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Abstract
A hybrid silver nanowires (AgNWs)/indium tin oxide (ITO) contact was used as a transparent back-electrode to fabricate a bifacial CdS/CdTe thin-film solar cell. The photovoltaic properties of the bifacial CdS/CdTe thin-film solar cell were investigated under front and back illumination conditions. The hybrid AgNWs/ITO back contact changed the average conversion efficiency from 0.4% (front) and 3.5% (rear) to 8.1% (front) and 0.9% (rear), respectively, improving the sum efficiency from 3.9% (ITO-only) to 9.0%. Our research demonstrates the use of a nanowire network as a transparent electrode in CdS/CdTe thin-film solar cells for bifacial or tandem applications.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Conventional solar cells generate energy by converting the incident light received on one side. This approach depends on the angle of incidence of the light according to the position of the sun during the day. Thus, a solar tracking system is commonly used to utilize the sunlight effectively without wasting light depending on the position of the sun [1]. The advantage of a solar tracking system is that it optimizes the exposure of the solar cell to sunlight and allows maximum energy to be generated. However, a solar tracking system is expensive and has an issue, including a low tracking performance in harsh environments. On the other hand, a bifacial solar cell is relatively inexpensive with less space required for installation and can provide a higher power conversion efficiency (PCE) than a monofacial solar cell without the need for the additional installation of a tracking system because it can utilize both the reflected light directed towards the back contact side and the wasted light of which the incidence angle is not suitable for the monofacial cell [2]. P-type mono-passivated emitter rear cell (PERC) bifacial solar cells provide efficiency gains between 10% and 30% that are albedo dependent. Ohtsuka et al. reported a single-crystalline bifacial Si solar cell with efficiencies of 21.3% and 19.8% under front and rear illumination, respectively [3].
Unlike Si-based solar cells, thin-film solar cells are attracting attention because of their versatile applications, high specific power, and low energy payback time [4,5]. Research of the bifacial thin-film cell structure is necessary to ensure the effective generation of electricity for the reasons described above and to optimize the structure of the thin-film solar cell as the top cell for tandem applications [6,7]. The tandem structure requires the light to penetrate the top cell and arrive at the bottom cell. In addition, the transparency and electrical conductivity of the back contact needs to be high. Bifacial thin-film solar cells have been reported using copper zinc tin sulfide (CZTS) and copper indium gallium selenide (CIGS) solar cells. For example, Ge et al. fabricated a CZTS bifacial solar cell and reported front and rear efficiencies of 2.9% and 1.3%, respectively [8]. Nakada et al. fabricated a CIGS bifacial solar cell with front and rear efficiencies of 12.6% and 4.9%, respectively [9]. However, bifacial CdTe thin-film solar cells have been rarely investigated.
Cadmium telluride (CdTe) is a semiconductor material that has been successfully commercialized among the numerous thin-film solar cells with a reported record cell efficiency of 22.1% [10]. In general, the CdTe solar cell has a superstrate structure consisting of a glass/TCO (transparent conducting oxide)/CdS/CdTe/back contact metal [11]. Cadmium sulfide (CdS) is an n-type semiconductor material with a ~2.4 eV band gap and is deposited using sputtering or chemical bath deposition (CBD) methods [12,13]. The CdS layer is used as a window layer to allow the light to pass through and form the p-n junction with the CdTe layer. CdTe has a ~1.5 eV direct band gap, high absorption coefficient (>5 × 105 cm−1), and is deposited using a method including close spaced sublimation (CSS), evaporation sputtering, and vapor transport deposition (VTD) [14–17]. The CdTe solar cell is estimated to have a theoretical power conversion efficiency (PCE) of ~29% [18]. In one of the reported bifacial studies for CdTe solar cells, Tiwari et al. applied ITO as a transparent back contact and reported front and rear efficiencies of 7.9% and 1%, respectively [19]. An important aspect of the fabrication of bifacial CdTe solar cells is to determine the transparent material for use in Ohmic contact with p-CdTe as the back contact. CdTe has an electron affinity of ~4.5 eV, and therefore, a metal with a high work function is required to form an Ohmic contact; thus, establishing sufficient Ohmic contact with p-CdTe is problematic [14]. Alternatively, producing a p+-CdTe layer by doping the CdTe with Cu, Ag, or As for the Ohmic contact has been widely used [20,21]. There are difficulties associated with forming a good back Ohmic contact with CdTe in bifacial solar cells using only ITO. Therefore, studies have been conducted to improve the contact properties by adding a p-type doping material or buffer layer between the CdTe and ITO layers. For instance, Romeo et al. added a thin copper layer between the CdTe and ITO and reported efficiencies of 10% and 3.5% at the front and rear, respectively [22]. Marsillac et al. obtained front and rear efficiencies of 5.7% and 5.0% using a ZnTe:N buffer layer and ITO as a transparent back-electrode [23].
Apart from establishing good Ohmic contact between the CdTe layer and the back-electrode, it is also important to ensure that this electrode has high optical transmittance and electrical conductivity. In the case of ITO, CNT, or a thin metal layer, the electrical conductivity improves as the thickness increases, but the loss in optical transmittance leads to a trade-off between the electrical conductivity and optical transmittance. As a solution to this problem, Kuang et al. reported a novel architecture consisting of transparent electrodes with a high-aspect-ratio metallic nanoribbon [24]. Also, silver nanowires (AgNWs) have been studied in an attempt to meet these requirements. NWs with a high aspect ratio are known to form percolating networks and have both high optical transmittance and low sheet resistance [25,26]. They also have a work function (~4.3 eV) similar to that of Cu (~4.7 eV) and can act as a p-type dopant [27]. The high mechanical flexibility of AgNWs makes them suitable for use in flexible and rollable applications [28]. Other advantages include easy scaling and a low manufacturing cost due to the possibility of roll-to-roll processing. A number of studies have employed AgNWs as transparent electrodes in organic solar cells; however, their incorporation in CdTe solar cells as a back contact electrode has not been reported. Similarly, Liang et al. used CuNWs and graphene as a back-electrode of a CdTe cell, but did not report a rear-side efficiency [29]. To the best of our knowledge, the use of NWs as a transparent back-electrode to fabricate bifacial CdTe thin-film solar cells has not yet been reported. In this study, we analyzed the optical transmittance and electrical conductivity of the hybrid AgNWs/ITO layer by controlling the amount of sprayed AgNWs. We then fabricated bifacial CdS/CdTe solar cells using the hybrid transparent electrodes and investigated the photovoltaic properties of the bifacial CdTe thin-film solar cells.
2. Experimental details
Figures 1(a) and 1(b) show schematics of the layers in a CdTe cell. This cell was fabricated according to the following process. A 2 cm × 3 cm borosilicate glass substrate (Corning Willow glass) with a 100-μm thickness was ultrasonically cleaned in acetone and isopropyl alcohol (IPA). Next, 200-nm-thick ITO and 120-nm-thick zinc oxide (ZnO) were successively deposited on a glass substrate using an RF sputtering system. Then, a CdS layer was deposited by the CBD method. Solutions of CdCl2 (6 mM), thiourea (90 mM), and NH4Cl (15 mM) with deionized (DI) water were stirred for 12 h. These solutions (60 mL) were mixed in a beaker and 28% NH4OH (7.6 mL) solution was added. The 300-nm CdS layer was deposited on the ZnO/ITO/ glass substrate at 75 °C with stirring at 350 rpm for 1 h. An H2SO4 solution was used to remove the CdS layer deposited on the rear-side of the glass substrate. Then, polycrystalline CdTe was deposited on the substrate using CdTe powder (99.999%, Johnson Matthey Co.). The deposition of the CdTe thin-film layer on the CdS layer was achieved using the close-spaced sublimation (CSS) method under ambient Ar atmosphere at a pressure of 0.5 torr for 3 min. The temperatures of the source and substrate were 600 and 540 °C, respectively. This was followed by post-deposition processes involving nitric-phosphoric (NP) etching and CdCl2 activation. The substrate was immersed into the NP etching solution, which was a mixture of nitric acid, phosphoric acid, and DI water (HNO3: H3PO4: H2O = 1/3: 71: 29 in volumetric ratio, Sigma-Aldrich), for 30 s. Then, the substrate was rinsed using DI water and dried under N2 gas. After the NP etching process, CdCl2 activation was carried out by immersing the samples (glass/ITO/i-ZnO/CdS/CdTe) in a saturated CdCl2 solution and annealing them at 385 °C, in an ambient air for 30 min. Then, the NP etching process was repeated on the CdCl2-activated samples. Subsequently, the AgNWs were dispersed in IPA to form a solution with a concentration of 0.25 wt%, which was used to form the AgNWs/ITO electrodes on the CdTe layer by a spray coating method. The substrate was placed on a hot plate of which the temperature was 60 °C, and the distance between the nozzle and the substrate was set to 20 cm. Then, a 200-nm ITO layer was deposited on the AgNW layer using an RF sputtering method [Fig. 1(b)]. Subsequent to the film deposition, the samples were annealed at 300 °C under N2 for 10 min. The working area of the device is 0.011 cm2. The fabricated devices with AgNWs/ITO were evaluated after storage in an air ambient for a week.
Fig. 1 Schematics of the layers in the bifacial CdS/CdTe thin-film solar cell. (a, b) The glass/ITO/i-ZnO/CdS/CdTe layers were deposited successively. (c) Optical microscope image of the spray-coated AgNWs, (d) The AgNWs were spray-coated onto the glass/ITO/i-ZnO/CdS/CdTe substrate and the ITO layer was subsequently deposited. (e) Energy band alignment of ITO/ZnO/CdS/CdTe/AgNW/ITO.
A spectrophotometer (Cary 5000, Varian) was used to measure the optical transmittance of the glass substrates onto which the AgNWs were sprayed and ITO was successively deposited. The sheet resistance of the hybrid AgNWs/ITO structure was compared before and after thermal annealing. Scanning electron microscopy (SEM; FESEM S-4700, Hitachi) was used to characterize the surface morphology and cross-sectional structure. The photovoltaic characteristics were assessed using a solar simulator (WACOM WXS-155S-10, AM1.5G, 100 mW/cm2) and a source meter (Keithley 2400). Scan direction was from + 1 V to −0.4 V with a scan rate of 0.1 V/sec. The measurement of the conversion efficiency under rear-side illumination was the same as that under front-side illumination. The solar cell was turned over, which only changed the direction of the incident light.
3. Results and discussion
Figure 1 shows the layers of the fabricated bifacial CdS/CdTe solar cell with the structure glass/ITO/i-ZnO/CdS/CdTe/AgNWs/ITO. The AgNWs, which formed a percolation network, were spray-coated on top of the CdTe layer [Fig. 1(c)]. Energy band alignment of the cell is shown in Fig. 1(e). The plot in Fig. 2(a) shows the optical transmittance of glass/AgNWs/ITO (annealed at 300 °C for 10 min in N2, which is the same conditions as the post-growth process of the CdTe solar cell). The optical transmittances of the ITO-only sample and AgNWs/ITO sample were comparable. A part of shading loss was compensated by the multi-scattering effects although the optical transmittance can decrease as more AgNWs are spray-coated.
Fig. 2 (a) optical transmittance of glass, glass/ITO and glass/AgNWs/ITO (b) Sheet resistances of ITO/glass and ITO/AgNWs/glass (c) photograph image of the bare glass and glass substrates coated with ITO-only and with AgNWs/ ITO, respectively.
The sheet resistances of the different structures (ITO/glass and ITO/AgNWs/glass) are shown in Fig. 2(b). Before thermal annealing, the sample with only ITO produced a lower sheet resistance than the samples with AgNWs/ITO. However, after annealing under N2 at 300 °C (post-growth process conditions), the sheet resistance of the ITO-only sample increased, which is consistent with the previous report [30]. On the other hand, the sheet resistance of the AgNWs/ITO transparent electrode decreased. Madaria et al. reported that a rapid decrease in sheet resistance occurs when the AgNWs network is heat-treated [25]. This is consistent with our results because the annealing step improves the junctions among NWs. In general, thermal annealing of NWs results in nanowelding at the junction points of the NWs. Garnett et al. showed that there is a sharp increase in the conduction properties after annealing [31]. Although the AgNWs are advantageous because of their high optical transmittance and electrical conductivity, their fast degradation under electrical stress or high temperatures remains an issue. As a solution to this problem, the fabrication of core-shell NWs using oxide materials has been studied to prevent the oxidation of AgNWs [32]. Similarly, we tried to prevent the degradation of AgNWs by depositing ITO as a transparent protection layer onto the network of spray-coated AgNWs. We fabricated the bifacial CdS/CdTe thin-film solar cell using different transparent back-electrodes: only ITO and AgNWs/ITO.
SEM was used to characterize the morphology of the AgNWs/ITO transparent electrodes deposited on the CdTe layer and a top view and cross-sectional images of the sample are shown in Figs. 3(a) and 3(b). The top view image [Fig. 3(a)] showed that the AgNWs/ITO transparent electrode was deposited on CdTe, and a percolating network and nanowelding at the junction points of the AgNWs were observed. The cross-sectional SEM image in Fig. 3(b) shows the glass/ITO/i-ZnO/CdS/CdTe/AgNWs/ITO structures. The CdTe layer was grown at a rate of ~1.4 μm/min for 3 min and had a thickness of ~4.2 μm, and the AgNWs/ITO electrode can be observed on the CdTe layer. Kim et al. used a transparent electrode consisting of a network of AgNWs to fabricate a CIGS solar cell and obtained a 0.1% efficiency [33]. They explained that the low efficiency was due to the loose contact between the one-dimensional NWs (round-shaped) and the CIGS layer and empty spaces between the NWs. This is because the empty spaces, that are longer than the charge carrier diffusion length between the NWs, interfere with the collection of free charge carriers. We used AgNWs/ITO hybrid transparent electrodes to overcome this problem and improve the stability of the AgNWs. The empty spaces within the network of AgNWs were filled by ITO, and their contact with the ITO was assisted by the AgNWs.
Fig. 3 (a) Top-view and (b) cross-sectional SEM image of the AgNWs/ITO transparent electrode on the CdTe layer.
Figure 4(a) shows the light current–voltage (light I–V) characteristics when light was incident on the glass side in the glass/ITO/i-ZnO/CdS/CdTe/AgNWs/ITO-structured bifacial CdTe solar cell. The average photovoltaic characteristics of the CdTe samples using only ITO as the back-electrode were VOC = 202 mV, JSC = 4.09 mA/cm2, FF = 45.5%, and a cell efficiency = 0.38%. Samples with AgNWs/ITO as the back-electrode produced average characteristics of VOC = 610 mV, JSC = 28.1 mA/cm2, FF = 46.9%, and a cell efficiency = 8.05%. The use of an AgNWs/ITO transparent electrode instead of an ITO-only back-electrode led to an improvement in the performance in VOC and JSC, which resulted in an increase in conversion efficiency. ITO has been widely used as a transparent and conductive electrode, but it cannot act as a p-type dopant upon CdTe, as it results in Ohmic loss within the CdTe layer. However, the efficiency of the solar cell is increased when AgNWs/ITO is used due to the p-doping effect of CdTe due to the incorporation of Ag. Chamonal et al. obtained a p-doped CdTe through Ag doping, with a carrier concentration of 2 × 1016 cm−3 [34]. More investigation is necessary because excess Ag can compensate p-CdTe by the reaction [Eq. (1)] [35].;
Fig. 4 Photovoltaic properties of the bifacial CdS/CdTe thin-film solar cells with AgNWs/ITO back contact under (a) front-side and (b) rear-side illuminations.
Figure 4(b) shows the light I-V characteristics of the bifacial CdTe solar cell with light incident on the AgNWs/ITO transparent back-electrode. A high JSC of ~25 mA/cm2 and 3.5% rear-side efficiency were obtained by using a transparent ITO back-electrode for the CdTe solar cell. Tiwari et al. fabricated a bifacial CdTe solar cell with an FTO/CdS/CdTe/ITO structure [19]. The photovoltaic characteristics of the fabricated front and rear-sides of the bifacial solar cell were measured with JSC = 3.4 mA/cm2 and an efficiency of 1.0%. Romeo et al. used a thin Cu layer followed by ITO as a back contact and obtained JSC = 12.2 mA/cm2 and PCE = 3.2% [22]. In comparison, our bifacial CdTe solar cell with the ITO back-electrode showed a high JSC and efficiency under the rear-side illumination. This indicates that a junction was formed between the ITO back-electrode and the CdTe layer. However, when AgNWs were added to fabricate the CdTe solar cell with the AgNWs/ITO back contact, the JSC decreased sharply, which is a major factor in the reduction of efficiency. These results can be explained by the disappearance of the junction between the ITO back-electrode and CdTe layer due to the Ag-induced p-doping of CdTe, which improved the contact properties between CdTe and AgNWs/ITO. The increase in the front-side efficiency induced the reduction in the rear-side efficiency due to the addition of AgNWs. A low efficiency of ~1% was obtained for the rear-side when an AgNWs/ITO back-electrode was used. This is because the light passing through the transparent back-electrode could not reach the CdS/CdTe junction owing to the high light absorption coefficient (>5 × 105 cm−1) of the CdTe, indicated by the QE data in Fig. 5(b). Compared with the QE data of the front-side of the bifacial CdTe solar cell, the rear-side QE data only reacted in the spectral region between 800 and 850 nm, with no response observed in the short wavelength region, which is attributed to the poor collection of charge carriers that are photo-generated far away from the CdS/CdTe junction [36]. A plot showing the sum efficiency of the front and rear-sides of our bifacial CdTe solar cell is shown in Fig. 5(c). The sum efficiency was calculated simply by adding the conversion efficiencies obtained in Figs. 4(a) and 4(b). The maximum sum efficiency was ~4.6% when only ITO was used as a transparent back-electrode. However, for the AgNWs/ITO transparent electrode, sum efficiencies of 9.0% (average) and 10.3% (maximum) were obtained. Therefore, the performance of the AgNWs/ITO transparent electrode was superior to that of the transparent electrode using only ITO in the bifacial CdTe solar cell. This study can facilitate the development of bifacial or tandem cells that require transparent top and bottom electrodes.
Fig. 5 (a) Illustration showing the incident angle of light according to the position of the sun during the day. (b) Quantum efficiency (QE) of the bifacial CdS/CdTe thin-film solar cell with AgNWs/ITO hybrid contact under front-side and rear-side illumination. (c) Sum of bifacial CdS/CdTe solar cell efficiencies under front-side and rear-side illumination.
4. Conclusion
We demonstrated bifacial CdS/CdTe thin-film solar cells by employing AgNWs/ITO as a transparent back contact electrode. The AgNWs were spray-coated to form a nanowire network followed by the deposition of ITO. Average efficiencies of 8.1% and 0.9% were obtained under front and rear-side illumination conditions, respectively. The diffusion of Ag into the CdTe caused a p-doping effect that enhanced the contact properties between the CdTe and the AgNWs/ITO back-electrode, resulting in vanishing the junction between CdTe and the back-electrode. The QE indicated that the efficiency in the rear-side resulted from long wavelengths. These results showed the potential of using AgNWs as a transparent and conductive electrode for CdTe-based bifacial or tandem cells.
Funding
Korea Institute of Energy Technology Evaluation & Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of Korea (No.20153030012110).
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