(318e) Band Alignment of Ternary Lead Chalcogenide Quantum Dots Via Composition and Size Tuning for Photovoltaic Applications

Lead chalcogenide quantum dots have recently been established as a strong candidate for quantum dot sensitized solar cells due to a low, tunable band gap and a large absorption coefficient. The band gap of bulk PbS and PbSe is 0.41 eV and 0.28 eV with large Bohr excitonic radius of 18 nm and 46 nm respectively. These band gaps can be increased through size quantization. Here we report the synthesis of ternary lead chalogenide (PbSe_xS_1-x) quantum dots by hot injection synthesis at temperatures below 150 °C that absorb in the range of 900 nm to 1200 nm. The band gaps of these quantum dots are tuned by changing the ratio of sulfur to selenium while maintaining a constant size. Furthermore, the effects of passivating ligands, injection temperature, as well as the growth time of these quantum dots were investigated to achieve better control over the size, band gap, and crystallinity of the particles. Ultraviolet photoemission spectroscopy and optical absorption spectroscopy were used to determine the exact positions of the valence band maxima and conduction band minima in order to improve the design of quantum dot solar cells.