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John O | January 2017

NUS scientists develop electrically conductive polymers for semiconductors


researchers at the national university of singapore (nus) have developed a new concept of doped conducting polymers with bonded ionic groups that provide unprecedented ohmic contacts and extreme work function, the minimum amount of energy needed to liberate an electron from the polymer surface into a vacuum.

 

nus_600

researchers work on conducting polymers that can provide unprecedented ohmic contacts for better performance
in a wide range of organic semiconductor devices. (seah zong long)

 

this new technology could lead to improved performance in plastic electronics, such as led, solar cells, and transistors, according to a report from the nus website.

 

the researchers were able to demonstrate work functions as high as 5.8 electron-volts and as low as 3.0 electron-volts because this new method chemically bonds counter-balancing ions and prevents the dissipation of doped mobile charges. this allows the polymers to maintain stability even with extreme work functions.

 

“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,” explained dr png rui-qi on the university website.

 

the work was recently published in nature. the abstract explained:

 

“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 range, 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 migration.

 

“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 polyelectrolytes 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 polymers, 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.”

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