Intraband Electron Cooling Mediated Unprecedented Photocurrent Conversion Efficiency of CdSxSe1-x Alloy QDs: Direct Correlation between Electron Cooling and Efficiency

摘要

Composition and size dependent band gap engineering with longer excited state charge carrier lifetime assist CdSxSe1-x alloy semiconductor quantum dots (QDs) as a promising candidate for quantum dot solar cell (QDSC). Colloidal CdSxSe1-x alloy QDs were synthesized using the hot injection method where a stoichiometric mixture of Se-TOP and Se-TOP were injected at 270 degrees C in a mixture of Cd-oleate. The electron decoupled from hole in the alloyed structure due to delocalization of electron in,electronically quasi type-II graded CdSxSe1-x alloyed structure. As a result, intraband electron cooling time increases from 100s of fs to sub 10 ps time scale in the alloyed graded structure. Extremely slow electron cooling time (similar to 8 ps) and less charge recombination (similar to 50% in >2 ns) as compared to both CdS and CdSe QDs are found to be beneficial for charge carrier extraction in QD solar cells. Using polysulfide electrolyte and Cu2S-deposited ITO glass plates as photocathode, the efficiency of the QD solar cell was measured to tie 1.1 (+/- 0.07)% for CdS, 3.36 (+/- 0.1)% for CdSe, and 3.95 (+/- 0.12)% for CdS0.7Se0.3 QDs. An additional nonepitaxial-CdS quasi-shell followed by ZnS passivation layer (TiO2/CdS0.7Se0.3 /quasi-CdS/ZnS) was deposited- on top of the CdS0.7Se0.3 film which showed a photo current conversion efficiency (PCE) of 4.5 (+/- 0.18) %. The overall 14%-increase of PCE is due to the-quasi CdS shell helps to separate more electrons through passivating the surface states of TiO2.