Raoul Schroeder

Absorption and photocurrent properties of thin ZnS films formed by pulsed-laser deposition on quartz

The absorption and photocurrent properties of thin film ZnS on quartz glass formed by pulsed-laser deposition have been studied experimentally and theoretically at room temperature. Using the Lorentzian function to describe the exciton density of states, we show that the absorption is strongly influenced by excitonic formation. The theory for the absorption, however, does not describe the PC spectra of the film since the exciton remains electrically neutral up to fields of 1 kV/cm due to the high binding energy of 36 meV. Therefore, the fundamental absorption according to density of states and Urbach rule determines the shape of the photocurrent spectrum.

Green single- and two-photon gap emission of thin-film CdS formed by infrared pulsed-laser deposition on glass

Thin-film CdS is formed by infrared pulsed-laser deposition (PLD) on glass. The film is excited with continuous-wave (CW) blue laser emission at 457.9 nm and with ultra-fast laser pulses of 200 fs at 801 nm. Both excitations cause green gap emission in the range from 2.43 to 2.45 eV at room temperature. Additionally, blue excitation evokes some sub-gap emission. The principle of detailed balance is used to describe the shape of the two-photon spectrum by modeling the absorption coefficient by the density of states and Urbachs rule. Spectra measured through the glass substrate are shifted 40 meV to lower energies with respect to the emission emitted from the front side of the film. Using Beers law, it is shown that the shift is caused by stronger absorption at the glass/CdS interface. This is confirmed with the lack of geometry dependence of an interface-free 50-/spl mu/m CdS platelet. The results show that two-photon spectroscopy is useful for revealing the interfacial absorption effects and PLD CdS exhibits outstanding emission properties, which are important for green light-emitting device fabrication.

High-electric-field photocurrent in thin-film ZnS formed by pulsed-laser deposition

Photocurrent spectra of thin-film ZnS on glass fabricated by pulsed-laser deposition have been studied employing electric fields up to 20 kV/cm. The spectra show a shift towards lower energy at and beyond 10 kV/cm. By modeling the film absorption with the density of states and the Urbach rule, it is shown that, without the involvement of the Franz–Keldysh effect and excitonic transitions, the slope of the Urbach tail depends on the electric field, owing to impurity ionization.

The influence of self-absorption on the photoluminescence of thin film CdS demonstrated by two-photon absorption

By means of two-photon excited photoluminescence, we demonstrate the influence of self-absorption on the emission properties of thin (1.5 microm) film CdS formed by laser ablation. The excitation of the sample is performed with 200 fs pulses at 804 nm (1.54 eV). The photoluminescence spectrum takes the form of a single peak centered at 510 nm (2.43 eV) at 300 K. The spectrum is shifted about 45 meV to lower energies with respect to the photoluminescence excited by one photon absorption. By fitting the photoluminescence spectra with the Roosbroeck-Shockley relation and Urbachs rule, it is shown by Beers law that the shift is caused by self-absorption. The results further provide evidence of low impurity concentration and excellent surface quality. They also confirm the outstanding optical properties of thin film CdS formed by pulsed-laser deposition and suggest the application of the films for effective up-conversation materials in ultra-fast experiments.

Preparation of thin film GaAs on glass by pulsed-laser deposition

One of the most straightforward methods possible is presented and investigated to form thin film GaAs. The film was deposited on unheated glass in vacuum (10-6 Torr) by the ablation from a GaAs wafer with the emission of a pulsed Nd:YAG laser (532 nm, 6 ns, 10 Hz). The photoluminescence, photocurrent, transmission and micro-Raman measurements of the films demonstrate that films with promising optoelectronic properties have been formed. Most importantly, from the viewpoint of light emitting and optoelectronic device production, the films show photoluminescence of comparable intensity with the bulk material without emissions owing to impurities, although the films show a rather flat absorption edge which indicates tail states. The observed photocurrent was in the μA/W range driven by rather moderate electric fields on the order of 100 V/cm. Concerning the material quality, the films have an extremely smooth surface as demonstrated with scanning electron microscopy. Grown GaAs films on glass substrates were amorphous evidenced by X-ray diffraction measurements, however, micro-Raman measurements showed crystalline phonon modes, suggesting that localized crystalline structure might co-exist in amorphous GaAs films.

Excitation density and photoluminescence studies of polyfluorene excited by two-photon absorption

Highly efficient photoluminescent organic materials are excellent alternatives to inorganic crystals for non-linear optics applications. One candidate, polyfluorene with an absorption onset of about 3 eV, was studied in this paper by means of transmission and photoluminescence measurements with an ultra-fast laser source at 1.54 eV with 250 fs pulses. We studied the two-photon absorption and emission properties of polyfluorene as a function of the concentration in toluene solvent at 300 K. It is demonstrated that both the two-photon absorption and the associated emission exhibit the same saturation intensity, which does not depend on the concentration. It is shown that although we achieve excitation rates of 1% to 10% of all repetition units, the excited units do not interfere before luminescent recombination occurs.

Photovoltaic cells based on ionically self-assembled nanostructures

We use the technique of ionically self-assembled monolayers (ISAMs) to produce photovoltaic devices of well-controlled thickness and composition. The ISAM nanostructure fabrication method simply involves the alternate dipping of a charged substrate into aqueous cationic and anionic solutions at room temperature. We have employed several approaches to combine the tetrahydrothiophenium precursor of poly(para-phenylene-vinylene) (PPV) with fullerenes and other organic materials. We apply modulation spectroscopy for the electro-optical characterization of the ISAM-devices. The modulation frequency dependence of the photocurrent can be assigned to the influence of trapped charges taking part in the photovoltaic process.

Control of excited state dynamics in ionically self-assembled monolayers of conjugated molecules

We report the fabrication of thin organic layers and photovoltaic devices made from them. Building thin layers of organic materials via the method of ionically self-assembled monolayers (ISAM) provides control over the layer thickness and composition of multilayer structures on a nanometer scale. This allows to accurately dope a photoluminescent host material with energy or charge accepting guests, changing the emissive character of the pure photoluminescent host film to a predominantly non-emissive, charge generating structure. We show that by varying the concentration of the guest Copper phthalocyanine and C60(OH)24 in poly-(para-phenylene-vinylene) we can measure the energy migration as well as dissociation of the exciton and can determine the lifetime and the diffusion radius of the exciton. Increasing the number of dopands in the host material, the photoluminescence emission spectra shift and decrease in intensity reflecting a decrease in the number of excitons transferring to neighboring chains or conjugation segments. For high dopand concentrations the recombination of excitons only happens on the same chain as the generation. Building a device to achieve the optimal guest/host ratio for optimal exciton dissociation is one important step in the design of high efficiency photovoltaic devices.

Analysis of single- and two-photon-excited green emission spectra of thin-film cadmium sulfide

Employing nanosecond and femtosecond laser pulses at 355 nm (3.49 eV) and 768 nm (1.61 eV), green single- and two-photon-excited interband emission at 2.412 and 2.381 eV of thin-film cadmium sulfide (CdS) on glass has been measured at room temperature. The spectra are fitted and analyzed by a theory based on the detailed balance principle. The results demonstrate that the interband emission is independent of the mode of excitation and that the energy shift of the spectra is solely due to self-absorption of the emission evoked by two-photon excitation. The work also addresses energy dissipation in thin-film CdS excited beyond the Mott transition.

Photocarrier generation quantum yield for ionically self-assembled monolayers

We have fabricated photovoltaic cells from organic donor/acceptor couples arranged as ionically self assembled monolayers. Making use of the morphological control on the nanometer level we were able to study the influence of the device structure and layer composition on the quantum yield of charge carrier photogeneration. The photoluminescence quenching and the photocurrent spectra reveal sample composition, charge carrier photogeneration and transport properties independently.