†Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States.
‡Department of Electrical and Computer Engineering Duke, University, Durham, North Carolina 27708, United States.
ACS Nano. 2015 Apr 28;9(4):4412-8. doi: 10.1021/acsnano.5b00938. Epub 2015 Apr 13.
Direct modification of exciton energy has been previously used to optimize the operation of organic optoelectronic devices. One demonstrated method for exciton energy modification is through the use of the solvent dielectric effects in doped molecular films. To gain a deeper appreciation of the underlying physical mechanisms, in this work we test the solid-state solvation effect in molecular thin films under applied external pressure. We observe that external mechanical pressure increases dipole-dipole interactions, leading to shifts in the Frenkel exciton energy and enhancement of the time-resolved spectral red shift associated with the energy-transfer-mediated exciton diffusion. Measurements are performed on host:dopant molecular thin films, which show bathochromic shifts in photoluminescence (PL) under increasing pressure. This is in agreement with a simple solvation theory model of exciton energetics with a fitting parameter based on the mechanical properties of the host matrix material. We measure no significant change in exciton lifetime with increasing pressure, consistent with unchanged aggregation in molecular films under compression. However, we do observe an increase in exciton spectral thermalization rate for compressed molecular films, indicating enhanced exciton diffusion for increased dipole-dipole interactions under pressure. The results highlight the contrast between molecular energy landscapes obtained when dipole-dipole interactions are increased by the pressure technique versus the conventional dopant concentration variation methods, which can lead to extraneous effects such as aggregation at higher doping concentrations. The present work demonstrates the use of pressure-probing techniques in studying energy disorder and exciton dynamics in amorphous molecular thin films.
直接修饰激子能量已被用于优化有机光电设备的性能。一种已被证实的激子能量修饰方法是通过利用掺杂分子薄膜中的溶剂介电效应。为了更深入地了解潜在的物理机制，在这项工作中，我们测试了施加外部压力下分子薄膜中的固态溶剂化效应。我们观察到外部机械压力增加了偶极-偶极相互作用，导致弗伦克尔激子能量发生位移，并增强了与能量转移介导的激子扩散相关的时间分辨光谱红移。我们在主体：掺杂分子薄膜上进行了测量，结果表明在压力增加下光致发光（PL）出现红移。这与基于主体基质材料力学性能的激子能态简单溶剂化理论模型一致，模型中的拟合参数考虑了溶剂化效应。我们发现随着压力的增加激子寿命没有明显变化，这与分子薄膜在压缩下聚集状态不变一致。然而，我们确实观察到压缩分子薄膜中激子光谱热化速率增加，表明在压力下偶极-偶极相互作用增强导致激子扩散增强。研究结果突出了在增加压力技术下增加偶极-偶极相互作用时获得的分子能量景观与传统掺杂浓度变化方法获得的分子能量景观之间的对比，后者可能导致在较高掺杂浓度下出现额外的效应，如聚集。本工作证明了在研究非晶态分子薄膜中的能量无序和激子动力学时使用压力探测技术的有效性。
