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. 2026 Mar 13;11(4):3164–3167. doi: 10.1021/acsenergylett.6c00156

Bandgap Tunable Two-Step Vapor-Deposited Perovskite Absorbers for Perovskite-Silicon Tandem Solar Cells

Austin G Kuba †,*, Kerem Artuk †, Mostafa Othman †, Deniz Turkay †, Chiara Ongaro †, Michele Debastiani ‡, Quentin Guesnay †, Mohammad Reza Golobostanfard †,§, Maryamsadat Heydarian ∥, Oliver Fischer ∥,⊥, Martin C Schubert ∥, Florent Sahli ‡, Yohann Ansel ‡, Quentin Jeangros ‡, Aïcha Hessler-Wyser †, Christophe Ballif †, Christian M Wolff †,*
PMCID: PMC13078313  PMID: 41988639

Abstract

We report a two-step thermal evaporation–close space sublimation deposition method for tunable bandgap lead halide perovskites with single junction solar cell efficiencies exceeding 20%. These absorbers are integrated into perovskite–silicon tandem solar cells with stabilized efficiencies of >29%.

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Photovoltaic energy generation continues to drive down the cost of electricity. As single junction (SJ) silicon solar cells approach their practical limits, next generation concepts are needed for continued progress. The most successful route to overcoming these limits is the use of multijunction solar cells. , Perovskite–silicon tandem solar cells (TSCs) have recently seen a dramatic rise in power conversion efficiency (PCE), achieving up to 34.9%. Lead halide perovskites (LHPs) have continued their progress in stability to the point that encapsulated devices can pass damp heat and have improved in accelerated aging tests meaning they may soon find broader adoption in the market. However, most available reports focus on solution processes, which are still under debate regarding their suitability for commercialization on an industrial scale. Despite being rare in the literature, vapor-processed LHPs have shown promise regarding scalability and stability. , They have also attained ∼26% PCE in SJ solar cells, approaching their solution-processed counterparts. However, there are still few reports of TSCs produced with fully vapor-processed LHP absorbers. − A recent breakthrough in coevaporated LHP processing has achieved >30% stabilized PCE but two-step vapor-processed TSCs are still limited to <27% , to date. One technique receiving increasing attention is two-step physical-vapor-deposition close-space-sublimation (PVD-CSS, Supporting Note 1). We recently reported a PVD-CSS process capable of creating p-i-n solar cells with a broad range of bandgaps and SJ devices with efficiencies approaching 17%. In this process, an inorganic lead halide-cesium halide template is deposited using thermal evaporation and then converted to a LHP by reaction with mixed organohalide vapors in a CSS configuration (Supporting Note 2). Here, we adapt this process, show the absorbers are compatible with standard passivation treatments, and incorporate these absorbers into LHP-silicon TSCs achieving >29% PCE from stabilized maximum power point (MPP) tracking.

First, a series of additives were screened. FACl produced the highest photoluminescence (PL) quantum yield (Figure S1) and has already shown promise in CSS reactions using pure formamidimium iodide (FAI) so it was chosen for the rest of the work. Next, the bandgap tunability was investigated by varying the ratio of FAI to formamidinium bromide (FABr) with a constant formamidinium chloride (FACl) addition using coevaporated CsI:PbI2 or CsBr:PbI2 templates. The achievable range of bandgaps is shown in Figure a. The bandgap shows excellent tunability from 1.55 to 1.90 eV, even using pure iodide templates (spectrophotometry and PL in Figure S2). The conversion time was set to 20 min at 175 °C, and the organohalide mass addition was tuned to optimize conversion and device performance (Figures S3 and S4). SJ solar cells with an area of 0.1 cm2 were made with CsI:PbI2 templates and FAI/(FAI+FABr) values of 1.0, 0.8, and 0.6 (by mass, see Supporting Note 3). The current density–voltage (JV) performance is shown in Figure b (JV parameters are given in Table S1). External quantum efficiency (EQE) is shown in Figure c. X-ray diffraction (XRD) is shown in Figure S5 and scanning electron micrographs (SEMs) are shown in Figure S6.

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(a) Bandgap tunability for varying Cs source (CsI vs CsBr) with varying values of FAI/(FAI+FABr). (b) JV and (c) EQE for devices with varying bandgaps achieved by using CsI in the template and varying FAI/(FAI+FABr) from 1.0 to 0.8 to 0.6 by mass. (d) Study of surface passivation for the 1.60 eV absorber.

The 1.60 eV absorber with a short circuit current density of ∼ 21 mA/cm2 was selected for further testing and integration into TSCs. This was expected to create good current matching with a slight bottom cell limitation based on our previous work showing a cumulative current density of 40–41 mA/cm2 in LHP-silicon TSCs with flat front, rear textured silicon bottom cells. SJ devices incorporating no passivation, propane-1,3-diammonium iodide (PDAI2), or bimolecular passivation of 4F-phenylethylammonium chloride (4F-PEACl) and piperazinium chloride (PCl) are shown in Figure d, with bimolecular passivation achieving a champion efficiency of >20% without optical management. Across two batches of six films with different passivation conditions, excellent repeatability and yield were shown (Figure S7).

Six LHP-silicon TSCs with an active area of 1 cm2 were made with PDAI2 passivation. Good consistency across the batch was shown, with a mean PCE of 29.1% ± 0.6 (n = 6, average of forward and reverse scans) and a minimum of 27.8%. Cross-section SEM is shown in Figure a and (top-view Figure S8), and a schematic of the devices is shown in Figure b. The champion device reached 30.7% PCE from the reverse JV scan (Figure c) with a MPP efficiency of 29.8% for the first 300 s and minor degradation to 29.7% over 1000 s of tracking unencapsulated in air (Figure d). The champion device also showed good shelf stability, retaining 99% of its MPP efficiency over 134 days of storage in a N2 environment (Figure S9). Subcell selective EQE of the champion device is shown in Figure e. Current mismatch measurements confirm that the top cell produces 1 mA/cm2 more than the bottom cell (Figure S10) which can be improved for marginal current gains by increasing the FABr:FAI ratio to fine-tune the LHP bandgap. Four of the TSCs were cross-tested at Fraunhofer ISE after 3 weeks of shelf storage for an independent confirmation of the device performance. The JV parameters of the champion device and the selected devices are given in Table S2 and Figure S11 at both institutes. The relative error in the PCE for the champion device is 2% from MPP tracking (Figure S12). Subcell selective PL-based iV OC images (Figure f,g) of the champion TSC show good uniformity with small point-like defects in the top cell and an average iV oc of 1.158 V for the top cell and 0.724 V from the bottom cell, in good agreement with the measured V oc from JV. To explore the suitability of this process for TSCs on textured silicon bottom cells, we made one batch of LHP-silicon TSCs on textured Si with pyramid sizes ∼1 μm (SEM shown in Figure S13). The champion device achieved a maximum PCE of 28.3% from reverse JV (Figure S14) with V oc = 1.86 V and J sc = 20.4 mA/cm2 but low FF = 75%. Sister devices on ohmic textured Si substrates show that the low FF is likely related to low shunt resistance (Figure S14), which must be improved in future work to enable high efficiency textured TSCs.

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(a) Cross section SEM of a LHP-silicon tandem device produced in this work focused on the LHP top cell. (b) Schematic of a tandem device. (c) JV curve of the champion tandem solar cell produced in this work. (d) MPP tracking of the champion device for 1000 s unencapsulated in air. (e) Subcell selective EQE for the top and bottom cell. (f) Subcell selective PL-based iV OC images of the LHP top cell and (g) silicon bottom cell (full image width 1 cm in both cases).

In conclusion, we report the fabrication of two-step vapor-deposited LHP-silicon tandem solar cells with good scalability potential and efficiencies >29% through a PVD-CSS process. This report represents a significant advance in the performance of two-step vapor-deposited LHP-silicon tandem solar cells. These results motivate further studies into the potential advantages or disadvantages that vapor-deposited LHPs may have, in terms of stability and scalability, for both single junction and multijunction solar cells, and whether their efficiencies can further approach solution-processed solar cells in record efficiency.

Supplementary Material

nz6c00156_si_001.pdf (1.9MB, pdf)

Acknowledgments

The authors acknowledge funding from the European Union’s Horizon 2020 research and innovation program (TRIUMPH, 101075725), the Swiss State Secretariat for Education Research and Innovation (SERI) (TRIUMPH, 101075725), the Swiss Federal Office of Energy (PERSISTARS, BESTOBOT), the “fonds électricité vitale vert des Services industriels de Genèves”, and the ETH Domain through an AM grant (AMYS). M.O., D.T., and A.G.K. acknowledge funding from the European Union’s Horizon 2020 research and innovation program under a Marie Skłodowska-Curie grant (945363 and 101034260). D.T. acknowledges the State Secretariat for Education, Research, and Innovation for an FCS/ESKAS Swiss Government Excellence Scholarship.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.6c00156.

  • Experimental details and additional data on LHP films, single junction, and tandem solar cells (PDF)

The authors declare no competing financial interest.

The version of this paper that was published ASAP March 13, 2026, contained a spelling error in the name of author Mohammad Reza Golobostanfard. The corrected version posted March 18, 2026.

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Associated Data

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Supplementary Materials

nz6c00156_si_001.pdf (1.9MB, pdf)

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