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In this work, a series of C60/C70 fullerene derivatives bearing non-aromatic and mono-, bi-, tri- and tetracyclic aryl-substituents was synthesized according to the modified Bingel method and characterized using spectral methods: ESI-MS, 1H NMR, 13C NMR, UV–Vis, FT-IR and cyclic voltammetry (CV). HOMO and LUMO energy levels and the band gaps for optimized structures of reported fullerene derivatives were determined according to the DFT functionals, B3LYP 6-31G(d) and PBE/6-311G(d,p). Results obtained from CV and UV–Vis measurements, which showed very good agreement, were compared to calculated theoretical values also revealing satisfactory level of compliance of the obtained results. We have studied the impact of presence and number of aromatic rings in malonate substituent of C60/C70 fullerene derivatives employed as acceptor materials on performance of BHJ solar cells prepared using PTB7-Th as donor material. We successfully prepared solar cells based on all the synthesized compounds, and the highest performance of the obtained photovoltaic devices was observed for fullerene derivatives bearing monocyclic and bicyclic aromatic moieties. The obtained voltage was about 0.8 V and current density was above 10 mA/cm2. Optical studies showed absorption edges at 1.9 eV and 1.8 eV for C60 and C70 derivatives, respectively, with absorption coefficients comparable to C60/C70 PCBM samples. Photocurrent spectroscopy showed 20–40% quantum efficiency. Long-term ageing measurements showed T80 time between 105 days and 115 days for derivatives with phenyl and naphthalene substituents. Taking into account that synthesis of reported fullerene derivatives is very convenient, the reported compounds are very promising materials for construction of BHJ solar cells.

A new unsymmetrical imine with four thiophene rings was synthesized in a one-step reaction, starting from the commercially available and relatively inexpensive reagents. The obtained imine in the form of thin films exhibited photoluminescence properties in the 1.8–2.4 eV energy range and a photoluminescence lifetime of about 0.3 ns. The HOMO and LUMO levels of the imine determined by cyclic voltammetry were at about −5.19 eV and −3.05 eV, respectively. The density functional theory was applied to calculate the geometric and electronic structure of the imine. The UV-Vis spectra showed that the absorption range of the imine overlaps with that of PC70BM, and the absorption peak at the maximum of the imine at 424 nm is located between the two maxima at 404 nm and 461 nm of the fullerene derivative. The electron acceptor and donor activity of the imine was tested in the solar cell architecture: glass/ITO/PEDOT:PSS/active layer/In/Al. The best photovoltaic parameters, with very good reproducibility for each 8 pixels in the cell, were found for the active layer based on ternary mixture PTB7:PC70BM:imine at a weight ratio 8 : 13 : 1, with the power conversion efficiency of about 4%. The external quantum efficiency of devices with the imine was found to be about 40% at 3.3 eV. The thermal imaging together with the recorded current response at increasing potential showed that the presence of imine in the composition has a beneficial impact in terms of current flow stability at temperatures above 200 °C, compared to two component layers with the same imine as an additive.

Thin layers of transition metal dichalcogenides have been intensively studied over the last few years due to their novel physical phenomena and potential applications. One of the biggest problems in laboratory handling and moving on to application-ready devices lies in the high sensitivity of their physicochemical properties to ambient conditions. We demonstrate that novel, in situ capping with an ultra-thin, aluminum film efficiently protects thin MoTe2 layers stabilizing their electronic transport properties after exposure to ambient conditions. The experiments have been performed on bilayers of 2H-MoTe2 grown by molecular beam epitaxy on large area GaAs(111)B substrates. The crystal structure, surface morphology and thickness of the deposited MoTe2 layers have been precisely controlled in situ with a reflection high energy electron diffraction system. As evidenced by high resolution transmission electron microscopy, MoTe2 films exhibit perfect arrangement in the 2H phase and the epitaxial relation to the GaAs(111)B substrates. After the growth, the samples were in situ capped with a thin (3 nm) film of aluminum, which oxidizes after exposure to ambient conditions. This oxide serves as a protective layer to the underlying MoTe2. Resistivity measurements of the MoTe2 layers with and without the cap, exposed to low vacuum, nitrogen and air, revealed a huge difference in their stability. The significant rise of resistance is observed for the unprotected sample while the resistance of the protected one is constant. Wide range temperature resistivity studies showed that charge transport in MoTe2 is realized by hopping with an anomalous hopping exponent of x ≃ 0.66, reported also previously for ultra-thin, metallic layers.

In this paper, we present a comparative analysis of the optical properties of non-polar and polar GaN/AlGaN multi-quantum well (MQW) structures by time-resolved photoluminescence (TRPL) and pressure-dependent studies. The lack of internal electric fields across the non-polar structures results in an improved electron and hole wavefunction overlap with respect to the polar structures. Therefore, the radiative recombination presents shorter decay times, independent of the well width. On the contrary, the presence of electric fields in the polar structures reduces the emission energy and the wavefunction overlap, which leads to a strong decrease in the recombination rate when increasing the well width. Taking into account the different energy dependences of radiative recombination in non-polar and polar structures of the same geometry, and assuming that non-radiative processes are energy independent, we attempted to explain the ‘S-shape’ behavior of the PL energy observed in polar GaN/AlGaN QWs, and its absence in non-polar structures. This approach has been applied previously to InGaN/GaN structures, showing that the interplay of radiative and non-radiative recombination processes can justify the ‘S-shape’ in polar InGaN/GaN MQWs. Our results show that the differences in the energy dependences of radiative and non-radiative recombination processes cannot explain the ‘S-shape’ behavior by itself, and localization effects due to the QW width fluctuation are also important. Additionally, the influence of the electric field on the pressure behavior of the investigated structures was studied, revealing different pressure dependences of the PL energy in non-polar and polar MQWs. Non-polar MQWs generally follow the pressure dependence of the GaN bandgap. In contrast, the pressure coefficients of the PL energy in polar QWs are highly reduced with respect to those of the bulk GaN, which is due to the hydrostatic-pressure-induced increase in the piezoelectric field in quantum structures and the nonlinear behavior of the piezoelectric constant.

Fullerene derivatives offer great scope for modification of the basic molecule, often called a buckyball. In recent years, they have been the subject of numerous studies, in particular in terms of their applications, including in solar cells. Here, the properties of four recently synthesized fullerene C60 derivatives were examined regarding their optical properties and the efficiency of the charge transfer process, both in fullerene derivatives themselves and in their heterojunctions with poly (3-hexylthiophene). Optical absorption, electron spin resonance (ESR), and time-resolved photoluminescence (TRPL) techniques were applied to study the synthesized molecules. It was shown that the absorption processes in fullerene derivatives are dominated by absorption of the fullerene cage and do not significantly depend on the type of the derivative. It was also found by ESR and TRPL studies that asymmetrical, dipole-like derivatives exhibit stronger light-induced charge transfer properties than their symmetrical counterparts. The observed inhomogeneous broadening of the ESR lines indicated a large disorder of all polymer–fullerene derivative blends. The density functional theory was applied to explain the results of the optical absorption experiments.

In the present work, we report the successful synthesis and characterization of six (two new) fullerene mono- and di-pyrene derivatives based on C60 and C70 fullerenes. The synthesized compounds were characterized by spectral methods (ESI-MS, 1H-NMR, 13C-NMR, UV-Vis, FT-IR, photoluminescence and photocurrent spectroscopy). The energy of HOMO and LUMO levels and the band gaps were determined from cyclic voltammetry and compared with the theoretical values calculated according to the DFT/B3LYP/6-31G(d) and DFT/PBE/6-311G(d,p) approach for fully optimized molecular structures at the DFT/B3LYP/6-31G(d) level. Efficiency of solar cells made of PTB7: C60 and C70 fullerene pyrene derivatives were analyzed based on the determined energy levels of the HOMO and LUMO orbitals of the derivatives as well as the extensive spectral results of fullerene derivatives and their mixtures with PTB7. As a result, we found that the electronic and spectral properties, on which the efficiency of a photovoltaic cell is believed to depend, slightly changes with the number and type of pyrene substituents on the fullerene core. The efficiency of constructed solar cells largely depends on the homogeneity of the photovoltaic layer, which, in turn, is a derivative of the solubility of fullerene derivatives in the solvent used to apply these layers by spincoating.

A new unsymmetrical imine with four thiophene rings was synthesized in a one-step reaction, starting from the commercially available and relatively inexpensive reagents. The obtained imine in the form of thin films exhibited photoluminescence properties in the 1.8-2.4 eV energy range and a photoluminescence lifetime of about 0.3 ns. The HOMO and LUMO levels of the imine determined by cyclic voltammetry were at about -5.19 eV and -3.05 eV, respectively. The density functional theory was applied to calculate the geometric and electronic structure of the imine. The UV-Vis spectra showed that the absorption range of the imine overlaps with that of PC

Thin layers of transition metal dichalcogenides have been intensively studied over the last few years due to their novel physical phenomena and potential applications. One of the biggest problems in laboratory handling and moving on to application-ready devices lies in the high sensitivity of their physicochemical properties to ambient conditions. We demonstrate that novel, in situ capping with an ultra-thin, aluminum film efficiently protects thin MoTe2 layers stabilizing their electronic transport properties after exposure to ambient conditions. The experiments have been performed on bilayers of 2H-MoTe2 grown by molecular beam epitaxy on large area GaAs(111)B substrates. The crystal structure, surface morphology and thickness of the deposited MoTe2 layers have been precisely controlled in situ with a reflection high energy electron diffraction system. As evidenced by high resolution transmission electron microscopy, MoTe2 films exhibit perfect arrangement in the 2H phase and the epitaxial relation to the GaAs(111)B substrates. After the growth, the samples were in situ capped with a thin (3 nm) film of aluminum, which oxidizes after exposure to ambient conditions. This oxide serves as a protective layer to the underlying MoTe2. Resistivity measurements of the MoTe2 layers with and without the cap, exposed to low vacuum, nitrogen and air, revealed a huge difference in their stability. The significant rise of resistance is observed for the unprotected sample while the resistance of the protected one is constant. Wide range temperature resistivity studies showed that charge transport in MoTe2 is realized by hopping with an anomalous hopping exponent of x ≃ 0.66, reported also previously for ultra-thin, metallic layers.

An analysis of the main recombination modes in nitrides, based on new method of data treatment is proposed for the determination of the carrier recombination processes in optically excited matter measured by time-resolved photoluminescence (PL). The analysis includes basic recombination modes: nonradiative Shockley-Read-Hall (SRH), radiative and Auger recombination in relation to monomolecular, bi-molecular, and tri-molecular processes of optical relaxation. The method is based on the introduction of instantaneous PL decay rate r P L plotted as a function of the PL intensity or of the time. Such an approach provides deep insight into the time evolution of the recombination of the optically excited semiconductor systems and can be applied to the time evolution of a variety of optically excited systems. The demonstration of its strength is given by the application to III-nitride based systems, including nitride highly doped and semi-insulating thick layers, polar and non-polar multi-quantum wells (MQWs). At low temperatures (5 K), the mono- and bi-molecular processes determine the carrier relaxation, and the tri-molecular Auger recombination contribution is negligible. At room temperature the data indicate an important contribution of Auger processes. It is also shown that asymptotic (low excitation), one-exponential recombination rate has different character depending on the presence of the electric fields across the structure.

Poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) are commonly used for the fabrication of organic photovoltaics (OPV). Efficiency limitations of OPVs could be circumvented by incorporation of inorganic nanostructures into organic blends. Again, integration of organic solar cells with well-developed silicon photovoltaic technology is ultimately desirable. In the present work, GaN nanowires with diameters of 25–50 nm and two lengths (200 and 500 nm) have been grown using molecular beam epitaxy technique. Solar-grade monocrystalline silicon wafers were used as substrates for nanowire synthesis. GaN nanostructures were incorporated into P3HT:PCBM photoactive layer in order to facilitate charge transfer between P3HT:PCBM and Si. Samples with and without nanowires were compared. Addition of nanowires led to the improvement in photovoltaic performance. Open circuit voltage has risen by 72% and short circuit current density by 200%. Series resistance has decreased 50 times, and power conversion efficiency has risen 20.7 times. Additional maxima are found in photocurrent spectrum corresponding to carriers being generated near GaN absorption edge. Moreover, external quantum efficiency peaks near GaN absorption edge, indicating the formation of current transfer channel via P3HT/GaN/Si cascade heterojunction. Mechanism explaining source of abovementioned improvement is proposed.

In this article, we present a wet ultrasonochemical synthesis of nanocrystalline TiO2 powders in the anatase form in ethanol solution and their surface decoration using uniformly dispersed Ag metallic nanoparticles of about 4 nm at concentration of n = 0.5 to 2.5 wt% and those of 10–20 nm larger at a higher concentration than 1 wt%. The structure of the prepared composites was characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman and X-ray photoelectron spectroscopy (XPS). Optical properties were determined using UV-vis spectroscopy. Six polymer solar cells with modification by an TiO2/n-Ag hole transporting layer were manufactured and studied under AM 1.5G-simulated solar illumination (100 mW cm−2). The best performance was obtained for a device with the ITO/(PEDOT:PSS):1.5%Ag in TiO2/P3HT:PCBM/Al architectures. Under these conditions, a PCE of 2.07% with open circuit voltage Voc = 0.612 V, short circuit current density Jsc = 5.94 mA cm−2, and fill factor FF = 0.57 was achieved. Additionally, devices were tested by electrochemical impedance spectroscopy under illumination and a well-fitted equivalent circuit was proposed. Finally, our idea of the linkage of TiO2/n-Ag with PEDOT:PSS and P3HT in the constructed devices taking into account the amount of Ag in TiO2 was proposed.

Detailed comparison of optical quality of GaN layers grown homoepitaxially on bulk Ga-polar and N-polar substrates by plasma-assisted molecular beam epitaxy was performed. One order of magnitude lower photoluminescence (PL) intensity and decay time at room temperature was observed for layers grown on N-polar substrates, realized using gallium-rich conditions. This nonradiative recombination channel was very efficient also at liquid helium temperatures, what was evidenced by no improvement in PL time decay. Optical quality of GaN layer (PL intensity, lifetimes) grown on N-polar substrates was greatly improved by using nitrogen-rich growth conditions. We attribute this improvement to suppressed formation of nitrogen vacancy-related point defects. Finally, linewidth below 1 meV and lifetime above 0.1 ns (DX line, He temperature) were obtained for layers grown on N-polar substrate.

Polyaniline was chemically synthesized and protonated (doped) with CSA in chloroform with different PANI:CSA molar ratios, ranging from 6:1 to 1:4. Protonation level of the polymer was monitored by means of UV–Vis spectra of the solution and FTIR measurements of thin films. The results were correlated with the changes of specific conductivity in a function of PANI:CSA molar ratio in the films. A broad maximum of conductivity was found in the range from 1:1 to 1:3, i.e. for over-protonated polymer. The solutions with various PANI doping levels were used as hole transport layers (HTLs) in the polymer-fullerene (PTB7:PC70BM) and perovskite (MAPbI3) solar cells. The highest power conversion efficiency (PCE=3.8%) obtained for the organic cell with molar ratio PANI:CSA=1:1 was only slightly lower than that of the reference cell with PEDOT:PSS HTL (4%). The best perovskite cells achieved PCE of about 4% which was markedly lower that the efficiency of cells with Spiro-OMeTAD HTL. However, some of the perovskite cells with PANI:CSA revealed very high short current density, of about 20 mA cm−2 but relatively low open circuit photovoltage (0.42 V). The changes of the PCE in a function of PANI protonation level were discussed in terms of alignment of HOMO and LUMO levels of HTL, donor and acceptor layers in the solar cells as well as quality of interfacial layers.

The potential fluctuations in III-nitride quantum wells lead to many effects like emission broadening and S-shape energy vs. temperature dependence. The best description of the energy dependence comes from calculations based on Gaussian density of states. However, in most of the published reports, changes of carrier lifetime with energy and temperature are not taken into account. Since experimental evidence shows that lifetime significantly depends on energy and temperature, here we propose a model that describes two basic parameters of luminescence: lifetime of carries and emission energy as a function of temperature in the case of quantum wells and layers that are characterized by potential fluctuations. Comparison of the measured energy and lifetime dependences on temperature in specially grown InGaN/GaN quantum wells and InAlGaN layer shows very good agreement with the proposed theoretical approach.

We report on molecular beam epitaxy growth and properties of rarely studied quaternary In0.4(Al0.75Ga0.25)0.6As self-assembled quantum dots, which show strong and efficient emission of red light from single quantum dots. The increased yield is, among others, due to efficient energy transfer from indirect band-gap Al0.75Ga0.25As barriers. To maximize photon energy emitted from quantum dots, low In composition, xIn = 0.4 was applied, which also lowered the lattice misfit close to the limit of 2D/3D transition in the Stranski–Krastanov growth mode. Time-resolved micro-photoluminescence shows emission at 650–750 nm. Well-resolved single quantum dot photoluminescence lines (decay time of ≈ 1−2 ns) are observed despite a high concentration ≈ 3 × 10 cm−2 of quantum dots. We discuss this observation assuming newly a role of carriers or excitons diffusion/tunneling between quantum dots at high surface concentration of dots and a possible role of lattice disorder inside the dot on the exciton lifetime.

Parallel in situ TEM and XRD heating experiments of LiFePO4 precursors obtained by sol-gel method were conducted to study changes and to understand structural and morphological evolution during synthesis annealing, which is one of the most critical stages in preparing rechargeable cathodes based on these materials. Raman spectroscopy and electrochemical testing were also performed and a basic optimization of the final step of the sol-gel process was demonstrated by comparing in situ heating data with the electrochemical performance of materials annealed at different temperatures. The results obtained from these in situ measurements, at different length scales, provided a detailed picture of the structural and morphological changes and provided a better understanding of the electrochemical behavior of the final LiFePO4 material. The study showed a strong dependence between the electrochemical performance of LiFePO4 synthesized by sol-gel method and annealing temperature. The best performance was obtained w...

Photoinduced charge transfer is a crucial process in the operation of organic solar cells. In this work we used two spectroscopic techniques, namely, time-resolved photoluminescence and light-induced electron spin resonance, which directly detect this phenomenon in blends composed of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and two air-stable polyazomethines with different chemical structures, one with thiophene ring and cardo moieties (25Th-cardo) and one with three thiophene rings (2252Th-DMB). For both polymers we observed a photoluminescence with a maximum around 2 eV. In a mixture with PCBM, the photoluminescence of both polymers was quenched. At the same time, the photoluminescence of PCBM was enhanced in a mixture with 25Th-cardo, whereas it was similar to the photoluminescence of pure PCBM in the mixture with 2252Th-DMB. Moreover, for 2252Th-DMB:PCBM, two overlapping lines of electron spin resonance were detected under illumination, one originating from the positive polaron and another fr...

For the development and application of GaN-based nanowire structures, it is crucial to understand their fundamental properties. In this work, we provide the nano-scale correlation of the morphological, electrical, and optical properties of GaN/AlGaN nanowire light emitting diodes (LEDs), observed using a combination of spatially and spectrally resolved cathodoluminescence spectroscopy and imaging, electron beam-induced current microscopy, the nano-probe technique, and scanning electron microscopy. To complement the results, the photo- and electro-luminescence were also studied. The interpretation of the experimental data was supported by the results of numerical simulations of the electronic band structure. We characterized two types of nanowire LEDs grown in one process, which exhibit top facets of different shapes and, as we proved, have opposite growth polarities. We show that switching the polarity of nanowires (NWs) from the N- to Ga-face has a significant impact on their optical and electrical properties. In particular, cathodoluminescence studies revealed quantum wells emissions at about 3.5 eV, which were much brighter in Ga-polar NWs than in N-polar NWs. Moreover, the electron beam-induced current mapping proved that the p&ndash, n junctions were not active in N-polar NWs. Our results clearly indicate that intentional polarity inversion between the n- and p-type parts of NWs is a potential path towards the development of efficient nanoLED NW structures.

We demonstrate that a GaN nanowire array can be used for efficient charge transfer between the organic photovoltaic layer and silicon in a Si/GaN/P3HT:PC71BM inverted hybrid heterostructure. The band alignment of such a material combination is favorable to facilitate exciton dissociation, carrier separation and electron transport into Si. The ordered nature of the GaN array helps to mitigate the intrinsic performance limitations of the organic active layer. The dependence of photovoltaic performance enhancement on the morphology of the nanostructure with nanowire diameters 30, 50, 60, 100 and 150 nm was studied in detail. The short circuit current was enhanced by a factor of 4.25, while an open circuit voltage increase by 0.32 volts was achieved compared to similar planar layers.

In this study new symmetrical and unsymmetrical imines based on 4-[4-(4-fluorophenyl)1,3-thiazol-2-yl]aniline and 2,2′,5′,2”-terthiophene-5,5”-dicarboxaldehyde or 2,2’:5′, 2’’-terthiophene-5-carboxaldehyde, respectively, were synthesized by melt condensation with the yield of 86% and 40%. The 5% weight loss of the imines was found in temperature range from 294 to 390 °C in nitrogen, depending on the symmetry of aldehyde used in synthesis. The band gap of symmetrical imine (PV-BLJ-SC13) is lower by about ∼0.33 eV than that of unsymmetrical PV-BLJ-SC14. For both imines the electron density of HOMO level is mainly located on thiophene moieties and is extended to imine bonds in thiazole rings and imine moieties. The theoretical calculations of their HOMO-LUMO levels by DFT methods were also provided. The UV–Vis absorption spectra and cyclic voltammograms of imines both in the form of thin films cast on the electrode as well as dissolved in 1,2-dichloromethane were studied to determine the electrochemical and optical band gap energies as well as correlate the positions of HOMO and LUMO levels obtained by DFT and electrochemical methods. The resistance of imines was approximately 113.8 Ω and 102.4 Ω for PV-BLJ-SC13 and PV-BLJ-SC14, respectively as determined by IR thermal images and have a tendency to decrease by about 17–24 Ω when mixed with PTB7 and PC71BM. Both imines showed very bright luminescence. Moreover, optical absorption, photoluminescence and IR thermal images were measured for imine:PTB7 or imine:PTB7:PC71BM composite. Finally, simple organic solar cells based on (PTB7:imine):PC71BM in weight ratio (4:1):8 and (8:1):13 w/w were constructed and characterized. The highest power conversion efficiency, which was equal to 0.42% was determined for the ITO/PEDOT:PSS/PTB7:PV-BLJ-SC13:PC71BM (4:1):8/Al device. In addition, the power conversion efficiency was strongly dependent on the weight ratio of imine in active layer.

Optical phenomena in an ensemble of high-quality GaN nanowires (NWs) grown on a Si substrate have been studied by reflectance and time-resolved luminescence. Such NWs form a structure that acts as a virtual layer that specifically reflects and polarizes light and can be characterized by an effective refractive index. In fact we have found that the NW ensembles of high NW density (high filling fraction) behave rather like a layer of effective medium described by the Maxwell Garnett approximation. Moreover, light extinction and strong depolarization are observed that we assign to scattering and interference of light inside the NW ensemble. The wavelength range of high extinction and depolarization correlates well with transverse localization wavelength estimated for such an ensemble of NWs, so we suppose that these effects are due to Anderson localization of light. We also report results of time-resolved measurements of polarization of individual emission centers including free and bound excitons (D0XA, 3.47 eV), inversion domain boundaries (IDB, 3.45 eV) and stacking faults (SF, 3.42 eV). The emission of the D0XA and SF lines is polarized perpendicular to GaN c-axis while the 3.45 eV line is polarized along the c-axis which supports a hypothesis that this line is emitted from IDBs. Time-dependent depolarization of luminescence is observed during the first 0.1 ns after excitation and is interpreted as the result of interaction of the emission centers with hot particles existing for a short time after excitation.

We present selected semiconductor (inorganic and organic) structures for which non-radiative recombination of excess charge carriers is very high, luminescence suppressed and its lifetime substantially shortened. Processes competitive with radiative energy emission are discussed. The importance of Shockley–Read–Hall recombination in materials with high impurity or defect concentration, applied in ultrafast devices, is shown. Also, charge transfer process in solar cells is discussed in the context of luminescence quenching of individual components of an active layer. A part of the shown research was a subject of our common work with Prof. Arūnas Krotkus during the period from 1994 till 2006.

International audience; The dependence on exciton density of the interwell coupling scheme in a series of InGaN/GaN symmetrical double quantum wells (DQWs) with varying central barrier width was observed. Continuous-wave photolu-minescence (cw-PL) and time-resolved photoluminescence (TRPL), measured at low temperature (6-13 K), allowed us to examine the competition between three optical recombination channels, namely, the recombination of (i) intrawell excitons (DXs), (ii) interwell indirect excitons (IXs), and (iii) presumably coupled well excitons (CWXs), built of electron and hole wave functions that are widely spread over the entire DQW structure. We demonstrate a rather abrupt switching effect that relies on the actual exciton density in the system. In cw-PL experiments as a function of the pumping laser power, this switching is characterized by a threshold laser power density above which we observe (i) a significant change of slope of both the power-dependent blueshift and intensity of the ground-state exciton and (ii) the appearance of higher-energy optical emissions. In TRPL, as the PL intensity decays with time, both these effects are visible but in opposite direction, including the PL intensity transfer from the higher-energy state to the ground state. The observed switching is assigned to a change of the dominant excitonic recombination regime: at low pumping densities the dominant emission arises from the extremely long-lived IX, whereas above threshold the dominant emission corresponds to DXs or CWXs, depending on the barrier width. The threshold power density (or threshold time for TRPL) presents a clearly exponential dependence upon the width of the central barrier, which demonstrates the role of carrier tunneling in the overall switching process. The comparison of IXs in nitride QWs with bias electric-field induced IXs in GaAs/AlGaAs DQWs shows that the spectral blueshifts are a few times larger in the former structures. This enhancement originates from the built-in electric field in group-III nitrides, which has truly microscopic character. Moreover, it is argued that the presence of the potential fluctuations due to alloy disorder and the fluctuations of the coupling barrier width have only a secondary effect on the observed switching.

Organic (10-camphorsulfonic acid, CSA), organic-inorganic (Ag-poly(vinylpyrrolidone), Ag-PVP with 10 and 20 nm size of Ag) or inorganic (Al, 18 nm) compounds were applied as new components of active layer in bulk heterojunction polymer solar cells based on a new polyazomethine (PAZ-Car-TPA) resulting in significant change of optical and electrical properties. Moreover, colloidal Ag (100 nm) and Ag-PVP in aqueous solution (10 nm) were tested as an addition to the hole transporting layer based on PEDOT:PSS in polyazomethine solar cells. CSA added to PAZ-Car-TPA decrease its energy gap from 1.91 to 1.20 eV and causes a significant bathochromic shift of the maximum of absorption band along with the change of the polymer color from yellow to red (electrochromic behavior). Photoluminescence maximum of PAZ-Car-TPA protonated with CSA showed 120 nm redshift from 500 to 620 nm in comparison to undoped PAZ-Car-TPA. HOMO-LUMO of PAZ-Car-TPA and its mixture with CSA were analyzed by cyclic voltammetry and quantum mechanical calculations using Density Functional Theory method. Refraction index and extinction coefficient of PAZ-Car-TPA and its mixtures with PC71BM as well as Ag or Al nanoparticles were investigated taking into consideration various thickness of polymer layer. The power conversion efficiency of the ITO/PEDOT:PSS/PAZ-Car-TPA:PCBM:CSA/Al device was five time higher than that of the device based on PAZ-Car-TPA:PCBM. Such an enhancement was found to be primarily due to the increase of the short-circuit current, suggesting that the charge collection increases upon the incorporation of CSA in the active layer. Moreover, presence of Ag-PVP (20 nm) or Al (18 nm) in device increased short circuit current of the constructed polymer solar cells. Additionally, devices were tested by external quantum efficiency measurements and electrochemical impedance spectroscopy in dark and under illumination. The polymer solar cell with PAZ-Car-TPA as donor in active layer showed good stability over 600 hours, when stored in a climatic chamber.

A recently synthesized 25Th-cardo polyazomethine (PAZ) and its photocurrent generating junction with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) have been examined as possible materials for construction of new-generation solar cells. Properties of a 25Th-cardo/PCBM system, where PAZ and PCBM act as donor and acceptor materials, respectively, have been studied theoretically by time-dependent density-functional theory (TD-DFT) and experimentally by means of optical absorption, photocurrent spectroscopy (PCS), and time-resolved luminescence (TRPL). Theoretical calculations show that highest occupied molecular orbital (HOMO) energy levels of PAZ and PCBM are almost equal (values of −6.01 and −5.98 eV were obtained with the B3LYP functional and the def2-TZVP orbital basis for PAZ and PCBM, respectively), what suggests a possibility of charge transfer in both directions for this system. The shape of the calculated absorption spectrum is in a qualitative agreement with the experiment. The PCS measurements of the new material show that the external quantum efficiency is the highest (about 0.3%) in the near UV range. The TRPL studies reveal a fast decay of a Langevin type (in picosecond range) of the PAZ-related photoluminescence, which accelerates in presence of PCBM, probably due to the charge transfer to PCBM. In addition, our measurements document a usually neglected process of the hole transfer from a donor to an acceptor. A leakage of holes from PAZ to PCBM is supported by a small energy difference of the HOMO energies, as predicted by theory.

Growth of high-quality nitride-based structures requires use of miscut substrates, which means that the growth plane is tilted with respect to the c lattice plane (0001). In order to investigate the miscut influence on grown layers, patterned GaN substrates with separate regions tilted between 0.34° and 0.97° to the c-plane were used to grow multiquantum well InGaN/GaN structures by metalorganic vapor phase epitaxy. The quantum wells had a nominal 6% indium content. Precise values of miscut in each region were determined by atomic-force microscopy. Recombination dynamics in each region was studied using time-resolved microphotoluminescence (PL) from 4 K to room temperature. It was found that the PL energy increases with miscut angle from 3.17 to 3.29 eV (at 4 K). The PL lifetime also correlated with the miscut angle, it decreased from 1.2 down to 0.8 ns (at 4 K). These results suggest that an increase of the angle causes a decrease of the indium content. Less indium naturally leads to a higher bandgap, and to reduction of electric field inside the quantum wells (QWs). Both effects lead to a PL energy increase. Moreover, the decrease of the electric field reduces the electron–hole separation that explains the shorter radiative recombination at 4 K at regions of high-miscut angles. The shape of temperature dependence of PL decay times suggests that at low temperatures PL decay is determined by the localization of carriers, whereas in room temperature LO phonon-assisted delocalization processes play a dominant role.

We synthesized two new, air-stable and environmentally friendly polyazomethines and characterized their photovoltaic and electro-chemical impedance spectroscopic properties. Two polyazomethines with one (25Th-DMB) or three (2252Th-DMB) thiophene ringswere synthesized and tested as donors in the active layer in solar cells. The investigated polymer solar cells contained different amountsof PCBM and PC 71 BM. The bulk heterojunction polymer solar cells were fabricated and studied, considering the architecture of thedevices (i.e., LiF, TiO 2 , and TiO 2 :Ag) and the fabrication conditions (i.e., air, inert gas, and annealing). The highest power conversionefficiency, which was equal to 0.52% under simulated 100 mW/cm 2 AM 1.5G irradiation, was determined for the device using polyazome-thine with three thiophene rings and PC 71 BM created in air with the ITO/PEDOT:PSS (100 cP)/2252Th-DMB:PC 71 BM (1:2)/Al archi-tecture. In addition, the power conversion efficiency was strongly dependent on the viscosity of the PEDOT:PSS used. 2015 Elsevier Ltd. All rights reserved.

Nanocrystalline powders: lithium-manganese oxide (LiMn 2 O 4 ) of spinel and lithium-manganese phosphate (LiMnPO 4 ) of olivine structure were synthesized by a modified sol–gel method. In this synthesizing process, lithium and manganese salts and complexing agent were used as reactants. The obtained powders were characterized by a number of methods: X-ray powder diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), atomic force microscopy (AFM) and Raman spectroscopy. The mean sizes of crystallites were about 30 nm for LiMn 2 O 4 and 60 nm for LiMnPO 4 nanoparticles. The influence of crystallographic structure on the stability of two manganese compounds was studied. The correlation between the structural and morphological results of spinel LiMn 2 O 4 and olivine LiMnPO 4 properties was examined for the first time in this work.

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A high-energy ball-milling process was used for the solid-state synthesis of single phase powders of spinel-structured lithium-titanium oxide (Li4Ti5O12). The influence of the high-energy ball-milling time on the structure, morphology, and electrochemical performance of synthesized powders was investigated. Remarkably, the electrochemical tests performed at high rates showed significantly improved specific capacity of the powders obtained with extended synthesis times. In particular, after 50 charging/discharging cycles at the current rate of 1C, the samples retained more than 94% of their initial specific capacity.

Evidences are shown that excitons at stacking faults (SFs) in GaN nanowires (NWs) behave like 2-dimentional particles in quantum wells. The SFs were studied in samples of coalescent GaN nanowires grown on silicon substrate. They emit strong luminescence at room temperature what promises possible applications of SF for light emitting devices. At 4 K, the NWs exhibit sharp near-bandgap luminescence with the strongest signal from donor-bound excitons (DX) at 3.471 eV and from stacking faults (SF) at 3.42 eV. Observations of the SF dynamics show spectral diffusion of energy in time (shift up to Δ E d =8 meV) and S-shaped energy–temperature dependence. Both these features are often reported as characteristic for quantum wells. Moreover we report on two dynamical effects, which confirm two-dimensional character of excitons in SFs. First, decrease of radiative recombination rate at higher temperature r R =1/(α T + t 0 ), shows that excitons have momentum. Second, the SF luminescence peak has an exponential slope, i.e. is similar to thermal distribution, which points to kinetic energy of the excitons. The effective temperature calculated from the shape of the SF peak was in the range of few tens of Kelvins. It decreased in time with the cooling time τ C =0.37±0.05 ns.

A lithium manganese oxide of spinel structure (LiMn 2 O 4 ) has been successfully synthesised by modified sol–gel method. Two complexing agents — citric acid and acetic acid — have been utilized in order to enable a better complexing of reactants and thus better stability of reaction mixture. It results in a highly homogenous powder with satisfactory electrochemical properties. The charge/discharge tests have revealed a strong dependence of specific discharge capacities on electrode thickness. The thinnest electrodes show the lowest capacity drop while raising the discharge current from 1 C to 30 C. Based on the absolute magnitude of capacity revealed by cells, the maximum specific capacity when discharged at 30 C revealed the cathode with 0.83 mg cm − 2 loading. As a result the electrode with 0.83 mg cm − 2 cathode loading possesses the best properties for construction of a high rate performance battery.

Semi-insulating (Cd,Mn)Te crystals are believed to be suitable to successfully replace the commonly used (Cd,Zn)Te and CdTe crystals as a material for manufacturing large-area X- and γ-ray detectors. The purpose of the studies was to elaborate the technology of the (Cd,Mn)Te crystal plates with high resistivity, uniform in the whole volume of the plate. The article presents a method of annealing of the monocrystalline (Cd,Mn)Te crystal plates, giving the desired, repeatable results. The semi-insulating (Cd,Mn)Te crystals, doped (for compensation) mainly by vanadium, but sometimes by Cl, In, or Cl and V, were grown by the Bridgman method, and the monocrystalline plates were cut. Subsequently the plates were annealed in Cd vapour. The resistivity of the plates was around 109 Ωcm, and the μτ-product was about 10-4 -10-3 cm2/V. Mapping of the resistivity was performed on both the as-grown crystals and the annealed crystal plates. The resistivity of the as-grown crystals was in the range of 105-106 Ωcm and inhomogeneous. After annealing the resistivity increased up to 109 Ωcm and better homogeneity could be seen. Annealing in the saturated Cd vapour influenced tellurium inclusions/precipitates. It was studied by IR transmission microscopy. For the as-grown samples the density of Te inclusions (>1 µm) was ≈ 4×105 cm-3, but for the annealed samples it was ≈ 104 cm-3. The low-temperature photoluminescence (PL) measurements indicated significantly lowered concentrations of acceptors after annealing. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A three-step solid state synthesis was used to produce powders of spinel phase Li4Ti5O12 with crystallite size in a few hundred nanometers range. This was followed by surface modification through the deposition of 2–10 nm Ag nanoparticles, as verified by scanning and transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. The electrochemical performance of these Li4Ti5O12/n-Ag composite powders was examined by chronopotentiometry in three-electrode Swagelok cells. These measurements showed excellent high-rate performance and remarkably good cyclability of the fabricated powders. Specifically, capacity retention in excess of 86% after raising the discharge current from 1C to 10C and less than 6% of capacity loss after 50 charge/discharge cycles at 1C current rate were measured.

The horizontal excitation energy transport in the range of tens of micrometers was measured in high quality homoepitaxial InGaN quantum wells (QWs) with the use of time and space resolved micro-photoluminescence as a function of (i) applied vertical electric field, (ii) temperature, and (iii) linear density of atomic steps. The investigated structure consisted of InGaN QWs inside a p-n junction. The indium content in QWs was designed to be different in defined areas of the sample (due to mastering of different off-cuts and atomic steps density) so that the wells could emit at energies from 2.6 to 2.86 eV. The horizontal transport range was sensitive to the vertical electric field, which means that it could not be just a radiation transfer, but charge carriers must have been involved as well. We found that the transport range decreased for higher slope angles, possibly due to stronger scattering on atomic steps when their linear density became higher. The diffusion coefficients reached 6 cm2/s and due to long lifetime of even 2 μs, the diffusion length was even LD = 30 μm in areas of low off-cut angles. We discuss possible mechanisms of transport and conclude that for such high diffusion constant, the most probable is the excitonic transport. The LD was maximum at 40 K and then decreased significantly with temperature, which was probably caused by thermal dissociation of excitons.

Pure and Sn-modified LiFePO 4 crystalline powders are successfully synthesized by sol-gel method. The addition of Sn to the precursor changes the morphology of the material causing partial formation of nanorods and leads to increase of average crystallite size from 33 nm up to ca. 40 nm. The X-ray photoelectron spectroscopy, transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy indicate that Sn outdiffuses to the LiFePO 4 crystal surface during calcination process and creates a few nanometer-thick SnO 2 surface layer. The TEM analysis combined with Raman spectroscopy indicates also that LiFePO 4 particles are embedded in amorphous carbon. Electrochemical testing shows an improved capacity, as well as better stability and cycling reversibility of Sn-modified materials compared to pure LiFePO 4 . Reduced material degradation during charging/discharging cycling and excellent lithium intercalation reversibility in Sn-modified LiFePO 4 is attributed to a positive surface modification effect of the SnO 2 layer.

We present results of Raman and infrared absorption spectroscopy research on phonons in LiMnPO 4 —a new material for high capacitance rechargeable lithium-ion batteries. There is a significant interest in the structural and electrical properties of this material, because the battery performance depends strongly on the rate of lithium diffusion. Nanopowder of LiMnPO 4 was obtained via a modified sol–gel method from salts of lithium and manganese. This method is cheap and effective so it is promising for the most popular applications. The material showed sharp phonon peaks in Raman and infrared spectra. In the Raman spectra, the strongest peak was A g ν 1 mode at energy 117.77 meV (950.1 cm −1 ), at 4 K. At room temperature, its energy decreased (due to phonon–phonon interaction) to 117.5 meV (947.5 cm −1 ). The Gruneisen parameter found for this oscillation mode was relatively low, γ A g ν 1 = 0.5 , at about 300 K. Since the mode consisted mainly of the symmetric PO 4 tetrahedra oscillations, the low γ A g ν 1 value indicated that the temperature influenced rather Li–O and Mn–O bonds than the P–O bonds forming the LiMnPO 4 structure. The thermal dependencies of the antisymmetric modes (A g ν 3 and A g ν 4 ) were stronger ( γ A g ν 3 = 0.7 , γ A g ν 4 = 1.4 ) what suggested that these modes experienced stronger coupling. The thermal broadening of the A g ν 1 mode could be described in wide temperature range by exponential dependence with activation energy of 65 meV (about two times smaller than the A g ν 1 energy), what suggested a symmetric two-phonon decay.

Graphene has been shown to be an interesting material with various possible applications. Since graphene is transparent and has good conductivity, it is considered to be a good candidate for replacing ITO in solar cells. On the other hand, it has been shown recently that nanowire structures can substantially increase efficiency of solar cells. Therefore, possibility of using graphene as a transparent electrode on GaN nanowires (NWs) is very attractive. In this paper we focus on experimental studies of graphene grown by Chemical Vapor Deposition method, and transferred onto GaN NWs grown by Molecular Beam Epitaxy. Electron Spin Resonance (ESR) and Raman spectroscopy have been used to study the properties of graphene. ESR was measured as a function of temperature. Observation of the characteristic peak related to weak-localization allowed us to estimate the coherence length changes from about 150 nm at 5 K to 120 nm at 25 K (Fig.1). Interband and intraband scattering lengths were found to be independent of temperature and equal 150 nm and 31 nm, respectively. Interestingly, the obtained coherence length was shorter, whereas elastic scattering lengths were similar to those obtained for graphene transferred to other substrates. It was found, that presence of GaN NWs substantially increases intensity of graphene Raman spectra, by more than one order of magnitude compared to as grown CVD material. Besides, as can be seen from Raman mapping, the observed changes of spectrum parameters (energy shifts and intensity) correlated with positions of individual NWs (Fig. 2). The intensity ratio of D to G peak was high, however could not be correlated with GaN NWs pattern, which suggested large enhancement of defect spectrum rather than defect generation by NWs. The obtained results are discussed in terms of the interaction of graphene with a polar medium (GaN, with free carrier distribution along NWs).

The paper provides a deeper understanding of key-parameters of epitaxial graphene growth techniques on SiC. At 16000C, the graphene layer is continuous and covers a large area of the substrate. Significant differences in the growth rate could be observed for different reactor pressures and the polarity of SiC substrates as well as for the substrate miscut and surface quality. In addition, graphene thickness uniformity and mechanism of ridges creation was examined.