Preparation of SnS2 colloidal quantum dots and their application in organic/inorganic hybrid solar cells
© Tan et al; licensee Springer. 2011
Received: 7 December 2010
Accepted: 5 April 2011
Published: 5 April 2011
Skip to main content
© Tan et al; licensee Springer. 2011
Received: 7 December 2010
Accepted: 5 April 2011
Published: 5 April 2011
Dispersive SnS2 colloidal quantum dots have been synthesized via hot-injection method. Hybrid photovoltaic devices based on blends of a conjugated polymer poly[2-methoxy-5-(3",7"dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and crystalline SnS2 quantum dots as electron acceptor have been studied. Photoluminescence measurement has been performed to study the surfactant effect on the excitons splitting process. The photocurrent of solar cells with the hybrid depends greatly on the ligands exchange as well as the device heat treatment. AFM characterization has demonstrated morphology changes happening upon surfactant replacement and annealing, which can explain the performance variation of hybrid solar cells.
Organic-inorganic hybrid bulk-heterojunction (BHJ) solar cell is an interesting alternative to the all-organic solar cells with replacement of the organic n-type material with inorganic nano-particles (NPs). This new kind of solar cell aims at combining the solution processability of conjugated polymers with high electron mobility and the relative environmental stability of inorganic semiconductors. Up to now, various NPs such as CdSe [1–3], PbS , TiO2 , ZnO , or heterojunction nano-crystals [7–9] have been applied in organic-inorganic BHJ solar cells. The best solar cell adopting inorganic nano-phase as the electron acceptor demonstrated a power conversion efficiency exceeding 3% using CdSe tetrapods . However, compared with this toxic material, a nontoxic, environmental-friendly alternative may be more attractive in the future application of this kind of cells.
Tin disulfide (SnS2) is a fullerene-like semiconductor with a band gap of about 2.35 eV . In this study, the consideration of SnS2 as electron acceptor in BHJ solar cell is based on its several advantages. Comparing with Cd-containing inorganic nanoparticles, SnS2 is easy to prepare, nontoxic, and environment-friendly, with an abundant content of row materials in the earth. Besides, it has an appropriate energy level distribution when forming hybrid with an electron donor such as MDMO-PPV; electrons transfer at the interface is convenient. Furthermore, as a fullerene-like semiconductor, it is easy to form a net-like interpenetrating connection between particles, which is greatly beneficial to electrons transportation. Up to now, extensive research has been focused on its application of gas-sensoring , lithium batteries , and electrical switching . As a photoconductive semiconductor, its potential application in solar cells is rear. Our previous study has shown that SnS2 nano-particles with a crystalline/amorphous blended phase demonstrated obvious photovoltaic property as an electron acceptor when blending with an organic semiconductor poly[2-methoxy-5-(3",7"-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) . However, the conversion efficiency may be influenced due to the not well-dispersed and well-crystallized SnS2 particles prepared at low temperature. Other commonly used methods such as solvothermal  or chemical and vapor deposition  usually generated very large particles, which is not suitable for hybrid BHJ solar cells. A dispersive and well-crystallized SnS2 nano-particle is of great necessity to form an interpenetrating electron tunneling path in the hybrid BHJ solar cells.
Herein, SnS2 colloidal quantum dot that is up to the role in organic/inorganic hybrid BHJ solar cell is prepared via hot-injection method [17, 18]. The hybrid solution containing SnS2 quantum dots and MDMO-PPV in chloroform or chlorobenzene is clear and transparency. Solar cells using these quantum dots as electron acceptor generate an improved photovoltaic property after replacing the insulating surfactant on the particles surface. The result of this research suggests this easy-prepared nontoxic semiconductor could be a promising candidate for BHJ solar cells.
All chemicals are used as-received without further treatment. In a typical reaction, 0.26 g of tin (IV) chloride anhydrous (SnCl4, 99.0+%, Sinopharm, Beijing, China) is added to 20 ml of oleylamine (OLA, 80+%, Aladdin, Shanghai) in a 50-ml three-neck flask. The mixture is purged with N2 for 30 min at 120°C and heated to 200°C. Then 3 ml of OLA containing 0.22 g of thioacetamide (TAA, 99.0+%, Sinopharm, Beijing, China) is injected into the mixture quickly. The reaction is kept at 200°C with N2 purging for 12 h and then cooled down to room temperature. The as-formed nanocrystals are isolated by precipitation with ethanol followed by centrifugation. The final product, SnS2 quantum dot, is washed three more times by solvent/antisolvent precipitation with chlorobenzene/ethanol.
For a better application in hybrid BHJ solar cell, the OLA ligands on the surface of SnS2 colloidal quantum dots are partly replaced by stirring the final product in anhydrous pyridine at 60°C for 1 h and ultrasonicated at 40°C for 1 h. After that, the particles are precipitated with hexanes at room temperature, recollected by centrifugation, and then dispersed into a mixture of chlorobenzene/pyridine (90:10, vol/vol) for further use.
The fabrication process of hybrid BHJ solar cells is as follows. Poly(thiophene) (3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) is spin coated at 2400 rpm onto the ITO substrates those are precleaned by soap water followed by deionized water, and then ultrasonicated in acetone and isopropanol. After the ITO/PEDOT:PSS is annealed at 140°C for 1 h, it is transferred into a glove box together with the organic/inorganic-blended solution. The mixture is prepared by blending SnS2 and MDMO-PPV in chlorobenzene with different mass ratios and then ultrasonicated for about 1 min to form a transparent solution. Hybrid BHJ films with an optimized thickness of about 120 nm are achieved by spin-coating the mixture on PEDOT:PSS at 1500 rpm for 30 s in a N2 atmosphere in a glove box. Afterward, onto the hybrid film, a ZnO buffer layer of about 20 nm is obtained by spin-coating a ZnO methanol solution (30 mg/ml, 3000 rpm) . The solar cells fabrication is then finished by thermally depositing a 100-nm aluminum cathode on top.
The crystalline phase pattern of SnS2 particles is characterized by X-ray diffraction (XRD) on a Rigaku D/max-gA X-ray diffractometer with Cu Kα radiation. Its morphology is given by a transmission electron microscopy (TEM) on a Hitachi H-800 at an acceleration voltage at 80 kV. Absorption spectrum (Abs) and photoluminescence (PL) measurements are carried out on Varian U-3000 model ultraviolet-visible spectrophotometer and Varian Cary Eclipse fluorescence spectrophotometer, respectively. The surface morphology of hybrid MDMO-PPV:SnS2 films is characterized on Solver P47 scanning probe microscopy (SPM). The current-voltage (I-V) measurements on the MDMO-PPV:SnS2 BHJ solar cells are performed on Keithley 2400 source in forward bias mode under AM 1.5 100 mW/cm2 illumination.
Compared with the homogeneous and flat MDMO-PPV film as shown in Figure 9A and a, the hybrid film containing OLA-linked SnS2 shows clearly two phases (Figure 9B). The SnS2 phase that is present as small protrusions (no higher than 20 nm shown in Figure 9b) are well dispersed and immersed into the homogeneous organic phase, which demonstrates that the OLA-linked SnS2 particles can form good contact with MDMO-PPV. The SnS2 particles show more enlarged aggregation and protrusion after pyridine treating, so that phase separation as caused is clearly observed (Figure 9C). The hybrid film surface becomes rougher due to large SnS2 aggregation (Figure 9c). This will cause decreased interface between SnS2 and PPV. However, Jsc characteristic of solar cells with pyridine-SnS2 as acceptor is superior to that of solar cells with OLA-SnS2. One can get the supposition that the excitons splitting efficiency should be compensated upon ligand exchange so that it will not be weakened due to phase separation and interface decreasing. The supposition can be demonstrated through photoluminescence property in Figure 5b that PL quenching happens more intensively than that without pyridine treatment. On the other hand, transportation process of free charges, especially electrons, may be favored from suitable phase separation due to enlarged and connected inorganic phase. After heat treatment, previous small SnS2 aggregates, formed during pyridine exchange, will further connect and partly fuse with the adjacent ones (Figure 9D). Thus, electrons can find a more convenient way to transport themselves to the electrode through interpenetrating networks. Besides, the film surface becomes much flatter after annealing (Figure 9d), which is beneficial to form a good contact between the active layer and the electrode. This is why the cell performance has an improvement after annealing. Noticed is that, the efficiency of our solar cell using SnS2 as the electron acceptor is relatively low comparing with other hybrid solar cells such as CdSe  and PbS . This is mainly attributed to un-optimized particle morphology as well as the particle aggregation in the hybrid film, increasing the series resistance and decreasing current. On the other hand, SnS2 particles enhanced light absorption in the hybrid film is not obvious. Maybe this is our further study to improve the device performance in next step.
Dispersive SnS2 colloidal quantum dots are synthesized and considered as electrons acceptor in hybrid hetero-junction solar cells containing a conjugated polymer (MDMO-PPV). It shows a best performance at the SnS2 weight ratio of 50%. OLA ligand on the particles should have an influence on the charges separation in the blends when characterizing through photoluminescence. Surfactant exchange using pyridine causes increased PL quenching, which suggests the enhanced excitons splitting efficiency. Thus, an obvious improvement in photocurrent as well as energy transversion efficiency is realized. Annealing treatment of the devices produces further increase in efficiency due to the enhancement of photocurrent. AFM studies have provided the insights into the variation of devices performance. Comparing with the uniformly distributed two phases in the OLA-SnS2:MDMO-PPV blended film, a phase separation in pyridine-SnS2:MDMO-PPV blending is appreciated on the photocurrent increase due to the pyridine ligand. Besides, not the isolated aggregates but the connected SnS2 networks after annealing are the best for the solar cells performance.
transmission electron microscopy
This study is supported by the National Natural Science Foundation of China (Contract Nos. 61076009, 60736034 and 50990064) and the National Basic Research Program of China (973 Program) under Grant No. 2010CB933800.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.