Four-Terminal All-Perovskite Tandem Solar Cells Achieving Power

We report on fabrication of 4-terminal all-perovskite tandem solar cells with power conversion efficiencies exceeding 23% by mechanically stacking ...
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Four-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*,† †

Department of Physics and Astronomy and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, Ohio 43606, United States ‡ Key Laboratory of Artificial Micro/Nano Structures of Ministry of Education, School of Physics and Technology, Wuhan University, Wuhan 430072, China § Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States S Supporting Information *

does not require current matching between the two subcells as the 2-T tandem configuration does. So far, efficient perovskite/ Si, perovskite/copper indium gallium diselenide (Cu(In,Ga)Se2), and perovskite/perovskite tandem cells have been reported,3−9 among which perovskite/perovskite tandem cells are particularly attractive due to the low-temperature process of all components and potentially low fabrication cost.2,8,9 To date, however, none of all-perovskite tandem cells reported in the literature has shown 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 100 mW/cm2 AM1.5G solar irradiation. To the best of our knowledge, this is the first time that allperovskite tandem cells demonstrate PCEs exceeding the worldrecord PCE of single-junction PVSCs. We used two strategies to accomplish the improved PCEs of 4T 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 all-perovskite tandem cells, the top cells used a 1.58 eV FA0.3MA0.7PbI3 perovskite absorber and MoOx/ Au/MoOx electrode, which limit the transmitted light reaching the bottom cell due to the perovskite absorption onset at around 800 nm and the parasitic absorption by the thin metal layer.9 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 a relatively shorter wavelength, as shown in Figure 1a. Additionally, the MoOx/Au/MoOx electrode is

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 lowbandgap (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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rganic−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 approaching their theoretical PCEs limited by the Shockley−Queisser (SQ) radiative efficiency. One proven approach to break the single-junction 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 lowbandgap (low-Eg) absorber,2 through either a two-terminal (2-T) or a four-terminal (4-T) tandem configuration. In principle, the 4-T tandem configuration (mechanically stacks the top and bottom subcells) can have slightly higher theoretical efficiency than the 2-T tandem configuration (monolithically integrates the top and bottom subcells) because the 4-T tandem configuration © 2018 American Chemical Society

Received: December 18, 2017 Accepted: January 4, 2018 Published: January 4, 2018 305

DOI: 10.1021/acsenergylett.7b01287 ACS Energy Lett. 2018, 3, 305−306

Energy Express

Cite This: ACS Energy Lett. 2018, 3, 305−306

Energy Express

ACS Energy Letters

In summary, we have fabricated highly efficient 4-T allperovskite tandem cells 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 ultrahigh PCEs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b01287. Experimental details and supplementary characterizations of materials and devices (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

*E-mail: [email protected]. ORCID

Dewei Zhao: 0000-0001-7914-6288 Zhaoning Song: 0000-0002-6677-0994 Kai Zhu: 0000-0003-0908-3909 Yanfa Yan: 0000-0003-3977-5789 Author Contributions #

D.Z. and C.W. contributed equally to this work.

Notes

replaced by the more transparent MoOx/indium tin oxide (ITO) electrode, which enhances transmittance, 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-E g 1.25 eV (FASnI3)0.6(MAPbI3)0.4 bottom cell. Second, we applied paraffin oil as an optical coupling spacer between the semitransparent wideEg top cell and the low-Eg bottom cell to minimize optical loss for the low-Eg bottom cell due to multiple reflections in between the air gap between the subcells,3 as shown in Figure S3. Our semitransparent wide-Eg top cell has a structure of FTO/SnO2/ C60-SAM/FA0.8Cs0.2Pb(I0.7Br0.3)3/spiro-OMeTAD/MoOx/ITO (FTO is fluorine-doped tin oxide (SnO2), C60-SAM is fullereneself-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 ITO/poly(3,4ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/ (FASnI3)0.6(MAPbI3)0.4/C60/2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (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,c and Table S1. 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 a 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 (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 PVSCs.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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), the National Science Foundation under Contract No. CHE-1230246, DMR1534686, and ECCS1665028, and the 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-AC3608-GO28308.

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DOI: 10.1021/acsenergylett.7b01287 ACS Energy Lett. 2018, 3, 305−306