaces of nanoparticles prior to sintering into a film. This approach would retain the advantages of nanoporous films in terms of high surface area, non-toxicity and processability while improving mobility. Such modifications have been attempted previously, but due to the lack of understanding on the origin of charge trapping or the effects of surface modification, success has been limited. Here, we propose to combine the predictive power of first principles theoretical modelling with structural, spectroscopic and photophysical materials characterisation, in order to quantify the factors responsible for charge trapping at surface and interfaces in nanoporous oxide films at an atomistic level. Once validated and refined on unmodified films, theoretical methods will be used to assess modification strategies to reduce charge-trapping. In particular, we will consider the incorporation/substitution of anions and cations near the surface of oxide nanoparticles to eliminate the problematic trapping sites. The ability to theoretically screen various possible modification routes (i.e. different cations and anions) is a key advantage of our proposed approach. Application, testing and optimisation of such strategies may offer a new paradigm for knowledge-led design of solar oxide materials.We aim to demonstrate the effectiveness of our approach by increasing the mobility of nanostructured TiO2 and ZrO2 to deliver an improvement in the efficiency of perovskite-sensitised solar cells, which are emerging as an attractive third generation photovoltaic technology. The size of the third generation photovoltaic market is predicted to grow to $38bn by 2022, making this an area with significant potential for economic impact. Improving the mobility of nanoporous oxides could bring the efficiency of these devices from their current level (about 20%) to closer to the theoretical maximum of about 30%. An increase in overall efficiency from 20% to only 23% percent would increase the total power output by 15%, which when coupled with lower manufacturing costs would make the technology very attractive. We will work with leading manufacturers of nano-TiO2 (Cristal) and perovskite-sensitised solar cells (Dyesol Limited) to test the performance of our modified films. More generally, the ability to tailor the electronic properties of interfaces in nanoporous films by controlled modification should find applications in other technologies including sensing, catalysis and electronics.