Electroabsorption Studies Research Articles

Electroabsorption as a Probe of Electrons Injected into Nanocrystalline SnO2/TiO2 Core/Shell Thin Films

Core/shell nanoparticles comprised of a SnO2 core and a TiO2 shell, SnO2/TiO2, present in a mesoporous thin film, are of practical interest for applications in dye-sensitized water splitting as they outperform sensitized materials based on SnO2 or TiO2 (anatase or rutile polymorphs) for the water oxidation half-reaction. Here we report electroabsorption studies designed to quantify the surface electric field and the ability of the shell to screen an electric field from a surface anchored dye. The metal-to-ligand charge transfer (MLCT) absorbance of [Ru(bpy)2(4,4′-(PO3H2)2-2,2′-bipyridine)](PF6)2, abbreviated RuP, was indeed sensitive to electron injection into the oxide support. When a fixed charge of 1 mC was injected, the electric field magnitude (in MV/cm) increased in the order 0.15 TiO2 (rutile) < 0.35 TiO2 (anatase) ≪ 1.4 SnO2 (rutile). This order tracks the reported bulk dielectric constants of the oxide materials. Comparative studies with SnO2/TiO2 core/shell materials, fabricated by atomic layer deposition (ALD) of a TiO2 shell on a mesoporous SnO2 thin film, diminished the field strength by about a factor of 4 to values less than or about equal to that measured for anatase TiO2. Heat treating some SnO2/TiO2 core/shell materials at 450 °C resulted in a crystalline rutile TiO2 shell that screened the field most effectively, more so even than rutile TiO2 alone. Taken together, the data indicate that injected electrons reside within the SnO2 core region.

Perovskite solar cells reveal excitonic optical transitions

Electroabsorption studies show an excitonic signature based on bound electron-hole pairs.

Electroabsorption studies of organic p-i-n solar cells: evaluating the built-in voltage

ABSTRACTWe investigate the built-in voltage in organic bulk heterojunction solar cells using electroabsorption spectroscopy based on the Stark effect, i.e. the variation of the absorption energies of a material caused by an electric field. Due to spectral contributions of permanent dipoles, a novel approach for evaluating the EA spectra is required. We use a fitting routine analyzing a broad spectral range instead of using only a single wavelength. A reliable quantitative determination of the built-in voltage is achieved.

Electroabsorption studies of organic p-i-n solar cells: Increase of the built-in voltage by higher doping concentration in the hole transport layer

The built-in voltage in solar cells has a significant influence on the extraction of photogenerated charge carriers. For small molecule organic solar cells based on the p-i-n structure, we investigate the dependence of the built-in voltage on the work function of both the hole transport layer and the electrode material. The model system investigated here consists of a planar heterojunction with N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) as donor and Buckminster Fullerene (C60) as acceptor material. A higher concentration of the dopant C60F36 in the hole transport layer induces a shift of the work function towards the transport level. The resulting increase of the built-in voltage is studied using electroabsorption spectroscopy, measuring the change in absorption (Stark effect) caused by an externally applied electric field. An evaluation of these electroabsorption spectra as a function of the applied DC voltage enables the direct measurement of the built-in voltage. It is also shown that an increased built-in voltage does lead to a larger short-circuit current as well as a larger fill factor.

Synthesis, Characterization, and Surface Initiated Polymerization of Carbazole Functionalized Isocyanides

We describe the design and synthesis of carbazole functionalized isocyanides and the detailed investigation of their properties. Characterization by solid state NMR, CD, and IR spectroscopic techniques reveals that the polymer has a well-defined helical architecture. Surface-initiated polymerization of the isocyanide monomers onto various surfaces was carried out to give polymer brushes up to 150 nm thick. Insights into the electronic properties of the materials were obtained by Kelvin probe force microscopy (KPFM) and electroabsorption studies

Differential electroabsorption studies of semiconductors and semiconductor heterostructures

AbstractElectroabsorption spectra of high mobility semiconductors and quantum wells are presented and compared with non‐perturbative theoretical models. Since even moderate fields mix eigenstates in these materials very strongly a differential technique is applied, where a dc voltage defines the field in the sample which then is altered by a small modulation voltage. The response to that modulation is generated from the most sensitive part of an electronic state, the envelope function, which facilitates the analysis of the spectral lineshape. The difference of the response of delocalized electrons in bulk semiconductors and spatially confined electrons will be elucidated, including the intermediate case of 0.1 µm‐thick single crystals and field‐localisation of electrons in a superlattice. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Organic thin films based on a dicyanovinyl-quaterthiophene: Influence of electrode configuration on third-order nonlinear optical properties measured by electroabsorption spectroscopy

Electroabsorption (EA) studies at room temperature on organic thin films based on a dicyanovinyl-quaterthiophene 4T-V(CN) 2 are reported. An electric field modulation is applied to the samples for two different electrode geometries, i.e. sandwich and coplanar versus the organic layer. Changes in optical absorption coefficient of 4T-V(CN) 2 based thin films are measured and analyzed to determine the character of the optical transition in the visible range (400–800 nm). Depending on the experimental electrode configuration, magnitude of electroabsorption responses are different, possibly due to different distribution of the externally applied electric field. The results indicate a higher resolution of EA response for the sandwich electrode configuration and confirm the charge transfer exciton character of 4T-V(CN) 2 in contrast to the unsubstituted quaterthiophene 4T. Finally, a third-order nonlinear susceptibility χ (3) (− ω; ω, 0, 0) of 16 × 10 −12 e.s.u. is obtained.

Electroabsorption studies of structurally modified fullerene thin films

The excited-state properties of C 60 have been extensively studied in recent years through a number of processes and a good understanding of the active processes has begun to emerge. However, a number of key questions remain about the nature of the inter- and intra- molecular excited states of C 60. The application of Electroabsorption (EA) spectroscopy to the study of the charge transfer characteristics of C 60 has been shown to be an extremely valuable probe of these excited states. However, the effect of controlling the relative contributions from inter- and intra-molecular interactions has not been, thoroughly investigated. This work hopes to extend the knowledge base by monitoring the changes in the electroabsorption spectrum of fullerene thin films which are subjected to thermal annealing, a processes known to modify the electronic properties of solid state C 60. It has been shown by comparison that while there is a large pronounced change in the absorption spectrum of annealed fullerene films, there is little or no change in the electroabsorption spectrum.

Comparative electroabsorption studies of organic and inorganic solids

Spectral changes induced by moderate electric fields provide detailed insight into the electronic states of organic and inorganic solids. Although the basic effects, Stark effect and Franz–Keldysh effect, are the same in both types of material, the electroabsorption spectra vary strongly in size and spectral lineshape due to competing interactions. The large variance of the effects is demonstrated by representative examples of high mobility semiconductors, quantum wells, π -conjugated polymers, and charge transfer transitions in single crystals, disordered films and a double-quantum well. It is shown that only high-quality samples reveal the quantum mechanics of field-induced effects which are very sensitive to disorder.

A high energy exciton in polydiacetylene chains, involving electrons localized on the triple bond

UV spectra of polydiacetylene (PDA) chains in their monomer single crystal matrix show an absorption near 4 eV and above, predominantly polarized perpendicular to the chain direction. It is shown by combining absorption and electroabsorption studies that the spectrum corresponds to an excitonic transition. This exciton has polarizability and binding energy comparable to those of the well-known exciton near 1.9 eV in PDAs, but an oscillator strength smaller by more than two orders of magnitude. It is assigned to the excitation of a π electron localized on an acetylenic bond to the well delocalized π* states. The transition gains intensity via resonant coupling to states of the continuum associated to the 1.9 eV exciton.

Bandgap and exciton binding energies in lead-iodide-based natural quantum-well crystals

We have performed optical absorption and electroabsorption studies on the lead-iodide-based natural quantum-well perovskite-type crystals with different well width (C6H13NH3)2(CH3NH3)m21PbmI3mþ1. With decreasing well thickness, m; the resonance energies of the lowest-energy excitons shift to higher energy due to the increase of the bandgap. The binding energies and oscillator strengths of the excitons drastically increase due to the spatial confinement and image charge effect with decreasing m: The bandgap of (C6H13NH3)2(CH3NH3)m21- PbmI3mþ1 (m ≥ 2) can be reproduced by the effective-mass approximations, while the effective-mass approach is not valid for (C6H13NH3)2PbI4 (m = 1).

Electroabsorption and photoconductivity studies of the Frenkel excitons and negative polarons in poly(2,5-pyridinediyl)

We present electroabsorption and photocurrent measurements on disordered and oriented films of the conjugated polymer poly(2,5-pyridinediyl) (PPY). The electroabsorption spectrum is dominated by a Stark-red-shift of the first allowed optical transition at 3.2 eV and by the emergence of a normally one-photon forbidden state at 3.7 eV. The spectrum was successfully fitted to a linear combination of the linear absorption spectrum and its first and second derivative revealing the transition at 3.2 eV to be due to Frenkel excitons. Polarized EA spectroscopy on oriented films allow us to separate the parallel and perpendicular components (with respect to the polymer chain axis) of the first allowed optical transition. Analysis of the spectra provide an indication for the existence of a small number of delocalized Wannier-like excitons photogenerated by light polarized perpendicular to the polymer chains. Photoconductivity measurements have been carried out on ITO/PPY/Au (semitransparent) sandwich cells using the four possible directions of the applied electric field and illumination directions. Analysis of the results shows that the photocurrent originates from the dissociation of excitons, which are photogenerated inside the polymer near the negative electrode and evidence is given for the electron transporting properties of PPY.

LiF/Al cathodes and the effect of LiF thickness on the device characteristics and built-in potential of polymer light-emitting diodes

We report the characteristics of a series of polymer light-emitting diodes, fabricated with LiF/Al cathodes and differing only by the thickness of the LiF interlayer (0 nm⩽d⩽11 nm). Electroabsorption studies of the internal electrostatic potential give direct evidence of a sizable reduction of the cathodic barrier height brought about by the LiF films. These results also correlate with photoemission experiments [S. E. Shaheen, G. E. Jabbour, M. M. Morrell, Y. Kawabe, B. Kippelen, N. Peyghambarian, M. F. Nabor, R. Schlaf, E. A. Mash, and N. R. Armstrong, J. Appl. Phys. 84, 2324 (1998)] and with the electroluminescence performance of the diodes.

Electroabsorption studies of phthalocyanine/perylene solar cells

We report electroabsorption (EA) studies of electric fields in bilayer molecular organic solar cells made from zinc phthalocyanine (ZnPc) and a methyl substituted perylene pigment (MPP). We have detected an electric field at the metal/organic interface which is sensitive to the external DC bias. The interface field has a different spectral signature from that of the bulk of the two layers, which we attribute to interface species such as charge transfer-induced dipoles. The electric field is proportional to the applied bias in devices containing only ZnPc or MPP, but rectifying behavior is observed in the bilayer solar cell.

Evidence of band bending observed by electroabsorption studies in polymer light emitting device with ionomer/Al or LiF/Al cathode

We report electroabsorption studies of indium–tin–oxide (ITO)/poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene] insulating layer/Al light-emitting devices. An ionomer and LiF were used as the insulating layer. The internal electric field was nullified at the same bias voltage of 0.6 V for different cathodes of Al, ionomer/Al, and LiF/Al. The bias voltage is close to the work-function difference between ITO and Al. Average electric field in the emitting layer was reduced by inserting the insulating layers. On the contrary, current density was increased significantly. These observations suggest band bending in the devices with ionomer/Al or LiF/Al cathode.

Comparative absorption, electroabsorption and electrochemical studies of intervalence electron transfer and electronic coupling in cyanide-bridged bimetallic systems: ancillary ligand effects

Electroabsorption or Stark spectroscopy has been used to evaluate the systems (NC) 5M II–CN–Ru III(NH 3) 5 1− and (NC) 5M II–CN–Ru III(NH 3) 4py 1−, where M II=Fe II or Ru II. When a pyridine ligand is present in the axial position on the Ru III acceptor, the effective optical electron transfer distance – as measured by the change in dipole moment, |Δ μ| – is increased by more than 35% relative to the ammine substituted counterpart. Comparison of the charge transfer distances to the crystal structure of Na[(CN) 5Fe–CN–Ru(NH 3) 4py] · 6H 2O reveals that the Stark derived distances are ∼50% to ∼90% of the geometric separation of the metal centers. The differences result in an upward revision in the Hush delocalization parameter, c b 2 , and of the electronic coupling matrix element, H ab , relative to those parameters obtained exclusively from electronic absorption measurements. The revised parameters are compared to those, which are obtained via electrochemical techniques and found to be in only fair agreement. We conclude that the absorption/electroabsorption analysis likely yields a more reliable set of mixing and coupling parameters.

Electroabsorption studies on quasi-three-dimensional and quasi-two-dimensional excitons in (Zn,Cd)Se/Zn(S,Se) structures

The electro-optical properties of (Zn,Cd)Se/Zn(S,Se) quantum well structures grown on GaAs substrates have been studied with differential electroabsorption spectroscopy at room temperature and compared to model calculations. (Zn,Cd)Se wells of 20, 10, and 4×5 nm are investigated, corresponding to well widths of four, two, and one times the exciton Bohr radius in this material system, respectively. We observe the quantum confined Stark effect for the 4×5 nm sample and find a Stark shift of 18 meV in the heavy-hole exciton peak for an electric field change from 82 to 175 kV/cm. In contrast, the 10 nm sample shows a rather weak and more Franz–Keldysh-like signal. We show that the 20-nm-thick quantum-well sample behaves like bulk material, i.e., the electro-absorption signal is well described by Franz–Keldysh oscillations.

Optical studies of electric fields in poly(2-methoxy-5-ethyl (2′-hexyloxy) para-phenylene vinylene) light-emitting diodes

We report electroabsorption studies of poly(2-methoxy-5-ethyl(2′-hexyloxy) para-phenylene vinylene) light-emitting diodes. An electric field develops during operation which opposes the field of the applied bias. The counter field builds up within 5 s of turning on the device, increases in magnitude with the operating voltage, and decays exponentially with a time constant between 15 and 32 s. We attribute the counter field to bulk carrier traps and discuss its relevance to the increase of the turn-on voltage as organic light-emitting diodes degrade.

Third Order Nonlinear Optical Properties of Polycrystalline Octithiophene Thin Films Studied by Electroabsorption Spectroscopy

Electroabsorption (EA) studies at room temperature on polycrystalline thin films of α-octithiophene (8T), the longest non-substituted model oligomer of polythiophene, are reported. The EA spectra obtained on isotropic 8T films deposited on top of coplanar electrodes exhibit several intense and narrow peaks in the absorption region and on the absorption edge. They are compared with the derivatives of absorption spectrum, and it is shown that the excitation in the longer wavelength absorption presents a Frenkel exciton character. Third-order nonlinear optical susceptibility x (3) (ωω, 0, 0) is evaluated to be in the order of 10−10 esu at its maximum.

Aspects of Intervalence Charge Transfer in Cyanide-Bridged Systems: Modulated Electric Field Assessment of Distances, Polarizability Changes, and Anticipated First Hyperpolarizability Characteristics

Electroabsorption studies of the systems (CN)5MII−CN−M‘III(NH3)51- (M = Fe, Ru, Os; M‘ = Ru, Os) reveal that the metal-to-metal or intervalence charge-transfer transitions are associated with a change in dipole moment, |Δμ|, ranging from 11 to 17 D. This change corresponds to a charge-transfer distance of ca. one-half to two-thirds of the geometric separation between the donor and acceptor metals. This result has consequences for electron transfer parameters, where electronic coupling energies are now upwardly revised to as high as 3000 cm-1. The result is also pertinent in the context of nonlinear optics, where Δμ can be utilized in a two level model to estimate wavelength-dependent molecular first hyperpolarizabilities. The change in polarizability (Δα) accompanying the optical intervalence transitions varies from −10 to +310 A3. Comparing the experimental results to a simple two state model suggests that the two state picture is seriously deficient for these systems and that a multitude of available ad...