Photo-ferroelectric perovskite interfaces for boosting VOC in efficient perovskite solar cells.

Interface engineering is the core of device optimization, and this is particularly true for perovskite photovoltaics (PVs). The steady improvement in their performance has been largely driven by careful manipulation of interface chemistry to reduce unwanted recombination. Despite that, PVs devices still suffer from unavoidable open circuit voltage (VOC) losses. Here, we propose a different approach by creating a photo-ferroelectric perovskite interface. By engineering an ultrathin ferroelectric two-dimensional perovskite (2D) which sandwiches a perovskite bulk, we exploit the electric field generated by external polarization in the 2D layer to enhance charge separation and minimize interfacial recombination. As a result, we observe a net gain in the device VOC reaching 1.21 V, the highest value reported to date for highly efficient perovskite PVs, leading to a champion efficiency of 24%. Modeling depicts a coherent matching of the crystal and electronic structure at the interface, robust to defect states and molecular reorientation. The interface physics is finely tuned by the photoferroelectric field, representing a new tool for advanced perovskite device design.

Recent Progress on Cross-Linkable Fullerene-based Electron Transport Materials for Perovskite Solar Cells.

Fullerene-based derivatives are frequently used as electron transport materials (ETMs) and interface buffers for perovskite solar cells (PSCs) due to their excellent properties, including high electron affinity and mobility, low recombination energy, tunable energy levels, and solution processability. However, significant challenges arise because fullerene derivatives tend to aggregate and dimerize, which reduces exciton dissociation and charge transport capacity. Additionally, their chemical compatibility with perovskite absorbers facilitates halide diffusion and degradation of PSCs. This overlap causes delamination and dissolution during device fabrication, hindering the performance enhancement of fullerene-based PSCs. To address these issues, researchers have developed cross-linkable fullerene materials. These materials have been shown to not only significantly improve the power conversion efficiency (PCE) of PSCs but also effectively enhance the device stability. In this review, we summarized recent research progress on cross-linkable fullerene derivatives as ETMs for PSCs. We systematically analyze the impact of these cross-linked ETMs on device performance and long-term stability, focusing on their molecular structures and working mechanisms. Finally, we discuss the future challenges that need to be overcome to advance the application of cross-linkable fullerene materials in PSCs.

All-perovskite tandem solar cells with 3D/3D bilayer perovskite heterojunction.

All-perovskite tandem solar cells promise higher power-conversion efficiency (PCE) than single-junction perovskite solar cells (PSCs) while maintaining a low fabrication cost1-3. However, their performance is still largely constrained by the subpar performance of mixed lead-tin (Pb-Sn) narrow-bandgap (NBG) perovskite subcells, mainly because of a high trap density on the perovskite film surface4-6. Although heterojunctions with intermixed 2D/3D perovskites could reduce surface recombination, this common strategy induces transport losses and thereby limits device fill factors (FFs)7-9. Here we develop an immiscible 3D/3D bilayer perovskite heterojunction (PHJ) with type II band structure at the Pb-Sn perovskite-electron-transport layer (ETL) interface to suppress the interfacial non-radiative recombination and facilitate charge extraction. The bilayer PHJ is formed by depositing a layer of lead-halide wide-bandgap (WBG) perovskite on top of the mixed Pb-Sn NBG perovskite through a hybrid evaporation-solution-processing method. This heterostructure allows us to increase the PCE of Pb-Sn PSCs having a 1.2-µm-thick absorber to 23.8%, together with a high open-circuit voltage (Voc) of 0.873 V and a high FF of 82.6%. We thereby demonstrate a record-high PCE of 28.5% (certified 28.0%) in all-perovskite tandem solar cells. The encapsulated tandem devices retain more than 90% of their initial performance after 600 h of continuous operation under simulated one-sun illumination.

Boosting Perovskite Solar Cells Efficiency and Stability: Interfacial Passivation of Crosslinked Fullerene Eliminates the "Burn-in" Decay.

Perovskite solar cells (PSCs) longevity is nowadays the bottleneck for their full commercial exploitation. Although lot of research is ongoing, the initial decay of the output power - an effect known as "burn-in" degradation happening in the first 100 h - is still unavoidable, significantly reducing the overall performance (typically of >20%). In this paper, the origin of the "burn-in" degradation in n-i-p type PSCs is demonstrated that is directly related to Li+ ions migration coming from the SnO2 electron transporting layer visualized by time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements. To block the ion movement, a thin cross-linked [6,6]-phenyl-C61-butyric acid methyl ester layer on top of the SnO2 layer is introduced, resulting in Li+ immobilization. This results in the elimination of the "burn-in" degradation, showing for the first time a zero "burn-in" loss in the performances while boosting device power conversion efficiency to >22% for triple-cation-based PSCs and >24% for formamidinium-based (FAPbI3 ) PSCs, proving the general validity of this approach and creating a new framework for the realization of stable PSCs devices.