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Fabrication and characterization of flexible solar cell from electrodeposited Cu2O thin film on plastic substrate
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ScienceDirect Solar Energy 122 (2015) 1193–1198 www.elsevier.com/locate/solener
Fabrication and characterization of flexible solar cell from electrodeposited Cu2O thin film on plastic substrate Mahmoud Abdelfatah a,b,c,⇑, Johannes Ledig a,c, Abdelhamid El-Shaer b, Alexander Wagner a, Azat Sharafeev d, Peter Lemmens c,d, Mohsen Mohamed Mosaad b, Andreas Waag a,c, Andrey Bakin a,c a Institute of Semiconductor Technology, University of Technology, Braunschweig, Germany Physics Department, Faculty of Science, KafrelSheikh University, KafrelSheikh 33516, Egypt c Laboratory for Emerging Nanometrology Braunschweig, University of Technology, Braunschweig, Germany d Institute for Condensed Matter Physics, University of Technology, Braunschweig, Germany b
Received 20 February 2015; received in revised form 6 May 2015; accepted 2 November 2015
Communicated by: Associate Editor Elias Stefanakos
Abstract We present here for the first time the fabracition of a p-Cu2O/ZnO/AZO flexible heterojunction solar cell by electrodeposition of Cu2O thin film on a plastic substrate and sputtering of ZnO:Al layer. The Atomic Layer Deposition (ALD) has been employed to insert 5 nm ZnO as buffer layer. The heterojunction solar cell was characterized by Raman spectroscopy and scanning electron microscopy that show pyramidal shape and phonon modes for Cu2O thin film. Current–voltage (J–V), capacitance–voltage (C–V) and the external quantum efficiency (EQE) measurements were performed to understand the heterojunction properties. The solar cell device exhibits a power conversion efficiency of 0.897 ± 0.005% with an open circuit voltage of Voc = 300 mV, a short circuit current density of Jsc = 6.819 ± 0.048 mA cm2 and a fill factor of FF = 0.439 ± 0.006. The values of the built-in potential and the acceptor concentration at the junction were estimated from the reverse bias C–V measurement to be 0.37 V and 6.67 1016 cm3, respectively. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Flexible solar cell; Sustainable photovoltaics; Electrodeposition; Cu2O thin film; Earth abundant materials; I–V/C–V/EQE characteristics
1. Introduction Flexible solar cells are one of the most promising technologies to produce the lightweight solar cells for space and aerospace applications, portable electronic devices and integrated photovoltaics (Lee et al., 2014a). This type of ⇑ Corresponding author at: Institute of Semiconductor Technology, University of Technology, Braunschweig, Germany. Tel.: +49 5313913776. E-mail addresses: [email protected], Mahmoud. [email protected] (M. Abdelfatah).
http://dx.doi.org/10.1016/j.solener.2015.11.002 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
solar cells will reduce the production and installation cost of the photovoltaics (Cook et al., 2010). The majority of flexible solar cells are still fabricated from silicon as well as the traditional PV solar cells using high temperatures, for example 250 °C for a-Si (amorphous Si), and 620 °C or higher for poly-Si (polycrystalline Si) (Lee et al., 2014a; Sun et al., 2014). In contrast, the melting temperature for the flexible plastic substrates is significantly lower than these temperatures (Lee et al., 2014a). At present one of the main challenges is to find the material and the cost-effective method to fabricate such solar cells. Earthabundant elements become very promising candidates for
M. Abdelfatah et al. / Solar Energy 122 (2015) 1193–1198
such photovoltaic applications due to their non-toxicity, chemical stability and availability of low cost manufacturing techniques (Lee et al., 2013). Metal oxides like cuprous oxide (Cu2O) and zinc oxide (ZnO) are formed from such elements. Cu2O as absorber layer in solar cell can theoretically achieve a power conversion efficiency of around 20% (Meyer et al., 2012; Olsen et al., 1979). Cu2O is a p-type direct band gap semiconductor with energy of about 2 eV and have a high absorption coefficient (Akimoto et al., 2006; Olsen et al., 1979). Electrodeposition is one of the most attractive methods for Cu2O thin films growth (De Jongh et al., 1999; El-Shaer and Abdelwahed, 2013; Jeong et al., 2008; Mukhopadhyay et al., 1992). It is a low cost and low temperature technique based on chemical solutions providing large area thin films manufacturing (Baek et al., 2013; El-Shaer and Abdelwahed, 2013). Cu2O/ZnO heterojunction-based solar cells are good choice for the flexible photovoltaic devices. Great efforts have been made to improve the conversion efficiency (Xie et al., 2013, 2014). A photovoltaic performance of 1.28% was reported as the highest efficiency for polycrystalline n-ZnO/p-Cu2O heterojunctions which were fabricated by electrodeposition of both ZnO and Cu2O layers (Izaki et al., 2007). Using an electrodeposited Cu2O thin film, an efficiency of 3.97% was obtained by inserting gallium oxide (Ga2O3) as a buffer layer (Lee et al., 2014c). In the present work, we report a new approach to fabricate a flexible solar cell employing the electrodeposition technique for growth of the Cu2O thin films on plastic foils. We employ measurements of current–voltage characteristic, capacitance–voltage characteristic and external quantum efficiency (EQE) to understand the heterojunction behavior. 2. Experimental methods The bottom contact of the solar cell was realized on top of the plastic foil by 30 nm of Ti as an adhesion layer and 300 nm of Au, both deposited by e-beam evaporation. Electrodeposition of the Cu2O film was carried out in a three electrode cell equipped with a silver–silver chloride (Ag/AgCl in 3 M KCl) reference electrode, the Au-coated plastic substrate as the working electrode, and a Pt wire as the counter electrode (McShane et al., 2010; Musselman et al., 2010; Septina et al., 2011). The deposition solution of Cu2O films was composed of 0.4 M copper sulfate anhydrous (CuSO4, purity P 99%, purchased from Merck) and 3 M lactic acid (purchased from Merck with a purity of about 90%) while the pH = 12.5 is controlled by addition of 4 M sodium hydroxide (NaOH, with purity P 99% purchased from Carl Roth) (McShane et al., 2010; Musselman et al., 2012, 2010). Reagent-grade chemicals were used and the solutions were filtered and stirred thoroughly during the whole process. A constant potential of 0.4 V was applied vs. the reference electrode using a Princeton Applied Research 2273 potentiostat. The solution temperature was kept at 60 °C during the
deposition. After the deposition was finished, the Cu2O film was rinsed with de-ionized water for several times to remove any solution from the surface (McShane et al., 2010). The buffer layer of 5 nm ZnO was inserted by ALD on the Cu2O film at temperature of 120 °C. ALD has been performed employing a Picosun R-200 PEALD setup. Finally step for the fabrication of the AZO/ZnO/ Cu2O solar cell, the ZnO:Al layer used as a window and a transparent front contact of the solar cell was deposited by d.c. magnetron sputtering of a ZnO target containing 5 wt% Al2O3 by an argon plasma (Wagner et al., 2014). The sputtering process was adjusted to have an AZO layer with a respective layer thickness of about 250 nm and a conductivity of q = 7.5 E4 Ocm. Optical lithography and wet chemical etching were employed to manufacture a solar cell with a diameter of 3 mm. Characterization of the device morphology by secondary electron (SE) imaging was performed inside a Cambridge S360 SEM. Raman spectra was measured using a LabRAM HR 550 system equipped with a solid state laser (532 nm). A Newport 94021A solar simulator with an AM1.5G spectrum was used for the current–voltage solar cell characterization. The intensity was calibrated to 100 mW/cm2 by using a standard silicon reference cell. The capacitance–voltage characteristics of the device were measured employing a Suess probe station and a Keithley 4200 semiconductor characterization system. The external quantum efficiency (EQE) of the devices was measured employing a 150 W Xe arc lamp as the light source, while the relative light power density was tracked by a Si photodiode. 3. Results and discussion A Raman spectrum of the solar cell device presented in Fig. 1 shows phonon modes with frequencies that are characteristic for crystalline Cu2O and ZnO. Since the thickness 10000
Intensity (arb. units)
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416
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630 645 515 579
•
274 308
*
0 200
300
400
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Fig. 1. Raman spectra of the AZO/ZnO/Cu2O solar cell. The observed maxima are attributed to phonon modes including second-order phonons of Cu2O (;). The modes marked by an asterisk (*) and a full dot (d) are assigned to CuO and ZnO, respectively.
M. Abdelfatah et al. / Solar Energy 122 (2015) 1193–1198
of the AZO film and the ZnO buffer is very small only the ZnO A1 (LO) phonon (d) at 579 cm1 is recovered (Koyano et al., 2002). In contrast, modes of the thicker Cu2O film are observed with larger intensity. This is due to the high light penetration depth of the Raman experiment. In addition, a frequency independent background is observed that may be due to defects in the crystalline components of the solar cell. According to the analysis given in reference (SolacheCarranco et al., 2008) the second-order Raman-allowed mode 2C 12 of the Cu2O phase leads to the most intense peak of the spectrum at 218 cm1. The broader, low intensity peaks at 308 and 515 cm1 correspond to the secondorder overtone mode 2C15 ð1Þ and the Raman-allowed mode 12 þ C25 Cþ 25 , respectively. The four-phonon mode 3C 1 leads to a peak at 416 cm . The peaks at 630 and 645 cm1 are attributed to infrared-active modes (Hsu et al., 2013; Solache-Carranco et al., 2008). The weaker peak (*) at 274 cm1 is attributed to CuO (Bello et al., 2014). By this we proved, that electrodeposition allows predominantly growth of the crystalline Cu2O phase, also a weak Raman peak corresponding to the CuO phase is observed after deposition of further layers on Cu2O and hence assigned to a phase formation during the deposition of ZnO or AZO. The manufactured flexible AZO/ZnO/Cu2O solar cell module with round solar cells on top of the gold electrode
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on plastic foil is shown in Fig. 2a. A top view SE image of the AZO/ZnO/Cu2O heterojunction structure is presented in Fig. 2b. The surface morphology of the grainy Cu2O film appears to be of a pyramidal shape which is favorable to reduce the optical reflection (Lee et al., 2013; Mahalingam et al., 2006). Side view SE image of a cleaved AZO/ZnO/Cu2O solar cell is shown in Fig. 2c. Corresponding schematic view of the device is shown in Fig. 2d for better understanding of the device structure. The thickness of the p-Cu2O thin film and AZO layer are about 3.4 lm and 0.25 lm, respectively. The solar cell exhibited a p–n junction characteristic with a rectifying behavior of the J–V curve measured under dark conditions. Fig. 3 shows AZO/ZnO/Cu2O solar cell parameters measured 10 times under 1-sun (AM1.5G) illumination at room temperature. It is clear from the figure that the solar cell show reproducible values for the whole set of 10 measurements. The power conversion efficiency of 0.897 ± 0.005% with an open circuit voltage of Voc = 300 mV, a short circuit current density of Jsc = 6.819 ± 0.048 mA cm2 and a fill factor of FF = 0.439 ± 0.006 were obtained for the solar cell. The electron diffusion from the n-AZO layer to the pCu2O film causes the built-in potential of the device that prevents the undesirable dark current flow and therefore provides the open-circuit voltage. Depletion regions are generated in both layers at the heterointerface and forming
Fig. 2. (a) Photographic image, (b) SEM top view, (c) side view SE image and (d) Schematic diagram of the AZO/ZnO/Cu2O solar cell on a plastic substrate.
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1,4
Jsc (mA/cm2)
6
1,0 0,8 Short circuit current density(J ) sc Open circuit voltage (V ) oc Efficiency Fill factor
4
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0,0 1
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Fig. 3. AZO/ZnO/Cu2O solar cell parameters: short circuit current density (Jsc), open circuit voltage (Voc), efficiency (g%), and fill factor (FF) measured 10 times under 1-sun (AM1.5G) illumination, showing very high reproducibility from measurement to measurement.
the built-in potential. The measured short circuit current density is above 6 mA cm2 which can be expected from theoretical calculations (Musselman et al., 2010). It is assumed that the Cu2O film is thicker than the depletion and allows to obtain the full built-in potential of the material, therefore the electrons which are generated at distances less than the charge collection length of minority carrier in Cu2O layer will contribute to the device current (Musselman et al., 2012). We found that a Cu2O film thickness of around 3.4 lm achieve the highest built-in potential, although the minority carrier diffusion length is within the micrometer range. The Jsc will increase if we prepare a thicker Cu2O film of several micrometers. But in contrast the Voc will decrease and therefore the efficiency will be lower (Marin et al., 2013). The schematic energy band diagram for the AZO/ZnO/ Cu2O flexible solar cell is shown in Fig. 4. Insertion of 5 nm of ZnO as a thin buffer layer increased the efficiency of the solar cell comparing to the solar cells based only on the AZO/Cu2O heterojunctions where Cu2O is also obtained by electrodeposition (Minami et al., 2006; Septina et al.,
2011). The photovoltaic properties of the heterojunction are affected by the conduction band discontinuity height, resulting from the difference in electron affinity between the n-semiconductor buffer layer and the Cu2O film in the heterojunction (Minami et al., 2014). So, we can suppose that the conduction band discontinuity of an nZnO/p-Cu2O heterojunction could be lower than of an AZO/p-Cu2O heterojunction. Additionally the increased efficiency of the solar cell with ALD ZnO buffer layer could be explained by the influence of the metallorganic source material (DEZn) on the Cu2O surface during the first stages of the ALD deposition. It is possible that a very thin CuO layer formed at the Cu2O deteriorates photovoltaic parameters since it has a smaller band gap (Eg 1.4 eV) than Cu2O. Also its conduction band energy near 4 eV from the vacuum level can produce deep states at the interface which increasing the interfacial recombination (Lee et al., 2014b). This CuO layer can be etched at the beginning of the buffer layer deposition. Fig. 5 shows External quantum efficiency (EQE) as a function of wavelength. The photocurrent generated mostly for the region from 380 nm to 500 nm since the absorption coefficient of Cu2O increases with decreasing the excitation wavelength from 500 to 350 nm (Musselman et al., 2010). Also the Cu2O optical depth is less than 150 nm for wavelengths below 460 nm, so almost all photogenerated charge carriers will be collected by the junction and contributes to the device. For wavelengths higher than 460 nm the optical depth increases to micrometer range and most of the electrons (minority charge carriers) generated by such photons will be too far from p-n-junction interface to be collected (Liu et al., 2011; Musselman et al., 2010). The absorption inside the n-ZnO and AZO layers can be the reason for the decrease of EQE for wavelengths below 380 nm (Lee et al., 2013). C–V and C–f measurements were performed at room temperature using different frequencies in order to characterize the properties of the AZO/ZnO/Cu2O heterojunction. Fig. 6 illustrates the change of capacitance versus different frequencies. At low frequencies, the capacitance of the 100
EQE (%)
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0 350
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Fig. 4. Schematic energy band diagram of the AZO/ZnO/Cu2O solar cell.
Fig. 5. Measured external quantum efficiency (EQE) of the AZO/ZnO/ Cu2O solar cell under room temperature.
M. Abdelfatah et al. / Solar Energy 122 (2015) 1193–1198 100
5,00E+016
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1/ C2 (1/F2)
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10 6 0,00E+000
Frequency (Hz)
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Fig. 6. Room-temperature capacitance – frequency characteristics of the AZO/ZnO/Cu2O solar cell measured with 0 V (DC) bias under dark conditions.
solar cell plateaus to a depletion capacitance (Cd). The value of Cd is mainly determined by the doping concentrations of the materials (including intrinsic and extrinsic doping) which will subsequently change the depletion region width. The depletion capacitance (Cd) of the solar cell is about 91 nF/cm2 at low frequencies. At high frequencies, the capacitance decrease and reach the value from 2 to 5 nF/cm2, in this case the heterojunction is considered as an insulator because of the dielectric freeze-out inside the Cu2O – resulting in a geometric capacitance (Cg) between the back contact and AZO (Lee et al., 2013). The acceptor concentration and built-in potential of the heterojunction can be calculated according to the following relations (Tripathi and Sharma, 2012): 1 2 ¼ ðV b V Þ 2 C ee0 qN A A2 NA ¼
2 ee0 qA2
d dV
ð1=C 2 Þ
ð1Þ
-0,3
-0,2
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0,0
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V (volt)
Fig. 7. Room-temperature capacitance–voltage characteristics of AZO/ ZnO/Cu2O solar cell under dark conditions.
potential and therefore the open-circuit voltage. The improvement of Voc will lead to higher solar cell efficiency. The value of the acceptor concentration (NA) calculated using the slope from Fig. 7 and Eq. (2), is about 6.67 1016 cm3. This value is two orders higher than the value reported previously for electrodeposited Cu2O thin films (Musselman et al., 2012). This result also suggests that the acceptor concentration (NA) is limiting the efficiency of the device. The width of the depletion region wd can be estimated using the following relation (Hussain et al., 2012): 1=2 2ee0 V b wd ¼ ð3Þ qN A By this, wd is found to be around 67 nm which is much smaller than the Cu2O thin film thickness. Therefore the total thickness which is contributing to the photocurrent will be in the range of about micrometer.
ð2Þ
where NA is acceptor concentration (doping density) [1/cm3], Vb is the built-in potential, q is the electronic charge (1.60219 1019 C), e0 is the permittivity of free space (8.85 1014 F/cm), e is the relative permittivity of the Cu2O layer (7.6), A is the area of the solar cell (0.0707 cm2), V is the applied voltage and C is the measured capacitance. The built-in potential (Vb) can be obtained through the extrapolation of the 1/C2 intercept along the x-axis as presented in Fig. 7. It is found that 1/C2 is linear versus the bias voltage, the capacitance increases because of the intrinsic states of the bulk material or impurity states that formed uniform space charge trapping (Hussain et al., 2012). The value of the Vb interpolated from the CV characteristic is 0.37 V which corresponds to the measured open-circuit voltage Voc for this solar cell. This result suggests that the built-in potential limits the Voc of the device. Variation of intrinsic point defects or doping of Cu2O thin films can increase the built in
4. Conclusion A new approach was introduced to fabricate a flexible solar cell employing the electrodeposition technique for growth of the Cu2O thin films on plastic substrate. A constant potential of 0.4 V was applied vs. the reference electrode to prepare Cu2O thin film from a lactate-stabilized copper sulfate aqueous. Thin buffer layer of 5 nm ZnO was inserted by ALD to improve the junction confinement. The best achieved parameters for the flexible solar cell were 0.904%, 300 mV, 6.77 mA cm2 and 44% for the efficiency, the open circuit voltage (Voc), the short circuit current and the fill factor (FF), respectively. The built-in potential of 0.37 V, the acceptor concentration of 6.67 1016 cm3 and the depletion region width of wd = 67 nm were obtained by the reverse biased C–V measurement. Applying this approach to fabricate a flexible solar cell is promising for fabrication of lightweight and even flexible solar cells and for reduction of the production and installation costs of photovoltaic systems.
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Acknowledgments The authors would like to thank A. Schmidt, D. Ru¨mmler, J. Arens, M. Karsten, and K.-H. Lachmund for their excellent technical support. Funding by the DFG (Project BA2869/9-1), by the STDF (Project ID: 1473), as well as by the Egyptian Government via a Joint mission program from the Ministry of Scientific Research and Higher Education of the scholarship for Mahmoud Abdelfatah are gratefully acknowledged. References Akimoto, K., Ishizuka, S., Yanagita, M., Nawa, Y., Paul, G.K., Sakurai, T., 2006. Thin film deposition of Cu2O and application for solar cells. Sol. Energy 80 (6), 715–722. Baek, S.K., Lee, K.R., Cho, H.K., 2013. Oxide pn heterojunction of Cu2O/ZnO nanowires and their photovoltaic performance. J. Nanomater. 2013 (2514103), 6. Bello, A., Dodoo-Arhin, D., Makgopa, K., Fabiane, M., Manyala, N., 2014. Surfactant assisted synthesis of copper oxide (CuO) leaf-like nanostructures for electrochemical applications. Am. J. Mater. Sci. 4 (2), 64–73. Cook, T.R., Dogutan, D.K., Reece, S.Y., Surendranath, Y., Teets, T.S., Nocera, D.G., 2010. Solar energy supply and storage for the legacy and nonlegacy worlds. Chem. Rev. 110 (11), 6474–6502. De Jongh, P., Vanmaekelbergh, D., Kelly, J., 1999. Cu2O: electrodeposition and characterization. Chem. Mater. 11 (12), 3512–3517. El-Shaer, A., Abdelwahed, A., 2013. Potentiostatic deposition and characterization of cuprous oxide thin films. ISRN Nanotechnol. 2013. Hsu, Y.-K., Yu, C.-H., Chen, Y.-C., Lin, Y.-G., 2013. Fabrication of coral-like Cu2O nanoelectrode for solar hydrogen generation. J. Power Sources 242, 541–547. http://dx.doi.org/10.1016/j.jpowsour. 2013.05.107. Hussain, S., Cao, C., Usman, Z., Chen, Z., Nabi, G., Khan, W.S., Mahmood, T., 2012. Fabrication and photovoltaic characteristics of Cu2O/TiO2 thin film heterojunction solar cell. Thin Solid Films 522, 430–434. http://dx.doi.org/10.1016/j.tsf.2012.08.013. Izaki, M., Shinagawa, T., Mizuno, K.T., Ida, Y., Inaba, M., Tasaka, A., 2007. Electrochemically constructed p-Cu2O/n-ZnO heterojunction diode for photovoltaic device. J. Phys. D Appl. Phys. 40 (11), 3326–3329. Jeong, S.S., Mittiga, A., Salza, E., Masci, A., Passerini, S., 2008. Electrodeposited ZnO/Cu2O heterojunction solar cells. Electrochim. Acta 53 (5), 2226–2231. http://dx.doi.org/10.1016/j.electacta. 2007.09.030. Koyano, M., QuocBao, P., ThanhBinh, L.t., Hongha, L., NgocLong, N., Katayama, S.i., 2002. Photoluminescence and Raman spectra of ZnO thin films by charged liquid cluster beam technique. Phys. Status Solidi A 193 (1), 125–131. Lee, C.H., Kim, D.R., Zheng, X., 2014a. Transfer printing methods for flexible thin film solar cells: basic concepts and working principles. ACS Nano 8 (9), 8746–8756. Lee, S.W., Lee, Y.S., Heo, J., Siah, S.C., Chua, D., Brandt, R.E., Gordon, R.G., 2014b. Improved Cu2O-based solar cells using Atomic Layer Deposition to control the Cu oxidation state at the p–n junction. Adv. Energy Mater. 4 (11). Lee, Y.S., Chua, D., Brandt, R.E., Siah, S.C., Li, J.V., Mailoa, J.P., Buonassisi, T., 2014c. Atomic layer deposited gallium oxide buffer layer enables 1.2 V open-circuit voltage in cuprous oxide solar cells. Adv. Mater. 26 (27), 4704–4710. Lee, Y.S., Heo, J., Siah, S.C., Mailoa, J.P., Brandt, R.E., Kim, S.B., Buonassisi, T., 2013. Ultrathin amorphous zinc-tin-oxide buffer layer
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ScienceDirect Solar Energy 122 (2015) 1193–1198 www.elsevier.com/locate/solener
Fabrication and characterization of flexible solar cell from electrodeposited Cu2O thin film on plastic substrate Mahmoud Abdelfatah a,b,c,⇑, Johannes Ledig a,c, Abdelhamid El-Shaer b, Alexander Wagner a, Azat Sharafeev d, Peter Lemmens c,d, Mohsen Mohamed Mosaad b, Andreas Waag a,c, Andrey Bakin a,c a Institute of Semiconductor Technology, University of Technology, Braunschweig, Germany Physics Department, Faculty of Science, KafrelSheikh University, KafrelSheikh 33516, Egypt c Laboratory for Emerging Nanometrology Braunschweig, University of Technology, Braunschweig, Germany d Institute for Condensed Matter Physics, University of Technology, Braunschweig, Germany b
Received 20 February 2015; received in revised form 6 May 2015; accepted 2 November 2015
Communicated by: Associate Editor Elias Stefanakos
Abstract We present here for the first time the fabracition of a p-Cu2O/ZnO/AZO flexible heterojunction solar cell by electrodeposition of Cu2O thin film on a plastic substrate and sputtering of ZnO:Al layer. The Atomic Layer Deposition (ALD) has been employed to insert 5 nm ZnO as buffer layer. The heterojunction solar cell was characterized by Raman spectroscopy and scanning electron microscopy that show pyramidal shape and phonon modes for Cu2O thin film. Current–voltage (J–V), capacitance–voltage (C–V) and the external quantum efficiency (EQE) measurements were performed to understand the heterojunction properties. The solar cell device exhibits a power conversion efficiency of 0.897 ± 0.005% with an open circuit voltage of Voc = 300 mV, a short circuit current density of Jsc = 6.819 ± 0.048 mA cm2 and a fill factor of FF = 0.439 ± 0.006. The values of the built-in potential and the acceptor concentration at the junction were estimated from the reverse bias C–V measurement to be 0.37 V and 6.67 1016 cm3, respectively. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Flexible solar cell; Sustainable photovoltaics; Electrodeposition; Cu2O thin film; Earth abundant materials; I–V/C–V/EQE characteristics
1. Introduction Flexible solar cells are one of the most promising technologies to produce the lightweight solar cells for space and aerospace applications, portable electronic devices and integrated photovoltaics (Lee et al., 2014a). This type of ⇑ Corresponding author at: Institute of Semiconductor Technology, University of Technology, Braunschweig, Germany. Tel.: +49 5313913776. E-mail addresses: [email protected], Mahmoud. [email protected] (M. Abdelfatah).
http://dx.doi.org/10.1016/j.solener.2015.11.002 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
solar cells will reduce the production and installation cost of the photovoltaics (Cook et al., 2010). The majority of flexible solar cells are still fabricated from silicon as well as the traditional PV solar cells using high temperatures, for example 250 °C for a-Si (amorphous Si), and 620 °C or higher for poly-Si (polycrystalline Si) (Lee et al., 2014a; Sun et al., 2014). In contrast, the melting temperature for the flexible plastic substrates is significantly lower than these temperatures (Lee et al., 2014a). At present one of the main challenges is to find the material and the cost-effective method to fabricate such solar cells. Earthabundant elements become very promising candidates for
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such photovoltaic applications due to their non-toxicity, chemical stability and availability of low cost manufacturing techniques (Lee et al., 2013). Metal oxides like cuprous oxide (Cu2O) and zinc oxide (ZnO) are formed from such elements. Cu2O as absorber layer in solar cell can theoretically achieve a power conversion efficiency of around 20% (Meyer et al., 2012; Olsen et al., 1979). Cu2O is a p-type direct band gap semiconductor with energy of about 2 eV and have a high absorption coefficient (Akimoto et al., 2006; Olsen et al., 1979). Electrodeposition is one of the most attractive methods for Cu2O thin films growth (De Jongh et al., 1999; El-Shaer and Abdelwahed, 2013; Jeong et al., 2008; Mukhopadhyay et al., 1992). It is a low cost and low temperature technique based on chemical solutions providing large area thin films manufacturing (Baek et al., 2013; El-Shaer and Abdelwahed, 2013). Cu2O/ZnO heterojunction-based solar cells are good choice for the flexible photovoltaic devices. Great efforts have been made to improve the conversion efficiency (Xie et al., 2013, 2014). A photovoltaic performance of 1.28% was reported as the highest efficiency for polycrystalline n-ZnO/p-Cu2O heterojunctions which were fabricated by electrodeposition of both ZnO and Cu2O layers (Izaki et al., 2007). Using an electrodeposited Cu2O thin film, an efficiency of 3.97% was obtained by inserting gallium oxide (Ga2O3) as a buffer layer (Lee et al., 2014c). In the present work, we report a new approach to fabricate a flexible solar cell employing the electrodeposition technique for growth of the Cu2O thin films on plastic foils. We employ measurements of current–voltage characteristic, capacitance–voltage characteristic and external quantum efficiency (EQE) to understand the heterojunction behavior. 2. Experimental methods The bottom contact of the solar cell was realized on top of the plastic foil by 30 nm of Ti as an adhesion layer and 300 nm of Au, both deposited by e-beam evaporation. Electrodeposition of the Cu2O film was carried out in a three electrode cell equipped with a silver–silver chloride (Ag/AgCl in 3 M KCl) reference electrode, the Au-coated plastic substrate as the working electrode, and a Pt wire as the counter electrode (McShane et al., 2010; Musselman et al., 2010; Septina et al., 2011). The deposition solution of Cu2O films was composed of 0.4 M copper sulfate anhydrous (CuSO4, purity P 99%, purchased from Merck) and 3 M lactic acid (purchased from Merck with a purity of about 90%) while the pH = 12.5 is controlled by addition of 4 M sodium hydroxide (NaOH, with purity P 99% purchased from Carl Roth) (McShane et al., 2010; Musselman et al., 2012, 2010). Reagent-grade chemicals were used and the solutions were filtered and stirred thoroughly during the whole process. A constant potential of 0.4 V was applied vs. the reference electrode using a Princeton Applied Research 2273 potentiostat. The solution temperature was kept at 60 °C during the
deposition. After the deposition was finished, the Cu2O film was rinsed with de-ionized water for several times to remove any solution from the surface (McShane et al., 2010). The buffer layer of 5 nm ZnO was inserted by ALD on the Cu2O film at temperature of 120 °C. ALD has been performed employing a Picosun R-200 PEALD setup. Finally step for the fabrication of the AZO/ZnO/ Cu2O solar cell, the ZnO:Al layer used as a window and a transparent front contact of the solar cell was deposited by d.c. magnetron sputtering of a ZnO target containing 5 wt% Al2O3 by an argon plasma (Wagner et al., 2014). The sputtering process was adjusted to have an AZO layer with a respective layer thickness of about 250 nm and a conductivity of q = 7.5 E4 Ocm. Optical lithography and wet chemical etching were employed to manufacture a solar cell with a diameter of 3 mm. Characterization of the device morphology by secondary electron (SE) imaging was performed inside a Cambridge S360 SEM. Raman spectra was measured using a LabRAM HR 550 system equipped with a solid state laser (532 nm). A Newport 94021A solar simulator with an AM1.5G spectrum was used for the current–voltage solar cell characterization. The intensity was calibrated to 100 mW/cm2 by using a standard silicon reference cell. The capacitance–voltage characteristics of the device were measured employing a Suess probe station and a Keithley 4200 semiconductor characterization system. The external quantum efficiency (EQE) of the devices was measured employing a 150 W Xe arc lamp as the light source, while the relative light power density was tracked by a Si photodiode. 3. Results and discussion A Raman spectrum of the solar cell device presented in Fig. 1 shows phonon modes with frequencies that are characteristic for crystalline Cu2O and ZnO. Since the thickness 10000
Intensity (arb. units)
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416
2500
630 645 515 579
•
274 308
*
0 200
300
400
Raman shift
500
600
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Fig. 1. Raman spectra of the AZO/ZnO/Cu2O solar cell. The observed maxima are attributed to phonon modes including second-order phonons of Cu2O (;). The modes marked by an asterisk (*) and a full dot (d) are assigned to CuO and ZnO, respectively.
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of the AZO film and the ZnO buffer is very small only the ZnO A1 (LO) phonon (d) at 579 cm1 is recovered (Koyano et al., 2002). In contrast, modes of the thicker Cu2O film are observed with larger intensity. This is due to the high light penetration depth of the Raman experiment. In addition, a frequency independent background is observed that may be due to defects in the crystalline components of the solar cell. According to the analysis given in reference (SolacheCarranco et al., 2008) the second-order Raman-allowed mode 2C 12 of the Cu2O phase leads to the most intense peak of the spectrum at 218 cm1. The broader, low intensity peaks at 308 and 515 cm1 correspond to the secondorder overtone mode 2C15 ð1Þ and the Raman-allowed mode 12 þ C25 Cþ 25 , respectively. The four-phonon mode 3C 1 leads to a peak at 416 cm . The peaks at 630 and 645 cm1 are attributed to infrared-active modes (Hsu et al., 2013; Solache-Carranco et al., 2008). The weaker peak (*) at 274 cm1 is attributed to CuO (Bello et al., 2014). By this we proved, that electrodeposition allows predominantly growth of the crystalline Cu2O phase, also a weak Raman peak corresponding to the CuO phase is observed after deposition of further layers on Cu2O and hence assigned to a phase formation during the deposition of ZnO or AZO. The manufactured flexible AZO/ZnO/Cu2O solar cell module with round solar cells on top of the gold electrode
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on plastic foil is shown in Fig. 2a. A top view SE image of the AZO/ZnO/Cu2O heterojunction structure is presented in Fig. 2b. The surface morphology of the grainy Cu2O film appears to be of a pyramidal shape which is favorable to reduce the optical reflection (Lee et al., 2013; Mahalingam et al., 2006). Side view SE image of a cleaved AZO/ZnO/Cu2O solar cell is shown in Fig. 2c. Corresponding schematic view of the device is shown in Fig. 2d for better understanding of the device structure. The thickness of the p-Cu2O thin film and AZO layer are about 3.4 lm and 0.25 lm, respectively. The solar cell exhibited a p–n junction characteristic with a rectifying behavior of the J–V curve measured under dark conditions. Fig. 3 shows AZO/ZnO/Cu2O solar cell parameters measured 10 times under 1-sun (AM1.5G) illumination at room temperature. It is clear from the figure that the solar cell show reproducible values for the whole set of 10 measurements. The power conversion efficiency of 0.897 ± 0.005% with an open circuit voltage of Voc = 300 mV, a short circuit current density of Jsc = 6.819 ± 0.048 mA cm2 and a fill factor of FF = 0.439 ± 0.006 were obtained for the solar cell. The electron diffusion from the n-AZO layer to the pCu2O film causes the built-in potential of the device that prevents the undesirable dark current flow and therefore provides the open-circuit voltage. Depletion regions are generated in both layers at the heterointerface and forming
Fig. 2. (a) Photographic image, (b) SEM top view, (c) side view SE image and (d) Schematic diagram of the AZO/ZnO/Cu2O solar cell on a plastic substrate.
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1,4
Jsc (mA/cm2)
6
1,0 0,8 Short circuit current density(J ) sc Open circuit voltage (V ) oc Efficiency Fill factor
4
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1,2
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2
0,2 0
0,0 1
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Fig. 3. AZO/ZnO/Cu2O solar cell parameters: short circuit current density (Jsc), open circuit voltage (Voc), efficiency (g%), and fill factor (FF) measured 10 times under 1-sun (AM1.5G) illumination, showing very high reproducibility from measurement to measurement.
the built-in potential. The measured short circuit current density is above 6 mA cm2 which can be expected from theoretical calculations (Musselman et al., 2010). It is assumed that the Cu2O film is thicker than the depletion and allows to obtain the full built-in potential of the material, therefore the electrons which are generated at distances less than the charge collection length of minority carrier in Cu2O layer will contribute to the device current (Musselman et al., 2012). We found that a Cu2O film thickness of around 3.4 lm achieve the highest built-in potential, although the minority carrier diffusion length is within the micrometer range. The Jsc will increase if we prepare a thicker Cu2O film of several micrometers. But in contrast the Voc will decrease and therefore the efficiency will be lower (Marin et al., 2013). The schematic energy band diagram for the AZO/ZnO/ Cu2O flexible solar cell is shown in Fig. 4. Insertion of 5 nm of ZnO as a thin buffer layer increased the efficiency of the solar cell comparing to the solar cells based only on the AZO/Cu2O heterojunctions where Cu2O is also obtained by electrodeposition (Minami et al., 2006; Septina et al.,
2011). The photovoltaic properties of the heterojunction are affected by the conduction band discontinuity height, resulting from the difference in electron affinity between the n-semiconductor buffer layer and the Cu2O film in the heterojunction (Minami et al., 2014). So, we can suppose that the conduction band discontinuity of an nZnO/p-Cu2O heterojunction could be lower than of an AZO/p-Cu2O heterojunction. Additionally the increased efficiency of the solar cell with ALD ZnO buffer layer could be explained by the influence of the metallorganic source material (DEZn) on the Cu2O surface during the first stages of the ALD deposition. It is possible that a very thin CuO layer formed at the Cu2O deteriorates photovoltaic parameters since it has a smaller band gap (Eg 1.4 eV) than Cu2O. Also its conduction band energy near 4 eV from the vacuum level can produce deep states at the interface which increasing the interfacial recombination (Lee et al., 2014b). This CuO layer can be etched at the beginning of the buffer layer deposition. Fig. 5 shows External quantum efficiency (EQE) as a function of wavelength. The photocurrent generated mostly for the region from 380 nm to 500 nm since the absorption coefficient of Cu2O increases with decreasing the excitation wavelength from 500 to 350 nm (Musselman et al., 2010). Also the Cu2O optical depth is less than 150 nm for wavelengths below 460 nm, so almost all photogenerated charge carriers will be collected by the junction and contributes to the device. For wavelengths higher than 460 nm the optical depth increases to micrometer range and most of the electrons (minority charge carriers) generated by such photons will be too far from p-n-junction interface to be collected (Liu et al., 2011; Musselman et al., 2010). The absorption inside the n-ZnO and AZO layers can be the reason for the decrease of EQE for wavelengths below 380 nm (Lee et al., 2013). C–V and C–f measurements were performed at room temperature using different frequencies in order to characterize the properties of the AZO/ZnO/Cu2O heterojunction. Fig. 6 illustrates the change of capacitance versus different frequencies. At low frequencies, the capacitance of the 100
EQE (%)
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0 350
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Wavelenght (nm)
Fig. 4. Schematic energy band diagram of the AZO/ZnO/Cu2O solar cell.
Fig. 5. Measured external quantum efficiency (EQE) of the AZO/ZnO/ Cu2O solar cell under room temperature.
M. Abdelfatah et al. / Solar Energy 122 (2015) 1193–1198 100
5,00E+016
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1/ C2 (1/F2)
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20 1,00E+016 0 10 3
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10 6 0,00E+000
Frequency (Hz)
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Fig. 6. Room-temperature capacitance – frequency characteristics of the AZO/ZnO/Cu2O solar cell measured with 0 V (DC) bias under dark conditions.
solar cell plateaus to a depletion capacitance (Cd). The value of Cd is mainly determined by the doping concentrations of the materials (including intrinsic and extrinsic doping) which will subsequently change the depletion region width. The depletion capacitance (Cd) of the solar cell is about 91 nF/cm2 at low frequencies. At high frequencies, the capacitance decrease and reach the value from 2 to 5 nF/cm2, in this case the heterojunction is considered as an insulator because of the dielectric freeze-out inside the Cu2O – resulting in a geometric capacitance (Cg) between the back contact and AZO (Lee et al., 2013). The acceptor concentration and built-in potential of the heterojunction can be calculated according to the following relations (Tripathi and Sharma, 2012): 1 2 ¼ ðV b V Þ 2 C ee0 qN A A2 NA ¼
2 ee0 qA2
d dV
ð1=C 2 Þ
ð1Þ
-0,3
-0,2
-0,1
0,0
0,1
0,2
0,3
0,4
V (volt)
Fig. 7. Room-temperature capacitance–voltage characteristics of AZO/ ZnO/Cu2O solar cell under dark conditions.
potential and therefore the open-circuit voltage. The improvement of Voc will lead to higher solar cell efficiency. The value of the acceptor concentration (NA) calculated using the slope from Fig. 7 and Eq. (2), is about 6.67 1016 cm3. This value is two orders higher than the value reported previously for electrodeposited Cu2O thin films (Musselman et al., 2012). This result also suggests that the acceptor concentration (NA) is limiting the efficiency of the device. The width of the depletion region wd can be estimated using the following relation (Hussain et al., 2012): 1=2 2ee0 V b wd ¼ ð3Þ qN A By this, wd is found to be around 67 nm which is much smaller than the Cu2O thin film thickness. Therefore the total thickness which is contributing to the photocurrent will be in the range of about micrometer.
ð2Þ
where NA is acceptor concentration (doping density) [1/cm3], Vb is the built-in potential, q is the electronic charge (1.60219 1019 C), e0 is the permittivity of free space (8.85 1014 F/cm), e is the relative permittivity of the Cu2O layer (7.6), A is the area of the solar cell (0.0707 cm2), V is the applied voltage and C is the measured capacitance. The built-in potential (Vb) can be obtained through the extrapolation of the 1/C2 intercept along the x-axis as presented in Fig. 7. It is found that 1/C2 is linear versus the bias voltage, the capacitance increases because of the intrinsic states of the bulk material or impurity states that formed uniform space charge trapping (Hussain et al., 2012). The value of the Vb interpolated from the CV characteristic is 0.37 V which corresponds to the measured open-circuit voltage Voc for this solar cell. This result suggests that the built-in potential limits the Voc of the device. Variation of intrinsic point defects or doping of Cu2O thin films can increase the built in
4. Conclusion A new approach was introduced to fabricate a flexible solar cell employing the electrodeposition technique for growth of the Cu2O thin films on plastic substrate. A constant potential of 0.4 V was applied vs. the reference electrode to prepare Cu2O thin film from a lactate-stabilized copper sulfate aqueous. Thin buffer layer of 5 nm ZnO was inserted by ALD to improve the junction confinement. The best achieved parameters for the flexible solar cell were 0.904%, 300 mV, 6.77 mA cm2 and 44% for the efficiency, the open circuit voltage (Voc), the short circuit current and the fill factor (FF), respectively. The built-in potential of 0.37 V, the acceptor concentration of 6.67 1016 cm3 and the depletion region width of wd = 67 nm were obtained by the reverse biased C–V measurement. Applying this approach to fabricate a flexible solar cell is promising for fabrication of lightweight and even flexible solar cells and for reduction of the production and installation costs of photovoltaic systems.
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Acknowledgments The authors would like to thank A. Schmidt, D. Ru¨mmler, J. Arens, M. Karsten, and K.-H. Lachmund for their excellent technical support. Funding by the DFG (Project BA2869/9-1), by the STDF (Project ID: 1473), as well as by the Egyptian Government via a Joint mission program from the Ministry of Scientific Research and Higher Education of the scholarship for Mahmoud Abdelfatah are gratefully acknowledged. References Akimoto, K., Ishizuka, S., Yanagita, M., Nawa, Y., Paul, G.K., Sakurai, T., 2006. Thin film deposition of Cu2O and application for solar cells. Sol. Energy 80 (6), 715–722. Baek, S.K., Lee, K.R., Cho, H.K., 2013. Oxide pn heterojunction of Cu2O/ZnO nanowires and their photovoltaic performance. J. Nanomater. 2013 (2514103), 6. Bello, A., Dodoo-Arhin, D., Makgopa, K., Fabiane, M., Manyala, N., 2014. Surfactant assisted synthesis of copper oxide (CuO) leaf-like nanostructures for electrochemical applications. Am. J. Mater. Sci. 4 (2), 64–73. Cook, T.R., Dogutan, D.K., Reece, S.Y., Surendranath, Y., Teets, T.S., Nocera, D.G., 2010. Solar energy supply and storage for the legacy and nonlegacy worlds. Chem. Rev. 110 (11), 6474–6502. De Jongh, P., Vanmaekelbergh, D., Kelly, J., 1999. Cu2O: electrodeposition and characterization. Chem. Mater. 11 (12), 3512–3517. El-Shaer, A., Abdelwahed, A., 2013. Potentiostatic deposition and characterization of cuprous oxide thin films. ISRN Nanotechnol. 2013. Hsu, Y.-K., Yu, C.-H., Chen, Y.-C., Lin, Y.-G., 2013. Fabrication of coral-like Cu2O nanoelectrode for solar hydrogen generation. J. Power Sources 242, 541–547. http://dx.doi.org/10.1016/j.jpowsour. 2013.05.107. Hussain, S., Cao, C., Usman, Z., Chen, Z., Nabi, G., Khan, W.S., Mahmood, T., 2012. Fabrication and photovoltaic characteristics of Cu2O/TiO2 thin film heterojunction solar cell. Thin Solid Films 522, 430–434. http://dx.doi.org/10.1016/j.tsf.2012.08.013. Izaki, M., Shinagawa, T., Mizuno, K.T., Ida, Y., Inaba, M., Tasaka, A., 2007. Electrochemically constructed p-Cu2O/n-ZnO heterojunction diode for photovoltaic device. J. Phys. D Appl. Phys. 40 (11), 3326–3329. Jeong, S.S., Mittiga, A., Salza, E., Masci, A., Passerini, S., 2008. Electrodeposited ZnO/Cu2O heterojunction solar cells. Electrochim. Acta 53 (5), 2226–2231. http://dx.doi.org/10.1016/j.electacta. 2007.09.030. Koyano, M., QuocBao, P., ThanhBinh, L.t., Hongha, L., NgocLong, N., Katayama, S.i., 2002. Photoluminescence and Raman spectra of ZnO thin films by charged liquid cluster beam technique. Phys. Status Solidi A 193 (1), 125–131. Lee, C.H., Kim, D.R., Zheng, X., 2014a. Transfer printing methods for flexible thin film solar cells: basic concepts and working principles. ACS Nano 8 (9), 8746–8756. Lee, S.W., Lee, Y.S., Heo, J., Siah, S.C., Chua, D., Brandt, R.E., Gordon, R.G., 2014b. Improved Cu2O-based solar cells using Atomic Layer Deposition to control the Cu oxidation state at the p–n junction. Adv. Energy Mater. 4 (11). Lee, Y.S., Chua, D., Brandt, R.E., Siah, S.C., Li, J.V., Mailoa, J.P., Buonassisi, T., 2014c. Atomic layer deposited gallium oxide buffer layer enables 1.2 V open-circuit voltage in cuprous oxide solar cells. Adv. Mater. 26 (27), 4704–4710. Lee, Y.S., Heo, J., Siah, S.C., Mailoa, J.P., Brandt, R.E., Kim, S.B., Buonassisi, T., 2013. Ultrathin amorphous zinc-tin-oxide buffer layer
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