Effect of Al2O3 Recombination Barrier Layers Deposited by Atomic Layer Deposition in Solid-State CdS Quantum Dot-Sensitized Solar Cells

Despite the promise of quantum dots (QDs) as a light-absorbing material to replace the dye in dye-sensitized solar cells, quantum dot-sensitized solar cell (QDSSC) efficiencies remain low, due in part to high rates of recombination. In this article, we demonstrate that ultrathin recombination barrier layers of Al 2 O 3 deposited by atomic layer deposition can improve the performance of cadmium sulfide (CdS) quantum dot-sensitized solar cells with spiro-OMeTAD as the solid-state hole transport material. We explored depositing the Al 2 O 3 barrier layers either before or after the QDs, resulting in TiO 2 /Al 2 O 3 /QD and TiO 2 /QD/Al 2 O 3 configurations. The effects of barrier layer configuration and thickness were tracked through current–voltage measurements of device performance and transient photovoltage measurements of electron lifetimes. The Al 2 O 3 layers were found to suppress dark current and increase electron lifetimes with increasing Al 2 O 3 thickness in both configurations. For thin barrier layers, gains in open-circuit voltage and concomitant increases in efficiency were observed, although at greater thicknesses, losses in photocurrent caused net decreases in efficiency. A close comparison of the electron lifetimes in TiO 2 in the TiO 2 /Al 2 O 3 /QD and TiO 2 /QD/Al 2 O 3 configurations suggests that electron transfer from TiO 2 to spiro-OMeTAD is a major source of recombination in ss-QDSSCs, though recombination of TiO 2 electrons with oxidized QDs can also limit electron lifetimes, particularly if the regeneration of oxidized QDs is hindered by a too-thick coating of the barrier layer.