On the multiplying factor for the estimation of the average grain size in thin films.

The lineal-intercept method is commonly used to estimate the average grain size, where the 'Mendelson' multiplying factor (1.60) that is typically used explicitly assumes tetrakaidecahedral grains. While this is appropriate for bulk equiaxed microstructures, grains in thin films where the grain size exceeds the film thickness, are closer to prismatic polyhedrons in shape. Thus, assuming log-normal distribution of hexagonal-prism-shaped grains, a new multiplying factor of 1.20 is derived which relates the average grain size and measured intercept size. This model is successfully validated using experimental measurements of three different coarse-grained thin films of halide perovskites used in solar cells. Image, graphical abstract [ABSTRACT FROM AUTHOR]