Sb2S3 CoreâShell Nanowire Heterostructure for Extremely

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A ZnO/TiO2/Sb2S3 Core Shell Nanowire Heterostructure for Extremely Thin Absorber Solar Cells Romain Parize, Atanas Katerski, Inga Gromyko, Laetitia Rapenne, Hervé Roussel, Erki Karber, Estelle Appert, Malle Krunks, and Vincent Consonni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00178 • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 26, 2017

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The Journal of Physical Chemistry

A ZnO/TiO2/Sb2S3 Core Shell Nanowire Heterostructure for Extremely Thin Absorber Solar Cells Romain Parize,1 Atanas Katerski,2 Inga Gromyko,2 Laetitia Rapenne,1 Hervé Roussel,1 Erki Kärber,2 Estelle Appert,1 Malle Krunks,2* and Vincent Consonni1* 1

2

Université Grenoble Alpes, CNRS, LMGP, F-38000 Grenoble, France

Laboratory of Thin Film Chemical Technologies, Department of Materials Science, Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia

Corresponding author E-mail: [email protected] and [email protected]

ABSTRACT Extremely thin absorber (ETA) solar cells integrating ZnO nanowires have been receiving increasing interest owing to efficient light trapping phenomena and charge carrier management, but the chemical instability of ZnO in acidic conditions limits its combination with a variety of absorbing semiconducting shells grown by chemical deposition techniques. By covering the ZnO nanowires grown by chemical bath deposition with a protective, passivating, conformal, thin, anatase-TiO2 layer by atomic layer deposition, we show that a uniform Sb2S3 absorbing shell is formed by chemical spray pyrolysis without structural degradation of the ZnO. The Sb2S3 absorbing shell consists of a very thin, conformal layer together with homogeneously distributed small clusters from the bottom to the top of the ZnO/TiO2 core shell nanowire arrays. The resulting ETA solar cells integrating these ZnO/TiO2/Sb2S3 core shell nanowire heterostructures with an Sb2S3 absorbing shell less than 10 nm-thick and P3HT as the hole ACS Paragon Plus Environment

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transporting materials has a photo-conversion efficiency of 2.3% with a promising short-circuit current density of 7.5 mA/cm2 and a high open circuit-voltage of 656 mV as one of the largest reported values in ZnO nanowire-based ETA solar cells. The present findings thus reveal the great potential of Sb2S3 as an absorbing semiconducting shell when coupled with ZnO/TiO2 core shell nanowire heterostructures, opening the way for new strategies to improve the performances of ZnO nanowire-based ETA solar cells fabricated by low-cost, surface scalable, easily implemented chemical deposition techniques.

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INTRODUCTION Over the last decade, increasing efforts have been dedicated to developing nanowire (NWs) solar cells with the axial or radial (i.e., core shell) junction architectures.1,2 The fabrication of NW solar cells with a relatively high photo-conversion efficiency (PCE) has been demonstrated using a number of indirect and direct band gap semiconductors. Putnam et al. reported the fabrication of NW solar cells based on core shell p-Si/n-Si NW heterostructures on p-Si wafer with a PCE of 7.93%.3 Misra et al. used core shell p-Si/a-Si:H NW heterostructures on glass to fabricate NW solar cells with a PCE of 8.13%.4 Axial p-i-n InP NW heterostructures on p-InP substrate and p-i-n GaAs NW heterostructures on n-GaAs substrate were also employed to fabricate NW solar cells with a PCE as high as 13.8 and 15.3%, respectively.5,6 In principles, the core shell junction architecture offers the largest number of assets, including, besides the low amount of materials and low-cost deposition techniques used, efficient light trapping (i.e., optically guided modes, optically radiated modes, Fabry-Perot resonances, …),2,7,8 and efficient charge carrier management (i.e., charge carrier separation over the small distance of the NW radius).2 In the core shell junction architecture, ZnO has emerged as a very promising core to act as a building block into NW solar cells, owing to its abundancy and non-toxicity, its unique ability to be grown as NWs by low-cost, low-temperature and easily implemented deposition techniques including wet chemistry, and its high electron mobility especially when compared to TiO2.9-11 As an n-type wide band gap semiconductor with an energy of 3.3 eV at room temperature,9 ZnO is typically combined with a p-type absorbing semiconducting shell to form type II p-n core shell heterojunctions. The type II band alignment is required to favor charge carrier separation, and occurs when ZnO has the energy minimum of both the conduction and valence bands in the core shell heterojunction, which is typically achieved with most of the direct band gap semiconductors.12 These core shell heterojunctions have received increasing interest for their integration into extremely thin absorber (ETA) solar cells,13,14 which have ACS Paragon Plus Environment

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previously been introduced by using mesoporous (mp) TiO2.15,16 Lévy-Clément et al. reported the first efficient ETA solar cells integrating ZnO/CdSe core shell NW heterostructures covered with CuSCN as a hole transporting material (HTM), leading to the PCE of 2.3%.17 Subsequently, the fabrication of these ETA solar cells has been achieved with a wide variety of absorbing semiconducting shells, such as CdTe,18,19 CdSe,20,21 CdS,22 ZnSe,23 ZnS,24 In2S3,25 TiO2/CuInSe2,26 In2S3/CuInSe2,27 and Cu2O,28 as well as with different HTMs like CuSCN or iodide/triiodide and poly-sulfur electrolytes. The present ETA solar cells have a typical PCE lying in the range of 1.5 to 5%.17,20-28 Following the pioneered works in the 1990’s,29,30 antimony trisulfide (Sb2S3) as a p-type V/VI direct band gap semiconductor has recently emerged as a highly promising, alternative absorber to the standard chemical dyes typically used into dye-sensitized solar cells. It has a high optical absorption coefficient of 7.5 x 104 cm-1 at 550 nm wavelength and a suitable band gap energy of 1.7 eV at room temperature.31-34 Itzhaik et al. reported the fabrication of Sb2S3-sensitized solar cells with a PCE of 3.37% by its combination with nanoporous (np) TiO2 and CuSCN as the HTM, showing its high potential for fabricating all solid semiconductor-sensitized solar cells.35 Later on, mp/np TiO2/Sb2S3sensitized solar cells incorporating a number of organic HTMs including Spiro-MeOTAD [2,22′,7,77′tetrkis (N,N-di-p-methoxyphenylamine)-9,99′-spirobi fluorine],36 P3HT [poly(3-hexylthiophène)],37 and PCPDTBT

[Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta

[2,1-b;3,4-b′]dithiophene)-alt-4,7(2,1,3-

benzothiadiazole)]38 have been fabricated, leading to the PCE of 6.3 % by combining both conjugated polymers (i.e., P3HT and PCPDTBT).39 Following a sulfurization process using a thioacetamide treatment to remove the Sb2O3 surface layer, the PCE of mp TiO2/Sb2S3-sensitized solar cells was even improved to 7.5%.40 Additionally, TiO2 nanostructures including NWs and nanofibers have also been used as an alternative to mp TiO2, but the PCE is typically lower and in the range of 2 to 5%.41-43 Alternatively to TiO2, ZnO is a promising candidate to be integrated into Sb2S3-sensitized solar cells, but its use represents a major difficulty owing to its chemical instability. The growth of Sb2S3 thin films is typically achieved by chemical deposition techniques including the widely-used chemical bath


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deposition (CBD),44-49 successive ionic layer adsorption and reaction,50,51 electrodeposition,52 and spray pyrolysis.53-58 All of these techniques offer a convenient low-cost and low-temperature way to deposit Sb2S3 thin films followed by a further heat treatment around 300°C under nitrogen atmosphere for their crystallization.59 Nevertheless, the (very) acidic conditions (i.e., low pH of the solutions used containing Sb3+ ions or corrosive by-products) when using solution deposition techniques are typically incompatible with their direct deposition onto bare ZnO.60 Accordingly, the only integration of ZnO into Sb2S3-sensitized solar cells was performed by depositing Sb2S3 using vacuum evaporation onto bare ZnO flat and nanostructured thin films, leading to a PCE of 2.4% and 2.9%, respectively.61,62 However, vacuum evaporation is not recommended for depositing Sb2S3 as an absorbing shell on top of ZnO NW arrays to fabricate ETA solar cells, because the physical vapor deposition techniques are typically unable to form a conformal and uniform shell on nanostructured surfaces with a high aspect ratio. Han et al. recently reported the conversion of the ZnO NW surface layers initially into a ZnS shell by immersion into thioacetamide followed by transformation into a Sb2S3 shell by a metal cation exchange process; still, the resulting ETA solar cells with P3HT as the HTM have a PCE of 1.32% with a low open-circuit voltage (VOC) of 440 mV, which is likely due to high surface recombinations at the different uncontrolled interfaces.63 In this article, we develop ZnO/TiO2/Sb2S3 core shell NW heterostructures grown by low-cost chemical deposition techniques. The protective, passivating, conformal, thin anatase-TiO2 layer deposited by atomic layer deposition (ALD) on ZnO NW arrays is favorable for the subsequent growth of the Sb2S3 absorbing shell by chemical spray pyrolysis (CSP). The present CSP is further found as a very relevant technique for depositing the uniform Sb2S3 absorbing shell with high crystalline and structural quality on top of ZnO/TiO2 core shell NW heterostructures with a high aspect ratio, representing an alternative to the standard CBD process. The resulting ETA solar cells integrating these ZnO/TiO2/Sb2S3 core shell NW heterostructures with P3HT as the HTM show their great potential and open the way for new strategies to improve the performances of ZnO NW-based ETA solar cells.


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METHODS Chemical deposition techniques and solar cell fabrication The ZnO/TiO2/Sb2S3 core shell NW heterostructures were grown by low-cost, surface scalable, and easily implemented chemical deposition techniques on FTO/glass substrate. Polycrystalline fluorinedoped SnO2 (FTO) thin films were deposited by ultrasonic spray pyrolysis on cleaned Corning C1737 borosilicate glass substrates heated at 420°C using a sprayed precursor solution containing 0.16 M of tin (IV) chloride (SnCl4·5H2O) and 0.04 M of ammonium fluoride (NH4F) mixed in pure methanol. 64 A flow rate of 1.25 mL/min with a total deposition time of 13min were used to grow 250 nm-thick FTO thin films. The growth texture of the FTO thin films was controlled, with the (100) planes parallel to the surface to minimize their roughness and to favor the structural ordering of the polycrystalline ZnO seed layer and NW arrays on top of them. 30 nm-thick ZnO seed layers were then deposited on these FTO thin films by sol-gel process using dip coating.65 The equimolar precursor solution was prepared by dissolving 0.375 M of zinc acetate (Zn(CH3COO)2·2H2O) and 0.375 M of monoethanolamine (MEA) in pure ethanol. It was stirred on a hot plate kept at 60°C for a couple of hours to get a clear solution and then at room temperature for 24h to complete zinc acetate dilution. The samples were slowly dipped into the solution and gently pulled out under controlled atmosphere (i.e., hygrometry < 15%) at a withdrawal speed of 3.3 mm/s. They were annealed on a hot plate kept at 300°C for residual organic compound evaporation and then on another plate kept at 500 °C for 1h for ZnO seed layer crystallization. ZnO nanowire arrays were grown on these ZnO seed layers by chemical bath deposition in an aqueous solution containing 0.03 M of zinc nitrate (Zn(NO3)2.6H2O) and 0.03 M of hexamethylenetetramine (HMTA).66 The samples were placed face down in a sealed beaker inside a regular oven kept at 90°C for 3h. The ZnO nanowire arrays were covered with an anatase-TiO2 layer by atomic layer deposition using a F200 Fiji reactor from Cambridge Nanotech. Sequential pulses of tetrakis dimethylamino titanium (TDMAT) and H2O of 0.1 s followed by a purge of 10 s after each


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pulse were used to alternately inject titanium and oxygen chemical precursors, respectively. The TDMAT chemical precursor was beforehand heated at 75°C. The equipment chamber was maintained at 200°C during the whole deposition and its pressure was set to 11 mTorr and increased up to 200 mTorr when each pulse was injected. A small number of 150 cycles was employed to grow 8 nm-thick anataseTiO2 layers. The samples were annealed in air in an oven kept at 300°C for 3h for crystallizing the amorphous TiO2 layer into the anatase-TiO2 phase. The extremely thin Sb2S3 absorbing shell was prepared using a solution containing SbCl3 (with Sb3+ concentration of 15 mmol/L) and SC(NH2)2 at a molar ratio of 1:3 in methanol. The SbCl3 was from Sigma-Aldrich (≥99.0%, p.a.) and SC(NH2)2 from Merck (≥99.0%, p.a.). The precursor solution was prepared inside a glove box with controlled humidity (less than 14 ppm) and then ultrasonically nebulized at 1.5 MHz by using compressed air as the carrier gas at a flow rate of 5L min−1. The deposition temperature was fixed to 220°C and six deposition cycles were used. After deposition, the samples were annealed at 300 °C for 5 min under flowing nitrogen (99.999 %) atmosphere. To deposit the P3HT as the hole transporting material, the samples were immersed into a room-temperature solution of 2 wt.% regioregular poly(3-hexylthiophene- 2,5-diyl) from Sigma-Aldrich in chlorobenzene, followed by its drying at 50 °C for 10 min in air and further drying in vacuum (4·10-6 Torr) at 170 °C for 5 min. The gold contact was deposited onto the P3HT by vacuum thermal evaporation of metallic gold (99.99 %) through a metal mask with 1.7 mm2 area for each hole. Characterization techniques Field-emission gun scanning electron microscopy (FESEM) images were collected with a ZEISS HR FESEM Ultra 55 microscope operating at an accelerating voltage of 4 kV. The transmission electron microscopy (TEM) specimens were prepared by scratching the surface of ZnO/TiO2/Sb2S3 core shell nanowire heterostructures using a diamond tip and put on a copper grid. TEM and high-resolution TEM (HRTEM) images were collected with a JEOL 2010 LaB6 microscope operating at 200 kV with a 0.19 nm point-to-point resolution. Energy-dispersive x-ray spectroscopy (EDS) mappings were recorded by


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scanning TEM (STEM) with a JEOL 2100F FEG microscope operating at 200 kV with a 0.2 nm resolution in the scanning mode. The novel JEOL SDD Centurio detector with a large solid angle of up to 0.98 steradians was employed for the EDS experiments. Out-of-plane and in-plane x-ray diffraction (XRD) patterns were recorded with a RIGAKU Smartlab diffractometer equipped with a 9kW rotating anode (using a Cu source) operating at 45kV and 200mA. Both Bragg-Brentano and in-plane configuration were employed with the Cu radiation (λ was the average of Kα(Cu) and equals 0.1544 nm). The θ-2θ XRD measurements following the standard Bragg-Brentano configuration were performed in the range of 10 to 60° (in 2-Theta scale) with a step of 0.008° and 0.1s by step. The inplane configuration was run on a 5 circle-goniometer that was specifically designed for in-plane acquisitions. The x-ray beam was about 5 mm parallel to the sample surface and 0.05 mm perpendicular to the sample surface. As the acquisition proceeded, the samples were kept horizontal and stuck with an ethanol droplet on a double tilt stage. Omega incidence was 0.15°. 2Theta-Chi/Phi XRD measurements were performed in the range of 10 to 60° with a step of 0.04° and a speed of 1.0°/min. The 00-041-1445, 00-036-1451, 00-021-1272, 00-042-1393, and 00-005-0534 files of the International Center for Diffraction Data (ICDD) were used for cassiterite SnO2, wurtzite ZnO, anatase TiO2, stibnite Sb2S3, and senarmontite Sb2O3, respectively. Raman scattering measurements were performed using a Jobin Yvon/Horiba Labram spectrometer equipped with a liquid nitrogen cooled CCD detector. The 632.8 nm excitation line of a He-Ne laser with a power lower than 114 µW at the sample surface was used. The light was focused to a spot size smaller than 1 µm2 using a 50 times long working distance objective. Raman spectra were calibrated using a silicon reference sample at room temperature with a theoretical position of 520.7 cm-1. The optical parameters, such as total transmittance (TT) and total reflectance (TR), were measured in the wavelength range of 300-900 nm with a Jasco V-670 spectrophotometer equipped with an integrating sphere. The absorption A was calculated according to the following equation: A = 100%-(TT+TR). Current-voltage (I-V) measurements from the solar cells in the dark and under the quartz-tungsten-halogen lamp illumination with an intensity of 100 mW/cm2 were used to obtain the main performances of the solar cells: the voltage at open circuit conditions (Voc), current ACS Paragon Plus Environment

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density at short-circuit conditions (JSC), fill factor (FF), and photo-conversion efficiency (η). The active area of the solar cell is defined by the back contact area of 1.7 mm2.

RESULTS AND DISCUSSION Principles and geometry of the ETA solar cell The radial architecture together with the schematic energy band diagram of the entire ETA solar cells integrating ZnO/TiO2/Sb2S3 core shell NW heterostructures is presented in Figure 1. The present ETA solar cell is formed by the stack of the transparent front electrode (i.e., fluorine-doped SnO2 (FTO) thin film) for collecting the electrons, the n-type ZnO (i.e., seed layer and NWs) as the electron transporting material (ETM), the n-type anatase-TiO2 layer, the p-type Sb2S3 layer as the absorbing shell, P3HT as the HTM, as well as the back contact (i.e., gold) for collecting the holes. The anatase-TiO2 layer plays the important roles of a passivating shell to reduce surface recombinations on the ZnO NWs as well as of a protective shell during the CSP of the Sb2S3 absorbing shell and to avoid any short-circuiting that may proceed through the ZnO seed layer. All these materials are separated each other by type II heterojunctions, which are required for charge carrier separation at the different interfaces. The photogenerated electron–hole pairs in the Sb2S3 absorbing shell under illumination are thus separated and collected by the transparent front electrode and back contact through the ETM and HTM, respectively.

Figure 1. (a) Radial architecture and (b) schematic energy band diagram of the ETA solar cell integrating ZnO/TiO2/Sb2S3 core shell NW heterostructures with P3HT as the HTM.


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Structural and optical properties The structural properties of the resulting ZnO/TiO2/Sb2S3 core shell NW heterostructures were investigated by FESEM and TEM imaging, EDS-STEM mapping, and XRD in Bragg-Brentano and inplane configurations. The optical properties of the ZnO/TiO2/Sb2S3 core shell NW heterostructures were studied by Raman scattering and UV-visible-NIR absorption measurements.

Figure 2. Top-view and cross-sectional FESEM images of (a-e) ZnO NW arrays, (b-f) ZnO/TiO2 core shell NW heterostructures, (c-g) as-grown ZnO/TiO2/Sb2S3 core shell NW heterostructures, and (d-h) ZnO/TiO2/Sb2S3 core shell NW heterostructures annealed at 300°C for 5 min under flowing nitrogen atmosphere, respectively. The structural morphology of the ZnO NWs, ZnO/TiO2 core shell NW heterostructures, and ZnO/TiO2/Sb2S3 core shell NW heterostructures are presented on the microsocpic scale in Figure 2 by FESEM imaging. The ZnO seed layers are composed of grains that are strongly oriented along the polar ±[0001] (i.e., c) axis.67 The 60% c-axis oriented grains with a typical density of 130 NPs/µm-2 act as the preferential sites for the homoepitaxial, heterogeneous nucleation of ZnO NWs.68 The ZnO NWs have a typical mean diameter and length of 80 and 900 nm, respectively, and are vertically aligned owing to the low mosaicity (i.e., the angle of the c-plane with the surface) of the c-axis oriented grains. Their mean density is approximately 62 NW/µm² while their mean tilt angle is smaller than 10° with respect to the normal to the surface. The protective, passivating anatase-TiO2 layer covers the ZnO NW arrays from their bottom to their top, as shown by their slight enlargement indicated by the increase in their diameter. Sb2S3 absorbing shell is also formed from the bottom to the top of the ZnO/TiO2 core shell ACS Paragon Plus Environment

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NW arrays with a remarkable penetration as indicated by the granular character on their top and on their vertical sidewalls as well as at their base.

Figure 3. (a) HAADF-STEM image of ZnO/TiO2/Sb2S3 core shell NW heterostructures annealed at 300°C for 5 min under flowing nitrogen atmosphere. Corresponding EDS-STEM elemental mapping of the (b) zinc (red), (c) titanium (blue), (d) antimony (green), and (e) sulfur (orange) elements, respectively. (f) Corresponding EDS-STEM elemental mapping superimposing the zinc, titanium, and antimony element signals. To investigate their structural morphology on the local scale, high-angle annular dark-field (HAADF)-STEM image and EDS-STEM mapping as well as high-resolution TEM (HRTEM) image are presented in Figure 3 and 4, respectively. The protective, passivating anatase-TiO2 layer is found to be 8 nm-thick and highly conformal from the bottom to the top of the ZnO NW arrays, as revealed in Figure 3c and 4. The Sb2S3 absorbing shell is composed of a very thin layer of few nanometers, which is highly uniform and conformal as seen in Figure 3d, together with homogeneously distributed small clusters with a typical size ranging from 5 to 10 nm as shown in Figure 3a and 4. This implies that the crystallization process of the Sb2S3 absorbing shell following the short annealing at 300°C for 5 min has not involved a significant mass transport of antimony and sulfur atoms through their surface diffusion. The superimposed EDS-STEM elemental map in Figure 3f together with the FESEM image in Figure 2b provide compelling evidence that the protective, passivating anatase-TiO2 layer and Sb2S3 absorbing ACS Paragon Plus Environment

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shell are highly uniform and conformal from the bottom to the top of the ZnO NW arrays, indicating that CSP as a more easily implemented deposition technique with a lower cost is thus worth developing to close the gap and achieve the high conformity and good penetration on high aspect ratio nanoscale structures, which is more typically associated with deposition techniques such as ALD.

Figure 4. HRTEM image of ZnO/TiO2/Sb2S3 core shell NW heterostructures annealed at 300°C for 5 min under flowing nitrogen atmosphere with a special focus on a small cluster in the Sb2S3 absorbing shell.


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Figure 5. XRD patterns in both (a) Bragg-Brentano and (b) in-plane configurations of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. The crystallinity and phases of as-grown and annealed ZnO/TiO2/Sb2S3 core shell NW heterostructures are depicted in Figure 5a and 5b by XRD patterns in Bragg-Brentano and in-plane configurations, respectively. The diffracting planes are parallel/perpendicular to the surface of the FTO/glass substrate in Bragg-Brentano/in-plane configurations, respectively, which is a strong asset to investigate the structural properties like growth texture for instance in high aspect ratio nanoscale structures. The tetragonal cassiterite FTO (00-041-1445 ICDD file), hexagonal wurtzite ZnO (00-0361451 ICDD file), and anatase-TiO2 (00-021-1272 ICDD file) phases are clearly identified in the XRD patterns in both configurations for as-grown and annealed ZnO/TiO2/Sb2S3 core shell NW heterostructures. The diffraction peaks of the FTO thin film are much less pronounced in the in-plane configuration because the x-ray beam was mainly focused on the top of ZnO/TiO2/Sb2S3 core shell NW heterostructures. The (0002) diffraction peak of ZnO at 34.4° predominates in the Bragg-Brentano configuration in Figure 5a, whereas its perpendicular (101ത0) diffraction peak at 31.7° is dominant in the in-plane configuration in Figure 5b, because the ZnO NWs are oriented along the polar c-axis and have non-polar m-plane vertical sidewalls. The as-grown Sb2S3 absorbing shell is mostly amorphous, but a small amount may have undergone a start of crystallization as indicated by the (101) diffraction peak at 24.4° in Figure 5a. The annealed Sb2S3 absorbing shell crystallizes into the orthorhombic stibnite (00-042-1393 ICDD file) phase belonging to the Pbnm space group, where one half of the antimony atoms are coordinated to three sulfur atoms while the other half is coordinated to five sulfur atoms. Owing to the presence of 20 atoms per Sb2S3 unit cell and to its polycrystalline nature, a wide number of diffraction peaks in both configurations occur in Figure 5a and 5b, indicating that the annealed Sb2S3 absorbing shell is crystallized on top and on the vertical sidewalls of ZnO/TiO2 core shell NWs. The diffraction peak broadening in the in-plane configuration is mainly due to the different optics and detection used. The annealed Sb2S3 absorbing shell is thus polycrystalline and slightly


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oriented according to the texture analysis (reporting (hkl) texture coefficients)63 achieved among the 26 and 15 Sb2S3 diffraction peaks in the Bragg-Brentano and in-plane configurations in Figure 5, respectively. The (110), (120), (220), and (320) planes parallel to the surface, respectively corresponding to the diffraction peaks at 11, 17.5, 22.2, and 28.5°, are predominant in the BraggBrentano configuration in Figure 5a, but do not directly indicate the growth texture of the Sb2S3 absorbing shell that is mostly located on the vertical sidewalls of ZnO/TiO2 core shell NWs and not on their top. In contrast, the (310) planes perpendicular to the surface as indicated by the diffraction peak at about 25.2° are dominant in the in-plane configuration in Figure 5b. This shows that the growth texture of the Sb2S3 absorbing shell is, to a certain extent, achieved with the (310) planes parallel to the nucleation surface, which is shown on the vertical sidewalls of ZnO/TiO2 core shell NWs and expected on their top. Also, the diffraction peaks corresponding to the cubic senarmontite Sb2O3 (00-005-0534 ICDD file) phase clearly occur on the XRD patterns of annealed ZnO/TiO2/Sb2S3 core shell NW heterostructures in both configurations. The diffraction peaks related to the Sb2O3 phase are however much less intense than those related to the Sb2S3 phase, showing that the Sb2O3 phase is minor.

Figure 6. Raman scattering spectra of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. Raman scattering spectra of as-grown and annealed ZnO/TiO2/Sb2S3 core shell NW heterostructures are shown in Figure 6. The as-grown ZnO/TiO2/Sb2S3 core shell NW heterostructures only exhibit the ACS Paragon Plus Environment

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Raman line at 143 cm-1 attributed to the low-frequency Eg phonon mode of the anatase-TiO2 layer.69 The broad band centered at 304 cm-1 can be assigned to the amorphous Sb2S3 absorbing shell.58 After annealing, a large number of Raman lines related to the orthorhombic stibnite phase of the Sb2S3 absorbing shell arises owing to the presence of 20 atoms per Sb2S3 unit cell resulting in 30 active Raman modes (ΓRaman = 10Ag + 5B1g + 10 B2g + 5 B3g):70 the Raman lines pointing at 61 (B1g/B3g), 71 (Ag), 100 (Ag), 128 (B2g), 156 (Ag), 190 (Ag), 207 (B1g/B3g), 237 (B1g/B3g), 282 (Ag), 300 (Ag), and 312 (Ag) cm-1 are detected in Figure 6. Owing to the CS site symmetry of the antimony and sulfur atoms, their motion exclusively occurs within the xy plane for the A1g and B2g phonon modes, and along the y axis for the B1g and B3g phonon modes. The Sb2O3 minor phase is also detected through the Raman lines pointing at 88 and 114 cm-1, most likely corresponding to the B2 and E phonon modes71 and in agreement with the XRD patterns in Figure 5. The role of the Sb2O3 minor phase is still open and has not been fully clarified in the Sb2S3 sensitized-solar cells. Its location on the surface of the Sb2S3 absorbing layer seems to be beneficial and is delibaretly induced for surface passivation.35,42,72,73 In contrast, when located in the bulk of the Sb2S3 absorbing layer, its effects are clearly detrimental as the effects of any foreign phase usually are. In the present case, the Sb2S3 absorbing shell is very thin, such that its location on the surface and/or in the bulk is not straightforward to be determined and hence surface/bulk effects are typically not distinguished.


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Figure 7. (a) UV-visible absorption measurements of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. (b) Corresponding Tauc plot (αhν)² vs hν. Additionally, the sample colour was switched from orange to dark brown after annealing, leading to a much higher absorption of incident photons in the wavelength range corresponding to the solar spectrum, as shown in Figure 7a. The optical bandgap energy of the Sb2S3 absorbing shell was accordingly reduced from 2.2 eV down to 1.79 eV as calculated from the spectra of the as-grown and annealed shells, respectively, following the Tauc plot in Figure 7b: (αhν) = A(hν – Eg)n where α is the absorption coefficient, A is a numerical constant, Eg is the optical bandgap energy, hν is the incident photon energy, and n equals 1/2 for direct optical transitions given that Sb2S3 is a direct band gap semiconductor. Both optical band gap energies are in good agreement with the theoretical and experimental values reported for amorphous and crystalline Sb2S3.74 Photovoltaic performances The annealed ZnO/TiO2/Sb2S3 core shell NW heterostructures were filled by P3HT as the HTM by dipping into the P3HT solution and contacted on the back side with gold by vacuum evaporation to fabricate the corresponding ETA solar cells over a surface area of 0.017 cm2. As a comparison, three additional types of solar cells integrating ZnO NWs, annealed ZnO/Sb2S3 core shell NW heterostructures, and ZnO/TiO2 core shell NW heterostructures were fabricated following the same ACS Paragon Plus Environment

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growth conditions and filled by P3HT as the HTM. The J-V curves collected under dark and AM 1.5G illuminated conditions are presented in Figure 8 and the PV performances of all the solar cells are summarized in Table I. The PV properties of the solar cells integrating ZnO NW heterostructures without the anatase-TiO2 layer are poor, even when the Sb2S3 absorbing shell is deposited. This clearly indicates that the anatase-TiO2 layer as a protective and passivating shell is beneficial in the present core shell NW heterostructures. The solar cells integrating ZnO/TiO2 core shell NW heterostructures without the Sb2S3 absorbing shell have a fairly high VOC of 530 mV, but a low JSC of 0.525 mA/cm2 since P3HT does not have significant absorption capabilities over the solar spectrum. In contrast, the PV properties of the ETA solar cells integrating ZnO/TiO2/Sb2S3 core shell NW heterostructures are much better. The deduced JSC of 7.5 mA/cm² of the ETA solar cell is relatively high, considering that the Sb2S3 absorbing shell is less than 10 nm-thick (and that P3HT does not have a significant contribution). The VOC of 656 mV represents one of the largest VOC reported in the ETA solar cells integrating ZnO core shell NW heterostructures.17,20-28 It is also comparable to the typical VOC reported in the Sb2S3 sensitized-solar cells integrating mp-TiO2.35-43 The calculated fill factor (FF) is found to be approximately 47% while the series (Rs) and shunt (Rsh) resistances amount to 1.6 Ω.cm² and 795 Ω.cm², respectively. The relatively low FF is likely the result of recombinations in the ETA solar cells. The resulting PCE of 2.3 % is highly promising and shows the great potential of ZnO/TiO2/Sb2S3 core shell NW heterostructures grown by chemical deposition techniques including CBD, ALD, and CSP.


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Figure 8. J-V measurements under dark and AM 1.5G illuminated conditions of the ETA solar cells integrating (a) ZnO NWs and ZnO/Sb2S3 core shell NW heterostructures as well as (b) ZnO/TiO2 and ZnO/TiO2/Sb2S3 core shell NW heterostructures. In all these ETA solar cells, P3HT is used as the HTM.

Heterostructures

Jsc (mA/cm²)

ZnO/P3HT

Voc (mV)

FF (%)

η (%)

No photovoltaic effect

ZnO/Sb2S3/P3HT

0.4

13

25

1.3x10-3

ZnO/TiO2/P3HT

0.525

530

41

0.12

ZnO/TiO2/Sb2S3/P3HT

7.5

656

47

2.31

Table I: PV performances of the four types of ETA solar cells with different heterostructure configurations

CONCLUSIONS In summary, we have demonstrated that the p-type direct band gap semiconductor Sb2S3 can be used as an absorbing shell, on top of ZnO NW arrays covered by a protective, passivating, conformal, thin anatase-TiO2 layer, using low-cost, surface scalable, easily implemented chemical deposition techniques for the fabrication of the whole stack of semiconductors. The resulting Sb2S3 absorbing shell is composed of a very thin, conformal layer together with homogeneously distributed small clusters from the bottom to the top of the ZnO/TiO2 core shell NW arrays. The resulting ETA solar cells integrating these ZnO/TiO2/Sb2S3 core shell NW heterostructures with P3HT as the HTM have a promising PCE of 2.3 % with a high JSC of 7.5 mA/cm2 when considering that the Sb2S3 absorbing shell is less than 10 nmACS Paragon Plus Environment

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thick and a high VOC of 656 mV as one of the largest reported VOC in these ZnO NW-based ETA solar cells. These findings show the great potential of ZnO/TiO2/Sb2S3 core shell NW heterostructures, opening the way for new strategies to improve the performances of ZnO NW-based ETA solar cells.

ACKNOWLEDGEMENTS The authors acknowledge the Estonian Research Council as well as MENESR and MAEDI French Ministries for their financial support through the Parrot program (n°33787YJ). This work was also supported by the LabEx Cemam under the contract ANR-10-LABX-44-01, by the Estonian Research Council (IUT19-4) and by the European Regional Development Fund (TK141 “Advanced materials and high-technology devices for energy recuperation systems”). R.P. held a doctoral fellowship from the LabEx Cemam. The authors further thank Dr. Odette Chaix-Pluchery and Thomas Cossuet for their experimental assistance in Raman scattering measurements and growth experiments, respectively, as well as Dr. Valdek Mikli for his experimental assistance in SEM experiments.

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(73) Reeja-Jayan, B.; Manthiram, A. Effects of Bifunctional Metal Sulfide Interlayers on Photovoltaic Properties of Organic-Inorganic Hybrid Solar Cells. RSC Adv. 2013, 3, 5412-5421. (74) Gonzalez-Lua, R.; Escorcia-Garcia, J.; Perez-Martinez, D.; Nair, M. T. S.; Campos, J.; Nair, P. K. Stable Performance of Chemically Deposited Antimony Sulfide-Lead Sulfide Thin Film Solar Cells under Concentrated Sunlight. ECS J. Solid State Sci.Tech. 2015, 4, Q9-Q16.


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Table of contents graphic


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Table of contents graphic 1274x762mm (96 x 96 DPI)


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Figure 1. (a) Radial architecture and (b) schematic energy band diagram of the ETA solar cell integrating ZnO/TiO2/Sb2S3 core shell NW heterostructures with P3HT as the HTM. 478x145mm (150 x 150 DPI)



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Top-view and cross-sectional FESEM images of (a-e) ZnO NW arrays, (b-f) ZnO/TiO2 core shell NW heterostructures, (c-g) as-grown ZnO/TiO2/Sb2S3 core shell NW heterostructures, and (d-h) ZnO/TiO2/Sb2S3 core shell NW heterostructures annealed at 300°C for 5 min under flowing nitrogen atmosphere, respectively. 48x14mm (300 x 300 DPI)


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(a) HAADF-STEM image of ZnO/TiO2/Sb2S3 core shell NW heterostructures annealed at 300°C for 5 min under flowing nitrogen atmosphere. Corresponding EDS-STEM elemental mapping of the (b) zinc (red), (c) titanium (blue), (d) antimony (green), and (e) sulfur (orange) elements, respectively. (f) Corresponding EDS-STEM elemental mapping superimposing the zinc, titanium, and antimony element signals. 85x58mm (244 x 244 DPI)



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Figure 4. HRTEM image of ZnO/TiO2/Sb2S3 core shell NW heterostructures annealed at 300°C for 5 min under flowing nitrogen atmosphere with a special focus on a small cluster in the Sb2S3 absorbing shell. 92x106mm (300 x 300 DPI)


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Figure 5. XRD patterns in both (a) Bragg-Brentano and (b) in-plane configurations of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. 65x49mm (600 x 600 DPI)



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Figure 5. XRD patterns in both (a) Bragg-Brentano and (b) in-plane configurations of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. 65x49mm (600 x 600 DPI)


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Figure 6. Raman scattering spectra of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. 65x49mm (600 x 600 DPI)



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(a) UV-visible absorption measurements of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. (b) Corresponding Tauc plot (αhν)² vs hν. 64x48mm (600 x 600 DPI)


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(a) UV-visible absorption measurements of as-grown (black solid line) and annealed (red solid line) ZnO/TiO2/Sb2S3 core shell NW heterostructures. (b) Corresponding Tauc plot (αhν)² vs hν. 65x49mm (600 x 600 DPI)



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Sb2S3 CoreâShell Nanowire Heterostructure for Extremely

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