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

Abstract

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 (SR) 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 SR, unless the layers were otherwise defective. We found SR to be a more sensitive indicator of the overall film quality, as varying SR values were still observed among the films that looked similar in their characteristics via other methods.