The novel perovskite solar cell (PSC) technology is expected to soon outshine the conventional silicon-based solar cell technology as its record efficiency to date is already reaching over 22% [1]. However, before the PSC technology can break into the market, issues related to the poor reproducibility of the currently used processing methods have to be solved. Dip coating is a scalable and low-cost method for preparing uniform films with tunable thickness and structure, and we have previously utilized it for studying various charge selective layers in organic solar cells [2].

Recently, we utilized dip coating to improve the quality and to better understand the charge extraction mechanism in compact TiO₂ layers in mesoscopic PSCs [3]. We chose TiCl₄ as the sol-gel precursor, as we earlier demonstrated it gave rise to superior device properties [4]. By varying the TiCl₄ concentration, we prepared ultra-thin (5−50 nm) compact TiO₂ layers on FTO substrates. AFM measurements revealed that the thinnest TiO₂ layers followed the texture of the underlying FTO substrates, while at thicknesses larger than ~25 nm the surface roughness substantially decreased. Furthermore, below this critical film thickness, the underlying FTO was partly exposed creating pinholes in the compact TiO₂ layer.

Based on the performance of these layers in mesoscopic PSCs, a similar critical compact TiO₂ layer thickness was identified. Devices without or with very thin compact TiO₂ layers displayed poor efficiencies and J-V curves with an “s-shaped” feature in the negative voltage regime. This could be attributed to immobilized negative ions at the electron-extracting interface. A strong correlation between the magnitude of the s-shape feature and the exposed FTO indicated that the s-shape was related to the extent of pinholes in the compact TiO₂ layer. The results highlight the importance of a uniform and pinhole-free electron selective TiO₂ layer in efficient perovskite solar cells.

[1] https://www.nrel.gov/pv/assets/images/efficiency-chart.png

[2] O.J. Sandberg, S. Sandén, A. Sundqvist, J.H. Smått and R. Österbacka, Phys. Rev. Lett., 118, 076601 (2017).

[3] M.T. Masood, C. Weinberger, J. Sarfraz, E. Rosqvist, S. Sandén, O.J. Sandberg, P. Vivo, G. Hashmi, P.D. Lund, R. Österbacka and J.H. Smått, ACS Appl. Mater. Interfaces, ACS Appl. Mater. Interfaces, 9, 17906 (2017).

[4] P. Vivo, A. Ojanperä, J.H. Smått, S. Sandén, S.G. Hashmi, K. Kaunisto, P. Ihalainen, M.T. Masood, R. Österbacka, P.D. Lund and H. Lemmetyinen, Org. Electron., 41, 287 (2017).