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Title: Highly air-stable, n-doped conjugated polymers achieved by dimeric organometallic dopants
Chemical doping is a key process for controlling the electronic properties of molecular semiconductors, including their conductivity and work function. A common limitation of n-doped polymers is their instability under ambient conditions, which has imposed restrictions on the characterisation and device application of n-doped polymers. In this study, sequential n-doping with organometallic dopants was performed on thin films of polymeric semiconductors with naphthalene diimide and perylene diimide-based backbones. Moderate ambient stability was achieved with (RuCp*Mes) 2 , {Cp* = pentamethylcyclopentadienyl; Mes = 1,3,5-trimethylbenzene}, which is in striking contrast to the unstable, n-doped state obtained with cobaltocene, a simple one-electron reductant. The highly cathodic, effective redox potential of (RuCp*Mes) 2 , ca. −2.0 V vs. ferrocene, suppresses the back electron transfer reaction and the subsequent dopant loss in air, which gives rise to the observed air stability. It also allows a perylene diimide-based polymer to be reduced to a state in which the repeat units are largely dianionic. Photoelectron measurements show that the ionization potential of the heavily doped polymer is ca. 3.9 eV. Our findings show that chemical doping with (RuCp*Mes) 2 is an effective method to produce highly stable, n-doped conjugated polymers.  more » « less
Award ID(s):
1807797 1729737
NSF-PAR ID:
10227509
Author(s) / Creator(s):
; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Journal of Materials Chemistry C
Volume:
9
Issue:
12
ISSN:
2050-7526
Page Range / eLocation ID:
4105 to 4111
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Abstract

    n‐Doping electron‐transport layers (ETLs) increases their conductivity and improves electron injection into organic light‐emitting diodes (OLEDs). Because of the low electron affinity and large bandgaps of ETLs used in green and blue OLEDs, n‐doping has been notoriously more difficult for these materials. In this work, n‐doping of the polymer poly[(9,9‐dioctylfluorene‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,7‐diyl)] (F8BT) is demonstrated via solution processing, using the air‐stable n‐dopant (pentamethylcyclopentadienyl)(1,3,5‐trimethylbenzene)ruthenium dimer [RuCp*Mes]2. Undoped and doped F8BT films are characterized using ultraviolet and inverse photoelectron spectroscopy. The ionization energy and electron affinity of the undoped F8BT are found to be 5.8 and 2.8 eV, respectively. Upon doping F8BT with [RuCp*Mes]2, the Fermi level shifts to within 0.25 eV of the F8BT lowest unoccupied molecular orbital, which is indicative of n‐doping. Conductivity measurements reveal a four orders of magnitude increase in the conductivity upon doping and irradiation with ultraviolet light. The [RuCp*Mes]2‐doped F8BT films are incorporated as an ETL into phosphorescent green OLEDs, and the luminance is improved by three orders of magnitude when compared to identical devices with an undoped F8BT ETL.

     
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