Optimization of SnO${}_2$ electron transport layer for efficient planar perovskite solar cells with very low hysteresis†

Nanostructured tin oxide (SnO${}_2$) is a very promising electron transport layer (ETL) for perovskite solar cells (PSCs) that allows low-temperature processing in the planar n–i–p architecture. However, minimizing current–voltage (J–V) hysteresis and optimizing charge extraction for PSCs in this architecture remains a challenge. In response to this, we study and optimize different types of single- and bilayer SnO${}_2$ ETLs. Detailed characterization of the optoelectronic properties reveals that a bilayer ETL composed of lithium (Li)-doped compact SnO${}_2$ (c(Li)-SnO${}_2$) at the bottom and potassium-capped SnO${}_2$ nanoparticle layers (NP-SnO${}_2$) at the top enhances the electron extraction and charge transport properties of PSCs and reduces the degree of ion migration. This results in an improved PCE and a strongly reduced J–V hysteresis for PSCs with a bilayer c(Li)-NP-SnO${}_2$ ETL as compared to reference PSCs with a single-layer or undoped bilayer ETL. The champion PSC with c(Li)-NP-SnO${}_2$ ETL shows a high stabilized PCE of up to 18.5% compared to 15.7%, 12.5% and 16.3% for PSCs with c-SnO${}_2$, c(Li)-SnO${}_2$ and c-NP-SnO${}_2$ as ETL, respectively.