Role of Surface Coverage and Film Quality of the TiO₂ Electron Selective Layer for Optimal Hole-Blocking Properties

Titanium dioxide (TiO₂) is a commonly used electron selective layer in thin-film solar cells. The energy levels of TiO₂ align well with those of most light-absorbing materials and facilitate extracting electrons while blocking the extraction of holes. In a device, this separates charge carriers and reduces recombination. In this study, we have evaluated the hole-blocking behavior of TiO₂ compact layers using charge extraction by linearly increasing voltage in a metal–insulator–semiconductor structure (MIS-CELIV). This hole-blocking property was characterized as surface recombination velocity (S_R) for holes at the interface between a semiconducting polymer and TiO₂ layer. TiO₂ layers of different thicknesses were prepared by sol–gel dip coating on two transparent conductive oxide substrates with different roughnesses. Surface coverage and film quality on both substrates were characterized using X-ray photoelectron spectroscopy and atomic force microscopy, along with its conductive imaging mode. Thicker TiO₂ coatings provided better surface coverage, leading to reduced S_R, unless the layers were otherwise defective. We found S_R to be a more sensitive indicator of the overall film quality, as varying S_R values were still observed among the films that looked similar in their characteristics via other methods.