Self-assembled monolayers (SAMs) based on carbazole units and phosphonic acid anchoring groups are used as hole transport layers in perovskite solar cells to achieve high power conversion efficiency. So far, SAMs have been deposited on the electrode with solution-based methods. Yet, vacuum-based deposition approaches could ease the formation of a conformal and uniform layer, especially on textured surfaces used in perovskite/silicon tandem solar cells. Vacuum-based methods also have high process yields and reproducibility as well as being already employed in large-scale industrial manufacturing. Now, Ahmed Farag, Ulrich Paetzold and colleagues across Germany fabricate hole transport layers via a vapour deposition method based on thermal evaporation of SAM molecules commonly used in perovskite photovoltaics. The solar cells they produce achieve efficiencies comparable to solution-processed devices.
By means of X-ray photoelectron and infrared spectroscopies, the researchers show that SAM molecules withstand the temperatures used during thermal evaporation and do not show signs of chemical degradation. The evaporated monolayers have lower water contact angles than solution-processed ones, indicating an improved wettability: Farag et al. are able to deposit a SAM molecule with methyl groups that only achieved poor surface coverage using solution-based methods. Finally, they demonstrate that SAMs thermally evaporated over textured surfaces could enable improved photovoltaic performance compared to conventional hole-transport layers based on metal oxides or a combination of metal oxide and solution-processed SAMs. The work is a step forward to fully vacuum-deposited perovskite solar cells.
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