Doping at the Limit: Improving Charge Injection in Organic Light-Emitting Diodes Using Molecular Dopants

Jun 24, 2022, 10:00 am11:30 am



Event Description

Incorporating molecular dopants in organic hole and electron transport layers (HTLs and ETLs) has been crucial for the development of high efficiency organic light-emitting diodes (OLEDs). These layers facilitate the transport of either an electron or a hole as it is injected into the active emissive layer of a device. Organic semiconductors often have low conductivities and poor charge transport, particularly the large bandgap molecules utilized for green and blue LEDs. The charge transport layers which are used with blue and green OLEDs have two correlated challenges: their large transport gaps make them both insulating and difficult to p- or n-dope.

These issues were addressed by incorporating highly reducing n-dopants and oxidizing p- dopants into materials with a challenging ionization energy (IE) and electron affinity (EA). The organometallic dimeric n-dopant (RuCp*Mes)2 addresses the pervasive challenge of oxygen sensitivity in n-dopants. Using (RuCp*Mes)2, the polymer F8BT was n-doped via solution processing; the doping of the film was confirmed using ultraviolet and inverse photoemission spectroscopy (UPS and IPES) to determine the shift in Fermi level closer to the lowest unoccupied molecular orbital (LUMO) of F8BT. The film’s conductivity also increased by four orders of magnitude after doping and irradiation to cleave the n-dopant dimer. The n-doped F8BT film was successfully employed as an ETL in green OLEDs, improving their efficiency by reducing the electron injection barrier between the electrode and the ETL.

Stable p-dopants with a sufficiently high EA are challenging to synthesize and use to p- dope organic semiconductors. I demonstrated the use of the strongest known molecular dopant, CN6-CP, to p-dope the host material POPy2, shifting the Fermi level close to the highest occupied molecular orbital (HOMO) of POPy2. Conductivity and contact potential difference measurements confirmed the doping of the film. Using both the CN6-CP and (RuCp*Mes)2, the host was doped to form what is currently known as the largest bandgap p-i-n organic homojunction diode, resulting in a large built-in potential. These diodes emitted blue light, indicating hole and electron injection into the undoped host, facilitated by the doped HTL and ETL.