Abstract
AbstractThe realization that polymers can be used as active material in opto‐electronic applications initiated substantial effort in the scientific community to explore new materials. Polymers can be made strong, flexible, lightweight, and can be mass produced. Furthermore, polymers can be processed at low temperatures, typically below 150 °C, creating the opportunity to use a range of plastic substrates instead of glass. Many polymers are soluble in organic solvents, making it possible to create electronically active “inks” that allow for solution‐processed electronic components as light‐emitting diodes and transistors. Examples of innovative new products based on semiconducting polymers are inkjet‐printed displays, non‐contact radio frequency identification tags, and sensors. However, the realization of polymeric displays is hindered by the relatively low efficiency of polymer‐based light‐emitting diodes. Major problems are the inability to realize multi‐layers from solution, insufficient harvesting of triplet excitons and the presence of defects. Printed circuits of organic transistors are still hampered by stability issues and relatively low charge carrier mobility of the organic semiconductors. More recently, organic electrochemical transistors have been employed as biosensors. Herein, the fundamentals and recent progress of polymer‐based light‐emitting devices and transistors are being discussed.
Highlights
The realization that polymers can be used as active material in opto-electronic circuits for application in radio-frequency applications initiated substantial effort in the scientific community to explore identification tags.[4]
Top-contact devices typically exhibit an improved injection compared to bottom ones and provide the opportunity to observe the interfacial microstructure of the organic semiconducting (OSC) film, which is important to understand the relation between molecular organization, film morphology and charge carrier transport.[49,50]
The electron trap depths appear to be directly related to the electron affinity of the polymer, revealing that the polymers exhibit a common trap distribution related to oxygen/ water complexes
Summary
A first requirement for an efficient PLED is the realization of efficient charge injection; charges injected by the electrodes must overcome or tunnel through a barrier between the HOMO/LUMO of the emissive polymer and the electrode work function. It has been shown that the trap-free hole transport at room temperature in most of the conjugated polymers can be described by the SCLC model but only at low voltages.[17] At higher voltage the current increases faster than the prediction of the SCLC model.[20] The occurrence of energetic disorder and corresponding distribution of localized states strongly affects the charge transport properties of conjugated polymers. In contrast to the SCL hole current observed in PPV derivatives the electron current is orders of magnitude lower and exhibits a much steeper voltage dependence (see Figure 3b).[17] This observation is a fingerprint for trap-limited transport, indicating the presence of impurities with energetically distributed energy levels in the band gap of the organic semiconductor (Figure 3a). Using trap-dilution they were able to fabricate PLEDs with balanced electron and hole transport and reduced SRH recombination and cathode quenching, leading to the predicted doubling of the efficiency at nearly ten times lower material costs.[33]
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