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Solution-processed perovskite-kesterite reflective tandem solar cells
Solar Energy 155 (2017) 35–38
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Brief Note
Solution-processed perovskite-kesterite reflective tandem solar cells Yan Li a, Hongwei Hu b, Bingbing Chen b, Teddy Salim b, Yeng Ming Lam b, Ningyi Yuan a,⇑, Jianning Ding a,c,⇑ a School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China b School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore c Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 9 April 2017 Received in revised form 7 June 2017 Accepted 9 June 2017 Available online 15 June 2017 Keywords: Perovskite solar cell Perovskite/CZTSSe tandem Reflective tandem
a b s t r a c t Solution-processed solar cells are promising as low-cost alternatives to the first-generation solar cells. Combining two solution-processed devices in a tandem structure can potentially achieve a high efficiency while maintaining a relatively low fabrication cost. Here we present a tandem combining solutionprocessed perovskite and CZTSSe solar cells in a reflective configuration. The perovskite cell with a large bandgap acts as a spectral filter reflecting the sub-bandgap photons to a low bandgap CZTSSe cell. A total efficiency of 16.1% has been obtained in a four-terminal measurement, higher than each of the sub-cells. These results show the potential of solution-processed tandem solar cell with perovskite and kesterite in achieving a high efficiency with a low fabrication cost. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Metal halide perovskite solar cells have gained significant attention as a promising alternative for the existing photovoltaic technologies because of their high power conversion efficiency and low processing cost (W.S. Yang et al., 2015; Y. Yang et al., 2015; W. Chen et al., 2015; C.C. Chen et al., 2015; Kojima et al., 2009; Salim et al., 2015; Zhou et al., 2014; Ball et al., 2013; Burschka et al., 2013; Liu et al., 2013; Mei et al., 2014). Their band gap can be easily tuned from 1.55 to 2.3 eV, making them an ideal candidate for tandem applications (Noh et al., 2013; Eperon et al., 2014). In a tandem device, two materials with different band gaps absorb photons from different spectral regions and maximize the harvest of the solar irradiative spectrum. Several narrow band gap photovoltaic materials such as silicon, CIGS and CZTSSe has been employed to construct tandem with perovskite in mechanically stacking or monolithically integrated tandems (McMeekin et al., 2016; Jiang et al., 2016; Song et al., 2016; Mailoa et al., 2015; Todorov et al., 2014; Bush et al., 2017). However, these tandem configurations require strict processing compatibility between two sub-cells, increasing the fabrication cost. Furthermore, the tunneling junction between two sub-cells or transparent electrodes increases parasitic absorption, limiting the tandem per⇑ Corresponding authors at: School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China (J. Ding). E-mail addresses: [email protected] (N. Yuan), [email protected] (J. Ding). http://dx.doi.org/10.1016/j.solener.2017.06.026 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
formance (W. Chen et al., 2015; C.C. Chen et al., 2015; Todorov et al., 2015; Bailie et al., 2015; Todorov et al., 2014; W.S. Yang et al., 2015; Y. Yang et al., 2015; Lang et al., 2015; Löper et al., 2015; Löper et al., 2014; Li et al., 2017). Due to the thermal and solvent sensitivity, the perovskite is easily damaged during deposition of the transparent electrode. As an alternative to the conventional tandem configuration, a reflective tandem applies perovskite solar cell as spectral filter to reflect long wavelength light to the low bandgap cell. This reflective tandem not only greatly simplifies the photovoltaic system by eliminating additional transparent electrodes but also collects all the light reflected from the front side of the perovskite solar cell. In our previous paper, a reflective tandem based on perovskite and silicon cell has been explored which shows a high efficiency of 23.1%, improved by 29% compared to that of perovskite cell (Li et al., 2017). However, the sophisticated process and high cost of bottom silicon cell limits these technologies in cost reduction and production growth. Using solution processed solar cells for both the top and bottom cells of a tandem holds the promise of low cost and high efficiency, a more competitive way in the future photovoltaic market. Solution-processed kesterite solar cells employing earth-abundant elements are promising for the next generation low-cost photovoltaics. Their narrow band gap (1.1 eV for CZTSSe) makes them attractive as bottom cells in tandems. Teodor et al. firstly explored monolithic perovskite/kesterite tandem solar cell by depositing perovskite solar cells on top of CZTSSe cell (Todorov et al., 2014). The device suffered a great optical loss at the top electrode and interlayer electrode, resulting a
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tive stoichiometric ratio and stirred at 60 °C for 3 h. Then, the solution was spin-coated on a molybdenum substrate coated soda-lime glass at 3000 rpm for 30 s, followed by preheating at 260 °C for 2 min. The process was repeated ten times and then put them into a tube furnace annealing in selenium atmosphere at 580 °C for 40 min. CdS buffer layer was deposited by the chemical bath deposition (CBD), ITO and i-ZnO layer were deposited by sputtering method, followed by evaporation of Al electrode. 2.3. Characterization The absorption and reflection was obtained by 5000 UV–VisNIR Spectrometer. The J-V characteristics were measured under an AM 1.5G condition by using a Keithley 2400 sourcemeter with the illumination of a solar simulator. The external quantum efficiency (EQE) spectra were carried out with a monochromated 450 W xenon lamp and a SR830 Lock-in amplifier. 3. Results and discussion Fig. 1. Schematic of perovskite/CZTSSe reflective tandem solar cell.
low tandem efficiency of 4.6% (Todorov et al., 2014). They also suggested that the optimal band gap for perovskite is around 1.7 eV in combination with the small bandgap of CZTSSe. Here, we fabricated CZTSSe solar cell using low-cost solution method and studied the perovskite/CZTSSe reflective tandem solar cell. The perovskite cell was based on a wide band gap mixedcation mixed halide perovskite (FA0.9Cs0.1Pb(I0.7Br0.3)3) in an inverted device structure. The tandem efficiency reached 16.1% using a four-terminal measurement, a significant gain compared each sub-cell. This is also the first report of an efficient solution processed tandem from perovskite and kesterite. 2. Experimental 2.1. Preparation of perovskite solar cells The detailed fabrication of the inverted planer perovskite cell can be found in a previous paper (Li et al., 2017). 2.2. Preparation of CZTSSe solar cell Firstly, the CZTS precursor solution was prepared by mixing with copper acetate monohydrate, zinc acetate dihydrate, tin chloride dehydrate and thiourea into 2-methoxyethanol with respec-
Fig. 1 shows the reflective perovskite/CZTSSe tandem configuration. The perovskite solar cell is oriented at 45° to the incident light, and the CZTSSe solar cell is oriented at 45° to the perovskite solar cell to ensure the reflected light is normally incident on the CZTSSe solar cell. The transmittance and reflection of a perovskite solar cell had been investigated in previous paper, the total reflectance of a perovskite cell could generate more photon current leading to a higher efficiency than transmission configuration (Li et al., 2017). The reflectance of perovskite solar cell is showed in Fig. 2(a), the perovskite cell reflection reach >50% in the near-infrared (NIR) spectral region. The bottom CZTSSe solar cell with a band gap of 1.12 eV was used in the tandem cell. For the top cell, a mixedcation mixed-halide perovskite (FA0.9Cs0.1Pb(I0.7Br0.3)3) was synthesized as the absorber layer with a wide band gap of approximately 1.70 eV, as measured from the UV–Vis absorption shown in Fig. 2(b). Fig. 3 shows the schematic (a) and SEM cross-section (b) of the inverted planer perovskite cell. The perovskite solar cell was fabricated on an ITO substrate, and the poly-TPD was first deposited as the hole transport layer. The perovskite layer was subsequently deposited through spin-coating, followed by annealing at 180 °C for 30 min. The electron extraction layer (PCBM) was prepared by spin coating, and Bathocuproine (BCP) and Ag were deposited by thermal evaporation. The CZTS bottom cell was prepared by a solution process, and the CZTS absorber was deposited on a Mo substrate coated soda-lime glass. After annealing the CZTS film in selenium atmosphere, the CdS buffer layer was deposited by
Fig. 2. The reflectance of the perovskite solar cell (a) and the band gap of perovskite (b).
Y. Li et al. / Solar Energy 155 (2017) 35–38
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Fig. 3. The schematic of inverted perovskite solar cell (a) and corresponding layers in cross-section scanning electron morphological image (b), The schematic of CZTSSe solar cell (c) and corresponding layers in cross-section scanning electron morphological image (d).
Fig. 4. EQE spectra of perovskite cell and filtered CZTSSe cell (a) and J-V characteristics of perovskite solar cells, CZTSSe cell (unfiltered) and CZTSSe cell (filtered) (b).
chemical bath deposition (CBD), then ITO and i-ZnO were deposited using the sputtering method, followed by evaporation of the Al electrode. The corresponding structure and SEM cross-section are shown in Fig. 3(c) and (d), respectively. The EQEs of the perovskite cell and the filtered CZTSSe cell is shown in Fig. 4(a). The perovskite absorbed the short wavelength light resulting in a Jsc of 17.56 mA/cm2, and reflected the long wavelength light to the filtered CZTSSe cell resulting in a Jsc of 8.96 mA/cm2. As seen from the Fig. 4(a), the reflective tandem achieved an EQE of 88% from 400 nm to 700 nm, which was benefited from the CZTSSe sub-cell also collected the reflected light from front of perovskite cell. The current-voltage (J-V) curves of various devices measured under AM 1.5 G illumination is shown in Fig. 4(b). The detailed parameters of various devices are listed in Table 1, and the perovskite cell is measured with an angle of 45° to the incident light. The stand-alone CZTSSe cell exhibits a PCE of 5.01%, with a short circuit current density (Jsc) of 25.56 mA/cm2, an open-circuit voltage (Voc) of 0.378 V, and a fill factor (FF) of 51.7%. The corresponding external quantum efficiency (EQE) spectra of the CZTSSe cell are shown in Fig. 5, which is in good agreement with the values obtained from the current-voltage (J-V) curve. From the intercept in the plot of [hv ln(1 EQE)]2, the band gap of CZTSSe is determined to be 1.12 eV. The perovskite cell measures under the same condition exhibits a PCE of 14.5%, with a Jsc of 18.1 mA/cm2, FF of 72%, and Voc of 1.12 V. Due to the wide band gap of perovskite, the cell has the low Jsc and high Voc. The high Voc of perovskite solar cell indicates the device possesses a small thermodynamic loss with the wide band gap absorber. After the perovskite cell itself acts as a spectral filter in the reflective tandem, the CZTSSe exhibits a Jsc of 9.01 mA/cm2 close to the photon current calculated from EQE curve. Due to a lower Jsc value, the Voc decreased to 0.353 V and a FF of 51.5%, the CZTSSe sub-cell reaches a PCE of 1.63%, leading to a total tandem efficiency of 16.1% with the perovskite subcell. This is higher than both sub-cells stand-alone. Although the tandem efficiency is lower than that of perovskite/Si tandem, this is the best efficiency reported for perovskite/CZTSSe tandem solar
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Table 1 J-V parameters of CZTSSe cells unfiltered and filtered by different perovskite cells. Device
Jsc (mA cm–2)
Voc (V)
FF
PCE (%)
FA0.9Cs0.1Pb (I0.7Br0.3)3 CZTSSe (unfiltered) CZTSSe (filtered) Reflective tandem
18.1 25.56 9.01
1.12 0.378 0.353
0.72 0.517 0.515
14.5 5.01 1.63 16.1
Fig. 5. EQE spectra of CZTSSe solar cells. The inset is the band gap determined to 1.12 eV by the plot of [hv ln(1 EQE)]2 vs hv.
cell. The CZTSSe bottom cell used here has a lower PCE as compared to the recorded highest PCE of 12.6%. If we apply the best CZTSSe cell with the high band gap of perovskite, the efficiency can be reached to 20%. 4. Conclusion We have demonstrated a reflective configuration with solutionprocessed kesterite and inverted perovskite absorbers with a bandgap of 1.7 eV. The perovskite cell itself acts as a spectral filter that absorbs the short-wavelength light and reflects the longwavelength light to the CZTSSe cell. The tandem efficiency reaches 16.1% by using a four-terminal measurement. This study demonstrated the potential of using a reflective tandem to improve the cell efficiency. This solution process and the reflective tandem configuration could dramatically reduce the material cost and module fabrication cost. Acknowledgements This work was supported by the National Natural Science Foundation of China (51272033, 51572037 and 51335002), the Priority
Academic Program Development of Jiangsu Higher Education Institutions, the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (14KJA430001). Y.M.L. acknowledges financial support from a MOE AcRF Tier 1 grant (RG99/14). References Bailie, C.D., Christoforo, M.G., Mailoa, J.P., Bowring, A.R., Unger, E.L., Nguyen, W.H., Burschka, J., Pellet, N., Lee, J.Z., Grätzel, M., Noufi, R., Buonassisi, T., 2015. Energy Environ. Sci. 8, 956–963. Ball, J.M., Lee, M.M., Hey, A., Snaith, H.J., 2013. Science 6, 1739–1743. Burschka, J., Pellet, N., Moon, S.J., Humphry-Baker, R., Gao, P., Nazeeruddin, M.K., Grätzel, M., 2013. Nature 499, 316–319. Bush, K.A., Palmstrom, A.F., Yu, Z.J., Boccard, M., Cheacharoen, R., Mailoa, J.P., Mcmeekin, D.P., Hoye, R.Z., Bailie, C.D., Leijtens, T., Peters, I.M., Minichetti, M.C., Rolston, N., Prasanna, R., Sofia, S., Harwood, D., Ma, W., Moghadam, F., Snaith, H. J., Buonassisi, T., Holman, Z.C., Bent, S.F., McGehee, M.D., 2017. Nature Energy 2, 17009. Chen, C.C., Bae, S.H., Chang, W.H., Hong, Z., Li, G., Chen, Q., Zhou, U., Yang, Y., 2015. Mater. Horiz. 2, 203–211. Eperon, G.E., Stranks, S.D., Menelaou, C., Johnston, M.B., Herz, L.M., Snaith, H.J., 2014. Energy Environ. Sci. 7, 982–988. Jiang, Y., Almansouri, I., Huang, S., Young, T., Li, Y., Peng, Y., Hou, Q.C., Spiccia, L., Bach, U., Cheng, Y.B., Green, M.A., Ho-Baillie, A., 2016. J. Mater. Chem. C 4, 5679– 5689. Kojima, A., Teshima, K., Shirai, Y., Miyasaka, T., 2009. J. AM. Chem. Soc. 131, 6050– 6051. Lang, F., Gluba, M.A., Albrecht, S., Rappich, J., Korte, L., Rech, B., Nickel, N.H., 2015. J. Phys. Chem. Lett. 6, 2745–2750. Li, Y., Hu, H.W., Chen, B.B., Salim, T., Zhang, J., Ding, J.N., Yuan, N.Y., Lam, Y.M., 2017. J. Mater. Chem. C 5, 134–139. Liu, M., Johnston, M.B., Snaith, H.J., 2013. Nature 501, 395–398. Löper, P., Niesen, B., Moon, S.J., De Nicolas, S.M., Holovsky, J., Remes, Z., Ledinsky, M., Haug, F.J., Yum, J.H., Wolf, S.D., Ballif, C., 2014. IEEE J. Photovol. 4, 1545–1551. Löper, P., Moon, S.J., De Nicolas, S.M., Niesen, B., Ledinsky, M., Nicolay, S., Bailat, J., Yum, J.H., Wolf, S.D., Ballif, C., 2015. Phys. Chem. Chem. Phys. 17, 1619–1629. Mailoa, J.P., Bailie, C.D., Johlin, E.C., Hoke, E.T., Akey, A.J., Nguyen, W.H., McGehee, M. D., Buonassisi, T., 2015. Appl. Phys. Lett. 106, 121105. McMeekin, D.P., Sadoughi, G., Rehman, W., Eperon, G.E., Saliba, M., Hörantner, M.T., Haghighirad, A., Sakai, N., Korte, L., Johnston, M.B., Herz, L.M., Snaith, H.J., 2016. Science 351, 151–155. Mei, A., Li, X., Liu, L., Ku, Z., Liu, T., Rong, Y., Xu, M., Hu, M., Chen, J., Yang, Y., Grätzel, M., Han, H.W., 2014. Science 345, 295–298. Noh, J.H., Im, S.H., Heo, J.H., Mandal, T.N., Seok, S.I., 2013. Nano Lett. 13, 1764–1769. Salim, T., Sun, S., Abe, Y., Krishna, A., Grimsdale, A.C., Lam, Y.M., 2015. J. Mater. Chem. A 3, 8943–8969. Song, Z.N., Werner, J., Shrestha, N., Sahli, F., De Wolf, S.D., Niesen, B., Watthage, S.C., Phillips, A.B., Ballif, C., Ellingson, R.J., Heben, M.J., 2016. J. Phys. Chem. Lett. 7, 5114–5120. Todorov, T., Gershon, T., Gunawan, O., Sturdevant, C., Guha, S., 2014. Appl. Phys. Lett. 105, 173902. Todorov, T., Gershon, T., Gunawan, O., Lee, Y.S., Sturdevant, C., Chang, L.Y., Guha, S., 2015. Adv. Energy Mater. 5, 1500799. Chen, W., Wu, Y., Yue, Y., Liu, J., Zhang, W., Yang, X., Chen, H., Bi, E., Ashraful, I., Grätzel, M., 2015. Science 350, 944–948. Yang, W.S., Noh, J.H., Jeon, N.J., Kin, Y.C., Ryu, S., Seo, J., Seok, S.I., 2015. Science 348, 1234–1237. Yang, Y., Chen, Q., Hsieh, Y.T., Song, T.B., Marco, N.D., Zhou, H.P., Yang, Y., 2015. ACS Nano 9, 7714–7721. Zhou, H., Chen, Q., Li, G., Luo, S., Song, T.B., Duan, H.S., Hong, Z., You, J., Yang, Y., 2014. Science 345, 542–546.
Contents lists available at ScienceDirect
Solar Energy journal homepage: www.elsevier.com/locate/solener
Brief Note
Solution-processed perovskite-kesterite reflective tandem solar cells Yan Li a, Hongwei Hu b, Bingbing Chen b, Teddy Salim b, Yeng Ming Lam b, Ningyi Yuan a,⇑, Jianning Ding a,c,⇑ a School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China b School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore c Micro/Nano Science and Technology Center, Jiangsu University, Zhenjiang 212013, China
a r t i c l e
i n f o
Article history: Received 9 April 2017 Received in revised form 7 June 2017 Accepted 9 June 2017 Available online 15 June 2017 Keywords: Perovskite solar cell Perovskite/CZTSSe tandem Reflective tandem
a b s t r a c t Solution-processed solar cells are promising as low-cost alternatives to the first-generation solar cells. Combining two solution-processed devices in a tandem structure can potentially achieve a high efficiency while maintaining a relatively low fabrication cost. Here we present a tandem combining solutionprocessed perovskite and CZTSSe solar cells in a reflective configuration. The perovskite cell with a large bandgap acts as a spectral filter reflecting the sub-bandgap photons to a low bandgap CZTSSe cell. A total efficiency of 16.1% has been obtained in a four-terminal measurement, higher than each of the sub-cells. These results show the potential of solution-processed tandem solar cell with perovskite and kesterite in achieving a high efficiency with a low fabrication cost. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Metal halide perovskite solar cells have gained significant attention as a promising alternative for the existing photovoltaic technologies because of their high power conversion efficiency and low processing cost (W.S. Yang et al., 2015; Y. Yang et al., 2015; W. Chen et al., 2015; C.C. Chen et al., 2015; Kojima et al., 2009; Salim et al., 2015; Zhou et al., 2014; Ball et al., 2013; Burschka et al., 2013; Liu et al., 2013; Mei et al., 2014). Their band gap can be easily tuned from 1.55 to 2.3 eV, making them an ideal candidate for tandem applications (Noh et al., 2013; Eperon et al., 2014). In a tandem device, two materials with different band gaps absorb photons from different spectral regions and maximize the harvest of the solar irradiative spectrum. Several narrow band gap photovoltaic materials such as silicon, CIGS and CZTSSe has been employed to construct tandem with perovskite in mechanically stacking or monolithically integrated tandems (McMeekin et al., 2016; Jiang et al., 2016; Song et al., 2016; Mailoa et al., 2015; Todorov et al., 2014; Bush et al., 2017). However, these tandem configurations require strict processing compatibility between two sub-cells, increasing the fabrication cost. Furthermore, the tunneling junction between two sub-cells or transparent electrodes increases parasitic absorption, limiting the tandem per⇑ Corresponding authors at: School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center for Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164, China (J. Ding). E-mail addresses: [email protected] (N. Yuan), [email protected] (J. Ding). http://dx.doi.org/10.1016/j.solener.2017.06.026 0038-092X/Ó 2017 Elsevier Ltd. All rights reserved.
formance (W. Chen et al., 2015; C.C. Chen et al., 2015; Todorov et al., 2015; Bailie et al., 2015; Todorov et al., 2014; W.S. Yang et al., 2015; Y. Yang et al., 2015; Lang et al., 2015; Löper et al., 2015; Löper et al., 2014; Li et al., 2017). Due to the thermal and solvent sensitivity, the perovskite is easily damaged during deposition of the transparent electrode. As an alternative to the conventional tandem configuration, a reflective tandem applies perovskite solar cell as spectral filter to reflect long wavelength light to the low bandgap cell. This reflective tandem not only greatly simplifies the photovoltaic system by eliminating additional transparent electrodes but also collects all the light reflected from the front side of the perovskite solar cell. In our previous paper, a reflective tandem based on perovskite and silicon cell has been explored which shows a high efficiency of 23.1%, improved by 29% compared to that of perovskite cell (Li et al., 2017). However, the sophisticated process and high cost of bottom silicon cell limits these technologies in cost reduction and production growth. Using solution processed solar cells for both the top and bottom cells of a tandem holds the promise of low cost and high efficiency, a more competitive way in the future photovoltaic market. Solution-processed kesterite solar cells employing earth-abundant elements are promising for the next generation low-cost photovoltaics. Their narrow band gap (1.1 eV for CZTSSe) makes them attractive as bottom cells in tandems. Teodor et al. firstly explored monolithic perovskite/kesterite tandem solar cell by depositing perovskite solar cells on top of CZTSSe cell (Todorov et al., 2014). The device suffered a great optical loss at the top electrode and interlayer electrode, resulting a
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Y. Li et al. / Solar Energy 155 (2017) 35–38
tive stoichiometric ratio and stirred at 60 °C for 3 h. Then, the solution was spin-coated on a molybdenum substrate coated soda-lime glass at 3000 rpm for 30 s, followed by preheating at 260 °C for 2 min. The process was repeated ten times and then put them into a tube furnace annealing in selenium atmosphere at 580 °C for 40 min. CdS buffer layer was deposited by the chemical bath deposition (CBD), ITO and i-ZnO layer were deposited by sputtering method, followed by evaporation of Al electrode. 2.3. Characterization The absorption and reflection was obtained by 5000 UV–VisNIR Spectrometer. The J-V characteristics were measured under an AM 1.5G condition by using a Keithley 2400 sourcemeter with the illumination of a solar simulator. The external quantum efficiency (EQE) spectra were carried out with a monochromated 450 W xenon lamp and a SR830 Lock-in amplifier. 3. Results and discussion Fig. 1. Schematic of perovskite/CZTSSe reflective tandem solar cell.
low tandem efficiency of 4.6% (Todorov et al., 2014). They also suggested that the optimal band gap for perovskite is around 1.7 eV in combination with the small bandgap of CZTSSe. Here, we fabricated CZTSSe solar cell using low-cost solution method and studied the perovskite/CZTSSe reflective tandem solar cell. The perovskite cell was based on a wide band gap mixedcation mixed halide perovskite (FA0.9Cs0.1Pb(I0.7Br0.3)3) in an inverted device structure. The tandem efficiency reached 16.1% using a four-terminal measurement, a significant gain compared each sub-cell. This is also the first report of an efficient solution processed tandem from perovskite and kesterite. 2. Experimental 2.1. Preparation of perovskite solar cells The detailed fabrication of the inverted planer perovskite cell can be found in a previous paper (Li et al., 2017). 2.2. Preparation of CZTSSe solar cell Firstly, the CZTS precursor solution was prepared by mixing with copper acetate monohydrate, zinc acetate dihydrate, tin chloride dehydrate and thiourea into 2-methoxyethanol with respec-
Fig. 1 shows the reflective perovskite/CZTSSe tandem configuration. The perovskite solar cell is oriented at 45° to the incident light, and the CZTSSe solar cell is oriented at 45° to the perovskite solar cell to ensure the reflected light is normally incident on the CZTSSe solar cell. The transmittance and reflection of a perovskite solar cell had been investigated in previous paper, the total reflectance of a perovskite cell could generate more photon current leading to a higher efficiency than transmission configuration (Li et al., 2017). The reflectance of perovskite solar cell is showed in Fig. 2(a), the perovskite cell reflection reach >50% in the near-infrared (NIR) spectral region. The bottom CZTSSe solar cell with a band gap of 1.12 eV was used in the tandem cell. For the top cell, a mixedcation mixed-halide perovskite (FA0.9Cs0.1Pb(I0.7Br0.3)3) was synthesized as the absorber layer with a wide band gap of approximately 1.70 eV, as measured from the UV–Vis absorption shown in Fig. 2(b). Fig. 3 shows the schematic (a) and SEM cross-section (b) of the inverted planer perovskite cell. The perovskite solar cell was fabricated on an ITO substrate, and the poly-TPD was first deposited as the hole transport layer. The perovskite layer was subsequently deposited through spin-coating, followed by annealing at 180 °C for 30 min. The electron extraction layer (PCBM) was prepared by spin coating, and Bathocuproine (BCP) and Ag were deposited by thermal evaporation. The CZTS bottom cell was prepared by a solution process, and the CZTS absorber was deposited on a Mo substrate coated soda-lime glass. After annealing the CZTS film in selenium atmosphere, the CdS buffer layer was deposited by
Fig. 2. The reflectance of the perovskite solar cell (a) and the band gap of perovskite (b).
Y. Li et al. / Solar Energy 155 (2017) 35–38
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Fig. 3. The schematic of inverted perovskite solar cell (a) and corresponding layers in cross-section scanning electron morphological image (b), The schematic of CZTSSe solar cell (c) and corresponding layers in cross-section scanning electron morphological image (d).
Fig. 4. EQE spectra of perovskite cell and filtered CZTSSe cell (a) and J-V characteristics of perovskite solar cells, CZTSSe cell (unfiltered) and CZTSSe cell (filtered) (b).
chemical bath deposition (CBD), then ITO and i-ZnO were deposited using the sputtering method, followed by evaporation of the Al electrode. The corresponding structure and SEM cross-section are shown in Fig. 3(c) and (d), respectively. The EQEs of the perovskite cell and the filtered CZTSSe cell is shown in Fig. 4(a). The perovskite absorbed the short wavelength light resulting in a Jsc of 17.56 mA/cm2, and reflected the long wavelength light to the filtered CZTSSe cell resulting in a Jsc of 8.96 mA/cm2. As seen from the Fig. 4(a), the reflective tandem achieved an EQE of 88% from 400 nm to 700 nm, which was benefited from the CZTSSe sub-cell also collected the reflected light from front of perovskite cell. The current-voltage (J-V) curves of various devices measured under AM 1.5 G illumination is shown in Fig. 4(b). The detailed parameters of various devices are listed in Table 1, and the perovskite cell is measured with an angle of 45° to the incident light. The stand-alone CZTSSe cell exhibits a PCE of 5.01%, with a short circuit current density (Jsc) of 25.56 mA/cm2, an open-circuit voltage (Voc) of 0.378 V, and a fill factor (FF) of 51.7%. The corresponding external quantum efficiency (EQE) spectra of the CZTSSe cell are shown in Fig. 5, which is in good agreement with the values obtained from the current-voltage (J-V) curve. From the intercept in the plot of [hv ln(1 EQE)]2, the band gap of CZTSSe is determined to be 1.12 eV. The perovskite cell measures under the same condition exhibits a PCE of 14.5%, with a Jsc of 18.1 mA/cm2, FF of 72%, and Voc of 1.12 V. Due to the wide band gap of perovskite, the cell has the low Jsc and high Voc. The high Voc of perovskite solar cell indicates the device possesses a small thermodynamic loss with the wide band gap absorber. After the perovskite cell itself acts as a spectral filter in the reflective tandem, the CZTSSe exhibits a Jsc of 9.01 mA/cm2 close to the photon current calculated from EQE curve. Due to a lower Jsc value, the Voc decreased to 0.353 V and a FF of 51.5%, the CZTSSe sub-cell reaches a PCE of 1.63%, leading to a total tandem efficiency of 16.1% with the perovskite subcell. This is higher than both sub-cells stand-alone. Although the tandem efficiency is lower than that of perovskite/Si tandem, this is the best efficiency reported for perovskite/CZTSSe tandem solar
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Table 1 J-V parameters of CZTSSe cells unfiltered and filtered by different perovskite cells. Device
Jsc (mA cm–2)
Voc (V)
FF
PCE (%)
FA0.9Cs0.1Pb (I0.7Br0.3)3 CZTSSe (unfiltered) CZTSSe (filtered) Reflective tandem
18.1 25.56 9.01
1.12 0.378 0.353
0.72 0.517 0.515
14.5 5.01 1.63 16.1
Fig. 5. EQE spectra of CZTSSe solar cells. The inset is the band gap determined to 1.12 eV by the plot of [hv ln(1 EQE)]2 vs hv.
cell. The CZTSSe bottom cell used here has a lower PCE as compared to the recorded highest PCE of 12.6%. If we apply the best CZTSSe cell with the high band gap of perovskite, the efficiency can be reached to 20%. 4. Conclusion We have demonstrated a reflective configuration with solutionprocessed kesterite and inverted perovskite absorbers with a bandgap of 1.7 eV. The perovskite cell itself acts as a spectral filter that absorbs the short-wavelength light and reflects the longwavelength light to the CZTSSe cell. The tandem efficiency reaches 16.1% by using a four-terminal measurement. This study demonstrated the potential of using a reflective tandem to improve the cell efficiency. This solution process and the reflective tandem configuration could dramatically reduce the material cost and module fabrication cost. Acknowledgements This work was supported by the National Natural Science Foundation of China (51272033, 51572037 and 51335002), the Priority
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