Munshi Joydeep, Dulal Rabindra, Chien TeYu, Chen Wei, Balasubramanian Ganesh
Department of Mechanical Engineering and Mechanics , Lehigh University , Bethlehem , Pennsylvania 18015 , United States.
Department of Physics and Astronomy , University of Wyoming , Laramie , Wyoming 82071 , United States.
ACS Appl Mater Interfaces. 2019 May 8;11(18):17056-17067. doi: 10.1021/acsami.9b02719. Epub 2019 Apr 18.
Mixtures of poly(3-hexyl-thiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) have been widely employed as donor and acceptor materials, respectively, for the active layer of the bulk heterojunction (BHJ) organic solar cells. Experiments are able to provide only limited insights on the dynamics of blend morphology of these organic materials because of the challenges in extracting microstructural characterization amidst the poor contrast in electron microscopy. We present results from coarse-grained molecular dynamics simulations (CGMD) describing the morphological evolution of P3HT/PCBM active layer under solution processing in chlorobenzene (CB). We examine the impact of various processing parameters such as weight ratio, degree of polymerization (DOP), thermal annealing, and preheating on the BHJ active layers using morphological characterizations from atomic trajectories. Simulated diffraction patterns are compared with experimental results of X-ray diffraction and Small Angle X-ray Scattering (SAXS). Both simulated scattering and experimental X-ray diffraction and X-ray scattering measurements reveal increase in crystallinity for P3HT upon annealing until PCBM weight fraction ∼50%. The solubility of PCBM being greater in CB than that of P3HT facilitates the phase separation of the polymer during early stages of solvent evaporation. An increase in the average size of the P3HT domain relative to the preannealed morphology, is due to phase segregation and crystallization of the polymer upon annealing. Percolation for PCBM remains unchanged until PCBM constitutes at least one-half of the composition. Although 1.0:2.0 weight ratio is predicted to be ideal for balanced charge transport, 1.0:1.0 weight ratio is the most beneficial of overall power conversion based on exciton generation and charge separation at the interface. DOP of P3HT molecules is another important design variable as larger P3HT molecules tend to entangle more often deteriorating molecular order of P3HT phase in the active layer. Preheating the ternary mixture of P3HT, PCBM, and CB modifies the structural order and morphology of the BHJ due to changes in PCBM diffusion into the P3HT phase.
聚（3-己基噻吩）（P3HT）和苯基-C61-丁酸甲酯（PCBM）的混合物已分别被广泛用作本体异质结（BHJ）有机太阳能电池活性层的供体和受体材料。由于在电子显微镜对比度较差的情况下提取微观结构特征存在挑战，实验只能对这些有机材料共混物形态的动力学提供有限的见解。我们展示了粗粒度分子动力学模拟（CGMD）的结果，该模拟描述了P3HT/PCBM活性层在氯苯（CB）溶液处理下的形态演变。我们使用原子轨迹的形态表征，研究了各种加工参数，如重量比、聚合度（DOP）、热退火和预热对BHJ活性层的影响。将模拟衍射图案与X射线衍射和小角X射线散射（SAXS）的实验结果进行比较。模拟散射以及实验X射线衍射和X射线散射测量均表明，退火后P3HT的结晶度增加，直至PCBM重量分数约为50%。PCBM在CB中的溶解度大于P3HT，这有利于在溶剂蒸发的早期阶段聚合物的相分离。相对于预退火形态，P3HT域平均尺寸的增加是由于退火后聚合物的相分离和结晶。在PCBM构成至少一半的组成之前，其渗流情况保持不变。尽管预测1.0:2.0的重量比对于平衡电荷传输是理想的，但基于界面处的激子产生和电荷分离，1.0:1.0的重量比对于整体功率转换最为有利。P3HT分子的DOP是另一个重要设计变量，因为较大的P3HT分子往往更容易缠结，从而恶化活性层中P3HT相的分子有序性。由于PCBM扩散到P3HT相的变化，预热P3HT、PCBM和CB的三元混合物会改变BHJ的结构有序性和形态。
