Void-Assisted Extrinsic Charge Trapping Governs Hole Transport in Organic Semiconductors.

Abstract

To realize electrically pumped organic lasing, single-layer devices without organic heterointerfaces have been employed to suppress efficiency rolloff. In such architectures, on the other hand, extrinsic trap states result in trap-assisted nonradiative recombination, thereby decreasing electroluminescence efficiency. Here, we elucidate the dominant factors governing extrinsic trap formation by combining current density (J)-voltage (V) characteristics of single-layer hole-only-devices (HODs) with molecular dynamics (MD) simulations of 6,6'-bibenzo[d]oxazole (BZ) derivatives exhibiting high photostability and low amplified spontaneous emission (ASE) thresholds. Drift-diffusion analysis of the J-V characteristics reveals that 2,2'-bis(4-(9-phenyl-9H-carbazol-2-yl)phenyl)-6,6'-bibenzo[d]oxazole (BZCz2) exhibits a higher density of extrinsic trap states than the other BZ derivatives studied. MD simulations further show that amorphous BZCz2 layers contain a high population of large voids arising from bulky substituents, potentially facilitating the incorporation of water clusters during vacuum deposition, thereby contributing to the formation of extrinsic hole traps. Furthermore, dispersing BZCz2 at low concentration in the host material of 4,4'-bis(carbazol-9-yl)biphenyl (CBP), which exhibits dense molecular packing and minimal void formation, effectively suppresses extrinsic hole trapping in host-guest systems. These findings provide a strategy for suppressing extrinsic trap states and pave the way for highly efficient single-layer EL devices.

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