Modification of metal–organic interface using F₄‐TCNQ for enhanced hole injection properties in optoelectronic devices

Abstract

Incorporation of thin layers of tetrafluro‐tetracyanoquinodimethane (F4‐TCNQ) has been found to modify the work functions of a number of substrates. Surface potential measurement using Kelvin probe method (KPM) has been used to monitor the change in work function of the modified substrates. The results support the integer charge transfer model by which the Fermi levels of the substrates are aligned with the negative polaron states of F4‐TCNQ. Further, we found that the work function of the substrates increases with increase in F4‐TCNQ thickness from 0 to 7 nm and then saturates for further increase in thickness. The variation in work function has been attributed to the low surface coverage of F4‐TCNQ islands on the substrates. The ITO and Au substrates with increased work functions were used as the electrode for hole only devices of common hole transport materials. The hole injection property has been found to increase with increase in F4‐TCNQ thickness and for ITO surface modified with 7 nm F4‐TCNQ layer, ohmic conduction has been achieved for HTLs with HOMO level up to 5.4 eV. When these modified substrates were used as hole injecting contacts in organic light emitting diodes (OLEDs), they gave substantially higher electroluminance, power efficiency and lower operating voltages.

Molecular Structure‐Dependent Charge Injection and Doping Efficiencies of Organic Semiconductors: Impact of Side Chain Substitution

“…The almost rigid shift induced by moderate F 4 -TCNQ doping could be interpreted by several reported mechanisms, including the integer charge transfer model [ 16 , 18,19 ] the energy level hybridization model, [ 20,21 ] and the effective electrode work function modifi cation due to the diffusion of F 4 -TCNQ molecules through the NPB host layers to the electrode/organic interface. [ 26,27 ] In all these three cases, the reduced HIBs (by ≈0.6 eV) will be capable of dramatically decreasing the drive voltage for devices with F 4 -TCNQ doping. In addition, the HIBs of doped DMFL-NPB and DPFL-NPB fi lms are still ≈0.1 eV lower than that of doped NPB fi lm.…”

Fine-Tuning the Electrostatic Properties of an Alkali-Linked Organic Adlayer on a Metal Substrate

“…ARTICLE already reported. 28,29 When coadsorbed with molecular donors, it reduces the hole-injection barrier between Au electrodes and several organic semiconductors. 30,31 We then consider the doubly substituted 2,5-difluoro-7,7,8,8-tetracyanoquinodimethane (F2-TCNQ, A = 4.59 eV) and fluoro-7,7,8,8-tetracyanoquinodimethane (F-TCNQ, A = 4.55 eV), where only one H is replaced (see the SI for the molecular structures on the substrate).…”

A Comparative Computational Study of the Adsorption of TCNQ and F4-TCNQ on the Coinage Metal Surfaces

“…28 Both molecules have been routinely used to decrease the hole injection barrier at the metal–organic interface either by doping the hole transport layer or as an intermediate buffer layer between the hole transport layer and the metal anode. 29−32 Recently, it has also been shown that doping the electron injection layer with TCNQ 33 or inserting a TCNQ buffer layer between the electron transport layer and the metal cathode 34 can also improve the performance of organic light-emmiting devices.…”