Energy level alignment at planar organic heterojunctions: influence of contact doping and molecular orientation

Abstract

Planar organic heterojunctions are widely used in photovoltaic cells, light-emitting diodes, and bilayer field-effect transistors. The energy level alignment in the devices plays an important role in obtaining the aspired gap arrangement. Additionally, the π-orbital overlap between the involved molecules defines e.g. the charge-separation efficiency in solar cells due to charge-transfer effects. To account for both aspects, direct/inverse photoemission spectroscopy and near edge x-ray absorption fine structure spectroscopy were used to determine the energy level landscape and the molecular orientation at prototypical planar organic heterojunctions. The combined experimental approach results in a comprehensive model for the electronic and morphological characteristics of the interface between the two investigated molecular semiconductors. Following an introduction on heterojunctions used in devices and on energy levels of organic materials, the energy level alignment of planar organic heterojunctions will be discussed. The observed energy landscape is always determined by the individual arrangement between the energy levels of the molecules and the work function of the electrode. This might result in contact doping due to Fermi level pinning at the electrode for donor/acceptor heterojunctions, which also improves the solar cell efficiency. This pinning behaviour can be observed across an unpinned interlayer and results in charge accumulation at the donor/acceptor interface, depending on the transport levels of the respective organic semiconductors. Moreover, molecular orientation will affect the energy levels because of the anisotropy in ionisation energy and electron affinity and is influenced by the structural compatibility of the involved molecules at the heterojunction. High structural compatibility leads to π-orbital stacking between different molecules at a heterojunction, which is of additional interest for photovoltaic active interfaces and for ground-state charge-transfer.

Energy alignment and recombination in perovskite solar cells: weighted influence on the open circuit voltage

“…This can arise from differences between solution and solid state, 18,[39][40][41] due to a chemical reactivity 42 or intermixing with the perovskite layer components. 19,43 When considering different HTMs for fabricating a perovskite solar cell, the resulting VOC will mainly relate to the respective HOMO energy level, 44,45 as well as on the recombination constant 16 and density of states disorder.…”

“…[14][15][16][17] Further insight on the role of the interface energetics is provided by analysing the work function (WF), determined from contact potential difference (CPD) measured by Kelvin probe force microscopy (KPFM) in order to contrast the results obtained by CE and validate that method. 18,19 Figure 2. Schematics of the energy diagram for the materials in the perovskite solar cells studied in this work.…”

Efficiency enhancement of small molecule organic solar cells using hexapropyltruxene as an interface layer

“…Such a template layer thus can be used to influence the molecular ordering to increase the PCE of OSCs because the ordering of the molecules in the photovoltaic active layer of an OSC can have profound consequences on their performance [26][27][28][29] , including improved absorption and more efficient exciton and charge carrier transport 26,[30][31][32] . Furthermore, the energy levels of the electrode materials can also be modified by the template layer 27,33,34 , which is important for device performance 33 . Research on the template growth effect has been conducted with numerous organic small molecules, e.g.…”

Effect of the Organic Semiconductor Side Groups on the Structural and Electronic Properties of Their Interface with Dopants

“…We note that the different change of the VL with respect to that of the bare substrate in the two cases is not well explained in the context of the integer charge transfer model, which predicts equal electronic equilibrium via alignment of the Fermi level and the LUMO of C 60 F 48 (negative polaron). 54 Differently from organic heterojunctions with unpinned intermediate layers, 12 here the HOMO of the OSC films is pinned to the LUMO of the dopant (via interface charge transfer) and, therefore, the OSC film is involved in the process of thermodynamic electronic equilibrium with the substrate. The larger Δϕ observed for C 60 F 48 /C8-BTBT in comparison to C 60 F 48 /DPh-BTBT, with a difference of ∼0.25 eV, is indicative of a larger surface dipole as expected from larger charge transfer at the OSC/dopant interface due to the lower ionization energy of C8-BTBT films (IP C8-BTBT = 5 eV vs IP DPh-BTBT = 5.3 eV).…”