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Vibronic control of organic electronic devices

Periodic Reporting for period 5 - VIBCONTROL (Vibronic control of organic electronic devices)

Reporting period: 2020-10-01 to 2021-05-31

The general goal of the project is to observe and harness so called vibronic-coupling phenomena in organic electronic materials and devices.
Organic- and bio-electronics is currently a lively, rapidly evolving field aiming at molecular systems soon to become the key building blocks for optoelectronic, computing, and sensing devices. In contrast to the conventional electronics based on inorganic semiconductors, organic electronics exploits the electronic functionality of carbon-based macro molecules making the devices solution-processable, flexible, adjustable, and cost efficient. Elegant examples of organic electronic applications so far include OLED smartphone displays, as now provided by Samsung, and new applications such as flexible light panels, rollable solar cells, or the developments towards battery-free artificial retina. Due to the ‘soft’ character of organic materials, molecular conductivity fundamentally depends on the vibronic coupling phenomena which arise from the interaction between electronic and nuclei dynamics in molecular systems. It was recently shown that such phenomena play critical role in many fundamental processes in electronic and biological systems including photosynthesis, protein motions, and charge transfer in organic semiconductors. It becomes clear that the next generation of molecular materials for electronic and optoelectronic applications (like flexible LED and thin-film organic transistors) may reach new functionalities based on the vibronic phenomena. The prospective benefits include faster performance, lower weight or reduced energy consumption, and therefore are of general importance for industry and society. The overall objectives of the project focus around the development of new experimental approach which can directly address the previously unexplored vibronic effects and apply those effects to actual organic electronic devices. The obtained expertise will be used for improving functionality of organic electronic systems.
In the first stage of the project we have been focusing on the development of a new experimental approach to selectively address the vibronic effects in the organic electronic devices. Specifically, we have proposed and tested new measurement technique called 'vibronic control' which combines an optical excitation of nuclear motion with the device-based detection of electron dynamics. This method also utilizes recent developments in coherent optical spectroscopy to generate and apply ultrafast IR radiation that is used to drive the structural dynamics of molecular systems.
The prototype experimental setup for vibronic control technique has been built and tested. We later applied this method in collaboration with AMOLF institute in Amsterdam to make the first proof-of-principle experiments. In the photoresistor devices based on the pentacene polycrystalline films we observed that the optical excitation of certain vibrational modes leads to the increase in photoconductivity. The results were explained in the frame of Miller-Abraham model and represent the first demonstration of vibronic control in solid state systems.
In the second stage of the project, we have focused on the development of new experimental setup specifically dedicated to observation of vibrational control effects in organic electronic devices.
As a result a fully functional ultrafast spectroscopy system have been built and the first text experiments in near-IR spectral range have been performed in August 2017. The new setup is using more powerful and higher-repetition rate utrafast optical pusles that will allow much faster and more reliable measurements of the system response. Upon successful testing, we applied the developed optical-control system to address electronic dynamics in organic-semiconductor based diodes and observed a number of new phenomena including barrier mediated charge separation.
In 2018 we focused on identifying the role of vibrational motions in electronic performance of hybrid organic-inorganic materials. We observed that the energy and availabity of vibrations influences the energetic losses in the motion of the electrons - ,for example, how fast hot charge carriers loose there energy. By increasing the material symmetry and reducing the number of vibrational modes we managed to slow down carrier cooling, which is important for the development of next generation solar cells. We are now extending this study towards hybrid nanomaterials important for light emitting applications.
In 2019 the work largely focused around on developing optical (THz radiation) probe for charge motion in the materials and on combining it with optical control approach. We managed the extend the probe range of THz setup into near IR region allowing the setup simultaneously detecting phonons and well as mobile electronic states. The technique was applied to hybrid perovskite materials revealing correlation between the electronic temperature and carrier mobility. We are now focusing on observing interplay between carrier mobility and vibrational population.
In 2020, despite the multiple interruption due to the COVID pandemic, we have completed the development of new spectroscopic approach named photocurrent-probe vibrationally promoted electronic resonance (PC-VIPER). This techniques allows for direct evaluation of coupling between electronic and vibrational modes and determination of characteristic timescales vibrational-electronic interactions. The technique has been successfully applied to reveal coupling between structural and electronic dynamics in perovskite materials and will be applied for organic electronic systems.
On the final stage of the project we introduce novel ultrafast visible pump – infrared push – terahertz probe spectroscopy to monitor the real-time conductivity dynamics of cooling carriers in methylammonium lead iodide. We find a mobility decrease upon optically re-exciting the carriers, but the conductivity recovery is incommensurate with the hot carrier population dynamics. These results reveal the importance of highly-localized lattice heating on the hot carrier mobility. This collective polaron-lattice phenomenon may contribute to the unusual photophysics of MHPs and should be accounted for in hot carrier devices.
We have performed the first demonstration of controlling molecular electronic device using vibrational excitation. Such demonstration was a long-standing problem in the field with first attempts to realize such control dating to 1980s. Though the observed effect was relatively modest it serves as an encouraging proof-of-principle example and may potentially initiate the development of new type of vibronic-based optoelectronic materials and devices.
ultrafast optical control setup