Scientists Develop Method to Detect Charge carrier traps in organic semiconductors
Charge carrier trapping processes are unavoidable in organic semiconductors, with the exception of extremely pure and defect-free single crystals. Scientists have developed a very sensitive method to detect the tiny signatures of so-called ‘charge traps’ in organic semiconductors. Trapping sites in organic semiconductors may have various origins; they can be formed by structural defects, dipoles, excimers, or guest molecules. Scientists at Swansea University have developed a very sensitive method to detect the tiny signatures of so-called ‘charge traps’ in organic semiconductors.
The research, published in Nature Communications and supported by the Welsh Government through the European Regional Development Fund, may change views about what limits the performance of organic solar cells, photodetectors, and OLEDs.
The weak intermolecular interactions inherent in organic semiconductors make them susceptible to defect formation, resulting in localized states in the band-gap that can trap charge carriers at different timescales. Charge carrier trapping is thus ubiquitous in organic semiconductors and can have a profound impact on their performance when incorporated into optoelectronic devices.
Charge carrier trapping usually limits the charge carrier transport properties of materials, however trapping may also affect, directly or indirectly, the processes of photogeneration and recombination of charge carriers. Organic semiconductors are materials mainly made of carbon and hydrogen which can be flexible, low weight, and colorful. They are the key components in OLED displays, solar cells, and photodetectors that can distinguish different colors and even mimic the rods and cones of the human eye. For these reasons, charge carrier trapping effects should always be taken into account in the analysis of the performance of electronic devices, such as organic light-emitting diodes (OLEDs), organic photovoltaic devices (OPVs), or organic field-effect transistors (OFETs).
Lead author Nasim Zarrabi, a Ph.D. student at Swansea University said: “For a long time, we guessed that some charges that are generated by the sunlight can be trapped in the semiconductor layer of the solar cell, but we’ve never really been able to prove it.
A high concentration of trapped charge carriers would lead to the modification of the internal electric field in the active layers of devices. “These traps make solar cells less efficient, photodetectors less sensitive and an OLED TV less bright, so we really need a way to study them and then understand how to avoid them – this is what motivates our work and why these recent findings are so important.”
Research lead, Dr. Ardalan Armin, a Sêr Cymru II Rising Start Fellow commented: “Ordinarily, traps are ‘dead ends’ so to speak; in our study, we see them also generating new charges rather than annihilating them completely.
“We’d predicted this could maybe happen, but until now did not have the experimental accuracy to detect these charges generated via traps.” Dr. Oskar Sandberg, the theorist behind the work said that he has been waiting for such experimental accuracy for several years.
“What we observed experimentally has been known in silicon and gallium arsenide as intermediate band solar cells, in organic solar cells it has never been shown that traps can generate charges,” he said. “The additional charges generated by the traps are not beneficial for generating lots of electricity because it is very tiny. “But it is sufficient to be able to study these effects and maybe find ways to control them in order to make genuine improvements in device performance.”
