Inverse Design of Tetracene Polymorphs with Enhanced Singlet Fission Performance by Property-Based Genetic Algorithm Optimization

The efficiency of solar cells may be improved by using singlet fission (SF), in which one singlet exciton splits into two triplet excitons. SF occurs in molecular crystals. A molecule may crystallize in more than one form, a phenomenon known as polymorphism. Crystal structure may affect SF performance. In the common form of tetracene, SF is experimentally known to be slightly endoergic. A second, metastable polymorph of tetracene has been found to exhibit better SF performance. Here, we conduct inverse design of the crystal packing of tetracene using a genetic algorithm (GA) with a fitness function tailored to simultaneously optimize the SF rate and the lattice energy. The property-based GA successfully generates more structures predicted to have higher SF rates and provides insight into packing motifs associated with improved SF performance. We find a putative polymorph predicted to have superior SF performance to the two forms of tetracene, whose structures have been determined experimentally. The putative structure has a lattice energy within 1.5 kJ/mol of the most stable common form of tetracene.利用单重态裂变（singlet fission, SF）可提升太阳能电池的效率——该过程指一个单重态激子分裂为两个三重态激子。单重态裂变可发生于分子晶体体系中。一种分子可存在多种结晶形式，该现象被称为多晶型（polymorphism）。晶体结构会对单重态裂变的性能产生显著影响。在常见的并四苯晶型中，实验已证实单重态裂变表现出微弱的吸能特性。研究人员还发现，并四苯的第二种亚稳态多晶型展现出更优异的单重态裂变性能。本研究采用适配同时优化单重态裂变速率与晶格能的适应度函数的遗传算法（genetic algorithm, GA），对并四苯的晶体堆积结构开展逆向设计。该基于物性的遗传算法成功生成了更多预测具备更高单重态裂变速率的晶体结构，并为阐明与更优单重态裂变性能相关的堆积基元提供了理论思路。本研究发现一种推定多晶型，其预测的单重态裂变性能优于两种已通过实验确定结构的并四苯晶型；且该推定结构的晶格能与最稳定的常见并四苯晶型的晶格能差值在1.5 kJ/mol以内。