Search for: All records

The mechanism and the nature of the species formed by molecular doping of the model polymer poly(3-hexylthiophene) (P3HT) in its regioregular (rre-) and regiorandom (rra-) forms in solution are investigated for three different dopants: the prototypical π-electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), the strong Lewis acid tris(pentafluorophenyl)borane (BCF), and the strongly oxidizing complex molybdenum tris[1-(methoxycarbonyl)-2-(trifluoromethyl)-ethane-1,2-dithiolene] (Mo(tfd-CO2Me)3). In a combined optical and electron paramagnetic resonance study, we show that the doping of rreP3HT in solution occurs by integer charge transfer, resulting in the formation of P3HT radical cations (polarons) for all the dopants considered here. Remarkably, despite the different chemical nature of the dopants and dopant-polymer interaction, the formed polarons exhibit essentially identical optical absorption spectra. The situation is very different for the doping of rraP3HT, where we observe the formation of a charge-transfer complex with F4TCNQ and formation of a “localized” P3HT polaron on non-aggregated chains upon doping with BCF, while there is no indication of dopant-induced species in case of Mo(tfd-CO2Me)3. We estimate the ionization efficiency of the respective dopants for the two polymers in solution and report the molar extinction coefficient spectra of the three different species. Finally, we observe increased spin delocalization in regioregular compared to regiorandom P3HT by electron nuclear double resonance, suggesting that the ability of the charge to delocalize on aggregates of planarized polymer backbones plays a significant role in determining the doping mechanism. 

Efficient electrical doping of organic semiconductors relies on identifying appropriate molecular dopants that are capable of ionizing semiconductor molecules with a high yield, thereby creating mobile charges. We explore the suitability of two different material parameters to predict ion pair formation for different sets of semiconductor–dopant combinations: (i) redox-potentials measured by cyclic voltammetry in solution and (ii) ionization energy (IE)/electron affinity (EA) measured on thin films by ultraviolet/inverse photoelectron spectroscopy. Our study suggests, at least for molecular semiconductors and dopants, that redox-potentials are better suited to identify matching material pairs and their ion pair formation yield than IE/EA values. This is ascribed to the dependence of IE/EA values on molecular orientation and film structure on and above the meso-scale. In contrast, cyclic voltammetry measurements, although performed on solutions rather than on thin films, capture dopant–semiconductor energy levels on the molecular scale, which is more relevant for doping even in the case of solid thin films. 

Abstract In₂O₃, an n‐type semiconducting transparent transition metal oxide, possesses a surface electron accumulation layer (SEAL) resulting from downward surface band bending due to the presence of ubiquitous oxygen vacancies. Upon annealing In₂O₃in ultrahigh vacuum or in the presence of oxygen, the SEAL can be enhanced or depleted, as governed by the resulting density of oxygen vacancies at the surface. In this work, an alternative route to tune the SEAL by adsorption of strong molecular electron donors (specifically here ruthenium pentamethylcyclopentadienyl mesitylene dimer, [RuCp*mes]₂) and acceptors (here 2,2′‐(1,3,4,5,7,8‐hexafluoro‐2,6‐naphthalene‐diylidene)bis‐propanedinitrile, F₆TCNNQ) is demonstrated. Starting from an electron‐depleted In₂O₃surface after annealing in oxygen, the deposition of [RuCp*mes]₂restores the accumulation layer as a result of electron transfer from the donor molecules to In₂O₃, as evidenced by the observation of (partially) filled conduction sub‐bands near the Fermi level via angle‐resolved photoemission spectroscopy, indicating the formation of a 2D electron gas due to the SEAL. In contrast, when F₆TCNNQ is deposited on a surface annealed without oxygen, the electron accumulation layer vanishes and an upward band bending is generated at the In₂O₃surface due to electron depletion by the acceptor molecules. Hence, further opportunities to expand the application of In₂O₃in electronic devices are revealed. 

Abstract The combination of inorganic and organic semiconductors in a heterojunction is considered a promising approach to overcome limitations of each individual material class. However, to date only few examples of improved (opto‐)electronic functionality have been realized with such hybrid heterojunctions. The key to unraveling the full potential offered by inorganic/organic semiconductor heterojunctions is the ability to deliberately control the interfacial electronic energy levels. Here, a universal approach to adjust the offset between the energy levels at inorganic/organic semiconductor interfaces is demonstrated: the interlayer method. A monolayer‐thick interlayer comprising strong electron donor or acceptor molecules is inserted between the two semiconductors and alters the energy level alignment due to charge transfer with the inorganic semiconductor. The general applicability of this method by tuning the energy levels of hydrogenated silicon relative to those of vacuum‐processed films of a molecular semiconductor as well as solution‐processed films of a polymer semiconductor is exemplified, and is shown that the energy level offset can be changed by up to 1.8 eV. This approach can be used to adjust the energy levels at the junction of a desired material pair at will, and thus paves the way for novel functionalities of optoelectronic devices.