Optimizing surface passivation: Unproductive charge recombination at surface defects can limit the efficiency of hybrid perovskite solar cells, but these defects can be passivated by the binding of small molecules. Wang et al. studied three such small molecules—theophylline, caffeine, and theobromine—that bear both carbonyl and amino groups. For theophylline, hydrogen bonding of the amino hydrogen to surface iodide optimized the carbonyl interaction with a lead antisite defect and improved the efficiency of a perovskite cell from 21 to 22.6%. Science , this issue p. 1509

Numerous defects on the surface of halide perovskite films considerably limit the photovoltaic efficacy of perovskite solar cells (PSCs). Herein, multifunctional carbon quantum dots (CQDs) were introduced to modify the perovskite film surface. The CQDs contain multiple functional groups, including C=O and –NH ₂ , that interact effectively with the uncoordinated Pb ²⁺ and organic cations on the perovskite film surface. This interaction enables defect passivation and energy-level alignment optimization, leading to an extension of the charge carrier lifetime and enhancement of carrier-selective transfer. As a result, the best CQD-modified PSCs, boasting an impressive fill factor of 84.5%, achieved a power conversion efficiency of 24.48%, surpassing the 22.31% of pristine devices. In addition, the unencapsulated PSCs exhibit excellent stability, retaining 83% of the initial efficiency after operating for over 1,000 h under simulated AM 1.5G illumination.

Charge transport and extraction in polycrystalline perovskite films are often hindered by inefficient carrier transfer across grain domain boundaries (GDBs). Herein, we present a universal post-treatment strategy leveraging supramolecular crown ether-assisted slow release and precise delivery of Rb⁺ cations to GDBs, achieving in-situ GDB bridging. The solid-state nuclear magnetic resonance (NMR), transmission electron microscopic (TEM), and time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses confirm that Rb⁺ forms a non-perovskite phase, primarily localized at the surface and GDBs. Ultrafast time-resolved photoluminescence mapping revealed accelerated carrier diffusion across the grain boundaries for the Rb⁺-treated perovskite thin films which enables photo-generated charge carriers to travels over two grain domain boundaries before recombination. As a result, perovskite solar cells treated with this strategy achieved a champion efficiency of 26.02% (certified as 25.77%) and demonstrated remarkable stability, retaining 99.2% of their initial efficiency after 1300 h of continuous one-sun illumination under maximum power point tracking (ISOS-L-1I).

Benefiting from good solubility of metal halide perovskites, low-temperature solution processes (including one- and two-step spin-coating) have become the most common approach for perovskite solar cells (PSCs). However, one confusing issue is that what are specifically effective strategies for the one-step process may not be applicable for the two-step process, and vice versa. Herein, the PSCs with the same perovskite composition and device configuration were prepared by one- and two-step processes, respectively. The results indicated that high performance PSCs with comparable power conversion efficiency over 22% were achieved by both methods, while the detailed photovoltaic parameters varied greatly depending on the one- or two-step process. Compared to the one-step counterpart, the two-step processed PSCs exhibit lower open-circuit voltage and fill factor, but superior short-circuit current, which was in-depth interpreted in terms of the crystalline growth mode, optical properties, defect types, and carrier transport mechanisms related to a perovskite film surface (including a top and a bottom surface). Understanding the root cause for such differences would be central toward identifying what is really crucial for further producing high performance PSCs.