How can wide-bandgap perovskite tandem solar cells reach 28% efficiency?

Why iodide and bromide crystallize unevenly

The front cell in this tandem uses a mixed-halide perovskite, FA0.67MA0.27Cs0.06Pb(I0.5Br0.5)3, with a bandgap near 1.85 eV. Iodide and bromide do not behave identically in that lattice: they nucleate at different rates and face different migration barriers. The paper's in situ photoluminescence tracking shows the consequence directly. In the control film, a bromide-rich emission at 650 nm appears as soon as the antisolvent is dropped, while iodide-rich emission at 750 nm emerges later and keeps growing; during annealing the iodide-rich signal appears and then fades. That sequence means the iodide-rich and bromide-rich phases form on different schedules, leaving an inhomogeneous film.

The same mismatch drives halogen-phase segregation, in which the halide distribution shifts over time and under operation. The paper links this to defects such as iodide vacancies, iodide interstitials and molecular iodine, which both accelerate ion migration and open non-radiative recombination paths. For a tandem cell, that matters twice: the front cell's voltage and fill factor set how much of the spectrum the organic bottom cell can convert, and any drift in the front cell's halide distribution erodes stability.

ITA acts only after reacting with methylamine

The additive is not the working species. ITA (2-iminothiolane) is added at 0.2 mg mL−1 to the precursor solution, where it preferentially reacts with methylamine generated from MACl. Nuclear magnetic resonance and high-resolution mass spectrometry identify the product: a peak at m/z 133.0794 matching C5H12N2S, the thiolation product 4-mercapto-N-methylformamidine, called MMF. The precursor is then spin-coated without isolating or purifying MMF, so the intermediate forms in place.

This ordering explains why the strategy is described as sequential. By capturing methylamine, ITA removes the partner that would otherwise undergo an addition-elimination side reaction with formamidinium, which the paper identifies as a source of a detrimental mixed MA-FA phase. The MMF that remains carries amidino and sulfhydryl groups that bind Pb2+ in the precursor, and the paper's infrared and X-ray photoelectron spectroscopy comparisons indicate that MMF interacts more strongly with PbI2 than ITA itself does. The same reducing intermediate is what later consumes iodine.

Slower nucleation yields larger, more uniform grains

The crystallization evidence is consistent across several measurements. X-ray diffraction and grazing-incidence wide-angle X-ray scattering show stronger (100) and (200) signals after ITA treatment, indicating higher crystallinity. Atomic force microscopy reports root-mean-square roughness falling from 28.19 to 19.01 nm, and scanning electron microscopy reports average grain size rising from 193.85 to 288.55 nm. Confocal photoluminescence maps show larger grains and more uniformly bright emission, with fewer sub-micron dark regions.

The in situ data explain the mechanism rather than just the outcome. In the ITA-treated film, neither the bromide-rich nor the iodide-rich emission appears during spin-coating or annealing, which the authors read as homogeneous crystallization. In situ ultraviolet-visible absorption shows a slower, smaller absorbance rise after antisolvent dripping, and the film reaches maximum absorption within 17 s of annealing versus 8 s for the control. The paper attributes this to MMF interacting with Pb-halide species and retarding nucleation, giving grains more time to grow. Grazing-incidence X-ray diffraction also shows residual stress switching from 9.53 MPa tensile to −25.47 MPa compressive.

Iodine is consumed and iodide migration is slowed

Two distinct chemical jobs are claimed for MMF. First, it scavenges molecular iodine and iodine radicals through a redox reaction; mass spectrometry shows a new peak at m/z 263.1359, consistent with a disulfide-containing oxidation product formed when MMF reacts with I2. Second, it anchors mobile iodide through hydrogen bonding at grain boundaries. Time-of-flight secondary ion mass spectrometry finds sulfur signals enriched near both termini of the perovskite film, where halide flux and iodine redox reactions are strongest, which the authors interpret as ITA sitting along the migration pathways rather than being buried inertly in the bulk.

Density functional theory supports the barrier argument. Calculated migration barriers rise along the vacancy-mediated paths, with the iodide vacancy barrier increasing from 0.22 to 0.32, 0.34 and 0.35 eV across the control, ITA, MMF and combined cases, and the bromide vacancy barrier from 0.26 eV for the control to 0.33 eV with ITA and 0.44 eV with ITA plus MMF. For interstitial paths, the combined treatment gives the highest barriers, 0.5 eV for iodide and 0.54 eV for bromide. Experimentally, the ion migration activation energy derived from temperature-dependent dark current measurements rises from 0.165 to 0.406 eV, and the calculated migration rate falls from 2.17 × 104 s−1 to 6.32 × 10−5 s−1.

The voltage gain comes from less non-radiative loss

The single-junction wide-bandgap cell improves from 18.74% to 20.45% power conversion efficiency, with open-circuit voltage rising from 1.337 to 1.391 V, fill factor from 83.88% to 86.66%, and short-circuit current density reported as 16.70 mA cm−2 for the control and 16.96 mA cm−2 for the treated device from the J-V curves. An integrated current density of 16.63 mA cm−2 from the external quantum efficiency curve is consistent with that value. These are small-area devices measured under an aperture mask with an effective area of 0.09 cm2.

The paper decomposes the voltage loss to show where the 54 mV gain originates. Both films share a bandgap of 1.854 eV and the same Shockley-Queisser limit voltage of 1.561 V, so the 293 meV gap between bandgap and limit is unchanged. The difference appears in non-radiative loss, which falls from 202.64 to 149.61 mV as the external electroluminescence quantum efficiency rises from 0.04% to 0.31%. Urbach energy drops from 20.62 to 16.33 meV, indicating a cleaner band edge with fewer tail states. Trap-filled limit voltages fall in both electron-only and hole-only devices, and the built-in potential from Mott-Schottky analysis rises from 1.10 to 1.24 V.

Reading the 28.20% tandem result correctly

The champion tandem pairs the treated wide-bandgap perovskite front cell with a narrow-bandgap organic bulk heterojunction bottom cell based on PM6, BTPSe-Ph4F and MO-IDIC-2F, joined by a 1 nm Au and 15 nm MoO3 interconnection layer. It reaches 28.20% efficiency, certified at 27.82%, with a quasi-steady-state power output of 28.08% (certified 26.96%). The stability figure comes from encapsulated devices tracked at maximum power point under one-sun white LED illumination at 25 °C for 700 h under the ISOS-L-1 protocol, retaining 90% of initial efficiency. The paper notes a brief interruption in that tracking curve caused by a laboratory power outage, after which the same device was measured under the original conditions.

These numbers should not be read as a field or module result. The efficiency is a champion small-area value, the certification covers the tandem cell rather than a module, and the stability test is an indoor ISOS-L-1 protocol rather than outdoor operation. The paper's own comparison point is that perovskite-organic tandems had exceeded 27% (certified over 26.5% at 1.0 cm2) while carrying voltage loss above 0.5 eV and fill factor below 86% in the wide-bandgap front cell; the reported 1.391 V and 86.66% in the single junction are the specific quantities this work moves.

The additive's value is in the front cell, not the stack

The most decision-relevant reading of this paper is that the tandem gain is traceable to one component: the wide-bandgap front cell. The bottom organic cell, the interconnection layers and the antireflection coating are described as fabrication steps, while the measured improvements — crystallization uniformity, suppressed halogen phase, higher migration barriers, lower non-radiative voltage loss — all belong to the ITA-treated perovskite. A reader comparing tandem approaches should therefore ask whether a candidate strategy raises the front cell's open-circuit voltage and fill factor while holding its halide distribution stable, because those are the two quantities the paper identifies as limiting.

The paper also sets a concrete threshold for what counts as progress here: a front-cell fill factor above 86% and a voltage loss below the 0.5 eV level it attributes to prior tandems. Its own single junction reaches 86.66% and 1.391 V at a 1.854 eV bandgap, and the tandem converts that into 28.20%. The unresolved question the evidence leaves open is how the in-situ-formed MMF behaves in larger-area coatings and under outdoor stress, since the reported stability is 700 h of indoor maximum-power-point tracking on encapsulated cells.

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