Organic–inorganic hybrid perovskites such as methylammonium lead halide (CH3NH3PbI3) have been employed in photoelectronic device applications such as light-emitting diodes, solar cells, field-effect transistors, and photodetectors. Combining ultraviolet photoemission spectroscopy(UPS), X-ray photoemission spectroscopy (XPS), atomic force microscopy(AFM), X-ray diffraction measurements (XRD), and device fabrication and characterization, we performed a systematic investigation on the correlation of energy level alignment, film growth and molecular orientation of a number of interfaces and their modifications in photodetectors based on CH3NH3PbI3 and FA0.4MA0.6PbI3.
There is a significant correlation in organic semiconductor thin films between the morphology and electronic structure, and both have strong effects on device performance. In this presentation, we describe our investigations on the relationship of the morphology and corresponding electronic structure and electrical properties using scanning probe microscopy techniques, photoemission spectroscopy, and transport measurements on organic semiconductor thin films. We also present investigations on the effects on optoelectronic device performance especially after controlling the morphology by quasi-epitaxial growth on representative organic semiconductors.
Organic-inorganic halide perovskites have attracted considerable attention in the past few years because of their remarkable performance in optoelectronic devices. However, long-term stability of the materials and devices remains the biggest challenge for realistic implementation of perovskite solar cells. Although significant efforts have been carried out on the causes of degradation at the device level, yet few measurements have been made at the surface analytical level to reveal the degradation mechanisms. I’ll present our work on the effects of environmental factors, such as O2, water, and light, on the perovskite layer by monitoring the intrinsic electronic structure and compositional changes in different aging tests. This work contributes in developing better understanding of the degradation mechanisms to improve the overall stability of perovskite light emitting diodes and solar cells.
Insertion of high work function (WF) transition metal oxide (TMO) layers between the anode
and the hole transport layer is established to substantially enhance the performance of organic
light emitting diodes (OLED). The high WF of transition metal oxide layer has been
demonstrated to be the most crucial for the enhancement. The WF of a TMO layer decreases
substantially with air exposure, and noticeably by the ambient even inside a low vacuum
system. In the present work we discuss various methods to protect and recover the high WF
after a TMO thin film has been exposed to air. We report covering a thin organic layer on top of
MoOx to protect the high work function. We found that a thin layer of 1-2 nm organic layer was
sufficient to protect the work function of MoOx thin film underneath. We further report
methods to recover already decreased TMO WF due to air exposure. We performed oxygen
plasma cleaning of air exposed MoOx film and found out that oxygen plasma could
substantially recover the WF of as deposited MoOx film. We also performed annealing of air
exposed MoOx film inside an ultra high vacuum system and observed a thin layer of oxygenrich
adsorbate layer, which desorbed upon annealing that in turn substantially recovered the
MoOx WF. We discuss the vacuum annealing and the effect of resulting surface on the interface energy level alignment.
Thermally deposited molybdenum oxide films are investigated with X-ray photo-emission spectroscopy, ultra-violet photoemission spectroscopy and inverse photoemission spectroscopy. The Interface between MoOx and copper pthalocynine (CuPc) is studied and the previously reported device performance improvement is explained, with the help of interface energy level alignment. The effect of oxygen and air exposure on MoOx films and growth of gap states with exposures are studied. The surface chemical compositions of MoOx films, of varying thicknesses from 1 nm to 50 nm, have also been investigated. For all the investigated film thicknesses, the thermally evaporated films are found to be oxygen deficient. It is believed, that the oxygen vacancies can be subdued to a great extent by annealing at elevated temperatures. We annealed the MoOx thin films in air, at 300 °C for 20 h, and investigated the changes induced by the air annealing.
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.
The electronic structure of the interface formed by Mg deposition onto 2,5-bis(6’-(2’,2"-bipyridyl))-1,1-dimethyl-3,4-diphenyl silacyclopentadiene (PyPySPyPy) was investigated using ultraviolet, inverse, and X-ray photoemission spectroscopies. PyPySPyPy is of interest for use as an electron injection/transport layer in high efficiency organic light-emitting diodes. Upon deposition of Mg onto PyPySPyPy there is a shift of the occupied energy level structure to higher binding energy, away from the Fermi level, and appearance of two energy levels within the energy gap of PyPySPyPy. The lowest unoccupied molecular orbital is also shifted to higher binding energy.
Pentacene, perylene, and sexithiophene are all materials being used in organic thin film transistors due to their relatively large mobilities. It has been suggested that the functional behavior in these devices occurs within the first few molecular layers of the organic at the interfaces between the organic and the dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation using photoemission spectroscopy to examine layer by layer growth of pentacene, perylene, and sexithiophene on conductors, dielectrics, and charge transfer agents and in some cases vice versa. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition. There appears to be a linear relation between the interface dipole and metal workfunction with the observed dipoles ranging from a 1 eV dipole at the interface between sexithiophene and gold to a -0.46 eV dipole at the interface between pentacene and calcium. We also observed that more complex material intermixing takes place during metal on organic deposition than during organic deposition onto metal and as a result, the electronic structure of the interface differs from that of organic on metal deposition. Possible charge transfer, dipole formation and energy level bending at these interfaces will be discussed.
The electronic structures of interfaces between metals and Copper phthalocyanine (CuPc) organic films are investigated using the combination of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) can be directly observed by IPES and UPS simultaneously. We found that the Fermi level, EF, in the organic film can be modified by metals through charge transfer or doping. The FERMI level at the Cs/CuPc interface is observed to shift to less than 0.2 eV below the CuPc LUMO. The IPES observation is the first direct confirmation of Fermi level pinning near the LUMO in organic films. The pinning of the Fermi level close to the LUMO can be explained by electron transfer from Cs to CuPc, which is supported by the presence of a gap state in CuPc as observed with UPS. On the other hand, the Au/CuPc interface is characterized by electron transfer from CuPc to Au, resulting in a reduced HOMO intensity shown in the UPS spectra and a new feature below the LUMO shown in the IPES spectra. These observations shed new light onto the understanding of interface formation in organic semiconductor devices.
We have investigated the evolution of the growth front of perylene, an organic semiconductor with high carrier mobility, on glass and Au substrates grown side-by-side by vapor deposition. The films were grown with gradually increasing thickness which allowed us to examine both the spatial and temporal correlation of the surface roughness using atomic force microscopy. Our results show that perylene growth on glass and Au substrates is non-stationary. However, the instability during the growth is shown to depend largely on the substrate. A roughness exponent of 0.82 is obtained for glass and 0.84 for Au. A growth exponent of 0.21 is obtained for glass and 0.74 for Au. The results indicate the strong influence of the substrate on the film morphology and point to possible ways to control and improve it.
We report the characterization of interface formation between Au(Au) and pentacene, an organic material used as an active material in Organic Thin-Film Transistors, using x- ray and ultraviolet photoelectron spectroscopy (XPS and UPS). XPS results indicate that there is no chemical reaction between Au and pentacene regardless of deposition order. UPS results indicate the presence of interface dipoles at both the pentacene/Au and the Au/pentacene interfaces. In addition there are indications of band- bending at the Au/pentacene interface.
The importance of interfacial properties in organic light emitting devices (OLED) is well recognized. Using mainly surface/interface analytical techniques in a well controlled ultra high vacuum (UHV) environment, we have studied the formation of interfaces that occurs in OLEDs using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The results have shown that the interface energy level alignment is intimately related to the charge injection and therefore to the device characteristics and performance. Relationship to interface charge transfer, chemical reaction, and dipole layer formation will also be discussed.
The charge transfer (CT) process in organic semiconductor thin film structures is an important problem for applications such as photoreceptors and light-emitting devices. The operation of a photoreceptor structure is based on a CT process between a donor molecule and an acceptor transport molecule. We have investigated such a structure formed by vacuum-grown thin films of two organic molecules, N,N'-diphenethyl-3,4,9,10-perylenetetracarboxylic-diimide (DPEP) and N,N'-diphenyl-N,N'-(3-methylphenyl)-1,1'- biphenyl-4,4'-diamine (TPD), with femtosecond time-resolved photoemission spectroscopy. By measuring the lifetimes of the excited electron states in the mixtures of these molecules we are able to determine the time-scale for the electron transfer (ET) between the excited states of the TPD and DPEP molecules. We show that the biexponential ET dynamics consist of a short component of less than 100 fs and a long component of several hundreds fs in length.
Indium tin oxide (ITO) is commonly used as a hole-injecting electrode in organic electroluminescent devices due to its transparency and good electrical conductivity. Various solutions of acid and base have been used to modify the work function of ITO. Depending on the solution, a negative or positive work function shift with respect to that of the standard ITO has been obtained. We have investigated the interface formation between treated ITO and N,N'-bis-(1- naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB), an organic materials often used as hole transport layer in OLEDs, using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The barrier for hole injection, which is defined as the energy difference between the Fermi level and the highest occupied molecular orbital of NPB, decreases from base-, to standard, to acid-treated ITO. No significant reactions is observed for NPB deposited on standard ITO. For the acid- treated ITO substrate, the XPS results point to the reaction between the proton of the adsorbed acid layer and the NPB nitrogen. XPS results also suggest that at low NPB coverage, islanding is occurred at the standard ITO surface.
Using x-ray and UV photoemission spectroscopy (XPS and UPS), we have investigated the early stages of the interface formation between metals, namely Al and Ca, and tris-(8- hydroxyquinoline) aluminum (Alq3). Both interfaces show signs of reaction between the metal and Alq3. However, the detailed behaviors of the two interfaces are very different. In the case of Al/Alq3 interface, the metal was found to react preferentially with the quinolate oxygen as soon as it was deposited onto Alq3. No evidence of reaction with the carbon was found. Unlike with Ca, little interaction between Al and nitrogen of the pyridyl was observed. UPS spectra show a quick disappearance of the Alq3 features as early as 0.7 angstrom of Al deposition, and also suggest the formation of a gap state induced by Al. In the case of Ca/Alq3, the interface is characterized by a staged interface reaction: for low Ca coverages, negatively charged Alq3 radical anions are formed by electron transfer from the Ca. The emergence of new states in the energy gap is observed in the UPS spectra. At higher overages, the Ca reacts with the phenoxide oxygen resulting in the decomposition of the Alq3 molecule.
Thermally stimulated luminescence has used to directly measure the trap states in intrinsic Alq3 and Alq3 doped with coumarin 6. For Alq3 doped with coumarin 6, we observe an increase in the trap energy from 0.25eV for undoped Alq3 to 0.32eV for doping concentrations as high as 2 percent. The origin of these trap states may be related to the relative energy level alignment between the Alq3 host and coumarin dopant. Using UV photoemission spectroscopy, we have measured the solid state energy alignment of the highest occupied molecular orbitals between Alq3 and coumarin 6. Finally, we report I-V curves for single layer devices as a function of doping with Al/LiF top and bottom contacts. The charge transport shows the trap states induced in the Alq3 films due to the presence of the coumarin 6 molecules decrease the carrier mobility and increase the energetic disorder. These results can be directly observed from the measured I-V curves.
The light emission from bi-layer organic devices (OLEDs) has been shown to be proportional to the current. Trap states have been speculated to contribute to the carrier transport in such devices. We will report on the bulk trap state properties of naphthyl-substituted benzidine derivative (NPB) and tris-8-(hydroxyquinoline) aluminum (Alq3) using thermally stimulated luminescence (TSL). Using a general order TSL expression, the four peaks in NPB were modeled with trap sates centered from 0.15 eV to 0.02 eV. The main Alq3 peak is modeled as a distribution of trap states from 0.25 to 0.15 eV with two additional peaks observed at lower trap energies. For both materials, the trapping mechanism involves a combination of first and second order emission. Using TSL,the evolution of the trap states in Alq3 has been studied as a function of coumarin 6 and NPB doping,k with doping levels from 0.1 percent to 2.0 percent. For Alq3 doped with coumarin 6, we observe an almost 0.1 eV increase in the width of the trap states. Conversely, the Alq3 samples doped with NPB do not show a change in the trap states. These trap depths are sufficient to support a trap charge limited model for the carrier transport in bilayer organic based light emitting diodes.
The importance of the interfacial properties in organic light emitting devices (OLED) is well recognized. We have studied the formation of interfaces that occurs in OLEDs using mainly surface/interface analytical techniques in a well controlled ultra high vacuum environment. The results have shown that microscopic surface and interface properties are intimately related to the device characteristics and performance. Specifically, the metal electrode material is observed to quench strongly the luminescence of the organic material in the interface region. Proper treatments of the interface may at least partially recover the quenched luminescence. The implications of these results in the design and operation of organic light-emitting devices are discussed.
We present a detailed description of ultrafast electron diffraction and its applications to study photoinduced molecular dynamics at single crystal surfaces. Experimental investigations for a new design of an ultrashort pulsed laser activated electron gun ((tau) < 5 ps) for time- resolved surface analysis are described. In addition, a novel electron detection and image analysis system, as it applies specifically to time-resolved reflection high-energy electron diffraction in the multiple-shot operation, are reviewed. The total experimental temporal resolution is discussed in terms of the electron pulse width and the time difference between an electron scattered at the front edge of the sample to an electron scattered at the trailing edge of the sample.
Femtosecond time resolved two photon photoemission and above- threshold photoemission (ATP) have been used to characterize the dynamics of photoexcited electrons at single crystal Cu surfaces. The two photon photoemission studies measure nonradiative relaxation pathways of electrons near the surface, and the ATP studies demonstrate that photoemission occurs even when using light that is far below the work function. These studies provide important information regarding the extent and duration of the interaction of photoexcited electrons with surface adsorbates.
Since the 1990 discovery that porous silicon emits bright photoluminescence in the red part of the spectrum, light-emitting devices (LEDs) made of light-emitting porous silicon (LEPSi) have been demonstrated, which could be used for optical displays, sensors or optical interconnects. In this paper, we discuss our work on the optical properties of LEPSi and progress towards commercial devices. LEPSi photoluminesces not only in the red- orange, but also throughout the entire visible spectrum, from the blue to the deep red, and in the infrared, well past 1.5 micrometers . The intense blue and infrared emissions are possible only after treatments such as high temperature oxidation or low temperature vacuum annealing. These new bands have quite different properties form the usual red-orange band and their possible origins are discussed. Different LED structures are then presented and compared and the prospects for commercial devices are examined.
The progress of thermal conversion of the thin films of poly(p-xylylene-(alpha) - tetrahydrosulfonium chloride) to poly(phenylene vinylene) (PPV) was investigated in situ under ultra-high vacuum by x-ray photoelectron spectroscopy (XPS). We found that at 150 - 200 degree(s)C chlorine was completely eliminated, sulfur was less than 1%, oxygen was 10 - 15% and carbon was about 85 - 90%. One could reduce the oxygen content to 3 - 5% by heating the samples at 320 degree(s)C. The interfaces of Ca or Al with the PPV films were then investigated using XPS. The interaction between Ca and PPV was very weak. Band bending of the Ca/PPV interface was very slow indicating the formation of Schottky barrier was a slow process. In contrast, Schottky barrier formation of Al/PPV was much faster. The slow barrier formation at the metal-PPV interface may be due to the shielding by the surface oxygen impurities, part of which underwent metal oxide formation at the interface. Our results may be highly relevant to the application of sulfonium precursor derived PPVs in optoelectronic layer devices where the interface chemistry is likely to govern the device performance.
Type II photo-oxidation depends on and consumes oxygen. Several factors, including the concentration of photosensitizer and the radiation fluence rate, determine the rate of oxygen consumption in tissue undergoing Photodynamic Therapy. If the tissue capillary density is sparse, as it is in many human tumors, our calculations indicate that for cells sufficiently distant from the nearest capillary, fluence rates commonly used in PDT (50 - 200 mW/cm2) deplete 3O2 levels below those necessary for 1O2 formation. The calculations suggest that under these conditions reduced fluence rates and radiation dose fractionation should be more effective than continuous radiation at high fluence rates in producing 1O2 throughout the treated tissue volume. These predictions are supported by results obtained in vivo. The data and their interpretation have implications for PDT dosimetry and offer the possibility of improved therapeutic ratio.
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