Optimisation of charge carrier mobility in nanoporous metal oxide films

由金属氧化物纳米颗粒烧结形成的高表面积纳米孔膜具有高度稳定、无毒和工业规模生产成本低廉的特点。他们在气体传感和催化中发现了广泛的应用，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。他们还在第三代太阳能电池中发现了电荷传输层的应用，例如染料或钙钛矿敏化电池，其中通过用吸光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，以及纳米多孔膜的其他重要应用，如燃料电池和光电化学电池中的电极，良好的载流子迁移率也是必不可少的要求。不幸的是，尽管纳米多孔氧化物薄膜有许多优点，但它们的电子迁移率通常很差。例如，纳米多孔TiO2、ZnO和SnO2薄膜的迁移率比相应单晶的迁移率小2到4个数量级。这种低迁移率是限制（光）电化学和光伏应用效率的关键因素，通常归因于纳米颗粒表面和界面处电荷载流子捕获的增加。由于电荷捕获与表面附近的离子有关，我们假设在烧结成薄膜之前，通过对纳米颗粒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孔膜在高表面积、无毒性和可加工性方面的优点，同时提高迁移率。以前曾尝试过这种修饰，但由于对电荷捕获的起源或表面修饰的影响缺乏了解，成功的情况有限。在这里，我们建议将第一性原理理论建模的预测能力与结构、光谱和光物理材料表征相结合，以便在原子水平上量化纳米多孔氧化物膜表面和界面上电荷捕获的因素。一旦在未经修饰的薄膜上得到验证和改进，理论方法将用于评估减少电荷捕获的修饰策略。特别是，我们将考虑在氧化物纳米颗粒表面附近的阴离子和阳离子的掺入/取代，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。这些策略的应用、测试和优化可能为以知识为主导的太阳能氧化物材料设计提供新的范例。我们的目标是通过增加纳米结构TiO2和ZrO2的迁移率来证明我们的方法的有效性，从而提高钙钛矿敏化太阳能电池的效率，钙钛矿敏化太阳能电池正在成为第三代光伏技术。到2022年，第三代光伏市场的规模预计将增长到380亿美元，使其成为一个具有巨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目前的水平（约20%）提高到接近理论最大值（约30%）。如果将总效率从20%提高到23%，则总功率输出将增加15%，再加上制造成本的降低，该技术将非常有吸引力。我们将与纳米tio2 （Cristal）和钙钛矿敏化太阳能电池（Dyesol Limited）的领先制造商合作，测试我们的改性薄膜的性能。更一般地说，通过控制修饰来定制纳米孔膜界面的电子特性的能力应该在传感、催化和电子学等其他技术中得到应用。

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.