M. Pranaitis

Second- and third-order nonlinearities of novel push-pull azobenzene polymers.

In this work, the second- and third-order nonlinear optical response of spin-deposited thin films of three different push-pull side chain azobenzene polymers is investigated by the second- and third-harmonic Maker fringes techniques using 30 ps laser pulses at a fundamental wavelength of 1064 nm. Measurements were carried out before and after aligning the chromophores by corona poling of the films, while different polarization configurations have been utilized. Strong dependence of the response upon the structure of the systems has been found, which is related to the different charge transfer within the molecules. The reported findings are compared with already published results.

Opportunities of deoxyribonucleic acid complexes composites for nonlinear optical applications

In this paper, we illustrate new functionalities for nonlinear optical applications of bio-molecular systems. This study presents DNA complex with new ionic surfactants. These surfactants enabled DNA solubility in solvents other than alcohols, like aromatic and chlorinated ones. Composites with two nonlinear optical (NLO) active dyes are subjects of the second and third harmonic generation experiments. The found effective nonlinear susceptibilities values are much higher than that for standard fused silica. We also demonstrate any influence of the surfactant on NLO properties.

Enhancement of linear and nonlinear optical properties of deoxyribonucleic acid-silica thin films doped with rhodamine

In this work, we present the linear and nonlinear optical properties of DNA as functional material, incorporated into a silica material matrix with rhodamine organic dye. We observed that even low concentration of DNA affects the aggregate behavior of the dyes in silica films. The samples with DNA showed higher transmittance and fluorescence efficiency. Moreover, the presence of DNA has been found to significantly enhance the nonlinear optical response of the systems. In this way, we prove that silica materials can provide suitable matrices for hybridization with functional molecules and can be utilized as active optical waveguide materials with enhanced nonlinear optical properties.

Charge Carrier Trapping in Organic Solar Cell Structures P3HT:PCBM

We have investigated carrier transport and trapping in blends of 6:5 wt. P3HT:PCBM that are important for the development of organic Solar cells. The devices with the inverted layer sequence and solar efficiency of 3.7% were analysed. We demonstrate that, though the fill factor of the IV characteristics is as high as 68%, carrier trapping is effectively involved in the transport phenomena. The evaluated trapping state activation energy is about 0.18 eV and their density is up to 1020 ÷ 7 × 1021 cm−3. At such high density these states may probably act as transport states, limiting carrier mobility. The results were analyzed by taking into account mobility variation according to the Gaussian disorder model as well as carrier thermal generation from traps. The mobility parameters obtained by both methods demonstrate good coincidence.

Determination of the charge trap energy distribution in conjugated polymer MDMO-PPV

The trap distribution depending on the exciting light spectral range and applied electrical field in the MDMO-PPV ([poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene) was investigated by the thermally stimulated current (TSC) technique. To ensure selective excitation of the defect states we have used the long-pass colour filters with cut-on energies from 1.77 up to 3.10 eV. In order to evaluate the trap parameters of MDMO-PPV, curve fitting was done. The analysis revealed the Gaussian distribution of the defect states; the estimated effective activation energies were about 0.22 and 0.40 eV.

Charge trapping in [poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene synthesized in different routes affected by the energetical distribution of trapping states

In this study, the thermally stimulated current measurements are used as a direct photoelectrical method to prove distribution of the trapping states in MDMO-PPV ([poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene) polymers synthesized in the “Gilch” and “Sulfinyl” routes. It was demonstrated that two traps with the Gaussian distribution of the states and effective mean activation energies of about 0.22 and 0.40 eV are prevailing in MDMO-PPV synthetized in the “gilch” route, meanwhile in the material synthesized in the “sulfinyl” route the best expressed traps appear at 0.28 eV.

Charge Carrier Mobility and Ageing of ZnPc/C60 Solar Cells

Cu and Zn Phtalocyanines (CuPc and ZnPc), and C60 are materials frequently used for organic Solar cell engineering. Their energy levels form a donor-acceptor junction, and they have high absorption coefficients and a complementary absorption for the Sun spectrum. We have investigated ageing properties of ZnPc/C60 Solar cells as they are influenced by the charge carrier mobility and variation of the potential barrier height of the ZnPc/C60 interface. The structures ITO/ZnPc/C60/C60:AOB/Al with a reasonable energy conversion efficiency of ∼1.5% were investigated. The samples were aged for 1300 hours upon illumination with blue LED, with peak emission at 475 nm, and incident light power density of 10 mW/cm2. The aged devices showed a strong and fast degradation of the short circuit current and of the fill factor after several hours followed by an almost constant behaviour of these values. The reference samples kept in the dark at the room temperature did show only very small changes in their I-V curves. Carrier mobility dependencies on electric field strength at different temperatures were measured by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. It was demonstrated that mobility values decrease during degradation as compared to the reference samples. Nevertheless only mobility changes cannot explain the observed drop of device current. The increase of the effective barrier height at the interface of ZnPc and C60 by about 0.1 eV from ∼0.55 eV up to ∼0.65 eV was observed in the aged samples. Meanwhile thermal activation energy values of the electrical conductivity grew from about 0.28 eV prior to degradation up to about 0.34 eV after ageing.

Negative Mobility Dependence in Different Regioregular Polythiophenes Revealed by the Charge Extraction by Linearly Increasing Voltage Method

Carrier mobility measurements were performed in the regioregular poly(3-hexylthiophene) (P3HT) and regioregular poly(3-octylthiophene) (P3OT) samples by the Charge Extraction by Linearly Increasing Voltage (CELIV) method. The samples were produced by the drop-casting and spin-coating methods on the Indium-Tin-Oxide (ITO) substrate and provided with Al contact at the top. We had observed the “negative” mobility dependence at low electric fields, i.e., the mobility used to increase with decreasing electric field. To our knowledge this is the first time, when such behaviour was confirmed by the CELIV method in P3OT samples, though it was reported earlier by the Time-of-Flight (TOF) in disordered materials.

Gaussian Distribution of the Charge Traps in the Energy Gap of MDMO-PPV and Its Dependence on Synthesis Route Revealed by the Thermally Stimulated Current Spectroscopy

Energetical distribution of the charge traps was investigated in MDMO-PPV ([poly-(2-methoxyl),5-(3,77dimethyloctyloxy)] paraphenylenevinylene). The thermally stimulated current spectroscopy was applied as a direct photoelectrical method immediately related with charge transport and trapping in materials with defect states. Possibility of the selective trapping states filling by the light with different spectra below and above the band gap was demonstrated. The analysis revealed the Gaussian distribution of the traps in the energy gap. The sensitivity of the technique is evidenced by the results, indicating that in MDMO-PPV synthesized by different routes – Gilch and Sulfinyl – different defect states were identified.

Temperature Dependence of the Bistable Photoconductivity of Thin DNA: PEDOT Films

Thin DNA:PEDT-PSS layers were investigated. The functionalization of DNA by PEDT-PSS rendered the material electrically active, its conductivity being about (1–5) × 10−10 Ω−1cm−1 at the room temperature. The samples remained ohmic down to 77 K. The thermal activation energy of the conductivity near the room temperature was about 0.033 eV, and it decreased under 0.014 eV below 170–180 K. The weak carrier trapping was identified by the Thermally Stimulated Current method, proving the recombination of light-generated carriers. Notably, by constant light excitation a “bistable” photoconduction below the room temperature was evidenced. The photosensitive state could be induced by the light from the spectral region from ∼ 500 nm up to ∼1000 nm, with a maximum effect in the range of 650–800 nm. A remarkable increase of the photocurrent could be observed below 145–155 K by cooling the samples. Meanwhile by heating the photosensitivity remained increased up to 235–245 K. The long characteristic relaxation times after the light excitation in this state were proportional to the relative photosensitivity of material. This indicates that such phenomenon could presumably be attributed to the light-induced changes associated with PEDT-PSS, i.e., modification of the sample material morphology and/or induced variation of carrier transport conditions.