OLED Pumping Enables Electrically Driven Semiconductor Lasing

Researchers at the University of St. Andrews demonstrated an electrically driven, organic semiconductor laser using a new approach that could improve the performance of this class of laser and further the development of ultrafast, organic optoelectronics applications.

The researchers used indirect OLED pumping to power the laser. They built an integrated device structure that efficiently coupled an OLED that had very high light output with a polymer-based, distributed feedback laser.

Most organic semiconductor lasers are optically pumped by another laser — a complicated, expensive process. Electrical pumping, which involves injecting a current to generate a population inversion, is challenging to achieve in organic semiconductors because the low charge-carrier mobility of organic semiconductors makes it difficult to inject high current densities. Doing so causes injected charges that have some absorption at the laser wavelength to accumulate.

The low mobility of organic semiconductors means the contacts must be very close to the gain medium, which can also lead to loss. Moreover, many injected charges form triplets that do not contribute to light emission in fluorescent materials, but that may absorb the emitted light and quench singlets.

To circumvent these challenges, the researchers devised a method for indirect electrical pumping by an OLED. First, the researchers separated the region where charges were injected from the region where the laser population inversion was formed. Then they excited the gain medium by electroluminescence from the charge-injection region.

This approach allowed the researchers to avoid the losses due to injected charges and reduce the losses due to triplets and contacts. The spatial separation between charge injection and lasing greatly reduced losses overall.

The researchers confirmed that their approach generated lasing when, under the electrical driving of the integrated structure, they observed a threshold in light output versus drive current, with a narrow emission spectrum and the formation of a beam above the threshold. The threshold behavior, spectral narrowing, and polarized beam emitted from the device provided evidence for lasing consistent with the properties of the gain medium and resonator used.

Among the benefits of organic semiconductors are their flexibility, visible light emission, and role in simple fabrication of electronic devices. They are used for solar cells, transistors, sensors, and OLED displays in mobile phones and TVs. The team showed that indirect electrical pumping by an OLED could be an effective way to realize an electrically driven, organic semiconductor laser for broad use.

Electrical pumping of organic semiconductor lasers, via indirect electrical pumping by an OLED, could also facilitate the development of visible lasers for spectroscopy, metrology, and sensing. “We expect this new laser to use less energy in its manufacture, and in the future it will generate laser light across the visible spectrum,” professor Graham Turnbull said.

The researchers’ approach to organic lasers requires the OLED to operate under exceptionally intense current injection in order to make a very fast, organic optoelectronic device. The team believes that the microscopic physics of OLEDs under such intense, short-pulse operation will need to be more fully explored to achieve practical implementation of their approach.

The researchers anticipate that their work will stimulate future studies to understand the dynamics of organic semiconductors in this regime.

With additional development, the process of integrated OLED pumping to electrically drive organic lasing could potentially be integrated with OLED displays, allowing communication between displays, or be used for spectroscopy applications for detecting diseases and environmental pollutants.

Researchers have been working on a solution to electrically powering organic semiconductor lasers for 30 years.

“Making an electrically driven laser from organic materials has been a huge challenge for researchers across the world,” said Ifor Samuel, a professor at the University of St. Andrews. “Now, after many years of hard work, we are delighted to have made this new type of laser.”

The research was published in Nature (www.doi.org/10.1038/s41586-023-06488-5).