Fluoride passivation of ZnO electron transport layers for efficient PbSe colloidal quantum dot photovoltaics

Abstract

Lead selenide (PbSe) colloidal quantum dots (CQDs) are suitable for the development of the next-generation of photovoltaics (PVs) because of efficient multiple-exciton generation and strong charge coupling ability. To date, the reported high-efficient PbSe CQD PVs use spin-coated zinc oxide (ZnO) as the electron transport layer (ETL). However, it is found that the surface defects of ZnO present a difficulty in completion of passivation, and this impedes the continuous progress of devices. To address this disadvantage, fluoride (F) anions are employed for the surface passivation of ZnO through a chemical bath deposition method (CBD). The F-passivated ZnO ETL possesses decreased densities of oxygen vacancy and a favorable band alignment. Benefiting from these improvements, PbSe CQD PVs report an efficiency of 10.04%, comparatively 9.4% higher than that of devices using sol-gel (SG) ZnO as ETL. We are optimistic that this interface passivation strategy has great potential in the development of solution-processed CQD optoelectronic devices.

Graphical abstract

Keywords

Zinc oxide / Surface passivation / Band alignment / Quantum-dot solar cells

Introduction

Molybdenum is a kind of important inorganic materials and can be widely used in the photoluminescence [1], microwave [2], fiber optic [3], scintillator [4], humidity sensors [5], magnetic [6], catalyst [7], and other aspects [8]. The previously reported synthetic methods of molybdate, mostly takes place at high temperature or require other harsh conditions, such as at 1000°C under solid state metal exchange reaction [9], or the sol-gel process [10]. There have been few reports on the synthesis of these materials by hydrothermal/solvothermal methods [11]. The molybdate (such as Ag₂M_O4O₁₃, Ag₂M_O2O₇ and Ag₂MoO₄₎ is made by MoO₃ and Ag₂O mixed in a certain proportion and then sintered with high temperature [12]. These materials have high electrical conductivity, which are normally applied in conductive glass [13]. Yu group reports the synthesis of single-crystal silver molybdate/tungstate nanowires with hydrothermal process, showing a selective synthesis of uniform single crystalline silver molybdate/tungstate nanorods/nanowires in large scale by a facile hydrothermalre crystallization technique [14]. Shao et al. have observed the ultrasensitive surface-enhanced Raman scattering (SERS) signals of four typical analytes on Ag nanoparticles (NPs) from β-silver vanadate and copper, even though the concentrations of these analytes were very low.

In this paper, we report on how to synthesize silver molybdate nanowires in the liquid phase at room temperature using 12-silicotungstic acid system. Then, the above mixture solution was irradiated with ultraviolet (UV) light. Finally, the silver nanoparticles coving on the surface of silver molybdate nanowires (SMNS) can be synthesized. The experimental device in this method is simple, easy to operate and applicable to a large scale synthesis. This new and universal method for preparing a substrate for the detection of surface-enhanced Raman spectroscopy is proposed.

Experiments

Materials

Tungstosilicate acid [H₄(SiW₁₂O₄₀), TSA], silver nitrate (AgNO₃), Na₂MoO₄ and ethanol were all of A.R. grade and obtained from Shanghai Reagent Co. and were used without further purification. p-aminothiophenol (PATP) was purchased from Sigma-Aldrich and used without further purification. Doubly distilled water was used throughout to prepare the solutions.

Characterizations

The X-ray diffraction (XRD) analysis was carried out with a MAP18AHF instrument (Japan MAC Science Co.). The morphologies and structures of the nanoparticles were examined by FEI Sirion-200 field emission scanning electron microscope (SEM) and JEOL 2010 transmission electron microscope (TEM). The macro-SERS spectra were recorded with a Renishaw Raman RM2000 equipped with the 514.5 nm laser line, an electrically refrigerated CCD camera, and a notch filter to eliminate the elastic scattering. The spectra shown here were obtained with a 30 mm focus length lens. The output laser power on the sample was about 2 mW. The spectral resolution was 4 cm^-1. The spectral scanning conditions were chosen to avoid sample degradation. The reported spectra were registered as single scans. Pyris-I TGA Analyzer (Perkin-Elmer Corporation), the temperature of the sample rose at constant speed (10°C / min) in N₂ atmosphere during heating.

Preparation of silver and silver molybdate complex

In a typical experiment, 2-propanol (2 mL) and deaerated solution of tungstosilicate acid (30 mL, 2 mM) were added to an aqueous deaerated solution of AgNO₃ (15 mL, 4 mM solution), and then the solution of Na₂MoO₄ (15 mL, 4 mM) was added to the above mixture with continuous stirring for 30 min and then was allowed to age for 2 h. A homogeneous light-yellow solution at room temperature was formed, indicating the formation of silver molybdate. When the above mixture was irradiated with UV light (Pyrex filter,>280 nm, 450 W Hanovia medium pressure lamp) for 2 h at the same time, the solution turned gray gradually, indicating the formation of the silver and silver molybdate complex. The yields were collected and washed several times using the double distilled water, dried in a vacuum oven at the 60°C for 6 h. The Ag NPs concentration was measured by UV-visible spectroscopy using the molar extinction coefficients at the wavelength of the maximum absorption of Ag colloid.

Results and discussion

XRD pattern shown in curve 1 of Fig. 1 was recorded from a drop coated film of the non UV-irradiated sample on a glass substrate. All reflection peaks of the products can be indexed as a pure structure with cell parameters a = 7.59, b = 8.31, c = 11.42 Å, which is in good agreement with the literature value (JCPDS Card: 72-1689). Pure Ag₆Mo₁₀O₃₃ phase can be obtained in TSA system at pH 2 [15]. Curve 2 of Fig. 1 shows the XRD pattern of the UV-irradiated sample. The (111), (200), (220), and (311) Bragg reflections of face-centered cubic (fcc) silver are clearly observed (indicated by an asterisk). Some of the Bragg reflections corresponding to the Ag₆Mo₁₀O₃₃ phase structure indicate that the formation of silver nanoparticles on the Ag₆Mo₁₀O₃₃ colloidal particle template does not disrupt the basic structure of silver molybdates.

Fig.1 XRD patterns recorded from drop-coated films on glass substrates of silver molybdate synthesized by reaction of TSA solution with aqueous AgNO3 and Na2MoO4, before (curve 1) and after (curve 2) UV irradiation (Bragg reflections marked * correspond to Ag)

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In this study, a solution method was employed to synthesize SMNs. The SEM image (Fig. 2(a)) reveals that the SMNs are generally ultralong with the length up to several hundred micrometers. When the reaction solution was irradiated by UV-light simultaneously, they produced fresh silver nanoparticles in situ. Figure 2(b) displays an SEM image of the SMNs covered with Ag nanoparticles. TEM image in Fig. 2(c) shows that the particles are also rod-like and the diameter is about 50 nm. The surface structures indicate that the crystal surface is smooth. The inset of Fig. 2(c) shows the associated selected-area electron diffraction (SEAD) pattern recorded by focusing the incident electron beam on an individual nanowire, and exhibits the single-crystalline nature of the wire. We have also carried out extensive investigations on more individual wires by using electron diffraction (ED), and the results demonstrate that the as-synthesized sample is single-crystalline. Based on the XRD pattern and the electron diffraction pattern, we concluded that the nanowires grow preferentially along c axis. The TEM image (Fig. 2(d)) shows that the SMNs were almost completely covered with Ag nanoparticles with an average diameter of 16 nm. The high resolution TEM images in Fig. 2(e) shows that very tiny nanoparticles sized from 10 to 25 nm are attached on the backbone of the SMNs. Interestingly, the silver nanoparticles are not observed to be separated from the SMNs in the solution, which indicates that the Ag⁺ reduction occurrs on the surface of the Ag₆Mo₁₀O₃₃ colloidal particles and suggests that the Ag₆Mo₁₀O₃₃ colloidal particles act as excellent templates. From the insets of Figs. 2(e) and 2(f), Ag NP and SMN was a single crystal, and it grows along the [111] and [022] axis. The crystal lattice of Ag and Ag₆Mo₁₀O₃₃ may be indexed as (111) and (022) crystal planes with 0.24 and 0.335 nm corresponding to Ag (111) plane and Ag₆Mo₁₀O₃₃ (022) plane, respectively.

Fig.2 A solution method was employed to synthesize SMNs. (a) SEM images of SMNs; (b) SEM images of SMNs covered with Ag nanoparticles; (c) TEM images of SMNs; (d) TEM images of SMNs covered with Ag nanoparticles; insets of (c) are individual SMN and electron diffraction pattern; (e) TEM image of individual SMN covered with Ag nanoparticles, inset shows crystal-lattice image of an individual Ag nanoparticle and SMN; (f) Crystal-lattice image of individual SMN

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It is interesting that more and more other tiny nanoparticles have appeared on the backbone of the Ag₆Mo₁₀O₃₃ nanowire after its exposure under electron beam irradiation. Clearly, this nanoline is unstable with electron beam irradiation. Nanoparticles on the surface get increased gradually as the time for the electron beam irradiation is prolonged. Accordingly, it is deduced that the melting point of this compound is likely low. Figure 3 shows the results of Ag₆Mo₁₀O₃₃ nanowires differential thermal analysis. It reveals that the compound melting point is about 236°C, which is not high,. This can explain the phenomenon: under electron beam irradiation. Large heat is generated because of electron beam bombardment. Nanoline partially melts and amorphous particles are formed after its structure is destroying. At the same time, electron diffraction results—whose data is not presented—show that these new nanoparticles in the experimental process is amorphous. This agrees with the phenomenon observed by Shuhong Yu [14].

Fig.3 Differential thermal analysis of Ag6Mo10O33 nanowires

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Herein, a unique approach to prepare a SERS substrate is proposed by utilizing silver molybdate as raw materials. Silver molybdate nanowires in large scale are synthesized through a simple solution approach at room temperature. Then, silver nanoparticles are obtained using UV light to irradiate silver molybdate solution, and Ag nanoparticles are covered on the surface of the silver molybdate in situ. Solution synthesis of the silver and silver molybdates nanowires complex is not found in literature up to now. The Ag-SMNs complex demonstrated the sensitivity in the SERS detection of PATP, which is similar to the SERS substrate of Ag nanoparticles covered on β-silver vanadate nanowires [14].

On the basis of theoretical calculations combined with the reports in literatures, Wu et al. [16] demonstrated that the PATP molecules adsorbed on nanoscale rough surfaces of noble metals and nanoparticles undergo a catalytic coupling reaction to selectively produce a new surface species p,p′-dimercaptoazobenzene (DMAB). There are no fundamentals found in their cluster model calculations, which matches the vibrational frequencies of 1142, 1391, and 1440 cm^-1 as observed in SERS of PATP on rough electrodes and nanoparticles of silver [17-19]. The peak positions of these intense SERS bands observed in experiments cannot be reproduced in their calculations of PATP interacting with various silver clusters. They [16] concluded that the experimentally observed bands do not arise directly from PATP adsorbed on silver surfaces. Therefore, the phenomenon in SERS of PATP should be with some new surface species, DMAB. In our study, we observed the same experiment phenomenon. Figure 4 is the SERS spectra from PATP modified Ag-SMNs complex compared with the Raman spectra of the solid PATP. The Raman spectrum of the solid PATP (curve 1) is different from that of PATP modified Ag-SMNs under the light irradiation (curve 2). The peaks at 1142, 1390, or 1435 cm^-1 as observed in our SERS spectra (curve 2) can be attributed to that the DMAB is produced from the catalytic coupling reaction of PATP on rough silver nanoparticles surfaces under the nanoparticle-assisted photonic catalytic oxidation, which is different from the results of the electrochemical process [20].

Fig.4 Raman and SERS spectra of solid PATP (curve 1), and SERS (curve 2) spectra from PATP modified Ag-SMNs complex

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Conclusion

In summary, silver and silver molybdate nanowires complex can be synthesized in the 12-silicotungstic acid system. Ag-Silver molybdate nanowires complex are demonstrated to be a new style SERS substrate.

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