Structure and optical properties of heterostructures based on MOCVD (Al x Ga1 − x As1 − y P y )1 − z Si z alloys

Epitaxial heterostructures produced by MOCVD on the basis of Al _x Ga_{1 ? x} As ternary alloys with the composition parameter x ≈ 0.20–0.50 and doped to a high Si and P atomic content are studied. Using the high-resolution X-ray diffraction technique, scanning electron microscopy, X-ray microanalysis, Raman spectroscopy, and photoluminescence spectroscopy, it is shown that the epitaxial films grown by MOCVD are formed of five-component (Al _x Ga_{1 ? x} As_{1 ? y} P _y )_{1 ? z} Si _z alloys.     

Band gap tailoring and yellow band emission of Cd0.9−xMnxZn0.1S (x=0 to 0.05) nanoparticles: Influence of Mn concentration

Cd_0.9-xMn_xZn_0.1S nanoparticles with x=0 to 0.05 were prepared by a simple chemical co-precipitation method at room temperature. Crystal structure and optical properties of the synthesized nanoparticles have been analyzed by X-ray diffraction (XRD) and UV–visible spectrophotometer. XRD confirmed the phase singularity of the synthesized material, which also confirmed the formation of Cd–Mn–Zn–S solid solution rather than secondary phase formation. Energy dispersive X-ray spectra showed the presence of Cd, Zn, Mn and S in the synthesized samples. The observed higher absorbance and lower transmittance of Mn-doped Cd_0.9Zn_0.1S than Cd_0.9Zn_0.1S is due to the size effect and also the defect states induced by Mn. The decrease in energy gap at Mn=0.01 is due the ‘sp–d’ exchange interactions between band electrons in CdS and the localized ‘d’ electrons of the Mn²⁺ ions. The increase in energy gap after Mn=0.01 can be explained by the excessive carriers generated by the impurity atoms. Fourier transform infrared spectroscopy (FTIR) illustrated the vibration modes of Cd–Zn–Mn–S between the wave number 530 cm⁻¹ and 780 cm⁻¹. Mn=0.01 doped sample exhibits a relatively high PL intensity and covers most of the visible region than the other samples; so desirable for LED application. The intensity ratio of the green band (GB) to Mn-related yellow band (YB) is decreased after Mn=0.01 which may be due the size effect or reduction of surface defect at higher doping concentrations.     

Study of the composition, structure, and optical properties of a-Si1 − x C x :H〈Er〉 films erbium doped from the Er(pd)3 complex compound

Rutherford backscattering, IR spectroscopy, ellipsometry, and atomic-force microscopy are used to perform an integrated study of the composition, structure and optical properties of a-Si_{1 ? x} C _x :H〈Er〉 amorphous films. The technique employed to obtain the a-Si_{1 ? x} C _x :H〈Er〉 amorphous films includes the high-frequency decomposition of a mixture of gases, (SiH₄) _a + (CH₄) _b , and the simultaneous thermal evaporation of a complex compound, Er(pd)₃. It is demonstrated that raising the amount of CH₄ in the gas mixture results in an increase in the carbon content of the films under study and an increase in the optical gap E _g ^opt from 1.75 to 2.2 eV. Changes in the composition of a-Si_{1 ? x} C _x :H〈Er〉 amorphous films, accompanied, in turn, by changes in the optical constants, are observed in the IR spectra. The ellipsometric spectra obtained are analyzed in terms of multiple-parameter models. The conclusion is made on the basis of this analysis that the experimental and calculated spectra coincide well when variation in the composition of the amorphous films with that of the gas mixture is taken into account. The existence of a thin (6–8 nm) silicon-oxide layer on the surface of the films under study and the validity of using the double-layer model in ellipsometric calculations is confirmed by the results of structural analyses by atomic-force microscopy.     

Synthesis,optical and electrochemical properties new donor–acceptor (D–A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b″] trithiophene donor and different acceptor units: Application as donor for photovoltaic devices

Two new conjugated D–A polymers P3 (PBTT-d-BTT) and P4 (PBTT-d-TPD) based on same benzo[1,2-b:3,4-b′:6,5-b″] trithiophene (BTT) donor and different acceptors monomers 5,8-dibromo-2-dodecanoylbenzo[1,2-b:3,4-b′:6,5-b″] trithiophene (d-BTT), and 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4]pyrrol-4,6-dione (d-TPD) respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), ¹H NMR, UV–Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. Photovoltaic properties of the polymers were studied by using the polymers as donor and PC₇₁BM as acceptor with a weight ratio of polymer:PC₇₁BM 1:1, 1:2 and 1:2.5. The optimized photovoltaic device was fabricated with an active layer of a blend P3:PC₇₁BM and P4:PC₇₁BM with a blend ratio of 1:2 showed PCE 3.16% and 2.42%, respectively under illumination of AM 1.5 at 100 mW/cm² with solar simulator. The PCE of the device based on P3:PC₇₁BM processed with DIO/o-DCB has been further improved up to 4.64% with J_sc of 10.52 mA/cm² and FF of 0.58 attributed to the increase in crystalline nature of active layer and more balanced charge transport in the device, induced by DIO additive.     

3,7-Bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT) based polymer with balanced ambipolar charge transport performance

A novel acceptor building block, 3,7-bis((E)-2-oxoindolin-3-ylidene)-3,7-dihydrobenzo[1,2-b:4,5-b′]dithiophene-2,6-dione (IBDT), is developed to construct a donor-acceptor polymer PIBDTBT-40. This polymer has favorable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels for balanced ambipolar charge transport. Organic thin film transistors (OTFTs) based on this polymer shows well-balanced ambipolar characteristics with electron mobility of 0.14 cm² V⁻¹ s⁻¹ and hole mobility of 0.10 cm² V⁻¹ s⁻¹ in bottom-gate bottom-contact devices. This polymer is a promising semiconductor for solution processable organic electronics such as CMOS-like logic circuits.     

Detailed analysis and contact properties of low-voltage organic thin-film transistors based on dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) and its didecyl and diphenyl derivatives

Low-voltage organic thin-film transistors (TFTs) based on four different small-molecule semiconductors (pentacene, DNTT (dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene), C₁₀-DNTT and DPh-DNTT) were fabricated, and a detailed comparison of the semiconductor thin-film morphology, of the current-voltage characteristics of transistors with channel lengths ranging from 100 to 1 μm, and of the contact resistances is provided. The three thienoacene derivatives DNTT, C₁₀-DNTT and DPh-DNTT all have significantly larger charge-carrier mobilities and smaller contact resistances than pentacene. In terms of the intrinsic channel mobility (determined using the transmission line method), C₁₀-DNTT and DPh-DNTT perform quite similarly and notably better than DNTT, suggesting that the decyl substituents in C₁₀-DNTT and the phenyl substituents in DPh-DNTT provide a similar level of enhancement of the charge-transport characteristics over DNTT. However, the DPh-DNTT TFTs have a substantially smaller contact resistance than both the DNTT and the C₁₀-DNTT TFTs, resulting in notably larger effective mobilities, especially in transistors with very small channel lengths. For DPh-DNTT TFTs with a channel length of 1 μm, an effective mobility of 0.68 cm²/V was determined, together with an on/off ratio of 10⁸ and a subthreshold swing of 100 mV/decade.