We investigate the counterintuitive phenomenon of inserting a transition metal oxide layer to improve hole injection or extraction in organic semiconductor devices using ultraviolet photoemission (UPS), x-ray photoemission (XPS), and inverse photoemission spectroscopy. We observe that metal oxides, such as MoO3 and WO3, substantially increase the work function when deposited on indium tin oxide. The increase lifts up the highest occupied molecular orbital (HOMO) of the hole transport layer and therefore reduces the energy barrier between the HOMO and the Fermi level of the anode. The uplift creates an interface band-bending-like region that results in a drift electric field, which encourages the collection of holes at the anode. The optimum thickness for the oxide layer is estimated to be 2 nm. We also investigate the effects of air and O2 exposure of MoOx films. We observe that while most of the electronic energy levels of the oxide remain largely intact, the work-function reduction is substantial. The UPS and XPS data indicate that chemisorption is the major contributor of the work-function reduction. The reduction can be seen in two stages: initially dominated by oxygen adsorption at exposure <1013 L, and finally saturated by moisture at 1014 L.
We have investigated the electronic structure, interface formation, and thin film growth dynamics of rubrene using
ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), angle-resolved
photoemission spectroscopy (ARPES), and atomic force microscopy (AFM). From UPS and IPES we obtained an
injection gap of 2.67 eV, which is derived from the onset separation of the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO), and a transport gap of 3.98 eV, which is derived from the peak
separation. The ARPES results indicate that the HOMO band dispersion along Γ-X is 0.25 eV, and the electron effective
mass is 1.3 times of the free electron one. These values can produce an estimate of the hole mobility of 15 Vs/cm2. The
investigation of the electronic structure of the interfaces between rubrene and various metals, such as Au, Ag, Al, and
Ca, shows that the Fermi level shifts linearly within the band gap as a function of metal workfunction, until it is been
pinned at the LUMO by a low workfunction metal like Ca. The growth morphology dependence on the film thickness,
deposition rate, and substrate temperature will also be discussed.
Inserting an ultra-thin interlayer has been an important means in modifying the performance of organic semiconductor
devices. Using photoemission and inverse photoemission spectroscopy (UPS, XPS and IPES), we have investigated the
electronic structure of a number of insertion layers widely used in organic semiconductor devices. We found that
inserting alkali metal compound thin layers such as LiF between the electron transport layer (ETL) and the cathode can
induce energy level shift in the ETL that reduces the electron injection barrier. The reduction is attributed to the release
of the alkali metal that n-doped the ETL, and as such it depends on the cathode material deposited on top of the insertion
layer. For thin metal oxide insertion layers, such as MoO3, between the anode and the hole transport layer (HTL),
reduction of the hole injection barrier is also observed. This reduction, however, is due to the large workfunction of the
oxide that subsequently moves the highest occupied molecular orbital (HOMO) toward the anode Fermi level. Effects of
other insertion layers, such as metal insertion layer in organic bistable device (OBD) and organic insertion layer in
bipolar organic thin film transistors (OTFT) will also be discussed.
We investigated the electronic structure of organic thin films doped with alkali metal using photoemission and inverse
photoemission spectroscopy (UPS, XPS and IPES). We found that doping induces energy level shift, which can be seen
as in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of
the doping of electrons from the alkaline metal to the organic, and the second stage is characterized by the significant
modification of organic energy levels such as the introduction of a new gap state, new core level components, and
change of binding energies with respect to the frontier orbital. In addition, we observed that the energy level shift in the
first stage depended approximately in a semi-logarithmic fashion on the doping concentration, whose slope could not be
explained by the conventional model used in inorganic semiconductors. The lowest unoccupied molecular orbital
(LUMO) is observed to diminish as doping progresses. Furthermore, we observed that the doping induced modification
can be compensated by depositing Au or O2 on alkali metal doped organic films. The modification of the electronic
structure by other inorganic or organic dopants will also be discussed.
There has been a considerable interest on forming ambipolar organic thin film transistors (OTFTs) because such devices
are advantageous for integrated circuits, such as lower power consumption, design and fabrication simplification, and
better immunity. Most recently, Shi et al. observed a substantial mobility improvement in ambipolar OTFTs based on
the heterojunction formed between copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl)
bithiophene (BP2T). Specifically, the hole and electron mobility are improved by 3 and 12 folds from the bulk values,
respectively. We examined the interface formation between F16CuPc and BP2T using ultraviolet photoemission (UPS)
and inverse photoemission spectroscopy (IPES). It is observed that in F16CuPc/BP2T the heterojunction is characterized
by band bending in both materials, while in BP2T/F16CuPc the band bending is confined in BP2T only. For
F16CuPc/BP2T, the band bending of BP2T and F16CuPc are 0.40 and 0.35 eV, respectively. The band bending region is
~15 nm in both materials, from which the Debye lengths of the materials can be deduced. The combination of the band
bending and finite Debye lengths offers an explanation to the observed improvement and thickness dependence of the
mobility in OTFTs based on such heterojunctions.
Despite its wide application in devices, the mechanism of improvement induced by the LiF insertion layer remains
controversial and to be fully resolved. We report our study of the interface formation when gold or Al is deposited onto
5 Å LiF covered Alq using X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). We found that initial Au
deposition produced a small shift of energy levels toward higher binding energy, which was reversed by subsequent Au
coverages. The energy level positions finally reach those of the pristine Alq, resulting in a flat-band situation in the
interface region. This is in sharp contrast to the Al/LiF/Alq interface, where ~1 eV downward shift of the energy levels
substantially reduces the electron injection barrier. The observation of the overall flat-band condition in the interface
region explains well why for thin LiF interlayer, the metal overlayer material is critical for the improvement of charge
injection. As we observed here for Au, the low reactivity of the deposited metal atoms do not result in substantial n-doping
of the Alq in the interface region, in contrast to more reactive metals like Al and Mg that can cause substantial n-doping
of Alq, signified by the ~1 eV energy level shifts toward higher BE and emergence of the gap state, and reduce
the electron injection barrier as a result.
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