Discussion on hole traps of amorphous films of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine ( α- NPD) deposited at different substrate temperatures

Abstract

The hole current in amorphous films of N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (α-NPD) strongly depends on substrate temperature during vacuum deposition (Tsub) and is the highest at a Tsub value of around 275 K. However, the reason for this enhancement of hole current at this Tsub is not clearly understood. In this study, we performed thermally stimulated current (TSC) measurements, which is a versatile method used to obtain information about carrier traps, on α-NPD films. The TSC results revealed that hole traps were uniformly distributed throughout the films and that hole traps were the shallowest for films fabricated at a Tsub value of around 275 K. Thus, the shallowest hole traps at this Tsub are believed to be one reason for the highest hole current for α-NPD films. This is the demonstration of how Tsub affects carrier traps, contributing to a better understanding of the underlying physics in organic amorphous films.

Ultrastable glasses: new perspectives for an old problem

“…Interestingly, the transfer integral between neighbor molecules was enhanced by 10% and the energetic disorder of the anisotropic and more stable vapordeposited glass was 10% lower relative to the conventional glass. While more work [31,32] needs to be done on larger molecules used in practical applications, this tendency supports the still scarce experimental evidence discussed above.…”

“…Most of the work on ultrastable glasses has involved molecular glasses. Since the first measurements in 2007 [1] with 1,3-bis-(1-naphthyl)-5-(2-naphthyl)benzene (TNB) (T g = 347 K) and indomethacin (IMC) (T g = 315 K), more than 45 different organic molecules, ranging from small molecules such as toluene [26][27][28] and ethylbenzene [28] to pharmaceuticals such as IMC and TNB and more recently to organic semiconductors like TPD [29,30], NPD [31,32] and TPBi [2], have shown their ability to form highly stable glasses upon growth by physical vapor deposition at the right processing conditions. Table 1 shows a comprehensive list of molecules and some of the outstanding properties of the vapor-deposited thin film ultrastable glasses obtained from them.…”

“…A narrow DOS will result in improved hopping due to the lower energy barrier for states at the center of the DOS distribution. In fact, in a recent work, Adachi and coworkers [32], carried out thermally stimulated current (TSC) measurements to unveil the reason for the change in the hole current density in amorphous (bulk-like, thickness 300 nm) films of alpha-NPD, a hole-transporting material, deposited at various substrate temperatures below the glass transition temperature (T g = 362 K). The highest value of the hole current was found at T sub = 0.75 T g .…”

“…8, 23, 5891-5897 (2017). Copyright {2017} by American Chemical Society and Esaki Y et al, Appl Phys Lett 114, 173301 (2019), AIP Publishing[31,32]…”

Concepts of the HOMO and LUMO Traps from the Carrier Dynamics of Organic Semiconductor Isomers α-NPB and β-NPB

“…In our previous report, the defect state in α-NPB has been determined to be a defect state of the electron kind, which indeed exists in the site of 0.515 eV above the LUMO and whose function, in fact, can be seen as a LUMO trap according to research in this report. Sometimes there are also the concepts of electron trap and hole trap in the literature. − Electron trap means the trap is close to the conduction band, and hole trap means the trap is close to the valence band. Because they both exist in the energy space between the HOMO and LUMO, they are also included in the concept of the geometric trap.…”