Researchers and scientists from the National University of Singapore (NUS) has successfully developed conducting polymer films that can provide "unprecedented" ohmic contacts to give superior performance in organic electronics, including organic light-emitting diodes, solar cells and transistors. The research findings have been recently published in the journal Nature (see abstract below).
According to the announcement the key finding these researchers discovered is to be able to design polymer films with the desired extreme work functions needed to generally make ohmic contacts. Work function is the minimum amount of energy needed to liberate an electron from the film surface into vacuum.
The researchers showed that work functions as high as 5.8 electron-volts and as low as 3.0 electron-volts can now be attained for films that can be processed from solutions at low cost.
The project outcome is a result of a collaboration with the materials chemistry team led by Associate Professor Chua Lay-Lay from the Department of Chemistry at the NUS Faculty of Science, the physics team led by Associate Professor Peter Ho from the Department of Physics from the same faculty, and scientists from Cambridge Display Technology - a subsidiary of Sumitomo Chemical.
Figure: NUS - Dr. Png Rui-Qi (left), Mervin Ang (middle) and Cindy Tang (right) working on conducting polymers
Dr Png Rui-Qi, a senior research fellow from the Department of Physics at the NUS Faculty of Science, who led the device research team, said, "To design such materials, we developed the concept of doped conducting polymers with bonded ionic groups, in which the doped mobile charges - electrons and holes - cannot dissipate away because their counter-balancing ions are chemically bonded." Dr Png Rui-Qi added, "As a result, these conducting polymers can remain stable despite their extreme work functions and provide the desired ohmic contacts."
Dr Png Rui-Qi continued, "The lack of a general approach to make ohmic contacts has been a key bottleneck in flexible electronics. Our work overcomes this challenge to open a path to better performance in a wide range of organic semiconductor devices." Dr Png Rui-Qi concluded, "We are particularly thrilled about this Singapore-led innovation."
Associate Professor Chua Lay-Lay, Department of Chemistry at the NUS Faculty of Science, said, "The close partnership of the chemists and physicists has made this innovation possible. We are now working with our industrial partner to further develop this technology."
Cindy G. Tang | Mervin C. Y. Ang | Kim-Kian Choo | Venu Keerthi | Jun-Kai Tan | Mazlan Nur Syafiqah | Thomas Kugler | Jeremy H. Burroughes | Rui-Qi Png | Lay-Lay Chua | Peter K. H. Ho
Nature 539, 536–540 (24 November 2016) | doi:10.1038/nature20133
Received 13 January 2016 | Accepted 03 October 2016 | Published online 23 November 2016
To make high-performance semiconductor devices, a good ohmic contact between the electrode and the semiconductor layer is required to inject the maximum current density across the contact. Achieving ohmic contacts requires electrodes with high and low work functions to inject holes and electrons respectively, where the work function is the minimum energy required to remove an electron from the Fermi level of the electrode to the vacuum level. However, it is challenging to produce electrically conducting films with sufficiently high or low work functions, especially for solution-processed semiconductor devices. Hole-doped polymer organic semiconductors are available in a limited work-function range1, 2, but hole-doped materials with ultrahigh work functions and, especially, electron-doped materials with low to ultralow work functions are not yet available. The key challenges are stabilizing the thin films against de-doping and suppressing dopant migration3, 4. Here we report a general strategy to overcome these limitations and achieve solution-processed doped films over a wide range of work functions (3.0–5.8 electronvolts), by charge-doping of conjugated polyelectrolytes5, 6, 7 and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer backbone in these materials are compensated by covalently bonded counter-ions. Although our self-compensated doped polymers superficially resemble self-doped polymers8, 9, they are generated by separate charge-carrier doping and compensation steps, which enables the use of strong dopants to access extreme work functions. We demonstrate solution-processed ohmic contacts for high-performance organic light-emitting diodes, solar cells, photodiodes and transistors, including ohmic injection of both carrier types into polyfluorene—the benchmark wide-bandgap blue-light-emitting polymer organic semiconductor. We also show that metal electrodes can be transformed into highly efficient hole- and electron-injection contacts via the self-assembly of these doped polyelectrolytes. This consequently allows ambipolar field-effect transistors to be transformed into high-performance p- and n-channel transistors. Our strategy provides a method for producing ohmic contacts not only for organic semiconductors, but potentially for other advanced semiconductors as well, including perovskites, quantum dots, nanotubes and two-dimensional materials.