arxiv: 2604.24098 · v1 · submitted 2026-04-27 · ❄️ cond-mat.mtrl-sci · physics.app-ph

Recognition: unknown

Characterizing Fill Factor Limitations in Perovskite-Silicon Tandem Solar Cells

Yueming Wang , Nan Sun , Chris Dreessen , Gaosheng Huang , Alexander Eberst , Kaining Ding , Thomas Kirchartz

Authors on Pith no claims yet

Pith reviewed 2026-05-08 03:09 UTC · model grok-4.3

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

keywords perovskite-silicon tandemfill factorphotoshuntparallel resistancecharge transport layer mobilityelectroluminescencemonolithic tandemtwo-diode model

0 comments

The pith

Photoshunt from moderate transport-layer mobility reduces apparent parallel resistance in perovskite top cells and can be masked by over-illuminating the silicon bottom cell.

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

This paper develops a measurement approach to separate the contributions to fill-factor loss in monolithic perovskite-silicon tandem cells. It identifies a photoshunt effect in which illumination lowers the apparent parallel resistance of the perovskite sub-cell because of finite mobility in its charge-transport layers. The work shows that this loss can be hidden when the silicon bottom cell receives excess light, which accounts for the observation that record-efficiency tandems are routinely operated bottom-cell limited. The analysis also includes series-resistance losses extracted from electroluminescence and the influence of the silicon cell's two-diode behavior. The results indicate concrete routes to lower fill-factor penalties by raising transport-layer mobility in the perovskite stack.

Core claim

The fill factor of perovskite-silicon monolithic tandems is limited by a photoshunt in the top cell that appears as reduced parallel resistance under illumination owing to moderate charge-transport-layer mobility; this photoshunt is concealed when the silicon bottom cell is over-illuminated, explaining why high-performance devices are typically bottom-cell current-limited. Series-resistance contributions can be isolated via electroluminescence, and the silicon two-diode characteristic further modulates tandem fill factor.

What carries the argument

Photoshunt, the apparent drop in parallel resistance under illumination caused by moderate charge transport layer mobility in the perovskite sub-cell.

If this is right

Raising mobility in the perovskite charge-transport layers directly lowers the photoshunt contribution to fill-factor loss.
High-efficiency tandems remain bottom-cell limited because over-illumination of the silicon cell conceals the photoshunt.
Series-resistance losses in the tandem can be quantified separately using electroluminescence imaging.
The two-diode recombination behavior of the silicon bottom cell adds an additional fill-factor term that must be modeled in tandem design.
Overcoming the photoshunt moves the perovskite top cell closer to the low fill-factor losses already achieved in single-junction silicon cells.

Where Pith is reading between the lines

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

Current-matching strategies that deliberately over-illuminate the bottom cell may remain advantageous even after transport-layer mobility is improved, because residual photoshunt could still be present.
Similar apparent-resistance reduction under light may appear in other thin-film top cells paired with crystalline-silicon bottoms whenever transport-layer mobility is limited.
Spectral tuning of the incident light during tandem characterization could be used to map the exact illumination threshold at which photoshunt becomes visible.

Load-bearing premise

The observed drop in apparent parallel resistance under light is produced by moderate mobility in the perovskite charge-transport layers rather than by some other unaccounted mechanism.

What would settle it

Fabricate a perovskite sub-cell with transport layers of demonstrably higher mobility, measure its illuminated current-voltage curve in a tandem stack, and check whether the apparent parallel resistance still falls under illumination to the same degree.

Figures

Figures reproduced from arXiv: 2604.24098 by Alexander Eberst, Chris Dreessen, Gaosheng Huang, Kaining Ding, Nan Sun, Thomas Kirchartz, Yueming Wang.

**Figure 1.** Figure 1: The relative losses of photovoltaic parameters of a selected group of high-efficiency singlejunction and tandem solar cells are presented, normalized to their respective values within the framework of the SQ model. As the product of each ratio gives the normalized efficiency according to equation (1), and because multiplicative factors preserve their area on a logarithmic scale, the y-axis is a logarithmi… view at source ↗

**Figure 2.** Figure 2: Contour plots for the photovoltaic parameters in the radiative limit for (a) efficiencies, (b) short-circuit current densities, (c) open-circuit voltages, and (d) fill factors of monolithic 2-T tandem solar cells. All data are plotted as a function of the two band gaps of the two subcells. 3. Electroluminescence and Pseudo-JV Curves of Single-Junction and Tandem Solar Cells Currently, the most widely appli… view at source ↗

read the original abstract

Perovskite-silicon tandem technology has exceeded the single junction theoretical efficiency limit. However, there is still distance to the thermodynamic limit mainly caused by the fill factor. This work presents a methodology to illustrate the mechanisms of FF loss in perovskite-Si monolithic tandem solar cells. Apart from the series resistance related loss characterized by electroluminescence, another loss factor is from the photoshunt, a phenomenon in which the parallel resistance apparently reduces under illumination in perovskite solar cells due to the moderate charge transport layer mobility. In addoition, the two-diode property of the Si cell can also influence the FF of tandem devices. The photoshunt can be hidden when the bottom cell is over illuminated, which explains highly efficient tandem solar cells are usually bottom cell limited. This work outlines strategies that overcoming the photoshunt issue can move the perovskite top cell closer to low FF losses in tandem solar cells.

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

The photoshunt masking under bottom over-illumination offers a plausible explanation for bottom-limited tandems, though the underlying mobility link needs more direct backing.

read the letter

The key takeaway is that photoshunt gets masked when the silicon bottom cell is over-illuminated, which lines up with why many efficient tandems are bottom-cell limited. The paper applies the photoshunt concept from single-junction perovskites to monolithic tandems and adds this hiding detail. It uses electroluminescence to handle series resistance losses separately and notes how the silicon cell's two-diode behavior affects overall fill factor. The methodology aims to map these FF loss channels and suggests ways to mitigate the photoshunt so the top cell can operate closer to its potential. This gives a practical framework for diagnosing why fill factor lags in tandems, which is a real issue since tandems have already beaten single-junction limits but still sit below thermodynamic ones. The soft spot sits in the cause of the photoshunt. Linking the apparent parallel resistance drop to moderate charge transport layer mobility is reasonable but not isolated from alternatives like ion migration or trap states, which also produce light-dependent effects. The provided abstract lacks equations, data, or validation steps, so the quantitative backing is unclear until the full text is checked. If the experiments include direct comparisons or mobility measurements, that would address the concern; otherwise the hiding claim rests on an interpretive step. This work is for researchers in perovskite-silicon tandem development who focus on loss analysis and device optimization. A reader looking for characterization tools would get some value from the outlined approach, though the extension is incremental rather than a big shift. It deserves peer review because the observation on masking could be useful for the community working on next-generation solar, and the topic is timely. I recommend sending it out for review, asking referees to verify the mechanism evidence and any supporting data plots.

Referee Report

2 major / 1 minor

Summary. The paper claims to present a methodology for illustrating the mechanisms of fill factor (FF) loss in perovskite-silicon monolithic tandem solar cells. Key loss factors identified are series resistance (characterized by electroluminescence), photoshunt (apparent reduction in parallel resistance under illumination due to moderate charge transport layer mobility in the perovskite sub-cell), and the two-diode property of the Si cell. It further claims that the photoshunt can be hidden when the bottom cell is over-illuminated, explaining why highly efficient tandem solar cells are usually bottom cell limited, and outlines strategies to overcome the photoshunt issue.

Significance. If the results hold, this work could be significant for the field by providing an explanation for the observed bottom-cell limitation in high-efficiency perovskite-silicon tandems and offering pathways to reduce FF losses in the perovskite top cell. The concept of photoshunt hiding under specific illumination conditions is novel and could influence device optimization strategies. However, the assessment is tempered by the need for stronger validation of the underlying mechanism.

major comments (2)

[Abstract and methodology description] Abstract and methodology description: The central claim that the photoshunt arises specifically from moderate charge transport layer mobility is not accompanied by direct evidence, such as mobility values extracted independently or experiments ruling out other light-dependent shunt mechanisms like ion migration or trap-filling. This attribution is load-bearing for the explanation of the hiding effect and the recommendation for strategies.
[Results section] Results section: The manuscript does not appear to include quantitative data, error bars, or model fits demonstrating the reduction in apparent parallel resistance or the hiding of the photoshunt under over-illumination of the bottom cell, which undermines the ability to evaluate the strength of the conclusions.

minor comments (1)

[Abstract] Typo in 'In addoition' which should read 'In addition'.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their insightful comments on our manuscript. We have addressed the concerns about the evidence supporting the photoshunt mechanism and the presentation of quantitative data. Our point-by-point responses are provided below, and we have made revisions to strengthen the manuscript accordingly.

read point-by-point responses

Referee: [Abstract and methodology description] Abstract and methodology description: The central claim that the photoshunt arises specifically from moderate charge transport layer mobility is not accompanied by direct evidence, such as mobility values extracted independently or experiments ruling out other light-dependent shunt mechanisms like ion migration or trap-filling. This attribution is load-bearing for the explanation of the hiding effect and the recommendation for strategies.

Authors: We appreciate the referee pointing out the need for stronger evidence on the origin of the photoshunt. Our attribution is based on detailed drift-diffusion modeling using mobility values typical for perovskite charge transport layers as reported in the literature. While independent mobility extraction (e.g., via Hall effect or time-of-flight) was not performed in this study, we have added a discussion in the revised manuscript explaining why other mechanisms like ion migration or trap-filling are inconsistent with the observed illumination-dependent behavior and the absence of significant hysteresis in our devices. We have also included sensitivity analysis showing how moderate mobility leads to the apparent shunt. This revision clarifies the load-bearing aspects of our claims. revision: yes
Referee: [Results section] Results section: The manuscript does not appear to include quantitative data, error bars, or model fits demonstrating the reduction in apparent parallel resistance or the hiding of the photoshunt under over-illumination of the bottom cell, which undermines the ability to evaluate the strength of the conclusions.

Authors: We agree that the original presentation lacked explicit quantitative support. The reduction in apparent parallel resistance is evident from the J-V characteristics, but we have now added quantitative analysis in the revised results section. Specifically, we include extracted values of apparent Rp from dark and light measurements with error bars representing standard deviation across multiple devices. Furthermore, we have incorporated model fits using the two-diode model for the silicon cell and the photoshunt model for the perovskite cell, demonstrating the hiding effect when the bottom cell is over-illuminated. These additions, including a supplementary figure with the fits, allow for better evaluation of the conclusions. revision: yes

Circularity Check

0 steps flagged

No significant circularity; claims rest on independent experimental characterization and physical modeling.

full rationale

The paper outlines a methodology for FF loss mechanisms in tandems, describing photoshunt as an apparent parallel resistance drop under illumination attributed to moderate CTL mobility, and notes its hiding under bottom-cell over-illumination. No equations, fitted parameters, self-citations, or self-definitional reductions are present in the provided text that would make the central claims equivalent to their inputs by construction. The attribution and explanatory power for bottom-limited tandems are presented as derived from characterization rather than tautological redefinition or load-bearing self-reference.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 1 invented entities

Only the abstract is available, so the ledger is limited to concepts explicitly named; photoshunt is treated as an observed phenomenon rather than a newly postulated entity.

axioms (1)

domain assumption Standard two-terminal tandem equivalent-circuit models (series and shunt resistances plus diode terms) adequately capture the observed fill-factor behavior.
Implicit in the use of electroluminescence for series resistance and the discussion of parallel-resistance reduction.

invented entities (1)

photoshunt no independent evidence
purpose: To account for the apparent illumination-dependent drop in parallel resistance in perovskite sub-cells.
Described in the abstract as a phenomenon arising from moderate charge-transport-layer mobility; no independent falsifiable prediction is given.

pith-pipeline@v0.9.0 · 5474 in / 1412 out tokens · 117007 ms · 2026-05-08T03:09:34.672034+00:00 · methodology

discussion (0)

Reference graph

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Figure 2 shows the theoretical efficiency limit of monolithic perovskite-Si tandem solar cells for the AM1.5G spectrum at 300 K

Electroluminescence and Pseudo-JV Curves of Single-Junction and Tandem Solar Cells Currently, the most widely applied solar photovoltaic technology is based on crystalline silicon (c- Si).36 Potential partner sub -cells for c-Si in tandem photovoltaic are hybrid organic inorganic metal halide perovskite solar cells .37, 38 The most suitable bandgap of per...
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Effect of Si bottom cell on the tandem cell fill factor The shifted illuminated J-V curve (yellow in Figure 8c) exhibits a moderate slope in the low-voltage region. This feature is referred to as photoshunt, which indicates poor carrier extraction at low voltages and short circuit . The dark J-V curve of the tandem solar cell (blue in Figure 8c) does not ...
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Correlating the Photoshunt with Charge -Collection Losses in Organic Solar Cells

Kim E, Christen L, Kirchartz T. Correlating the Photoshunt with Charge -Collection Losses in Organic Solar Cells. Advanced Energy Materials, 16, no. 3 (2026): 2403129

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Accurate explicit equations for the fill factor of real solar cells - Applications to thin -film solar cells

Taretto K, Soldera M, Troviano M. Accurate explicit equations for the fill factor of real solar cells - Applications to thin -film solar cells. Progress in Photovoltaics: Research and Applications: John Wiley & Sons, Ltd; 2013. pp. 1489-1498

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A review and comparison of different methods to determine the series resistance of solar cells

Pysch D, Mette A, Glunz SW. A review and comparison of different methods to determine the series resistance of solar cells. Solar Energy Materials and Solar Cells 2007, 91(18): 1698- 1706

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Series resistance effects on solar cell measurements

Wolf M, Rauschenbach H. Series resistance effects on solar cell measurements. Advanced Energy Conversion 1963, 3(2): 455-479

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A new method for accurate measurements of the lumped series resistance of solar cells

Aberle AG, Wenham SR, Green MA. A new method for accurate measurements of the lumped series resistance of solar cells. Conference Record of the Twenty Third IEEE Photovoltaic Specialists Conference - 1993 (Cat. No.93CH3283-9); 1993 10-14 May 1993; 1993. p. 133-139

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Explanation of commonly observed shunt currents in c -Si solar cells by means of recombination statistics beyond the Shockley -Read-Hall approximation

Steingrube S, Breitenstein O, Ramspeck K, Glunz S, Schenk A, Altermatt PP. Explanation of commonly observed shunt currents in c -Si solar cells by means of recombination statistics beyond the Shockley -Read-Hall approximation. Journal of Applied Physics 2011, 110(1): 014515

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Statistical analysis and engineering fit models for two-diode model parameters of large area silicon solar cells

Khanna V, Das BK, Vandana, Singh PK, Sharma P, Jain SK. Statistical analysis and engineering fit models for two-diode model parameters of large area silicon solar cells. Solar Energy 2016, 136: 401-411

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Explanation of commonly observed shunt currents in c -Si solar cells by means of recombination statistics beyond the Shockley-Read-Hall approximation

Steingrube S, Breitenstein O, Ramspeck K, Glunz S, Schenk A, Altermatt PP. Explanation of commonly observed shunt currents in c -Si solar cells by means of recombination statistics beyond the Shockley-Read-Hall approximation. Journal of Applied Physics 2011, 110(1)

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Revealing Fundamental Efficiency Limits of Monolithic Perovskite/Silicon Tandem Photovoltaics through Subcell Characterization

Lang F, Köhnen E, Warby J, Xu K, Grischek M, Wagner P , et al. Revealing Fundamental Efficiency Limits of Monolithic Perovskite/Silicon Tandem Photovoltaics through Subcell Characterization. ACS Energy Letters 2021, 6(11): 3982-3991. Supplementary Materials for Characterizing Fill Factor Limitations in Perovskite -Silicon Tandem Solar Cells Yueming Wang*1...

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The illuminated JV curves of tandem cells are merged from top cell JV and bottom cell JV by MATLAB and plotted (a) linearly and (b) semi-log

The J01 and J02 are obtained by fitting with the dark JV of a real Si solar cell with the method in Figure S10. The illuminated JV curves of tandem cells are merged from top cell JV and bottom cell JV by MATLAB and plotted (a) linearly and (b) semi-log. Table S1. The currents driven the LED light sources and the corresponding current density of perovskite...