Interfacial synthesis of SnSe quantum dots for sensitized solar cells

Abstract

A new interfacial synthesis of colloidal SnSe quantum dots (QDs) was realized with use of common precursors under a moderate temperature (95 °C). SnSe QD-sensitized solar cells were fabricated to show an improved power conversion efficiency (>5 times) with a high fill factor of 0.71.

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Semiconductor colloidal quantum dots (QDs) have been widely explored in both fundamental research and applications such as bio-imaging, light emitting diodes (LEDs) and photovoltaic devices, owing to their unique optical and electrical properties originating from their size-dependent quantum confinement effect.^1–5 Among these, cadmium and lead chalcogenides have gained special interest because of their excellent optical and photovoltaic potential.^6,7 However, the potential exposure of cadmium and lead is considered to be a significant threat to public health and the environment,⁸ and the preparation and development of a new generation of QDs still remains a challenge partly due to the lack of a robust synthetic route, which motivates us to develop a new kind of QDs by a suitable method.

SnSe is a IV–VI p-type semiconductor with narrow band gaps, which has an indirect band gap of 0.9 eV and a direct band gap of 1.3 eV in its bulk form.⁹ Recently, much effort has been focused on this material due to its potential in photovoltaic application and phase-change memory alloys.^10,11 However, there are only a few examples of synthesis of colloidal SnSe nanocrystals,^12–15 and successful preparation of colloidal SnSe QDs remains as a difficulty probably due to the fast nucleation and growth in the current strategy and the relatively high reactivity of Sn precursors. Besides, expensive and toxic organophosphorus or organoselenium compounds, and high reaction temperature are generally required for the previous methods, along with relatively large particle size (usually >10 nm) for the resulting nanocrystal products.^12–15 Challenges remain in the synthesis of SnSe QDs from common starting materials under mild conditions. Furthermore, the application of SnSe nanocrystals in solar cells has been demonstrated,¹² while the improvement in the power conversion efficiency of these sensitized solar cells is still highly needed.

A liquid–liquid interface offers an ideal platform for fabrication of fine nanoparticles driven by a minimization of interfacial energy.¹⁶ Brust et al. first reported the preparation of gold nanoparticles via the liquid–liquid interfacial method.¹⁷ Ji et al. and An et al. further developed the Brust route to synthesis other noble and semiconductor nanocrystals.^18–20 In this work, we report a mild interfacial (toluene–glycerol) synthesis of SnSe QDs with particle size of ∼3 nm for the first time, as well as the application of the resulting QDs in solar cells with highly improved power conversion efficiency. This synthetic route offers several advantages over those reported previously: the control of reacting rate by the interface enables the preparation of SnSe QDs with very small particle size; relatively mild conditions (below 100 °C); common starting materials can be adopted, that is, stannous octoate and selenium powder in this case. This method enables the efficient control over the nucleation and growth process, thus, particle size, and provides new insights into the preparation of other new QDs. In addition, we further investigated the photovoltaic properties of SnSe QDs by applying them to QD-sensitized TiO₂ solar cells. The final power conversion efficiency (η) of 0.33% with a high fill factor of 0.71 under AM1.5G solar irradiation, which is much improved compared with previous results (η = 0.06% and 0.0104% respectively),^12,21 demonstrates the potential application of SnSe QDs for photovoltaic devices and building blocks for other devices.

In a typical synthesis of SnSe QDs, stannous octoate (1.5 mmol) and oleylamine (OLA, 6 mL) were first dissolved in 30 mL toluene, and 30 mL glycerol was added to form an interface. The solution was purged with nitrogen for about 30 min, after which the temperature was raised to 95 °C. A fresh solution of sodium hydrogen selenide (NaHSe) prepared by the reaction of sodium borohydride (3.5 mmol) and selenium powder (1.5 mmol) in 3 mL deionized water was then injected to the above solution under stirring. The reaction was maintained at 95 °C for 8 h and then cooled to room temperature. The as-prepared SnSe QDs in the toluene phase was precipitated with ethanol and redispersed in toluene for further use (see details in ESI†).

The typical transmission electron microscope (TEM) image of SnSe QDs is shown in Fig. 1a. As can be seen, the QDs are quasi-spherical and dispersed uniformly without obvious aggregation. The average size is about 2.5 nm with a relatively narrow size distribution (Fig. 1a inset). The high resolution transmission electron microscope (HRTEM) image in Fig. 2b reveals the high crystalline nature of these QDs. The lattice fringes with the spacing d = 0.333 nm can be clearly observed in the inset, which corresponds to (210) lattice planes of orthorhombic SnSe. This is consistent with the anisotropic crystal structure of SnSe as illustrated in the crystallographic model in Fig. 1c inset, which shows a weak bonding of (010) plane.¹³ The XRD pattern shown in Fig. 1c demonstrates the orthorhombic crystal nature of the QDs. The major diffraction peaks at 2θ values of 25.3°, 30.6°, 37.8°, 43.5° and 49.5° and the relative peak intensities well match with the standard orthorhombic SnSe (JCPDS no. 048-1224). The weak and broad diffraction peaks imply the small particle size of the QDs. And the average grain size calculated from the major diffraction peak (111) (2θ = 30.6°) via Scherrer's equation is 3.0 nm, which is in good agreement with the result obtained from the TEM images.

Fig. 1 (a) TEM image of SnSe QDs. Inset: histogram with experimental size distribution of the sample. (b) HRTEM image. (c) XRD pattern of SnSe QDs. Inset: (100) and (001) projections of a model SnSe unit cell. (d) XPS survey spectrum of the SnSe QDs. (e) High resolution Sn 3d spectrum. (f) Experimental (black) and simulated (coloured) Se 3d spectra.

Fig. 2 (a) Absorption spectrum of as-prepared SnSe QDs. (c) Plot of (αhν)² vs. hν for the direct transition. Inset: plot of α^1/2 vs. hν for the indirect transition.

X-ray photoelectron spectroscopy (XPS) was further performed to clarify the composition and investigate the valence states of tin and selenium in SnSe QDs. The survey spectrum shown in Fig. 1d confirms the existence of C, H, N, Sn, Se and O. Fig. 1e and f are the high resolution spectra of Sn 3d and Se 3d. The binding energy of Sn 3d_3/2 and Sn 3d_5/2 are 494.8 ev and 486.4 eV respectively, with a characteristic peak separation of 8.4 eV.¹² And both the binding energy of Se 3d and Sn 3d show a shift to higher values, indicating the partial oxidation of Se in the sample during the sample handling.

The optical property of the SnSe QDs was also investigated. Fig. 2a is the UV-vis absorption spectrum of SnSe QDs. A strong absorption shoulder can be observed at about 410 nm, demonstrating the narrow size distribution of the QDs, which is consistent with the TEM result. Based on the UV-vis spectrum, we further studied the quantum confinement effect according to the band gaps of the QDs. As reported previously,²² by extrapolating the linear part of the α^1/2 and (αhν)² curves to α = 0, where α is the absorption coefficient and hν is the photon energy, both indirect and direct band gaps of SnSe QDs can be obtained. As shown in Fig. 2b and the inset, the indirect band gap is therefore 2.27 eV and direct band gap is 2.78 eV. The large blue-shift of the bandgaps with respect to those of bulk SnSe (0.9 eV and 1.3 eV, respectively) demonstrates a strong quantum confinement of SnSe QDs, which shows the evidence of small particle size of QDs.

To go insight into the formation of SnSe QDs, the temporal evolution of the reaction process was investigated via the ultraviolet-visible (UV-vis) spectra of the samples taken from different intervals, as shown in Fig. S3†. In the initial stage (0–2 h), no absorption peak was observed. And the absorption peak appeared at 4 h at about 390 nm, indicating the generation and narrowing of QDs. After 8 h, the absorption peak became more obvious, and slowly shifted to 410 nm, implying the growth of the QDs. The reaction was slower and an improved size distribution was achieved in comparison with those previously reported via other methods,^12–15 as indicated from the UV-vis spectra. The slow nucleation and growth rate as well as the separation of the nucleation and growth stages in the liquid–liquid biphase approach might be responsible for the formation of monodisperse small-sized QDs (Fig. S4†).¹⁹ Besides, the introduction of glycerol as the aqueous phase in the preparation is also crucial to obtain pure-phase and well-defined QDs. The samples prepared in the toluene–water system resulted in impure crystals with the existence of SnO₂, since Sn(II) is active and prone to hydrolyse in water (Fig. S5†). Control experiments were also conducted by replacing OLA with oleic acid (OA) and tri-n-octylphosphine (TOP), respectively. Despite their success as stabilizing ligands in the preparation of II–VI QDs such as CdSe and CdTe,^23,24 a large amount of a black precipitate formed immediately upon the injection of the NaHSe solution in both cases, which was identified as selenium powders. This phenomenon suggests the failure in the fabrication of SnSe QDs, which might be ascribed to the stronger binding between Sn²⁺ and OA (or TOP) ligand.^13,25

The successful synthesis of SnSe QDs allowed us to further assess their potential photovoltaic properties. We demonstrate here the preliminary work of SnSe QD-sensitized solar cells (Fig. 3a). The photoanode was fabricated by the direct absorption method (see details in the ESI†).²⁶ The J–V curve of the solar cell sensitized by SnSe QDs is shown in Fig. 3b (red curve), which shows a short-circuit current density (J_sc) of 0.70 mA cm⁻² and an open-circuit voltage (V_oc) of 0.54 V with a fill factor (FF) of 0.7 and a power conversion efficiency (η) of 0.27%. The power conversion efficiency and FF are much better than those previously reported for SnSe nanocrystals hybrid solar cell and SnSe nanowire hybrid solar cell (η = 0.06% and 0.0104% respectively).^12,21 FF usually depends on the series and shunt resistance of the devices and the high FF here may benefit from the good alignment between QDs and TiO₂. However, the J_sc is low, which may be caused by the finite association between the individual SnSe QDs and long transportation distance between QDs and TiO₂ films, since the long hydrocarbon chains used as ligands in most QDs inhibit the transportation of excited electrons from QDs to TiO₂ films.³

Fig. 3 (a) Schematic representation of the QDs sensitized solar cell. (b) J–V characteristics of solar cell devices under simulated AM1.5 irradiation before and after ligand exchange. Inset: brown colloidal dispersion of SnSe QDs undergoes the phase transfer from toluene to formamide (FA) upon exchange of the original OLA ligand with S²⁻.

As demonstrated by the work of Sargent and Talapin, the replacement of traditional ligands containing long hydrocarbon chains with small inorganic ligands can facilitate the electron transportation between individual QDs.^3,27 Besides, small inorganic ligands reduce the transportation distance between QDs and TiO₂ films and may improve the charge transfer between them as well. Therefore, a ligand exchange process was introduced before device fabrication. A toluene solution of SnSe QDs stabilized by OLA was mixed with a formamide (FA) solution containing (NH₄)₂S, and then the mixture was stirred for 30 min leading to the complete phase transfer. The colour of the toluene phase changed from brown to transparent and the FA solution from chartreuse to brownish red, indicating the successful transfer of QDs from toluene to FA (Fig. 3b inset). The following fabrication procedure was similar. The corresponding J–V curve of the solar cell is shown in Fig. 3 (purple curve). An improvement of 35.7% in J_sc was observed while the FF remained almost the same. Obviously, the substitution of long hydrocarbon chains with small inorganic molecules not only facilitates the electron transportation between individual QDs, but also boosts the charge transfer from QDs to TiO₂ films by reducing the transportation distance, and hence improves the photocurrent. Thus a remarkable 22.2% enhancement in η to 0.33% was achieved. This demonstrated the success of the ligand exchange strategy, which may find its application in fabricating other QDs sensitized solar cells. However, the power conversion efficiency is still relatively low if compared with other QDs sensitized solar cell,^3,4 which may be caused by the poor match of the absorption spectrum with the solar spectrum, so further bang gap engineering or other strategy is essential to improve the efficiency.

In summary, we have demonstrated a successful synthesis of colloidal SnSe QDs via a mild interfacial strategy for the first time. The adoption of toluene–glycerol interface enables the successful synthesis of monodisperse and well crystallized sub-5 nm SnSe QDs and will open a new avenue for the preparation of other new QDs. In addition, the SnSe QDs possess the potential to be sensitizers in QD-sensitized solar cells, and the introduction of a ligand exchange process enhanced the power conversion efficiency by 22.2%, with a final η of 0.33% and fill factor of 0.71, which is the highest value ever reported for SnSe. We believe this work will bring new insight into synthesizing and modifying QDs for optical and electronic devices.

Acknowledgements

This work was supported by the National High Technology Research and Development Program of China (863 Program) (2012AA030313), National Natural Science Foundation of China (21474052) Industrial Project in the Science and Technology Pillar Program of Jiangsu Province (BE2012181), and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and additional characterization data. See DOI: 10.1039/c4ra10392k

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