Open Access
Add to BibSonomy
Abstract
A bifacial CdS/CdTe thin-film solar cell with a superstrate configuration was demonstrated by using copper nanowire (CuNW)/indium tin oxide (ITO) back contacts as a transparent and conductive electrode (TCE). CdS and CdTe were deposited by chemical bath deposition and close-spaced sublimation techniques, respectively. The CuNWs acted both as an acceptor dopant and TCE for the p-CdTe, improving the total cell efficiency compared to a copper-free back contact. CuNW/ITO back contacts with high optical transmittance (72.3% at 550 nm) and low sheet resistance (47.1 Ω/sq.) were obtained. The average cell efficiency of the bifacial CdS/CdTe thin-film cells with the optimized CuNW/ITO back contact was 10.0% (front-side illumination) and 0.55% (rear-side illumination). The quantum efficiencies under front-side and rear-side illumination were studied. The prepared bifacial cell can facilitate full usage of incoming sunlight (direct or diffused), enhancing the output power under cloudy conditions.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Thin-film solar cells are attractive because they require less material to absorb incoming sunlight, which makes them competitive when compared to conventional mono-/multi-crystalline silicon solar cells. Copper indium gallium (di)selenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells have been intensively investigated as second generation solar cells. Although both achieved outstanding conversion efficiency, as high as 22.6% (CIGS) and 22.1% (CdTe), CdTe is the only material to have been successfully commercialized due to its simple structure, easy scalability, and low manufacturing cost [1]. The energy bandgap of CdTe (direct bandgap of ~1.5 eV) is ideal as an absorber layer for a single p-n junction structure, where a CdTe layer of a few μm is sufficient for absorbing most of the incoming sunlight due to its high absorption coefficient of the order of 105 cm−1 [2]. To date, the superstrate configuration has been preferred over the substrate configuration due to the difficulty of forming a good ohmic contact with the p-CdTe absorber layer. In a conventional superstrate configuration using a glass substrate, n-CdS with an energy bandgap of ~2.4 eV is widely used as a window layer, which needs to be made as thin as possible due to its high absorption at short wavelengths and high electrical resistance [3].
Bifacial photovoltaic devices can offer superior performance to monofacial devices for building-integrated photovoltaics (BIPVs) and for applications with reflected or diffuse light because the bifacial solar cells can capture light from both sides of the device. A promising application of these devices is as an acoustic barrier integrated with bifacial PV modules, e.g., to block noise from a road or railway. The total output power generated by a bifacial PV module is often higher than that of a monofacial module as it can use the light reflected from surfaces such as the snow on the ground, a rooftop, or an adjacent solar module to generate power. Bifacial cells can outperform monofacial cells under cloudy conditions due to the increased level of scattering and diffuse light. In addition, the structure of the glass-to-glass sealing in a bifacial PV module offers better fire protection and long-term reliability [4]. Romeo et al. fabricated bifacial CdTe thin-film solar cells with an indium tin oxide (ITO) back contact and investigated the effect of the thickness of CdTe absorber layers for optimizing cell performance under rear-side illumination [5]. Kwon et al. prepared bifacial CdTe thin-film solar cells using Ag nanowires as a transparent back-electrode, achieving a cell conversion efficiency of 8.05% and 1.0% under front-side and back-side illumination, respectively [6].
The nature of p-type semiconductors requires the metal electrode with a high work function to form a good ohmic contact with p-CdTe. However, an appropriate contact metal is difficult to prepare because of the high electron affinity (4.28 eV) and large energy bandgap (~1.5 eV) of CdTe. Therefore, opaque Cu-based contacts have been widely used due to their acceptor behavior and high work function (~4.7 eV). Romeo et al. demonstrated a high efficiency of ~16% for CdTe solar cells with Cu-based back contacts. They suggested that a non-rectifying contact with CdTe was achieved via a CuXTe layer [7]. However, the back-contact electrode must be either transparent or semi-transparent to enable the CdTe layer to absorb light on both faces, which cannot be achieved using conventional opaque Cu contacts. Over the past decades, nanowire (NW) networks have been intensively studied as transparent and conductive materials, offering low sheet resistance and high optical transmittance [8,9]. In particular, Cu nanowire-based transparent electrodes exhibit high flexibility and stability. Manikandan et al. fabricated a graphene-coated CuNW network with a low sheet resistance of ˂25Ω/sq [10]. In this study, we incorporated CuNWs into a CdTe solar cell as a back-electrode, which facilitated good ohmic contact with p-CdTe and high optical transparency.
2. Experimental details
The fabrication process of the bifacial CdS/CdTe solar cells is shown in Fig. 1(a). First, 2 cm × 3 cm ITO-coated glass was prepared as a transparent substrate. A CdS layer was then deposited using a chemical bath deposition (CBD) method. Cadmium chloride (CdCl2), thiourea, and ammonium chloride (NH4Cl) powder were dissolved in deionized water and stirred for ˃12 h. Subsequently, the solutions were mixed with an ammonium hydroxide solution. The ITO-coated glass substrates were immersed in the mixture for 1.25 h at 75 °C to deposit a CdS window layer on the ITO/glass. Subsequently, a CdTe layer (~3.3 µm in thickness) was grown on the CdS layer using a close-spaced sublimation (CSS) method with CdTe powder (99.999%, Alfa Aesar). In the CSS chamber at a pressure of 66.7 Pascal argon, the CdS/ITO/glass substrate was placed facing the CdTe powder with a 3 mm distance between the CdTe powder and the sample. The temperature for the substrate and source were set to 540 and 600 °C, respectively. As a post-growth treatment, pre-nitric-phosphoric (NP) etch, CdCl2 activation, and post-NP etch steps were performed. The NP etch solution was prepared with a mixture of nitric acid, phosphoric acid, and deionized water (HNO3:H3PO4:H2O = 1:213:87 volumetric ratio; Sigma-Aldrich). The samples were dipped in the mixture for 25 s and subsequently rinsed with deionized water. The samples were dip-coated in a saturated CdCl2 solution and annealed at 385 °C for 30 min under an air atmosphere for activation. Post-NP etching was performed using the same procedure as the pre-NP etching; then the samples were subsequently rinsed with deionized water.
Fig. 1 (a) Flow diagram of the fabrication process of bifacial CdS/CdTe solar cells. (b) Cross-sectional schematic of the bifacial CdS/CdTe solar cell. ITO/n-CdS/p-CdTe layers were deposited on glass substrate and the back contact was formed on the CdTe layer by spray-coating CuNWs, followed by deposition of ITO.
Subsequently, the back contact was formed by spray-coating the CuNWs, followed by ITO sputtering. The CuNWs (Sigma-Aldrich) were diluted in isopropyl alcohol (IPA) to 2.5 mg/mL. After spray coating, a 200 nm ITO layer was deposited on the back-contact area by RF sputtering (ULDiS-4540SV, ULVAC). The samples were then annealed under a nitrogen atmosphere at 300 °C for 10 min. The optical transmittance of the CuNW/ITO hybrid electrode was measured using a spectrophotometer at 550 nm (Cary 5000, Varian). A four-point probe system connected to a Keithley 2400 SourceMeter was used to measure the sheet resistance of the hybrid electrode. The morphology of the CdS/CdTe solar cell with the hybrid electrode was characterized by scanning electron microscopy (SEM; Quanta 250 FEG, FEI). The photovoltaic properties of the prepared devices were measured using a solar simulator (WACOM WXS-155S-10, AM1.5G, 100 mW/cm2). To investigate the bifacial photovoltaic properties, the current density–voltage (J–V) characteristics were measured under front and rear side illumination, respectively.
3. Results and discussion
The structure of the fabricated bifacial CdS/CdTe solar cells is shown in Fig. 1(b). ITO was used as a front contact on the glass substrate and an n-CdS/p-CdTe junction was formed. A CuNW/ITO hybrid electrode was used as the back contact, allowing light to pass through the rear side of the cell. As a control, samples with ITO as both the front and back contacts were fabricated. The surface and cross-sectional SEM images of the bifacial CdS/CdTe solar cell with CuNW/ITO hybrid electrode are shown in Figs. 2(a) and 2(b). In the surface SEM image, CuNWs were clearly observed on the polycrystalline CdTe thin film. The CuNWs had a length distribution of 5 to 40 μm with an average of ~15 μm. The thickness of the ITO and CdS layers were both 200 nm, and the growth rate of the CdS layer was ~160 nm/h. The CdTe layer was grown to a thickness of 3.3 μm at a growth rate of 1.1 μm/min. The grain diameter of the CdTe layer was a few µm with a maximum of ~4 μm. Figure 2(c) shows the room temperature photoluminescence (PL) spectrum of the CdTe layer. The CdTe band gap obtained from the PL peak was 1.51 eV, which is consistent with the literature [2].
Fig. 2 (a) Surface SEM image of the CuNW/ITO hybrid electrode deposited on polycrystalline CdTe. (b) Cross-sectional SEM image of the bifacial CdS/CdTe solar cell with a CuNW/ITO hybrid back electrode. (c) PL spectrum of p-CdTe layer at room temperature
After the thermal annealing at 300 °C, the sheet resistance of ITO-only film without CuNWs was increased from 19.7 to 23.4 Ω/sq. However, the sheet resistance of CuNWs/ITO films was improved from 33.4 to 22.7 Ω/sq. An important consideration for a transparent electrode of a bifacial solar cell is the simultaneous exhibition of high optical transparency and high electrical conductivity. The ITO-only electrode showed good properties as a transparent electrode with a high optical transmittance of 75.9% at 550 nm and a low sheet resistance of 23.4 Ω/sq. (Fig. 3(a)). However, ITO is difficult to form an ohmic contact with p-CdTe. The CuNWs are suitable as a transparent electrode, showing high optical transmittance in the IR-visible-UV region while being highly conductive [11]. Moreover, the transmittance of the CuNWs can be controlled by their concentration. Im et al. developed crystalline-ITO/CuNW hybrid electrodes that were stable after 100 cycles of bending and 240 h of thermal annealing [12]. In addition, ITO deposited onto CuNWs can assist in carrier collection from the space between the CuNWs and the formation of better contacts. The CuNW/ITO hybrid electrode with a low density of CuNWs exhibited high optical transmittance and high sheet resistance; after 5 cycles of spray coating, the sheet resistance was 79.8 Ω/sq. and optical transmittance was 74.5%. Increasing the number of spray cycles to 10, i.e., CuNW(10)/ITO, the sheet resistance was 47.1 Ω/sq. and optical transmittance was 72.3%, demonstrating a drastic decrease in sheet resistance, while the high transmittance was preserved. The CuNW(30)/ITO electrode exhibited an average sheet resistance of 21.6 Ω/sq., which was lower than that of the ITO-only electrode; meanwhile, its average optical transmittance was 65.1%, which was ~11% lower than the ITO-only electrode. The sheet resistance decreased to 16.6 Ω/sq. for the electrode prepared with 100 spray cycles; however, its transmittance at 550 nm dropped to 4.1%. Because of this trade-off between sheet resistance and optical transmittance, it is important to optimize the amount of CuNWs used to the form the back contact. Photographs of the hybrid electrodes fabricated on the glass substrates are shown in Fig. 3(b). The hybrid electrode samples with a high density of CuNWs showed lower optical transmittance. However, the transmittance of the 10-cycle spray-coated CuNW/ITO sample did not decrease significantly compared to that using the ITO electrode.
Fig. 3 (a) Sheet resistance vs. optical transmittance for the CuNW/ITO hybrid transparent electrodes with different CuNW densities compared to an ITO-only electrode. A higher density of CuNWs resulted in low sheet resistance and optical transmittance. (b) Photograph of the CuNW/ITO electrodes with different CuNW densities.
The photovoltaic parameters of the bifacial CdS/CdTe solar cells prepared using a transparent back electrode were investigated by varying the CuNW density. Figure 4 shows the average values of each parameter and its standard deviation. For the solar cells using only ITO as the back electrode, the average values of each parameter under front illumination were: open circuit voltage (VOC) of 0.633 V; short circuit current (JSC) of 24.9 mA/cm2; fill factor (FF) of 48.8%; and power conversion efficiency (PCE) of 7.75%. The 10-cycle spray-coated samples exhibited increased VOC, JSC, FF, and PCE (VOC = 0.739 V; JSC = 26.9 mA/cm2; FF = 50.53%; and PCE = 10.0%) compared to those of the samples with only an ITO back contact. When CuNWs were used, Cu acted as a substitutional dopant for Cd, increasing the hole concentration [13] (EVCd-ECuCd = 1.36 eV), which also contributed to an increase in the VOC [14].
Fig. 4 Comparison of the photovoltaic parameters of the bifacial CdS/CdTe solar cells illuminated from front and rear sides of the cell. The CuNW density used on the back contact was defined by the number of spray coating cycles.
Forming an Ohmic contact with CdTe is challenging as it is a p-type material with high electron affinity, which reduces the fill factor and decreases the overall efficiency of the cell [15,16]. When copper is introduced to the back contact, a Cu2-XTe layer is formed, which helps lower the energy barrier. In addition, p + doping of the CdTe layer using Cu is effective in reducing the contact resistance [17] (e.g., ZnTe:Cu [18]). However, as shown in Eqs. (1) and (2), when excessive Cu is introduced to the back contact, donor-type defects which result in compensation doping in CdTe can be created [19].
The lower PCE of the 30- and 50-cycle spray-coated samples can be attributed to the compensation effect (CuNW(30)/ITO: VOC = 0.708 V; JSC = 23.7 mA/cm2; FF = 45.90%; and PCE 7.86%; while for CuNW(50)/ITO: VOC = 0.541 V; JSC = 22.8 mA/cm2; FF = 36.27%; and PCE = 4.82%). Excessive Cu diffusion may generate a shunt path and recombination centers, causing low shunt resistance (RSH) [20].Under rear-side illumination, the sample with only ITO as a back electrode exhibited the highest efficiency, and the samples with a low density of CuNWs showed better efficiency than those with a higher CuNW density in the back contact. Similar to the front-side illumination, the VOC of the 10-cycle spray-coated samples were higher than that of the ITO-only electrode and decreased as the concentrations of CuNWs increased. The samples containing CuNW/ITO hybrid electrodes exhibited lower JSC values than those fabricated with ITO-only electrodes under rear-side illumination, owing to their lower optical transmittance. Despite the improvements in VOC and FF, the 10-cycle spray-coated samples showed lower efficiency than the ITO-only electrode, due to their low JSC under rear-side illumination.
Figures 5(a) and 5(b) show J–V curves under front- and rear-side illumination of the CuNW(10)/ITO and ITO-only electrodes. A reduction in RSH was not observed in the CuNW(10)/ITO electrode sample under front-side illumination. For the CuNW/ITO hybrid electrode, a roll-over occurred near the VOC region. However, under rear-side illumination, this roll-over was not observed; such behavior was previously reported for a single-walled carbon nanotube (SWCNT) electrode as the carriers generated near the back contact screen the potential barrier and prevent roll-over behavior [21]. When illuminated from the rear, despite the lower FF and VOC than the CuNW(10)/ITO back electrode, solar cells with an ITO back electrode exhibited the highest efficiency due to their high JSC. The quantum efficiency (QE) of the bifacial cell with a CuNW(10)/ITO back contact is shown in Fig. 5(c). Under front-side illumination, the QE curve appeared at 300–900 nm. The lower QE at shorter wavelengths was due to loss of light at the front electrode and CdS window layer. Carriers are not generated at IR wavelengths longer than 900 nm as the energy of the light is lower than that of the band gap of CdTe. When the sample is illuminated from the rear-side, a weak QE signal was observed around 830 nm. The high absorption coefficient of CdTe at visible wavelengths results in the generation of electron-hole pairs further away from the junction than the minority carrier diffusion length. The thinner CdTe layer can improve the PCE under rear-side illumination [5].
Fig. 5 J–V curves of the bifacial solar cells under (a) front-side illumination and (b) rear-side illumination. (c) QE from the bifacial solar cell with a 10-cycle spray-coated back electrode. (d) Total PCE (sum of the PCE values obtained under front- and rear-side illumination).
Figure 5(d) shows the total PCE obtained by simply summing the PCE values under front- and rear-side illumination (from Fig. 4). For the samples with an ITO-only back electrode, the average total efficiency was 8.42%, which is comparable to the previous report [22]. The highest average total PCE was 10.6% for the samples with a CuNW(10)/ITO hybrid back electrode. The maximum total PCE of the cell with the CuNW(10)/ITO hybrid electrode was 12.0%, which was higher than that of a bifacial solar cell prepared using a SWCNT [21] or ZnTe:N/ITO [23] back contact, owing to the effect of Cu doping. In addition, the bifacial solar cell with CuNW/ITO back contacts showed better efficiency than those with AgNW/ITO back contacts [6], which is attributed to Cu doping and the higher work function of Cu than that of Ag. The total PCE could be further optimized by decreasing the thickness of the CdTe absorber layer. Bifacial solar cells are a promising technology as they can generate more energy per unit area than conventional monofacial solar cells and have significant potential for use as a top cell in tandem solar cells.
4. Conclusion
A bifacial CdS/CdTe thin-film solar cell was fabricated by employing a CuNW/ITO hybrid transparent conducting back electrode. The average total PCE of the samples with CuNW(10)/ITO back contacts was 10.6%, higher than that of the cells with an ITO-only or hybrid back contact with AgNW contents. The improved PCE was attributed to p-doping by Cu and its high work function, although excess CuNWs lowered the total PCE. The QE values indicated that thinner CdTe layers could potentially increase the PCE. Hence, CuNW/ITO hybrid electrode back contacts could be applied to bifacial CdS/CdTe solar cells to manufacture more economical PV cells.
Funding
Korea Institute of Energy Technology Evaluation & Planning (KETEP); Ministry of Trade, Industry & Energy (MOTIE) of Korea (grant Nos. 20153030012110, 20173010012970).
References and links
1. M. A. Green, Y. Hishikawa, E. D. Dunlop, D. H. Levi, J. Hohl-Ebinger, and A. W. Y. Ho-Baillie, “Solar cell efficiency tables (version 51),” Prog. Photovolt. Res. Appl. 26(1), 3–12 (2018). [CrossRef]
2. A. Morales-Acevedo, “Thin film CdS/CdTe solar cells: Research perspectives,” Sol. Energy 80(6), 675–681 (2006). [CrossRef]
3. K. Nakamura, M. Gotoh, T. Fujihara, T. Toyama, and H. Okamoto, “Influence of CdS window layer on 2-μm thick CdS/CdTe thin film solar cells,” Sol. Energy Mater. Sol. Cells 75(1-2), 185–192 (2003). [CrossRef]
4. Q. Wei, C. Wu, X. Liu, S. Zhang, F. Qian, J. Lu, W. Lian, and P. Ni, “The Glass-glass Module Using n-type Bifacial Solar Cell with PERT Structure and its Performance,” Energy Procedia 92, 750–754 (2016). [CrossRef]
5. A. Romeo, G. Khrypunov, S. Galassini, H. Zogg, and A. N. Tiwari, “Bifacial configurations for CdTe solar cells,” Sol. Energy Mater. Sol. Cells 91(15–16), 1388–1391 (2007). [CrossRef]
6. Y. Kwon, J. Seo, Y. Kang, D. Kim, and J. Kim, “Bifacial CdS/CdTe thin-film solar cells using a transparent silver nanowire/indium tin oxide back contact,” Opt. Express 26(2), A30–A38 (2018). [CrossRef] [PubMed]
7. N. Romeo, A. Bosio, S. Mazzamuto, A. Romeo, and L. Vaillant-Roca, “High efficiency CdTe/CdS thin film solar cells with a novel back contact,” in Proceedings of 22nd European Photovoltaic Solar Energy Conference (2007), pp. 3–7.
8. J. Park, Z. Ouyang, C. Yan, K. Sun, H. Sun, F. Liu, M. Green, and X. Hao, “Hybrid Ag nanowire–ITO as transparent conductive electrode for pure sulfide kesterite Cu2ZnSnS4 solar cells,” J. Phys. Chem. C 121(38), 20597–20604 (2017). [CrossRef]
9. T. Kim, A. Canlier, G. H. Kim, J. Choi, M. Park, and S. M. Han, “Electrostatic spray deposition of highly transparent silver nanowire electrode on flexible substrate,” ACS Appl. Mater. Interfaces 5(3), 788–794 (2013). [CrossRef] [PubMed]
10. A. Manikandan, L. Lee, Y.-C. Wang, C.-W. Chen, Y.-Z. Chen, H. Medina, J.-Y. Tseng, Z. M. Wang, and Y.-L. Chueh, “Graphene-coated copper nanowire networks as a highly stable transparent electrode in harsh environments toward efficient electrocatalytic hydrogen evolution reactions,” J. Mater. Chem. A Mater. Energy Sustain. 5(26), 13320–13328 (2017). [CrossRef]
11. A. R. Rathmell, S. M. Bergin, Y.-L. Hua, Z.-Y. Li, and B. J. Wiley, “The growth mechanism of copper nanowires and their properties in flexible, transparent conducting films,” Adv. Mater. 22(32), 3558–3563 (2010). [CrossRef] [PubMed]
12. H.-G. Im, S. Jeong, J. Jin, J. Lee, D.-Y. Youn, W.-T. Koo, S.-B. Kang, H.-J. Kim, J. Jang, D. Lee, H.-K. Kim, I.-D. Kim, J.-Y. Lee, and B.-S. Bae, “Hybrid crystalline-ITO/metal nanowire mesh transparent electrodes and their application for highly flexible perovskite solar cells,” NPG Asia Mater. 8(6), e282 (2016). [CrossRef]
13. K. K. Chin, “p-Doping limit and donor compensation in CdTe polycrystalline thin film solar cells,” Sol. Energy Mater. Sol. Cells 94(10), 1627–1629 (2010). [CrossRef]
14. R. A. Sinton and A. Cuevas, “Contactless determination of current–voltage characteristics and minority–carrier lifetimes in semiconductors from quasi-steady-state photoconductance data,” Appl. Phys. Lett. 69(17), 2510–2512 (1996). [CrossRef]
15. C. R. Corwine, A. O. Pudov, M. Gloeckler, S. H. Demtsu, and J. R. Sites, “Copper inclusion and migration from the back contact in CdTe solar cells,” Sol. Energy Mater. Sol. Cells 82(4), 481–489 (2004).
16. H. C. Chou, A. Rohatgi, N. M. Jokerst, E. W. Thomas, and S. Kamra, “Copper migration in CdTe heterojunction solar cells,” J. Electron. Mater. 25(7), 1093–1098 (1996). [CrossRef]
17. K. Durose, P. R. Edwards, and D. P. Halliday, “Materials aspects of CdTe/CdS solar cells,” J. Cryst. Growth 197(3), 733–742 (1999). [CrossRef]
18. T. A. Gessert, P. Sheldon, X. Li, D. Dunlavy, D. Niles, R. Sasala, S. Albright, and B. Zadler, “Studies of ZnTe back contacts to CdS/CdTe solar cells,” in Proceedings of the Twenty Sixth IEEE Photovoltaic Specialists Conference (1997), pp. 419–422. [CrossRef]
19. E. Kučys, J. Jerhot, K. Bertulis, and V. Bariss, “Copper impurity behaviour in CdTe films,” Phys. Status Solidi 59(1), 91–99 (1980). [CrossRef]
20. I. Rimmaudo, A. Salavei, and A. Romeo, “Ageing of CdTe device by copper diffusion,” in Proceedings of 27th European Photovoltaic Solar Energy Conference and Exhibition (2012), pp. 2828–2832.
21. R. R. Khanal, A. B. Phillips, Z. Song, Y. Xie, H. P. Mahabaduge, M. D. Dorogi, S. Zafar, G. T. Faykosh, and M. J. Heben, “Substrate configuration, bifacial CdTe solar cells grown directly on transparent single wall carbon nanotube back contacts,” Sol. Energy Mater. Sol. Cells 157, 35–41 (2016). [CrossRef]
22. A. N. Tiwari, G. Khrypunov, F. Kurdzesau, D. L. Bätzber, A. Romeo, and H. Zogg, “CdTe solar cell in a novel configuration,” Prog. Photovolt. Res. Appl. 12(1), 33–38 (2004). [CrossRef]
23. S. Marsillac, V. Y. Parikh, and A. D. Compaan, “Ultra-thin bifacial CdTe solar cell,” Sol. Energy Mater. Sol. Cells 91(15–16), 1398–1402 (2007). [CrossRef]
