Electroabsorption Measurements Research Articles

Degradation in blue-emitting conjugated polymer diodes due to loss of ohmic hole injection

We report studies of the evolution of the hole injection and transport characteristics of fluorene-based polymer diodes subjected to electrical stressing. Dark injection (DI) transient measurements show that the polyethylenedioxythiophene/polystyrenesulphonate (PEDOT:PSS)-topolymer contact is initially ohmic, but as stressing proceeds, the transients shift to longer times and lose their characteristic temporal profile. A comparison with time-of-flight transient photocurrent measurements led us to conclude that the DI transient is modified by a loss of ohmic injection. Electroabsorption measurements show a drastic reduction in the built-in potential from 1.4 V to 0.6 V. Device simulation shows this to be consistent with a change in the PEDOT:PSS work function, and the introduction of an interfacial resistance at the PEDOT:PSS-to-polymer contact.

Contact optimization in polymer light‐emitting diodes

AbstractTo fully exploit the properties of light‐emitting polymers (LEPs) in electroluminescent applications, it is of paramount importance to develop efficient electrical contacts. An ideal electrode is highly conductive, stable, provides a low barrier to carrier injection, and does not degrade the LEP upon contact. It is difficult to find a single homogeneous material that satisfies all of these requirements. Hence, contact optimization has often required the development of multilayer structures. In particular, indium tin oxide covered by a film of poly(ethylene‐dioxythiophene):poly(styrene sulfonic acid) {ITO/PEDOT:PSS} has become a favorite combination for the transparent anode, and heterostructures of LiF and CsF with metals (Al and Ca) have proven to be efficient electron‐injecting contacts. Here we review our progress in the understanding of the operation of light‐emitting diodes incorporating such contacts, in particular by gauging the materials' energy‐level lineup via electroabsorption measurements. Among the series of LEDs investigated, using a high‐energy‐gap blue polyfuorene polymer, CsF/Ca/Al and LiF/Ca/Al electrodes lead to the best improvements in electron injection. The most promising performance for applications, where a high luminance (∼1600 cd/m2 at 5 V) is also accompanied by a high maximum efficiency (∼3 lm/W), was obtained with LiF/Ca/Al cathodes and ITO/PEDOT:PSS anodes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2649–2664, 2003

Electric field in phthalocyanine/perylene heterojunction solar cells studied by electroabsorption and photocurrent measurements

The electric-field distribution in Au/Zn-phthalocyanine(ZnPc)/3,4,9,10-perylene-tetracarboxylbis-benzimidazole(PTCBI)/In/Al heterojunction solar cells is studied by electroabsorption and photocurrent measurements. In the PTCBI surface region near the PTCBI/In interface, a large anomalous electric field exists which is nearly bias independent and does not contribute to the photocurrent generation. This anomalous electric field exists only for the PTCBI-film thickness ⩾35 nm. The electric fields in the bulk of the PTCBI and ZnPc layers are responsible for the photocurrent generation. The bulk electric field in the PTCBI layer changes its sign at a forward bias, Vbias, of 0.3 V, while that in the ZnPc layer disappears for Vbias⩾0.5 V. The latter may indicate that holes are hardly trapped in the ZnPc layer.

Direction-dependent band nonparabolicity effects on high-field transient electron transport in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p–i–n diode provide evidence of electron velocity overshoot at fields as low as ∼130 kV/cm for transport in the c-direction of wurtzite GaN. Theoretical Monte Carlo calculations employing a full band structure indicate that at fields below ∼300 kV/cm, this velocity overshoot is associated primarily with band nonparabolicity in the Γ valley related to a negative electron effective mass rather than intervalley transfer. Similar calculations of transport in the basal plane indicate that in this case, both a higher threshold field for velocity overshoot and a lower steady-state velocity at a given field are expected.

Electronic line-up in light-emitting diodes with alkali-halide/metal cathodes

The electronic nature of metal-semiconductor contacts is a fundamental issue in the understanding of semiconductor device physics, because such contacts control charge injection, and therefore play a major role in determining the electron/hole population in the semiconductor itself. This role is particularly important for organic semiconductors as they are generally used in their pristine, undoped form. Here, we review our progress in the understanding of the energy level line-up in finished, blue-emitting, polyfluorene-based light-emitting diodes, which exploit LiF and CsF thin films in combination with Ca and Al to obtain cathodes with low injection barriers. We have used electroabsorption measurements, as they allow the noninvasive determination of the built-in potential when changing the cathode. This provides precious experimental information on the alteration of the polymer/cathode interfacial energy level line-up. The latter is found to depend strongly on the electrode work function. Thus, the Schottky–Mott model for the energy level alignment is found to be a better first-order approximation than those models where strong pinning or large interface dipoles determine the alignment (e.g., Bardeen model), except for electrodes that extensively react with the polymer, and introduce deep gap states. In addition, we show results that validate the approximation of rigid tilting of polymer energy levels with bias (for biases for which no significant injection of carriers occurs). To investigate further the consequences of the electronic line-up on device operation, we complemented the electroabsorption measurements with characterization of the emissive and transport properties of the light-emitting diodes, and confirmed that the cathodic barrier lowering in CsF/Ca/Al and LiF/Ca/Al electrodes leads to the best improvements in electron injection. We found that luminance and overall current are greatly affected by the barrier-reducing cathodes, indicating a truly bipolar transport, with comparable electron and hole currents. We also found significant indications of CsF/Ca/Al cathodes strongly reacting with the polymer, which is suggestive of CsF dissociation and diffusion in the bulk of the polymer.

Polarization-resolved electro-absorption in InAs/GaAs quantum dots in waveguide structures—modeling of size, shape and In-content

Integrating InAs self-assembled quantum dots into p–i–n (and n–i–p) waveguide structures allows for polarization resolved electro-absorption measurements of quantum dot single excitonic transitions. In this geometry, the field dependence of three electron heavy-hole and three electron light-hole transitions has been investigated. The light-hole ground state exciton is separated from the heavy-hole ground state transition by 160 meV . A zero field polarization of the heavy-hole excitons is found, which compensates at −75 kV/cm . The light hole excitons, however, do not indicate any dipole moment. In contrast to the heavy-hole transitions they show a strongly pronounced field dependence. Modeling these extensive results, the parameter space with respect to shape, size and In-content of the quantum dot turns out to be very narrow. As a key issue, the light-/heavy-hole splitting is strongly influenced by the biaxial strain, which in turn depends on the width/height ratio of the dot. The best agreement with the experimental data has been achieved by assuming a lens-shaped dot with a trumpet-shaped In-distribution.

Quantitative Analysis of Absorption and Field-Induced Absorption Changes in InGaN/GaN Quantum Wells

Results of absorption and electroabsorption measurements are reported for wurtzite InGaN/GaN quantum wells. Despite the relatively high exciton binding energy of 25 meV in GaN no pronounced excitonic features could be observed and the absorption spectra are very similar to those obtained for bulk material. The measured field-induced absorption changes are as large as 7000 cm -1 below and 20000 cm -1 above the band edge. A quantitative analysis of electroabsorption data allows the precise determination of the strong internal fields in InGaN/GaN heterostructures originating from strain-induced polarization and differences in the spontaneous polarization. The fields measured on functioning diodes vary between 1.1 MV cm -1 and 1.4 MV cm -1 for indium concentrations in the InGaN quantum wells ranging from about 7 to 9%.

Quantitative analysis of the polarization fields and absorption changes in InGaN/GaN quantum wells with electroabsorption spectroscopy

Electroabsorption measurements are reported for wurtzite InGaN/GaN quantum wells. The electroabsorption technique allows exact quantitative analysis of absorption and absorption changes in InGaN quantum wells and barrier layers, with recorded field-induced absorption changes as large as 7000 cm−1 below and almost 20000 cm−1 above the band edge. The technique thus allows precise determination of the strong internal fields that originate from strain-induced polarization and differences in spontaneous polarization. The fields measured on functioning diodes vary between 1.1 and 1.4 MV/cm for indium concentrations in InGaN quantum wells ranging from 7% to 9%.

Femtosecond pump and probe spectroscopy of optical nonlinearities in an InGaN/GaN heterostructure

ABSTRACTThe magnitudes and evolutions of two photoinduced absorption nonlinearities, absorption bleaching and field screening, were compared and investigated by employing electroabsorption and femtosecond pump-probe spectroscopy in a biased InGaN/GaN p-i-n double heterostructure. Steady state electroabsorption measurements indicate the field induced absorption coefficient changes in this structure are caused by the Franz-Keldysh effect. The temporal resolution of the absorption bleaching spectra suggests that the photoinduced carriers rapidly relaxed to the InGaN band edge within several picoseconds. As the applied reverse bias field was increased, the transition of the differential absorption spectral signature from the signature for absorption bleaching to the signature for field screening was observed. The magnitude of the change in absorption due to photoinduced carrier screening of the applied field is quantified and compared to absorption bleaching.

Band Structure Effects on the Transient Electron Velocity Overshoot in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p-i-n photodiode have been used to study the transient electron velocity overshoot for transport in the c-direction in wurtzite GaN. The velocity overshoot increases with electric field up to ∼320 kV/cm, at which field a peak velocity of 7.25 x 10 7 cm/s is attained within the first 200 fs after photoexcitation. However, theoretical Monte Carlo calculations incorporating a GaN full-zone band structure show that the majority of electrons do not attain sufficient energy to effect intervalley transfer until they are subjected to higher fields (> 325 kV/cm). Insight into this behavior can be gleaned from the band nonparabolicity deduced from the constant energy surfaces in the Γ valley, which shows that the effective mass in the c-direction can be viewed as becoming larger at high k values.

Electroabsorption Spectroscopy ? Direct Determination of the Strong Piezoelectric Field in InGaN/GaN Heterostructure Diodes

We report on room temperature electroabsorption measurements on (In)GaN heterostructure p-i-n diodes. This technique allows a precise determination of the strong changes of internal fields at the InGaN/GaN hetero interface due to differences in the spontaneous polarisation and strain-induced piezoelectric fields. The measured fields vary between 1 and 1.4 MV/cm for thin InGaN layers with an In content ranging from approximately 7% to 9%. Absorption spectroscopy is also well suited to investigate the optical properties of GaN layers. In contrast to Si-doped or intrinsic material Mg-doped GaN films reveal a pronounced absorption tail below the band edge with absorption coefficients of about 30 cm -1 at typical emission wavelengths (λ 400 nm) of InGaN laser diodes.

Electroabsorption Spectroscopy — Direct Determination of the Strong Piezoelectric Field in InGaN/GaN Heterostructure Diodes

We report on room temperature electroabsorption measurements on (In)GaN heterostructure p-i-n diodes. This technique allows a precise determination of the strong changes of internal fields at the InGaN/GaN hetero interface due to differences in the spontaneous polarisation and strain-induced piezoelectric fields. The measured fields vary between 1 and 1.4 MV/cm for thin InGaN layers with an In content ranging from approximately 7% to 9%. Absorption spectroscopy is also well suited to investigate the optical properties of GaN layers. In contrast to Si-doped or intrinsic material Mg-doped GaN films reveal a pronounced absorption tail below the band edge with absorption coefficients of about 30 cm -1 at typical emission wavelengths (λ 400 nm) of InGaN laser diodes.

Characterization of Chiral H and J Aggregates of Cyanine Dyes Formed by DNA Templating Using Stark and Fluorescence Spectroscopies

A series of studies is presented to characterize the photophysical properties of a novel type of aggregate formed by the spontaneous noncovalent assembly of numerous cofacial dimers of cyanine dyes (DiSC3+(5)) to the minor groove of poly(dI−dC) DNA. The dimensions of these helical aggregates, first synthesized and characterized by Armitage and co-workers (J. Am. Chem. Soc. 2000, 122, 9977−9986), are restricted to the width of the dye dimer because of steric constraints in the minor groove, though the length of the aggregate can extend essentially for the full length of the DNA template. These unique species exhibit both H- and J-type absorption bands that are shifted from the absorption maximum of the monomeric dye by +1650 and −1275 cm-1, respectively, because of the stacking interactions between the dyes composing the dimers. Additional splittings are seen because of head-to-head interactions between adjacent dye dimers. Here, we present the low-temperature (77 K) absorption, fluorescence, and electroabsorption spectra of these aggregates as well as measurements of the fluorescence lifetime of the monomer and of the J-type emission at 10 °C. The electroabsorption measurements yield values of the average difference polarizability on excitation, 〈Δα〉, for the H and J bands of −74 and −34 Å3, respectively. These are between 2 and 6 times larger than that of the monomer. Both bands exhibit similar values for the difference dipole moment on excitation of between 0.6 and 0.7 D that are somewhat smaller than that of the monomer (1.1 D). The absorption and fluorescence experiments show that the line width of the J band is ∼4 times narrower than the experimental fwhm of the DiSC3+(5) monomer while the fluorescence decay of the aggregate is roughly a factor of 2 faster. Implications of all of these measurements for determining the number of dyes that are excited cooperatively upon light absorption are discussed.

Optical pumping in ferromagnet-semiconductor heterostructures: Magneto-optics and spin transport

Epitaxial ferromagnetic metal - semiconductor heterostructures are investigated using polarization-dependent electroabsorption measurements on GaAs p-type and n-type Schottky diodes with embedded In1-xGaxAs quantum wells. We have conducted studies as a function of photon energy, bias voltage, magnetic field, and excitation geometry. For optical pumping with circularly polarized light at energies above the band edge of GaAs, photocurrents with spin polarizations on the order of 1 % flow from the semiconductor to the ferromagnet under reverse bias. For optical pumping at normal incidence, this polarization may be enhanced significantly by resonant excitation at the quantum well ground-state. Measurements in a side-pumping geometry, in which the ferromagnet can be saturated in very low magnetic fields, show hysteresis that is also consistent with spin-dependent transport. Magneto-optical effects that influence these measurements are discussed.

Development of infrared electroabsorption spectroscopy for liquids

An infrared spectroscopic system has been constructed that is specifically designed for electroabsorption measurements of liquids. The system can detect electric-field-induced absorbance changes as small as one part in a million, two orders of magnitude better than the conventional difference spectrum method using FT-IR. Electroabsorption spectra of liquid 1,2-dichloroethane have been measured. The observed absorbance changes have been explained in terms of two dominant mechanisms; the change in the trans–gauche equilibrium, and the orientational polarization of the gauche conformer. It is demonstrated that infrared electroabsorption spectroscopy provides useful information on otherwise inaccessible electric properties of molecules in liquids.

Optical and photovoltaic properties of indium selenide thin films prepared by van der Waals epitaxy

Indium selenide thin films have been grown on p-type gallium selenide single crystal substrates by van der Waals epitaxy. The use of two crucibles in the growth process has resulted in indium selenide films with physical properties closer to these of bulk indium selenide than those prepared by other techniques. The optical properties of the films have been studied by electroabsorption measurements. The band gap and its temperature dependence are very close to those of indium selenide single crystals. The width of the fundamental transition, even if larger than that of the pure single crystal material, decreases monotonously with temperature. Exciton peaks are not observed even at low temperature, which reveals that these layers still contain a large defect concentration. The current–voltage characteristic of indium selenide thin film devices was measured under simulated AM2 conditions. The solar conversion efficiency of these devices is lower than 0.6%. The high concentration of defects reduces the diffusion length of minority carriers down to values round to 0.2 μm.

Conformational effects on optical charge transfer in the emeraldine base form of polyaniline from electroabsorption measurements and semiempirical calculations

The electroabsorption (Stark) spectrum of the emeraldine base form of polyaniline (EB) in a polymethyl methacrylate (PMMA) matrix at room temperature is presented. For the lowest-energy (1.98 eV) band, the absolute value of the change in dipole moment on excitation (|Δμ⃗|) is 6.1 D and the trace of the change in polarizability (Tr Δα⇊) is 200 Å3. Similar values were obtained for the corresponding electronic transition of a five-ring oligomer of EB that is symmetrically terminated with phenyl rings, also in a PMMA matrix. In order to understand the origin of the substantial value of |Δμ⃗| measured for these nominally symmetric species, a series of calculations were performed in which the geometries of model oligomers were varied in a systematic fashion and resulting effects on the values of |Δμ⃗| were determined. Numerous low-energy conformers were identified by this method having substantial values of |Δμ⃗|, suggesting that the large |Δμ⃗| measured experimentally results from conformational heterogeneity. Our findings suggest that Stark spectroscopy is a useful probe of structural disorder in this technologically important polymer in the solid state.

Time-resolved electroabsorption measurement of the transient electron velocity overshoot in GaN

A femtosecond time-resolved electroabsorption technique employing an AlGaN/GaN heterojunction p–i–n diode with a p-type AlGaN window layer and a semitransparent p contact has been used to measure the transient electron velocity overshoot in GaN. A peak transient electron velocity of 7.25×107 cm/s within the first 200 fs after photoexcitation has been observed at a field of 320 kV/cm. The increase in electron transit time across the device with increasing field beyond 320 kV/cm provides experimental evidence for a negative differential resistivity region of the steady-state velocity-field characteristic in this high field range.

A non-ideal bias dependence of the inner electric field in Au/Zn-phthalocyanine/Al photovoltaic cells studied by electroabsorption and photocurrent measurements

Electroabsorption (EA) and photocurrent measurements are done for Au/Zn-phthalocyanine (ZnPc)/Al sandwich cells in order to study the bias dependence of the inner electric field in the ZnPc layer. The bias dependence of the EA signal intensity and photocurrent is not explained by the conventional models for the inner electric field in the following features. 1) The EA intensity and the photocurrent disappear at different bias voltages, i.e. 1.24 and 0.47 V, respectively. 2) The EA intensity does not have a simple linear dependence on the bias voltage. 3) The photocurrent is kept zero with the bias voltage higher than 0.47 V. The results are consistently explained by a model in which an interfacial layer and surface states exist at the ZnPc/Al junction.

Electroabsorption measurements and built-in potentials in amorphous silicon–germanium solar cells

We report electromodulated reflectance spectra in n-i-p solar cells with hydrogenated amorphous silicon–germanium alloy absorber layers. At lower photon energies the spectra are determined by bulk electroabsorption, and exhibit peaks near the optical gap of the absorber layers. Voltage scaling of the electroabsorption spectra indicate a built-in potential of Vbi=1.17 V in cells with absorber layer band gaps of 1.50 eV; in conjunction with earlier work, this value argues against a systematic decline in Vbi with an absorber layer band gap. At higher photon energies the spectra are due to direct electroreflectance; the voltage scaling was consistent with model predictions for the electric field at the interface of the p-type and absorber layers.