Hole Transport in Triphenylamine Based OLED Devices: From Theoretical Modeling to Properties Prediction

Abstract

For the series of para-substituted triphenylamines, optimized geometries, HOMO and LUMO energy levels, ionization potentials Ip, reorganization energies for hole transport λ(+), and frontier orbital contours have been calculated by means of ab initio computations. Relationships between them and the Hammett parameter are presented. According to calculations, electron releasing substituents increase the HOMO and LUMO energies of TPA, while electron withdrawing ones decrease it. This behavior is reflected in subsequent decreasing and increasing of ionization potentials of substituted TPAs. Calculations show that there exists also a strong substituent effect on the reorganization energy λ(+), which is a dominating factor of hole mobility. It is concluded that proper tuning of the HOMO and LUMO levels (and, as a result, ionization potential, Ip) and reorganization energy λ(+) (consequently, hole mobility) of the triphenylamine can be done by alteration of the TPA electronic structure by an appropriate substitution. It is demonstrated that the proper adjustment of the HOMO levels of HTM facilitates the reduction of an energy barrier at the interface of ITO/HTL and HTL/EL and ensure the high hole injection and hole transport rate. On the other hand, appropriate adjustment of the LUMO level prevents an electron leak from the EL into the HTM layer. Results of these calculations can be useful in the process of designing new HTM materials of desired properties (high efficiency, stability, and durability).

Optoelectronic and nonlinear optical properties of triarylamine helicenes: a DFT study

“…The bond length of the C 2 -N bond increased slightly from 1.400 to 1.429Å. The central moiety retains planarity, indicating that there is little affect on structural stability as the Scheme 2 Sketch of triarylaminehelicene (TAH) derivatives in this study using DFT at B3PW91/6-311G (2d, 2p) ++ level of theory optimized cationic geometries show only small structural changes compared to the natural molecules, which are close to triarylamine [75]. From plots of change in interplanar angle versus the Hammett parameter, we observed that, upon extraction of one negative charge from the system, the amplitude of structural modification from TAH derivatives increases with electron-donating substituents compared to TAH derivatives with electron-withdrawing substituents, signifying an apparent substitution effect on the TAH derivatives (see Fig.…”

“…The molecular orbitals contributions are important in determining the charge-separated states of TAH derivatives. To create an efficient charge-separated state, the HOMO must be localized on the donor subunits and LUMO on the acceptor subunits [75]. In the TAH(2) derivative, the HOMO comprises both helical conjugate and substituent, which is indicative of enhanced CT efficiency.…”

“…The total static dipole moment (μ), average linear polarizability (α) and first-order hyperpolarizability (β) using the x, y, z components of the compound from Gaussian 03 W [60] output were calculated by Eqs. 4-6 [61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77].…”

A cross-linkable triphenylamine derivative as a hole injection/transporting material in organic light-emitting diodes

“…Both compounds provided a single absorption 25 band at 306 nm, which we attribute to the energy level of their triphenylamine functional groups. 16,42,43 The band gap energies (E g ) of the films of TPABZ and P-TPABZ, obtained from their onset absorption wavelengths in the UV-Vis absorption spectra, were 3.49 and 3.48 eV, respectively (Eg = 1240/λ). Fabrication of 30 multilayer OLEDs and PLEDs through solution-processing requires a film to be insoluble in the solvent used for the subsequent deposition process.…”

“…15 The triphenylamine functional group has been investigated thoroughly; it is considered a promising HITL material because of its appropriate energy level and good hole injection/transporting ability. 16 The small 65 molecules for OLED devices are regularly incorporated using high-vacuum vapor deposition, which allows the fabrication of well-defined multilayers with relatively simple structures. Small organic compounds also can be purified, through crystallization or sublimation, to a high degree prior to deposition.…”