Analytical and Numerical Study of Photocurrent Transients in Organic Polymer Solar Cells

This article is an attempt to provide a self consistent picture, including existence analysis and numerical solution algorithms, of the mathematical problems arising from modeling photocurrent transients in Organic-polymer Solar Cells (OSCs). The mathematical model for OSCs consists of a system of nonlinear diffusion-reaction partial differential equations (PDEs) with electrostatic convection, coupled to a kinetic ordinary differential equation (ODE). We propose a suitable reformulation of the model that allows us to prove the existence of a solution in both stationary and transient conditions and to better highlight the role of exciton dynamics in determining the device turn-on time. For the numerical treatment of the problem, we carry out a temporal semi-discretization using an implicit adaptive method, and the resulting sequence of differential subproblems is linearized using the Newton-Raphson method with inexact evaluation of the Jacobian. Then, we use exponentially fitted finite elements for the spatial discretization, and we carry out a thorough validation of the computational model by extensively investigating the impact of the model parameters on photocurrent transient times.Comment: 20 pages, 11 figure

Do ultrafast exciton-polaron decoherence dynamics govern photocarrier generation efficiencies in polymer solar cells?

All-organic-based photovoltaic solar cells have attracted considerable attention because of their low-cost processing and short energy payback time. In such systems the primary dissociation of an optical excitation into a pair of photocarriers has been recently shown to be extremely rapid and efficient, but the physical reason for this remains unclear. Here, two-dimensional photocurrent excitation spectroscopy, a novel non-linear optical spectroscopy, is used to probe the ultrafast coherent decay of photoexcitations into charge-producing states in a polymer:fullerene based solar cell. The two-dimensional photocurrent spectra are interpreted by introducing a theoretical model for the description of the coupling of the electronic states of the system to an external environment and to the applied laser fields. The experimental data show no cross-peaks in the two-dimensional photocurrent spectra, as predicted by the model for coherence times between the exciton and the photocurrent producing states of 20\,fs or less

Systematic derivation of a surface polarization model for planar perovskite solar cells

Increasing evidence suggests that the presence of mobile ions in perovskite solar cells can cause a current-voltage curve hysteresis. Steady state and transient current-voltage characteristics of a planar metal halide CH$_3$NH$_3$PbI$_3$ perovskite solar cell are analysed with a drift-diffusion model that accounts for both charge transport and ion vacancy motion. The high ion vacancy density within the perovskite layer gives rise to narrow Debye layers (typical width $\sim$2nm), adjacent to the interfaces with the transport layers, over which large drops in the electric potential occur and in which significant charge is stored. Large disparities between (I) the width of the Debye layers and that of the perovskite layer ($\sim$600nm) and (II) the ion vacancy density and the charge carrier densities motivate an asymptotic approach to solving the model, while the stiffness of the equations renders standard solution methods unreliable. We derive a simplified surface polarisation model in which the slow ion dynamic are replaced by interfacial (nonlinear) capacitances at the perovskite interfaces. Favourable comparison is made between the results of the asymptotic approach and numerical solutions for a realistic cell over a wide range of operating conditions of practical interest.Comment: 32 pages, 7 figure

Nitriles in Prebiotic Chemistry and Astrobiology

Life appeared on Earth within a billion years of the planet’s formation. How? - no one knows. Theories regarding the origin of life involve reactions of molecules predicted to have existed on early Earth in what is called prebiotic chemistry. In this thesis, I use computational methods to investigate hypotheses in prebiotic chemistry and astrobiology. With computational chemistry, it is possible to predict the thermodynamics and kinetics of chemical processes. The long-term goal of this line of research is furthering our understanding of the origin of life.The first part of this thesis is devoted to hydrogen cyanide (HCN) chemistry. HCN is believed to have been present on early Earth. Molecular building blocks of DNA, RNA, and proteins have been detected among HCN reaction products. However, because of HCN’s reactivity, the molecule forms numerous other compounds as well. One such set of proposed reaction products are HCN-derived polymers - a diverse group of structures which have been proposed to form in many ways. Here I present a thermodynamic landscape of HCN-derived molecules and polymers. Using the thermodynamic map, some hypothesized reaction pathways are proven to be unfeasible. Polyaminoimidazole is estimated to be one of the most stable polymers, while the nucleobase adenine is computed as the most stable of all studied structures. We also investigate the first steps in the formation of two proposed HCN reaction products: diaminomaleonitrile and polyimine. Our results reveal that all studied competing reactions have similar activation barriers. These results open for the possibility of a diverse beginning to HCN oligomerization under kinetic control. The estimated timescale of HCN oligomerization suggests that reactions in low-temperature environments as cold as 200 K could occur within thousands of years. I discuss the implications of the predicted reaction rates for HCN chemistry in astrochemical environments like comets and Saturn’s moon Titan. The second part of the thesis investigates an astrobiological hypothesis: the possibility for cryogenically operable membranes in the seas of Titan. It is concluded that a previously suggested polarity-inverted membrane made from acrylonitrile, a so-called azotosome, cannot spontaneously self-assemble and is therefore unlikely to exist on Titan

Comprehensive Study of 3-Level Stabilizer in Hybrid PV and Wind Energy Systems- A Review

When variables are bounded so that almost the whole constraints are satisfied, power system stability is typically characterized as an electric power system's ability to restore a situation of functioning equilibrium after being subjected to a physical disturbance.This paper presents a thorough investigation of three-level stabilizers in a hybrid PV Wind Energy system that is integrated with the power system. From a hybrid renewable energy system that is linked to the grid and studied with a stabilizer that uses a variety of control algorithms based on artificial intelligence algorithms developed by several researchers