Dilute-Donor Organic Solar Cells: Breaking the Fullerene Monopoly

University Of Texas At Dallas, Richardson TX

Investigators

Abstract

Solar energy is among the most attractive renewable energy sources, motivating fundamental research to enable low-cost technologies that are adaptable for a variety of applications. Organic photovoltaic solar cells (OPVs), based on small organic molecules and polymers, are a potentially suitable option. OPVs are lightweight, made of environmentally benign materials, and can be manufactured by inexpensive methods. This technology provides a path towards a lightweight, flexible platform for integration of renewable electricity production capacity into building infrastructure. OPV technology can also generate electricity for indoor displays, automobile sensors, health monitors, and the internet of things. A new kind of OPV, the dilute-donor organic solar cell, has recently been demonstrated to overcome previous technological shortcomings such as the cell's current-versus-voltage trade off. To date, there is limited fundamental scientific understanding of how electricity is generated in dilute-donor OPVs. This project will investigate the established fullerene-based dilute-donor OPV systems and then use this information to rationally design new systems with other materials for improved performance and solar conversion efficiency. In addition to technical advancements, the project includes educational activities on energy science for high school and undergraduate students, career guidance for graduate students, and outreach activities to K-12 students and the general public in the Northern Texas area through a partnership with UT Dallas Science and Engineering Education Center. The focus of this fundamental research project is to expand the understanding of photogenerated charge carrier transport dynamics in dilute donor organic photovoltaics. The active layers in current high-performing OPV systems are based on the bulk heterojunction (BHJ) nanostructure, i.e. a blend of donor (D) and acceptor (A) molecules in roughly equal amounts. BHJ based cells have two fundamental drawbacks: (1) there is a trade-off between short circuit current density and open circuit voltage; and (2) the performance is highly sensitive to the nanoscale morphology, which to date has been optimized by trial and error. Recently, a different kind of OPV system which circumvents the trade-off between short circuit current density and open circuit voltage has gained attention. Fullerene-based OSCs, also called dilute-donor OPVs, contain only a small amount of donor material embedded in the fullerene matrix and show a substantial short circuit current density while maintaining a high open circuit voltage. While the high voltage value in the dilute-donor system is largely accepted to be determined by the Schottky barrier height between the metal electrode and the acceptor matrix, the origin of a high current density without percolating paths for holes was unknown. This project lays out a joint experimental and theoretical modeling study to investigate the photo-generation, transport, and recombination of carriers in dilute-donor OPV systems. So far, all known dilute-donor OPV studies use fullerene acceptors as the matrix material. However, the low absorption coefficient and low carrier mobilities of fullerenes are a fundamental weakness. To address this, the project will investigate ambipolar materials with smaller band gaps and higher carrier mobilities as matrix materials, to increase light absorption and improve carrier transport, respectively. Drift-diffusion calculations using Technology Computer-Aided Design combined with kinetic Monte Carlo simulations will be performed to complement the experimental activities. The Monte Carlo analysis will reveal the fundamentals behind the relative importance of various material properties and device structures in dilute-donor OPVs. The theoretical results will help guide the experimental selection of materials. The joint effort will lead to the rational design of dilute-donor OPVs with high short circuit current density and high open circuit voltage. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.