Perovskite tandem solar cells (TSCs) have become a research hotspot in the photovoltaic field due to their high theoretical efficiency (>40%) and low-cost fabrication processes. However, wide-bandgap (WBG) perovskites face significant challenges such as crystallization inhomogeneity, phase separation, and high defect density caused by bromine doping, which severely limit the efficiency and stability of large-area devices. Existing research has mainly focused on optimizing narrow-bandgap perovskites, with limited control methods for WBG systems, and the efficiency gap (>1%) between small-area and large-area devices hinders their commercialization.
Summary of Findings
To address these challenges, the team led by Wei-Jun Ke from Wuhan University (the sole corresponding author) reported the application of a novel crystal modifier, piracetam, in wide-bandgap (WBG) perovskite films for efficient large-area all-perovskite tandem solar cells (TSCs). Piracetam regulates the nucleation process of perovskites through its amide and pyrrolidone groups, promoting the growth of large-sized grains and preferential orientation of the (110) crystal plane, while reacting with residual PbI₂ during post-treatment to form one-dimensional (PI) PbI₃ nanorods, significantly reducing defects and enhancing optoelectronic performance. The single-junction WBG (1.77 eV) device achieved an open-circuit voltage (Voc) of 1.36 V and a certified efficiency (PCE) of 20.35%.
Based on this, single-terminal all-perovskite TSCs achieved PCEs of 28.71% (certified 28.13%) and 28.20% (certified 27.30%) at aperture areas of 0.07 cm² and 1.02 cm², respectively, with an area expansion efficiency loss of only 0.51%. Additionally, piracetam shows universality for various perovskite components, improving the efficiency of 1.56 eV narrow-bandgap single-junction devices from 23.56% to 25.71%. This strategy provides an effective pathway for the scalable fabrication of high-efficiency all-perovskite TSCs. The related research results were published in Nature Nanotechnology under the title “Piracetam shapes wide-bandgap perovskite crystals for scalable perovskite tandems.” PhD student Shi-Qiang Fu from Wuhan University is the first author, with PhD students Shun Zhou, Guang Li, and Weiwei Meng as co-first authors.
Visual Guide
Figure 1 Growth Mechanism of WBG Perovskite Films
Figure 1 illustrates the regulatory effect of piracetam on the growth mechanism of WBG perovskite films. Comparing the growth processes of films without (a, control group) and with piracetam (b, target group), piracetam preferentially adsorbs on the (110) crystal plane (binding energy Eb=−1.54 eV vs. −1.19 eV for the (100) crystal plane), suppressing grain tilting and promoting (110) preferential orientation (d-e). DFT calculations show that the surface energy of the (110) crystal plane (0.33 J m⁻²) is slightly lower than that of the (100) crystal plane (0.37 J m⁻²) (f), combined with the selective adsorption of piracetam, forming micron-sized large grains (h, SEM) and uniform photoluminescence (PL) distribution (j). This mechanism significantly enhances the crystallinity and carrier lifetime of the films (g: 37.28 ns → h: 735.06 ns).
Figure 2 Characterization of WBG Perovskite Films
Figure 2 reveals the dynamic regulation of perovskite crystallization kinetics by piracetam through in situ GIWAXS and XRD. After adding piracetam, the diffraction intensity of the (110) crystal plane significantly increased (b-d), and one-dimensional (PI) PbI₃ nanorods were formed at high concentrations (15 mg ml⁻¹) (c). Fourier-transform infrared spectroscopy (FTIR) shows that the C=O groups of piracetam coordinate with Pb²⁺ (e, 1652→1667 cm⁻¹), passivating uncoordinated defects. Transient absorption (TA) and steady-state PL indicate a reduction in non-radiative recombination in the target films, with carrier lifetime increasing from 4.66 ns to 5.33 ns (h-i), confirming their excellent optoelectronic performance.
Figure 3 Performance of Single-Junction WBG PSCs
Figure 3 compares the performance differences of single-junction WBG devices. After adding piracetam, the device’s Voc increased from 1.27 V to 1.34 V, and PCE increased from 18.51% to 20.35% (b). Space charge limited current (SCLC) tests show that the trap state density decreased from 7.04×10¹⁵ cm⁻³ to 2.03×10¹⁵ cm⁻³ (g). Energy band alignment analysis indicates that the conduction band of the target films (−4.02 eV) matches better with the C60 layer (−4.05 eV) (h), facilitating electron transport. Additionally, piracetam also shows universality in 1.56 eV narrow-bandgap devices, improving efficiency from 23.56% to 25.71% (i).
Figure 4 Performance of 2T All-Perovskite TSCs
Figure 4 demonstrates the performance of all-perovskite tandem devices (2T-TSCs). Based on optimized WBG sub-cells, the tandem devices achieved PCEs of 28.71% and 28.20% at areas of 0.07 cm² and 1.02 cm², respectively (c, e), with an efficiency loss of only 0.51% (g). EQE curves show that the integrated current densities of WBG and NBG sub-cells are 16.23 mA cm⁻² and 15.97 mA cm⁻², respectively (d). MPP tracking indicates that the target tandem devices maintained 90% of their initial efficiency after continuous operation for 663 hours at 55°C (h), attributed to the passivation of grain boundaries and suppression of ionic migration by one-dimensional (PI) PbI₃.
Conclusion and Outlook
This study demonstrates that piracetam, as a crystal modifier, can significantly optimize the crystal orientation, defect density, and surface smoothness of WBG perovskite films, and further suppress ionic migration and phase separation by forming one-dimensional (PI) PbI₃. This strategy successfully reduces the Voc loss of WBG devices, achieving PCEs of 28.71% and 28.20% for all-perovskite TSCs at areas of 0.07 cm² and 1.02 cm², respectively, with the smallest efficiency difference reported to date. Although further optimization is needed in phase separation suppression and interface engineering, the introduction of piracetam provides important technical support for achieving efficient, stable, and scalable all-perovskite tandem devices.
Author Biography
Wei-Jun Ke, Professor and PhD supervisor at the School of Physics Science and Technology, Wuhan University, selected for the National Youth Talent Program. H-index 67, with the highest single paper citation >1200 times and total citations >17000 times. Representative academic achievements have been published as corresponding author (including co-corresponding) and first author in top academic journals such as Nature, Nat. Nanotechnol., Nat. Photonics, Nat. Commun., Sci. Adv., AM, JACS, Joule, EES, etc., with a total of 15 papers as corresponding/first author selected as ESI highly cited, and 6 papers selected as ESI hot papers. He has published a monograph titled “Tin Oxide and Perovskite Solar Cells” (Science Press). His research mainly focuses on the theoretical, design, preparation, and application of new semiconductor optoelectronic materials and devices, especially in the fields of new perovskite solar cells, detectors, and light-emitting diodes.
Image Source: Wuhan University
Reference Information
Fu, S., Zhou, S., Meng, W. et al. Piracetam shapes wide-bandgap perovskite crystals for scalable perovskite tandems. Nat. Nanotechnol. (2025). https://doi.org/10.1038/s41565-025-01899-z
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