Direct Investigation of the Reversible Charge Carrier Trapping Process
Organic light-emitting diodes (OLEDs) are established as a mainstream light source for display applications and can now be found in a plethora of consumer electronic devices used daily. OLEDs can be fabricated on flexible substrates, unlike their rigid inorganic counterparts. Despite their wide application, there has been ongoing research to innovate novel classes of functional materials, chromophores, and device architectures to enhance further the performance and application of OLEDs. Specifically, there has been growing research interest in stimuli-responsive organic devices that can be remotely controlled by external stimuli such as light.
The most popular means for introducing light responsivity in organic thin film devices is by embedding a photochromic compound in its functional layers. Consequently, irradiating these devices with photons with sufficient energy can cause reversible modulation of the properties. Diarylethenes (DAEs) and azobenzenes are the most common photochromic compounds that have been effectively utilized in photo-switchable technologies. In particular, DAEs exhibit several properties desirable for designing and fabricating different responsive systems.
Embedding DAEs in electronic/photoelectronic devices induces a light-responsive behavior that results in a dramatic energy shift in the molecular orbitals of the dopant and offers the possibility of reversible optical control. There have been numerous attempts to modulate the performance of OLEDs by incorporating DAEs. Despite the good work, previous studies faced several challenges that must be addressed to realize DAE-based optically switchable OLEDs (OSOLEDs).
On this account, University College London scientists Dr. Giovanni Cotella, Dr. Giuseppe Carnicella, Professor Alessandro Minotto, and Professor Franco Cacialli together with Professor Aurelio Bonasera and Professor Stefan Hecht from the Humboldt University of Berlin reported the design, fabrication, and characterization of OSOLED devices. These new devices were fabricated by incorporating an active layer comprised of the commercially available light-emitting polymer poly(9,9′-dioctylfluorene-alt-benzothiadiazole) (F8BT) blended with a DAE derivative. The photochromic activity of the dopant was investigated in the solid-state via photoluminescence spectroscopy and UV absorption, while the morphology of the different blends was characterized through atomic force microscopy. They also investigated the effects of DAE isomerization on the transport of charged carriers. Their work is currently published in the journal Advanced Optical Materials.
The research team demonstrated the fabricated OSOLEDs’ ability to be controlled remotely via optical stimuli, reporting a maximum reversible threshold voltage shift of 4 V at doping loadings of 1, 5, and 10 wt%. The best device performance exhibited a maximum ON/OFF ratio of ≈ 20 for current density ≈ 90 and for luminance, and it was achieved at dopant loading of 5 wt%. Furthermore, the design of multicomponent emissive layer (EML) for generating optically switchable trap sites for electron and hole carriers enabled direct characterization of the trapping process of charged carriers. The results confirmed and demonstrated the asymmetry of the photoresponsive trapping of electrons and holes on DAE. Under the same conditions, the reversible modulation of the electron transport was over 3.4 times larger than that of hole transport.
In summary, the study successfully reported the design and fabrication of the impact of DAE isomerization OSOLEDs with a simple device architecture based on DAE and F8BT blends. Most importantly, it allowed direct investigation of the reversible charger carrier trapping process by characterizing the single-carrier switchable devices, which provided a better understanding of the impact of DAE isomerization on carrier transport. The findings also provided more insights into the design path and underlying mechanism of light-responsive devices. In a statement to Advances in Engineering, Professor Franco Cacialli said that their findings will improve the design of multi-functional OSOLEDs for smart display applications.
Cotella, G., Bonasera, A., Carnicella, G., Minotto, A., Hecht, S., & Cacialli, F. (2022). Diarylethenes in Optically Switchable Organic Light‐Emitting Diodes: Direct Investigation of the Reversible Charge Carrier Trapping Process. Advanced Optical Materials, 10(2), 2101116.