Hierarchical SnO2 hollow sub-microspheres for panchromatic PbS quantum dot-sensitized solar cells

Journal of Alloys and Compounds 709 (2017) 187e196

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Hierarchical SnO2 hollow sub-microspheres for panchromatic PbS quantum dot-sensitized solar cells Shuhao Pan a, Ru Zhou a, b, *, Haihong Niu a, Lei Wan a, Bin Huang a, Yuanzhang Huang a, Fengwei Ji a, Jinzhang Xu a a b

School of Electrical Engineering and Automation, Hefei University of Technology, Hefei, 230009, PR China School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 January 2017 Received in revised form 24 February 2017 Accepted 14 March 2017 Available online 16 March 2017

In contrast with the commonly used TiO2 or ZnO electron transporters, SnO2, which possesses relatively low conduction band and high electron-mobility, is expected to facilitate the extraction of photogenerated electrons from quantum dots (QDs) to oxides in quantum dot-sensitized solar cells (QDSCs), especially for those narrow band gap QDs (e.g., PbS). In this work, hierarchical SnO2 hollow submicrospheres have been synthesized by a facile one-step hydrothermal process, and further studied for near infrared responsive PbS QDSCs. Morphology and structure characterizations reveal that these sub-micrometer-sized spheres (150e200 nm) with hollow interiors are assembled by numerous packed nanograins. The nanometer-sized grains ensure large specific surface area (~69.2 m2 g
1) and pore size (~19 nm) for high QD loading, while the sub-micrometer-sized spheres function as efficient light scatters and robust electron transporting structures. As a result, these superior features make such hierarchical SnO2 architectures very promising candidates for photovoltaic application. Based on the multifunctional photoelectrode constructed with hierarchical SnO2 hollow architectures, an appreciable power conversion efficiency up to 1.34% has been achieved for a PbS QDSC, coupled with interface engineering through TiO2 coating and CdS passivation. This work offers a promising design for developing high performance QDSCs. © 2017 Elsevier B.V. All rights reserved.

Keywords: Quantum dot-sensitized solar cells PbS SnO2 Sub-microspheres Hierarchical nanostructure

1. Introduction Inorganic semiconductor quantum dots (QDs) are among promising candidates for the next generation of photovoltaic technologies [1
6]. They possess multiple extraordinary optical and electrical properties, such as tunable size-dependent bandgap, high absorption coefficient, large intrinsic dipole moment, and a splendid feature of multiple exciton generation (MEG) [2,7,8]. A remarkable theoretical power conversion efficiency (PCE) up to 44%, exceeding the traditional S-Q limit of 32% for single-junction semiconductor solar cells, has motivated many researchers to develop high performance quantum dot-sensitized solar cells (QDSCs) [1
3,9]. The most widely used QDs can be broadly classified into three types: (i) cadmium chalcogenide QDs, including CdS, CdSe, CdTe and their alloyed nanocystals, (ii) lead chalcogenide

* Corresponding author. School of Electrical Engineering and Automation, Hefei University of Technology, Hefei, 230009, PR China. E-mail address: [email protected] (R. Zhou). http://dx.doi.org/10.1016/j.jallcom.2017.03.147 0925-8388/© 2017 Elsevier B.V. All rights reserved.

QDs, including PbS, PbSe, PbTe and their alloyed nanocystals, and (iii) new type of QDs, for example, Sb2S3, Ag2S, SnS, ternary CuInS2, etc. [1
3,6,10
12]. In addition, some co-sensitization systems (e.g., CdS/CdSe) have also been explored to enhance the light harvesting and reduce the charge recombination for photovoltaic application [2,13,14]. As we known, one of important issues limiting the performance enhancement for a variety of QDSCs is the insufficient light harvesting due to the narrow spectral response confined to UVevisible region [1,6]. For this reason, the success to take advantage of infrared light beyond 700 nm, which accounts for nearly half of solar energy, should contribute to a significant performance improvement of photovoltaics. Therefore, among all these candidates, lead chalcogenide QD-based photovoltaics, especially PbS QDSCs, have been extensively studied in recent years in view of their near-infrared (NIR) responsive light harvesting. PbS has narrow band gap (Eg ¼ 0.41 eV) and large exciton Bohr raddi (r ¼ 18 nm), which enables the sufficient utilization of solar energy and great potential to realize MEG effect [1,7,15,16]. For example, Tian et al. demonstrated enhanced performance of PbS QDSCs up to 4.01% via optimizing precursor solation and electrolytes [15]. Lee at

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al. reported an unprecedentedly high photocurrent density more than 30 mA/cm2 through doping strategy [16]. The MEG effect has also been observed in a sensitized photovoltaic system based on PbS QDs [7]. Recently, a remarkable PCE as high as ~10% has been reached for a colloidal PbS QD photovoltaic [17]. To date, intensive work has been devoted to the utilization of mesoporous nanocrystalline TiO2 or ZnO as electron transporting materials in QDSCs and other types of photovoltaics [2,18
20]. However, the low electron injection efficiency from QDs to the traditional TiO2 or ZnO remains as one of main drawbacks for QDSCs, especially for those narrow band gap QDs, (e.g., PbS) in view of their relatively low conduction band (CB) energy levels [4,6,21
25]. Other semiconductor oxides as potential substitutes for TiO2 and ZnO are being searched. In particular, SnO2 as an p-type wide band gap semiconductor with favorable chemical and physical properties, offers two advantageous features compared to TiO2 for QDSC applications: (i) SnO2 possesses a larger band gap (3.6 eV) than that of TiO2 (3.2 eV) and a more negative CB minimum (CBM ¼
4.5 eV vs vacuum), which should facilitate the electron injection from QDs to the oxide materials, and (ii) SnO2 exhibits a high electron mobility (100e500 cm2 V
1 s
1), two orders of magnitude higher than that of TiO2 (0.1e10 cm2 V
1 s
1), which would improve the charge transport and further reduce the charge recombination in the photoelectrodes [26
32]. However, the commonly studied SnO2-based QDSCs constructed with typical nanometer-sized (20e40 nm) nanoparticles were explored with less success, demonstrating relatively poor photovoltaic performance than those of TiO2-based devices. As a critically important part of QDSC, an excellent photoelectrode configuration generally requires high surface area, strong light scattering and quick electron transport, so as to ensure adequate carrier generation and efficient charge collection. Since the invention of QDSCs, considerable efforts have been spent on the development of a variety of multifunctional oxide materials for the construction of photoelectrodes, such as nanoparticles, nanorods, nanofibers, nanosheets, nanoflowers, hollow microspheres and various other hierarchical nanostructures [2,20]. The efficient photoelectrodes composed of these nanostructured oxides with distinctive features have been demonstrated to improve the photovoltaic performance of nanocrystalline solar cells. Particularly, in the seek of hierarchical nanostructures which can take advantage of nanometer-sized effects for high surface area and submicrometer-sized assemblies for quick charge transport, mesoporous architectures consisting of densely packed nanocrystallites have received extensive attention to construct the photoelectrodes for solar cells [13,33
35]. For example, our previous work has elaborated mesoporous TiO2 beads consisting nanocrystallites of ~17 nm for high efficiency CdS/CdSe QDSCs [13,34]. TiO2 coated multilayered hollow SnO2 microspheres prepared by a chemically induced self-assembly reaction of aqueous sucrose/SnCl4 solution under hydrothermal condition have also been reported to deliver considerable performance for dye sensitized solar cells [35]. Therefore, exploring favorable hierarchical mesoporous SnO2 architectures is of great importance to develop high performance photovoltaics, coupled with NIR responsive PbS QDs. In this work, we have demonstrated the synthesis of hierarchical mesoporous SnO2 hollow sub-microspheres, consisting of numerous nanograins via one-step hydrothermal process for high performance PbS QDSCs. As a result of large specific area and strong light scattering for high light harvesting, reasonable CB level for favorable electron injection, and high electron mobility for efficient charge transport, a PCE up to 1.34% has been reached for a PbSbased QDSC employing a multifunctional photoelectrode constructed with these promising hierarchical SnO2 hollow submicrospheres.

2. Experimental section 2.1. Chemicals and materials Tin(II) sulfate (SnSO4, 99.0%, Sinopharm), titanium tetrachloride (TiCl4, 98.0%, Sinopharm), terpineol (C10H18O, Sinopharm), ethyl cellulose ([C6H7O2(OC2H5)3]n, Sinopharm), lead acetate trihydrate ((Pb(CH3COO)2$3H2O), 99.5%, Sinopharm), cadmium acetate dihydrate (Cd(CH3COO)2$2H2O, 98%, Sinopharm), zinc acetate dihydrate (Zn(CH3COO)2$2H2O, 99.0%, Sinopharm), sodium sulfide nonahydrate (Na2S$9H2O, 98.0%, Sinopharm), sulfur (S, purified by sublimation, 99.5%, Sinopharm), brass foil (alloy 260, 0.3 mm thick, Alfa Aesar), hydrochloric acid (HCl, mass fraction ¼ 36.5e38.0%, Sinopharm), methanol (CH3OH, 99.5%, Sinopharm) and ethanol (CH3CH2OH, 99.5%, Sinopharm) were all used as received. 2.2. Synthesis of hierarchical SnO2 hollow sub-microspheres Hierarchical SnO2 hollow sub-microspheres were obtained from a one-step hydrothermal process [36]. Typically, 0.18 g of SnSO4 was dissolved in 80 mL of deionized (DI) water, following by the ultrasonication for 5 min to produce a white suspension. Then the resulting white suspension was transferred into a 100 mL autoclave and heated at 120  C in a Muffle furnace for a certain period of time (i.e., 20 min, 2 h and 20 h). After centrifugation and ethanol washing for several times, the air-dried products were calcined at 500  C for 2 h, resulting in the formation of hierarchical SnO2 submicrospheres for photovoltaic application. 2.3. Preparation of SnO2 photoelectrodes F-doped tin oxide glass (FTO, sheet resistance ~15 U sq
1) was employed as transparent conducting substrates for the fabrication of SnO2 photoelectrodes. Firstly, to prepare SnO2 paste, 0.2 g assynthesized SnO2 products were mixed with 0.7 g a-terpineol and 0.1 g ethyl cellulose dissolved in 5.0 mL ethanol, and then undergone sonication and stirring for a certain time to form a slurry after removing the ethanol. Secondly, the photoelectrodes were prepared through doctor-blading of the as-prepared SnO2 paste on FTO substrates, followed by calcination at 500  C for 30 min in air with a heating rate of 5  C min
1. As for a control SnO2 photoelectrode modified through an ultrathin TiO2 layer, the posttreatment with the TiCl4 aqueous (50 mM, 70  C, 30 min) and another calcination of 500  C for 30 min were conducted before the loading of QDs. The thickness of the photoelectrodes, measured from a step profiler, was ca. 11 mm. 2.4. In situ loading of QDs The successive ionic layer adsorption and reaction (SILAR) process was conducted for in situ loading of PbS QDs, and passivation layers of CdS and ZnS in this work. For a typical SILAR process, the bare films were sequentially immersed into the as-prepared cationic and anionic precursors for allowing the corresponding ions to penetrate into the mesopores of photoelectrodes, resulting in in situ loading of QDs. A typical SILAR process to load PbS QDs has been schematically illustrated in Fig. S1, and the subsequent CdS and ZnS deposition share the similar fabrication procedure. Specifically, the deposition of PbS QDs involves the use of cationic methanol precursor, i.e., 0.02 M Pb(CH3COO)2 and 0.02 M anionic Na2S precursor (dissolved in the solvent of water and methanol with the volume ratio of 1/1, v/v). As for the loading of CdS and ZnS, the cationic methanol precursors have been replaced by 0.1 M Cd(CH3COO)2 and 0.1 M Zn(CH3COO)2, respectively, and the anionic

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Na2S precursors share the same concentration as those of corresponding cationic solutions. A single SILAR cycle consisted of 1 min dip-coating of the photoelectrode into cationic and anionic solutions, respectively. Following each immersion, thorough rinsing was performed using methanol to remove the excess of precursor solutions and dried in air. In this work, a certain number of SILAR cycles, 4 for PbS, 4 for CdS and 2 for ZnS, were employed to obtain a desired amount of the corresponding QDs loaded on the photoelectrodes. 2.5. Fabrication of solar devices A typical solar cell was assembled by sandwiching the asprepared photoelectrode and Cu2S counterelectrode using a scotch tape spacer (ca. 50 mm thick) and further permeating the assembly with the polysulfide electrolyte for measurements [9,37]. In this work, the most widely used polysulfide electrolyte and Cu2S counter electrode were chosen for the construction of QDSCs. The polysulfide electrolyte composed of 1 M S and 1 M Na2S in DI water. The Cu2S counter electrode was fabricated on a brass foil as follows: a brass foil was immersed into HCl solution (mass fraction ¼ 36.5e38.0%) at ca. 70  C for ca. 30 min, following by rinsing with water and drying in air; the etched brass foil was then dipped into the as-prepared polysulfide electrolyte for ca. 5 min, leading to the formation of a black Cu2S layer on the foil. 2.6. Characterization of materials and solar devices Morphologies of the samples were characterized by scanning electron microscope (SEM, SU8020) and transmission electron microscope (TEM, JEM-2100F), both equipped with an energy dispersive X-ray (EDX) system to analyze the element content and distribution. X-ray diffraction (XRD) measurements were performed on an X'Pert PRO MPD diffractometer (Panalytical B. V.) using the Cu Ka irradiation. Nitrogen adsorptionedesorption isotherms were measured on an Autosorb iQ Station 3 system. The multi-point BrunauereEmmetteTeller (BET) method was used to calculate the specific surface area, and the pore size distributions were derived from the desorption branches of the isotherms based on the BaretteJoynereHalenda (BJH) model. X-ray photoelectron spectroscopy (XPS) was measured on an ESCALAB220Xi electron spectrometer from Thermo Scientific. Optical absorption spectra were collected on an Agilent UVeviseNIR spectrophotometer (CARY 5000) fitted with an integrating sphere accessory. Current density-voltage (J-V) characteristics of solar devices with an active area of ~0.196 cm2 were performed by using a digital source meter (Keithley, 2400) under AM 1.5 (100 mW cm
2) illumination provided by a solar simulator (Oriel Sol 3A Solar Simulator, USA). 3. Results and discussion SEM images of the as-synthesized hierarchical SnO2 submicrospheres through one-step hydrothermal treatment of SnSO4 for different durations (20 min, 2 h and 20 h) are presented in Fig. 1. As shown, the primary products at the initial synthetic stage (e.g., 20 min, Fig. 1a and b) displays diameters of ~100 nm, exhibiting a little polydispersity. A portion of these architectures seem to glue together with each other. The corresponding XRD patterns as given in Fig. S2 reveal that these products are mainly tin hydroxide oxide sulfate (Sn3(OH)2OSO4), the precursor which has not oxidized into SnO2 yet. When the hydrothermal reaction time reaches 2 h (Fig. 1c and d), the resultant hierarchical nanostructures are torispherical with sub-micrometer sizes ranging from 150 to 200 nm, showing more pronounced rough and granular surfaces. The XRD patterns further evidence that such a hydrothermal duration of 2 h, or even

189

longer, has resulted in the formation of SnO2. It is evident that these sub-microspheres are built up by spindle-like individual nanograins with a length of 30e60 nm and a width of 5e10 nm. With the further increase of reaction time (e.g., 20 h, Fig. 1e and f), the hierarchical architectures of SnO2 are largely retained; however, the excessive hydrothermal treatment seems to result in a certain degree of destruction and adhesion for distinct SnO2 submicrospheres, as revealed by the images. Since SEM images do not allow the distinction of the innermost structure of hierarchical SnO2 sub-microspheres, we resort to TEM images for further examinations. The typical TEM images of assynthesized SnO2 architectures are shown in Fig. 2aec. It is evident that the clear contrast between dark edges and the pale centers (Fig. 2a) confirms that the as-synthesized hierarchical architectures are highly porous with hollow interiors. The notable porous feature of such architectures allows for easy penetration of precursor solutions into the interior of SnO2 sub-microspheres, favoring the high and uniform loading of QDs [13]. Moreover, the hierarchical SnO2 hollow architectures, with abundant intercrystalline pores throughout the sphere-shells, are assembled by numerous small nanograins (Fig. 2b), in accordance with the SEM images. These agglomerated nanograins serve as building blocks for the formation of hierarchical SnO2 superstructures with robust stability. The high-resolution TEM (HRTEM) image (Fig. 2c) clearly allows the identification of a well-resolved lattice pattern with the interplanar spacing of d110 ¼ 0.33 nm, consistent with the relevant inter-planar distance for the tetragonal rutile SnO2, suggesting that the nanograins grow along the preferential growth direction of [001] crystallographic orientation [36,38]. The ring-like selective area electron diffraction (SAED) pattern as shown in Fig. 2d reveals that the SnO2 architectures are polycrystalline in nature, and the obvious discrete spots in the SAED pattern and clear lattice fringes indicate the good crystallinity and rutile phase of SnO2. The crystalline nature of SnO2 would facilitate efficient charge transport in the hierarchical architectures. As described in the experimental part, the hierarchical SnO2 hollow sub-microspheres were obtained through one-step hydrothermal treatment of SnSO4 as the single precursor. The possible reaction mechanism involving the hydrolyzation and oxidization of SnSO4 in such a hydrothermal process to produce hierarchical SnO2 can be expressed by Eqns (1) and (2):

3SnSO4 þ3H2 O/Sn3 ðOHÞ2 OSO4 þ2H2 SO4

(1)

Sn3 ðOHÞ2 OSO4 þ2O2 /3SnO2 þH2 SO4

(2)

On the basis of the morphology evolution as revealed by SEM and TEM images, it can be speculated that the hierarchical SnO2 hollow architectures are transformed from solid spheres which are composed of nanograins, other than grown by continuous collection of SnO2 nanocrystals. Such a formation mechanism for hollow SnO2 might involve a result of the Ostwald ripening process [36,39e41]. Generally, at the initial reaction stage of the hydrothermal process, the solid aggregated SnO2 architectures are formed due to the oxidation and hydrolysis of Sn2þ. With the prolonged reaction time, the subsequent recrystallization process occurs, accompanied by the growth of small SnO2 nanograins on the surface of sub-microspheres; meanwhile, the nanograins lying below the surface region are dissolved and turned into the new nucleation sites at the surface. As the reaction further proceeds, the continual evacuation depletes the materials in the interior of the spheres, thereby forming the hierarchical hollow SnO2 architectures. Herein SnO2 recrystallizes to spindle-like nanograins other than nanosheets which have been reported by Yin et al. This might be ascribed to the slight difference in the chemical environment of

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Fig. 1. SEM images of the as-synthesized hierarchical mesoporous SnO2 sub-microspheres through one-step hydrothermal process for different durations: (a, d) 20 min, (b, e) 2 h, and (c, f) 20 h.

Fig. 2. (a,b) TEM, (c) HRTEM images and (d) SAED pattern of the as-synthesized hierarchical mesoporous SnO2 sub-microspheres through one-step hydrothermal process for a certain duration of 2 h.

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the reaction conditions associated with the sensitive Ostwald ripening process for the recrystallization of SnO2 [36,39]. The hydrothermal processed hierarchical SnO2 were further calcined at 500  C for photovoltaic application. Fig. 3a gives the XRD patterns of the calcined SnO2 products obtained under the hydrothermal treatment for certain durations of 20 min (S1), 2 h (S2) and 20 h (S3). As shown, all the XRD patterns show well-resolved diffraction peaks of (110), (101), (200) and (210) indexed to the tetragonal rutile structure of SnO2 (JCPDS card no. 41-1445), agreeing with that is revealed in TEM characterization [36,38]. No diffraction peaks arisen from impurities were detected. The average sizes of SnO2 nanograins for these calcined samples as well as the direct products without calcination, derived from Scherrer's equation, are all estimated to be ~5 nm. The similar nanograin size for these SnO2 products processed under the hydrothermal treatment for different durations echoes the proposed formation mechanism of hierarchical SnO2 architectures, attributed to the Ostwald ripening process. The result is also consistent with the observation from SEM and TEM images. Fig. 3b shows the XPS survey spectrum of calcined SnO2 products, and the characteristic peaks for Sn and O elements are identified. The Sn 3d and O 1s high resolution spectra are depicted in Fig. 3c and d through spectrum deconvolution. For instance, prominent doublet peaks locate at 487.0 eV for Sn 3d5/2 and 495.4 eV for Sn 3d3/2 with a peak separation of ~8.4 eV are observed, which corresponds to Sn4þ [42]. The O 1s peak centered at 530.9 eV is assigned to oxygen bound to Sn4þ ions, while a smaller shoulder at ~532.1 eV is related to OH groups partially covered onto the SnO2 surface, similar to the case for TiO2 [43]. To explore a further insight into the porous structure of hierarchical SnO2, we performed nitrogen adsorption and desorption isotherms to determine the surface areas and pore size distributions of SnO2 sub-microspheres, as displayed in Fig. 4a and b. The corresponding surface area, pore volume and pore size are summarized in Table 1. As shown, the specific surface area of

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hierarchical SnO2 exhibits an evident increase with the prolonged hydrothermal duration from 20 min to 2 h, coupled with the significant enlargement for pore volume and pore size. That is, a longer treatment duration leads to a more porous feature for hierarchical SnO2 at the initial stage of hydrothermal process. This fact is in good agreement with the result revealed by the morphology evolution from SEM images. As reaction time further increases, the average pore size does not keep growing while the surface area and pore volume both drop a little instead. In this work, the hierarchical SnO2 produced through a hydrothermal treatment duration of 2 h delivers an optimized surface area of 69.2 m2 g
1, along with a pore volume of 0.228 cm2 g
1 and an average pore size distribution centered at 19.13 nm. Such a considerable surface area is much larger than the typical value for the commonly used 20e30 nm nanoparticles in photovoltaics [34]. In addition, the adsorption isotherms for the hierarchical SnO2 appear to be type IV curves with observed hysteresis loops of type III (Fig. 3c), characteristic of the presence of mesopores in the size range of 2e50 nm for our hierarchical SnO2. The obtained pore sizes are evidently resulted from the mesopores between SnO2 nanograins packed in the shells of torispherical hollow architectures. The porous properties of hierarchical SnO2 are consistent with the results reported by Yang et al. [44]. In conclusion, the mesoporous SnO2 architectures are consisted of randomly packed nanograins, thereby possessing large specific surface area and pore size. Such ideal surface area and pore size would afford a favorable loading of light harvesting materials, i.e., QDs or dye molecules in the inner pores of porous structures, and superior infiltration of electrolytes in the case of solar cell application [38]. Therefore, the hierarchical SnO2 hollow sub-microsphere offers a promising choice of efficient electron transporting material for high performance QDSCs. The photoelectrodes constructed with these hierarchical hollow SnO2 sub-microspheres were fabricated for QDSC application. The SEM image of PbS/CdS sensitized SnO2 photoelectrode as shown in Fig. 5a reflects that the mesoporous structure benefits the

Fig. 3. (a) XRD patterns of the calcined SnO2 products obtained under the hydrothermal treatment for certain durations of 20 min (S1), 2 h (S2) and 20 h (S3), and (b) XPS survey spectrum and (c,d) Sn 3d and O 1s high resolution spectra of as-synthesized SnO2 products.

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Fig. 4. (a) Nitrogen sorption isotherms and (b) the corresponding BJH pore size distributions of calcined SnO2 products obtained from the hydrothermal treatment for certain durations of 20 min (S1), 2 h (S2) and 20 h (S3). The isotherms of samples S2 and S3 are shifted by 50 and 100 cm2 g
1, respectively.

Table 1 The surface area, pore volume and pore size of calcined hierarchical SnO2 products. Sample

Surface area (m2 g
1)

Pore volume (cm2 g
1)

Pore size (nm)

S1 S2 S3

48.8 69.2 56.0

0.089 0.228 0.163

3.51 19.13 19.11

penetration of the precursor solutions and loading of QDs. In addition, the solution deposition method, i.e., SILAR, employed in our experiments is also ideal for allowing the infiltration of the reactants into the film pores [14]. Fig. 5b gives the corresponding EDX spectrum of QD-sensitized film, which demonstrates the successful deposition of PbS and CdS. It is observed that, besides the energy-dispersion peaks of Sn, Ti and O, there also exist the characteristic peaks assigned to Pb, Cd and S for as-prepared photoelectrodes. Herein typical peaks of Ti come from the thin coating layer of TiO2 for the TiO2 modified SnO2 photoelectrode, obtained through TiCl4 treatment. Furthermore, the estimated atomic ratios of elements are as follows: 4.5% Pb, 3.4% Cd, 8.1% S, indicting the

nearly stoichiometric formation of PbS and CdS onto the photoelectrodes. The profile drawing of PbS QD-sensitized hierarchical SnO2 hollow sub-microsphere, as shown in Fig. 5c, describes the uniform loading of QDs throughout the porous structure. Herein we only depict the dominating sensitizer, i.e., PbS, in the illustration. The corresponding insets presenting the SEM images of SnO2 submicrospheres before and after QD sensitization further reveals that QD loading lead to a little blurry of SnO2 surfaces. Fig. 6a and b presents TEM and high resolution TEM (HRTEM) images of PbS/CdS sensitized SnO2 sub-microspheres (S2) with the aim of imaging the QDs adsorbed on the surface of SnO2. As shown, the small black dots, i.e., QDs, are homogeneously distributed over the larger SnO2 nanograins, similar to the cases reported for QD sensitized TiO2 [15,25]. The mean particle sizes for the achieved QDs are ca. 4.0 nm. HRTEM further allows the identification of lattice fringes for SnO2, TiO2, PbS and CdS. The thin TiO2 layer coated on SnO2, as illustrated by the white dotted lines, comes from the treatment of photoelectrodes through TiCl4 solutions. The interplanar spacings of ~0.30 nm and ~0.34 nm are correlated with the (200) plane of PbS and (111) plane of CdS, respectively [13,45].

Fig. 5. (a) The SEM image and (b) EDX spectrum of PbS/CdS sensitized SnO2 photoelectrode and (c) schematic of the light scattering effect and photon localization within a photoelectrode consisting of SnO2 sub-microspheres and the profile drawing of mesoporous hierarchical SnO2 hollow sub-microsphere loaded with PbS QDs.

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Fig. 6. (a) TEM and (b) HRTEM images of PbS/CdS sensitized SnO2 sub-microspheres, and (ceh) elemental maps displaying the spatial elemental distribution of Pb, Cd, S, Sn, Ti and O, respectively.

Fig. 6ceh present elemental maps displaying the spatial elemental distribution of Pb, Cd, S, Sn, Ti and O, confirming the homogeneous distribution of PbS and CdS QDs onto SnO2 nanograins. It is well known that the sub-micrometer-sized spheres should serve as the light scatters [13,46]. Therefore, the photoelectrode made of SnO2 sub-microspheres can drastically enhance the photon capturing capability in view of the presence of multiple light reflecting and scattering in-between the hierarchical spheres. As illustrated in Fig. 5c, the diffuse reflection of incident light significantly extends the traveling distance within the photoelectrodes and consequently increases the opportunities for incident photons to be captured by the QD sensitizers. A photon-localization effect may also occur when the diffuse reflection is confined in a closed loop [13]. The light harvesting capability of a QD-sensitized photoelectrode can be evaluated by the UVeviseNIR absorption spectrum, which highlights two significant features: the absorption range and the optical absorbance [14]. Fig. 7a shows the absorption spectra of photoelectrodes constructed with hierarchical SnO2. The pure SnO2 photoelectrode displays a typical absorption edge of SnO2 at ca. 350 nm [27], while the PbS/CdS QD-sensitized photoelectrodes exhibit light absorptions over wide wavelength ranges extending to the NIR region, which are attributed to the absorption of PbS QDs. Moreover, among all the photoelectrodes, the one based on S2 delivers a highest optical absorbance, echoing its largest surface area. That is, a photoelectrode with larger surface area accommodates more QDs, assuming that the surface properties for various SnO2 samples are the same and the pore size allows the access of QDs into the inner pores. Accordingly, the improved QD

loading and enhanced light absorption would increase the photocurrent density. Furthermore, the effective band gap of SnO2 and PbS QDs can be estimated by extrapolating the linear portion of the (Ahy)2 versus hy plots at A ¼ 0 according to Eqn (3), which gives the relationship between the optical bandgap (Eg) for direct inter-band transition and the absorption coefficient (A) near the absorption edge [9,37]:

ðAhyÞ2 ¼ c hy
Eg




(3)

where y is the frequency, h is Planck constant and c is a constant. As performed in Fig. 7b, the band gap of SnO2 is estimated to be 3.6 eV, consistent with the reported values [28,32]. For PbS, the QDs deposited on the photoelectrodes of S2 and S3 displays similar band gaps of ~1.1 eV, which is ~0.2 eV smaller than that of QDs deposited on the photoelectrode of S1 (~1.3 eV). These band gaps of as-prepared PbS QDs are much larger than that of bulk PbS (0.41 eV) due to the quantum size effect [1,15]. We further deduce the valence band (VB) of SnO2 on the basis of the XPS valence spectrum through the linear interpolation of the leading edge of the SnO2 film, as performed in Fig. 7c [31]. It is evident that the obtained VB(SnO2) ¼
3.6e4.5 (eV) ¼
8.1 (eV, Energy vs. vacuum), and therefore CB(SnO2) ¼ VB(SnO2) þ Eg(SnO2) ¼
8.1 þ 3.6 (eV) ¼
4.5 eV, since Eg(SnO2) ¼ 3.6 eV as obtained based on the absorption spectrum. Fig. S3 shows the same operation was carried out for TiO2. The energy band structures of PbS QDs (1.1 eV) relative to SnO2 and TiO2 are depicted in Fig. 7d. The energy levels of PbS QDs are compiled according to references, and the estimated particle sizes of QDs in

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Fig. 7. (a) UVeviseNIR absorption spectra, (b) (Ahy)2 vs hy plots for determining absorption onsets of pure and PbS/CdS QD-sensitized hierarchical SnO2 photoelectrodes, (c) XPS valence spectrum and the linear interpolation of the leading edge of the SnO2 film, and (d) schematic diagram illustrating energy band structures of PbS QDs (1.1 eV) relative to SnO2 and TiO2.

this work are ca. 4.3 nm (Eg ¼ 1.1 eV) and ca. 3.7 nm (Eg ¼ 1.3 eV), agreeing with the results from TEM characterizations [45]. The smaller size for PbS QDs deposited on the photoelectrode S1 might be bound up with the smaller pore size for sample S1 as listed in Table 1, which would confine the further growth of QDs. As demonstrated in the diagram, SnO2 exhibits a larger band gap (3.6 eV) and a more negative CB minimum (CBM ¼
4.5 eV) as expected. Therefore, in contrast to TiO2, it is reasonable to achieve a more efficient extraction of photogenerated from PbS to SnO2 due to a larger energy level difference. A schematic drawing as shown in Fig. 8 demonstrates the device architecture for a PbS sensitized SnO2 solar cell and the charge transfer processes in the device. The device consists of a photoelectrode based on hierarchical SnO2 hollow sub-microspheres, PbS QDs, polysulfide electrolyte and Cu2S counter electrode. Upon the illumination of sunlight, the photons are captured by PbS sensitizers, which generate the electron-hole pairs. The electron-hole pairs are quickly separated, followed by the injection of excited electrons in the VB of QDs into SnO2 and reduction of holes left in the VB of QDs. The injected electrons are further collected by the FTO conducting substrate. Thus photocurrents can be generated in the resultant closed circuit. Besides the electron injection processes (ket), the charge transfer processes also include some electron recombination, as represented by ker, which mainly involves the back reaction of electrons in the CB of QDs and SnO2 with S2
n ions in the polysulfide electrolyte [14]. As a result, the efficient electron injection and suppressed electron recombination should be responsible for a high performance photovoltaic device [2e4,6]. Fig. 9a presents photocurrent density-voltage (J-V) curves of PbS QDSCs based on the photoelectrodes constructed with the hierarchical SnO2 hollow architectures, measured under the illumination of one sun (AM 1.5, 100 mW cm
2). The corresponding short circuit

Fig. 8. Schematic drawing of the device architecture for PbS sensitized SnO2 solar cell and the charge transfer processes in the device. Herein ket and ker refer to electron injection and electron recombination processes, respectively.

current density (Jsc), open circuit voltage (Voc), fill factor (FF) and PCE (h) are summarized in Table 2. For the devices based on pure SnO2 processed under hydrothermal treatment for different durations, the PCE first increases and then decreases with the prolonged reaction time. The most remarkable difference for the photovoltaic parameters of these devices relies on Jsc. It is well known that the amount of QDs loaded on the photoelectrode affects the current density of QDSCs [14,37]. Therefore, the larger value of Jsc for the device based on the photoelectrode S2 should be attributed to the more effective generation of photoexcited electrons, reflected by the stronger absorbance of the corresponding photoelectrode. Moreover, as demonstrated above, the photoelectrode consisting of

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195

Fig. 9. Photocurrent density-voltage (J-V) curves of PbS QDSCs based on the photoelectrodes constructed with the hierarchical SnO2 hollow architectures, measured under (a) the illumination of one sun (AM 1.5, 100 mW cm
2) and (b) dark conditions.

Table 2 Photovoltaic parameters of PbS QDSCs based on the photoelectrodes constructed with the hierarchical SnO2 hollow architectures, measured under the illumination of one sun (AM 1.5, 100 mW cm
2). Device

Jsc (mA cm
2)

Voc (V)

FF

h (%)

S1 S2 S3 S2/TiO2

4.90 6.45 5.72 9.06

0.32 0.35 0.33 0.39

0.32 0.34 0.34 0.38

0.51 0.75 0.65 1.34

numerous tightly packed hierarchical SnO2 hollow submicrospheres should benefit the charge transport through a longer distance with less diffusive hindrance, and the high electron mobility of SnO2 would also contribute to the efficient electron collection [13,45]. As a result, the solar device based on the photoelectrode S2 shows a highest PCE of 0.75%, coupled with Jsc of 6.45 mA cm
2, Voc of 0.35 V and FF of 0.34. Furthermore, it is well known that pure SnO2 serving as the photoelectrode commonly exhibits relatively poor photovoltaic performance owing to the severe interfacial charge recombination. Herein we further coated an ultrathin TiO2 layer on the hierarchical SnO2 through in situ hydrolysis (i.e., TiCl4 treatment) to modify the photoelectrochemical properties of the SnO2 surface, thereby enhance the electron injection efficiency from QDs into oxides and suppress the electron recombination at the SnO2/QDs interface [35]. As shown, with the aid of TiO2 coating layer, the corresponding solar cell exhibits considerably improved Jsc of 9.06 mA cm
2, Voc of 0.39 V and FF of 0.38 and yields a remarkable PCE of 1.34%, around 76% improvement in contrast to the solar device based on pure SnO2. Furthermore, the photocurrent density-voltage (J-V) curves measured under dark as shown in Fig. 9b reveals that, the dark current for the device based on TiO2 coated SnO2 is much smaller than those for the devices based on pure SnO2. The superior passivation effect should be responsible for the significant improvement of Voc for TiO2 modified device in view of the J-V characteristics. Therefore, it is concluded that thin TiO2 layer is effective in passivating the recombination centers inhibiting the charge recombination at the interfaces. In summary, it is well known that the photovoltaic performance of QDSCs is the collective results of several processes: light harvesting, electron injection and charge transport. For a PbS QDSC based on hierarchical SnO2 hollow sub-microspheres in this work, the following merits make it a promising candidate of high performance photovoltaics: 1) the panchromatic light absorption of PbS QDs, high QD loading due to large specific surface area, and strong light scattering arising from sub-micrometer sized spheres ensure adequate light harvesting of device; 2) the relatively low CB

of SnO2 facilitates the favorable electron injection from QDs to electron transporters; 3) the high electron mobility of SnO2 and robust hierarchical hollow architecture consisting of tightly numerous packed nanograins afford efficient charge transport. In addition, as for the big issue involving severe interfacial electron recombination in such a device, it can be significantly suppressed through interface engineering, such as coating a thin TiO2 layer on SnO2 and reducing the defects on the surface of PbS by CdS passivation. Consequently, it is of great prospect to achieve further improvements in photovoltaic performance through optimization and development of fabrication technology. 4. Conclusions This work has demonstrated the synthesis of hierarchical hollow SnO2 sub-microspheres, consisting of numerous packed nanograins, through hydrothermal treatment and the use of them for QDSC application. Such a hierarchical structure not only offers high internal surface area and large pore size for adequate QD loading, but also affords strong light scattering and efficient charge transport capabilities. Moreover, the low CB and the high electron mobility of SnO2 significantly contribute to the efficient electron injection and transport. Eventually, a panchromatic PbS QDSC, based on a multifunctional photoelectrode constructed with hierarchical SnO2 hollow sub-microspheres, delivers a PCE up to 1.34%. The superior performance is ascribed to a combined effect of high light harvesting, favorable electron injection and efficient charge transport. Acknowledgements This work was financially supported by the National Natural Science Foundation (NSF) of China (Nos. 51602088 and 51372061), the Natural Science Foundation of Anhui Province (Nos. 1608085QE92 and 1608085ME101), the Fundamental Research Funds for the Central Universities (No. 2015HGQC0200), the China Postdoctoral Science Foundation (No. 2016M590566), and the Postdoctoral Science Foundation of Jiangsu Province (No. 1601095B). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2017.03.147. References [1] G.H. Carey, A.L. Abdelhady, Z.J. Ning, S.M. Thon, O.M. Bakr, E.H. Sargent,

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