Rational design of solid-state semiconductor-sensitized solar cells: from materials modelling to device fabrication

由于全球对能源的需求日益增长，开发有效利用太阳能的方法已成为一项关键的科学挑战。在前景看好的硅光伏低成本替代品中，与许多新的低成本概念相比，基于多孔性金属氧化物薄膜的纳米结构太阳能电池因其相对较高的能量转换效率而获得了突出的地位。尽管材料界面在这些先进的太阳能电池概念中扮演着重要的角色，但人们对其纳米级的电子和光学性质知之甚少，目前的大多数研究依赖于爱迪生的反复试验方法。该项目的核心思想是开发一种合理的方法来设计和制造基于半导体无机敏化剂的纳米结构太阳能电池，使用量子力学原子材料建模、材料合成和表征、器件制造和表征以及先进的光谱技术。事实上，表征技术和计算机建模如今可以解决类似的长度尺度(亚纳米到几纳米)，因此现在是协同使用实验和建模以加速纳米级太阳能研究发现的最佳时机。支持该项目的愿景是，在10年内，将有可能以类似于理性药物设计和生物信息学的方式设计、优化和制造纳米结构太阳能电池。为了实现这一目标，我们的战略资产将是领先的材料模型师、纳米技术专家和设备工程师之间的密切合作。新型太阳能电池的合理设计需要对敏化剂的光学性质、界面能级取向、电荷注入/复合速率、载流子迁移率等多个方面进行计算研究和实验控制。在这个项目中，我们沿着这个方向迈出了第一步，主要集中在敏化剂/氧化物界面的电子能级排列。界面能级排列与敏化太阳能电池的开路电压直接相关，是提高电池效率的关键设计参数。我们提出的合理设计将包括以下步骤：(I)通过计算模型确定有希望的敏化剂，(Ii)合成和表征选定的材料，(Iii)制造和优化太阳能电池，以及(Iv)进行先进的光谱分析，以了解整个太阳能电池的基本操作和限制因素。到目前为止，在纳米光伏研究中还没有尝试过第一原理建模和实验的协同使用，并有可能给该领域带来革命性的变化。由于我们的技能互补，我们的研究团队在英国和欧盟领域是独一无二的，该项目有望彻底改变我们对纳米级敏化太阳能电池的理解，并引入和开发范式转换技术。在这个项目中，我们将特别关注固态半导体敏化太阳能电池。这些器件是染料敏化太阳能电池概念的演变，即染料敏化剂被半导体量子点或纳米级半导体薄膜取代。这种选择有三个优点：(I)用胶体合成得到的廉价的光吸收材料取代昂贵的过渡金属基染料敏化剂；(Ii)利用量子尺寸效应可以调节敏化剂的光学性质；(Iii)与传统的薄膜光伏相比，具有更广泛的材料库，可以有效地用作半导体敏化剂。

Due to the growing global demand for energy, the development of efficient ways of harnessing solar power has become a key scientific challenge. Among promising low-cost alternatives to silicon photovoltaics, nanostructured solar cells based on porous metal oxides films coated with an extremely thin film of light absorbing semiconductors have gained prominence due to their relatively high energy conversion efficiencies, as compared to many new low cost concepts. Despite the prominent role of materials interfaces in these advanced solar cell concepts, very little is known about their electronic and optical properties at the nanoscale, and most of the current research relies on a Edisonian trial-and-error approach. The key idea of this project is to develop a rational approach to the design and fabrication of nanostructured solar cells based on semiconducting inorganic sensitizers, using a combination of quantum-mechanical atomistic materials modelling, materials synthesis and characterization, device fabrication and characterization, and advanced spectroscopy. Indeed characterization techniques and computer modelling can nowadays address similar length-scales (sub-nm to a few nm's), hence it is now the perfect time to use experiment and modelling synergistically in order to accelerate discovery in nanoscale solar energy research. The vision underpinning this project is that within 10 years it will be possible to design, optimize, and fabricate nanostructured solar cells in a way similar to what happens in rational drug design and bioinformatics. In order to achieve this goal our strategic asset will be a very close cooperation between leading materials modellers, nanotechnologists, and device engineers. The rational design of new solar cells will require the computational study and the experimental control of many aspects, including the optical properties of the sensitizer, the interfacial energy-level alignment, the charge injection/recombination rates, and the carrier mobilities. In this project we take the first step along this direction by focussing primarily on the electronic energy-level alignment at the sensitizer/oxide interface. The interfacial energy level alignment is directly related to the open-circuit voltage of sensitized solar cells and is a key design parameter for improving cell efficiencies. Our proposed rational design will consist of the following steps: (i) identify promising sensitizers via computational modelling, (ii) synthesize and characterize the selected materials, (iii) fabricate and optimise the solar cells, and (iv) perform advanced spectroscopy to understand the fundamental operation and limiting factors to performance in complete solar cells. This synergistic use of first-principles modelling and experiment has not been attempted so far in nano-photovoltaics research and has the potential of revolutionizing the field. Owing to our complementary skills, our research team is unique in the UK and EU arenas and this project holds the promise for revolutionizing our understanding of sensitized solar cells at the nano scale, and introducing and developing paradigm-shifting technology. In this project we will focus specifically on solid-state semiconductor-sensitized solar cells. These devices are an evolution of the concept of dye-sensitized solar cells whereby the dye sensitizer is replaced by a semiconductor quantum dot or a nanoscale semiconducting film. This choice has three advantages: (I) the expensive transition-metal based dye sensitizer is replaced by a inexpensive light-absorber obtained by colloidal synthesis (ii) the optical properties of the sensitizer can be tuned by exploiting quantum size effects, and (iii) in comparison to conventional thin film photovoltaics, there is a much broader library of materials which may work effectively as semiconductor sensitizers.