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



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.