Physics

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The relationship between charge carrier lifetime and mobility in a bulk heterojunction based organic solar cell, utilizing diketopyrrolopyrole- naphthalene co-polymer and PC71BM in the photoactive blend layer, is investigated using the photoinduced charge extraction by linearly increasing voltage technique. Light intensity, delay time, and temperature dependent experiments are used to quantify the charge carrier mobility and density as well as the temperature dependence of both. From the saturation of photoinduced current at high laser intensities, it is shown that Langevin-type bimolecular recombination is present in the studied system. The charge carrier lifetime, especially in Langevin systems, is discussed to be an ambiguous and unreliable parameter to determine the performance of organic solar cells, because of the dependence of charge carrier lifetime on charge carrier density, mobility, and type of recombination. It is revealed that the relation between charge mobility (μ) and lifetime (τ) is inversely proportional, where the μτ product is independent of temperature. The results indicate that in photovoltaic systems with Langevin type bimolecular recombination, the strategies to increase the charge lifetime might not be beneficial because of an accompanying reduction in charge carrier mobility. Instead, the focus on non-Langevin mechanisms of recombination is crucial, because this allows an increase in the charge extraction rate by improving the carrier lifetime, density, and mobility simultaneously. © 2013 AIP Publishing LLC.

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(SoNS) School of Natural Sciences

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Journal of Applied Physics

Relation between charge carrier mobility and lifetime in organic photovoltaics

Scattering is an important phenomenon which is observed in systems ranging from the micro- to macroscale. In the context of nuclear reaction theory, the Heidelberg approach was proposed and later demonstrated to be applicable to many chaotic scattering systems. To model the universal properties, stochasticity is introduced to the scattering matrix on the level of the Hamiltonian by using random matrices. A long-standing problem was the computation of the distribution of the off-diagonal scattering-matrix elements. We report here an exact solution to this problem and present analytical results for systems with preserved and with violated time-reversal invariance. Our derivation is based on a new variant of the supersymmetry method. We also validate our results with scattering data obtained from experiments with microwave billiards. © 2013 American Physical Society.

Physical Review Letters

Distribution of scattering matrix elements in quantum chaotic scattering

The capillary flow in SU8 based microfluidic devices fabricated in SF-100 by lithographic technique has been recorded and analysed in this report. Leakage free microfluidic flow is achieved by indirect bonding between polymethyl methacrylate (PMMA) coated glass lid surface and SU8 channel substrate during hard baking. The working liquids are dyed ethylene glycol, dyed ethanol, dyed isopropyl alcohol (IPA) and dyed liquid mixture (water and IPA). The capillary flow of dyed ethylene glycol is found to be slower in each device due to higher viscosity and lower surface wettability as compared to other working liquids. Square SU8 micropillar arrays (80 μm and 120 μm of side lengths) are fabricated on the glass bottom wall surface of SU8 based microchannel. The capillary flow of any particular working liquid is observed to be faster through the square micropillars of smaller side length due to their lower surface area to volume ratio in the device. Also, the capillary flow of polar liquids is found to be faster on the microchannel surface of higher polar component of surface free energy. The above results are highly significant to control the speed of capillary flow in lab-on-a-chip systems. © 2012 Elsevier Ltd.

International Journal of Adhesion and Adhesives

Effects of liquid viscosity, surface wettability and channel geometry on capillary flow in SU8 based microfluidic devices

31P nuclear-magnetic-resonance (NMR) study in the trimer spin chain compound Ca3Cu2Ni(PO4)4 is performed in the presence of different external magnetic fields in the temperature range 4-300 K. The results suggest that the three magnon mediated scattering process which was the dominant contribution to 1/T1 in the pure compound Ca3Cu3(PO4)4, has negligible contribution in Ca3Cu2Ni(PO4) 4, with the two magnon mediated Raman process being dominant. The linear variation of nuclear spin lattice relaxation rate 1/T1T with the magnetic susceptibility $\chi$ indicates that the Q component of magnetic susceptibility $\chi$(Q) is nearly proportional to $\chi$, where Q is the antiferromagnetic wave vector. A change in the slope of 1/T1T with respect to $\chi$ near 15 K suggests a change in the antiferromagnetic spin fluctuation spectrum. Below 15 K, the spin diffusion mechanism governs the 1/T1 process. This could be a signature of the development of short range magnetic correlation or due to the contribution arising from the paramagnetic impurity spins. {\textcopyright} 2013 Elsevier B.V. All rights reserved.

Physica B: Condensed Matter

Low energy spin dynamics in trimer spin chain compound Ca 3Cu2Ni(PO4)4: 31P NMR study

We investigate the blend morphology and performance of bulk heterojunction organic photovoltaic devices comprising the donor polymer, pDPP-TNT (poly{3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1, 4-dione-alt-naphthalene}) and the fullerene acceptor, [70]PCBM ([6,6]-phenyl C71-butyric acid methyl ester). The blend morphology is heavily dependent upon the solvent system used in the fabrication of thin films. Thin films spin-coated from chloroform possess a cobblestone-like morphology, consisting of thick, round-shaped [70]PCBM-rich mounds separated by thin polymer-rich valleys. The size of the [70]PCBM domains is found to depend on the overall film thickness. Thin films spin-coated from a chloroform:dichlorobenzene mixed solvent system are smooth and consist of a network of pDPP-TNT nanofibers embedded in a [70]PCBM-rich matrix. Rinsing the films in hexane selectively removes [70]PCBM and allows for analysis of domain size and purity. It also provides a means for investigating exciton dissociation efficiency through relative photoluminescence yield measurements. Devices fabricated from chloroform solutions show much poorer performance than the devices fabricated from the mixed solvent system; this disparity in performance is seen to be more pronounced with increasing film thickness. The primary cause for the improved performance of devices fabricated from mixed solvents is attributed to the greater donor-acceptor interfacial area and resulting greater capacity for charge carrier generation. © The Royal Society of Chemistry 2013.

RSC Advances

Nanoscale phase domain structure and associated device performance of organic solar cells based on a diketopyrrolopyrrole polymer

We explore the Jacobi last multiplier (JLM) as a means for deriving the Lagrangian of a fourth-order differential equation. In particular, we consider the classical Pais-Uhlenbeck problem and write down the accompanying Hamiltonian. We then compare such an expression with our alternative derivation of the Hamiltonian that makes use of the Ostrogradski's method and show how a mapping from the one to the other is achievable by variable transformations. © 2013 World Scientific Publishing Company.

Modern Physics Letters A

Comments on the structural features of the Pais-Uhlenbeck oscillator

We propose a new mechanism for generating a CP phase via a Higgs vacuum expectation value originating from geometry of an extra dimension. A twisted boundary condition is the key to produce an extra-dimension coordinate-dependent vacuum expectation value, which contains a CP phase degree of freedom and can be a new source of a CP phase in higher-dimensional gauge theories. As an illustrative example, we apply our mechanism to a five-dimensional gauge theory with point interactions and show that our mechanism can dynamically produce a nontrivial CP-violating phase with electroweak symmetry breaking, even though the five-dimensional model does not include any CP-violating phases of Yukawa couplings in the five-dimensional Lagrangian because of a single generation of five-dimensional fermions. We apply our mechanism to a model with point interactions, which has no source of CP-violating phases in the couplings of the higher-dimensional action, and show that a nontrivial CP phase dynamically appears. © 2013 American Physical Society.

Physical Review D - Particles, Fields, Gravitation and Cosmology

CP phase from twisted Higgs vacuum expectation value in extra dimension

We propose a new model which can simultaneously and naturally explain the origins of fermion generation, quark mass hierarchy, and the Cabibbo-Kobayashi-Maskawa matrix from the geometry of an extra dimension.We take the extra dimension to be an interval with point interactions, which are additional boundary points in the bulk space of the interval. Because of the Dirichlet boundary condition for fermions at the positions of point interactions, profiles of chiral fermion zero modes are split and localized, and then we can realize three generations from each five-dimensional Dirac fermion. Our model allows fermion flavor mixing but the form of the non-diagonal elements of fermion mass matrices is found to be severely restricted due to the geometry of the extra dimension. The Robin boundary condition for a scalar leads to an extra coordinate-dependent vacuum expectation value, which can naturally explain the fermion mass hierarchy. © The Author(s) 2013.

Progress of Theoretical and Experimental Physics

Quark mass hierarchy and mixing via geometry of extra dimension with point interactions

Confronting Precision Cosmology with the Minimal Supersymmetric Standard Model

75As and 31P NMR studies are performed in PrCoAsO and NdCoPO respectively. The Knight shift data in PrCoAsO indicate the presence of an antiferromagnetic interaction between the 4f moments along the c axis in the ferromagnetic state of Co 3d moments. We propose a possible spin structure in this system. The 75As quadrupolar coupling constant, $\nu$Q, increases continuously with decrease of temperature and is found to vary linearly with the intrinsic spin susceptibility, Kiso. This indicates the possibility of the presence of a coupling between charge density and spin density fluctuations. Further, the 31P NMR Knight shift and spin-lattice relaxation rate (1/T1) in the paramagnetic state of NdCoPO indicate that the differences of LaCoPO and NdCoPO from SmCoPO are due to the decrement of the interlayer separation and not due to the moments of the 4f electrons. The nuclear spin-lattice relaxation time (T1) in NdCoPO shows weak anisotropy at 300 K. Using the self-consistent renormalization (SCR) theory of itinerant ferromagnets, it is shown that in the ab plane, the spin fluctuations are three-dimensional ferromagnetic in nature. From SCR theory the important spin-fluctuation parameters (T0, T A, ) are evaluated. The similarities and dissimilarities of the NMR results in As and P based systems with different rare earths are also discussed. {\textcopyright} 2013 IOP Publishing Ltd.