Date of Award
Marco J. Castaldi
ultrasmall quantum dots, solar cells, kinetics of N-oleoylmorpholine, dielectric environment, ligation
Traditional, or 1st generation, solar cells have a theoretical upper limit of 33% power conversion efficiency based on the thermodynamic limitation of the single p-n junction cell design. This limitation is known as the Shockley-Queisser limit and methods to design solar cells to circumvent it have been creative, with a significant part of the field focused on quantum dot (QD) based systems. With theoretical efficiencies ranging between 44 and 67% for one sun concentration, QD solar cell designs are clearly superior to the limits of 1st generation solar cells. However, maximum achieved efficiencies for QD-based designs are far lower at present, with the current record at ~13%. These low efficiencies are caused in part by the fact that these cell types absorb in relatively narrow regions of the solar spectrum, thereby using only a fraction of the photons incident upon the earth’s surface. A broad absorption solar device would present an opportunity for unparalleled absorption capability.
In that vein, this thesis work has focused on the investigation of the electronic band structure of ultrasmall CdSe QDs. Below 2 nm in diameter, approximately 90% of the atoms in ultrasmall QDs are on the surface, making them highly disordered structures. When excited by UV light, CdSe QDs in the ultrasmall size regime emit white light, i.e. all colors of the visible spectrum. The cause of this unpredicted behavior has been attributed to two possible hypotheses: 1) a multitude of defect states or 2) fluxionality, a process in which bonds between neighboring atoms are constantly being made and broken, resulting in conformational and electronic fluctuations. Determining which model most accurately describes these QDs will allow for the strategic manipulation of this single material that possesses either many midgap energy states (defect hypothesis) or a constantly fluctuating bandgap (fluxionality hypothesis) with the goal of a broad absorption solar cell in mind.
Ultrasmall CdSe QDs were synthesized in the non-commercially available solvent, N-oleoylmorpholine (NOM). Differences in the purity of NOM batches had the capability to alter QD radiative relaxation pathways and therefore the kinetics of NOM formation were investigated. Using a test matrix with a range of reaction temperatures (110-170 °C) and morpholine feed rates (250-1000 μL/hr), the rate law for the formation of NOM was derived, in addition to chemical analysis via infrared (IR) spectroscopy and structural analysis of NOM via solution state nuclear magnetic resonance (NMR) spectroscopy. Importantly, this contribution to the kinetics of an open-air, low-temperature, single-size synthesis route for ultrasmall QDs should allow for more facile scale up.
Next, the sensitivity of the QD band structure based on the environment of the surface of the QDs was examined by 1) ligation and 2) alteration of dielectric environment. First, QDs were ligated with one of the following: dodecanethiol, oleic acid, piperidine, or benzenesulfonic acid monohydrate. Second, the absorption and emission characteristics and stability of the QDs in solvents with dielectric constants ranging from 2.4 (toluene) to 78.4 (water) were analyzed both via experimental methods and simulation. Following initial studies, the QDs in these different dielectric systems were taken to Brookhaven National Labs (BNL) and examined via x-ray absorption spectroscopy (XAS), which yielded information on the nature of the Cd-Se bond as a function of the QD surface environment. These findings are compared to simulations of thermodynamic and optoelectronic properties of CdSe clusters conducted across a range of temperatures and dielectric constants.
Finally, ultrasmall CdSe QDs were incorporated into metal oxide mesoporous structures, which served as the active material in QD-sensitized solar cells. The performance of these solar cells was evaluated via linear sweep voltammetry, which allows for the measurement of the open circuit voltage (VOC), the short circuit current (ISC), the power conversion efficiency (PCE), and the fill factor (FF). It was found that the greater the overlap of the valence band of the QDs with the metal oxide substrate, the higher the PCE of the cell; this is attributed to hole (h+) transport. The PCE of QD cells was found to be highly correlated to the location of the valence band in relation to the energy level of absolute vacuum (0 eV) by an exponential decay function.
Webster, Megan, "Investigation of the Synthesis and Optoelectronic Properties of Ultrasmall CdSe Quantum Dots" (2019). CUNY Academic Works.