Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts

A general strategy for producing solution-processed doped polymers with the extreme work functions that are required to make good ohmic contacts to semiconductors is demonstrated in high-performance light-emitting diodes, transistors and solar cells. Device-ready doped polymer semiconductors Electronic and optoelectronic devices made from organic polymers can be produced via solution processing, which is inexpensive, offers a high degree of design control and is especially suitable for producing flexible materials. An outstanding challenge is to make good 'ohmic' electrical contact with metal connections. This requires polymers with high charge doping content, but it is difficult to obtain highly doped polymer films from solution that retain their doping charges and are sufficiently stable. Peter Ho and colleagues present a general approach to produce polymers with stable, high doping content, by including a self-compensation mechanism that involves covalently bonded counter ions to block the migration of dopants. They apply this to a range of polymers and demonstrate devices including high-performance light-emitting diodes and ambipolar field-effect transistors. 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 1 , 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 migration 3 , 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 polyelectrolytes 5 , 6 , 7 and then internal ion-exchange to give self-compensated heavily doped polymers. Mobile carriers on the polymer b