Characterizing The Energetic Landscape In Solution Processable Solar Cells Via Frequency-Dependent Impedance Measurements
Nanoscience and Nanotechnology
This thesis presents measurements and analyses aimed at describing charge transport dynamics in quantum dot (QD) photovoltaics (PVs). Due to their solution processability and unique size-dependent optoelectronic properties, ensembles of electronically coupled QDs (QD solids) provide an exciting platform for next generation PV devices. However, the structural disorder associated with the formation of conductive QD solids gives rise to a complicated density of states (DOS) emerging from the distribution of mesoscale charge dynamics occurring in these materials. I present phenomological models to describe the DOS in the disordered energetic and spatial landscape of QD solids that relies on a suite of frequency-domain measurements known as impedance spectroscopy (IS). Though specific applications of IS such as thermal admittance spectroscopy (TAS) have been applied to the capacitance characteristics of QD solids, a fuller picture of the DOS in these materials is afforded by analysis of the time-scales evident in the full impedance characteristics of QD devices. I heuristically propose extensions of charge transport models developed for capacitance-voltage (CV) measurements of bulk semiconductors to describe the frequency-dependent capacitance and conductance response of a variety of QD solar cell device architectures. In Chapter 3, I show how TAS and drive level capacitance profiling (DLCP) characterization of a QD Schottky junction is linked to charge hopping processes observed in AC conductance data. This allows me to map the time scales detected in these data to the DOS in the QD solid. I then suggest how the observed DOS translates onto macroscale device properties like the diode current. In Chapter 4, I apply these techniques to a QD heterojunction device. I use forward biased IS characterization to suggest the presence of a defect state at the junction interface, and calculate the associated distribution of carrier lifetimes. In Chapter 5, I attempt to extend this model to a QD p-i-n heterojunction solar cell, and obtain a response consistent with interfacial trapping and carrier transport. Though unambiguous identification of the origin of these responses proves beyond the scope of this thesis, I use illuminated TAS and DLCP measurements to show the presence of an interfacial trap for photogenerated electrons.