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