A Modified NiOx Layer for Robust and Photostable Organic Solar Cells

Metal-oxide transport layers have become an essential building block for fabricating thin-film solar cells. In particular, the robustness, inert nature, easy processability from solution, and high carrier mobilities offered by several both n-type, such as SnO₂ or ZnO, and p-type, like NiO_X or MoO_X, have made them promising candidates for improving performance and scaling up organic solar cells^1,2. Nevertheless, the inherent formation of surface traps during the fabrication process, originating either from structural defects or from ligand residuals, can influence negatively the solar cell characteristics and device stability, and ultimately make them fail to become a standard transport layer.<br/>Herein, we have studied independently two different systems: firstly an n-i-p structure using SnO₂ as the bottom n-type layer and, secondly, a p-i-n structure with a bottom NiO_X as the p-type layer. In both cases, commercially available well-known colloidal inks were employed.<br/>For the first case, s-shaped J-V curves and poor device stability evidenced the suboptimal interface quality with SnO₂. We identified the presence of potassium ions as stabilizing ligands, staying on the film surface as trap sites. By removing them with a simple washing with deionized water, we remove the s-shape and improve the device efficiency from 12.82% to 16.26% and the stability to maintain up to ≈87% after 100 hours³.<br/>In the second case, a low V_OC is found when NiO_X is used as the hole transport layer with the same active layer. Through the combination of multiple spectroscopic methods, we pinpoint the specific NiO_Xstructural surface defects that are limiting the charge extraction, and we propose a passivation strategy using carbazole-based self-assembly monolayers (SAMs). We demonstrate the passivation mechanism and we generalize it by screening out several SAMs. After all the optimization, solar cells with around 18% of power conversion efficiency are demonstrated.<br/><br/><br/>[1] Fakharuddin, A., Vasilopoulou, M., Soultati, A., Haider, M.I., Briscoe, J., Fotopoulos, V., Di Girolamo, D., Davazoglou, D., Chroneos, A., Yusoff, A.R.b.M., Abate, A., Schmidt-Mende, L. and Nazeeruddin, M.K., Sol. RRL, 2021, <b>5</b>: 2000555.<br/>[2] R. Sorrentino , E. Kozma , S. Luzzati and R. Po , Energy Environ. Sci., 2021,<b> 14</b> , 180 —223<br/>[3] Garcia Romero, D., Di Mario, L., Yan, F., Ibarra-Barreno, C. M., Mutalik, S., Protesescu, L., Rudolf, P., Loi, M. A., Adv. Funct. Mater., 2023, 2307958.