Atomic Layer Deposition of Tin Oxide Electron Transport Layer for High-Performance Organic Solar Cells with Inverted Structure

OSCs with the inverted n-i-p structure are considered more suitable for commercialization compared to those with conventional p-i-n structure, due to the superior stability of the materials used. However, the highest efficiencies (close to 20%) are currently achieved exclusively using the conventional structure. In fact, efficiencies above 17% from OSCs with the inverted structure are scarcely reported, mainly due to the low values of fill factor (FF) which characterize devices in this configuration. Electron and hole transport layers play a crucial role in determining the performance of OSCs. Therefore, a proper selection of them is essential to achieve higher efficiency in OSCs with inverted structure and to close the gap with devices with conventional structure.<br/>Transparent conductive oxides are usually employed as electron transport layers (ETLs) in OSCs with inverted structure. Among them, zinc oxide (ZnO) is the most studied and adopted, since it provides a good band alignment with the organic semiconductors and it can be easily deposited from solution. Tin oxide (SnO₂) represents a valid alternative to ZnO as ETL, offering superior transparency to the visible light and higher electron mobility. Moreover, unlike ZnO, SnO₂ is reported to have an excellent ambient stability and to not cause any photocatalytic degradation of the organic materials [1].<br/>A broad use of SnO₂ as ETL in OSCs is currently hindered by the difficulty in fabricating films of the material with low defect density. SnO₂ films for OSCs are often obtained from solution processing, starting from nanoparticle colloidal dispersions, with the result of a high defect density, especially on the surface. Moreover, organic ligands, used to stabilize the nanoparticle dispersions, often leave residuals in the deposited film, which can compromise the interface with the active layer. As a result, the overall performance of OSCs with SnO₂ as ETL is often poor, with in particular low values of FF [2].<br/>In this work, highly efficient OSCs with inverted structure are demonstrated by using as ETL SnO₂ deposited by atomic layer deposition (ALD). ALD is an industrial grade technique which can be applied at the wafer level and also in a roll-to-roll configuration. ALD allows obtaining compact layers of SnO₂ with exceptional quality and low defect density. SnO₂ fabricated by ALD already attracted attention in the research field of photovoltaic, for perovskite [3] and tandem solar cells [4]. Nonetheless, its use in OSCs is scarcely reported and its potential not yet fully exploited. By using ALD to deposit the SnO₂ ETL, we fabricated OSCs with PM6:L8-BO active layer, reaching a champion efficiency of 17.26% and a record FF of 79%. The devices fabricated outperform not only solar cells using SnO₂ nanoparticles casted from solution (PCE 16.03%, FF 74%) but also those utilizing the more common ZnO (PCE 16.84%, FF 77%). Furthermore, the OSCs with ALD SnO₂ show a higher stability under illumination in comparison with those utilizing ZnO. The outstanding results achieved are demonstrated to arise from a superior quality of the interface between the ALD SnO₂ film and the active layer and, thus, from a reduced charge carrier recombination [5].<br/><br/>[1] Y. Jiang, et al., <i>Mater. </i><i>Horiz.</i>, <b>2019</b>, 6, 1438—1443.<br/>[2] L. Hu, et al., <i>J. Mater. Chem. C</i>, <b>2020</b>, 8, 12218-12223.<br/>[3] L. Xiong, et al., <i>Adv. Funct. Mater.</i>, <b>2018</b>, 28, 1802757.<br/>[4] K. O. Brinkmann, et al., <i>Nature</i>, <b>2022</b>, 604, 280–286.<br/>[5] L. Di Mario, et al., <i>Adv. Mater.</i>, <b>2023</b>, 2301404.