Vibrational Spectroscopy of Electronic Processes in Emerging Photovoltaic Materials

Abstract

Molecules affect the electronic properties of many emerging materials, ranging from organic thin film transistors and light emitting diodes for flexible displays to colloidal quantum dots (CQDs) used in solution processed photovoltaics and photodetectors. For example, the interactions of conjugated molecules not only influence morphological and charge transport properties of organic photovoltaic (OPV) materials, but they also determine the primary photophysical events leading to charge generation. Ligand-nanocrystal interactions affect the density and energetic distributions of trap states, which in turn influence minority carrier transport in CQD photovoltaics. Therefore, it is critical for scientists to understand how the underlying molecular structures and morphologies determine the electronic properties of emerging materials. Recently, chemists have used vibrational spectroscopy to study electronic processes in emerging materials, and been able to directly measure the influence molecular properties have on those processes. Time-resolved vibrational spectroscopy is uniquely positioned to examine molecular species involved in electronic processes because it combines ultrafast time resolution with measurement of the vibrational spectra of molecules. For instance, molecules at the electron donor/acceptor interfaces in OPV materials have unique vibrational features because vibrational frequencies of molecules are sensitive to their local molecular environments. Through ultrafast vibrational spectroscopy, researchers can directly examine the dynamics of charge transfer (CT) state formation and dissociation to form charge separated states specifically at donor/acceptor interfaces. Vibrational modes of ligands are also sensitive to their bonding interactions with nanocrystal surfaces, which enables chemists to directly probe the molecular nature of charge trap states in colloidal quantum dot solids. Because of the ability to connect electrical properties with the underlying molecular species, scientists can use ultrafast vibrational spectroscopy to address fundamental challenges in the development of emerging electronic materials. In this Account, we focus on two applications of vibrational spectroscopy to examine electronic processes in OPV and CQD photovoltaic materials. In the first application, we examine archetypal classes of electron acceptors in OPV materials and reveal how their molecular structures influence the dynamics and energetic barriers to CT state formation and dissociation. In the second application, we discuss the surface chemistry of ligand-nanocrystal interactions and how they impact the density and energetic distribution of charge trap states in CQD photovoltaic materials. Through direct observations of the vibrational features of ligands attached to surface trap states, we can obtain valuable insights into the nature of charge traps and begin to understand pathways for their elimination. We expect that further examination of electronic processes in materials using ultrafast vibrational spectroscopy will lead to new design rules in support of continued materials development efforts.