Ultrafast Hole- and Electron-Transfer Dynamics in CdS-Dibromofluorescein (DBF) Supersensitized Quantum Dot Solar Cell Materials

Ultrafast charge-transfer (CT) dynamics has been demonstrated in CdS quantum dot (QD)-4',5'-dibromofluorescein (DBF) composite materials, which form a strong CT complex in the ground state. Charge separation in the CdS DBF composite was found to take place in three different pathways, by transferring the photoexcited hole of CdS to DBF, electron injection from photoexcited DBF to the CdS QD, and direct electron transfer from the HOMO of DBF to the conduction band of the CdS QD. CT dynamics was monitored by direct detection of the DBF cation radical and electron in the QD in the transient absorption spectra. Electron injection and the electron-transfer process are found to be pulsewidth-limited (<100 fs); however, the hole-transfer time was measured to be 800 fs. Charge recombination dynamics has been found to be very slow, confirming spatial charge separation in the CdS DBF supersensitized quantum dot system. Grand charge separation process suggests that the CdS-DBF supersensitized quantum dot system can be used as superior materials for quantum dot solar cells (QDSCs).