A new publication from Opto-Electronic Advances; 10.29026/oea.2025.240207 , discusses how light boosts exciton transport in organic molecular crystal.
Organic semiconductors, composed of organic molecules or polymers, offer advantages such as low cost, flexibility, lightweight design, and tunable structural-functional properties. They have significant applications in OLED displays (e.g., smartphones, TVs), organic photovoltaic cells (flexible solar panels), and flexible sensors. In organic semiconductors, excitons—carriers of excited-state energy—play a critical role in photoelectric conversion and energy transfer. Thus, the transport capability of excitons greatly influences the performance of organic semiconductor optoelectronic devices. Unfortunately, compared to inorganic semiconductors, exciton transport in organic semiconductors is generally less efficient, posing a bottleneck for improving device performance. Enhancing exciton transport and developing organic semiconductor materials with superior transport properties is therefore crucial.
Previous studies have focused on chemical synthesis to design new material systems with improved transport characteristics, yielding notable achievements. For example, high-quality nanostructures synthesized via self-assembly have demonstrated efficient exciton transport in isolated systems. However, isolated nanostructures are unsuitable for practical device applications, and transport parameters, such as diffusion coefficient exceeding 1 cm2 s-1, are often measured on ultrafast timescales (tens of picoseconds), which may not reflect sustained exciton transport efficiency under equilibrium conditions over their lifetime. Thus, achieving persistent, efficient exciton transport in device-relevant material systems remains challenging.
Considering that the structure of a material determines its properties, chemically synthesizing different materials is naturally the most direct way to alter the structure. Besides this, is it possible to explore other methods to modify the structure of organic semiconductors in order to more effectively enhance exciton transport properties?
A collaborative team led by Prof. Hongbo Sun and Assoc. Prof. Honghua Fang from Tsinghua University's Department of Precision Instruments, and Prof. Bin Xu from Jilin University's College of Chemistry, has achieved a breakthrough in enhancing exciton transport by modifying organic molecular crystal structures via light irradiation.
Using transient photoluminescence-microscopy, the team characterized exciton transport in organic molecular crystal of 2,2'-(2,5-bis(2,2-diphenylvinyl)-1,4-phenylene) dinaphthalene (BDVPN). Through iterative irradiation and characterization, they observed a progressive enhancement of exciton transport: the diffusion coefficient increased by three orders of magnitude (from ~10-3 cm2 s-1 to over 1 cm2 s-1), and the diffusion length extended from below 50 nm to nearly 1 µm. Environmental factors during irradiation were systematically ruled out, confirming that the improvement stems from structural changes in the BDVPN molecular crystal under light exposure. Key structural features of BDVPN—such as its twisted, rotationally flexible molecular framework and intermolecular C–H···π and H···H interactions—facilitate light-induced structural reorganization while maintaining stacking order.
Notably, both laser and cost-effective incoherent LED irradiation induced this enhancement, with the improved transport properties persisting for months. This demonstrates the feasibility and practicality of post-synthesis irradiation treatments for optimizing exciton transport. Future work will focus on resolving the structural modifications induced by irradiation and elucidating their mechanistic link to transport enhancement—a critical step for extending this approach to broader material systems.
Keywords: organic semiconductor / enhanced transport properties / exciton diffusion / time-resolved photoluminescence microscopy
