Photoluminescence and photovoltaic properties of cesium lead bromide nanocrystals with linoleic acid ligand

Cesium lead bromide (CsPbBr3) nanocrystals (NCs) were synthesized using the ligand-assisted reprecipitation (LARP) method, with linoleic acid (LA) and oleylamine (OlAm) as ligands. The crystal structures of the samples depend on the ligand used in the synthesis. Sample-1, with LA-OlAm as the ligand, showed an orthorhombic crystal structure, whereas Sample-2, with OlAm only, showed a tetragonal crystal structure. These samples exhibited slightly different photoluminescence (PL) characteristics, where the PL peak of Sample-1 appeared at 521.4 nm, whereas the PL peak of Sample-2 appeared at 524.1 nm. Sample-1 showed brighter PL than that of Sample-2. These differences are strongly related to their crystal structures and degree of crystallization, which is also evident from the differences in the chemical states of the Br and Pb ions on the NCs surface, as depicted by the X-Ray Photoelectron Spectroscopy (XPS) spectra. In addition, despite exhibiting PL characteristics, these CsPbBr3 NCs also exhibit photovoltaic properties.


Introduction
Nanocrystals (NCs) of all-inorganic perovskite materials, namely cesium lead halide (CsPbX3; X = Cl, Br, I), show promising potential as active materials in optoelectronic devices, such as light-emitting diodes (LED) and solar cells [1-2].Cesium lead bromide (CsPbBr3) perovskite materials have excellent optical and electronic properties, such as an absorption coefficient, high carrier mobility, high stability, and long diffusion length [3,4].However, the properties of these NCs may be affected by the ligand used in their synthesis, which serves as a control agent in determining the size and shape of the NCs [5].The crystal size and shape of these inorganic NCs affect their electronic and optical properties, particularly their band structures [6].There are several methods for synthesizing CsPbBr3 NCs, such as hot injection [7], ligand-assisted reprecipitation (LARP) [8], indirect (two-step) synthesis [9], and microwave-assisted synthesis [10].In this paper, cesium lead bromide (CsPbBr3) nanocrystals (NCs) were synthesized using a ligand-assisted reprecipitation (LARP) method with linoleic acid (LA) and oleylamine (OlAm) as the ligands and toluene as the antisolvent.The crystal structure and photoluminescence (PL) characteristics of the synthesized CsPbBr3 NCs were investigated, along with their solar cell characteristics.

Nanocrystals synthesis
CsPbBr3 NCs were synthesized using cesium bromide (CsBr) and lead bromide (PbBr2) as precursor materials, and LA and oleylamine as ligands.A solvent mixture of DMSO and DMF at a ratio of 3:7 was used as the solvent to form a 0.1 M precursor solution.The LARP method was employed to synthesize CsPbBr3 NCs with two ligand variations, namely, a mixture of LA and oleylamine for Sample-1 and oleylamine only for sample 2 at room temperature with the same antisolvent (toluene).

Characterizations
The X-ray diffraction (XRD) patterns are measured by using an X-ray diffractometer (Bruker D8 Advance) at a scan speed of 10 min -1 .X-ray Photoelectron Spectroscopy (XPS) (PHI 5000 Versa Probe II) was used to measure the binding energy of the core electrons in the atoms of those NCs.The photoluminescence spectra of the NCs samples of CsPbBr3 were measured using a spectrophotometer (Hitachi F-2700 FL 2957-004).Ivium Vertex.S was used to measure the current density-voltage (J-V) and Electrical Impedance Spectroscopy (EIS) of the fabricated solar cell.

Results and discussion
Figure 1(a) shows the appearance of different PL spectral shapes measured from Sample-1 and Sample-2.Based on their PL intensities, the PL of Sample-1 is much brighter than that of Sample-2.From the XRD measurement results, as shown in Figure 1(b), Sample-1 has an orthorhombic crystal structure with space group Pnma, whereas Sample-2 has a tetragonal crystal structure with space group P4/mmm.Sample-1 has several diffraction peaks in the range of 20-30, which agrees with the orthorhombic crystal structure, although these peaks are much higher than those in the PDF-01-072-7929 reference data.These higher peaks could be caused by the effects of the LA and OlAm ligands.Sample-2 has one high peak in the range of 20-25, which is similar to the PDF-01-074-6645 reference data.However, as indicated by the S/N ratio of the curves in Figure 1(b), the XRD peaks of Sample-2 appear to be weaker than those of Sample-1, indicating a lower degree of crystallinity in Sample-1.X-Ray Photoelectron Spectroscopy (XPS) measurements were performed to investigate the chemical and electronic states of each element that formed the two types of perovskite samples.Figure 2(a) shows the XPS results, which indicate significantly different chemical states for the bromide ion in these two samples (Figure 2b-d).Sample-2 exhibited a lower intensity and broader spectral shape of the XPS peaks.This may be related to, in the absence of the LA ligands, the excessive interaction between the OlAm ligand and Br/Pb ions on the NCs surface and leading to the destruction of the crystallinity order on the surface.This situation then suppressed the PL characteristics of Sample-2.To investigate the photovoltaic properties, we then prepared CsPbBr3 NCs with LA ligand only.The synthesized NCs were then used as the active layer for a solar cell device, with the structure and energy level diagram shown in Figure 3.The conduction band (ECB) and valence band (EVB) were taken from the literature for CsPbBr3 NCs, namely -4.2 and -6.5 eV [11][12][13].The energy levels of the other layers, namely, FTO, TiO2, and the carbon layer, were also obtained from the literature [14,15].This energy level difference (HOMO and LUMO) can cause the transfer of electric charge between the layers in the single-layer perovskite solar cell device designed in this study.The EHOMO and ELUMO values for CsPbBr3 nanocrystals are between the work function of carbon (-5.0 eV) and the valence band of TiO2 (-4.0 eV).The J-V and EIS characteristics of this single-layer perovskite solar cell device are shown in Figure 3c  and 3d.Based on its J-V curve, the power conversion efficiency is estimated to be approximately 4.7%.

Conclusions
The present experimental results show the significant effect of the ligands (LA and OlAm) on the formed crystal structure of CsPbBr3 NCs and their PL characteristics, as evident from their XRD data.The XPS measurement results revealed significant differences in the chemical states of Br and Pb atoms.The weaker and broader XPS peaks in Sample-2 may be related to the strong interactions between Br and Pb atoms with the OlAm ligand and a lower degree of crystallinity due to the absence of LA ligands, leading to lower PL characteristics.On the other hand, CsPbBr3 NCs with LA ligands exhibited photovoltaic characteristics with a power conversion efficiency of 4.7%.The present results show the significant effect of ligands on the optical and electronic properties of these perovskites, with crucial impacts on their applications.

Figure 3 .
Figure 3. CsPbBr3 NCs solar cell (a) Device structure (b) Energy level diagram (c) J-V curve (d) EIS characteristics and its equivalent circuit.