Enhancing the Hole-Conductivity of Spiro-OMeTAD without Oxygen or Lithium Salts by Using Spiro(TFSI)₂ in Perovskite and Dye-Sensitized Solar Cells

2,2′,7,7′-Tetrakis­(<i>N,N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), the prevalent organic hole transport material used in solid-state dye-sensitized solar cells and perovskite-absorber solar cells, relies on an uncontrolled oxidative process to reach appreciable conductivity. This work presents the use of a dicationic salt of spiro-OMeTAD, named spiro­(TFSI)₂, as a facile means of controllably increasing the conductivity of spiro-OMeTAD up to 10^–3 S cm^–1 without relying on oxidation in air. Spiro­(TFSI)₂ enables the first demonstration of solid-state dye-sensitized solar cells fabricated and operated with the complete exclusion of oxygen after deposition of the sensitizer with higher and more reproducible device performance. Perovskite-absorber solar cells fabricated with spiro­(TFSI)₂ show improved operating stability in an inert atmosphere. Gaining control of the conductivity of the HTM in both dye-sensitized and perovskite-absorber solar cells in an inert atmosphere using spiro­(TFSI)₂ is an important step toward the commercialization of these technologies

Molecular Engineering of Organic Dyes for Improved Recombination Lifetime in Solid-State Dye-Sensitized Solar Cells

A major limitation of solid-state dye-sensitized solar cells is a short electron diffusion length, which is due to fast recombination between electrons in the TiO₂ electron-transporting layer and holes in the 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) hole-transporting layer. In this report, the sensitizing dye that separates the TiO₂ from the Spiro-OMeTAD was engineered to slow recombination and increase device performance. Through the synthesis and characterization of three new organic D-π-A sensitizing dyes (WN1, WN3, and WN3.1), the quantity and placement of alkyl chains on the sensitizing dye were found to play a significant role in the suppression of recombination. In solid-state devices using Spiro-OMeTAD as the hole-transport material, these dyes achieved the following efficiencies: 4.9% for WN1, 5.9% for WN3, and 6.3% for WN3.1, compared to 6.6% achieved with Y123 as a reference dye. Of the dyes investigated in this study, WN3.1 is shown to be the most effective at suppressing recombination in solid-state dye-sensitized solar cells, using transient photovoltage and photocurrent measurements

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 Al₂O₃ 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 Al₂O₃ barrier layers either before or after the QDs, resulting in TiO₂/Al₂O₃/QD and TiO₂/QD/Al₂O₃ 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 Al₂O₃ layers were found to suppress dark current and increase electron lifetimes with increasing Al₂O₃ 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 TiO₂ in the TiO₂/Al₂O₃/QD and TiO₂/QD/Al₂O₃ configurations suggests that electron transfer from TiO₂ to spiro-OMeTAD is a major source of recombination in ss-QDSSCs, though recombination of TiO₂ 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