Spectroscopic Characterization of Interfacial Charge Transfer and Recombination in Polymer/Quantum Dot Blends

Abstract

Considerable research interest in solution-processed semiconductors, including conjugated polymers and colloidal quantum dots, has been primarily motivated by the desire to produce optoelectronic devices using low-cost fabrication methods. Realization of this goal requires both improvements in material properties, and an extensive understanding of the factors regulating device operation and efficiency. This thesis is concerned with spectroscopically probing the fundamental mechanisms of photoinduced interfacial charge transfer in polymer/quantum dot composites, and exploring the role of quantum dot surface chemistry in governing the electronic properties photovoltaic devices. Specifically, we study blends of conjugated polymers with low band gap PbS quantum dots with applications to solar cells and photodetectors. Quantum dot surface ligands play an important role in mediating the spatial and electronic barrier for charge transfer at the donor/acceptor interface in bulk heterojunction polymer/quantum dot blends. The first part of this thesis explores the effects of postdeposition ligand exchange on the device performance and electronic properties of conjugated polymer composites with PbS quantum dots. We tested a series of ligand exchanges with small bidentate organic molecules, and inorganic iodide ions, and determined that the different treatments influence device performance mainly through changes in open-circuit voltage and fill factor. We found that treatment with 3-mercaptopropionic acid (MPA) yields the highest device efficiencies, and iodide treatment gives the lowest efficiencies. Photoinduced absorption (PIA) experiments showed that MPA treatment results in both greater long-lived polaron populations and longer average polaron lifetimes. We further employed transient photovoltage (TPV) and charge extraction (CE) techniques on solar cell devices to determine that MPA treatment yields greater open-circuit voltages and higher charge carrier densities by promoting longer carrier recombination lifetimes compared to the other ligand treatments. We speculate that these observations are due to differences in carrier mobility and lifetime, in addition to changes in quantum dot energy level alignment relative to the polymer with the different ligands. Interfacial charge transfer between donor and acceptor materials is a fundamental process for the operation of bulk heterojunction solar cells. The second part of this thesis provides verification of a hole transfer mechanism from photoexcited QDs to the organic host polymer in polymer/quantum dot blends. We used PIA spectroscopy to show that selective excitation of the PbS quantum dot species results in the formation of long-lived charges (polarons) on the polymer chains due to the transfer of photoexcited holes. We also showed that higher photon-energy excitation of both polymer and quantum dot components produces greater charge transfer yields compared to selective excitation of the quantum dots with lower photon-energy. We hypothesized that either electron transfer is more efficient than hole transfer, or that hole transfer efficiency is wavelength-dependent, suggesting that the excess energy of “hot” (nonthermalized) excitations facilitates hole transfer to the polymer. Building on these results, we used transient absorption (TA) spectroscopy to investigate the role of pump photon-energy on hole transfer rates in polymer/PbS quantum dot blends through selective excitation of the quantum dot component at two different wavelengths. We showed that higher photon-energy excitation of the blends produces a significantly greater prompt hole transfer yield compared to lower photon-energy excitation, on timescales consistent with carrier cooling rates in the quantum dots. This result provides direct evidence to support the hypothesis that the excess energy of hot carriers in the quantum dots, resulting from excitation with higher photon-energy, facilitates hole transfer to the polymer on subpicosecond timescales. Furthermore, we demonstrate that relaxed carriers on the quantum dots can also transfer to the polymer, at slower rates and with reduced efficiency.