Importance of ligands on TiO2 nanocrystals for perovskite solar cells

doi:10.1088/1674-1056/27/1/018401

Abstract The fabrication of high-quality electron-selective layers at low temperature is a prerequisite to realizing efficient flexible and tandem perovskite solar cells (PSCs). A colloidal-quantum-dot ink that contains TiO₂ nanocrystals enables the deposition of a flat film with matched energy level for PSCs; however, the selection of ligands on the TiO₂ surface is still unexplored. Here, we systematically studied the effect of the titanium diisopropoxide bis(acetylacetonate) (TiAc₂) ligand on the performance of PSCs with a planar n-i-p architecture. We prepared TiO₂ nanocrystals from TiCl₄ and ethyl alcohol with Cl^- ligands attached on its surface and we found that a tiny amount of TiAc₂ treatment of as-prepared TiO₂ nanocrystals in a mixed solution of chloroform and methyl alcohol can enhance PSC power conversion efficiency (PCE) from 14.7% to 18.3%. To investigate the effect of TiAc₂ ligand on PSCs, TiO₂ samples with different TiAc₂ content were prepared by adding TiAc₂ into the as-obtained TiO₂ nanocrystal solution. We use x-ray photoelectron spectroscopy to identify the content of Cl so as to reveal that Cl ligands can be substituted by TiAc₂. We speculate that the improvement in PCE originates from amorphous TiO₂ formation on the TiO₂ nanocrystal surface, whereby a single-molecule layer of amorphous TiO₂ facilitates charge transfer between the perovskite film and the TiO₂ electronic transport layer, but excessive TiAc₂ lowers the PSC performance dramatically. We further prove our hypothesis by x-ray diffraction measurements. We believe the PCE of PSCs can be further improved by carefully choosing the type and changing the content of surface ligands on TiO₂ nanocrystal.

Keywords: TiAc₂ TiO₂ nanocrystal surface ligand perovskite solar cells

Received: 27 September 2017
Revised: 21 November 2017
Accepted manuscript online:

PACS:	84.60.Jt	(Photoelectric conversion)
	88.40.H-	(Solar cells (photovoltaics))
	84.60.Bk	(Performance characteristics of energy conversion systems; figure of merit)

Fund: Project supported by the National Natural Science Foundation of China (Grant Nos. 51622201, 61571015, and 91433102).

Corresponding Authors: Qing Zhao
E-mail: zhaoqing@pku.edu.cn

Cite this article:

Yao Zhao(赵耀), Yi-Cheng Zhao(赵怡程), Wen-Ke Zhou(周文可), Rui Fu(伏睿), Qi Li(李琪), Da-Peng Yu(俞大鹏), Qing Zhao(赵清) Importance of ligands on TiO₂ nanocrystals for perovskite solar cells 2018 Chin. Phys. B 27 018401

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Importance of ligands on TiO2 nanocrystals for perovskite solar cells

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Importance of ligands on TiO₂ nanocrystals for perovskite solar cells