Comparative study of dielectric functions of complex organic heterostructures

Abstract

Organic/organic heterostructures and mixed layers were prepared by organic molecular beam deposition (OMBD) in high vacuum (HV) on hydrogen passivated (111) oriented silicon. The substrates were kept at room temperature during the deposition. The organic superstructures consisting in alternative layers of tris--diamine (α-NPD) and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)/copper phthalocyanine (CuPc) were investigated by means of spectroscopic ellipsometry in the 0.73 -5 eV spectral range. Additionally reflection infra-red (IR) measurements were performed using p-and s-polarisation.Taking into account the dielectric function of the single layers the optical response of the Alq 3 /α-NPD superstructure can be modeled assuming sharp interfaces. However, for the PTCDA/CuPc superstructure the optical response requires a more sophisticated approach than simply superimposing the responses of the individual layers. The deviation between simulated and experimental data is assigned to the molecular interaction at the interfaces between PTCDA and CuPc. This can affect the molecular orientation and the optical properties.

Optical properties of organic thin films deposited via organic vapour phase deposition

“…The analysis of the measured 〈 ε ( ω )〉 spectra has revealed that the fundamental energy gap of the Alq 3 films has the value of ω g = 2.7 eV. Also, the existence of four individual electronic transitions has been found at energies ω 01 ≈ 3.0 eV, ω 02 ≈ 4.6 eV and ω 03 ≈ 6.2 eV in accordance with other published results 1, 2, 10. Moreover, the optical response of the Alq 3 films (energy position) has been found to be independent of thin film thickness.…”

“…Organic semiconductors and their heterostructures have attracted considerable attention due to their application in organic light emitting devices (OLEDs) which are expected to play a significant role in various lighting and flat‐panel displays applications 1–4. These materials combine high brightness with low cost and allow large‐area fabrication onto flexible and rigid substrates 5.…”

“…The optical response of single and multi‐layered structures can provide valuable information about the preparation quality, thickness and optical properties of materials (refractive index, absorption coefficient). In this way, the deposition parameters of the fabrication process can be optimized and so that the final product has an improved quality 1, 4, 11, 12.…”

Vibrational mode contribution to the dielectric permittivity of disordered small-molecule organic semiconductors

“…We note that for Alq 3 , the isomer or the isomer ratio in the thin films studied is not specified in the references to the experimental work given. [17,18,20,22], at wavelengths λ = 1000-1500 nm. i Ref.…”

“…For many small-molecule organic semiconductor materials, the dielectric permittivity at optical frequencies is known from thin-film ellipsometry measurements [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26], electron energy loss spectroscopy [27], and from theory using quantum-chemically calculated molecular polarizabilities and the Clausius-Mossotti relationship [28,29]. The relative dielectric constant at quasistatic conditions may be obtained from low-frequency capacitance-voltage (C-V) studies of sandwich-type devices [30][31][32][33][34][35][36][37][38][39].…”