EAGER: Characterization of the Red-Shift Effect Observed in Solutions of Photoluminescent Down-Shifting Carbon Quantum Dots and CdTe/Carbon Quantum Dots

University Of Texas At San Antonio, San Antonio TX

Investigators

Abstract

The development of higher efficiency solar cells is considered a requirement to reduce the dependence on fossil fuels. This is the main impetus for a variety of research schemes including the utilization of nanoparticles for performance improvement, particularly in silicon because it remains the most widely employed material in solar cell manufacturing. However, one of the problems affecting silicon structures is associated with relatively high energy photons that tend to interact with phonons in the silicon lattice rather than producing electron-hole pairs. Thus, improving the capture of high energy photons that are otherwise lost, could lead to a dramatic increase in the efficiency of photovoltaic structures. In order to address this shortcoming a variety of quantum dots are frequently explored because of their ability to capture high energy photons and subsequently emitting lower energy photons more suitable for producing electron-hole pairs in silicon, i.e., down-shifting. In addition to the aforementioned red-shift of quantum dots, large red-shifts have been observed in solutions containing a collection of photoluminescent carbon quantum dots (CQDs) of different synthesized radii. The observed red-shift effect is anticipated to have a dramatic impact on the power conversion efficiency of solar cells since it can be construed as an enhancement of the down-shifting effect associated with photoluminescent quantum dots. Specifically, with an excitation wavelength of 390 nm, individual colloidal solutions of CQDs of a specific radius, exhibited down-shifted emission peaks centered at ~420 nm. However, the solution comprised of the mixture of the previously synthesized colloidal solutions of Carbon QDs of different radii, was observed to have an emission red-shifted to ~515 nm. Furthermore, the red-shift effect was also observed in CdTe QDs when added to the solution with the aforementioned collection of Carbon QDs. Thus, whereas a colloidal solution solely comprised of CdTe QDs of different radii, exhibited a down-shifted photoluminescence centered at ~555 nm, the peak was observed to be further red-shifted to ~580 nm when combined with the previously mentioned mixed solution of Carbon QDs of different radii. However, upon spin casting the quantum dot mixtures either directly on a silicon substrate, or dispersing the quantum dots in poly-methyl-methacrylate (PMMA) prior to spin casting, the red-shift effect is lost. Therefore, we propose the extensive utilization of spectroscopic methods to investigate (i.) the origin of the merged and shifted spectrum observed upon mixing the QDs (both different sized CQDs and mixed CQD/CdTe QDs), (ii.) the dependence and/or stability of the emission spectra on the temperature and gaseous atmosphere of the colloidal solutions and (iii.) the disappearance of the red-shift observed upon processing the QDs with different solvents either directly on silicon substrates or when dispersing the synthesized QDs in PMMA.