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Molecular Electrical Doping of Organic Semiconductors: Fundamental Mechanisms and Emerging Dopant Design Rules Ingo Salzmann,† Georg Heimel,† Martin Oehzelt,§ Stefanie Winkler,† and Norbert Koch*,†,§,# †
Humboldt-Universität zu Berlin, Institut für Physik & IRIS Adlershof, Brook-Taylor Straße 6, 12489 Berlin, Germany Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Bereich Solarenergieforschung, Albert-Einstein-Straße 15, 12489 Berlin, Germany # Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, P.R. China §
CONSPECTUS: Today’s information society depends on our ability to controllably dope inorganic semiconductors, such as silicon, thereby tuning their electrical properties to applicationspecific demands. For optoelectronic devices, organic semiconductors, that is, conjugated polymers and molecules, have emerged as superior alternative owing to the ease of tuning their optical gap through chemical variability and their potential for low-cost, large-area processing on flexible substrates. There, the potential of molecular electrical doping for improving the performance of, for example, organic light-emitting devices or organic solar cells has only recently been established. The doping efficiency, however, remains conspicuously low, highlighting the fact that the underlying mechanisms of molecular doping in organic semiconductors are only little understood compared with their inorganic counterparts. Here, we review the broad range of phenomena observed upon molecularly doping organic semiconductors and identify two distinctly different scenarios: the pairwise formation of both organic semiconductor and dopant ions on one hand and the emergence of ground state charge transfer complexes between organic semiconductor and dopant through supramolecular hybridization of their respective frontier molecular orbitals on the other hand. Evidence for the occurrence of these two scenarios is subsequently discussed on the basis of the characteristic and strikingly different signatures of the individual species involved in the respective doping processes in a variety of spectroscopic techniques. The critical importance of a statistical view of doping, rather than a bimolecular picture, is then highlighted by employing numerical simulations, which reveal one of the main differences between inorganic and organic semiconductors to be their respective density of electronic states and the doping induced changes thereof. Engineering the density of states of doped organic semiconductors, the Fermi−Dirac occupation of which ultimately determines the doping efficiency, thus emerges as key challenge. As a first step, the formation of charge transfer complexes is identified as being detrimental to the doping efficiency, which suggests sterically shielding the functional core of dopant molecules as an additional design rule to complement the requirement of low ionization energies or high electron affinities in efficient n-type or p-type dopants, respectively. In an extended outlook, we finally argue that, to fully meet this challenge, an improved understanding is required of just how the admixture of dopant molecules to organic semiconductors does affect the density of states: compared with their inorganic counterparts, traps for charge carriers are omnipresent in organic semiconductors due to structural and chemical imperfections, and Coulomb attraction between ionized dopants and free charge carriers is typically stronger in organic semiconductors owing to their lower dielectric constant. Nevertheless, encouraging progress is being made toward developing a unifying picture that captures the entire range of doping induced phenomena, from ion-pair to complex formation, in both conjugated polymers and molecules. Once completed, such a picture will provide viable guidelines for synthetic and supramolecular chemistry that will enable further technological advances in organic and hybrid organic/inorganic devices.
1. INTRODUCTION
to other differently doped semiconductors. It is these interfacial phenomena, present in, for example, p−n junctions or fieldeffect transistors, that allow for the manifold of device functionalities exploited in silicon-based electronics. Doping inorganic materials is achieved by controllably introducing impurity atoms (at typical concentrations of 10−6−10−3) into
1.1. Inorganic Semiconductor Doping
The initial discovery of semiconducting materials and, in particular, the technological breakthrough of gaining control over their electrical properties via doping constitute the very basis for the multitude of electronic devices that pervade today’s information society. Doping an inorganic semiconductor allows changing the energy alignment of its electronic bands relative to those of metal contacts as well as © XXXX American Chemical Society
Received: September 28, 2015
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DOI: 10.1021/acs.accounts.5b00438 Acc. Chem. Res. XXXX, XXX, XXX−XXX
Article
Accounts of Chemical Research
Figure 1. (a) Sheet conductivities (σ) of poly(3-hexylthiophene) (P3HT) and quaterthiophene (4T) thin films upon p-doping with the strong electron acceptor 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (F4TCNQ); dopant ratio refers to the number of dopant molecules divided by the sum of monomer units and dopant molecules (P3HT) or divided by total number of all molecules (4T). (b) Schematic illustration of the p-in concept in an OLED. Adapted with permission from ref 13. Copyright 2012 by John Wiley and Sons.
emitting devices (OLEDs),8 photovoltaic cells,9 and organic field-effect transistors in the late 1980s.10,11 Interestingly, however, applications at this time were almost exclusively based on intrinsic (i.e., not intentionally doped) OSCs, which is primarily because, in contrast to inorganic semiconductors, halides and alkali metals employed as dopants are not covalently bonded to the host and, therefore, exhibit a strong tendency to diffuse. This rendered, for example, doped injection layers for ohmic contacts or p−n junctions unstable under operating conditions.12
an otherwise highly pure and crystalline semiconductor. Through covalent interaction with the host atoms, electronic “defect” states arise in the fundamental gap that lie a few tens of millielectronvolts (ref 1) above or below the valence or conduction band edge for p- or n-type doping. Because all available electronic states are occupied following Fermi−Dirac statistics, this altered density of states (DOS) leads, for example, for p-doping, to (i) a shift of the electron chemical potential (often referred to as the Fermi level, EF) from midgap down toward the valence band edge, (ii) essentially all (by design shallow) acceptor states being occupied with electrons at room temperature, and, as a consequence, (iii) mobile holes in the valence band. n-Type doping proceeds in full analogy, now with deliberately introduced shallow donor states closely below the conduction band edge with their complete ionization at room temperature leading to mobile electrons instead. The high efficiency of this very process (typically generating one free charge carrier per dopant atom) leads to a dramatic increase in semiconductor conductivity1 even at ultralow doping ratios (
