Fig. 6. (a) Effective spiro-OMeTAD conductivity and UV-vis absorption spectra (inset) as a function of the LiTFSI content. These films were left for 78 hours in air before measurement. The solid-line is simply to aid the eye. Reprinted with permission from ref. 86. (b) Photographs of a HTL solution used for spin coating. (A) Before adding tBP and (B) after adding tBP. Top-view SEM images of the freshly prepared HTL (C) without tBP, (D) with tBP, and (E) with tBP after overnight vacuum treatment (10^–4 Pa). Reprinted with permission from ref. 85. (c) Contact angle measurements of water droplets on films of spiro-OMeTAD doped with LiTFSI and spiro-OMeTAD⁺TFSI^– salt. Reprinted with permission from ref. 88. (d) UV-vis spectra of a TiO₂/perovskite film and tBP-treated film, a photo of the films is shown in the inset. Reprinted with permission from ref. 90. (e) Schematic representation of the chemical reaction of the hole transporting material with tBP which occurs during the doping procedure resulting in new products with different properties. Reprinted with permission from ref. 91.