PbS Quantum Dots: Size, Ligand Dependent Energy Level Structures and Their Effects on the Performance of Heterojunction Solar Cells

doi:10.15541/jim20160017

Abstract

Abstract:

The conduction band minimum and valence band maximum of colloidal PbS quantum dots with different sizes (2.6-4.5 nm) and different surface ligands (oleic acid or tetrabutylammonium iodide, TBAI) were determined by cyclic voltammetry and absorption measurements. Furthermore, the effect of quantum dot sizes on the performance of PbS quantum dots/TiO₂ heterojunction solar cells prepared in air was also studied. The results show that energy level structures of PbS quantum dots strongly depend on the size and ligand on the surface. As the quantum dot size increases from 2.6 nm to 4.5 nm, the conduction band minimum of pristine PbS quantum dots with oleic acid ligands decreases from -3.67 eV to -4.0 eV, while their valence band maximum increases from -5.19 eV to -4.97 eV. However, for PbS quantum dots passivated by TBAI ligands, their conduction band minimum and valence band maximum change from -4.15eV and -5.61 eV to -4.51eV and -5.46 eV, respectively. Devices made of the PbS quantum dots with size of 3.9 nm show the highest power conversion efficiency of 2.32%, which can be ascribed to their proper bandgap, crystal quality and favorable energy-level alignment at the PbS quantum dot /TiO₂ interface.

Key words: PbS quantum dot, cyclic voltammetry, energy level structure, solar cell

CLC Number:

TQ174

WANG Heng, ZHAI Guang-Mei, ZHANG Ji-Tao, YANG Yong-Zhen, LIU Xu-Guang, LI Xue-Min, XU Bing-She. PbS Quantum Dots: Size, Ligand Dependent Energy Level Structures and Their Effects on the Performance of Heterojunction Solar Cells[J]. Journal of Inorganic Materials, 2016, 31(9): 915-922.

Figures/Tables 8

Fig. 1 Absorption spectra and morphology of PbS QDs (a) Absorption spectra of PbS QDs with different sizes; (b) Absorption spectra of PbS QD-films after TBAI ligand exchange; (c) TEM image of colloidal PbS QDs with average size of 4.5 nm; (d) Size distribution of PbS QDs with an average size of 4.5 nm measured by dynamic light scattering

Fig. 2 (a) FT-IR spectra of PbS QDs film before and after TBAI ligand-exchange, and (b) XPS spectrum (I3d) of the TBAI-exchanged PbS QDs

Fig. 3 TEM images of (a) PbS QDs capped with oleic acid and (b) PbS QDs after ligand exchange with TBAI

Fig. 4 (a) Cyclic voltammetry curve of ferrocene in acetonitrile solution, (b) Cyclic voltammetry curves of 3.9 nm PbS QDs capped by OA and TBAI; The energy level diagrams of (c) as-synthesized PbS QDs with OA ligands and (d) PbS QDs after ligand exchange with TBAI

Fig. 5 Schematic diagram (a) of the PbS/TiO2 heterojunction solar cell, AFM images of (b) TiO2 compact layer, (c) mesoporous TiO2 nanoparticle layer and (d) PbS QD film passivated by TBAI

Fig. 6 (a) Light current density-voltage curves and (b) dark current density-voltage curves of the PbS QDs/TiO2 heterojunction solar cells made of different sizes of QDs

Table 1 Parameters of solar cells made of different sizes of PbS QDs with TBAI ligands

Size/nm	V_oc/V	J_sc/(mA·cm^-2)	FF/%	η/%
2.6	0.43	11.0	25	1.18
3.1	0.37	13.7	29	1.47
3.9	0.35	15.4	43	2.32
4.5	0.33	6.93	35	0.80

Fig. 7 Energy level diagram (a) of the materials used in PbS/TiO2 heterojunction solar cells and schematic diagram of the energy levels in the heterojunction devices made of (b) 3.9 nm PbS QDs and (c) 4.5 nm PbS QDs.

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