The inverted (n-i-p) structure is considered more suitable for the industrial production of organic solar cells (OSCs), due to the superior stability of the materials used in this configuration. Transparent conductive oxides are usually employed as electron transport layer (ETL) in OSCs with the inverted structure. Among them, zinc oxide (ZnO) is the most studied and adopted, since it is easy to obtain with relative good crystalline quality and it provides a good band alignment. Tin oxide (SnO₂) could represent a valid alternative to ZnO as ETL, with superior transparency to the visible light and higher electron mobility. Most importantly, unlike ZnO, SnO₂ is reported to have an excellent ambient stability and to not cause any photocatalytic degradation of the organic materials [1].

The main drawback that hinder an extended use of SnO₂ as ETL in OSCs is the difficulty to fabricate films of the material with high crystalline quality. SnO₂ films are often fabricated by solution processing, starting from a nanoparticle colloidal dispersion. This method is lacking in control over the material quality, giving rise to films with high density of defects, especially on the surface. Moreover, organic ligands, used to stabilize the nanoparticle dispersion, often leave residuals on the deposited film, which can compromise the interface with the active layer. As a result, the overall performance of the OSCs with SnO₂ as ETL is often inferior to that of devices with ZnO, due to substantially lower values of fill factor (FF).

An alternative method to fabricate SnO₂ films, with the desired crystalline quality, is the use of atomic layer deposition (ALD). ALD is a deposition technique widely used in the fabrication of electronic devices, since it is scalable and allows obtaining compact layers the material with exceptional quality. SnO₂ deposited by ALD is attracting attention in the research field of photovoltaic and it is currently used in perovskite [2] and tandem solar cells [3]. Nonetheless, its use in OSCs is scarcely reported and its potential in these devices still need to be evaluated.

In this work, we fabricated OSCs with SnO₂ deposited by ALD as ETL. We compared the performance of the solar cells with those of devices with either SnO₂ or ZnO deposited from solution. The scalability of the performance with the device active area and device stability have also been investigated. Our results show the higher efficiency of the OSCs with SnO₂ deposited by ALD, thanks to a superior FF. The improvement of FF has been verified on OSCs with different composition of the active layer. Values of FF close to 80% have been achieved, an exceptional result for OSCs with the inverted structure, which demonstrate the great potential of SnO₂ deposited by ALD as ETL for high-performance OSCs.