RESEARCH

Overview

“Establishing the function of organic semiconductors as promising materials for solar-to-fuel energy conversion.”

Direct conversion of solar energy into solar fuels as a form of chemical energy at molecular level (e.g., solar H­2 production) by photoelectrodes is a promising approach to reduce our dependence on fossil fuels and enable long-term energy storage, given the stability and transportability of chemical fuels. Optoelectronic Nanomaterials Engineering Laboratory (ONEL) is directed to designing novel organic semiconductors (OSs) and establishing their roles as promising materials for various solar fuel productions. More importantly, ONEL employs an all-in-one concept incorporating material design, preparation, characterization, and demonstration of diverse platforms for solar-driven energy conversion in a single lab. By leveraging this, ONEL carries out autonomous and systematic research with the aim of demonstrating next-generation OS-photoelectrodes and OS-photocatalysts for efficient and long-lasting production of solar fuels. 

Motivation

To mitigate the climate change effect, achieving a sustainable energy economy cannot be overstated. In this context, attaining H2 economy is one of the promising approaches as H2 can be converted into electrical energy using a fuel cell or transformed into chemical feedstocks on a global scale. However, when it comes to conventional hydrogen production, there is a controversy. The most common method for producing hydrogen is steam-methane reforming where 9.3 kg of carbon dioxide is generated along with 1 kg of H2 production. Clearly, it’s not a sustainable system. In this regard, for the last few decades, research efforts have been dedicated to the demonstration of H2 production by utilizing renewable energy. Among possible approaches, producing H2 by splitting water using sunlight is a sustainable way, considering abundant water and enormous solar energy. This type of H2 production is in line with the vision of UNIST as well.

Then, how do we produce H2 using sunlight? Of the possible approaches, photoelectrochemical (PEC) cells and photocatalysts (PCs) are promising ways over the currently feasible way where conventional solar cells are combined with standard electrolyzers (PV-EC approach). The PEC and PC approaches have intrinsically thermodynamic advantages as semiconductors are utilized to convert solar energy directly into electrochemical potential capable of driving water electrolysis. In these systems, H2 is instantly generated in the cathodic electrode (or cathodic particles) while O2 is produced in the anodic electrode (or anodic particles) as a counter reaction. However, ideal light harvesting semiconducting materials that combine economic viability, scalability, optoelectronic tunability, high efficiency, and stability for PEC cells and PCs have not been identified yet.

Outlook

Organic semiconductors are exceptional alternative semiconductors to tackle the given challenge. The promising aspect of organic semiconductors is that their chemical structures and energy levels affecting light harvesting ability can be easily controlled by molecular engineering. Moreover, they have a clear advantage when they are mixed into this unique structure referred as bulk-heterojunction (BHJ) where donor and acceptor materials are intermixed at a few tens of nanometers scale. Upon illumination, these semiconducting materials form a strongly bound hole-electron pair, which is called exciton. This photogenerated exciton can be dissociated into free charge carriers at the donor/acceptor interface and the resulting charges are transported through each domain, which ensures high photocurrent compared to a single component system. Furthermore, they are solution processable at low temperature, which enables the demonstration of various platforms. However, the use of organic semiconductors in these systems is still in its infancy.



The vision of ONEL is to open a new era of solar fuel production using organic semiconductors. ONEL incorporates the competences of material design, preparation, characterization, additional molecular engineering, and demonstration of diverse platforms for the solar-driven photo-electrocatalysis. One major research stream is the demonstration of artificial leaves and the other one is the demonstration of photocatalyst systems. Consequently, ONEL aims at achieving the ambitious goal of inexpensive and highly efficient artificial photosynthesis of hydrocarbon fuels and commodity feedstock chemicals such as ammonia (value-added product, VAP, in the figure above) using organic semiconductors.