V. Janonis

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.

Correlation between temperature activation of charge-carrier generation efficiency and hole mobility in small-molecule donor materials.

In organic solar cells, free charge carriers are generated at the interface between an electron-donating and an electron-accepting material. The detailed mechanisms of the generation of free charge carriers are still under discussion. In this work, we investigate the influence of temperature on the generation efficiency of free charge carriers in blends of dicyanovinyl substituted oligothiophene (DCVnT) molecules and C60 by quasi-steady-state photoinduced absorption (PIA) measurements. The observed positive temperature dependence of charge-carrier generation can be directly correlated to the charge-transport behavior. The determined activation energy scales inversely with the hole mobility for all investigated DCVnT derivatives, suggesting higher dissociation probability of bound interfacial charge pairs at high mobility. Furthermore, the energetic disorder parameter, σ, determined by CELIV (charge extraction by linearly increasing voltage) measurements for a DCV6T derivative, matches the activation energy from the PIA measurements. In conclusion, these results underline the need for high-mobility donor materials for optimal charge-pair dissociation in organic solar cells.

Influence of cation-vacancy imperfection on the electrical and photoelectric properties of the Cu1 − xZnxInS2 alloy

The growth technology of single crystals of Cu1 − xZnxInS2 alloys (x = 0–12) of n-type conductivity is developed. The formation mechanism of the alloy is investigated by X-ray structural analysis. It is shown that the single crystals have chalcopyrite structure, and the unit-cell parameters depend on the alloy composition. The temperature dependence of the electrical conductivity in the temperature range T = 27–300 K and the spectral distribution of the photoconductivity at T ≈ 30 K are investigated. Induced photoconductivity is found for CuInS2-ZnIn2S4 with a content of ∼8 and ∼12 mol % ZnIn2S4 and thermally stimulated currents are investigated.

Specific features of the low-temperature conductivity and photoconductivity of CuInSe2-ZnIn2Se4 alloys

Abstractn-type CuInSe2-ZnIn2Se4 alloy single crystals are grown by the horizontal variant of the Bridgman method. The slight temperature dependence of the conductivity, high electron concentration, and the low photoconductivity of single crystals containing a low (5–10 mol %) fraction of ZnIn2Se4 are indicative of the nearly degenerate state of the crystals. It is established that, in the CuInSe2-ZnIn2Se4 single crystals containing 15 and 20 mol % of ZnIn2Se4, the hopping mechanism of conductivity is dominant at temperatures of T ∼ 27–110 K. At T ≥ 110 K, hopping conductivity gives way to activated conductivity. It is found that the specific feature of the low-temperature (27–77 K) photoconductivity spectrum of single crystals with ∼15 and 20 mol % of ZnIn2Se4 is a single narrow peak at a wavelength of λmax = 1190–1160 nm.

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.

CuInS2-ZnIn2S4 Solid Solutions: Growth, Physical and Photo-electrical Properties

The horizontal modification of the Bridgman–Stockbarger method was applied to grow the solid solutions of CuInS2-ZnIn2S4, containing 4 to 16 mol% of ZnIn2S4. The crystal structure was analyzed by the X-Ray analysis. The induced photoconduction was identified in the single crystals containing 8 to 12 mol% of ZnIn2S4 by measuring the photocurrent spectra at T ≈ 30 K. The model of two recombination centers with different capture cross sections was proposed to explain it. Indium vacancies or substitutional defects CuIn were assumed to be the fast recombination centers, meanwhile copper vacancies VCu behave as the slow recombination centers. The measurements of the temperature dependencies of electrical conductivity and thermally stimulated currents confirmed presence of the electrically active shallow defects in these n-type samples.

Electrical Measurements on P3HT:CdSe Blends for Photovoltaic Applications

Hybrid photovoltaic structures made of P3HT and branched CdSe nanocrystals (NCs) with 50 wt%–90 wt% ratio were investigated. Charge Extraction by Linearly Increasing Voltage (CELIV) mobility and temperature dependent (photo-)conductivity measurements were used to analyze the charge transport. The samples with 50 wt% were photosensitive, but just a minor PV effect was observed. The photovoltaic phenomenon was pronounced in the samples containing 75 wt% or more of NCs. The Open Circuit Voltages up to 1 V were obtained. The recombination of the generated carriers was fast and could be a limiting factor of the cell current. Carrier mobility was up to (3−4) × 10−4 cm2/Vs. From the fitting of mobility data the best carrier transport conditions were found in the samples with 75 wt% and 83 wt% of NCs.