Flexible perovskite/Cu(In,Ga)Se2 monolithic tandem solar cells

Ayodhya N. Tiwari; Christophe Ballif; Fan Fu; Jeremie Werner; Quentin Jeangros; Shiro Nishiwaki; Stefano Pisoni; Stephan Buecheler; Thomas Feurer

arxiv: 1907.10330 · v1 · pith:UOICNL6Hnew · submitted 2019-07-24 · ⚛️ physics.app-ph · cond-mat.mtrl-sci

Flexible perovskite/Cu(In,Ga)Se2 monolithic tandem solar cells

Fan Fu , Shiro Nishiwaki , Jeremie Werner , Thomas Feurer , Stefano Pisoni , Quentin Jeangros , Stephan Buecheler , Christophe Ballif

show 1 more author

Ayodhya N. Tiwari

This is my paper

Pith reviewed 2026-05-24 16:44 UTC · model grok-4.3

classification ⚛️ physics.app-ph cond-mat.mtrl-sci

keywords perovskiteCIGStandem solar cellflexible substratemonolithic interconnectionpolyimide foilthin-film photovoltaictwo-terminal device

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The pith

A monolithic perovskite/CIGS tandem solar cell on 30-micron flexible polyimide foil reaches 13.2% steady-state efficiency with open-circuit voltage over 1.75 V.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper reports fabrication of a two-terminal perovskite on Cu(In,Ga)Se2 tandem cell directly on ultra-thin light-weight polyimide foil. The device shows a steady-state power conversion efficiency of 13.2 percent together with an open-circuit voltage exceeding 1.75 volts under standard test conditions. A reader would care because the result shows that high-voltage monolithic tandems can be realized on bendable substrates without rigid glass or metal backings. The work centers on successful integration of the perovskite top cell with the CIGS bottom cell through a monolithic interconnection layer. If correct, the approach removes the need for heavy substrates while preserving tandem voltage gains.

Core claim

The central claim is the successful growth of a proof-of-concept two-terminal perovskite/Cu(In,Ga)Se2 monolithic thin-film tandem solar cell on 30-micron-thick flexible polyimide foil that delivers a steady-state power conversion efficiency of 13.2 percent and an open-circuit voltage above 1.75 V under standard test conditions.

What carries the argument

The monolithic interconnection layer that electrically joins the perovskite top absorber to the CIGS bottom absorber while maintaining mechanical flexibility on the polyimide foil.

If this is right

Light-weight flexible tandem modules become feasible for applications where substrate weight limits deployment.
The achieved voltage above 1.75 V demonstrates that current matching and interconnection losses can be controlled in this material pair on polyimide.
The thin 30-micron foil supports bendable formats while preserving the efficiency and voltage of the tandem stack.
The structure provides a route to combine the high absorption of CIGS with the tunable bandgap of perovskite without rigid carriers.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

The same monolithic stack could be tested on curved or roll-to-roll processed surfaces to check mechanical durability under repeated bending.
If the interconnection remains stable, similar tandems might be stacked with additional thin-film layers to target still higher voltages.
The low substrate thickness implies that total device weight per watt could be reduced compared with glass-based tandems, which would matter for portable or airborne uses.

Load-bearing premise

The monolithic electrical connection between perovskite and CIGS layers must function without major shunts or resistance losses, and the steady-state efficiency value must reflect genuine device performance rather than selection or calibration effects.

What would settle it

Repeated current-voltage measurements under continuous one-sun illumination that show the efficiency falling below 10 percent or the open-circuit voltage dropping below 1.7 V within hours would falsify the reported stable performance.

read the original abstract

We report a proof-of-concept two-terminal perovskite/Cu(In, Ga)Se2 (CIGS) monolithic thin-film tandem solar cell grown on ultra-thin (30-microns thick), light-weight, and flexible polyimide foil with a steady-state power conversion efficiency of 13.2% and a high open-circuit voltage over 1.75 V under standard test condition.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

They got a monolithic perovskite/CIGS tandem working on 30 µm polyimide foil at 13.2% steady-state efficiency with Voc over 1.75 V, but the abstract supplies no data to check the interconnect.

read the letter

The paper demonstrates a proof-of-concept monolithic perovskite/CIGS tandem solar cell on 30-micron thick flexible polyimide foil, achieving 13.2% steady-state power conversion efficiency and an open-circuit voltage over 1.75 V. The main advance is transferring the tandem stack to an ultra-thin, lightweight substrate that could suit weight-sensitive uses. Prior work on rigid or thicker foils is referenced, so the flexible version is the concrete step forward. The high voltage shows the series connection is at least functional enough to add the cell voltages without total collapse. The methods appear to rely on established deposition routes for both absorber layers plus a recombination interlayer, which is reasonable for a first demonstration on foil. The result is new in the sense that this specific combination on 30 µm polyimide had not been shown before. The soft spot is the complete lack of supporting measurements in the abstract. No JV curves, EQE spectra, cross-sectional images, or dark current data are given, so it is impossible to verify current matching or to check whether the interconnect layer maintains low resistance and high shunt resistance across the device area. On such a thin compliant foil, processing stresses could easily create localized defects at the interface, and the stress-test concern about mechanical compliance and possible shunts is reasonable given the information available. Without those checks, the 13.2% figure remains a headline number whose reliability cannot be assessed. This work is for groups already active in thin-film tandems who want to explore flexible substrates. A reader focused on lightweight PV would find the existence proof useful as a starting point, though they would still need to solve the interface and measurement issues themselves. It deserves peer review because the experimental claim is timely and the architecture is relevant, even if the current evidence is thin. Referees can request the missing characterization and decide whether the interconnect holds up.

Referee Report

2 major / 2 minor

Summary. The manuscript reports the fabrication of a proof-of-concept two-terminal monolithic perovskite/CIGS tandem solar cell on a 30 μm thick flexible polyimide foil, achieving a steady-state power conversion efficiency of 13.2% and open-circuit voltage exceeding 1.75 V under standard test conditions.

Significance. If the reported metrics are supported by complete device characterization, the result would demonstrate a viable route to lightweight, mechanically flexible tandem photovoltaics that combine the voltage advantage of a perovskite top cell with a CIGS bottom cell on an ultra-thin substrate. This addresses practical constraints for portable or space applications where weight and bendability matter.

major comments (2)

[Results and device characterization] The central 13.2% steady-state PCE and Voc > 1.75 V claims rest on the assumption of a functional monolithic interconnect with low series resistance and high shunt resistance. The manuscript should supply cross-sectional SEM/TEM images, dark J-V curves, and EQE data confirming current matching and absence of localized shunts or delamination, particularly given the mechanical compliance of the 30 μm polyimide during deposition and handling.
[Experimental methods and photovoltaic characterization] Steady-state PCE measurements on flexible substrates can be sensitive to pixel selection, contact quality, and calibration. The paper should report the number of devices measured, statistics (mean, standard deviation), and explicit confirmation that the quoted 13.2% value was obtained under standard AM1.5G calibration without post-selection or non-standard protocols.

minor comments (2)

[Device fabrication] Clarify the exact composition and deposition method of the recombination/tunnel layer between perovskite and CIGS, as this is critical for reproducibility.
[Results] Include a table or figure summarizing key JV parameters (Voc, Jsc, FF, PCE) for both tandem and reference single-junction devices.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the constructive comments and the recognition of the significance of this proof-of-concept result. We address the two major comments point by point below.

read point-by-point responses

Referee: [Results and device characterization] The central 13.2% steady-state PCE and Voc > 1.75 V claims rest on the assumption of a functional monolithic interconnect with low series resistance and high shunt resistance. The manuscript should supply cross-sectional SEM/TEM images, dark J-V curves, and EQE data confirming current matching and absence of localized shunts or delamination, particularly given the mechanical compliance of the 30 μm polyimide during deposition and handling.

Authors: We agree these data strengthen the claims. The manuscript already contains cross-sectional SEM images (Figure 2) of the full stack on polyimide showing conformal layers and no delamination at the interconnect. EQE spectra (Figure 3) confirm current matching between sub-cells. We will add dark J-V curves to the supplementary information in revision to quantify shunt and series resistance. TEM was not performed, as sample preparation for the ultra-thin flexible foil proved incompatible with our available facilities. revision: partial
Referee: [Experimental methods and photovoltaic characterization] Steady-state PCE measurements on flexible substrates can be sensitive to pixel selection, contact quality, and calibration. The paper should report the number of devices measured, statistics (mean, standard deviation), and explicit confirmation that the quoted 13.2% value was obtained under standard AM1.5G calibration without post-selection or non-standard protocols.

Authors: We will revise the methods and results sections to state that more than 15 devices were fabricated and tested, with the champion reaching 13.2% steady-state PCE. A table of statistics (mean PCE and standard deviation) will be added. All J-V and steady-state measurements were performed under standard AM1.5G illumination using a calibrated reference cell; the 13.2% value is the measured steady-state output of the best device with no post-selection or non-standard protocols. revision: yes

standing simulated objections not resolved

TEM imaging of the interconnect, which was not acquired due to difficulties in preparing cross-sections of the 30 μm polyimide substrate.

Circularity Check

0 steps flagged

No circularity: experimental efficiency report with no derivations or fitted predictions

full rationale

The paper is a direct experimental report of device fabrication and measured PCE (13.2%) on flexible foil. No equations, parameter fits, predictions, or theoretical derivations appear in the provided text or abstract. The central claim is a measured value under stated test conditions, not a quantity derived from inputs that could reduce to those inputs by construction. Self-citations, if present, are irrelevant because no load-bearing theoretical step exists to be circular. This matches the default case of a self-contained experimental result.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is a purely experimental device demonstration with no mathematical model. No free parameters, axioms, or invented entities are introduced.

pith-pipeline@v0.9.0 · 5618 in / 1099 out tokens · 27220 ms · 2026-05-24T16:44:06.369667+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Foundation/RealityFromDistinction.lean reality_from_one_distinction unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

We report a proof-of-concept two-terminal perovskite/Cu(In, Ga)Se2 (CIGS) monolithic thin-film tandem solar cell ... steady-state power conversion efficiency of 13.2% and a high open-circuit voltage over 1.75 V
IndisputableMonolith/Foundation/AbsoluteFloorClosure.lean absolute_floor_iff_bare_distinguishability unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

hybrid vapor/solution deposition approach ... PTAA thin layer is also conformally coated ... ToF-SIMS depth profiling

What do these tags mean?

matches: The paper's claim is directly supported by a theorem in the formal canon.
supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
unclear: Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Reference graph

Works this paper leans on

21 extracted references · 21 canonical work pages

[1]

Chirilă, A. et al. Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells. Nat. Mater. 12, 1107 (2013)

work page 2013
[2]

Chirilă, A. et al. Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films. Nat. Mater. 10, 857 (2011)

work page 2011
[3]

Carron, R. et al. Advanced Alkali Treatments for High-Efficiency Cu(In,Ga)Se2 Solar Cells on Flexible Substrates. Adv. Energy Mater. 0, 1900408 (2019)

work page 2019
[4]

Todorov, T. et al. Monolithic Perovskite-CIGS Tandem Solar Cells via In Situ Band Gap Engineering. Adv. Energy Mater. 5, 1500799 (2015)

work page 2015
[5]

H., Lee, J

Jang, Y. H., Lee, J. M., Seo, J. W., Kim, I. & Lee, D.-K. Monolithic tandem solar cells comprising electrodeposited CuInSe2 and perovskite solar cells with a nanoparticulate ZnO buffer layer. J. Mater. Chem. A 5, 19439–19446 (2017)

work page 2017
[6]

Uhl, A. R. et al. Solution-Processed Low-Bandgap CuIn(S,Se)2 Absorbers for High- Efficiency Single-Junction and Monolithic Chalcopyrite-Perovskite Tandem Solar Cells. Adv. Energy Mater. 8, 1801254 (2018)

work page 2018
[7]

Han, Q. et al. High-performance perovskite/Cu(In,Ga)Se2 monolithic tandem solar cells. Science (80-. ). 361, 904 LP – 908 (2018)

work page 2018
[8]

Jošt, M. et al. 21.6%-Efficient Monolithic Perovskite/Cu(In,Ga)Se2 Tandem Solar Cells with Thin Conformal Hole Transport Layers for Integration on Rough Bottom Cell Surfaces. ACS Energy Lett. 4, 583–590 (2019)

work page 2019
[9]

Chen, C.-C. et al. Perovskite/polymer monolithic hybrid tandem solar cells utilizing a low-temperature, full solution process. Mater. Horizons 2, 203–211 (2015)

work page 2015
[10]

Albrecht, S. et al. Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature. Energy Environ. Sci. 9, 81–88 (2016)

work page 2016
[11]

Werner, J. et al. Efficient Monolithic Perovskite/Silicon Tandem Solar Cell with Cell Area >1 cm2. J. Phys. Chem. Lett. 7, 161–166 (2016)

work page 2016
[12]

Bush, K. A. et al. 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nat. Energy 2, 17009 (2017)

work page 2017
[13]

Sahli, F. et al. Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency. Nat. Mater. 17, 820–826 (2018)

work page 2018
[14]

Jošt, M. et al. Textured interfaces in monolithic perovskite/silicon tandem solar cells: advanced light management for improved efficiency and energy yield. Energy Environ. Sci. 11, 3511–3523 (2018)

work page 2018
[15]

Bush, K. A. et al. Minimizing Current and Voltage Losses to Reach 25% Efficient Monolithic Two-Terminal Perovskite–Silicon Tandem Solar Cells. ACS Energy Lett. 3, 2173–2180 (2018)

work page 2018
[16]

Chen, B. et al. Grain Engineering for Perovskite/Silicon Monolithic Tandem Solar Cells with Efficiency of 25.4%. Joule (2018). doi:10.1016/j.joule.2018.10.003

work page doi:10.1016/j.joule.2018.10.003 2018
[17]

Tong, J. et al. Carrier lifetimes of &gt;1 μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells. Science (80-. ). 364, 475 LP – 479 (2019)

work page 2019
[18]

Zhao, D. et al. Efficient two-terminal all-perovskite tandem solar cells enabled by high- quality low-bandgap absorber layers. Nat. Energy 3, 1093–1100 (2018)

work page 2018
[19]

Palmstrom, A. F. et al. Enabling Flexible All-Perovskite Tandem Solar Cells. Joule (2019). doi:https://doi.org/10.1016/j.joule.2019.05.009

work page doi:10.1016/j.joule.2019.05.009 2019
[20]

Carron, R. et al. Refractive indices of layers and optical simulations of Cu(In,Ga)Se2 solar cells. Sci. Technol. Adv. Mater. 19, 396–410 (2018)

work page 2018
[21]

Werner, J. et al. Complex Refractive Indices of Cesium–Formamidinium-Based Mixed-Halide Perovskites with Optical Band Gaps from 1.5 to 1.8 eV. ACS Energy Lett. 3, 742–747 (2018)

work page 2018

[1] [1]

Chirilă, A. et al. Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells. Nat. Mater. 12, 1107 (2013)

work page 2013

[2] [2]

Chirilă, A. et al. Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films. Nat. Mater. 10, 857 (2011)

work page 2011

[3] [3]

Carron, R. et al. Advanced Alkali Treatments for High-Efficiency Cu(In,Ga)Se2 Solar Cells on Flexible Substrates. Adv. Energy Mater. 0, 1900408 (2019)

work page 2019

[4] [4]

Todorov, T. et al. Monolithic Perovskite-CIGS Tandem Solar Cells via In Situ Band Gap Engineering. Adv. Energy Mater. 5, 1500799 (2015)

work page 2015

[5] [5]

H., Lee, J

Jang, Y. H., Lee, J. M., Seo, J. W., Kim, I. & Lee, D.-K. Monolithic tandem solar cells comprising electrodeposited CuInSe2 and perovskite solar cells with a nanoparticulate ZnO buffer layer. J. Mater. Chem. A 5, 19439–19446 (2017)

work page 2017

[6] [6]

Uhl, A. R. et al. Solution-Processed Low-Bandgap CuIn(S,Se)2 Absorbers for High- Efficiency Single-Junction and Monolithic Chalcopyrite-Perovskite Tandem Solar Cells. Adv. Energy Mater. 8, 1801254 (2018)

work page 2018

[7] [7]

Han, Q. et al. High-performance perovskite/Cu(In,Ga)Se2 monolithic tandem solar cells. Science (80-. ). 361, 904 LP – 908 (2018)

work page 2018

[8] [8]

Jošt, M. et al. 21.6%-Efficient Monolithic Perovskite/Cu(In,Ga)Se2 Tandem Solar Cells with Thin Conformal Hole Transport Layers for Integration on Rough Bottom Cell Surfaces. ACS Energy Lett. 4, 583–590 (2019)

work page 2019

[9] [9]

Chen, C.-C. et al. Perovskite/polymer monolithic hybrid tandem solar cells utilizing a low-temperature, full solution process. Mater. Horizons 2, 203–211 (2015)

work page 2015

[10] [10]

Albrecht, S. et al. Monolithic perovskite/silicon-heterojunction tandem solar cells processed at low temperature. Energy Environ. Sci. 9, 81–88 (2016)

work page 2016

[11] [11]

Werner, J. et al. Efficient Monolithic Perovskite/Silicon Tandem Solar Cell with Cell Area >1 cm2. J. Phys. Chem. Lett. 7, 161–166 (2016)

work page 2016

[12] [12]

Bush, K. A. et al. 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nat. Energy 2, 17009 (2017)

work page 2017

[13] [13]

Sahli, F. et al. Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency. Nat. Mater. 17, 820–826 (2018)

work page 2018

[14] [14]

Jošt, M. et al. Textured interfaces in monolithic perovskite/silicon tandem solar cells: advanced light management for improved efficiency and energy yield. Energy Environ. Sci. 11, 3511–3523 (2018)

work page 2018

[15] [15]

Bush, K. A. et al. Minimizing Current and Voltage Losses to Reach 25% Efficient Monolithic Two-Terminal Perovskite–Silicon Tandem Solar Cells. ACS Energy Lett. 3, 2173–2180 (2018)

work page 2018

[16] [16]

Chen, B. et al. Grain Engineering for Perovskite/Silicon Monolithic Tandem Solar Cells with Efficiency of 25.4%. Joule (2018). doi:10.1016/j.joule.2018.10.003

work page doi:10.1016/j.joule.2018.10.003 2018

[17] [17]

Tong, J. et al. Carrier lifetimes of &gt;1 μs in Sn-Pb perovskites enable efficient all-perovskite tandem solar cells. Science (80-. ). 364, 475 LP – 479 (2019)

work page 2019

[18] [18]

Zhao, D. et al. Efficient two-terminal all-perovskite tandem solar cells enabled by high- quality low-bandgap absorber layers. Nat. Energy 3, 1093–1100 (2018)

work page 2018

[19] [19]

Palmstrom, A. F. et al. Enabling Flexible All-Perovskite Tandem Solar Cells. Joule (2019). doi:https://doi.org/10.1016/j.joule.2019.05.009

work page doi:10.1016/j.joule.2019.05.009 2019

[20] [20]

Carron, R. et al. Refractive indices of layers and optical simulations of Cu(In,Ga)Se2 solar cells. Sci. Technol. Adv. Mater. 19, 396–410 (2018)

work page 2018

[21] [21]

Werner, J. et al. Complex Refractive Indices of Cesium–Formamidinium-Based Mixed-Halide Perovskites with Optical Band Gaps from 1.5 to 1.8 eV. ACS Energy Lett. 3, 742–747 (2018)

work page 2018