Understanding the Surface Chemistry of SnO2 Nanoparticles for High Performance and Stable Organic Solar Cells

In organic solar cells, the interfaces between the photoactive layer and the transport layers are critical in determining not only the efficiency but also their stability. When solution-processed metal oxides are employed as the electron transport layer, the presence of surface defects can downgrade the charge extraction, lowering the photovoltaic parameters. Thus, understanding the origin of these defects is essential to prevent their detrimental effects. Herein, it is shown that a widely reported and commercially available colloidal SnO2 dispersion leads to suboptimal interfaces with the organic layer, as evidenced by the s-shaped J-V curves and poor stability. By investigating the SnO2 surface chemistry, the presence of potassium ions as stabilizing ligands is identified. By removing them with a simple washing with deionized water, the s-shape is removed and the short-circuit current is improved. It is tested for two prototypical blends, TPD-3F:IT-4F and PM6:L8:BO, and for both the power conversion efficiency is improved up to 12.82% and 16.26%, from 11.06% and 15.17% obtained with the pristine SnO2 , respectively. More strikingly, the stability is strongly correlated with the surface ions concentration, and these improved devices maintain approximate to 87% and approximate to 85% of their initial efficiency after 100 h of illumination for TPD-3F:IT-4F and PM6:L8:BO, respectively.

Automated summary

The interfaces between the photoactive layer and the transport layers play a critical role in determining the efficiency and stability of organic solar cells. This study reveals that a commercially available colloidal SnO2 dispersion leads to suboptimal interfaces and poor stability. Removing potassium ions as stabilizing ligands improves the interfaces and enhances the efficiency and stability of the solar cells.