Hou Xueyan, Coker Jack F, Yan Jun, Shi Xingyuan, Azzouzi Mohammed, Eisner Flurin D, McGettrick James D, Tuladhar Sachetan M, Abrahams Isaac, Frost Jarvist M, Li Zhe, Dennis T John S, Nelson Jenny
Department of Physics, Imperial College London, London SW7 2AZ, U.K.
School of Physical and Chemical Sciences, Queen Mary University of London, London E1 4NS, U.K.
Chem Mater. 2023 Dec 28;36(1):425-438. doi: 10.1021/acs.chemmater.3c02353. eCollection 2024 Jan 9.
Higher adducts of a fullerene, such as the bis-adduct of PCBM (bis-PCBM), can be used to achieve shallower molecular orbital energy levels than, for example, PCBM or C Substituting the bis-adduct for the parent fullerene is useful to increase the open-circuit voltage of organic solar cells or achieve better energy alignment as electron transport layers in, for example, perovskite solar cells. However, bis-PCBM is usually synthesized as a mixture of structural isomers, which can lead to both energetic and morphological disorder, negatively affecting device performance. Here, we present a comprehensive study on the molecular properties of 19 pure bis-isomers of PCBM using a variety of characterization methods, including ultraviolet photoelectron spectroscopy, thermal gravimetric analysis, differential scanning calorimetry, single crystal structure, and (time-dependent) density functional theory calculation. We find that the lowest unoccupied molecular orbital of such bis-isomers can be tuned to be up to 170 meV shallower than PCBM and up to 100 meV shallower than the mixture of unseparated isomers. The isolated bis-isomers also show an electron mobility in organic field-effect transistors of up to 4.5 × 10 cm/(V s), which is an order of magnitude higher than that of the mixture of bis-isomers. These properties enable the fabrication of the highest performing bis-PCBM organic solar cell to date, with the best device showing a power conversion efficiency of 7.2%. Interestingly, we find that the crystallinity of bis-isomers correlates negatively with electron mobility and organic solar cell device performance, which we relate to their molecular symmetry, with a lower symmetry leading to more amorphous bis-isomers, less energetic disorder, and higher dimensional electron transport. This work demonstrates the potential of side chain engineering for optimizing the performance of fullerene-based organic electronic devices.
富勒烯的高阶加合物，如PCBM的双加合物（双-PCBM），可用于实现比例如PCBM或C更浅的分子轨道能级。用双加合物替代母体富勒烯有助于提高有机太阳能电池的开路电压，或在例如钙钛矿太阳能电池中作为电子传输层实现更好的能量排列。然而，双-PCBM通常作为结构异构体的混合物合成，这可能导致能量和形态上的无序，对器件性能产生负面影响。在此，我们使用多种表征方法，包括紫外光电子能谱、热重分析、差示扫描量热法、单晶结构以及（含时）密度泛函理论计算，对19种纯PCBM双异构体的分子性质进行了全面研究。我们发现，此类双异构体的最低未占据分子轨道可比PCBM浅达170 meV，比未分离异构体的混合物浅达100 meV。分离出的双异构体在有机场效应晶体管中的电子迁移率高达4.5×10 cm/(V s)，比双异构体混合物的迁移率高一个数量级。这些性质使得能够制造出迄今为止性能最佳的双-PCBM有机太阳能电池，最佳器件的功率转换效率为7.2%。有趣的是，我们发现双异构体的结晶度与电子迁移率和有机太阳能电池器件性能呈负相关，我们将其与分子对称性相关联，较低的对称性导致更多非晶态双异构体、更少的能量无序以及更高维度的电子传输。这项工作展示了侧链工程在优化基于富勒烯的有机电子器件性能方面的潜力。
