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.

Automated summary

In this study, the hole-blocking behavior of TiO2 compact layers was evaluated and characterized using surface recombination velocity (S-R). Thicker TiO2 coatings provided better surface coverage, resulting in reduced S-R. S-R was found to be a sensitive indicator of the overall film quality.