Organic Solar Cells with Solution-Processed Carbon Top Electrodes

Organic solar cells (OSCs) continue to show rapid performance improvements. The interest in OSCs is nurtured by the prospects of a sustainable technology that allows for low-cost, large-area solution processability. In reality, most OSCs still rely on thermally evaporated metal electrodes, such as silver or gold. Here, we demonstrate the first OSCs utilizing a fully solution-processed, doctor-bladed carbon top electrode as a low-cost and recyclable alternative to established metal contacts. A robust electron transport layer (ETL) architecture comprising a thin film of solution-processed aluminum-doped zinc oxide nanoparticles (AZO-NPs) and a 20 nm-thick tin oxide (SnO x ) layer, grown by atomic layer deposition (ALD) at 80 °C, is introduced to protect the underlying organic layers against the otherwise detrimental solvent from the carbon paste. The AZO-NP layer is of critical importance to improve the nucleation of the ALD-SnO x layer and afford the necessary solvent barrier properties. To improve the electronic interface between the carbon top electrode and the ETL, we inserted an ultrathin indium oxide (InO x ) layer (1.5 nm), grown in the same ALD process. The metallic properties of the ALD-InO x improved reproducibility, and we also confirmed that the 2 orders of magnitude lower sheet resistance of ALD-InO x (5 × 10 6 Ω/sq) vs ALD-SnO x (5 × 10 8 Ω/sq) enabled sufficient lateral transport of photogenerated electrons to be extracted by the carbon top electrode, that is found to only make pointlike contact to the adjacent ETL. The resulting OSCs with the doctor-bladed carbon top electrode achieved an encouraging power conversion efficiency of 11%, which provides a very promising direction toward sustainable, scalable, and resource-efficient OSC manufacturing.