Oxygen-Related Band Gap State in Single Crystal Rubrene

Abstract

A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Twophoton excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene. DOI: 10.1103/PhysRevLett.97.166601 PACS numbers: 72.80.Le, 71.20.Rv, 71.35.ÿy Charge transport in organic molecular crystals is notably different from inorganic semiconductors. Despite a large number of investigations, understanding the fundamental processes governing charge transport remains a central issue for organic-based devices. In particular, the origin of band gap states and their effect on electronic and optical characteristics are both controversial and important questions [1,2]. Rubrene (C 42 H 28 ) in single crystal form has emerged as an important aromatic molecular solid, showing the highest field effect transistor (FET) mobility to date among organics (20 cm 2 =V s) [3]. In addition, several studies suggest that bandlike transport is operative [3][4][5][6]. Electronic properties in rubrene are, however, highly sensitive to both atmospheric environment as well as material treatment. As is true for inorganic semiconductors, device applications demand the understanding and control of such environmental factors, but the situation in rubrene is far from clear to date. Oxidation of organic crystals often results in modification of transport characteristics [7][8][9][10][11], and recent studies indicate that the presence of ambient oxygen also affects the conductivity of crystalline rubrene [12,13]. Furthermore, chemical analysis of naturally oxidized crystalline rubrene shows that the relative concentration of rubrene peroxide (C 42 H 28 O 2 ) can be as much as 1% at a depth of 50 nm [14], a length scale greater than a typical FET channel depth. Such a high impurity concentration will likely have a significant effect on charge transport, particularly if the impurity forms a band gap state.In this Letter, we demonstrate the formation of a band gap state in oxidized rubrene crystals and address the impact of the state on charge transport and exciton recombination. A spectral signature consistent with the molecular exciton model is established and investigated under different ambient conditions. A well-defined photoluminescence (PL) band at an energy lower than the highest exciton recombination transition energy develops in the spectra of oxidized rubrene. The PL band reflects an additional radiative recombination path through the oxygenrelated state in the band gap. The PL depth profile indicates that the oxygen-related band is emitted only from a surface region. Correlation between the ...

First-principles studies on organic electronic materials

“…Oxygen and hydrogen species produce also levels in the energy band gap of pentacene. [25,26] These theoretical findings are consistent with pertinent experimental observations about oxygen and hydrogen-related carrier traps in rubrene [27,28] and pentacene. [29] Hybrid systems with mixtures of organic and inorganic materials are currently being investigated intensively in pursuit of the optimal balance between the normally superior physical properties of inorganic systems and the low cost and flexibility of their organic counterparts.…”

Reliability of Organic Field‐Effect Transistors

“…The state is at an energy of 0.24 eV above the valence-band edge, and gives rise to a distinct defect photoluminescence signature and to increased dark and photoconductivity. [86] Jurchescu found that pentacene single crystals grown from as received source material contain about 0.1% of pentacene quinone impurities, which can act as scattering centers. The impurity concentration can be reduced to 0.03% by rigorous purification, resulting in a significant increase in the bulk charge-carrier mobility extracted from space-charge limited current (SCLC) measurements.…”

Optical spectra obtained from amorphous films of rubrene: Evidence for predominance of twisted isomer

“…We found that these data can be described very well using the optical constants of rubrene on silicon obtained by ellipsometry (and those of glass measured sep- arately with variable angle ellipsometry). The minor differences between the data and the model might be due to oxidation of some rubrene molecules in air [16,17]. …”