Improvement of antimony sulfide photo absorber performance by interface modification in Sb2S3–ZnO hybrid nanostructures

Author’s Accepted Manuscript Improvement of Antimony Sulfide photo absorber performance by interface modification in Sb2S3ZnO hybrid nanostructures Asad Ali, Syed Khurshid Hasanain, Tahir Ali, Muhammad Sultan www.elsevier.com/locate/physe

PII: DOI: Reference:

S1386-9477(16)30949-3 http://dx.doi.org/10.1016/j.physe.2016.11.002 PHYSE12640

To appear in: Physica E: Low-dimensional Systems and Nanostructures Received date: 26 August 2016 Revised date: 27 October 2016 Accepted date: 3 November 2016 Cite this article as: Asad Ali, Syed Khurshid Hasanain, Tahir Ali and Muhammad Sultan, Improvement of Antimony Sulfide photo absorber performance by interface modification in Sb2S3-ZnO hybrid nanostructures, Physica E: Lowdimensional Systems and Nanostructures, http://dx.doi.org/10.1016/j.physe.2016.11.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improvement of Antimony Sulfide photo absorber performance by interface modification in Sb2S3-ZnO hybrid nanostructures Asad Ali 1,2,3, Syed Khurshid Hasanain1,2, Tahir Ali 4, Muhammad Sultan1 1 Nanosciences and Technology Department, National Centre for Physics, Quaid-i-Azam University Campus, Islamabad, Pakistan 2 Department of Physics, Quaid-i-Azam University, Islamabad, Pakistan. 3. Preston Institute of Nanoscience and Technology, Preston University, Islamabad, Pakistan. 4 Physics Division, PINSTECH, Nilore, Islamabad, Pakistan Abstract Metal-oxide chalcogenide nanostructures as part of hybrid systems are very important for photovoltaic and optoelectronic applications. It is however known that the various interfaces within the hybrid structures play a crucial role in limiting the efficiency of these devices. Here we report on the improvement of Sb2S3 structure through modification of interface between Znoxide nanostructures and chalcogenides. ZnO nanorods were grown on fluorine doped tin oxide (FTO) substrate by chemical bath deposition (CBD) method. X-ray diffraction (XRD) and SEM analysis confirmed the single phase wurtzite structure and c-axis orientation of the ZnO nanorod arrays. Antimony tri-sulfide (Sb2S3) was deposited on ZnO nanords by CBD and subsequently annealed at 300 0C in argon environment for 30 min. XRD and the XPS analysis of ZnO-Sb2S3 system showed the dominant presence of Sb2O3 rather than Sb2S3. Since oxidation of Sb2S3 is understood to proceed mainly from the ZnO-Sb2S3 interface, a ZnS interlayer was introduced between ZnO nanorods and Sb2S3 by chemical route. The subsequent structural and optical properties of the ZnO-ZnS-Sb2S3 system are analyzed in detail. The introduction of sulfide interlayer prevents the oxidation of Sb2S3 which is evident from reduced oxide phase in Sb2S3. Significant improvement in the structural and optical properties of Sb2S3 are reported as compared to the parent ZnO-Sb2S3 system. This gain in the optical properties of hybrid ZnOZnS-Sb2S3 nanostructures is explained as being related to successful prevention of Sb2O3 formation at the Sb-ZnO interface and stabilization of the desired Sb2S3.

1. Introduction Sb2S3 is one of the most promising candidates for the fabrication of extremely thin absorber solar cell [1-6]. Its high absorbance coefficient(∝ = 7.3×10-4 cm-1 at 600 nm), optimum band gap (≈ 1.7 eV ) [7] and efficient charge extraction make it a suitable candidate for a photovoltaic material. By sandwiching Sb2S3 absorber between electron and hole transport materials a hybrid solar cell can be developed. Thin film solar cells developed using Sb2S3 are still far from commercial application. One of the possible ways to improve the efficiency of HSCs is to

Corresponding Author: [email protected]

incorporate the nanowire array (NA) structure in the device architecture [8-10]. The introduction of NA has several advantages for solar energy conversion. Firstly, the large surface area of NA results in an enormous increase in the charge carrier generation and collection regions in the photovoltaic (PV) devices. It has been reported in the literature that by incorporating three dimensional n-type CdS nanopillers into the polycrystalline thin films of p-type CdTe results in the enhanced carrier collection efficiency [9]. Secondly the charge carrier transport properties improve [8], and finally the light harvesting ability of the device improves due to the light trapping effects in the NA structures [11]. Accordingly it is of obvious interest to use NA geometry in Sb2S3 based solar cell to improve the device efficiency. Moreover engineering hybrid structures at nanometer scale is very important for emerging areas such as Nanoarchitectonics[12-14] Recently Kemat et al showed that sulfide radical species in Sb2S3 result in providing hole trap sites. These trap sites caused slow extraction of holes as compared to electrons which ultimately result in high recombination processes at the TiO2/Sb2S3 interface [15]. Deep level transient spectroscopy (DLTS) showed trap site for charge carrier recombination at the TiO2/Sb2S3 interface in Sb2S3 [16]. These trap sites were located around 1.3 eV below the conduction band. Therefore to improve the device performance it is important to remove these traps which may act as recombination centers. There are three factors which contribute to the defects in Sb2S3 sensitized solar cells. Firstly, during the crystallization process of Sb2S3 film sulfur vacancies may form as a result of sulfur loss [17-18]. Secondly, oxidation of Sb2S3 film quickly occurs if we expose it to air and forms Sb2O3 and sulphates [19-20]. Thirdly, the interfacial diffusion of oxygen leads to oxidation of the Sb as well when the Sb2S3 is in direct contact with oxide surfaces. This effect predominantly contributes in nanostructures where large surface area containing S, is exposed to oxygen. For solar cell application one of the most popular methods to deposit Sb2S3 thin film is by chemical bath deposition (CBD). This method of deposition inevitably includes some impurities like SbOCl, Sb2O3, Sb2(SO3)3 [21-23]. In this work on the synthesis of Sb2S3:ZnO nanostructure for photovoltaic application, we have decorated Sb2S3 on ZnO nanorods by using chemical bath deposition and the electrical and structural features of this nanostructure were studied. The role of oxide defect states in deterioration of the photovoltaic properties was inferred. To address this problem we have introduced an intermediate metal sulfide layer between oxide and chalcogenide layers to avoid the oxygen penetration from oxide to chalcogenide layer. The subsequent improvement in structural and electronic properties has been investigated. 2. Experimental Procedure Zinc nitrate, hexamethylene tetra amine (HMTA), antimony trichloride, sodium thiosulfate, and sodium sulfide were used as starting materials. FTO substrates were sequentially washed in baths of ethonal, acetone and in distilled water using ultra sonic agitation. A 0.4 M solution of Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, Aldrich, 98%) was prepared in ethylene glycol and the

solution turned milky after few minutes stirring. Few drops of di-ethanolamine were added in the solution with stirring till the solution turned transparent. The obtained solution was than spin coated on the clean FTO glass at the spinning speed of 3000 rpm for 30 s and the samples were dried in oven for 15 minutes at 300 0C. This process was repeated for three times and finally ZnO seed layer was annealed at 500 0C for one hour [24]. The solution for the growth of ZnO nanorods was prepared by dissolving equimolar (0.1M) aqueous solution of Zinc nitrate hexahydrate and hexamethylene tetramine. The growth of ZnO was carried out for three hours at 90 0C. After the growth the samples were washed with distilled water and annealed at 400 0C for one hour [25-26]. ZnO-ZnS core shell nanostructures were prepared by a self-assembling route. Typically ZnO nanorods were immersed at 60 0C in 0.16 M Na2S aqueous solution for 2h and then in 0.16 mol Zinc nitrate hexahydrate aqueous solution for 1h for ZnS growth, this whole process was repeated for three times to grow ZnS on the surface of ZnO [27]. Sb2S3 thin film was deposited on ZnO nanorods and ZnO/ZnS core shell nanostructures by chemical bath deposition [28-30], In brief the chemical bath was prepared by dissolving separately prepared solution of 650 mg of Sb2cl3 in 2.5 ml acetone and 25 ml 1 molar aqueous solution of Na2S2O3 .The volume of the solution adjusted up to 100 ml by adding distilled water. After the deposition the samples were annealed at 300 0C for 30 minutes in argon environment. In characterization, Conventional D8 Discover diffractometer (Cu–Kα radiation, λ = 1.54 A°) was utilized for XRD structural analysis. Optical properties were measured using Perkin Elmer Lembda-950 UV/Vis/NIR spectrometer. I(V) measurements were made by using Keithley solar simulator model SSB1K2. Two probe method was used for I(V) measurements using silver paste to make electrical contact with the sample. XPS analysis was used to study the surface composition of the samples. The XPS spectroscopy was performed in ultra high vacuum conditions using standard omicron system equipped with monochromatic Al Kα 1.4867 Kev X-ray source and Argus hemispherical electron spectrometer with 128 channels MCP detector. XPS software was used for data analysis and curve fitting. C1s was used for the calibration of binding energy. 3. Results and discussion Figure 1 shows the XRD pattern of ZnO nanorods on FTO glass substrate. In this spectrum we can see different diffraction peaks which can be labeled as (100), (002), (101), (102),(103), and (004) at 31.80, 34.40, 36.30, 47.60, 630 and 72.80. All the peaks were matched with the JCPDS card number 790207 and correspond to the hexagonal phase of the ZnO having the space group P63mc. The intensity of (002) peak is much larger compared to the other observed peaks. This is due to the fact that the formation energy of (002) plane is less compared to the other planes, hence the preferential growth of the ZnO nanorods is along the c-axis perpendicular to the FTO substrate. In order to see the morphology of the grown ZnO nanorods, SEM of the samples was performed. Figure 2a and 2b shows the SEM of the grown ZnO nanorods. The SEM micrographs depict the hexagonal morphology of the nanorods and showed uniform coverage of the substrate with ZnO nanorods. There are some regions where the nanorods are well aligned and perpendicular to the substrate as suggested by the strong (002) plane peak of the XRD data. The

average diameter of the ZnO nanorods is approximately equal to 80-100 nm. Figure 3 shows the optical transmittance of ZnO nanorods. The sample showed high transmittance in the visible range and a sharp dip is observed at about 374 nm. This dip corresponds to the band edge of the ZnO showing band to band transition with band gap of 3.3 eV.

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Figure 3: Optical transmittance of ZnO nanorods on FTO substrate To carry out the structural investigation of ZnO-Sb2S3 system XRD of the sample was performed. Figure 4 shows the XRD spectrum of Sb2S3 incorporated ZnO nanorods. In this pattern we identified two peaks corresponding to Sb2S3 at 32.30 and 33.40. These observed peaks can be indexed as (221) and (301) planes of orthorhombic crystal structure. In addition to Sb2S3 peaks we also observed peaks corresponding to oxide phase of antimony namely Sb2O3 at 27.5o, 28.2o, 46o, and 54.5o. This shows that some part of Sb2S3 deposited on ZnO nanorods exists in oxidized form. To investigate the surface composition of the Sb2S3 we performed the x-ray photoelectron spectroscopy of the Sb2S3 grown ZnO nanorods. Figure 5a shows the XPS spectra of the sample. Peaks observed at the binding energy of 530.2 eV and 539.7 eV corresponds to the Sb 3d5/2 and 3d3/2 levels respectively, while the peak appearing at the binding energy of 531.3 eV, which is not spin-orbit splitted, corresponds to the O1s. The oxygen peak originates in adventitious physical adsorption of oxygen or OH species. Comparing our results with the previous experimental reports, the dominant contribution in the XPS is from Sb2O3 state since Sb 3d5/2 peak is closer to the oxide phase at 530.5 eV as reported earlier [31]. The antimony sulfide may also contribute to the peak however from resolution of the current data it is not possible to distinguish between different species of the antimony. The contribution of Sb2O3 is also coming in the bulk of the structure which appeared in XRD results. Figure 6 presents the optical absorbance of ZnO-Sb2S3 system. The sample showed high absorbance in the visible range which shows the presence of Sb2S3 since the band gap of Sb2S3 is 1.7 eV (when annealed at 300 0C). The observation of a continuous broad region of absorption in the visible range is consistent with earlier reports [31-33]. The reason for this broad absorbance, as opposed to a sharp absorbance edge, could be the varying particle sizes of Sb2S3 nanostructure in the system. At the nanoscale, particles of different sizes can differ in their band gap leading to

a broad region of absorbance. It has been reported in the literature that the absorbance of Sb2S3 is red shifted with the increase in the annealing temperature due to the increase in particle size as the annealing temperature increases. At annealing temperatures greater than 300 0C the absorbance of Sb2S3 deteriorates either due to oxidation or due to evaporation of Sb2S3 [32] . 3.1 I(V) characteristics of ZnO-Sb2S3 system: We performed I(V) characteristic of ZnO-Sb2S3 heterojunction both in light and dark conditions due to the known absorbance of Sb2S3 in the visible range. Figure 7 shows the I-V characteristics of the ZnO-Sb2S3 nanostructures. The sample showed diode like behavior in both light and dark conditions. However, in the presence of light there was more current flowing through the device which shows that the photogenerated carriers also contributed to the current. But as is evident from the figure, there is no shifting of current output to the negative axis indicating that no photocurrent is generated. The absence of a photocurrent indicates that the photo generated carriers (e and h) in Sb2S3 are not being separated and collected at the two electrodes. The reason for this could be that a weak electric field exists across the junction and/or the presence of trap

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Figure 7: I-V characteristics of ZnO-Sb2S3 nanostructures in light and dark states due to the oxidation of Sb2S3 which latter results in the recombination of the generated carriers [34]. 3.2 Introduction of ZnS intermediate layer To address the issue of the oxidation of the Sb2S3 we grew an intermediate layer of ZnS on ZnO nanorods. Figure 8 shows the XRD pattern of the ZnO-ZnS nanostructures. In the XRD spectrum of ZnO-ZnS nanostructure an extra hump was observed at 28.8o in addition to the ZnO peaks. This extra peak at 28.8o can be indexed as (111) plane of the Zinc blend structured ZnS [27]. The appearance of (111) plane shows that ZnS layer has been grown on ZnO nanorods. Optical transmittance and XPS analysis of the ZnO-ZnS system also supported the XRD data and confirmed the growth of ZnS on ZnO nanorods. Figure 9 shows the optical transmittance of ZnO-ZnS nanostructures. In optical transmittance the sample showed high transmittance in the visible range and two sharp band edges have been observed. The first dip at around 374 nm corresponds to the band edge of the ZnO while the other edge at about 317 nm corresponds to the ZnS indicating band gap energy of 3.9 eV. Figure 10 shows the XPS spectrum of ZnO nanorods and ZnO-ZnS nanostructures. Figure 10a presents the O1s spectrum of pure ZnO nanorods. This spectrum is deconvoluted to two components at the binding energy of 530 eV and 531.5 eV. The first peak is attributed to the O2- ions in the hexagonal structure of ZnO lattice surrounded by zinc atoms, while the second peak is associated with chemisorbed or dissociated oxygen or OH species on the surface of ZnO nanorods [35]. After the growth of ZnS layer on the ZnO nanorods only one O 1s peak was observed at about 531.5 eV while the other peak of ZnO at the

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Figure 8: XRD spectrum of ZnO-ZnS nanostructure on FTO substrate Binding energy of 530 eV disappeared. This result shows that after the growth of ZnS no bare ZnO crystal exists in the ZnS shell layer and the thickness of ZnS layer is greater than 10 nm. Figure 10b shows the XPS spectra of Zn 2p core level of ZnO and ZnO-ZnS core shell nanostructures. The observed binding energies of Zn 2p3/2 and 2p1/2 core levels in case of ZnO are 1021.7 eV and 1044.8 eV while in case of ZnO-ZnS core shell structures, the observed binding energies are 1022 eV and 1045.3 eV. The increase in the binding energy of Zn 2p core levels in case of ZnO-ZnS hybrid structure confirms that Zn is present in the form of ZnS at the outer surface of ZnO-ZnS core shell structure. Figure 10c further confirms the presence of ZnS on ZnO nanorods. In this figure we can see a sulpher 2S peak at the binding energy of 225.8 eV which is absent in case of pure ZnO nanorods. Figure 10d shows the valence band spectra of ZnO nanorods and ZnO-ZnS hybrid nanostructures. The broad structure in the VB XPS observed for both ZnO nanorods and ZnO-ZnS nanostructure at the binding energy of 10 eV is mainly attributed to Zn 3d bands while the states closer to the Fermi level (2-8eV) arise from the oxygen 2p valence band and hybrid Zn 4s-O1s bands [36]. In this graph we can see that the valence band edge of ZnO-ZnS hybrid structure is more close to the Fermi level as compared to the pure ZnO nanorods. This result is consistent with the known higher valence band edge of ZnS as compared to ZnO, and indicates that the valence band XPS of the ZnO-ZnS nanostructure includes contributions from both ZnO and ZnS further confirming the presence of ZnS on the ZnO nanorods.

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BE (eV) Figure 10d: Valence band spectrum of ZnO and ZnO-ZnS nanostructures 3.3 ZnO-ZnS-Sb2S3 hybrid system: As discussed earlier, after the growth of ZnS layer on ZnO nanorods Sb2S3 was grown on ZnOZnS nanostructures to see the effect of ZnS layer in minimizing the oxidation. Figure 4 shows the XRD pattern of ZnO-ZnS-Sb2S3 nanostructures. XRD pattern showed the improved crystal structure of Sb2S3 as compared to ZnO-Sb2S3 sample by showing an extra diffraction plane (310) at 25o. Also it is important to note that all the Sb2O3 peaks which were observed in ZnO-Sb2S3 system at 2 theta values of 27.5 o, 28.2 o, 46 o, 54.5 o respectively are not observed in the ZnOZnS-Sb2S3 system. This shows that the ZnS layer helped in preventing the oxidation of Sb2S3. To study the surface composition of ZnO-ZnS-Sb2S3 nanostructure, XPS of the sample was performed. Figure 5b shows the XPS spectra of ZnO-ZnS-Sb2S3 nanostructure. XPS spectrum of this sample is similar to the ZnO-Sb2S3 system. This result shows that the surface composition of the two samples is almost same and confirms that the oxidation of Sb2S3 that we observed in ZnO-Sb2S3 system is occurring at the interface, since XPS is surface sensitive technique and shows dominantly single Sb peak, it is assigned to the Sb2O3 dominant contribution Figure 6 shows the optical absorbance of ZnO-ZnS-Sb2S3 system. Optical absorbance showed improved absorbance as compared to ZnO-Sb2S3 nanostructures. Comparing the optical absorbance of ZnO-Sb2S3 with ZnO-ZnS-Sb2S3 system we see that both the spectra are very similar with both samples showing high absorbance in the visible range. However the sample that contains the ZnS layer showed an increase absorbance by almost 40% compared to the nanostructure without the ZnS layer. Also it is important to note that at each wavelength in the visible range this sample showed an increased absorbance. The improved absorbance by the

Sb2S3 indicates in the ZnS containing hybrid structure there is an overall larger Sb2S3 content because of the absence of the oxidation and conversion into Sb2O3. 3.4 I(V) characteristics of the ZnO-ZnS-Sb2S3 system: Figure 11 (a) shows the I-V characteristics of ZnO-ZnS-Sb2S3 heterojunctions. The sample showed diode like behavior in dark while in light there is clear shift of the curve down to the fourth quadrant which shows the photovoltaic effect which was absent in ZnO-Sb2S3 system. Although the observed photo response of the sample is very small but the clear difference as compared to the pure ZnO-Sb2S3 system confirms that this system is now capable of photovoltaic response and more efficient charge carrier extraction. In Figure 11(b) we compare the I-V curves of the ZnO-Sb2S3 and ZnO-ZnS-Sb2S3 nanostructure in light. This figure clearly shows the I-V curve of ZnO-Sb2S3 sample passes through the origin while in case of ZnO-ZnS-Sb2S3 system IV curve shifts downward due to the photocurrent flowing through the device. Comparing the photo-response of the two systems, and based on our XPS, XRD and optical results we suggest that the difference between the two systems lies in the presence of trap states in the ZnO-Sb2S3 originating in the oxidized Sb ions i.e. the Sb2O3. Due to the absence or drastic reduction in the oxidized Sb these trap states are effectively eliminated leading to much smaller e-h recombination processes and comparatively larger photocurrent[37].

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Voltage Figure 11 (b) I-V curves of ZnO-Sb2S3 and ZnO-ZnS-Sb2S3 in light 4. Conclusion We have found that growing the Sb2S3 directly on the ZnO nanostructures does not lead to a system with any photovoltaic response and this is attributed to the formation of Sb2O3 at the ZnO-Sb2S3 interface. This conclusion is borne out by various characterization techniques employed. The problem has been successfully addressed by introduction of a ZnS interface between the ZnO nanostructures and the absorber material, Sb2S3. This leads to prevention of oxidation and much improved, albeit still weak photocurrent. The latter limitation is due to incomplete device architecture, not optimized film parameters like thicknesses etc. However the important part of work is the elimination of the oxidation of the chalcogenides. Work is now underway to address the limiting factors for weak photovoltaic response in the ZnO-ZnS-Sb2S3 system. We note that typically the single phase Sb2S3 thin films are being deposited on metal oxide semiconductors by using expensive experimental techniques. Our method is an inexpensive method for the deposition of pure Sb2S3 on metal oxide semiconductor which could have important applications in optoelectronics, considering the importance of Sb2S3 as one of the efficient light absorbing materials. 5. References 1.

Chang, J.A., et al., High-Performance Nanostructured Inorganic−Organic Heterojunction Solar Cells. Nano letters, 2010. 10(7): p. 2609-2612.

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Highlights  



ZnO- Sb2S3 nanostructures were grown on FTO glass substrate by chemical bath deposition method. Structural, Optical, and XPS analysis of the ZnO- Sb2S3 nanostructures showed dominant oxide phase of Sb, namely Sb2O3 and no photovoltaic effect was observed. Zinc sulfide interlayer was grown between ZnO-Sb2S3 system and the subsequent structural, optical and I(V) characteristics of the ZnO-ZnS- Sb2S3 nanostructures indicated reduced oxide phase of Sb and showed photovoltaic effect.