Electron. Mater. Lett., Vol. 11, No. 2 (2015), pp. 171-179 DOI: 10.1007/s13391-014-4236-x
Facile Linker Free Growth of CdS Nanoshell on 1-D ZnO: Solar Cell Application Archana Kamble,1 Bhavesh Sinha,2 Kookchae Chung,3 Anup More,4 Sharad Vanalakar,1 Chang Woo Hong,1 Jin Hyeok Kim,1,* and Pramod Patil4,* 1
Dept. of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Korea 2 Centre for Nanoscience and Nanotechnology, University of Mumbai, Mumbai, Maharashtra, India 3 Functional Powder Division, Korea Institute of Material Science, Changwon 642-831, Korea 4 Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416004 (M.S.), India (received date: 10 August 2014 / accepted date: 28 October 2014 / published date: 10 March 2015)
One dimensional type-II core/shell heterostructures are widely employed in solar cells because of their adventitious role in both light absorption and charge separation. Here we report a facile two step chemical approach to synthesizing ZnO/CdS core/shell nanorod arrays. ZnO nanorods (ZNR) with a high aspect ratio were grown using a hydrothermal technique where a uniform CdS shell was deposited using a facile, linker free, one pot, Hexamethylenetetramine (HMTA) based reflux technique for the first time. Though the reflux technique is quite similar to the chemical bath deposition technique (CBD), we obtained more uniform CdS coating and improved solar cell performance with the ZnO/ CdS heterostructure compared to CBD-grown ZnO/CdS heterostructures. To obtain a conformal coating of CdS, we optimized the CdS deposition time. Formation of pure phase ZnO/CdS core/shell heterostructure was confirmed by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy (XPS) depth analysis. Improved solar cell performance of 1.23% was obtained for ZnO/CdS core/shell structures with ZnS surface treatment. Keywords: core/shell, ZnO/CdS, semiconductor sensitized solar cell, nanorods
1. INTRODUCTION The semiconductor sensitized solar cell (SSSC) belongs to the third generation of solar cells, which represents a class of solar cells with low production cost and acceptable efficiency. Among the third generation solar cells, organic dye sensitized solar cells (DSSC) with a maximum photo conversion efficiency of ~12% have been heavily investigated, but they are limited by low absorbance with a dye monolayer and low efficiency with a dye multilayer.[1] DSSC also has poor stability. On other hand SSSC has prominent advantages over DSSC such as tunable band gap over a wide range, high extinction coefficients, improved light-harvesting capability, multiple exciton generation capability, large intrinsic dipole *Corresponding author:
[email protected] *Corresponding author:
[email protected] ©KIM and Springer
moments and stability. These advantages have made the development of SSSC a focal point for researchers.[2] Though SSSC has many advantages over DSSC, the working principle of SSSC is quite similar to that of DSSC. According to the operating principle, efficient performance of SSSC requires appreciable exciton generation and facile charge transport capability. Further, an appreciable quantity of exciton generation requires maximum photon absorption which can be furnished by maximum loading of the sensitizer. Facile charge transport however, requires an intact interface between an efficient electrical path providing the WBSC and the sensitizer material. A one dimensional (1-D) WBSC/ sensitizer core/shell structure is the legitimate solution to the above requirements because; (I) compared to discrete nanoparticles, the shell of the sensitizer material ensures maximum loading density of the sensitizer and an intact interface between the sensitizer and WBSC (II) Carrier separation takes place in the radial rather than the longer
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axial direction, (III) smaller carrier collection distance comparable to the minority carrier diffusion length, (IV) a 1D core assists in fast electron transport by avoiding particle to particle hopping, (V) compared to the particulate system, a 1-D core/shell structure provides more active surface area i.e. interface between electrode and electrolyte.[3] Here in this work, we grew 1-D ZnO/CdS core/shell nanorod array photoelectrodes for SSSC application. The preferred use of ZnO and CdS lies in the fact that ZnO is a multifunctional wide band gap (3.37 eV) semiconductor with large exciton binding energy and better electron mobility (115 - 155 cm2V−1S−1) than TiO2 (10−5 cm2V−1S−1). In particular, 1-D ZNR array can provide a direct pathway for photo-generated electrons. Hence they can be rapidly collected and transported. Moreover, 1-D ZNR arrays can be easily prepared by various cost effective and simple chemical methods.[4] The CdS is an attractive choice among the most widely used semiconductor sensitizers due to its direct narrow band gap, its ability to grow with various simple chemical techniques and its higher conduction band edge (−4.3 eV) compared to ZnO (−4.1 eV in a vacuum). Also, CdS is usually reported as having a high incident photon to current conversion efficiency among the most widely used sensitizers. The ZnO/CdS core/shell heterostructure forms a type-II band alignment that facilitates the injection of excited electrons from CdS to ZnO and reduces the rate of recombination between electron-hole pairs.[5] Moreover, WBSC/CdS heterostructures have small tPL (effective life time of charge), which also suggests superior charge injection ability compared to other sensitizers.[6] Herein, the ZNR arrays were grown by popular hydrothermal techniques whereas the CdS nanoshell was grown by controlled, one pot, linker free, HMTA based reflux technique. There are several reports on the sensitization of ZNR with CdS by an ammonia based regular chemical bath deposition (CBD) technique.[7] Compared to those reports, we observed that HMTA based sensitization provides finely controlled and uniform sensitization without dissolution of ZnO. For CdS sensitization, we adopted the reflux technique because it furnishes higher reproducibility than CBD on account of uniform heating of the solution and maintenance of constant growth conditions. Unlike CBD, the precursor solution is not wasted through evaporation, which helps to maintain constant growth conditions such as pH, concentration and volume of precursors. All these advantages along with use of HMTA based growth solution may have contributed to the growth of a uniform ZnO/CdS core/shell heterostructure based photoelectrode with improved solar cell efficiency. We also report high resolution transmission electron microscopy analysis (HR-TEM) and X-ray photoelectron spectroscopy (XPS) depth profiling of the synthesised ZnO/CdS core/shell structures to investigate the presence of any impurity phase
in the synthesized ZnO/CdS core/shell heterostructure as well as continuous growth of the shell.
2. EXPERIMENTAL PROCEDURE The ZnO/CdS core-shell nanorod thin films were prepared by a two-step chemical technique. The ZNR were grown by a hydrothermal technique using optimized conditions as mentioned in an earlier report.[8] For the growth of the CdS layer over ZNR, we adopted the reflux technique. In a typical experiment, 0.025 M aqueous Cadmium sulphate solution was used as a cadmium ion precursor. A solution of 0.05 M aqueous Hexamethylenetetraamine (HMTA) was added to the above solution and the mixture was stirred for 10 min. Then, 0.1 M thiourea solution was combined with the above mixture to form the CdS growth solution. The ZNR thin films were immersed in the CdS growth solution and the bath was allowed to reflux at 90 ± 3°C. Further, with the goal of optimizing parameters for the growth of the ZnO/ CdS core/shell structure, the deposition time was varied from 5 min to 30 min in steps 5, 10, 15, 20, and 30 and the corresponding films were denoted as ZCS-05, ZCS-10, ZCS-15, ZCS-20 and ZCS-30 respectively. The deposited films were rinsed in distilled water, dried at room temperature and used for further characterizations.
3. CHARACTERIZATION The morphological study of the ZnO-CdS core-shell nanorod thin film was carried out by field emission scanning electron microscopy (FESEM) using a Hitachi S-4700 instrument with an accelerating voltage of 30 kV. Detailed study of nanostructures and crystal phases were done using TEM and HRTEM images collected from Jeol JEM 2100F. X-ray photoemission spectroscopy (XPS) analysis on the as synthesized thin films was carried out using a Theta Probe AR-XPS System from Thermo Fisher Scientific. Monochromated Al Kα X-rays of 1486.6 eV were used to scan the surface of the film with a spot size selected at 400 μm. Optical absorption studies of the samples were done by means of UV-vis absorbance spectra of the samples, obtained from a UV-vis spectrophotometer (UV1800 Shimadzu, Japan). Crystal structure identification was carried out from the X-ray diffraction pattern collected using Rigaku DMAX 2200 diffractometer with CuKα1 radiation. The I-V characteristics of films were recorded using a semiconductor characterization system (SCS-4200 Keithley, Germany) with a two-electrode configuration, under the illumination of a halogen lamp (27 mW/cm−2). Photo-electrochemical solar cell performance measurements were performed using the cell configuration FTO/ZnO-CdS/Na2S-NaOH-S/Pt. The average area of the film used for PEC measurement was 1 cm2.
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4. REACTION MECHANISM The reactions involved in the growth of ZNR are given in our earlier report.[8] The reactions culminating in the growth of the CdS nanoshell are given below.[8,9] (1) First, the cadmium sulfate salt is dissolved in water, resulting in the formation of cadmium hydrated ions in the bath as represented in the equation (2) (3) From reaction (2)-(3), we observe that HMTA hydrolyses to form formaldehyde and ammonia, further formed ammonia provides hydroxide ions.
(4) Finally, the formed hydroxide ions, thiourea and hydrated cadmium ions react with each other to form CdS.
5. RESULTS AND DISCUSSIONS The XRD patterns of all films are shown in Fig. 1. The XRD patterns of CdS sensitized ZNR clearly indicate the deposition of hexagonal phase CdS nanoparticles with (100), (002), (101), (110) and (112) plane peaks at 2θ values of 24.8°, 26.5°, 28.1°, 43.7°, and 51.8° respectively. These
Fig. 1. X-ray diffraction pattern of bare ZnO and CdS sensitized ZNR array thin films.
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peaks are well matched to the hexagonal CdS, JCPDS data card of no. 41-1049. It is apparent that as the deposition time of CdS increases, the peak intensity also increases, depicting the enhancement in CdS nanoparticle loading density with respect to time. The peaks at 31.7°, 34.4°, 36.2°, 47.5°, 62.8° and 72.5° belongs to (100), (002), (101), (102), (103) and (004) planes respectively from the ZnO hexagonal wurtzite crystal structure. The highly intense peak at 34.4° belonging to the (002) plane of ZnO confirms the c-axis oriented growth and hence the formation of ZNR. The peaks marked with stars belong to the FTO substrate. Figure 2 shows FESEM images of the top and crosssectional views of CdS sensitized ZNR arrays. Surface morphological details of bare ZNR are given in our earlier report.[8] From Fig. 2, we can observe the uniform distribution of CdS sensitized ZNR arrays over the surface. The crosssectional images indicate that as the CdS deposition time increases, the loading density of CdS nanoparticles over ZnO surface increases. In the ZCS-05 (Fig. 2(a) & (b)) film, a few nanometer sized CdS particles were sparsely deposited on the ZNR, while in the ZCS-10 (Fig. 2(c) & (d)) film, the loading density of particles is found to be enhanced. The cross-sectional view (Fig. 2(b) & (d)) reveals the deposition of CdS nanoparticles along the entire length of ZNR. However, there is no continuous deposition of CdS, as we can see inter CdS particle spaces showing the bare ZnO surface. When CdS sensitization is performed for 20 min as in (Fig. 2(e) and (f)), the total ZnO surface gets covered by a continuous layer of CdS, leading to the formation of a ZnO/ CdS core-shell structure that is highly significant for SSSC application. This structure entails a higher loading density of absorber material with perpetuation of a higher active surface area. It also inhibits recombination losses between photo generated charge carriers in ZnO and electrolyte species. Further increase in CdS sensitization time for 30 min (Fig. 2(g) and (h)) leads to growth of a very thick CdS layer on the ZnO surface as well as in inter-rod spacing, resulting in the formation of dense and compact film, implying a reduction in active surface area. Thus we observe that by simply varying the deposition time, we can obtain a uniform nanoshell of CdS over ZNR without dissolution of ZnO. This can be attributed to the occurrence of CdS growth at low pH, with nanoscale nucleation. Further insight study on the formation of the core-shell structure was done by TEM and XPS analysis. Figure 3(a) shows the TEM image of CdS sensitized ZNR from ZCS-20 film. The inset in Fig. 3(a) shows the magnified section of sensitized ZNR. The image provides a substantial view and fulfils our expectation of continuous coating of CdS over the entire ZNR surface rather than discrete nanoparticles adhered on the surface. This is well in agreement with the FE-SEM analysis showing a relatively rough CdS layer coated over ZNR. A clear picture on uniform distribution of CdS shell
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Fig. 2. Field emission scanning electron microscopy images of surface and cross-sections of ZNR arrays sensitized with CdS for different time durations.
rather than discrete nanoparticles sitting over ZNR can be gauged by elemental mapping. Figure 3(b) shows the TEMelemental mapping of CdS sensitized ZNR from the ZCS-20 film. Here we observe the uniform presence of Cd and S elements all over the ZNR, which confirms that there is a continuous coating of CdS over ZNR, leading to formation of the ZnO/CdS core/shell structure. This elemental mapping supports the TEM results and verifies that CdS forms a
complete yet consistent coating over the entire ZNR surface. To further understand and analyze the core/shell structure, HRTEM analysis is presented in Fig. 3(c) and (d). The image reveals the lattice arrangements in the ZNR core as well as in the shell at randomly selected locations. The atomic lattice in the ZNR core is arranged with a uniform inter planer spacing of 0.26 nm that belongs to (002) planes of ZnO wurtzite crystal structure. However, the coating shows a polycrystalline
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Fig. 3. (a) Transmission electron microscopy (TEM) image of ZnO/CdS core/shell nanorod, (b) EDAX elemental mapping for Zn, O, Cd, and S ions, (c-d) High resolution TEM image of ZnO/CdS core/shell nanorod.
structure that includes a lattice arrangement comprising different orientations as indicated by fast Fourier transform images. We found different lattice spacings 0.20 nm, 0.31 nm and 0.24 nm, at three different places that respectively correspond to inter planer spacing in (110), (101) and (102) planes from the hexagonal crystal structure of CdS as revealed in the XRD patterns. Thus the TEM and HRTEM results confirm the formation of uniform and continuous CdS coating over ZNR. The ZnO/CdS photoelectrode of SSSC consists of a nano dimensional coating of one semiconductor over another. The necessary insight on the chemical environment and distinctness of the core and shell assembly provides information on the quality of the synthesized structure. XPS is an important legitimate surface scanning technique for effectively analysing the surface composition and chemical states of the proposed structure. Figure 4(a) and (b) show survey scans for bare ZNR and CdS sensitized ZNR (ZCS-20) thin films respectively. The XPS of pristine ZnO is added for the sake of convenience in comparative analysis. The X-ray photoemission peaks for ZnO in Fig. 4(a) are duly indexed according to the earlier reported results for ZnO.[10] The survey spectrum contains various XP emissions from different electronic
Fig. 4. (a-b) X-ray photoelectron survey spectrums of pure ZnO and CdS sensitized ZnO.
levels in Zn with dominating Zn2p states along with O1s state for Zn-O bonding. The absence of any other peak belonging to counter ions from precursors signifies a complete reaction process culminating in the clean growth of ZNR. These as grown ZNR were further sensitized by CdS
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Fig. 5. (a-b) The core level XP spectrum of Zn 2p states and O1s states. (c) Systematic deconvolution of O1s state before and after CdS sensitization. (d-e) Core level XP spectrum of Cd 3d and S 2p oxidation states from ZCS-20 thin film.
to form the ZnO/CdS core-shell structure. As a result, the dominating peaks of the Zn2p state are found to be duly submerged against various emissions from CdS grown on the surface of ZNR. The survey spectrum of sample ZCS-20 is as shown in Fig. 4(b). The spectrum comprises of all the peaks relevant to CdS, along with Zn2p and O1s states from the ZnO core beneath the CdS shell. The core level emission spectrum for individual elements was studied to understand the chemical ambiance in the core/shell nanostructure. Figure 5(a-e) deals with the core level spectrum of Zn, O, Cd and S. A comparative plot of the core level emission from Zn in ZNR before and after CdS coating is shown in Fig. 5(a). Zn2p3/2 and Zn2p1/2 states are clearly visible at binding energies of 1022 and 1045.1 eV. The spin orbit splitting in between the two states is observed to be 23.1 eV, which mainly exists in Zn ions present in the form of ZnO.[11] The intensity of the Zn3p3/2 peak has decreased considerably, which is obviously due to CdS coating, while the FWHM remains the same at 1.8 eV before and after CdS coating. There is no chemical shift in the XP spectrum of Zn ions in ZNR after CdS coating, ruling out the possibility of intermediate phase formation between the core and shell. The core level spectra for Cd and S are shown in Fig. 5(d) and (e) respectively. Cd3d5/2 and Cd3d3/2 emission peaks are observed at 405.1 eV and 411.8 eV respectively. These XP emission peaks for Cd3d state are devoid of chemical shifts and bear 6.7 eV of spin orbit doublet separation, which corresponds to the formation of a clean CdS phase.[12] Similarly, the core level spectrum of S comprises of two peaks corresponding to S2p3/2 and S2p1/2 observed at 161 eV and 162.2 eV with a spin-orbit separation of about 1.2 eV that again belongs to the CdS phase.[13] Further, in the
nanostructured assembly of semiconductors consisting of elements with a tendency to combine with oxygen, analysis of the electronic state of oxygen is of crucial importance. The core level XP spectrum of the O1s state of ZNR before and after CdS sensitization (ZCS-20) is shown in Fig. 5(b). The peak for O1s states in pristine ZNR appears at 530.3 eV and is asymmetric, with a convoluted shoulder at higher binding energies. The deconvolution of this peak is shown in Fig. 5(c). The main peak centered at 530.3 eV is accompanied by a shoulder peak at 531.7 eV. This high intensity main peak at 530.3 eV belongs to the O2− oxide state, which in the present case means stoichiometric Zn-O bonds in the ZnO wurtzite structure.[14] The shoulder peak based at 531.7 eV comes from the oxygen deficient region in the ZnO structure.[15] It can be understood from surface analysis that the pristine ZNR comprises an oxygen deficient region. For CdS sensitized ZNR, it is interesting to note that the XP response of the O1s state is symmetric and precisely shifted to 531.7 eV while the peak at 530.3 eV is absent. At this stage, it is significant to note that a symmetric peak remarkably sits at the same place as and is almost identical to the deconvoluted peak at 531.7 eV for pristine ZNR. Further, it should be taken into account that no peak is observed at lower (ca. 530 eV O2- states) or higher (ca. 532 eV adventitious atmospheric oxygen) binding energies around this peak.[16] Thus it can be categorically stated that this peak at 531.7 eV belongs to the oxygen deficient surface of ZNR on which the CdS layer is coated. The CdS coating on ZNR completely blocked the excitations from O1s states belonging to the stoichiometric Zn-O bonding. To confirm the purity of the CdS nanoshell along the entire depth of the shell, it is necessory to analyse it by XPS depth analysis. So we carried out XPS depth analysis using argon ion etching. The
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Fig. 6. UV-VIS absorption spectra of bare ZnO and CdS sensitized ZNR.
evolution in the core level spectrum of Zn, O, Cd and S with respect to the etching time for ZCS-20 thin film is given in ESI-1. Figure 6 shows the optical absorption spectra of bare ZnO and all CdS sensitized ZnO thin films recorded in the wavelength range of 350 to 750 nm at room temperature. Here we observe that the band edge of bare ZNR located around 400 nm is shifted in the visible range upon CdS sensitization. The absorption edge of ZnO/CdS structures shows a red shift from ~480 nm to ~520 nm with an increase in CdS loading time. Because, as the deposition time increases, the particle size and amount of CdS loading also increases and the absorption edge shifts to longer wavelengths due to the size quantization effect. There is little difference in the absorption edge of ZCS-20 and ZCS-30 compared to the difference between ZCS-10 and ZCS-20. This is because after covering the entire surface of ZnO with CdS in ZCS20, the CdS growth reaction slows down and eventually stops as the reactive surface of ZnO as well as the inter rod space is completely consumed by the CdS layer in the ZCS30 film. The photo-electrochemical (PEC) properties of all ZnO/CdS photoelectrodes were studied by measuring the current density-voltage (J-V) characteristics. Figure 7 shows a schematic representation of the solar cell device formed with the ZnO/CdS photoelectrode. This figure illustrates the working principle of the ZnO/CdS core/shell structure based phototelectrode. The red arrows indicate the flow of electrons and the inset image shows band structure. When light falls on the photoelectrode, the electrons in the sensitizer (i.e. CdS) gets excited, leading to the formation of electron hole pairs. This process can be represented in reaction form as; (5) Then type-II band alignment between sensitizer CdS and
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Fig. 7. Schematic diagram of ZnO/CdS core/shell based Semiconductor sensitized solar cell, inset figure shows formation of type-II band alignment at ZnO/CdS interface.
wide band gap semiconductor (WBSC) ZnO injects the photo-generated electrons into a ZnO. (6) Electrons injected in ZnO arrive at counter electrodes via the external circuit. Meanwhile, the holes generated due to excitation of electrons are injected into the electrolyte (or the reduced species in the electrolyte donates an electron to the hole in the valence band of CdS) which acts as a holetransporter medium. (7) The reaction taking place in the electrolyte is given as (8) 2−
The resulting holes/oxidised species ( Sx ) are transported to the counter electrode, where they are reduced by acceptance of an electron at the counter electrode coming from the photoelectrode through an external circuit [2(a)]. (9) These steps culminate the procedure of energy generation, which continues until the photoelectrode is exposed to light. Figure 8 shows the J-V response of bare ZNR and CdS sensitized ZNR thin films. The image reveals that the PEC performance of ZNR increases significantly on CdS sensitization, and the CdS deposition time greatly influences PEC performance. All the solar cell output parameters calculated from J-V curves are given in Table 1. In the ZCS-5 film,
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1.23% to 0.64% with increased CdS loading time from 20 min to 30 min. Here we observe that the PEC performance of bare ZnO increases significantly upon sensitization by CdS nanoparticles. This can be attributed to enhanced visible light absorption and the formation of type-II band alignment at heterostructures between ZnO and CdS as shown in the Fig. 7 inset. The enhanced light absorption leads to generation of a maximum number of charge carriers and hence the rise in current density. The type-II band alignment assists in the readily separation of photo-generated charge carriers and their efficient transport, as the energy difference between the conduction band edges of CdS sensitizer and ZnO acts as a driving force for the injection of photogenerated electrons from CdS into ZnO.[6] Also, the formation of ZnO/CdS typeII band alignment shifts the conduction band edge of ZnO to more negative potentials, which leads to an increment in Voc. This is because during the formation of the ZnO/CdS type-II heterostructure, electrons from the higher conduction band of CdS will be transferred to the lower conduction band of ZnO. This charge distribution would raise the conduction band of ZnO and thus could increase the Voc of SSSC.[17] The maximum Voc is then determined by the energy difference in the quasi-Fermi level of the ZnO under irradiation and the Fermi level in the dark, which is the same as the potential of the redox couple in the electrolyte.[18]
Fig. 8. PEC performance of bare ZnO and all CdS sensitized ZnO thin films.
sparse loading of CdS nanoparticles over ZNR leads to less optical absorption and hence lower PEC performance. In the ZCS-10 film, increased CdS loading over ZnO rods results in enhanced PEC performance compared to the ZCS5 film. Further, upon increasing the CdS loading time to 20 min, there is formation of a uniform shell of CdS thin layer over ZNR, leading to formation of ZnO/CdS core-shell heterostructure. This enhances optical absorption and reduces recombination losses between electrons in ZnO and electrolyte species by preventing contact between ZnO and electrolyte. Thus the best performance with short circuit current density (JSC) 1.44 mA, open circuit voltage (VOC) 589 mV, and photo-conversion efficiency (η) 1.23% is observed for films prepared with 20 min. CdS sensitization. The photoelectrochemical performance was not improved further by increasing the CdS deposition time. This is because increasing CdS deposition time up to 30 min results in the formation of dense and compact film, which reduces the surface area of the film, thereby reducing the electrode electrolyte interface. Along with this, the formation of a thick CdS layer extends the migration distance of photo-generated electrons to the ZNR and increases the possibility for carrier recombination. Hence solar cell efficiency is found to be reduced from
5. CONCLUSIONS A fascinating 1-D ZnO/CdS type-II core/shell structure was synthesized by a cost effective, template free, surfactant free, and binder-less facile chemical approach. A core/shell structure ensures higher loading density of sensitizer with maintenance of the maximum active surface area. In the present work, CdS loading for 20 min. led to the formation of a ZnO/CdS Core/shell structure. Here we report the growth of a CdS nanoshell over ZnO with a HMTA based solution using the reflux technique. Compared to ammonia based CBD-grown ZnO/CdS heterostructures, we have grown a more uniform core/shell structure with improved solar cell performance. HR-TEM and EDAX elemental mapping analysis reveals the formation of the conformal coating of CdS over ZnO. XPS depth analysis study rules
Table 1. Solar cell output parameters of bare ZnO and CdS sensitized ZnO thin films calculated from J-V characteristics. J (mA/cm )
V (mV)
R (Ω)
R (Ω)
FF
η (%)
ZnO
0.27
446
994
2291
0.32
0.54
ZCS5
0.79
544
355
1113
0.37
0.59
ZCS10
1.19
574
207
1252
0.34
0.86
ZCS20
1.44
589
139
1242
0.39
1.23
ZCS30
1.13
530
141
1024
0.29
0.64
Sample code
2
sc
oc
S
Sh
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out the probability of impurity phase formation. The solar cell performance of the ZNR array is boosted from 0.54% to 1.23% on application of the CdS shell.
ACKNOWLEDGEMENTS One of the authors Miss Archana Kamble is thankful to University Grants Commission (UGC) for the Rajiv Gandhi Senior Research Fellowship award. One of the authors Dr. B. B. Sinha is grateful to the Department of Science and Technology India for the Inspire fellowship award. This work was partly supported by the Converging Research Center program funded by the Ministry of science, ICT and Future Planning (2013K000407) and Human Resource Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (no.: 20124010203180).
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Electron. Mater. Lett. Vol. 11, No. 2 (2015)
