Organic photovoltaics: tracking the open-circuit voltage of bulk-heterojunction solar cells comprising non-fullerene ternary blends

Abstract

Organic solar cells, with attractive qualities, such as mechanical flexibility and solution processability, have the potential to be useful alternatives to their silicon counterparts in unique form factor applications if their power conversion efficiencies can be improved. One processing method shown to improve OPV efficiency is ternary blend active layers, comprised of two electron acceptors and a donor, or two electron donors and an acceptor. It has been proven that the open circuit voltage (VOC) can be tuned in ternary blends comprised of two polymer donors and a small molecule acceptor or of one donor and two fullerene acceptors; the tunability of the VOC is dependent on the chemical and compatibility of the two polymer donors or the two fullerene acceptors. In this thesis, the behavior of binary and ternary blends comprised of a single donor (P3HT, a common polymer donor) and three structurally similar non-fullerene acceptors was characterized. Two ternary systems were demonstrated to exhibit tunable VOCs across the acceptor composition range explored, while holding P3HT fraction constant. The binary and ternary systems were characterized from an electronic as well as morphological perspective through methods including ionization potential (IP) measurements, UV-visible spectroscopy (UV-vis), differential scanning calorimetry (DSC), grazing-incidence X-ray diffraction (GIXD), and Flory-Huggins interaction (χ) parameters. In agreement with the literature, both acceptor pairs were found to be highly miscible and to have small energetic offsets, meeting the requirements for VOC tunability.