Unraveling the Structure–Property Relationship of Molecular Hole-Transporting Materials for Perovskite Solar Cells

Clarifying the structural basis and microscopic mechanism lying behind electronic properties of molecular semiconductors is of paramount importance in further material design to enhance the performance of perovskite solar cells. In this paper, three conjugated quasilinear segments of 9,9-dimethyl-9H-fluorene, 9,9-dimethyl-2,7-diphenyl-9H-fluorene, and 2,6-diphenyldithieno­[3,2-b:2′,3′-d]­thiophene are end-capped with two bis­(4-methoxyphenyl)­amino groups for structurally simple molecular semiconductors Z1, Z2, and Z3, which crystallize in the monoclinic P21/n, triclinic P1̅, and monoclinic C2/c space groups, respectively. The modes and energies of intermolecular noncovalent interactions in various closely packed dimers extracted from single crystals are computed based on the quantum theory of atoms in molecules and energy decomposition analysis. Transfer integrals, reorganization energies, and center-of-mass distances in these dimers as well as band structures of single crystals are also calculated to define the theoretical limit of hole transport and microscopic transport pictures. Joint X-ray diffraction and space-charge-limiting current measurements on solution-deposited films suggest the dominant role of crystallinity in thin-film hole mobility. Photoelectron spectroscopy and photoluminescence measurements show that an enhanced interfacial interaction between the perovskite and Z3 could attenuate the adverse impact of reducing the energetic driving force of hole extraction. Our comparative studies show that the molecular semiconductor Z3 with a properly aligned highest occupied molecular orbital energy level and a high thin-film mobility can be employed for efficient perovskite solar cells, achieving a good power conversion efficiency of 20.84%, which is even higher than that of 20.42% for the spiro-OMeTAD control.阐明分子半导体电子性质背后的结构基础与微观机制，对于优化钙钛矿太阳能电池性能的后续材料设计而言至关重要。本文中，研究者以双(4-甲氧基苯基)氨基对9,9-二甲基-9H-芴、9,9-二甲基-2,7-二苯基-9H-芴以及2,6-二苯基二噻吩并[3,2-b:2′,3′-d]噻吩三种共轭准线性片段进行端基封端，得到结构简洁的分子半导体Z1、Z2与Z3；三者分别以单斜晶系P2₁/n、三斜晶系P1̄以及单斜晶系C2/c空间群结晶。基于分子中原子量子理论（Quantum Theory of Atoms in Molecules, QTAIM）与能量分解分析，研究者计算了从单晶中提取的多种紧密堆积二聚体的分子间非共价相互作用模式与能量。同时计算了上述二聚体的传输积分、重组能以及质心距离，以及单晶的能带结构，以明确空穴传输的理论极限与微观传输图像。对溶液沉积薄膜开展的X射线衍射与空间电荷限制电流联合测试结果表明，结晶度对薄膜空穴迁移率起到决定性作用。光电子能谱与光致发光测试结果显示，钙钛矿与Z3之间增强的界面相互作用，能够削弱降低空穴提取能量驱动力所带来的负面影响。本研究的对比分析表明，具备匹配最优最高占据分子轨道（Highest Occupied Molecular Orbital, HOMO）能级与高薄膜迁移率的分子半导体Z3，可用于制备高效钙钛矿太阳能电池，实现了20.84%的优异光电转换效率，甚至高于对照样品spiro-OMeTAD的20.42%效率。