Characterizing Upconversion Efficiency and Photophysics in CdSe(Te)/CdS/CdSe Nanostructures for High-Efficiency Photovoltaics

Semiconductor nanostructures that upconvert low-energy light to high-energy light have demonstrated properties advantageous for various applications, such as bioimaging and high-efficiency photovoltaics. By modifying the spatial dependence of the band structure within these nanostructures through heterostructure engineering, we can control carrier recombination pathways for preferential emission of high-energy photons. We aim to optimize this design for photovoltaic use by engineering CdSe(Te)/CdS/CdSe nano-rods, where CdSe(Te) serves as an absorber quantum dot and CdSe as an emitter quantum dot in a CdS rod. In the absorber, quantum confinement allows for two-photon excitation of holes, which can then migrate to the emitter for recombination. Increasing the Tellurium (Te) doping in the absorber increases the structures’ absorption bandwidth, a salient feature for high-efficiency photovoltaics. However, its effects on device efficiency, or upconversion quantum yield (UCQY), is unknown. To study this effect, we implement a procedure for determining the quantum yield of luminescent colloidal solutions to study rod-like CdSe(Te)/CdS/CdSe double quantum dot nanostructures with varying Te doping. Photoluminescence (PL) from the nanostructures is collected in an integrating sphere and coupled to a spectrometer via fiber bundles. The system is calibrated using rhodamine 101, a known 100% quantum yield standard, and by measuring the incident power of the exciting laser. The internal and external quantum yield of each sample are then calculated by comparing the PL of the sample to the standard. Photophysics responsible for trends in quantum yield is probed with (a) time-resolved photoluminescence (TRPL) using a pulsed laser excitation source and time-correlated avalanche photodiode detector and (b) dependence of upconversion photoluminescence intensity on excitation laser power. We find that nanostructures with the lowest Te doping in the core quantum dot, at 4% Te, produce the highest upconversion quantum yield, approximately 0.001%, while the highest Te doping of 40% produces the least efficient structures. However, neither demonstrate the quadratic power dependence expected of two-photon upconverters, indicating the presence of thermally-assisted excitation processes. A sample with 10% Te doping shows the greatest quadratic quality in power dependence studies, and TRPL indicates that it also produces the highest ratio of core/absorber dot exciton lifetime to emitter dot exciton lifetime, possibly due to greater quantum confinement in the cores of this sample. Although increasing Te doping diminishes upconversion quantum yield, relationships between two-photon quality and quantum yield are not evident.