Singh Ranbir, Shivanna Ravichandran, Iosifidis Agathaggelos, Butt Hans-Jürgen, Floudas George, Narayan K S, Keivanidis Panagiotis E
Centre for Nanoscience and Technology@PoliMi, Fondazione Istituto Italiano di Tecnologia , Via Giovanni Pascoli 70/3, 20133 Milano, Italy.
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research , Bangalore 560064, India.
ACS Appl Mater Interfaces. 2015 Nov 11;7(44):24876-86. doi: 10.1021/acsami.5b08224. Epub 2015 Oct 29.
Perylene diimide (PDI)-based organic photovoltaic devices can potentially deliver high power conversion efficiency values provided the photon energy absorbed is utilized efficiently in charge transfer (CT) reactions instead of being consumed in nonradiative energy transfer (ET) steps. Hitherto, it remains unclear whether ET or CT primarily drives the photoluminescence (PL) quenching of the PDI excimer state in PDI-based blend films. Here, we affirm the key role of the thermally assisted PDI excimer diffusion and subsequent CT reaction in the process of PDI excimer PL deactivation. For our study we perform PL quenching experiments in the model PDI-based composite made of poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiophene)-2-6-diyl] (PBDTTT-CT) polymeric donor mixed with the N,N'-bis(1-ethylpropyl)-perylene-3,4,9,10-tetracarboxylic diimide (PDI) acceptor. Despite the strong spectral overlap between the PDI excimer PL emission and UV-vis absorption of PBDTTT-CT, two main observations indicate that no significant ET component operates in the overall PL quenching: the PL intensity of the PDI excimer (i) increases with decreasing temperature and (ii) remains unaffected even in the presence of 10 wt % content of the PBDTTT-CT quencher. Temperature-dependent wide-angle X-ray scattering experiments further indicate that nonradiative resonance ET is highly improbable due to the large size of PDI domains. The dominance of the CT over the ET process is verified by the high performance of devices with an optimum composition of 30:70 PBDTTT-CT:PDI. By adding 0.4 vol % of 1,8-diiodooctane we verify the plasticization of the polymer side chains that balances the charge transport properties of the PBDTTT-CT:PDI composite and results in additional improvement in the device efficiency. The temperature-dependent spectral width of the PDI excimer PL band suggests the presence of energetic disorder in the PDI excimer excited state manifold.
基于苝二亚胺（PDI）的有机光伏器件若能在电荷转移（CT）反应中高效利用吸收的光子能量，而非在非辐射能量转移（ET）步骤中消耗，则有望实现高功率转换效率。迄今为止，在基于PDI的共混薄膜中，究竟是ET还是CT主要驱动PDI准分子态的光致发光（PL）猝灭尚不清楚。在此，我们证实了热辅助PDI准分子扩散及随后的CT反应在PDI准分子PL失活过程中的关键作用。在本研究中，我们在由聚[4,8 - 双（5 - （2 - 乙基己基）噻吩 - 2 - 基）苯并[1,2 - b；4,5 - b']二噻吩 - 2,6 - 二基 - 交替 - （4 - （2 - 乙基己酰基） - 噻吩并[3,4 - b]噻吩） - 2 - 6 - 二基]（PBDTTT - CT）聚合物供体与N，N'-双（1 - 乙基丙基） - 苝 - 3,4,9,10 - 四羧酸二亚胺（PDI）受体混合而成的模型PDI基复合材料中进行了PL猝灭实验。尽管PDI准分子PL发射与PBDTTT - CT的紫外 - 可见吸收之间存在强烈的光谱重叠，但两个主要观察结果表明在整体PL猝灭中不存在显著的ET成分：（i）PDI准分子的PL强度随温度降低而增加；（ii）即使存在10 wt%含量的PBDTTT - CT猝灭剂，其PL强度仍不受影响。温度依赖的广角X射线散射实验进一步表明，由于PDI域尺寸较大，非辐射共振ET极不可能发生。具有30:70 PBDTTT - CT:PDI最佳组成的器件的高性能验证了CT相对于ET过程的主导地位。通过添加0.4 vol%的1,8 - 二碘辛烷，我们验证了聚合物侧链的增塑作用，其平衡了PBDTTT - CT:PDI复合材料的电荷传输性能，并导致器件效率进一步提高。PDI准分子PL带的温度依赖光谱宽度表明在PDI准分子激发态流形中存在能量无序。
