Size Effects in the Interface Level Alignment of Dye-Sensitized TiO₂ Clusters

The efficiency of dye-sensitized solar cells (DSCs) depends critically on the electronic structure of the interfaces in the active region. We employ recently developed dispersion-inclusive density functional theory (DFT) and GW methods to study the electronic structure of TiO2 clusters sensitized with catechol molecules. We show that the energy level alignment at the dye-TiO2 interface is the result of an intricate interplay of quantum size effects and dynamic screening effects and that it may be manipulated by nanostructuring and functionalizing the TiO2. We demonstrate that the energy difference between the catechol LUMO and the TiO2 LUMO, which is associated with the injection loss in DSCs, may be reduced significantly by reducing the dimensions of nanostructured TiO2 and by functionalizing the TiO2 with wide-gap moieties, which contribute additional screening but do not interact strongly with the frontier orbitals of the TiO2 and the dye. Precise control of the electronic structure may be achieved via “interface engineering” in functional nanostructures.染料敏化太阳能电池（dye-sensitized solar cells, DSCs）的效率关键取决于其活性区域内界面的电子结构。我们采用新近开发的包含色散校正的密度泛函理论（dispersion-inclusive density functional theory, DFT）与GW方法，研究儿茶酚分子修饰的二氧化钛团簇的电子结构。研究表明，染料-二氧化钛界面处的能级对齐是量子尺寸效应与动态屏蔽效应复杂相互作用的结果，且可通过对二氧化钛进行纳米结构化与功能化来调控。我们证实，与DSCs注入损耗相关的儿茶酚最低未占据分子轨道（Lowest Unoccupied Molecular Orbital, LUMO）与二氧化钛最低未占据分子轨道之间的能级差，可通过减小纳米结构化二氧化钛的尺寸，以及用宽隙基团功能化二氧化钛的方式得到显著降低——这类宽隙基团可提供额外的屏蔽作用，却不会与二氧化钛及染料的前线轨道发生强相互作用。通过功能化纳米结构中的界面工程，可实现电子结构的精准调控。