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

Abstract

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.

Charge transfer from an adsorbed ruthenium-based photosensitizer through an ultra-thin aluminium oxide layer and into a metallic substrate

“…5 Hence, aluminium oxide layers between a dye and TiO 2 substrate have been shown to increase power conversion efficiency, 6 by reducing dark current and increasing electron lifetimes. 7 An optimal oxide layer of 14 ± 2 Å has been shown to improve efficiency via the reduction recombination rates. 8 Since an aluminium oxide layer suppresses the injection of an electron into the substrate, 9 an ultra-thin oxide may provide an attractive option for future DSSC devices by passivation of the surface, therefore reducing recombination effect with minimal disruption to charge injection.…”

Alumina Coatings on Fluorine-Doped Tin Oxide@Titanium Dioxide as Photoanode for Dye-Sensitized Solar Cells

“…3b). The thicker Al 2 O 3 layer leads to a more efficient barrier layer for recombination and a higher recombination resistance [32]. Fig.…”

Recent progress on quantum dot solar cells: a review

“…104 As discussed earlier for ZnS coating, these wide bandgap semiconductor coating layers act as energy barriers to prevent the photoexcited electrons in QDs and TiO 2 from recombination with oxidized species in electrolyte or solid-state HTMs.…”

“…[99][100][101][102][103][104] ZnS deposition with SILAR method was first applied to PbS/CdS cosensitized QDSC, 99 and it has attracted much attention since it was proven to double the PCE of CdSe QDSSCs by Toyoda's group. 100,101 Nowadays, ZnS passivation has become a routine surface passivation method for various QDSSCs, such as CdSe/CdS, and Mn-doped CuInS 2 , and the PCE of the QDSSCs has been improved largely.…”