Intraband electron cooling mediated unprecedented photocurrent conversion efficiency of CdS_xSe_1–x alloy QDs: direct correlation between electron cooling and efficiency

Abstract

Composition and size dependent band gap engineering with longer excited state charge carrier lifetime assist CdS_xSe_1–x alloy semiconductor quantum dots (QDs) as a promising candidate for quantum dot solar cell (QDSC). Colloidal CdS_xSe_1–x alloy QDs were synthesized using the hot injection method where a stoichiometric mixture of S-TOP and Se-TOP were injected at 270 °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 CdS_xSe_1–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 (∼8 ps) and less charge recombination (∼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 be 1.1 (±0.07)% for CdS, 3.36 (±0.1)% for CdSe, and 3.95 (±0.12)% for CdS_0.7Se_0.3 QDs. An additional nonepitaxial CdS quasi-shell followed by ZnS passivation layer (TiO₂/ CdS_0.7Se_0.3 /quasi-CdS/ZnS) was deposited on top of the CdS_0.7Se_0.3 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.