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arxiv: 2604.05855 · v1 · submitted 2026-04-07 · ❄️ cond-mat.mtrl-sci

Loss analysis of Low Bandgap (Ag,Cu)(In,Ga)Se2 Solar Cells for Tandem Applications

Francesco Lodola , Sevan Gharabeiki , Maximilian Krause , Shiro Nishiwaki , Romain Carron , Susanne Siebentritt This is my paper

Pith reviewed 2026-05-10 19:41 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords chalcopyrite solar cellslow-bandgap absorbersloss analysistandem photovoltaicsnon-radiative recombinationdiode factorspace charge regionphotoluminescence
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The pith

Non-radiative recombination in the absorber and space-charge-region recombination cause the main efficiency losses in 1.0 eV bandgap (Ag,Cu)(In,Ga)Se2 solar cells.

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

The paper conducts a loss analysis on (Ag,Cu)(In,Ga)Se2 solar cells with 1.0 eV bandgap and efficiencies near 18.5 percent that are candidates for bottom cells in tandem devices. Absolute photoluminescence, electroluminescence, current-voltage, and external quantum efficiency measurements are performed on both the bare absorber and finished cells to separate losses in short-circuit current, open-circuit voltage, and fill factor. Current losses trace mainly to incomplete absorption in the absorber, but voltage losses arise from non-radiative recombination inside the absorber and fill-factor losses arise from an elevated diode factor. The diode factor measured on finished cells exceeds the value measured on the absorber alone, pointing to additional recombination inside the space charge region. A reader would care because the breakdown shows which physical processes must be addressed to raise performance in these narrow-bandgap devices.

Core claim

The authors establish that the largest losses occur in open-circuit voltage from non-radiative recombination within the absorber and in fill factor from a diode factor that is markedly higher in the completed cell than in the absorber. This difference indicates that recombination in the space charge region dominates the fill-factor limitation. Absorption losses limit short-circuit current but remain secondary. The conclusions rest on absolute photoluminescence and electroluminescence data that quantify the radiative deficit together with current-voltage characteristics that extract the diode factor before and after cell completion.

What carries the argument

Comparison of diode factor and recombination currents extracted from the bare absorber versus the finished cell, using absolute photoluminescence, electroluminescence, and current-voltage measurements to locate recombination in the absorber bulk versus the space charge region.

If this is right

  • Reducing non-radiative recombination inside the absorber would raise open-circuit voltage.
  • Lowering recombination in the space charge region would reduce the diode factor and raise fill factor.
  • Light-management structures would be required to cut absorption losses and increase short-circuit current.
  • Addressing these two dominant mechanisms would improve suitability of the cells as bottom junctions in tandem stacks.

Where Pith is reading between the lines

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

  • Interface or buffer-layer engineering could reduce space-charge-region recombination without changing absorber quality.
  • In a tandem stack the identified voltage deficit would constrain the operating point and current matching of the overall device.
  • The same measurement sequence applied to other narrow-bandgap absorbers could test whether space-charge-region recombination is a general limitation.

Load-bearing premise

That the rise in diode factor from absorber to finished cell can be attributed solely to space-charge-region recombination without contributions from interfaces, contacts, or measurement differences.

What would settle it

A set of cells in which the space charge region is deliberately modified (for example by buffer-layer changes or interface passivation) while absorber properties remain fixed and the diode factor stays unchanged would falsify the attribution of fill-factor loss to space-charge-region recombination.

read the original abstract

Tandem solar cells can better harness the energy of the solar spectrum. Chalcopyrite solar cells have drawn attention, being the only highly efficient devices with bandgap around 1.0 eV, suitable for bottom cells. In the quest for better efficiencies, we conduct a complete loss analysis of 1.0 eV bandgap (Ag,Cu)(In,Ga)Se2 cells with efficiencies around 18.5%. We perform absolute photoluminescence, electroluminescence, JV and EQE measurements on the absorber and the finished cells to analyze losses of short-circuit current, open-circuit voltage and fill factor. The relevant losses in current are due to absorption losses in the absorber and could only be mitigated by light management structures. But the most significant losses are found in the voltage, due to non-radiative recombination in the absorber, and the fill factor, due to a high diode factor. The diode factor of the cells is significantly higher than in the absorber alone, indicating a strong influence of recombination in the space charge region.

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.

Referee Report

1 major / 2 minor

Summary. The manuscript conducts a loss analysis of ~1.0 eV bandgap (Ag,Cu)(In,Ga)Se2 solar cells (~18.5% efficiency) for tandem applications. Absolute PL, EL, JV, and EQE measurements are performed on both bare absorbers and finished devices to quantify losses in Jsc, Voc, and FF. Current losses are attributed to absorption in the absorber; the dominant voltage losses to non-radiative recombination within the absorber; and FF losses to an elevated diode factor. The diode factor is reported to be markedly higher in completed cells than in the absorber alone, which is interpreted as evidence that space-charge-region recombination dominates in the device.

Significance. If the central attributions hold, the work supplies actionable guidance for improving low-bandgap CIGS bottom cells by targeting absorber non-radiative recombination and clarifying the origin of the high diode factor. The use of complementary absolute PL/EL and JV/EQE data on the same samples is a methodological strength that allows direct comparison between absorber and device behavior. The findings are relevant to the tandem-photovoltaics community, where 1 eV CIGS remains one of the few mature options for the bottom junction.

major comments (1)
  1. [Diode-factor comparison and FF-loss attribution (results/discussion)] The key inference that the increase in diode factor from absorber to finished cell signals dominant space-charge-region recombination rests on the untested premise that the absorber-only measurement (PL or EL intensity-voltage dependence) fully excludes interface, buffer, and contact contributions. No control experiments, interface-variation studies, or modeling of series-resistance distortion in the cell JV curves are described to rule out these confounders. Because this attribution directly supports the partitioning of FF losses, the claim requires either additional data or explicit justification that alternative mechanisms are negligible.
minor comments (2)
  1. The abstract states efficiencies 'around 18.5%' and refers to 'significantly higher' diode factors without providing numerical values, uncertainties, or the number of devices measured. Inclusion of these quantitative details (with error bars) would allow readers to assess the magnitude and reproducibility of the reported losses.
  2. Clarify the precise method used to extract the diode factor from the absorber (e.g., PL intensity vs. quasi-Fermi-level splitting) versus the cell (JV or EL), including any assumptions about ideality-factor extraction ranges or series-resistance corrections.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive feedback on our loss analysis of low-bandgap (Ag,Cu)(In,Ga)Se2 solar cells. We address the major comment point by point below, providing clarifications on our methodology while acknowledging areas where the manuscript can be strengthened.

read point-by-point responses

  1. Referee: The key inference that the increase in diode factor from absorber to finished cell signals dominant space-charge-region recombination rests on the untested premise that the absorber-only measurement (PL or EL intensity-voltage dependence) fully excludes interface, buffer, and contact contributions. No control experiments, interface-variation studies, or modeling of series-resistance distortion in the cell JV curves are described to rule out these confounders. Because this attribution directly supports the partitioning of FF losses, the claim requires either additional data or explicit justification that alternative mechanisms are negligible.

    Authors: We appreciate the referee's scrutiny of this central attribution. The absorber-only PL and EL measurements were performed on bare (Ag,Cu)(In,Ga)Se2 layers prior to buffer and contact deposition, so buffer interfaces and back-contact contributions are absent by experimental design. The extracted diode factor in the absorber therefore captures recombination within the absorber material (bulk and free surfaces). In the completed device the diode factor increases markedly (approaching values near 2), which we interpret as evidence for additional space-charge-region recombination at the p-n junction, consistent with standard thin-film solar cell behavior. We did not perform dedicated interface-variation experiments or series-resistance modeling because the study focused on direct comparative loss analysis using identical samples for absorber and device measurements. We agree, however, that the manuscript would benefit from explicit discussion of why alternative mechanisms are unlikely to dominate the observed change. We will therefore revise the relevant results and discussion sections to include a concise justification based on the magnitude of the diode-factor shift, the absence of buffer layers in the absorber data, and typical literature values for SCR recombination. This constitutes a partial revision (added explanatory text, no new data or experiments). revision: partial

Circularity Check

0 steps flagged

No significant circularity; loss partitioning derived from independent experimental measurements.

full rationale

The paper's central claims rest on direct comparisons of absolute PL, EL, JV, and EQE data collected separately on bare absorbers versus completed cells. Diode-factor values are extracted from fits to these measured curves and then contrasted; the inference that elevated cell diode factor signals SCR recombination follows from the numerical difference between those two data sets rather than from any self-referential definition, fitted parameter renamed as prediction, or load-bearing self-citation. No equation or section reduces the reported voltage or fill-factor losses to the inputs by construction. The analysis therefore remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard domain assumptions of solar-cell characterization without introducing free parameters, new physical entities, or ad-hoc postulates.

axioms (2)
  • domain assumption Absolute photoluminescence and electroluminescence measurements can quantitatively separate radiative and non-radiative recombination rates
    Invoked to attribute voltage losses to non-radiative processes in the absorber.
  • domain assumption An elevated diode factor in the finished cell versus the bare absorber indicates dominant recombination inside the space-charge region
    Used to interpret the fill-factor loss mechanism.

pith-pipeline@v0.9.0 · 5509 in / 1468 out tokens · 46120 ms · 2026-05-10T19:41:29.980282+00:00 · methodology

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