Efficiency limitations in a low band-gap diketopyrrolopyrrole-based polymer solar cell

We report the performance and photophysics of a low band-gap diketopyrrolopyrrole-based copolymer used in bulk heterojunction devices in combination with PC71BM. We show that the short lifetime of photogenerated excitons in the polymer constitutes an obstacle towards device efficiency by limiting the diffusion range of the exciton to the donor-acceptor heterojunction. We employ ultrafast transient-probe and fluorescence spectroscopy techniques to examine the excited state loss channels inside the devices. We use the high boiling point solvent additive 1,8-diiodooctane (DIO) to study the photoexcited state losses in different blend morphologies. The solvent additive acts as a compatibiliser between the donor and the acceptor material and leads to smaller domain sizes, higher charge formation yields and increased device efficiency.

New low band gap 2-(4-(trifluoromethyl)phenyl)-1H-benzod]imidazole and benzo1,2-c; 4,5-c `]bis1,2,5]thiadiazole based conjugated polymers for organic photovoltaics

Two new low band gap D-A structured conjugated polymers, PBDTTBI and PBDTBBT, based on 2-(4-(trifluoromethyl)phenyl)-1H-benzod]imidazole and benzo1,2-c; 4,5-c']bis1,2,5]thiadiazole acceptor units with benzo1,2-b; 3,4-b']dithiophene as a donor unit have been designed and synthesized via a Stille coupling reaction. The incorporation of the benzo1,2-c; 4,5-c']bis1,2,5]thiadiazole unit into PBDTBBT has significantly altered the optical and electrochemical properties of the polymer. The optical band gap estimated from the onset absorption edge is similar to 1.88 eV and similar to 1.1 eV, respectively for PBDTTBI and PBDTBBT. It is observed that PBDTBBT exhibited a deeper HOMO energy level (similar to 4.06 eV) with strong intramolecular charge transfer interactions. Bulk heterojunction solar cells fabricated with a configuration of ITO/PEDOT: PSS/PBDTBBT: PC71BM/Al exhibited a best power conversion efficiency of 0.67%, with a short circuit current density of 4.9 mA cm(-2), an open-circuit voltage of 0.54 V and a fill factor of 25%.

TiO2/ZnO core/shell nano-heterostructure arrays as photo-electrodes with enhanced visible light photoelectrochemical performance

The present article reports a facile method for preparing the vertically-aligned 1D arrays of a new type of type II n-n TiO2/ZnO core/shell nano-heterostructures by growing the nano-shell of ZnO on the electrochemically fabricated TiO2 nanotubes core for visible light driven photoelectrochemical applications. The strong interfacial interaction at the type II heterojunction leads to an effective interfacial charge separation and charge transport. The presence of various defects such as surface states, interface states and other defects in the nano-heterostructure enable it for improved visible light photoelectrochemical performance. The presence of such defects has also been confirmed by the UV-vis absorption, cathodoluminescence, and crystallographic studies. The TiO2/ZnO core/shell nano-heterostructures exhibit strong green luminescence due to the defect transitions. The TiO2/ZnO core/shell nano-heterostructures photo-electrode show significant enhancement of visible light absorption and it provides a photocurrent density of 0.7 mA cm(-2) at 1 V vs. Ag/AgCl, which is almost 2.7 times that of the TiO2/ZnO core/shell nano-heterostructures under dark conditions. The electrochemical impedance spectroscopy results demonstrate that the substantially improved photoelectrochemical and photo-switching performance of the nano-heterostructures photo-anode is because of the enhancement of interfacial charge transfer and the increase in the charge carrier density caused by the incorporation of the ZnO nano-shell on TiO2 nanotube core.

Double Gaussian distribution of barrier height observed in densely packed GaN nanorods over Si (111) heterostructures

GaN nanorods were grown by plasma assisted molecular beam epitaxy on intrinsic Si (111) substrates which were characterized by powder X-ray diffraction, field emission scanning electron microscopy, and photoluminescence. The current-voltage characteristics of the GaN nanorods on Si (111) heterojunction were obtained from 138 to 493K which showed the inverted rectification behavior. The I-V characteristics were analyzed in terms of thermionic emission model. The temperature variation of the apparent barrier height and ideality factor along with the non-linearity of the activation energy plot indicated the presence of lateral inhomogeneities in the barrier height. The observed two temperature regimes in Richardson's plot could be well explained by assuming two separate Gaussian distribution of the barrier heights. (C) 2014 AIP Publishing LLC.

Growth and electrical transport properties of InGaN/GaN heterostructures grown by PAMBE

InGaN epitaxial films were grown on GaN template by plasma-assisted molecular beam epitaxy. The composition of indium incorporation in single phase InGaN film was found to be 23%. The band gap energy of single phase InGaN was found to be similar to 2.48 eV: The current-voltage (I-V) characteristic of InGaN/GaN heterojunction was found to be rectifying behavior which shows the presence of Schottky barrier at the interface. Log-log plot of the I-V characteristics under forward bias indicates the current conduction mechanism is dominated by space charge limited current mechanism at higher applied voltage, which is usually caused due to the presence of trapping centers. The room temperature barrier height and the ideality factor of the Schottky junction were found to 0.76 eV and 4.9 respectively. The non-ideality of the Schottky junction may be due to the presence of high pit density and dislocation density in InGaN film. (C) 2014 Elsevier Ltd. All rights reserved.

Electronic Structure of CH3NH3PbX3 Perovskites: Dependence on the Halide Moiety

A combination of measurements using photoelectron spectroscopy and calculations using density functional theory (DFT) was applied to compare the detailed electronic structure of the organolead halide perovskites CH3NH3PbI3 and CH3NH3PbBr3. These perovskite materials are used to absorb light in mesoscopic and planar heterojunction solar cells. The Pb 4f core level is investigated to get insight into the chemistry of the two materials. Valence level measurments are also included showing a shift of the valence band edges where there is a higher binding energy of the edge for the CH3NH3PbBr3 perovskite. These changes are supported by the theoretical calculations which indicate that the differences in electronic structure are mainly caused by the nature of the halide ion rather than structural differences. The combination of photoelectron spectroscopy measurements and electronic structure calculations is essential to disentangle how the valence band edge in organolead halide perovskites is governed by the intrinsic difference in energy levels of the halide ions from the influence of chemical bonding.

Determination of band offsets at the Al:ZnO/Cu2SnS3 interface using X-ray photoelectron spectroscopy

The Al:ZnO/Cu2SnS3 semiconductor heterojunction was fabricated. The structural and optical properties of the semiconductor materials were studied. The band offset at the Al:ZnO/Cu2SnS3 heterojunction was studied using X-ray photoelectron spectroscopy technique. From the measurement of the core level energies and valence band maximum of the constituent elements, the valence band offset was calculated to be -1.1 +/- 0.24 eV and the conduction band offset was 0.9 +/- 0.34 eV. The band alignment at the heterojunction was found to be of type-I. The study of Al:ZnO/Cu2SnS3 heterojunction is useful for solar cell applications. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Efficient visible light photocatalytic activity and enhanced stability of BiOBr/Cd(OH)(2) heterostructures

Novel BioBr/Cd(OH)(2) heterostructures were synthesized by a facile chemical bath method under ambient conditions. A series of BiOBr/Cd(OH)(2) heterostructures were obtained by tuning the Bi/Cd molar ratios. The obtained heterostructures were characterized by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Optical properties were studied by UV-visible spectroscopy, diffuse reflectance spectroscopy and photoluminescence (PL). Photocatalytic studies on rhodamine B (RhB) under visible light irradiation showed that the heterostructures are very efficient photocatalysts in mild basic medium. Scavenger test studies confirmed that the photogenerated holes and superoxide radicals (O-2(center dot-)) are the main active species responsible for RhB degradation. Comparison of photoluminescence (PL) intensity suggested that an inhibited charge recombination is crucial for the degradation process over these photocatalysts. Moreover, relative positioning of the valence and conduction band edges of the semiconductors, O-2/O-2(center dot-) and (OH)-O-center dot/H2O redox potentials and HOMO-LUMO levels of RhB appear to be responsible for the hole-specificity of degradation. Photocatalytic recycling experiments indicated the high stability of the catalysts in the reaction medium without any significant loss of activity. This study hence concludes that the heterojunction constructed between Cd(OH)(2) and BiOBr interfaces play a crucial role in influencing the charge carrier dynamics and subsequent photocatalytic activity.

Binary group III-nitride based heterostructures: band offsets and transport properties

In the last few years, there has been remarkable progress in the development of group III-nitride based materials because of their potential application in fabricating various optoelectronic devices such as light emitting diodes, laser diodes, tandem solar cells and field effect transistors. In order to realize these devices, growth of device quality heterostructures are required. One of the most interesting properties of a semiconductor heterostructure interface is its Schottky barrier height, which is a measure of the mismatch of the energy levels for the majority carriers across the heterojunction interface. Recently, the growth of non-polar III-nitrides has been an important subject due to its potential improvement on the efficiency of III-nitride-based opto-electronic devices. It is well known that the c-axis oriented optoelectronic devices are strongly affected by the intrinsic spontaneous and piezoelectric polarization fields, which results in the low electron-hole recombination efficiency. One of the useful approaches for eliminating the piezoelectric polarization effects is to fabricate nitride-based devices along non-polar and semi-polar directions. Heterostructures grown on these orientations are receiving a lot of focus due to enhanced behaviour. In the present review article discussion has been carried out on the growth of III-nitride binary alloys and properties of GaN/Si, InN/Si, polar InN/GaN, and nonpolar InN/GaN heterostructures followed by studies on band offsets of III-nitride semiconductor heterostructures using the x-ray photoelectron spectroscopy technique. Current transport mechanisms of these heterostructures are also discussed.

Vesicular Nanostructure Formation by Self-Assembly of Anisotropic Penta-phenol-Substituted Fullerene in Water

A study on self-assembly of anisotropically substituted penta-aryl fullerenes in water has been reported. The penta-phenol-substituted amphiphilic fullerene derivative C60Ph5(OH)(5)],exhibited self-assembled vesicular nanostructures in water under the experimental conditions. The size of the vesicles Was observed to depend upon the kinetics of self-assembly and could be varied from similar to 300 to similar to 70 nm. Our mechanistic study indicated that the self-assembly of C60Ph5(OH)(5) is driven by extensive intermolecular as well as water-mediated hydrogen :bonding along with fullerene-fullerene hydrophobic interaction in water. The cumulative effect of these interactions is responsible for the stability of vesicular structures even on the removal of solvent. The substitution of phenol with anisole resulted in different packing and interaction of the fullerene derivative, as Indicated in the molecular dynamics studies, thus resulting in different self-assembled nanostructures. The hollow vesicles were further encapsulated with a hydrophobic conjugated polymer and water-soluble dye as guest molecules. Such confinement of pi-conjugated polymers in fullerene has significance in bulk heterojunction devices for efficient exciton diffusion.