Optimisation of charge carrier mobility in nanoporous metal oxide films

由金属氧化物纳米颗粒烧结而成的高比表面积纳米多孔膜具有高度稳定、无毒、工业化生产成本低等优点。它们在气体传感和催化中得到了广泛的应用，在这些领域中，高表面积对于最大化分子与薄膜的相互作用至关重要。它们还被用作第三代太阳能电池的电荷传输层，例如染料或钙钛矿敏化电池，其中电子和空穴的有效光注入是通过在纳米多孔薄膜上涂上光吸收材料来确保的。对于太阳能电池，以及纳米多孔薄膜的其他重要应用，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具有许多优点，但总的来说，其电子迁移率非常低。例如，纳米多孔二氧化钛、氧化锌和二氧化锡薄膜的迁移率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)电化学和光伏应用效率的一个关键因素，通常归因于表面和纳米粒子之间界面上电荷载流子陷阱的增加。由于电荷陷阱与表面附近的离子有关，我们假设在将纳米粒子烧结成薄膜之前，应该可以通过适当的化学修饰来消除这些陷阱。这种方法将保留纳米多孔薄膜在高比表面积、无毒和可加工性方面的优势，同时提高迁移率。以前也尝试过这样的修饰，但由于对电荷捕获的起源或表面修饰的影响缺乏了解，成功受到了限制。在这里，我们建议将第一性原理理论模型的预测能力与结构、光谱和光物理材料的表征相结合，以便在原子水平上量化导致纳米多孔氧化物薄膜表面和界面电荷陷阱的因素。一旦在未经修饰的薄膜上进行验证和改进，将使用理论方法来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加入/取代阴离子和阳离子，以消除有问题的陷阱位置。从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)的能力是我们所提出的方法的一个关键优势。这些策略的应用、测试和优化可能会为以知识为导向的太阳能氧化物材料设计提供一个新的范例。我们的目标是通过增加纳米结构的二氧化钛和氧化锆的迁移率来提高钙钛矿型敏感太阳能电池的效率，从而证明我们的方法的有效性。钙钛矿型太阳能电池正成为一种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至380亿美元，这将使该领域成为一个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目前的水平(约20%)更接近理论上约30%的最大值。总效率从20%提高到23%将使总发电量增加15%，再加上较低的制造成本，这将使这项技术非常有吸引力。我们将与纳米二氧化钛(Cristal)和钙钛矿敏化太阳能电池(Dyesol Limited)的领先制造商合作，测试我们改性薄膜的性能。更广泛地说，通过可控修饰来定制纳米多孔薄膜中界面的电子性质的能力应该会在其他技术中得到应用，包括传感、催化和电子学。

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.