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4-Terminal All-Perovskite Tandem Solar Cells Achieving Power Conversion Efficiencies Exceeding 23% Dewei Zhao, Changlei Wang, Zhaoning Song, Yue Yu, Cong Chen, Xingzhong Zhao, Kai Zhu, and Yanfa Yan ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01287 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Energy Letters

4-Terminal All-Perovskite Tandem Solar Cells Achieving Power Conversion Efficiencies Exceeding 23% Dewei Zhao,1,# Changlei Wang,1,2,# Zhaoning Song,1 Yue Yu,1 Cong Chen,1 Xingzhong Zhao,2 Kai Zhu,3 Yanfa Yan1,* 1

Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and

Commercialization, The University of Toledo, Toledo, Ohio 43606, United States 2

Key Laboratory of Artificial Micro/Nano Structures of Ministry of Education, School of Physics

and Technology, Wuhan University, Wuhan 430072, China 3

Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado

80401, United States

Author Contributions ﹟D.Z. and C.W. contributed equally to this work.

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Abstract:

We report on fabrication of 4-terminal all-perovskite tandem solar cells with power conversion efficiencies exceeding 23% by mechanically stacking semitransparent 1.75 eV wide-bandgap FA0.8Cs0.2Pb(I0.7Br0.3)3 perovskite top cells with 1.25 eV low-bandgap (FASnI3)0.6(MAPbI3)0.4 bottom cells. The top cells use MoOx/ITO transparent electrodes and achieve transmittance up to 70% beyond 700 nm.

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ACS Energy Letters

Organic-inorganic perovskite solar cells (PVSCs) have attracted intensive attention in the past few years since the record power conversion efficiency (PCE) of single-junction PVSCs has rapidly increased from 3.8% to certified 22.7%,1-2 approaching their theoretical PCEs limited by the Shockley-Queisser (SQ) radiative efficiency.3 One proven approach to break the singlejunction SQ efficiency limits is to form tandem devices by combining a top subcell with a widebandgap (wide-Eg) absorber with a bottom subcell with a low-bandgap (low-Eg) absorber,4-5 through either a two-terminal (2-T) or a four-terminal (4-T) tandem configuration. In principle, 4-T tandem configuration (mechanically stacks the top and bottom subcells) can have slightly higher theoretical efficiency than 2-T configuration (monolithically integrates the top and bottom subcells), since 4-T tandem configuration does not require current matching between the two subcells as 2-T tandem configuration does. So far, efficient perovskite/Si, perovskite/copper indium gallium diselenide (Cu(In,Ga)Se2), perovskite/perovskite tandem cells have been reported,6-10 among which perovskite/perovskite tandem cells are particularly attractive due to the low-temperature process of all components and potentially low fabrication cost.4,

11-13

Recently, MeGehee and Snaith and co-workers have reported an all-perovskite 4-T tandem cell with a PCE of 20.3% and 2-T tandem cell with a PCE of 17%.10 Bolink and co-workers combined solution-processed wide-Eg perovskite top cell with vacuum-deposited normalbandgap perovskite bottom cell to build 2-T tandem cells with a champion PCE of 18.1%.8 Jen and co-workers have achieved the PCE of 18.5% for a 2-T all-perovskite tandem cell using a 1.82 eV wide-Eg top cell and a 1.22 eV low-Eg bottom cell.9 By mechanically stacking a semitransparent 1.58 eV wide-Eg perovskite top cell and a 1.25 eV low-Eg perovskite bottom cell, our group has boosted the PCE of 4-T all-perovskite tandem cells to 21% due to the significantly enhanced efficiency of low-Eg (1.25 eV) bottom cells.14-15 To date, however, none of all-


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ACS Energy Letters

perovskite tandem cells reported in literature has showed PCEs higher than or close to the worldrecord PCE of single-junction PVSCs. In this work, we fabricate efficient 4-T all-perovskite tandem cells by mechanically stacking semitransparent 1.75 eV wide-Eg FA0.8Cs0.2Pb(I0.7Br0.3)3 (FA = Formamidinium, Cs = Cesium, Pb = Lead, I = Iodide, and Br = Bromide) top cells with 1.25 eV low-Eg (FASnI3)0.6(MAPbI3)0.4 (Sn = Tin and MA = Methylammonium) bottom cells. The champion device shows a PCE of 23.1% measured under reverse voltage scan and a steady-state efficiency (SSE) of 22.9% under a 100 mW/cm2 AM1.5G solar irradiation. To the best of our knowledge, this is the first time that all-perovskite tandem cells demonstrate PCEs exceeding the world-record PCE of singlejunction PVSCs.

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Transmittance Absorbance

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Absorbance (a.u.)

Transmittance (%)

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Current density (mA/cm )

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Semitransparent 1.75 eV top cell 1.25 eV bottom cell Filtered 1.25 eV bottom cell

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0 1100

0 0.0

Wavelength (nm)

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0.4

0.6

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1.0

1.2

Voltage (V)

24

100

1.75 eV top cell Filtered 1.25 eV bottom cell

22.9% 20

80

EQE (%)

2

PCE (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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15.5% 1.75 eV top cell Filtered 1.25 eV bottom cell 4-Terminal tandem cell

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17.1 mA/cm 60

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11.7 mA/cm

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7.4% 20

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(d)

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ACS Energy Letters

Figure 1. (a) Absorption spectrum of a 1.75 eV wide-Eg perovskite film and transmittance spectrum of a semitransparent 1.75 eV wide-Eg perovskite top cell. (b) J–V curves of the semitransparent 1.75 eV wide-Eg perovskite top cell illuminated from Glass/FTO side and the 1.25 eV low-Eg perovskite bottom cell with and without the semitransparent 1.75 eV wide-Eg perovskite top cell as an optical filter. (c) Steady-state efficiencies of the semitransparent wideEg (1.75 eV) perovksite top cell, the filtered low-Eg (1.25 eV) perovskite bottom cell, and the summated 4-T all-perovskite tandem cell. (d) EQE spectra of the 1.75 eV wide-Eg perovskite top cell and the filtered 1.25 eV low-Eg perovskite bottom cell.

Table 1. Summary of main performance metrics of the semitransparent wide-Eg (1.75 eV) perovskite top cell, the low-Eg (1.25 eV) perovskite bottom cell, the filtered low-Eg (1.25 eV) perovskite bottom cell, and the summated 4-T all-perovskite tandem cell. 2

Cells Wide-Eg cell (1.75 eV)

Voc (V)

Jsc (mA/cm )

1.204

17.6

74.6

15.7

15.5

Low-Eg cell (1.25 eV)

0.845

28.6

72.4

17.5

17.2

Filtered low-Eg cell

0.810

12.1

75.3

7.4

7.4

23.1

22.9

(1.25 eV) 4-T tandem cell

FF (%) PCE (%) SSE (%)

We used two strategies to accomplish the improved PCEs of 4-T all-perovskite tandem cells. First, we developed efficient semitransparent 1.75 eV wide-Eg FA0.8Cs0.2Pb(I0.7Br0.3)3 perovskite top cells, allowing more infrared light to reach the bottom cells. In our previous 4-T allperovskite tandem cells, the top cells used 1.58 eV FA0.3MA0.7PbI3 perovskite absorber and MoOx/Au/MoOx electrode, which limit the transmitted light reaching bottom cell due to the perovskite absorption onset at around 800 nm and the parasitic absorption by the thin metal layer.15 To circumvent these issues, our new semitransparent top cells use a 1.75 eV FA0.8Cs0.2Pb(I0.7Br0.3)3 perovskite absorber moving the absorption onset to the relatively shorter wavelength as shown in Figure 1a. Additionally, the MoOx/Au/MoOx electrode is replaced by the more transparent MoOx/Indium tin oxide (ITO) electrode, which enhances transmittance,


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especially in the wavelength range beyond 700 nm (Figure S1). As a result, the light transparency beyond 700 nm of the new top cell is increased up to 70% (Figure S2), which significantly enhances the performance of the filtered low-Eg 1.25 eV (FASnI3)0.6(MAPbI3)0.4 bottom cell. Secondly, we applied paraffin oil as an optical coupling spacer between the semitransparent wide-Eg top cell and the low-Eg bottom cell to minimize the optical loss for the low-Eg bottom cell due to the multiple reflections in between the air gap between the subcells,6 as shown in Figure S3. Our semitransparent wide-Eg top cell has a structure of FTO/SnO2/C60SAM/FA0.8Cs0.2Pb(I0.7Br0.3)3/Spiro-OMeTAD/MoOx/ITO (FTO is fluorine-doped tin oxide (SnO2), SnO2 serves as the electron selective layer (ESL), C60-SAM is fullerene-self-assembled monolayer,

Spiro-OMeTAD

is

2,2’,7,7’-tetrakis(N,N-bis(p-methoxy-phenyl)amino)-9,9’-

spirobifluorene), and the low-Eg bottom cell has a structure of indium-doped tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene

sulfonate

(PEDOT:PSS)/(FASnI3)0.6(MAPbI3)0.4/C60/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Ag. The J–V curves, SSE tracking, and PV parameter summary of the semitransparent wide-Eg top cell, the low-Eg bottom cell, and the filtered low-Eg bottom cell are shown in Figure 1b and c and Table 1. The integrated Jsc’s from the measured external quantum efficiency (EQE) (Figure 1d) of the semitransparent top cell and the filtered bottom cell are 17.1 and 11.7 mA/cm2, respectively, in agreement with the Jsc’s obtained from the J–V curves. The EQE spectrum of the unfiltered bottom cell is shown in Figure S4. By mechanically stacking two subcells, the best 4-T all-perovskite tandem cell achieves a PCE of 23.1% measured under the reverse voltage scan and an SSE of 22.9% measured by maximum power point tracking, which are higher than those of the wide-Eg semitransparent top cell, 15.7% and 15.5%, and those of filtered low-Eg


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ACS Energy Letters

(FASnI3)0.6(MAPbI3)0.4 bottom cell, 7.4% and 7.4%. For the first time, an all-perovskite tandem solar cell shows a PCE higher than the world-record PCE of single-junction PVSC. In summary, we have fabricated highly efficient 4-T all-perovskite tandem cell with PCEs exceeding 23% by mechanically stacking semitransparent 1.75 eV wide-Eg perovskite top cells with 1.25 eV low-Eg perovskite bottom cells. Our results demonstrate the potential of low-cost all-perovskite tandem solar cells for achieving ultra-high PCEs.

ASSOCIATED CONTENT Supporting Information. Experimental details and supplementary characterizations of materials and devices. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is financially supported by the U.S. Department of Energy (DOE) SunShot Initiative under the Next Generation Photovoltaics 3 program (DE-FOA-0000990), National Science Foundation under contract no. CHE−1230246, DMR−1534686, and ECCS1665028, and the


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Ohio Research Scholar Program.The work at the National Renewable Energy Laboratory is supported by the U.S. Department of Energy SunShot Initiative under the Next Generation Photovoltaics 3 program (DE-FOA-0000990) under Contract No. DE-AC36-08-GO28308.

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ACS Energy Letters

(11) Hörantner, M. T.; Leijtens, T.; Ziffer, M. E.; Eperon, G. E.; Christoforo, M. G.; McGehee, M. D.; Snaith, H. J. The Potential of Multijunction Perovskite Solar Cells. ACS Energy Lett. 2017, 2, 2506-2513. (12) Eperon, G. E.; Hörantner, M. T.; Snaith, H. J. Metal halide perovskite tandem and multiplejunction photovoltaics. Nature Reviews Chemistry 2017, 1, 0095. (13) Song, Z.; McElvany, C. L.; Phillips, A. B.; Celik, I.; Krantz, P. W.; Watthage, S. C.; Liyanage, G. K.; Apul, D.; Heben, M. J. A technoeconomic analysis of perovskite solar module manufacturing with low-cost materials and techniques. Energy Environ. Sci. 2017, 10, 12971305. (14) Liao, W.; Zhao, D.; Yu, Y.; Shrestha, N.; Ghimire, K.; Grice, C. R.; Wang, C.; Xiao, Y.; Cimaroli, A. J.; Ellingson, R. J., et al. Fabrication of Efficient Low-Bandgap Perovskite Solar Cells by Combining Formamidinium Tin Iodide with Methylammonium Lead Iodide. J. Am. Chem. Soc. 2016, 138, 12360-12363. (15) Zhao, D.; Yu, Y.; Wang, C.; Liao, W.; Shrestha, N.; Grice, C. R.; Cimaroli, A. J.; Guan, L.; Ellingson, R. J.; Zhu, K., et al. Low-bandgap mixed tin–lead iodide perovskite absorbers with long carrier lifetimes for all-perovskite tandem solar cells. Nat. Energy 2017, 2, 17018.


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