Molecular Environment-Tailored, Self-Assembled and Nanomorphology-Controlled Electron Acceptors for High-Performance Solar Cells

University Of Washington, Seattle WA

Investigators

Abstract

PI: Ma, Hong Proposal Number: 1236272 Institution: University of Washington Title: Molecular Environment-Tailored, Self-Assembled and Nanomorphology-Controlled Electron Acceptors for High-Performance Solar Cells So far, the most efficient architecture to build polymeric solar cells (PSCs) is the bulk heterojunction (BHJ) structure prepared by mixing electron-rich polymers and electron-deficient fullerides such as [6,6]-phenyl-C61 (or C71) butyric acid methyl ester (PCBMs) with a power-conversion efficiency of over 8%. However, fullerene derivatives have undesirable properties such as a relatively large bandgap and low absorption coefficient in the visible spectrum to lack ideal absorption of solar radiation, and excessively deep lying LUMO level to result in needless energy loss during electron transfer and so limit the efficiencies of the final devices, in particular their open-circuit voltages. Thus, there is an urgent need for alternative acceptor materials that - like PCBMs - possess favorable electron-transporting and processing properties, but which also absorb strongly in the solar spectrum, have energy levels significantly different from those of fullerene-based acceptors, and exhibit diversification regarding derivatization and functionalization. In addition, it is desirable to have electron acceptors with the combination of tunable molecular structures, controlled nanomorphology in electron donor (D)/acceptor (A) blends, and tailored D/A interfaces in optimized device architectures, which allow maximized exciton generation/migration and charge separation/transport/collection. The main objectives of this project are 1) development of electron acceptors through tailoring of pi-conjugated core/bridge and peripheral group to have tunable energetics (bandgap, HOMO, LUMO), controlling molecular dielectric, dipolar and steric environment to favor efficient D/A interfacial charge separation, and introducing thermo-/photo-removable, hydrogen-bonding and surface-binding groups to allow processability, monolayer-templated self- assembly and nanophase formation for optimized charge transport and collection; 2) photophysical, electronic and morphological studies of these electron acceptors in solution/thin film and their blends with variable ratios of low-bandgap p-type polymers, and establishment of a correlation between molecular structures and material properties such as light absorption, electronic property, photoluminescence quenching, and charge separation/transport dependent on molecular environment and nanophase control; 3) fabrication and testing of conventional and inverted PSC devices by utilizing optimized electron acceptors as component of active layer through spin-cast bulk heterojunction or patterned monolayer-induced assembly of D/A nanophases toward high power-conversion efficiency above 10% and good stability. Rational design, synthesis and processing of novel electron acceptors with tailorable molecular environment, self- assembling property and controlled nanophase, and their device fabrication and testing will provide a transformative approach to fulfill these requirements, and establish a new platform in fundamental understanding of relationship among molecular structure, processing, property and device performance in particular on D/A interfacial charge separation dependent on dielectric, dipolar and steric parameters. This project will have broad impact through the training of new professionals who are skilled with state-of-the-art research techniques and are sensible, creative, and devoted to solve the social need. This project will include education activities at different levels covering K-12 to graduate students.