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Enhanced optoelectronic coupling for perovskite/silicon tandem solar cells

Nature volume 623, pages 732–738 (2023)Cite this article

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Abstract

Monolithic perovskite/silicon tandem solar cells are of great appeal as they promise high power conversion efficiencies (PCEs) at affordable cost. In state-of-the-art tandems, the perovskite top cell is electrically coupled to a silicon heterojunction bottom cell by means of a self-assembled monolayer (SAM), anchored on a transparent conductive oxide (TCO), which enables efficient charge transfer between the subcells1,2,3. Yet reproducible, high-performance tandem solar cells require energetically homogeneous SAM coverage, which remains challenging, especially on textured silicon bottom cells. Here, we resolve this issue by using ultrathin (5-nm) amorphous indium zinc oxide (IZO) as the interconnecting TCO, exploiting its high surface-potential homogeneity resulting from the absence of crystal grains and higher density of SAM anchoring sites when compared with commonly used crystalline TCOs. Combined with optical enhancements through equally thin IZO rear electrodes and improved front contact stacks, an independently certified PCE of 32.5% was obtained, which ranks among the highest for perovskite/silicon tandems. Our ultrathin transparent contact approach reduces indium consumption by approximately 80%, which is of importance to sustainable photovoltaics manufacturing4.

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Fig. 1: Homogenized surface potential of the interconnection junction.
Fig. 2: FF analysis of the tandem cells.
Fig. 3: Improved current density of perovskite/silicon tandem solar cells.
Fig. 4: Perovskite/silicon tandem cell characteristics.

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All data are available in the main text or the supplementary materials. Further data are available from the corresponding author on reasonable request.

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Acknowledgements

This work was supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under award nos. OSR-2021-4833, OSR-CARF/CCF-3079, IED OSR-2020-4611, IED OSR-2019-4580, OSR-CRG2020-4350, OSR-2020-CPF-4519, OSR-CRG2019-4093 and IED OSR-2019-4208. We acknowledge the use of the KAUST Solar Center and KAUST Core Labs facilities and support from A. ur Rehman, S. Sarwade and A. Pininti for their contribution to either the development or the fabrication of silicon bottom cells. We thank S. Kralj and M. Morales-Masis for the insightful discussions on TCOs, KPFM analysis and SAMs coverage. Figure 4a was created by H. Hwang, a scientific illustrator at King Abdullah University of Science and Technology (KAUST).

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Author notes

  1. These authors contributed equally: Erkan Aydin, Esma Ugur, Bumin K. Yildirim

Authors and Affiliations

  1. Material Science and Engineering Program (MSE), KAUST Solar Center (KSC), Physical Sciences and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal, Kingdom of Saudi Arabia

    Erkan Aydin, Esma Ugur, Bumin K. Yildirim, Thomas G. Allen, Pia Dally, Arsalan Razzaq, Lujia Xu, Badri Vishal, Aren Yazmaciyan, Ahmed A. Said, Shynggys Zhumagali, Randi Azmi, Maxime Babics & Stefaan De Wolf

  2. Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China

    Fangfang Cao & Chuanxiao Xiao

  3. Ningbo New Materials Testing and Evaluation Center Co., Ltd., Ningbo, China

    Fangfang Cao & Chuanxiao Xiao

  4. Fraunhofer Institute for Solar Energy Systems, Freiburg, Germany

    Andreas Fell

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Contributions

E.A. conceived the project idea. E.A. and E.U. planned the experiments. B.K.Y. performed the optimization of the thin recombination junctions. E.A. developed the rear reflector design and T.G.A. contributed to its development. E.U. fabricated perovskite/silicon tandem solar cells. E.A. and E.U. conducted the device performance analysis. E.U. performed the absolute luminescence measurements and constructed J–V analysis of the devices. E.A. and B.K.Y. developed the MPPT system and performed the stability measurements. B.K.Y. performed the electrical characterization of the thin films. F.C. and C.X. performed the surface and cross-section KPFM measurements. P.D. carried out the XPS and UPS analysis. L.X. performed the optical loss analysis. B.V. performed the cross-sectional TEM measurements. A.Y. optimized the antireflection layer on tandem cells. A.A.S., S.Z. and R.A. contributed to the interpretation of the coverage mechanisms of SAMs. A.F. carried out the Quokka3 simulations. M.B. encapsulated the tandem cells. T.G.A. and A.R. fabricated the bottom cells. E.A. and E.U. wrote the manuscript and all authors contributed to the writing. S.D.W. supervised the overall project and secured the funding.

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Correspondence to Erkan Aydin, Esma Ugur or Stefaan De Wolf.

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Nature thanks Qi Chen, Jianghui Zheng and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Energy levels of the various TCOs before and after 2PACz deposition.

UPS spectra of the Si/TCO substrates showing WF (a) and valance band maximum (b). Si/TCO/2PACz substrates showing WF (c) and valance band maximum (d). e, Tabulated values of the WF and valence band maximum shifts on tandems.

Extended Data Fig. 2 Surface-potential mapping of the various TCOs.

Atomic force microscopy surface morphology and KPFM surface-potential mapping of the various TCO layers before and after 2PACz coverage. Here Ra refers to the arithmetic average roughness of the potential values of the scanned area for the selected regions. The table reports the aligned surface potentials before and after HTL deposition.

Extended Data Fig. 3 XPS analysis and coverage factors.

a, High-resolution XPS analysis of the O 1s of ITO and IZrO, respectively for 5-nm and 20-nm thicknesses. b, C 1s, which demonstrates the environment of the O and C atoms. c, Calculated area of C 1s and In 3d5/2 of the six substrates TCO/2PACz. The coverage factor is calculated for the six substrates. The P/In atomic ratio is calculated by the means of XPS high-resolution spectra of P and In 3d5/2. d, The surface composition of the TCO/SAM samples with three oxygen species and –OH/In–O peak ratio. Note that the measurements here are for the average of three different measurements from different regions to increase the accuracy of the results.

Extended Data Fig. 4 Influence of the interconnecting TCO layers on the cell performances.

Voc (a), Jsc (b), FF (c) and PCE (d) of the perovskite/silicon tandem cells with ITO, IZrO and IZO recombination layers for 20-nm and 5-nm thickness values. Note that we have not yet used the ultrathin rear TCO design here and the front-side IZO thickness is still 70 nm. The devices were fabricated within the same batch and, apart from recombination-layer TCOs, the other layers are identical. The mean values are also superimposed on the parameters. e, SEM cross-section images of the tandem cells on various TCOs. There is no marked change in the crystallinity of the perovskite absorbers by altering the TCO recombination junctions.

Extended Data Fig. 5 Quokka3 simulations.

Simulation results for variation of PCE of the tandem cells for different shunt scenarios.

Extended Data Fig. 6 Front IZO thickness optimization.

a, Simulated 1 − R spectra of tandem device stack with different front IZO thicknesses. b, The sheet resistivity of IZO films with used thicknesses. c, LT-SPICE simulation of PCE evaluation with different finger spacing for 40-nm-thick front IZO. d, Weighted absorptance (calculated from 280 to 800 nm) in relation to sheet resistance for the different IZO thicknesses. The dashed lines indicate curves of constant figure of merit (FOM). e, EQE and 1 − R spectra of perovskite/silicon tandem devices with 40-nm and 70-nm IZO TCOs. f, SEM image of silver finger. Scale bar, 20 µm. g, The statistical distribution of device parameters of 40-nm and 70-nm IZO thicknesses. Note that rear reflector optimizations have not yet been applied to the device stack.

Extended Data Fig. 7 Rear reflector contact optimization.

a, Statistical distribution of the device characteristics of the perovskite/silicon tandem cells with various rear IZO thicknesses. Perovskite processing conditions are identical and fabricated within the same batch. However, the bottom cells are from different batches and we assign the change of the FF and Voc to the variations in the quality of the bottom cell. b, Simulation results for the ideal thickness of the rear MgF2 thicknesses. After 150 nm, the reflectance does not change substantially. c, Photograph of the rear side of tandem cells (including silver layers) with and without rear reflectors.

Extended Data Fig. 8 Simulated EQE.

a, Simulated EQE and 1 − R spectra of perovskite/silicon tandem devices for optimized and non-ideal optics. b, Breakdowns of the optical simulation (the values are mA cm−2). Used thickness values (c) and n and k values (d) in optical simulation. Note that non-ideal optic-device simulation parameters are retrieved from our previous publication1.

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This file contains Supplementary Figs. 1–6, Supplementary Tables 1 and 2 and Supplementary Notes 1–3.

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Aydin, E., Ugur, E., Yildirim, B.K. et al. Enhanced optoelectronic coupling for perovskite/silicon tandem solar cells. Nature 623, 732–738 (2023). https://doi.org/10.1038/s41586-023-06667-4

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