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

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Titanium dioxide (TiO 2) is a commonly used electron selective layer in thin-film solar cells. The energy levels of TiO 2 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 TiO 2 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 TiO 2 layer. TiO 2 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 TiO 2 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.

Original languageEnglish
Pages (from-to)11688-11695
Number of pages8
JournalACS Omega
Issue number14
Publication statusPublished - 12 Apr 2022
MoE publication typeA1 Journal article-refereed


  • solar cell
  • Titanium dioxide
  • X-ray photoelectron spectroscopy (XPS)
  • atomic force microscopy (AFM)


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