Theoretical studies of charge transfer excitations, absorption, and polarisation in organic photovoltaic materials

To optimise organic photovoltaic devices, design rules relating chemical and physical structure to the ability of the active material to absorb light, separate charges and transport charges, are required. We assess the predictive power of computational modelling techniques, comparing results with experiment. In Chapter 4, we calculate the influence of chemical structure on conformation and light absorption in conjugated polymers, and explain the remarkably high optical absorption in poly-thieno[3,2b]-thiophene-diketo-pyrrolopyrrole-co-thiophene (PDPP-TT-T) in terms of its high persistence length. We calculate that diketopyrrolopyrrole does not act as an electron acceptor in PDPP-TT-T or other oligomers, and find no correlation between transition dipole moment and degree of charge transfer between units. Chapter 5 reviews recent developments in modelling charge pair generation, focussing on electronic structure calculations of interfacial states, electrostatic models, excited state dynamics, and remaining challenges in achieving a predictive approach. In Chapter 6, we study the influence of chemical structure and mutual position of oligomer:fullerene pairs on interfacial state properties using time-dependent density functional theory, reproducing energy trends measured by electroluminescence in corresponding polymer:fullerene systems. For several isoindigo and diketopyrrolopyrrole based cooligomers, we calculate a lowest excitation energy close in energy to that of pairs with fullerene. The natures and energies of these pair states are strongly dependent on fullerene position. We calculate the effect of thiophene substitution in PDPP-TT-T, and rationalise observed charge separation efficiency of corresponding experimental systems. In Chapter 7, we calculate the influence of chemical structure on electronic polarisabilities of functionalised fullerenes using the Hartree Fock method, and the influence of molecular polarisability, lattice structure, and direction of charge separation on dielectric constants in C60 and PCBM, using a polarisable dipole model. We calculate the influence of electronic polarisation on charge delocalisation, and its dependence upon chemical structure and packing, and discuss relevance to charge transport properties.Open Acces

Decision making for transformative change: exploring model use, structural uncertainty and deep leverage points for change in decision making under deep uncertainty

Moving to a low carbon society requires pro-active decisions to transform social and physical systems and their supporting infrastructure. However, the inherent complexity of these systems leads to uncertainty in their responses to interventions, and their critical societal role means that stakes are high. Techniques for decision making under deep uncertainty (DMDU) have recently begun to be applied in the context of transformation to a low carbon society. Applying DMDU to support transformation necessitates careful attention to uncertainty in system relationships (structural uncertainty), and to actions targeting deep leverage points to transform system relationships. This paper presents outcomes of a structured literature review of 44 case studies in which DMDU is applied to infrastructure decisions. Around half of these studies are found to neglect structural uncertainty entirely, and no study explicitly considers alternative system conceptions. Three quarters of studies consider actions targeting only parameters, a shallow leverage point for system transformation. Where actions targeting deeper leverage points are included, models of system relationships are unable to represent the transformative change these interventions could effect. The lack of attention to structural uncertainty in these studies could lead to misleading results in complex and poorly understood systems. The lack of interventions targeting deep leverage points could lead to neglect of some of the most effective routes to achieving transformative change. This review recommends greater attention to deeper leverage points and structural uncertainty in applications of DMDU targeting transformative change

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

The Impact of Shale Gas on the Cost and Feasibility of Meeting Climate Targets—A Global Energy System Model Analysis and an Exploration of Uncertainties

There exists considerable uncertainty over both shale and conventional gas resource availability and extraction costs, as well as the fugitive methane emissions associated with shale gas extraction and its possible role in mitigating climate change. This study uses a multi-region energy system model, TIAM (TIMES integrated assessment model), to consider the impact of a range of conventional and shale gas cost and availability assessments on mitigation scenarios aimed at achieving a limit to global warming of below 2 °C in 2100, with a 50% likelihood. When adding shale gas to the global energy mix, the reduction to the global energy system cost is relatively small (up to 0.4%), and the mitigation cost increases by 1%–3% under all cost assumptions. The impact of a “dash for shale gas”, of unavailability of carbon capture and storage, of increased barriers to investment in low carbon technologies, and of higher than expected leakage rates, are also considered; and are each found to have the potential to increase the cost and reduce feasibility of meeting global temperature goals. We conclude that the extraction of shale gas is not likely to significantly reduce the effort required to mitigate climate change under globally coordinated action, but could increase required mitigation effort if not handled sufficiently carefully

Energy system changes in 1.5 °C, well below 2 °C and 2 °C scenarios

Meeting the Paris Agreement's goal to limit global warming to well below 2 °C and pursuing efforts towards 1.5 °C is likely to require more rapid and fundamental energy system changes than the previously-agreed 2 °C target. Here we assess over 200 integrated assessment model scenarios which achieve 2 °C and well-below 2 °C targets, drawn from the IPCC's fifth assessment report database combined with a set of 1.5 °C scenarios produced in recent years. We specifically assess differences in a range of near-term indicators describing CO2 emissions reductions pathways, changes in primary energy and final energy across the economy's major sectors, in addition to more detailed metrics around the use of carbon capture and storage (CCS), negative emissions, low-carbon electricity and hydrogen. © 2018 The Author

Influence of Chemical Structure on the Charge Transfer State Spectrum of a Polymer:Fullerene Complex

Charge transfer (CT) state properties at the donor:acceptor interface in organic photovoltaic materials are widely regarded as crucial in determining the charge separation efficiency of organic photovoltaic devices. In this work, we use time dependent density functional theory (TDDFT) with B3LYP/6-31g* to study the influence of chemical structure on excitation energies, oscillator strengths, and electronic structure. We vertically excite states in a series of polymer:fullerene blends, modeling each blend as an oligomer:fullerene pair in vacuo. Our method reproduces experimentally observed trends in CT state energy with chemical structure as measured by electroluminescence. For oligothiophene:PCBM (PCBM = [6,6]-phenyl C61-butyric acid methyl ester), we find that Coulomb binding tends to reduce in higher excited CT states. In the case of several isoindigo and diketopyrrolopyrrole (DPP) based donors, we find that the first excited state of the pair lies close in energy to the first singlet of the oligomers, and low excited states have hybrid, or incomplete charge transfer, character. The natures and energies of these states are dependent on the fullerene position. We discuss the effect of thiophene substitution in a DPP polymer on charge generation in terms of the calculated CT state properties and rationalize the observed charge separation efficiency of corresponding experimental systems in terms of these calculations

The role of mini-grids for electricity access and climate change mitigation in India

Sustainable Development Goal 7 aims to achieve access to sustainable, affordable, reliable, and modern energy for all. Access to electricity is critical for development and economic growth and can support productive livelihoods and power critical community services such as for healthcare and education. Solar mini-grids can offer the most cost-effective option for rural and remote communities not yet connected to the grid and can deliver reliable, high-quality power which is able to serve multiple uses and meet growing demand over time