Article pubs.acs.org/JPCC
Large-Scale Synthesis of PbS−TiO2 Heterojunction Nanoparticles in a Single Step for Solar Cell Application Stephanie B. Bubenhofer, Christoph M. Schumacher, Fabian M. Koehler, Norman A. Luechinger, Robert N. Grass, and Wendelin J. Stark* Functional Materials Laboratory, Institute for Chemical and Bioengineering, ETH Zurich, CH-8093 Zurich, Switzerland S Supporting Information *
ABSTRACT: The demand for low cost solar energy technology calls for manufacturing processes using economic liquid- or gas-phase synthesis of the corresponding materials. In this regard, manufacturing of quantum dot-sensitized solar cells is particularly complicated through multiple-step preparations. Material pairs such as TiO2−PbS heterojunctions have shown high absorption of visible light and good electron transfer properties. However, traditional solution processing requires extensive surface functionalization or the use of surfactants to obtain well-defined films. Such surfactants, unfortunately, often lower electron hopping/tunneling in the system (surfactants are usually insulators) and therefore have to be removed or exchanged before completing device fabrication. Similarly, the so far presented processes to deposit PbS directly on TiO2 are very time consuming. In this paper, we present a single-step, large-scale, operable process to synthesize PbS−TiO2 heterojunction particles by aerosol synthesis using reducing flame spray pyrolysis. Nanopowders with different lead sulfide to titanium dioxide ratios were produced and characterized. Thermodynamic equilibrium calculations of the gaseous environment during the combustion process show that the process is robust with regard to usual process changes or fluctuations. We further showed how this approach allowed us to vary the structure and size of the PbS−TiO2 heterojunction particles, as long as an excess of sulfur species (S/Pb = 2.5) was applied during processing.
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such nanocrystalline solar cells.7,8 Several research groups have reported about sensitizing wide band gap semiconductors as TiO2 and ZnO with CdS, CdSe, CdTe, InP, PbS, and many other sulfide or selenide quantum dots.9−13 A solution-based device structure consisting of two stacked layers (wide band gap semiconductor and quantum dots, the so-called depleted heterojunction cell) was very recently presented to increase the photon absorption in the layers.14−17 Moreover, such quantum dot solar cells can theoretically show conversion efficiencies exceeding the Shockley−Queisser limit of 30%18 using photogenerated hot electrons.19−22 The major drawback of totally inorganic electron−hole transport systems is the interface between the two heterojunction materials. Classical solution processing (surfactant stabilization with subsequent spin or dip coating) normally introduces large, insulating molecules to the interface of the nanoparticles in the assembly. In some cases, they can be substituted by smaller or bifunctional linker molecules,23,24 which reduce electron transfer resistance between the two particles.25 However, the organic molecules usually form a tunneling barrier between the light-absorbing and the electronconducting nanoparticles. A direct contact of the sensitizing quantum dot and the nanocrystals would therefore be the preferred configuration. Recently, Acharya et al.26 and Lee et
INTRODUCTION The continuously growing energy demand asks for clean technologies.1 Next to wind and water, the sun is the most promising renewable energy source, and light conversion continues to challenge materials sciences. Single-house photovoltaics or assemblies in power plants today mainly rely on single- or multicrystalline silicon solar cells.2 In the last 10 years, a drift to new photovoltaic technologies has provided often flexible, thin film solar cells (CIGS and similar materials). Processing costs in both cases, however, are determined by their considerable energy demand. As a result, the current development focuses on more cost efficient, often solutionbased device fabrication and materials from less energy consuming processes. The U.S. Department of Energy has even lanced a program called “Sun-Shot” (about half a billion dollars granted in 2012) to bring the costs of solar energy down to 0.06 U.S. dollar per 1 kWh.3,4 A most interesting alternative became available in 1991 when Oregan and Grätzel5 presented nanocrystalline TiO2 solar cells produced in a simple solution-based process and sensitized with organic dye molecules (so-called Grätzel cell) for sun light harvesting that very recently demonstrated efficiencies up to 12.3%.6 Bleaching of the organic dyes in such systems is not negligible, and further optimization of the sensitizer mostly involves the synthesis of a totally new organic dye molecule (ongoing today). Semiconducting quantum dots with tunable, size-controlled band gaps in the visible light or near-infrared are more photostable and have been suggested as light absorbers in © 2012 American Chemical Society
Received: April 17, 2012 Revised: June 13, 2012 Published: July 11, 2012 16264
dx.doi.org/10.1021/jp3036814 | J. Phys. Chem. C 2012, 116, 16264−16270
The Journal of Physical Chemistry C
Article
Scheme 1. Heterojunction PbS−TiO2 Nanoparticle Productiona
In a single step, a liquid lead, titanium, and sulfur precursor is converted into the composite nanoparticles using a flame as a high-temperature reactor. Control on the oxygen content allows to us fine tune the chemistry in the system as to enable selective formation of titania support particles (formally an oxide formation) and lead sulfide quantum dots (formally a sulfide formation). The redox chemistry further determines the necessary, intermediate presence of H2S in the synthesis gas. a
al.27 have presented the synthesis of PbS particles directly grown on TiO2 nanocrystals using a so-called SILAR (successive ionic layer adsorption and reaction) process, which involves a lot of repetitive steps and is therefore less suitable for economic large-scale production. Even pure lead sulfide quantum dot production for depleted heterojunction solar cells involves multiple processing steps and is quite time consuming.16,28 Here, we present the large-scale synthesis of PbS quantum dots on TiO2 nanoparticles as a heterojunction material in a single-step flame spray synthesis process. Even at laboratory scale, the here-used setup was able, in a nonoptimized version, to manufacture up to 15 g of heterojunction particles per hour. More specifically, flame spray synthesis today provides access to a huge variety of oxide and salts,29 heterostructured and core/ shell materials,30−34 next to Janus shaped and spiked particles.35−39 The introduction of chemically reducing production atmosphere by Grass et al.40 has opened access to nanoparticles of metals, alloys, and sulfides.41 In this study, we investigated direct synthesis of three different, representative volume ratios of PbS to TiO2 (1:2, 1:4, and 1:10) and characterized the heterojunction materials by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The thermodynamics of the combustion and aerosol processing during coproduction of sulfide and oxide particles confirmed that the here-used aerosol process is rather robust with regard to PbS−TiO2 of different constituent ratios and structures.
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total metal concentration of 4 wt % was kept constant for all experiments. The thiophene content of the precursor was calculated to match the lead concentration, while excess sulfur was used to ensure complete lead sulfidation.42 The combustion of the above PbS−TiO2 precursor was carried out in an enclosed box under nitrogen purge to provide an oxygen-poor atmosphere (avoid oxidation of the sulfide species). The off-gas composition was continuously monitored by a mass spectrometer (OmniStar GSD 320 O1, Pfeiffer Vacuum AG), and the oxygen concentration in the system was held below 140 ppm O2 during production. The above precursor (5 mL min−1) was then dispersed by 4−4.5 L min−1 O2 (through a nozzle) into a premixed flame (2.4 L min−1 O2, 1.13 L min−1 CH4), and nanoparticles were collected on a glass fiber filter placed in the off-gas above the flame. Particle Characterization. TEM images were taken on a FEI Tecnai F30 (FEG cathode, operated at 300 kV, point resolution ∼2 Å). Particle size distributions were determined from several TEM images (analyzing 140−250 particles each). In specific cases, the particles were heat treated for 1 h at 500 °C in an oven (Nabertherm, L5/11). Samples for heat treatment under vacuum were previously fused under vacuum into a glass tube. XRD analysis was carried out on a X'Pert PRO-MPD (Cu Kα radiation, X'Celerator linear detector system, step size of 0.05°, 45 kV, 40 mA, ambient conditions). Background subtraction and Rietveld refinements were done with the X'Pert HighScore Plus software (Granularity 27, Bending Factor 1). The specific surface area was determined according to Brunauer−Emmett−Teller (BET, Tristar, Micromeritics Instruments).
EXPERIMENTAL METHODS
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Materials. Lead(II) 2-ethylhexanoate (40.5−42.5% Pb) was purchased from ABCR-Chemicals, titanium(IV) isopropoxide (purum) was from Fluka-Chemie AG, and tetrahydrofuran (THF) (high-performance liquid chromatography grade) was from Fisher Scientific. Thiophene (≥99%) was purchased from Aldrich Fine Chemicals, and technical grade 2-ethylhexanoic acid was used as a cofuel throughout this study. Particle Synthesis. Reducing flame spray synthesis40 was used to investigate manufacturing of PbS−TiO2 heterojunction nanoparticles. Precursors consisted of corresponding amounts of Ti-isopropoxide, Pb-2-ethylhexanoate, and thiophene in 2ethylhexanoic acid diluted 2:1 (weight:weight) with THF. A
RESULTS AND DISCUSSION TiO2−PbS Nanoparticle Production. Three precursor compositions were prepared to produce composite nanoparticles with different volume ratios of PbS to TiO2 (1:10, hence, 9 vol % PbS; 1:4, hence, 20 vol % PbS; and 1:2, hence, 33 vol % PbS). The total metal concentration for all precursors was held constant at 4 wt %. The addition of thiophene to the precursor provided the required sulfur for lead sulfide formation. A lead/sulfur molar ratio of about 2.5 was chosen to provide a sulfur excess and to suppress the formation of oxide impurities (see later sections on thermodynamics of the 16265
dx.doi.org/10.1021/jp3036814 | J. Phys. Chem. C 2012, 116, 16264−16270
The Journal of Physical Chemistry C
Article
be achieved on a lab scale using the above-described single-step flame spray synthesis reactor. Characterization of Heterojunction Nanoparticles. TEM images of the as-prepared particles are shown in Figure 2a−c. Clearly, the high contrast, small nanoparticles represent the lead-containing quantum dots (darker spots) on top of the titanium dioxide particles (lighter element with lower electron beam deflection/scattering). The quantum dots are quite homogeneously spread on top of the larger support nanospheres. An increase in lead sulfide loading can be optically followed when going from Figure 2a (9 vol % PbS) to Figure 2c (33 vol % PbS). Independent of the Pb−Ti ratio, a similar quantum dot particle size was observable and quantitatively confirmed through analysis of the full particle size distribution (Figure 2d−f) by optically analyzing TEM images. As theoretically expected from such aerosol processes, we observed a log-normal size distribution around 2 nm for the lead sulfide quantum dots and 10 nm for the TiO2 support particles (see the Supporting Information, Table 1). The specific surface area of all nanocomposites independently was confirmed above the particle size range and stayed in line with similar size values for all three samples (Supporting Information, Table 1). The lead to titanium ratio in the precursor apparently only has a minor influence on the form and size of the different constituents in the here-used range. Note that only the product volume ratio of the two materials was changed, while the total metal concentration in the precursor was kept constant. To confirm the identity and phase composition of the hereprepared materials, powder XRD patterns of all materials were collected. Additionally, the as-prepared particles were heated
process). Running the combustion process under wellcontrolled conditions (chemical composition, particularly sulfur and oxygen content) in an enclosed flame spray synthesis set up under a nitrogen protection atmosphere allows in process formation of hot hydrogen sulfide that directly afforded sulfidation of the lead during particle synthesis (Scheme 1). Combustion of Ti-isopropoxide, Pb-2-ethylhexanoate, and thiophene multicomponent precursors under oxygen-controlled atmosphere (free O2 after the flame
