Here we demonstrate a CdS quantum dot (QD) sensitized solar cell with significantly enhanced stability and depressed recombination in I⁻/I₃⁻ electrolyte. The CdS QDs were deposited in a mesoporous TiO₂ film using chemical bath deposition. Following the coating of an ultrathin TiO₂ protection layer using atomic layer deposition (ALD), the performance and stability of CdS QD sensitized solar cells were pronouncedly enhanced. Current–voltage measurements and electrochemical impedance spectroscopy analysis show that the ALD-TiO₂ coating slows down the charge recombination and protects the QDs from the photocorrosion of the I⁻/I₃⁻ electrolyte. A systematic study shows that a ∼2 nm ALD-TiO₂ layer can best promote solar cell efficiency by balancing protection with charge transfer. We have obtained a photoconversion efficiency of 1.41% and stable operation (no degradation in photocurrent) for at least 20 min under full sun illumination on CdS QD solar cells. The photocurrent gradually decreased down to ∼70% of its original value after 1 h of operation.