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Solar Conversion Efficiency Performance of a High Temperature Alloy over a Low Temperature One: Comprehending Interfaces through Excitonics Study

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journal contribution
posted on 08.05.2018, 00:00 by Tushar Debnath, Kausturi Parui, Sourav Maiti, Hirendra N. Ghosh
To take account of the interface in the nanocrystal (NC) materials, we have synthesized high quantum yield gradient CdZnSSe alloy NC having minimal involvement of interface (G-300) through high temperature (300 °C) pyrolysis and investigated the charge carrier dynamics. The performance was unraveled through femtosecond transient absorption studies. A gradient alloy of CdZnSSe (G-250) at low alloying temperature (250 °C) was also synthesized where several interfaces were present in the form CdSe/CdS/ZnSe/ZnS within the alloy material along with other deep traps as well as surface defects. The successful formulation of minimal involvement of interface in G-300 alloy has been envisaged through its blue-shifted optical absorption spectrum as compared to the G-250 alloy due to interionic diffusion of less reactive Zn and S toward the core of the material at elevated reaction temperatures that widen the band gap. Unlike the G-250 analogue, no charge transfer (CT) state was observed in the G-300 alloy, which also suggests the nonexistence of a CdSe/CdS gradient type structure otherwise present for the G-250 analogue. The slow electron cooling time of 4 ps in the G-250 alloy is found to be absent in the G-300 alloy, which can be attributed to minimal involvement of gradient structure otherwise, where electron–hole decoupling leads to slower electron cooling. It has been observed that although the absorption cross-section of G-300 alloy is lower in the solar spectrum as compared to the G-250 analogue, photocurrent conversion efficiency (PCE) measurements of G-300 show promising 4.5% PCE due to smooth electron transfer to TiO2 through the interface free NCs whereas the G-250 analogue shows only 3.5% PCE. Our investigation suggests that engineering with alloys having less gradient structure and without any boundary restrictions can lead us to new perceptions regarding the design and development of higher efficient quantum dot sensitized solar cell (QDSSC).