Charge Transfer at Organic/Inorganic Interfaces and the Formation of Space Charge Regions Studied with Infrared Light

Abstract

We present in situ infrared spectroscopy as a powerful tool for the qualitative and quantitative analysis of the charge transfer through the prototypical interface between the organic semiconductor 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and MoO 3 that in organic electronic devices is often used to improve their performance. Due to the different infrared vibrational spectra, charged and neutral species of CBP molecules can be well distinguished, which allows the measurement of the amount of charged species in the vicinity of the interface. The quantitative analysis of CBP thickness-dependent infrared transmission spectra delivered the extension of the space charge region from the interface into the CBP on a nanometer scale. The clear influence of the deposition sequence on these interface properties was clarified by further studies of the inverted layer structures. ■ INTRODUCTIONIn organic electronics the energy level alignment at interfaces between electrodes and organic semiconductors is crucial for the effective barrier heights for charge extraction (organic photovoltaics (OPVs)) and/or injection (organic field-effect transistors (OFETs)/organic light-emitting diodes (OLEDs)) and thus for the overall device performance. Inserting suitable interlayer materials is one approach to modify the energetics at such interfaces. For example, for many years, thin transitionmetal oxide (TMO) layers between organic semiconductors and the metal electrodes have been used to improve device performance. 1,2 For these systems the increased efficiency has been explained in terms of a decreased effective hole injection barrier due to a change of the work function of the contact and the energy level alignment at the interface between the TMO and the organic layer. 3−6 The origin of these effects and their experimental evaluation as well as their description by theoretical models based on molecular properties such as the density of states of the involved frontier orbitals are topics of recent publications. 7−12 Different experimental techniques are used to investigate the evolution of energy levels at the interface and the associated interfacial charge transfer (CT) that is also termed contact doping. In addition to the frequently used UV photoemission spectroscopy (UPS) and Kelvin probe measurements, which measure the potential profile as a consequence of the transferred charges, 1,7,13 CT at interfaces also has been investigated with differential-reflection spectroscopy in the UV−vis range. 12 Especially for the latter method, the broad spectral features complicate an easy evaluation of these. Very recently, Shallcross et al. showed that X-ray photoemission spectroscopy (XPS) can be used to investigate the extent of contact doping at interfaces of high work function materials, such as indium tin oxide (ITO) or MoO x -covered ITO and solution-processed thin layers of poly(3-hexylthiophene-2,5-diyl) (P3HT). 11 Since a better understanding of CT effects on the molecular level seems to be crucial for the optimal device design, fur...

Doping mechanism of MoO3in 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl: A photoelectron spectroscopic study

“…This observation suggests that there is an increased amount of CBP + cations at the very interface, which is in good agreement to IR investigations on the MoO 3 /CBP interface. In this study, charged and neutral CBP molecules could be discriminated by their specific vibrational modes . Corresponding to Papadopoulos et al in the Mo3d spectra, the additional component on the low energetic side of the doublet and between the two emission lines of the doublet, which is attributed to a formal Mo 5+ oxidation state, raises the more CBP is deposited.…”

Structure–Property Relationship of Phenylene-Based Self-Assembled Monolayers for Record Low Work Function of Indium Tin Oxide

“…Considering the tilt angles extracted from IR measurements, the theoretical values for the work function shift of each molecule are estimated with the equation: with the vacuum permittivity ϵ 0 and ϵ SAM , the dielectric constant of the investigated SAM. ϵ SAM is approximated by the dielectric background of 2.9. , The coverage is estimated by a general coverage of ρ = 0.8 molecule/nm ,,, scaled by the integral ratio of the corresponding N 1s signal N i of the molecule in relation to a reference integral N 0 . The value of N 1s for the maximum coverage N 0 is estimated from the highest N 1s integral in the case of XPA (see the SI for further information).…”

“…ϵ SAM is approximated by the dielectric background of 2.9. 57,58 The coverage is estimated by a general coverage of ρ = 0.8 molecule/nm 23,24,50,56 scaled by the integral ratio of the corresponding N 1s signal N i of the molecule in relation to a reference integral N 0 . The value of N 1s for the maximum coverage N 0 is estimated from the highest N 1s integral in the case of XPA (see the SI for further information).…”

Pristine Poly(para-phenylene): Relating Semiconducting Behavior to Kinetics of Precursor Conversion

“…Similar observations were made for doped organic semiconductors and the sequential deposition of both MoO 3 onto layers of CBP (4,4′-bis( N -carbazolyl)-1,1′-biphenyl) and small molecular dopants onto the polymer P3HT. 35−38 Therefore, a p-type doping of PPP with MoO 3 seems reasonable, and an enhanced electrical conductivity upon doping is expected (vide infra).…”