Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
Phys Chem Chem Phys. 2013 Jun 28;15(24):9562-74. doi: 10.1039/c3cp51477c. Epub 2013 May 14.
The ongoing depletion of fossil fuels has led to an intensive search for additional renewable energy sources. Solar-based technologies could provide sufficient energy to satisfy the global economic demands in the near future. Photovoltaic (PV) cells are the most promising man-made devices for direct solar energy utilization. Understanding the charge separation and charge transport in PV materials at a molecular level is crucial for improving the efficiency of the solar cells. Here, we use light-induced EPR spectroscopy combined with DFT calculations to study the electronic structure of charge separated states in blends of polymers (P3HT, PCDTBT, and PTB7) and fullerene derivatives (C60-PCBM and C70-PCBM). Solar cells made with the same composites as active layers show power conversion efficiencies of 3.3% (P3HT), 6.1% (PCDTBT), and 7.3% (PTB7), respectively. Upon illumination of these composites, two paramagnetic species are formed due to photo-induced electron transfer between the conjugated polymer and the fullerene. They are the positive, P(+), and negative, P(-), polarons on the polymer backbone and fullerene cage, respectively, and correspond to radical cations and radical anions. Using the high spectral resolution of high-frequency EPR (130 GHz), the EPR spectra of these species were resolved and principal components of the g-tensors were assigned. Light-induced pulsed ENDOR spectroscopy allowed the determination of (1)H hyperfine coupling constants of photogenerated positive and negative polarons. The experimental results obtained for the different polymer-fullerene composites have been compared with DFT calculations, revealing that in all three systems the positive polaron is distributed over distances of 40-60 Å on the polymer chain. This corresponds to about 15 thiophene units for P3HT, approximately three units for PCDTBT, and about three to four units for PTB7. No spin density delocalization between neighboring fullerene molecules was detected by EPR. Strong delocalization of the positive polaron on the polymer donor is an important reason for the efficient charge separation in bulk heterojunction systems as it minimizes the wasteful process of charge recombination. The combination of advanced EPR spectroscopy and DFT is a powerful approach for investigation of light-induced charge dynamics in organic photovoltaic materials.
化石燃料的持续消耗导致人们对其他可再生能源进行了深入研究。基于太阳能的技术可以提供足够的能源，以满足不久的将来全球经济的需求。光伏 (PV) 电池是最有前途的人造设备，可直接利用太阳能。在分子水平上了解 PV 材料中的电荷分离和电荷输运对于提高太阳能电池的效率至关重要。在这里，我们使用光致电子顺磁共振 (EPR) 光谱结合密度泛函理论 (DFT) 计算来研究聚合物 (P3HT、PCDTBT 和 PTB7) 和富勒烯衍生物 (C60-PCBM 和 C70-PCBM) 混合物中电荷分离态的电子结构。以相同复合材料作为活性层的太阳能电池的功率转换效率分别为 3.3%（P3HT）、6.1%（PCDTBT）和 7.3%（PTB7）。在这些复合材料的照射下，由于共轭聚合物和富勒烯之间的光致电子转移，形成了两种顺磁性物质。它们分别是聚合物主链和富勒烯笼上的正、P(+)和负、P(-)极化子，对应于自由基阳离子和自由基阴离子。使用高频 EPR（130 GHz）的高光谱分辨率，解析了这些物质的 EPR 光谱，并分配了 g 张量的主要分量。光致脉冲 ENDOR 光谱允许确定光生正、负极化子的 (1)H 超精细耦合常数。用 DFT 计算对不同的聚合物-富勒烯复合材料进行了比较，结果表明，在所有三个系统中，正极化子在聚合物链上的分布距离为 40-60 Å。对于 P3HT，这对应于大约 15 个噻吩单元，对于 PCDTBT，大约三个单元，对于 PTB7，大约三个到四个单元。EPR 未检测到相邻富勒烯分子之间的自旋密度离域。正极化子在聚合物给体上的强离域是体异质结系统中有效电荷分离的重要原因，因为它最小化了电荷复合的浪费过程。先进的 EPR 光谱与 DFT 的结合是研究有机光伏材料中光致电荷动力学的强大方法。
