Viscosity, surface tension, density and contact angle of selected PbI2, PbCl2 and methylammonium lead halide perovskite solutions used in perovskite solar cells

Abstract

Perovskite solar cells (PSCs) are currently under vigorous research and development, owing to their compelling power conversion efficiencies. PSCs are solution-processed and, therefore, are fabricated using casting and printing methods, such as spin, spray and blade coating. The coating characteristics significantly depend on the physical and rheological properties of the solutions. Thus, due to the scarcity of such properties, in this work, we report the surface tension, viscosity, density, and contact angle of selected methylammonium lead halide perovskite solutions, in order to gain insight into the behavior of the perovskite solutions and the range of such physical properties. The contact angles were measured on PEDOT:PSS and compact TiO2 (c-TiO2) substrates, commonly used as the underneath layers of the perovskite film. In total, 12 solutions of CH3NH3PbI3 and CH3NH3PbI3-xClx dissolved in common solvents, as well as solutions of PbI2, PbCl2, and CH3NH3I were tested. Among the results, it is shown that the tested perovskite solutions are Newtonian, the apparent contact angles on the mesoporous TiO2 (m-TiO2) are close to zero, on the PEDOT:PSS are around 10°, and on the c-TiO2 are around 30°. Also, contact angle hysteresis is observed in the case of the c-TiO2 substrates. Representative impact dynamics and spreading of perovskite solution droplets are also studied, to demonstrate the importance of the solution properties and process parameters on the coating process.

Efficient Inverted Planar Perovskite Solar Cells Using Ultraviolet/Ozone‐Treated NiOx as the Hole Transport Layer

“…The slight recovery of hydrophobicity with longer UVO exposure time is probably attributed to loss of low molecular weight organic material that has been created on the surface during UVO treatment and reorientation of side groups on the molecules due to the burying of newly formed polar groups in the exposure region . The smallest contact angle 24.2° is obtained by exposure under UVO for 5 min, which facilitates the formation of a more uniform perovskite layer . The current–voltage ( I–V ) characteristics have been measured under dark condition to determine the effect of UVO treatment on the electrical conductivity of the NiO x film (Figure c).…”

Core–Shell Nanoparticle Interface and Wetting Properties

“…Modifications of the CA by less than 5° were shown to directly result in higher conductive conjugated polymer‐based contacts in organic solar cells 64. For perovskite solar cells, it was reported that small variations in CA yield to macroscopic differences in film formation which directly affects the efficiency 65. Menzies and Jones correlated slight changes in CAs to immediate changes in cell–cell interactions in tissue engineering 21…”

Solvent and Spinning Speed Effects on CH3NH3PbI3 Films Deposited by Spin Coating

“…However, in the latter process, the synthesis is difficult to control as the perovskite grows extremely fast in the solution. Therefore, dipping times are usually less than 1 min . Actually, perovskite thin films, prepared on a flat substrate by the spin‐coating technique, suffer from pinholes and the reduced surface coverage, which may affect photovoltaic cell performance.…”

“…Actually, perovskite thin films, prepared on a flat substrate by the spin‐coating technique, suffer from pinholes and the reduced surface coverage, which may affect photovoltaic cell performance. Since these methods are liquid‐based methods, the liquid properties such as viscosity, surface tension, and density of the solution affect seriously the resulting films' properties, especially the growth mechanism and film morphology . Beside this, in spin coating, the radial force due to spinning velocity controls the liquid dynamics and film thinning.…”

“…For example, spin‐coated film's thickness is proportional to the square root of solution viscosity and the inverse of the squared spin speed . A recent report highlighted the influence of the choice of lead salt on perovskite formation . The growth mechanism of perovskite films on a flat substrate is not yet well understood.…”