On Geometric Phase from Pure Projections

The geometric phase is usually treated as a quantity modulo 2\pi, a convention carried over from early work on the subject. The results of a series of optical interference experiments involving polarization of light, done by the present author (reviewed in R.Bhandari, Phys. Rep. 281 (1997) p.1) question the usefulness of such a definition of the geometric phase in that it throws away useful and measurable information about the system, for example strengths of singularities giving rise to the geometric phase. Such singularities have been directly demonstrated by phase-shift measurement in interference experiments. In this paper, two recent polarization experiments (Hariharan et.al., J.Mod.Opt. 44 (1997)p.707 and Berry and Klein, J.Mod.Opt. 43 (1996)p.165) are analysed and compared with previous experiments and potentially detectible singularities in these experiments pointed out.Comment: Latex, 15 pages, 6 figures; ([email protected]

Comment on "Neutron Interferometric Observation of Noncyclic Phase"

A critique of a recent experiment [Wagh et.al., Phys.Rev.Lett.81, 1992 (7 Sep 1998)] to measure the noncyclic phase associated with a precessing neutron spin in a neutron interferometer, as given by the Pancharatnam criterion, is presented. It is pointed out that since the experiment measures, not the noncyclic phase itself, but a quantity derived from it, it misses the most interesting feature of such a phase, namely the different sign associated with states lying in the upper and the lower hemispheres, a feature originating in the existence of a phase singularity. Such effects have earlier been predicted and seen in optical interference experiments using polarization of light as the spinor [Bhandari, Phys.Rep.281, 1 (Mar 1997)].Comment: 5 pages, 0 figures, submitted to Phys.Rev.Let

Observable Dirac-type singularities in Berry's phase and the monopole

The physical reality and observability of 2n\pi Berry phases, as opposed to the usually considered modulo 2\pi topological phases is demonstrated with the help of computer simulation of a model adiabatic evolution whose parameters are varied along a closed loop in the parameter space. Using the analogy of Berry's phase with the Dirac monopole, it is concluded that an interferometer loop taken around a magnetic monopole of strength n/2 yields an observable 2n\pi phase shift, where n is an integer. An experiment to observe the effect is proposed.Comment: 12 pages Latex, 3 postscript figures; submitted to Physical Review Letters 15 September 2000; revised 19 November 200

On singularities of the mixed state phase

A recent proposal of Sjoqvist et.al. to extend Pancharatnam's criterion for phase difference between two different pure states to the case of mixed states in quantum mechanics is analyzed and the existence of phase singularities in the parameter space of an interference experiment with particles in mixed states pointed out. In the vicinity of such singular points the phase changes sharply and precisely at these points it becomes undefined. A closed circuit in the parameter space around such points results in a measurable phase shift equal to 2n\pi, where n is an integer. Such effects have earlier been observed in interference experiments with pure polarization states of light, a system isomorphic to the spin-1/2 system in quantum mechanics. Implications of phase singularities for the interpretation of experiments with partially polarized and unpolarized neutrons are discussed. New kinds of topological phases involving variables representing decoherence (depolarization) of pure states are predicted and experiments to verify them suggested.Comment: 4 pages Latex, 1 postscript figure; submitted to Physical Review Letters 12 Dec 2000; Revised on 13 August 200

Integrating Diverse Materials for Carbon Perovskite Solar Cells: Examining the Performance and Stability

The emergence of perovskite solar cells (PSCs) in a "catfish effect" of other conventional photovoltaic technologies with the massive growth of power conversion efficiency (PCE) with a simple manufacturing process has given a new direction to the entire solar energy field. Usually, PSC components such as electron transport material (ETM), perovskite sensitizer, hole transport material (HTM), and electrode materials need to be appropriately aligned according to the electron transfer and recombination process in order to achieve the best out of the device. Despite the enormous amount of research, the stability, reactivity, and cost issues of noble metal (Au, Ag) electrode-based traditional PSC devices are becoming obstacles to marketization. Due to the low fabrication cost and enhanced ambient stability, carbon counter electrode-based PSC (CPSC) evolved as a suitable alternative in such scenarios. These CPSCs are still in a stage of development where different fabrication engineering, designs and materials are being investigated to attain a comparable state with the standard commercialized photovoltaics. To date, hardly any report is available on ambient CPSC with PCE over 15% and stability of ~1000h without encapsulation, which opens up the window for more research. The fundamental objective of this thesis work was to develop high-performance ambient CPSC with PCE > 15% under 1SUN AM 1.5 illumination, maintaining the stability of ~1000h. This was achieved using alternative ETM and HTM with strategic incorporation instead of traditional ones. Noticeably, the temperature is a crucial parameter to attain and, at the same time, retain the aimed PV performance and stability. Therefore, a physico-thermal investigation was performed to understand the effect of temperature on the fabricated CPSC devices. The understanding further helped to examine the possible futuristic application of CPSC as the semi-transparent device for energy savings build environment. To achieve the goals of this thesis, the 1st step involved finding out suitable combination of HTM and carbon counter electrodes highlighted in chapter 3. For the first time, the fully printable mesoporous CPSCs are demonstrated with concentration-dependent WO3 (5, 7.5, and 10% by volume) nanoparticles incorporated in carbon electrodes fabricated under ambient conditions. The highest PCE ~10.5% was obtained with the 7.5% WO3/carbon device; however, the 10% WO3/carbon device exhibited better ambient stability of ~600h. Besides, graphene/ poly(3,4-ethylene dioxythiophene) polystyrene sulfonate (PEDOT: PSS) was introduced as an alternative HTM with novel light soaking and surface wettability strategies, and an enhanced PV performance with PCE >11% was achieved. In search of an alternative HTM/carbon combination with more superior performance, a novel and cost-effective synthesis process of graphitic CNP as a suitable counter electrode and its combination with NiO was visualized. The stability test of the high-temperature counter electrode strategy of CNP/NiO showed ~1000 h air stability with negligible efficiency loss having a maximum PCE of 13.2%, whereas the low-temperature strategy of CNP/NiO devices showed 14.2% PCE with ~650 h air stability. Thus CNP/NiO combination achieved performance very close to the aims of this thesis, which was enhanced to the required performance by introducing alternative ETM for the devices. Chapter 4 describes the strategic incorporation of morphology modulated BaSnO3 (BSO) and brookite TiO2 (BTO) nanostructures in place of conventional anatase TiO2 as ETM to successfully achieve PCE >13.5% and >15%, respectively, with stability >1000 h. The enhanced electron transport and reduced charge recombination by rod-based nanostructures of BSO and BTO displayed the best performance for the types to date in CPSC. Along with performance improvement, the understanding of CPSC’s temperature behaviour was considered in this thesis to understand the real-world feasibility of CPSC for the first time. The temperature coefficients (TC) of photovoltaic parameters for MAPbI3-based devices are demonstrated in chapter 5 with a detailed physico-chemical understanding. Besides CH3NH3PbI3, other perovskites such as CH3NH3PbI3-xClx and Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 were applied as an alternative sensitizer for the CPSCs and studied their temperature coefficients across a wide range of real-world temperatures to obtain behavioural differences between the halide perovskites. Finally, the suitability of semi-transparent CPSC for fenestration integration was evaluated for the first time via fabrication engineering and thickness control with the highest reported average visible transmittance/PCE combination to date, as discussed in chapter 6. Finally, in this thesis work, CPSC devices are explored, which highlights fascinating ambient fabrication processes and significant device performance with new series of HTMs, ETMs and designs for futuristic applications