UNS: Fundamental studies of charge transfer states at organic donor-acceptor interfaces for photovoltaics

Stanford University, Stanford CA

Investigators

Abstract

PI: Alberto Salleo Proposal Number: 1510481 The sun represents the most abundant potential source of sustainable energy on earth. Solar cells that use organic conducting polymers to convert light to electricity - organic photovoltaic (OPV) devices - offer a potentially low-cost route for renewable electricity production. However, in order to achieve parity with other solar photovoltaic technologies, organic solar cells must increase their power conversion efficiency past the current 10.5% world record. A principal reason for this low power output is that the electric charge transfer between the organic polymer that converts the sun's photons into electrons, and the nanostructured carbon which accepts the generated electron, does not work efficiently at the interface of these two materials. This proposed research will develop a fundamental scientific understanding of this process at the molecular level through advanced spectroscopic analysis techniques. The proposed educational activities include development of online videos for material science courses, and an undergraduate student project to use the spectroscopic methods developed in the research to characterize artifacts at the Cantor Arts Center at Stanford University. The goal of this research is to develop a fundamental understanding of the structure-property relationships for the donor-acceptor interface in organic polymer based photovoltaic (OPV) devices at the molecular level. The fundamental photocurrent generation mechanism in OPV involves splitting a tightly-bound electron-hole pair. Prior to the formation of separated charges, the electron transfer from the light absorbing polymer donor molecule to the electron acceptor molecule, typically fullerene or similar nanostructured carbon, generates a charge transfer state, where the electron and hole are formally on two different molecules and yet are still able to interact across the donor-acceptor interface. The solar energy conversion efficiency of OPV is due to low voltage generation, which is directly linked to energy of the charge transfer state, and it is hypothesized the disorder of the charge transfer state may be a causal factor. To better understand this process, model materials will be used to measure systematically how molecular orientation affects the energy of the charge transfer state. Towards this end, Fourier transform photocurrent spectroscopy will measure generated photocurrent, and a nanoscale electroluminescence technique will be developed to map the charge transfer state spatially and correlate it to well-known donor-acceptor interface configurations. Dilute ternary blends, where two mutually miscible fullerenes are blended with a low concentration of a donor polymer, will serve as model materials to investigate how compositional and structural disorder affects the energy of the charge transfer state. Complementary studies using donors with different degrees of aggregation will provide information on the dependence of this process on polymer aggregation and crystallinity. Overall, these studies will establish the structure-property relationships of the donor-acceptor interface and suggest new approaches for materials and interface design to increase voltage and power conversion efficiency of OPV devices. This fundamental knowledge may also provide insights on how to refine theoretical tools used to predict the performance and properties of organic polymer based optoelectronic materials. Research on OPV devices will be used to develop online video materials for an organic semiconductors course at Stanford University. The project will also involve undergraduate students to work with the Cantor Arts Center at Stanford University to spectroscopically characterize artifacts using the techniques developed in the proposed research.