Electron and hole transport in the organic small molecule α-NPD

Abstract

Electron and hole transport properties of the organic small molecule N,N 0-Di(1-naphthyl)-N,N 0diphenyl-(1,1 0-biphenyl)-4,4 0-diamine are investigated by space-charge-limited current measurements. The hole transport shows trap-free behavior with a mobility of 2.3 Â 10 À8 m 2 /Vs at vanishing carrier density and electric field. The electron transport, on the other hand, shows heavily trap-limited behavior, which leads to highly unbalanced transport. A trap concentration of 1.3 Â 10 24 m À3 was found by modeling the electron currents, similar to the universal trap concentration found in conjugated polymers. This indicates that electron trapping is a generic property of organic semiconductors, ranging from vacuum-deposited small-molecules to solution-processed conjugated polymers.

The Path to 20% Power Conversion Efficiencies in Nonfullerene Acceptor Organic Solar Cells

“…[191][192][193][194] The electron mobility measurements are further prone to traps and it is important to take the trap parameter into account if there are indications of traps in the currentvoltage behavior. [195][196][197][198] Additionally, charge carrier densities and electric-fields can be larger in the SCLC regime than the normal operating conditions of an OSC. Furthermore, the motion of injected carriers is measured in the dark, in contrast to the photogenerated carriers in operational OSCs.…”

Molecular Origin of the Charge Carrier Mobility in Small Molecule Organic Semiconductors

“…While water molecules and other molecules with intrinsic dipole moments can cause a broadening of the density of states and even result in exponential tail states (shallow traps), they cannot explain the experimental observation of deep traps in the band gap and the universal trap level found in many organic semiconducting materials. 5,9,38 For this reason, we performed a systematic search for potential impurities that can explain the experimental findings. A universal trap level as found in Nicolai et al 5 can hardly be explained by impurities such as side products, non-reacted educts or (parts) of the catalysts.…”

“…In thermally-evaporated films of Bis(8-hydroxy-2-methylquinoline)-(4phenylphenoxy)aluminum (BAlq) 14 and N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) the electron transport could be described by incorporating electron-trapping sites with a concentration of 10 24 m -3 , which is substantially higher than the typical charge concentrations under operating conditions in diodes, indicating that electron transport is heavily trap limited in these systems. [citation needed] 9 Here, we investigate two potential trapping effects, broadening of the density of states (Figure 1a) and active trapping of charges (Figure 1b) and their influence on electron transport in three different small molecule materials. Using a multiscale simulation approach, we demonstrate that water molecules can broaden the density of states and introduce an exponential tail in the density of states, which significantly reduces the charge mobility in amorphous organic semiconductors.…”

Large bipolaron density at organic semiconductor/electrode interfaces

“…1 a 12 . We take σ b = 0.1 eV based on bulk transport measurements of the closely related molecule NPD 24 , 25 and σ i = 0.35 eV in accord with estimates of the interfacial distribution obtained from internal photoemission and impedance spectroscopy 26 , 27 .…”