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Solution-processed Cu2ZnSnS4 superstrate solar cell using vertically aligned ZnO nanorods
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Nanotechnology Nanotechnology 25 (2014) 065401 (8pp)
doi:10.1088/0957-4484/25/6/065401
Solution-processed Cu2ZnSnS4 superstrate solar cell using vertically aligned ZnO nanorods Dongwook Lee and Kijung Yong Surface Chemistry Laboratory of Electronic Materials (SCHEMA), Department of Chemical Engineering, POSTECH, Pohang 790-784, Korea E-mail: [email protected] Received 3 October 2013, revised 27 November 2013 Accepted for publication 11 December 2013 Published 16 January 2014
Abstract
One-dimensional (1D) zinc oxide (ZnO) nanostructures are considered to be promising materials for use in thin film solar cells because of their high light harvesting and charge collection efficiencies. We firstly report enhanced photovoltaic performances in Cu2 ZnSnS4 (CZTS) thin film solar cells prepared using ZnO nanostructures. A CdS-coated, vertically well-aligned ZnO nanorod (NR) array was prepared via a hydrothermal reaction and nanocrystal layer deposition (NCLD) and was used as a transparent window/buffer layer in a CZTS thin film photovoltaic. A light absorber CZTS thin film was prepared on the CdS/ZnO NRs in air by depositing a non-toxic precursor solution that was annealed in two steps at temperatures up to 250 ◦ C. The crystallized CZTS phase completely infiltrated the CdS/ZnO NR array. The nanostructured ZnO array provided improved light harvesting behavior compared to a thin film configuration by measuring UV–vis transmittance spectroscopy. The prepared CZTS/CdS/ZnO NR device exhibited a solar energy conversion efficiency of 1.2%, which is the highest efficiency yet reported for nanostructured superstrate CZTS solar cells. Keywords: CZTS, ZnO, nanorod, molecular precursor solution, thin film solar cell, superstrate (Some figures may appear in colour only in the online journal)
1. Introduction
recent high prices of rare metals, such as indium and gallium, have driven the thin film device industry to seek methods for improving the efficiency with which such materials are used. Cu2 ZnSnS4 (CZTS) is an intriguing substitute as a lowcost light absorber for CIGS-based thin film solar cells because of the use of abundant chemical elements [6]. Each component of CZTS is abundant in the Earth’s crust (Cu: 50 ppm, Zn: 75 ppm, Sn: 2.2 ppm, S: 260 ppm) and shows a low toxicity relative to the elements present in CIGS (indium and selenium are present in the Earth’s crust at concentrations of less than 0.05 ppm) [7]. Kesterite CZTS has a direct band gap (1.4–1.5 eV) and a strong optical absorption coefficient (104 cm−1 ) [8–10]. This band gap energy is considered to be suitable for use in single-junction thin film solar cells. The structural analogy between CIGS and CZTS thin film solar cells has motivated the testing of methods for depositing
Inorganic semiconductors have attracted attention in recent years in the context of developing clean sustainable energy technologies. Compared to silicon-based photovoltaics, inorganic compound semiconducting materials have tunable direct band gaps and a high light absorption coefficient that permits the conversion of over 90% of incident sunlight into electric energy within a layer thickness of less than a few micrometers [1]. Among the inorganic semiconductors tested thus far, chalcopyrite Cu(In1−x Gax )(Se1−y Sy )2 (CIGS) offers a promising absorber material for thin film solar cells. The best CIGS thin film solar cell has yielded a 20.4% power conversion efficiency [2]. This cell was prepared using vacuum-based deposition processes that currently do not permit low-cost mass production and scaled-up fabrication processes [3–5]. The 0957-4484/14/065401+08$33.00
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Nanotechnology 25 (2014) 065401
D Lee and K Yong
Table 1. Current status of the photovoltaic performances of superstrate-type CZTS solar cells prepared using metal oxide nanostructures
(3-D CZTS solar cell). Structure Carbon paste/CZTS/In2 S3 /TiO2 NP/FTO Carbon paste/CZTS/TiO2 NP/TiO2 NP/TCO Mo/CZTS/In2 S3 /TiO2 NP/FTO Graphite/CZTS/CdS/TiO2 NP/FTO
Au/CZTS/CdS/ZnO NR/ITO
Deposition method Screen printing
Efficiency (%) Voc (mV) 0.60 250
Jsc (mA cm−2 ) 8.76
FF 0.27
Ref. [28]
Spray deposition
0.51
564
2.85
0.43
[29]
Doctor blading
0.55
240
7.82
0.29
[30]
Precursor solution deposition (spray deposition) Precursor solution deposition (spin-coating)
1.131
445.9
6.794
0.374
[31]
1.2
679.2
4.1
0.438
CIGS thin films for the fabrication of CZTS thin films. Among the various deposition methods, non-vacuum coating techniques, which are expected to facilitate the large-scale, high-throughput process, have been applied to the preparation of CZTS films. Solution-based deposition approaches can be advantageous in that they offer the efficient use of a precursor and are compatible with a broad selection of substrates, unlike vacuum-based deposition methods involving evaporation and sputtering [4]. Until now, a power conversion efficiency gap between CIGS and CZTS solar cells has persisted. A variety of synthetic processes have been proposed in an effort to improve the photovoltaic performances of CZTS thin film solar cells. For example, high-quality CZTS absorber films with fine grain structures have been obtained via a solution process by tuning the identities of the metal precursor, chalcogen precursor, solvents, additives, annealing temperature and annealing gas [11–14]. The current champion cell efficiency of 8.4% was recorded by IBM using a vacuum thermal evaporation method [15]. Unfortunately, this hydrazine-based synthetic method is not optimal from a manufacturing perspective. Hydrazine-based precursor solutions pose several safety and environmental concerns, and the deposition and crystallization processes must be handled entirely under an inert atmosphere to prevent oxygen incorporation. Safe and non-toxic solution-based methods for growing CZTS thin films have been proposed by Yuxiu et al based on the use of an ethanol solution containing amine additives to produce CZTS thin films [16]. Their device exhibited a power conversion efficiency of 5.36%. Another group demonstrated an environmentally benign route in which metal salts and thiourea were dissolved into dimethyl sulfoxide as a non-toxic solvent, yielding a power conversion efficiency of 4.1% after selenization [11]. The conversion efficiencies of thin film solar cells may be further improved by combining the advantages of a nanostructure with thin film photovoltaics [17]. These approaches are currently in the early stages of development. Nanostructures with controllable dimensions and aligned morphologies can potentially provide more efficient charge generation, collection and transfer due to the large junction area and the short collection distance to the interface [18]. Semiconductor metal oxides,
for example SnO2 , TiO2 and ZnO, have attracted attention as possible materials for use in photoanodes in quantum dot or dye-sensitized solar cells and bulk heterojunction solar cells [19–24]. Low-quality solution-processed CIGS or CZTS thin film absorbers tend to be characterized by short minority carrier diffusion lengths and a high recombination loss at grain boundaries [25, 26]. These issues can be ameliorated by introducing an interpenetrated absorber material with a nanostructured photoanode. Superstrate cell structures may be used to form nanojunctions between an absorber and a buffer/window structure [20, 22, 27]. Several groups have developed CZTS solar cells using three-dimensional (3-D) TiO2 nanoparticles [28–31]. TiO2 is the most commonly used metal oxide photoanode in a variety of solar cells. Table 1 summarizes the recent methodologies and photovoltaic performances of the nanostructured CZTS thin film devices fabricated by solution approaches. Most nanostructured CZTS superstrate photovoltaics are prepared in a configuration similar to the following: metal electrode/CZTS absorber/buffer layer/metal oxide nanostructure/metal electrode. The open-circuit voltage (Voc ) and fill factor (FF) of these reported cells remain low and hinder the preparation of high-efficiency cells. Herein, we report a route to the fabrication of a CZTS superstrate structure using well-aligned CdS-coated ZnO nanostructures. Our goal was to improve the photovoltaic performances of nanostructured CZTS solar cells using hydrothermally grown ZnO NRs and a low heat treatment temperature for the preparation of CZTS absorbers. The CdS and CZTS were deposited without the use of a vacuum environment via nanocrystal layer deposition (NCLD) and molecular precursor solution methods, respectively. The synthetic procedure used for the solar cell fabrication process and the device operating mechanism is illustrated schematically in figure 1. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis revealed that the morphologies and crystal phases of the nanostructures and the evolution of the CZTS absorber film from an as-coated precursor film. The optical properties were observed using UV–visible (UV–vis) transmittance spectroscopy. The current density and external quantum efficiency spectra of the CZTS superstrate solar cell were collected using a sweeping applied voltage under irradiation. Our superstrate CZTS solar cell recorded a 1.2% efficiency, the highest value yet obtained from nanostructured CZTS superstrate cells. 2
Nanotechnology 25 (2014) 065401
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Figure 1. Schematic illustration of the synthetic procedure used for the fabrication of ZnO NR-based CZTS thin film solar cells.
2. Experimental details
were maintained at this temperature for 10 min. All annealing processes were conducted in air. After annealing, the CZTScoated samples were cooled naturally to room temperature in an air environment. This coating–drying–annealing cycle was repeated twice. Complete photovoltaic devices were fabricated by depositing a patterned 100 nm thick gold film by RF-magnetron sputtering. Finally, silver paste was squeezed onto the Au and ITO electrodes and then dried in a preheated electric oven at 60 ◦ C. The total active area was 0.12 cm2 .
2.1. Materials
All chemicals were purchased from Sigma-Aldrich and were used as received. Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O, 98%), ammonium hydroxide (28 wt% NH3 in water, 99.99%), cadmium chloride (CdCl2 , technical grade), thioacetamide (TAA, C2 H5 NS, 99%), copper iodide (CuI, 99.999%), zinc acetate (Zn(OAc)2 , 99.99%), tin chloride (II) (SnCl2 , 98%) and pyridine (C5 H5 N, ≥ 99.0%) were used as received. Prior to material deposition, soda lime glass (SLG) substrates were ultrasonically cleaned for 10 min with detergent, acetone and ethanol, respectively.
2.4. Characterization
The morphologies of the nanostructures and films were investigated using field emission scanning electron microscopy (FE-SEM, XL30S, Philips). The crystal structure properties were analyzed using x-ray diffraction (XRD, Max-2500V, Rigaku) techniques. The optical properties of the nanostructures were evaluated by measuring the transmittance spectra using UV–visible spectroscopy (Jasco, V-530). The performance of the solar cell was measured using a solar simulator equipped with a 300 W Xenon lamp (Newport, USA) and an electrical analysis system (Keithley 2400). The power of the simulated light was calibrated to AM 1.5 (100 mW cm−2 ) using a standard silicon solar cell (PV Measurement Inc.). The incident photon-to-current conversion efficiency (IPCE) as a function of wavelength was measured using a photomodulation spectroscopic setup (model Merlin, Oriel).
2.2. Preparation of CdS-coated ZnO nanorod arrays
The ZnO NRs and a CdS buffer layer were grown on an ITO-coated SLG substrate via a hydrothermal reaction and NCLD [32]. The experimental conditions are described in detail in a previous report [22]. The NCLD deposition time was increased to 50 min in the current work to enable the preparation of a uniform CdS shell layer coating on the ZnO NRs. 2.3. Fabrication of the CZTS/CdS/ZnO photovoltaic devices
A CZTS light absorber layer was coated on a CdS-coated ZnO NRs array. The metal precursors (Cu, Zn and Sn) and chalcogen (S) precursors were dissolved in pyridine solvent [33]. SnCl2 (0.16 M), TAA (3.2 M), CuI (0.32 M) and Zn(OAc)2 (0.16 M) were mixed in pyridine one at a time to prevent the undesired formation of a precipitant. A yellowish transparent precursor solution was obtained by stirring for about 10 min. The Cu:Zn:Sn:S ratio was set at 2:1:1:20 to compensate for sulfur loss during annealing. One milliliter of the precursor solution was dropped onto the samples, which were then spin-coated at 1300 rpm for 30 s. The samples were placed on a hot plate preheated at 150 ◦ C for 10 min. After the first drying procedure, the samples were immediately moved to a hot plate preheated at 250 ◦ C and
3. Results and discussion
Figure 2 shows the morphological changes of the bare ZnO NR arrays after the deposition of CdS and CZTS layers via NCLD and non-vacuum solution deposition methods, respectively. A hydrothermal reaction of aqueous Zn(NO3 )2 and ammonium hydroxide was used for ZnO NRs growth at 95 ◦ C for 60 min. The ZnO thin film was sputtered on a transparent ITO-coated SLG substrate as a seed layer prior to NR growth. The CdS buffer layer was selectively deposited via NCLD on the ZnO NRs at room temperature for 50 min to form a shell layer on the ZnO NRs. It is well known that the n-type CdS is extensively 3
Nanotechnology 25 (2014) 065401
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Figure 2. Cross-sectional and top-view SEM images of (a), (b) bare ZnO NRs, (c), (d) CdS-coated ZnO NRs prepared via NCLD with a 50 min reaction time and (e), (f) CdS/ZnO NRs prepared after multiple spin-coating of CZTS.
figures 2(e) and (f). After 1 CZTS coating cycle, the degree to which the CZTS film penetrated the NR array was insufficient to extend to the bottom of the ZnO NRs, and the resultant film surface displayed frequent defects, such as cracks and voids, which reduced the Voc and FF values [34]. Two CZTS coating cycles provided a uniformly infiltrated film with no cracks or voids, as shown in figures 2(e) and (f). The deposited CZTS absorber layer resembled a pile of very small nanocrystals a few nanometers in size. An unnecessary thick CZTS overlayer formed if more than three coating cycles were applied. The crystal structures of the nanostructures and the evolution of the CZTS film from a precursor state were characterized using XRD analysis, and the results are presented in figures 3(a) and (b). Bare ZnO nanorods exhibited a sharp peak at 34.4 ◦ , corresponding to preferential growth of the ZnO NRs along the c axis (JCPDS-792205) [35]. After NCLD deposition, hexagonal, polycrystalline CdS peaks were observed (JCPDS-411049). These results confirmed that the uniformly coated shell layers on the ZnO nanorods were composed of polycrystalline CdS. A CZTS film was deposited on the CdS/ZnO nanorod array using the thioacetamide (TAA)-based
employed as a buffer layer in highly efficient CZTS thin film solar cells to provide enhancements in the device performances by avoiding shunt path and widening the depletion region [4]. In the superstrate configuration, incident light passes through the glass, ZnO film and ZnO NRs, respectively. Therefore, minimizing light loss, for example, by minimizing reflection, is important for the solar energy harvesting efficiency. For this reason, we aimed to prepare a transparent ZnO template 500–600 nm in length. At short reaction times, well-aligned ZnO NRs oriented along the c axis were grown on a seed layer (figures 2(a) and (b)). The gap distance between each NR tip was estimated to be about 1–200 nm. As shown in figures 2(c) and (d), after a 50 min NCLD reaction period, a uniform 30 nm CdS shell layer had formed on the ZnO NRs. The CdS shell coating layer uniformly increased the nanorod’s diameter. We previously reported the use of NCLD techniques (25 min reaction period) for the fabrication of CIS solar cells. In the current study, the NCLD time was increased to 50 min to obtain a more uniform and thicker CdS shell layer. After CZTS deposition via the molecular precursor solution method, morphological changes were observed, as shown in 4
Nanotechnology 25 (2014) 065401
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Figure 4. Raman analysis of the spin-coated CZTS films on glass substrate annealed at 150 ◦ C and 250 ◦ C.
to Cu, Zn, Sn, S or Si. The dissolved Cu ions may have combined with four TAA molecules to form a chelated metal–organic complex. Upon annealing the precursor film in air at 150 ◦ C, the intensities of the precursor decreased due to the decomposition of the TAA-complex. The CZTS and CuS structures formed simultaneously after drying at 150 ◦ C (JCPDS-260575). The intensity and breadth of the main CZTS peaks ((112), (220) and (312)) and the presence of a binary metal phase suggested that the conversion from a precursor to a CZTS phase was incomplete. Samples treated with a second annealing process at 250 ◦ C displayed an XRD pattern that agreed well with the reference CZTS kesterite phase and did not include precursor or impurity peaks. The estimated average grain size of the (112)-oriented CZTS crystals was about 5–6 nm using the Scherrer equation. Larger highly crystalline CZTS layers may be obtained by annealing at higher temperatures; however, the possibility of thermal inter-diffusion of CdS must be carefully considered [36, 37]. Because the deposition of CZTS was carried out under air conditions at low temperature, Raman analysis was used to reveal the presence of secondary phases such as metal chalcogenides including ZnS, Cux Sy and SnS, which are not well distinguished in XRD analysis. As shown in figure 4, the sample treated at 150 ◦ C exhibits a broad shoulder near 287 cm−1 and a peak located at 335 cm−1 . Fernandes et al reported a shoulder between 250 cm−1 and 300 cm−1 is related to the convolution of peaks corresponding to Sn2 S3 , CZTS, Cu2−x S and ZnS [38]. Upon annealing the film at 250 ◦ C, this broad shoulder disappeared and only the CZTS peak was observed. Synthetically, these XRD and Raman analysis results indicate the phase purity of the final CZTS absorber. The UV–visible light transmittance spectra over the range 300–1100 nm were used to compare the optical absorbance properties of the planar and nanostructured samples (figure 5). The quantity of the deposited CZTS was fixed by applying two coating cycles. The incident light was focused on the back side of the SLG. The observed spectra indicated that the ZnO nanorods were more transparent in the visible range than the flat ZnO seed layer. These results indicated that the scattering loss of visible light in the nanostructured ZnO could be reduced
Figure 3. (a) XRD patterns of nanostructures obtained through various deposition routes: bare ZnO NRs, CdS-coated ZnO NRs and CZTS-infiltrated CdS/ZnO NRs. (b) XRD patterns of the CZTS-coated Si annealed at room temperature, 150 ◦ C and 250 ◦ C, revealing the evolution of the weakly oriented polycrystalline CZTS peaks.
solution reaction method. Achim et al used thioacetamide (TAA) as a sulfur precursor to minimize organic impurities and to reduce the temperature required for synthesis [33]. Their analysis suggested that the CZTS formed upon heating at approximately 105 ◦ C. They did not report the solar cell performance in that study. A yellowish transparent CZTS precursor solution containing CuI, Zn(OAc)2 , SnCl2 and TAA yielded a pH of 5. The CdS and ZnO templates did not appear to deteriorate, even after the substrate temperature had been increased to 250 ◦ C. Figure 3(a) shows that no additional peaks appeared after the second annealing step during CZTS film deposition aside from the peaks corresponding to the CdS-coated ZnO sample. This observation was attributed to the concealment of the CZTS peaks by the relatively strong CdS and ZnO peaks due to the small sizes of the CZTS crystallites. Proof of the formation of a CZTS phase was sought by spin-coating a CZTS precursor solution onto a Si substrate and analyzing this layer using temperature-dependent XRD measurements (figure 3(b)). The as-coated precursor film, which included metal and chalcogen precursors, displayed peaks corresponding to copper tetrakis-thioacetamide chloride (JCPDS-130613). Several unknown peaks were also observed, which did not match any reported materials corresponding 5
Nanotechnology 25 (2014) 065401
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Figure 5. UV–visible light transmittance spectra of the various nanostructures, including SLG, ZnO film, ZnO NRs, CdS-coated ZnO NRs, CZTS/CdS/ZnO film and CZTS/CdS/ZnO NRs.
prior to reaching the CdS/CZTS interface. The deposition of CdS on ZnO increased the visible light absorption of the core/shell NRs, due to the low band gap energy of CdS (2.4 eV). Deposition of the CZTS absorber enhanced the light harvesting at the well-aligned ZnO nanostructured interface, indicating that the incident light was more efficiently trapped in the absorber than in the flat film structure. The estimated band gap energy of the CZTS film (1.6–1.7 eV) was larger than the bulk value (1.4–1.5 eV). The existence of undesired secondary phases or non-stoichiometric composition can shift the band gap energy to the higher energy side. As we mentioned above, there is no evidence for the existence of secondary phases in the final CZTS film. Using energy-dispersive x-ray spectroscopy (EDX), Cu-rich (Cu/(Zn+Sn) ∼ 1.1) and Zn-rich (Zn/Sn ∼ 1.5) stoichiometry was found. Also, the larger band gap energy was attributed to quantum confinement effects in the small CZTS crystals fabricated by low thermodynamic energy [39]. The band gap energy of CZTS can be shifted to blue wavelengths due to the quantum confinement effect by ensuring that they have dimensions on the order of (or smaller than) the Bohr exciton radius [40]. The photovoltaic performance of the CZTS superstrate solar cell was evaluated under irradiation of AM 1.5, and the results are shown in figure 6(a). A gold thin film top electrode was patterned onto the CZTS film. We chose a thin gold film as a top electrode to collect photogenerated charge carriers. Considering our synthetic process temperature, gold contact is better than Mo contact because gold has a higher work function than Mo and should make a better contact metal for the p-type absorber [41]. The active area of the solar cell was 0.12 cm2 . The maximum power conversion efficiency of the cell was 1.2% with a short-circuit current density (Jsc ) of 4.1 mA cm−2 , an open-circuit voltage (Voc ) of 679.2 mV and a fill factor (FF) of 0.438. As discussed in a previous report, CIGS, NCLD of the CdS layer on a planar sputtered ZnO film was inadequate for depositing a uniform film. Practically, NCLD of CdS layer on a planar sputtered ZnO film was inadequate for depositing a conformal film. Also, unfortunately, the cell performance of the Au/CZTS/SILAR-processed CdS/ZnO film/ITO did not show diode characteristics. Thus, the cell performances
Figure 6. (a) Current density–voltage curves of the complete device
(Au/CZTS/CdS/ZnO NRs/ITO). The photovoltaic performance was measured under AM 1.5 (100 mW cm−2 ) conditions. (b) IPCE spectra of the CZTS/CdS/ZnO NRs device.
could not be compared directly to the planar cell structures; however, our light absorption spectra clearly revealed that the nanostructured CZTS cells displayed better light harvesting properties than a planar configuration. A further qualitative and quantitative study should be followed to compare the effects of planar and nanostructure configurations. Figure 6(b) shows the IPCE spectra obtained from a CZTS superstrate solar cell fabricated using a CdS/ZnO NR array. Jsc was calculated by convoluting the IPCE response using the following equation: Z Jsc = qφ ph IPCE(λ) dλ, where φph is the spectral photon flux density of the AM 1.5 spectral intensity distribution as a function of the incident light wavelength (λ) and q is the charge of the electron [42]. The calculated Jsc value from the IPCE spectra is 4.0 mA cm−2 , close to the measured Jsc value (4.1 mA cm−2 ) obtained from the current density–voltage curve, as shown in figure 6(a). The low IPCE value in the UV region indicated that no photocurrent was produced below 380 nm due to the absorption of UV light by the ZnO seed layer and ZnO NRs. Photocurrents were generated below 720 nm as a result of band-edge absorption in the CZTS film. These results agreed well with the UV–vis absorbance curve shown in figure 5. However, the IPCE 6
Nanotechnology 25 (2014) 065401
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These solar cells displayed improved cell performances. A homogeneous solution containing Cu, Zn, Sn and S molecular precursors was spin-coated onto CdS-coated ZnO NRs. A two-step annealing procedure was applied by heating in air at 150 ◦ C and 250 ◦ C to transform the precursors into a CZTS absorber layer, which completely infiltrated the CdS/ZnO NR array. The phase evolution of the CZTS layer was characterized at each synthetic step. The CZTS film showed a red shift in the absorption edge and the optical properties of the planar and nanostructured devices were analyzed to compare the light harvesting properties. This novel structured CZTS superstrate solar cell exhibited an improved efficiency of 1.2%, which is the highest power conversion efficiency among previously reported studies. Through minimizing the electrical losses at the bulk absorber region, photovoltaic performances can be expected to increase the development of low-cost solar devices.
values dropped continuously over the range 480–720 nm. A decrease at wavelengths higher than 480 nm was interpreted as indicative of a high probability of recombination in the bulk absorber [26, 43]. A comparison to the photovoltaic performances of other CZTS superstrate solar cells, as shown in table 1, indicated that our cell displayed remarkably enhanced Voc and FF values. The homogeneous precursor liquid coating may have facilitated the complete infiltration of the absorber material into the porous CdS/ZnO NR array without the formation of shunt paths that could deteriorate Voc and FF; however, our cell generated a rather low Jsc , which was attributed to the insufficient CZTS grain size (5–6 nm) due to the low annealing temperature and narrow space between NRs. In the bulk absorber region, grain boundaries can play an important role as recombination centers. The photogenerated charges should rapidly be collected to a p–n depleted junction from the bulk absorber film without the loss by bulk recombination. Small-sized grains of the solutionprocessed thin film absorber generally suffer from the low photocurrent problem [44, 45]. Previous studies involved the deposition of pre-synthesized CZTS nanoparticles onto TiO2 nanostructures via screen printing, spray deposition or doctor blading. Thermal heat treatment at high temperatures was then applied after deposition to induce sintering of individual CZTS nanoparticles. Our synthetic procedures were carried out at relatively low temperatures (250 ◦ C) to avoid undesired disruption of the CZTS/CdS interface due to CdS inter-diffusion. We have also tried to form a CZTS film at higher temperature than 250 ◦ C: however, cell performances were drastically degraded or showed electrically short behavior. Also, the conductivity of the ITO film can be degraded by thermal treatment in air [46, 47]. Despite the low thermal budget in the current process, our cell yielded the highest cell efficiency compared to previous studies. An improved solar cell performance, especially an improved photocurrent, may potentially be achieved by optimizing the deposition process for densifying and growing large CZTS crystallites. The non-stoichiometric elemental ratio between Cu, Zn, Sn and S should be controlled to guarantee a high cell performance. The atomic concentration analysis of each element revealed that the final CZTS film has Cu-rich (Cu/(Zn+Sn) ∼ 1.1) and Zn-rich (Zn/Sn ∼ 1.5) stoichiometry. Also, the overall contents of the residual organics or halogens originating from solvent, additives or functional groups of metal salt precursors, which can act as the recombination centers in bulk absorber and deteriorate the charge transport efficiency, were not detected over the detection limit. Although Cu-poor and Zn-rich compositions have been considered as an appropriate stoichiometry for high performance CZTS photovoltaics, further work is required to optimize the composition of the CZTS film to obtain a better photovoltaic performance [11]. In addition, control over the nanostructure dimension will be helpful for optimizing the light harvesting, charge recombination characteristics and photovoltaic performances.
Acknowledgments
This research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2013K000189) and National Research Foundation of Korea projects (2013R1A2A2A05005344, 2012R1A1A2039604 and 220-2011-1-C00033). References [1] Aldakov D, Lefrancois A and Reiss P 2013 J. Mater. Chem. C 1 3756–76 [2] Green M A, Emery K, Hishikawa Y, Warta W and Dunlop E D 2013 Prog. Photovol. 21 827–37 [3] Habas S E, Platt H A S, van Hest M F A M and Ginley D S 2010 Chem. Rev. 110 6571–94 [4] Ramasamy K, Malik M A and O’Brien P 2012 Chem. Commun. 48 5703–14 [5] Romanyuk Y E, Fella C M, Uhl A R, Werner M, Tiwari A N, Schnabel T and Ahlswede E 2013 Sol. Energy Mater. Sol. Cells C 119 181–9 [6] Li Z Q, Shi J H, Liu Q Q, Chen Y W, Sun Z, Yang Z and Huang S M 2011 Nanotechnology 22 265615 [7] Katagiri H, Jimbo K, Maw W S, Oishi K, Yamazaki M, Araki H and Takeuchi A 2009 Thin Solid Films 517 2455–60 [8] Huang S, Luo W J and Zou Z G 2013 J. Phys. D: Appl. Phys. 46 235108 [9] Patel M, Mukhopadhyay I and Ray A 2013 Semicond. Sci. Technol. 28 055001 [10] Zaberca O, Oftinger F, Chane-Ching J Y, Datas L, Lafond A, Puech P, Balocchi A, Lagarde D and Marie X 2012 Nanotechnology 23 185402 [11] Ki W and Hillhouse H W 2011 Adv. Energy Mater. 1 732–5 [12] Sun Y X et al 2013 J. Mater. Chem. A 1 6880–7 [13] Cho J W, Ismail A, Park S J, Kim W, Yoon S and Min B K 2013 ACS Appl. Mater. Inter. 5 4162–5 [14] Woo K, Kim Y and Moon J 2012 Energ. Environ. Sci. 5 5340–5 [15] Shin B, Gunawan O, Zhu Y, Bojarczuk N A, Chey S J and Guha S 2013 Prog. Photovol. 21 72–6 [16] Sun Y et al 2013 J. Mater. Chem. A 1 6880–7 [17] Kamat P V, Tvrdy K, Baker D R and Radich J G 2010 Chem. Rev. 110 6664–88
4. Conclusion
All-solution routes to the fabrication of CZTS thin film solar cells were reported using aligned ZnO nanostructures. 7
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[34] Todorov T and Mitzi D B 2010 Eur. J. Inorg. Chem. 2010 17–28 [35] Tak Y and Yong K 2005 J. Phys. Chem. B 109 19263–9 [36] Rau U and Schmidt M 2001 Thin Solid Films 387 141–6 [37] Minemoto T, Harada S and Takakura H 2012 Curr. Appl. Phys. 12 171–3 [38] Fernandes P A, Salom´e P M P and da Cunha A F 2009 Thin Solid Films 517 2519–23 [39] Tosun B S, Chernomordik B D, Gunawan A A, Williams B, Mkhoyan K A, Francis L F and Aydil E S 2013 Chem. Commun. 49 3549–51 [40] Liu W, Guo B, Wu X, Zhang F, Mak C and Wong K 2013 J. Mater. Chem. A 1 3182–6 [41] Akhavan V A, Goodfellow B W, Panthani M G, Reid D K, Hellebusch D J, Adachi T and Korgel B A 2010 Energ. Environ. Sci. 3 1600–6 [42] Merdes S, Saez-Araoz R, Ennaoui A, Klaer J, Lux-Steiner M C and Klenk R 2009 Appl. Phys. Lett. 95 213502–5 [43] Guo L, Zhu Y, Gunawan O, Gokmen T, Deline V R, Ahmed S, Romankiw L T and Deligianni H 2013 Prog. Photovol. [44] Panthani M G, Akhavan V, Goodfellow B, Schmidtke J P, Dunn L, Dodabalapur A, Barbara P F and Korgel B A 2008 J. Am. Chem. Soc. 130 16770–7 [45] Steinhagen C, Panthani M G, Akhavan V, Goodfellow B, Koo B and Korgel B A 2009 J. Am. Chem. Soc. 131 12554–5 [46] Nguyen T P, Le Rendu P, Dinh N N, Fourmigue M and Meziere C 2003 Synth. Met. 138 229–32 [47] Hu Y L, Diao X G, Wang C, Hao W C and Wang T M 2004 Vacuum 75 183–8
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Solution-processed Cu2ZnSnS4 superstrate solar cell using vertically aligned ZnO nanorods
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 065401 (http://iopscience.iop.org/0957-4484/25/6/065401) View the table of contents for this issue, or go to the journal homepage for more
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Nanotechnology Nanotechnology 25 (2014) 065401 (8pp)
doi:10.1088/0957-4484/25/6/065401
Solution-processed Cu2ZnSnS4 superstrate solar cell using vertically aligned ZnO nanorods Dongwook Lee and Kijung Yong Surface Chemistry Laboratory of Electronic Materials (SCHEMA), Department of Chemical Engineering, POSTECH, Pohang 790-784, Korea E-mail: [email protected] Received 3 October 2013, revised 27 November 2013 Accepted for publication 11 December 2013 Published 16 January 2014
Abstract
One-dimensional (1D) zinc oxide (ZnO) nanostructures are considered to be promising materials for use in thin film solar cells because of their high light harvesting and charge collection efficiencies. We firstly report enhanced photovoltaic performances in Cu2 ZnSnS4 (CZTS) thin film solar cells prepared using ZnO nanostructures. A CdS-coated, vertically well-aligned ZnO nanorod (NR) array was prepared via a hydrothermal reaction and nanocrystal layer deposition (NCLD) and was used as a transparent window/buffer layer in a CZTS thin film photovoltaic. A light absorber CZTS thin film was prepared on the CdS/ZnO NRs in air by depositing a non-toxic precursor solution that was annealed in two steps at temperatures up to 250 ◦ C. The crystallized CZTS phase completely infiltrated the CdS/ZnO NR array. The nanostructured ZnO array provided improved light harvesting behavior compared to a thin film configuration by measuring UV–vis transmittance spectroscopy. The prepared CZTS/CdS/ZnO NR device exhibited a solar energy conversion efficiency of 1.2%, which is the highest efficiency yet reported for nanostructured superstrate CZTS solar cells. Keywords: CZTS, ZnO, nanorod, molecular precursor solution, thin film solar cell, superstrate (Some figures may appear in colour only in the online journal)
1. Introduction
recent high prices of rare metals, such as indium and gallium, have driven the thin film device industry to seek methods for improving the efficiency with which such materials are used. Cu2 ZnSnS4 (CZTS) is an intriguing substitute as a lowcost light absorber for CIGS-based thin film solar cells because of the use of abundant chemical elements [6]. Each component of CZTS is abundant in the Earth’s crust (Cu: 50 ppm, Zn: 75 ppm, Sn: 2.2 ppm, S: 260 ppm) and shows a low toxicity relative to the elements present in CIGS (indium and selenium are present in the Earth’s crust at concentrations of less than 0.05 ppm) [7]. Kesterite CZTS has a direct band gap (1.4–1.5 eV) and a strong optical absorption coefficient (104 cm−1 ) [8–10]. This band gap energy is considered to be suitable for use in single-junction thin film solar cells. The structural analogy between CIGS and CZTS thin film solar cells has motivated the testing of methods for depositing
Inorganic semiconductors have attracted attention in recent years in the context of developing clean sustainable energy technologies. Compared to silicon-based photovoltaics, inorganic compound semiconducting materials have tunable direct band gaps and a high light absorption coefficient that permits the conversion of over 90% of incident sunlight into electric energy within a layer thickness of less than a few micrometers [1]. Among the inorganic semiconductors tested thus far, chalcopyrite Cu(In1−x Gax )(Se1−y Sy )2 (CIGS) offers a promising absorber material for thin film solar cells. The best CIGS thin film solar cell has yielded a 20.4% power conversion efficiency [2]. This cell was prepared using vacuum-based deposition processes that currently do not permit low-cost mass production and scaled-up fabrication processes [3–5]. The 0957-4484/14/065401+08$33.00
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c 2014 IOP Publishing Ltd
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Nanotechnology 25 (2014) 065401
D Lee and K Yong
Table 1. Current status of the photovoltaic performances of superstrate-type CZTS solar cells prepared using metal oxide nanostructures
(3-D CZTS solar cell). Structure Carbon paste/CZTS/In2 S3 /TiO2 NP/FTO Carbon paste/CZTS/TiO2 NP/TiO2 NP/TCO Mo/CZTS/In2 S3 /TiO2 NP/FTO Graphite/CZTS/CdS/TiO2 NP/FTO
Au/CZTS/CdS/ZnO NR/ITO
Deposition method Screen printing
Efficiency (%) Voc (mV) 0.60 250
Jsc (mA cm−2 ) 8.76
FF 0.27
Ref. [28]
Spray deposition
0.51
564
2.85
0.43
[29]
Doctor blading
0.55
240
7.82
0.29
[30]
Precursor solution deposition (spray deposition) Precursor solution deposition (spin-coating)
1.131
445.9
6.794
0.374
[31]
1.2
679.2
4.1
0.438
CIGS thin films for the fabrication of CZTS thin films. Among the various deposition methods, non-vacuum coating techniques, which are expected to facilitate the large-scale, high-throughput process, have been applied to the preparation of CZTS films. Solution-based deposition approaches can be advantageous in that they offer the efficient use of a precursor and are compatible with a broad selection of substrates, unlike vacuum-based deposition methods involving evaporation and sputtering [4]. Until now, a power conversion efficiency gap between CIGS and CZTS solar cells has persisted. A variety of synthetic processes have been proposed in an effort to improve the photovoltaic performances of CZTS thin film solar cells. For example, high-quality CZTS absorber films with fine grain structures have been obtained via a solution process by tuning the identities of the metal precursor, chalcogen precursor, solvents, additives, annealing temperature and annealing gas [11–14]. The current champion cell efficiency of 8.4% was recorded by IBM using a vacuum thermal evaporation method [15]. Unfortunately, this hydrazine-based synthetic method is not optimal from a manufacturing perspective. Hydrazine-based precursor solutions pose several safety and environmental concerns, and the deposition and crystallization processes must be handled entirely under an inert atmosphere to prevent oxygen incorporation. Safe and non-toxic solution-based methods for growing CZTS thin films have been proposed by Yuxiu et al based on the use of an ethanol solution containing amine additives to produce CZTS thin films [16]. Their device exhibited a power conversion efficiency of 5.36%. Another group demonstrated an environmentally benign route in which metal salts and thiourea were dissolved into dimethyl sulfoxide as a non-toxic solvent, yielding a power conversion efficiency of 4.1% after selenization [11]. The conversion efficiencies of thin film solar cells may be further improved by combining the advantages of a nanostructure with thin film photovoltaics [17]. These approaches are currently in the early stages of development. Nanostructures with controllable dimensions and aligned morphologies can potentially provide more efficient charge generation, collection and transfer due to the large junction area and the short collection distance to the interface [18]. Semiconductor metal oxides,
for example SnO2 , TiO2 and ZnO, have attracted attention as possible materials for use in photoanodes in quantum dot or dye-sensitized solar cells and bulk heterojunction solar cells [19–24]. Low-quality solution-processed CIGS or CZTS thin film absorbers tend to be characterized by short minority carrier diffusion lengths and a high recombination loss at grain boundaries [25, 26]. These issues can be ameliorated by introducing an interpenetrated absorber material with a nanostructured photoanode. Superstrate cell structures may be used to form nanojunctions between an absorber and a buffer/window structure [20, 22, 27]. Several groups have developed CZTS solar cells using three-dimensional (3-D) TiO2 nanoparticles [28–31]. TiO2 is the most commonly used metal oxide photoanode in a variety of solar cells. Table 1 summarizes the recent methodologies and photovoltaic performances of the nanostructured CZTS thin film devices fabricated by solution approaches. Most nanostructured CZTS superstrate photovoltaics are prepared in a configuration similar to the following: metal electrode/CZTS absorber/buffer layer/metal oxide nanostructure/metal electrode. The open-circuit voltage (Voc ) and fill factor (FF) of these reported cells remain low and hinder the preparation of high-efficiency cells. Herein, we report a route to the fabrication of a CZTS superstrate structure using well-aligned CdS-coated ZnO nanostructures. Our goal was to improve the photovoltaic performances of nanostructured CZTS solar cells using hydrothermally grown ZnO NRs and a low heat treatment temperature for the preparation of CZTS absorbers. The CdS and CZTS were deposited without the use of a vacuum environment via nanocrystal layer deposition (NCLD) and molecular precursor solution methods, respectively. The synthetic procedure used for the solar cell fabrication process and the device operating mechanism is illustrated schematically in figure 1. Scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis revealed that the morphologies and crystal phases of the nanostructures and the evolution of the CZTS absorber film from an as-coated precursor film. The optical properties were observed using UV–visible (UV–vis) transmittance spectroscopy. The current density and external quantum efficiency spectra of the CZTS superstrate solar cell were collected using a sweeping applied voltage under irradiation. Our superstrate CZTS solar cell recorded a 1.2% efficiency, the highest value yet obtained from nanostructured CZTS superstrate cells. 2
Nanotechnology 25 (2014) 065401
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Figure 1. Schematic illustration of the synthetic procedure used for the fabrication of ZnO NR-based CZTS thin film solar cells.
2. Experimental details
were maintained at this temperature for 10 min. All annealing processes were conducted in air. After annealing, the CZTScoated samples were cooled naturally to room temperature in an air environment. This coating–drying–annealing cycle was repeated twice. Complete photovoltaic devices were fabricated by depositing a patterned 100 nm thick gold film by RF-magnetron sputtering. Finally, silver paste was squeezed onto the Au and ITO electrodes and then dried in a preheated electric oven at 60 ◦ C. The total active area was 0.12 cm2 .
2.1. Materials
All chemicals were purchased from Sigma-Aldrich and were used as received. Zinc nitrate hexahydrate (Zn(NO3 )2 ·6H2 O, 98%), ammonium hydroxide (28 wt% NH3 in water, 99.99%), cadmium chloride (CdCl2 , technical grade), thioacetamide (TAA, C2 H5 NS, 99%), copper iodide (CuI, 99.999%), zinc acetate (Zn(OAc)2 , 99.99%), tin chloride (II) (SnCl2 , 98%) and pyridine (C5 H5 N, ≥ 99.0%) were used as received. Prior to material deposition, soda lime glass (SLG) substrates were ultrasonically cleaned for 10 min with detergent, acetone and ethanol, respectively.
2.4. Characterization
The morphologies of the nanostructures and films were investigated using field emission scanning electron microscopy (FE-SEM, XL30S, Philips). The crystal structure properties were analyzed using x-ray diffraction (XRD, Max-2500V, Rigaku) techniques. The optical properties of the nanostructures were evaluated by measuring the transmittance spectra using UV–visible spectroscopy (Jasco, V-530). The performance of the solar cell was measured using a solar simulator equipped with a 300 W Xenon lamp (Newport, USA) and an electrical analysis system (Keithley 2400). The power of the simulated light was calibrated to AM 1.5 (100 mW cm−2 ) using a standard silicon solar cell (PV Measurement Inc.). The incident photon-to-current conversion efficiency (IPCE) as a function of wavelength was measured using a photomodulation spectroscopic setup (model Merlin, Oriel).
2.2. Preparation of CdS-coated ZnO nanorod arrays
The ZnO NRs and a CdS buffer layer were grown on an ITO-coated SLG substrate via a hydrothermal reaction and NCLD [32]. The experimental conditions are described in detail in a previous report [22]. The NCLD deposition time was increased to 50 min in the current work to enable the preparation of a uniform CdS shell layer coating on the ZnO NRs. 2.3. Fabrication of the CZTS/CdS/ZnO photovoltaic devices
A CZTS light absorber layer was coated on a CdS-coated ZnO NRs array. The metal precursors (Cu, Zn and Sn) and chalcogen (S) precursors were dissolved in pyridine solvent [33]. SnCl2 (0.16 M), TAA (3.2 M), CuI (0.32 M) and Zn(OAc)2 (0.16 M) were mixed in pyridine one at a time to prevent the undesired formation of a precipitant. A yellowish transparent precursor solution was obtained by stirring for about 10 min. The Cu:Zn:Sn:S ratio was set at 2:1:1:20 to compensate for sulfur loss during annealing. One milliliter of the precursor solution was dropped onto the samples, which were then spin-coated at 1300 rpm for 30 s. The samples were placed on a hot plate preheated at 150 ◦ C for 10 min. After the first drying procedure, the samples were immediately moved to a hot plate preheated at 250 ◦ C and
3. Results and discussion
Figure 2 shows the morphological changes of the bare ZnO NR arrays after the deposition of CdS and CZTS layers via NCLD and non-vacuum solution deposition methods, respectively. A hydrothermal reaction of aqueous Zn(NO3 )2 and ammonium hydroxide was used for ZnO NRs growth at 95 ◦ C for 60 min. The ZnO thin film was sputtered on a transparent ITO-coated SLG substrate as a seed layer prior to NR growth. The CdS buffer layer was selectively deposited via NCLD on the ZnO NRs at room temperature for 50 min to form a shell layer on the ZnO NRs. It is well known that the n-type CdS is extensively 3
Nanotechnology 25 (2014) 065401
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Figure 2. Cross-sectional and top-view SEM images of (a), (b) bare ZnO NRs, (c), (d) CdS-coated ZnO NRs prepared via NCLD with a 50 min reaction time and (e), (f) CdS/ZnO NRs prepared after multiple spin-coating of CZTS.
figures 2(e) and (f). After 1 CZTS coating cycle, the degree to which the CZTS film penetrated the NR array was insufficient to extend to the bottom of the ZnO NRs, and the resultant film surface displayed frequent defects, such as cracks and voids, which reduced the Voc and FF values [34]. Two CZTS coating cycles provided a uniformly infiltrated film with no cracks or voids, as shown in figures 2(e) and (f). The deposited CZTS absorber layer resembled a pile of very small nanocrystals a few nanometers in size. An unnecessary thick CZTS overlayer formed if more than three coating cycles were applied. The crystal structures of the nanostructures and the evolution of the CZTS film from a precursor state were characterized using XRD analysis, and the results are presented in figures 3(a) and (b). Bare ZnO nanorods exhibited a sharp peak at 34.4 ◦ , corresponding to preferential growth of the ZnO NRs along the c axis (JCPDS-792205) [35]. After NCLD deposition, hexagonal, polycrystalline CdS peaks were observed (JCPDS-411049). These results confirmed that the uniformly coated shell layers on the ZnO nanorods were composed of polycrystalline CdS. A CZTS film was deposited on the CdS/ZnO nanorod array using the thioacetamide (TAA)-based
employed as a buffer layer in highly efficient CZTS thin film solar cells to provide enhancements in the device performances by avoiding shunt path and widening the depletion region [4]. In the superstrate configuration, incident light passes through the glass, ZnO film and ZnO NRs, respectively. Therefore, minimizing light loss, for example, by minimizing reflection, is important for the solar energy harvesting efficiency. For this reason, we aimed to prepare a transparent ZnO template 500–600 nm in length. At short reaction times, well-aligned ZnO NRs oriented along the c axis were grown on a seed layer (figures 2(a) and (b)). The gap distance between each NR tip was estimated to be about 1–200 nm. As shown in figures 2(c) and (d), after a 50 min NCLD reaction period, a uniform 30 nm CdS shell layer had formed on the ZnO NRs. The CdS shell coating layer uniformly increased the nanorod’s diameter. We previously reported the use of NCLD techniques (25 min reaction period) for the fabrication of CIS solar cells. In the current study, the NCLD time was increased to 50 min to obtain a more uniform and thicker CdS shell layer. After CZTS deposition via the molecular precursor solution method, morphological changes were observed, as shown in 4
Nanotechnology 25 (2014) 065401
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Figure 4. Raman analysis of the spin-coated CZTS films on glass substrate annealed at 150 ◦ C and 250 ◦ C.
to Cu, Zn, Sn, S or Si. The dissolved Cu ions may have combined with four TAA molecules to form a chelated metal–organic complex. Upon annealing the precursor film in air at 150 ◦ C, the intensities of the precursor decreased due to the decomposition of the TAA-complex. The CZTS and CuS structures formed simultaneously after drying at 150 ◦ C (JCPDS-260575). The intensity and breadth of the main CZTS peaks ((112), (220) and (312)) and the presence of a binary metal phase suggested that the conversion from a precursor to a CZTS phase was incomplete. Samples treated with a second annealing process at 250 ◦ C displayed an XRD pattern that agreed well with the reference CZTS kesterite phase and did not include precursor or impurity peaks. The estimated average grain size of the (112)-oriented CZTS crystals was about 5–6 nm using the Scherrer equation. Larger highly crystalline CZTS layers may be obtained by annealing at higher temperatures; however, the possibility of thermal inter-diffusion of CdS must be carefully considered [36, 37]. Because the deposition of CZTS was carried out under air conditions at low temperature, Raman analysis was used to reveal the presence of secondary phases such as metal chalcogenides including ZnS, Cux Sy and SnS, which are not well distinguished in XRD analysis. As shown in figure 4, the sample treated at 150 ◦ C exhibits a broad shoulder near 287 cm−1 and a peak located at 335 cm−1 . Fernandes et al reported a shoulder between 250 cm−1 and 300 cm−1 is related to the convolution of peaks corresponding to Sn2 S3 , CZTS, Cu2−x S and ZnS [38]. Upon annealing the film at 250 ◦ C, this broad shoulder disappeared and only the CZTS peak was observed. Synthetically, these XRD and Raman analysis results indicate the phase purity of the final CZTS absorber. The UV–visible light transmittance spectra over the range 300–1100 nm were used to compare the optical absorbance properties of the planar and nanostructured samples (figure 5). The quantity of the deposited CZTS was fixed by applying two coating cycles. The incident light was focused on the back side of the SLG. The observed spectra indicated that the ZnO nanorods were more transparent in the visible range than the flat ZnO seed layer. These results indicated that the scattering loss of visible light in the nanostructured ZnO could be reduced
Figure 3. (a) XRD patterns of nanostructures obtained through various deposition routes: bare ZnO NRs, CdS-coated ZnO NRs and CZTS-infiltrated CdS/ZnO NRs. (b) XRD patterns of the CZTS-coated Si annealed at room temperature, 150 ◦ C and 250 ◦ C, revealing the evolution of the weakly oriented polycrystalline CZTS peaks.
solution reaction method. Achim et al used thioacetamide (TAA) as a sulfur precursor to minimize organic impurities and to reduce the temperature required for synthesis [33]. Their analysis suggested that the CZTS formed upon heating at approximately 105 ◦ C. They did not report the solar cell performance in that study. A yellowish transparent CZTS precursor solution containing CuI, Zn(OAc)2 , SnCl2 and TAA yielded a pH of 5. The CdS and ZnO templates did not appear to deteriorate, even after the substrate temperature had been increased to 250 ◦ C. Figure 3(a) shows that no additional peaks appeared after the second annealing step during CZTS film deposition aside from the peaks corresponding to the CdS-coated ZnO sample. This observation was attributed to the concealment of the CZTS peaks by the relatively strong CdS and ZnO peaks due to the small sizes of the CZTS crystallites. Proof of the formation of a CZTS phase was sought by spin-coating a CZTS precursor solution onto a Si substrate and analyzing this layer using temperature-dependent XRD measurements (figure 3(b)). The as-coated precursor film, which included metal and chalcogen precursors, displayed peaks corresponding to copper tetrakis-thioacetamide chloride (JCPDS-130613). Several unknown peaks were also observed, which did not match any reported materials corresponding 5
Nanotechnology 25 (2014) 065401
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Figure 5. UV–visible light transmittance spectra of the various nanostructures, including SLG, ZnO film, ZnO NRs, CdS-coated ZnO NRs, CZTS/CdS/ZnO film and CZTS/CdS/ZnO NRs.
prior to reaching the CdS/CZTS interface. The deposition of CdS on ZnO increased the visible light absorption of the core/shell NRs, due to the low band gap energy of CdS (2.4 eV). Deposition of the CZTS absorber enhanced the light harvesting at the well-aligned ZnO nanostructured interface, indicating that the incident light was more efficiently trapped in the absorber than in the flat film structure. The estimated band gap energy of the CZTS film (1.6–1.7 eV) was larger than the bulk value (1.4–1.5 eV). The existence of undesired secondary phases or non-stoichiometric composition can shift the band gap energy to the higher energy side. As we mentioned above, there is no evidence for the existence of secondary phases in the final CZTS film. Using energy-dispersive x-ray spectroscopy (EDX), Cu-rich (Cu/(Zn+Sn) ∼ 1.1) and Zn-rich (Zn/Sn ∼ 1.5) stoichiometry was found. Also, the larger band gap energy was attributed to quantum confinement effects in the small CZTS crystals fabricated by low thermodynamic energy [39]. The band gap energy of CZTS can be shifted to blue wavelengths due to the quantum confinement effect by ensuring that they have dimensions on the order of (or smaller than) the Bohr exciton radius [40]. The photovoltaic performance of the CZTS superstrate solar cell was evaluated under irradiation of AM 1.5, and the results are shown in figure 6(a). A gold thin film top electrode was patterned onto the CZTS film. We chose a thin gold film as a top electrode to collect photogenerated charge carriers. Considering our synthetic process temperature, gold contact is better than Mo contact because gold has a higher work function than Mo and should make a better contact metal for the p-type absorber [41]. The active area of the solar cell was 0.12 cm2 . The maximum power conversion efficiency of the cell was 1.2% with a short-circuit current density (Jsc ) of 4.1 mA cm−2 , an open-circuit voltage (Voc ) of 679.2 mV and a fill factor (FF) of 0.438. As discussed in a previous report, CIGS, NCLD of the CdS layer on a planar sputtered ZnO film was inadequate for depositing a uniform film. Practically, NCLD of CdS layer on a planar sputtered ZnO film was inadequate for depositing a conformal film. Also, unfortunately, the cell performance of the Au/CZTS/SILAR-processed CdS/ZnO film/ITO did not show diode characteristics. Thus, the cell performances
Figure 6. (a) Current density–voltage curves of the complete device
(Au/CZTS/CdS/ZnO NRs/ITO). The photovoltaic performance was measured under AM 1.5 (100 mW cm−2 ) conditions. (b) IPCE spectra of the CZTS/CdS/ZnO NRs device.
could not be compared directly to the planar cell structures; however, our light absorption spectra clearly revealed that the nanostructured CZTS cells displayed better light harvesting properties than a planar configuration. A further qualitative and quantitative study should be followed to compare the effects of planar and nanostructure configurations. Figure 6(b) shows the IPCE spectra obtained from a CZTS superstrate solar cell fabricated using a CdS/ZnO NR array. Jsc was calculated by convoluting the IPCE response using the following equation: Z Jsc = qφ ph IPCE(λ) dλ, where φph is the spectral photon flux density of the AM 1.5 spectral intensity distribution as a function of the incident light wavelength (λ) and q is the charge of the electron [42]. The calculated Jsc value from the IPCE spectra is 4.0 mA cm−2 , close to the measured Jsc value (4.1 mA cm−2 ) obtained from the current density–voltage curve, as shown in figure 6(a). The low IPCE value in the UV region indicated that no photocurrent was produced below 380 nm due to the absorption of UV light by the ZnO seed layer and ZnO NRs. Photocurrents were generated below 720 nm as a result of band-edge absorption in the CZTS film. These results agreed well with the UV–vis absorbance curve shown in figure 5. However, the IPCE 6
Nanotechnology 25 (2014) 065401
D Lee and K Yong
These solar cells displayed improved cell performances. A homogeneous solution containing Cu, Zn, Sn and S molecular precursors was spin-coated onto CdS-coated ZnO NRs. A two-step annealing procedure was applied by heating in air at 150 ◦ C and 250 ◦ C to transform the precursors into a CZTS absorber layer, which completely infiltrated the CdS/ZnO NR array. The phase evolution of the CZTS layer was characterized at each synthetic step. The CZTS film showed a red shift in the absorption edge and the optical properties of the planar and nanostructured devices were analyzed to compare the light harvesting properties. This novel structured CZTS superstrate solar cell exhibited an improved efficiency of 1.2%, which is the highest power conversion efficiency among previously reported studies. Through minimizing the electrical losses at the bulk absorber region, photovoltaic performances can be expected to increase the development of low-cost solar devices.
values dropped continuously over the range 480–720 nm. A decrease at wavelengths higher than 480 nm was interpreted as indicative of a high probability of recombination in the bulk absorber [26, 43]. A comparison to the photovoltaic performances of other CZTS superstrate solar cells, as shown in table 1, indicated that our cell displayed remarkably enhanced Voc and FF values. The homogeneous precursor liquid coating may have facilitated the complete infiltration of the absorber material into the porous CdS/ZnO NR array without the formation of shunt paths that could deteriorate Voc and FF; however, our cell generated a rather low Jsc , which was attributed to the insufficient CZTS grain size (5–6 nm) due to the low annealing temperature and narrow space between NRs. In the bulk absorber region, grain boundaries can play an important role as recombination centers. The photogenerated charges should rapidly be collected to a p–n depleted junction from the bulk absorber film without the loss by bulk recombination. Small-sized grains of the solutionprocessed thin film absorber generally suffer from the low photocurrent problem [44, 45]. Previous studies involved the deposition of pre-synthesized CZTS nanoparticles onto TiO2 nanostructures via screen printing, spray deposition or doctor blading. Thermal heat treatment at high temperatures was then applied after deposition to induce sintering of individual CZTS nanoparticles. Our synthetic procedures were carried out at relatively low temperatures (250 ◦ C) to avoid undesired disruption of the CZTS/CdS interface due to CdS inter-diffusion. We have also tried to form a CZTS film at higher temperature than 250 ◦ C: however, cell performances were drastically degraded or showed electrically short behavior. Also, the conductivity of the ITO film can be degraded by thermal treatment in air [46, 47]. Despite the low thermal budget in the current process, our cell yielded the highest cell efficiency compared to previous studies. An improved solar cell performance, especially an improved photocurrent, may potentially be achieved by optimizing the deposition process for densifying and growing large CZTS crystallites. The non-stoichiometric elemental ratio between Cu, Zn, Sn and S should be controlled to guarantee a high cell performance. The atomic concentration analysis of each element revealed that the final CZTS film has Cu-rich (Cu/(Zn+Sn) ∼ 1.1) and Zn-rich (Zn/Sn ∼ 1.5) stoichiometry. Also, the overall contents of the residual organics or halogens originating from solvent, additives or functional groups of metal salt precursors, which can act as the recombination centers in bulk absorber and deteriorate the charge transport efficiency, were not detected over the detection limit. Although Cu-poor and Zn-rich compositions have been considered as an appropriate stoichiometry for high performance CZTS photovoltaics, further work is required to optimize the composition of the CZTS film to obtain a better photovoltaic performance [11]. In addition, control over the nanostructure dimension will be helpful for optimizing the light harvesting, charge recombination characteristics and photovoltaic performances.
Acknowledgments
This research was supported by the Converging Research Center Program through the Ministry of Education, Science and Technology (2013K000189) and National Research Foundation of Korea projects (2013R1A2A2A05005344, 2012R1A1A2039604 and 220-2011-1-C00033). References [1] Aldakov D, Lefrancois A and Reiss P 2013 J. Mater. Chem. C 1 3756–76 [2] Green M A, Emery K, Hishikawa Y, Warta W and Dunlop E D 2013 Prog. Photovol. 21 827–37 [3] Habas S E, Platt H A S, van Hest M F A M and Ginley D S 2010 Chem. Rev. 110 6571–94 [4] Ramasamy K, Malik M A and O’Brien P 2012 Chem. Commun. 48 5703–14 [5] Romanyuk Y E, Fella C M, Uhl A R, Werner M, Tiwari A N, Schnabel T and Ahlswede E 2013 Sol. Energy Mater. Sol. Cells C 119 181–9 [6] Li Z Q, Shi J H, Liu Q Q, Chen Y W, Sun Z, Yang Z and Huang S M 2011 Nanotechnology 22 265615 [7] Katagiri H, Jimbo K, Maw W S, Oishi K, Yamazaki M, Araki H and Takeuchi A 2009 Thin Solid Films 517 2455–60 [8] Huang S, Luo W J and Zou Z G 2013 J. Phys. D: Appl. Phys. 46 235108 [9] Patel M, Mukhopadhyay I and Ray A 2013 Semicond. Sci. Technol. 28 055001 [10] Zaberca O, Oftinger F, Chane-Ching J Y, Datas L, Lafond A, Puech P, Balocchi A, Lagarde D and Marie X 2012 Nanotechnology 23 185402 [11] Ki W and Hillhouse H W 2011 Adv. Energy Mater. 1 732–5 [12] Sun Y X et al 2013 J. Mater. Chem. A 1 6880–7 [13] Cho J W, Ismail A, Park S J, Kim W, Yoon S and Min B K 2013 ACS Appl. Mater. Inter. 5 4162–5 [14] Woo K, Kim Y and Moon J 2012 Energ. Environ. Sci. 5 5340–5 [15] Shin B, Gunawan O, Zhu Y, Bojarczuk N A, Chey S J and Guha S 2013 Prog. Photovol. 21 72–6 [16] Sun Y et al 2013 J. Mater. Chem. A 1 6880–7 [17] Kamat P V, Tvrdy K, Baker D R and Radich J G 2010 Chem. Rev. 110 6664–88
4. Conclusion
All-solution routes to the fabrication of CZTS thin film solar cells were reported using aligned ZnO nanostructures. 7
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