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Posted: Sep 05, 2011

Researchers gain insight into the process of light emission in organic semiconductors

(Nanowerk News) Organic light-emitting diodes (OLEDs) are efficient, inexpensive and flexible lighting elements that are found in a range of personal electronics devices. However, the emission of light in an OLED occurs in the middle of a stack of organic semiconducting layers that are capped by metal electrodes, making it hard to observe the emission process directly. Satria Bisri and colleagues from Tohoku University and other institutions in Japan and have now gained insight into the process of light emission in organic semiconductors by constructing a device specifically for that purpose ("p-i-n Homojunction in Organic Light-Emitting Transistors").
Schematic of an ambipolar light-emitting transistor
Schematic of an ambipolar light-emitting transistor. (© 2011 Wiley-VCH)
The researchers studied an ambipolar light-emitting transistor (LET), which consists of a single thin-film organic semiconductor between two in-plane electrodes (see image). The semiconductor transports electrons from one electrode, and at the same time transports holes from the other. The process of light emission, which occurs when the opposite charges recombine, takes place in the exposed semiconductor, and can be observed by optical microscopy.
One of the key parameters of the recombination process is the spatial width over which electrons and holes recombine. Previous work had measured this width to be between 10 and 100 times larger than theoretical predictions, and this discrepancy was attributed in part to the complex material properties, such as grain boundaries, of the organic semiconductor itself. Bisri and his colleagues constructed their LET using single-crystal semiconductors with no grain boundaries, allowing them to avoid this issue.
They found that the recombination width was much larger than could be explained by the simplest relevant model, in which a positively charged region immediately abuts a negatively charged region. The also found that the width depends on the device configuration and operating conditions in a way that is inconsistent with this model.
This led the researchers to propose the existence of an undoped 'intrinsic' region between the positive and negative regions. Taking this intrinsic region into account, Bisri and his colleagues were able to successfully model the recombination width. This 'p-i-n' junction formed in the LET has potential advantages for optoelectronic devices, including efficient charge injection and transport. The research highlights the usefulness of LET devices for both the observation of fundamental science and the fabrication of practical devices.
Source: Tokyo Institute of Technology
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