Organic optoelectronic devices, reminiscent of organic light-emitting diodes (OLEDs), use molecules with specific structures arranged on thin movies. Moreover, the arrangement of those molecules on any surface is crucial for various processes that occur inside these devices. This arrangement is guided by two primary aspects: the deposition rate (how briskly the molecules are placed) and the surface temperature. Slower deposition rates and better temperatures facilitate the correct arrangement, leading to more stable structures. Finding the proper time scale for this process can be critical, and researchers at the moment are in search of ways to manage these aspects for optimal molecular arrangement on surfaces.
In a brand new study, a team from Japan led by Prof. Hisao Ishii from the Graduate School of Science and Engineering and the Center for Frontier Science at Chiba University, together with Masahiro Ohara from Chiba University and Dr. Yuya Tanaka from the Graduate School of Science and Technology at Gunma University, has introduced a brand new approach to deposition that achieves suitable molecular arrangement. The article was made available online on December 4, 2023, and published in Volume 15, Issue 49 of the journal ACS Applied Materials and Interfaces on December 13, 2023. “When depositing organic molecules by vacuum deposition, the orientation of the molecules is modified over time by pausing the deposition. Furthermore, by changing the deposition conditions, it is feasible to invert the orientation of each the head- and tail-end of the molecules,” explains Prof. Ishii.
Of their study, the team found a straightforward yet ingenious option to control the orientation of molecules deposited on aluminum and benzene-containing thin movies, denoted as Alq3 and TPBi, respectively. They used a way called “intermittent deposition,” which introduces breaks in the course of the deposition process, and developed an updated version of a tool called “rotary Kelvin probe” (RKP). This was used to measure the surface potential (voltage on the fabric’s surface) during and after the deposition in real-time. By repeatedly opening and shutting the deposition shutter at specific intervals, the researchers could change the polarization (the distribution of charges), influencing how the molecules were oriented on the movies.
The brand new approach of intermittent deposition created a relaxed and stable surface layer with controllable polarization. The study also revealed how surface rest affected molecular orientation and the formation of a possible valley (shaped like a “V”). The truth is, this deposition method enables the creation of an arbitrary potential profile for desired molecular orientations on the skinny film of interest.
When it comes to applications, this intermittent deposition technique can enhance the effectivity and lifelong of OLED materials. As well as, it will possibly even be used for non-polar organic molecules, making it useful for devices like organic photovoltaic cells and transistors. Prof. Ishii emphasizes, “This method is anticipated to further improve the efficiency and lifelong of OLEDs. Beyond OLEDs, it also promotes the event of other organic devices, reminiscent of organic memory devices. Due to this fact, replacing conventional inorganic devices with organic devices will make lightweight and versatile devices available.”
In summary, this study explores the relief processes that impact the orientation of molecules on the surface of organic thin movies and utilizes intermittent deposition to create a stable surface layer effectively. Moreover, an RKP tool is developed to research changes in surface potential over time. The proposed deposition method is anticipated to work with various organic molecules (not only polar ones) and will pave the best way for the development of existing organic devices and the event of recent ones.
