Designing Surface Passivators Through Intramolecular Potential Manipulation for Efficient and Stable Perovskite Solar Cells.

The conjugation of terminal ammonium salt groups with perovskite surfaces is a frequently employed technique that aims to enhance the overall performance of perovskite materials, encompassing both bulk and surface properties. Particularly, it exhibits heightened efficacy when applied to surface modification, due to its ability to mitigate defect accumulation and facilitate facile binding with the receptive sites inherent to the perovskite structure. However, the interaction of the bulk ammonium group with PbI2 has the potential to form a low-dimensional phase of perovskite, which may obstruct carrier extraction at the interface. Therefore, the surface passivators (MeO-PFACl) are designed through intramolecular potential manipulation. The combinations of the electron-donating methoxy group and π-π conjugation of the phenyl ring reduce the local potential at the reactive site of formamidinium group, making it less likely to form a low-dimension phase with perovskite. This surface passivation strategy effectively suppresses the surface nonradiative recombination and promotes the interface carrier extraction. The devices treated with MeO-PFACl have demonstrated exceptional performance, achieving a peak power conversion efficiency (PCE) of 25.88%, with an average PCE of 25.37%. These works offer a novel principle for enhancing both the efficiency and stability of PSCs using ammonium-incorporated molecules without the induction of an additional phase layer.

Shallow defects and variable photoluminescence decay times up to 280 µs in triple-cation perovskites.

Quantifying recombination in halide perovskites is a crucial prerequisite to control and improve the performance of perovskite-based solar cells. While both steady-state and transient photoluminescence are frequently used to assess recombination in perovskite absorbers, quantitative analyses within a consistent model are seldom reported. We use transient photoluminescence measurements with a large dynamic range of more than ten orders of magnitude on triple-cation perovskite films showing long-lived photoluminescence transients featuring continuously changing decay times that range from tens of nanoseconds to hundreds of microseconds. We quantitatively explain both the transient and steady-state photoluminescence with the presence of a high density of shallow defects and consequent high rates of charge carrier trapping, thereby showing that deep defects do not affect the recombination dynamics. The complex carrier kinetics caused by emission and recombination processes via shallow defects imply that the reporting of only single lifetime values, as is routinely done in the literature, is meaningless for such materials. We show that the features indicative for shallow defects seen in the bare films remain dominant in finished devices and are therefore also crucial to understanding the performance of perovskite solar cells.

Homogenizing out-of-plane cation composition in perovskite solar cells.

Perovskite solar cells with the formula FA1-xCsxPbI3, where FA is formamidinium, provide an attractive option for integrating high efficiency, durable stability and compatibility with scaled-up fabrication. Despite the incorporation of Cs cations, which could potentially enable a perfect perovskite lattice1,2, the compositional inhomogeneity caused by A-site cation segregation is likely to be detrimental to the photovoltaic performance of the solar cells3,4. Here we visualized the out-of-plane compositional inhomogeneity along the vertical direction across perovskite films and identified the underlying reasons for the inhomogeneity and its potential impact for devices. We devised a strategy using 1-(phenylsulfonyl)pyrrole to homogenize the distribution of cation composition in perovskite films. The resultant p-i-n devices yielded a certified steady-state photon-to-electron conversion efficiency of 25.2% and durable stability.

Regulating buried interface properties and alleviating micro-strain of crystals for efficient perovskite solar cells.

Surface properties of SnO2 and their effects on the growth of perovskite films play a crucial role for perovskite solar cells (PSCs). Herein, a facile strategy to synchronously regulate the buried interface defects and energy level arrangement, as well as improve the crystallinity of perovskite films with alleviated micro-strain by pre-modifying the SnO2 surface with ammonium hexafluorophosphate (NH4PF6) is proposed. The device achieved the promising PCE of 22.50% and improved stability.

Supple Formamidinium-Based Low-Dimension Perovskite Derivative for Sensitive and Ultrastable X-ray Detection.

Halide perovskite materials possess excellent optoelectronic properties and have shown great potential for direct X-ray detection. Perovskite wafers are particularly attractive among various detection structures due to their scalability and ease of preparation, making them the most promising candidates for X-ray detection and array imaging applications. However, device instability and current drift caused by ionic migration are persistent challenges for perovskite detectors, especially in polycrystalline wafers with numerous grain boundaries. In this study, we examined the potential of one-dimensional (1D) δ-phase (yellow phase) formamidinium lead iodide (δ-FAPbI3) as an X-ray detection material. This material possesses a suitable band gap of 2.43 eV, which makes it highly promising for X-ray detection and imaging using compact wafers. Moreover, we found that δ-FAPbI3 has low ionic migration, low Young's modulus, and excellent long-term stability, making it an ideal candidate for high-performance X-ray detection. Notably, the yellow phase perovskite derivative exhibits exceptional long-term atmospheric stability (RH of ≈70 ± 5%) over six months, as well as an extremely low dark current drift (3.43 × 10-4 pA cm-1 s-1 V-1), which is comparable to that of single-crystal devices. An X-ray imager with a large-size δ-FAPbI3 wafer integrated on a thin film transistor (TFT) backplane was further fabricated. Direct 2D multipixel radiographic imaging was successfully performed, demonstrating the feasibility of δ-FAPbI3 wafer detectors for sensitive and ultrastable imaging applications.

Bidirectional Anions Gathering Strategy Afford Efficient Mixed PbSn Perovskite Solar Cells.

Mixed lead-tin (PbSn) perovskite solar cells (PSCs) possess low toxicity and adjustable bandgap for both single-junction and all-perovskite tandem solar cells. However, the performance of mixed PbSn PSCs still lags behind the theoretical efficiency. The uncontrollable crystallization and the resulting structural defect are important reasons. Here, the bidirectional anions gathering strategy (BAG) is reported by using Methylammonium acetate (MAAc) and Methylammonium thiocyanate (MASCN) as perovskite bulk additives, which Ac- escapes from the perovskite film top surface while SCN- gathers at the perovskite film bottom in the crystallization process. After the optoelectronic techniques, the bidirectional anions movement caused by the top-down gradient crystallization is demonstrated. The layer-by-layer crystallization can collect anions in the next layer and gather at the broader, enabling a controllable crystallization process, thus getting a high-quality perovskite film with better phase crystallinity and lower defect concentration. As a result, PSCs treated by the BAG strategy exhibit outstanding photovoltaic and electroluminescent performance with a champion efficiency of 22.14%. Additionally, it demonstrates excellent long-term stability, which retains ≈92.8% of its initial efficiency after 4000 h aging test in the N2 glove box.

Negligible Ion Migration in Tin-Based and Tin-Doped Perovskites.

Ion migration is a notorious phenomenon observed in ionic perovskite materials. It causes several severe issues in perovskite optoelectronic devices such as instability, current hysteresis, and phase segregation. Here, we report that, in contrast to lead halide perovskites (LHPs), no ion migration or phase segregation was observed in tin halide perovskites (THPs) under illumination or an electric field. The origin is attributed to a much stronger Sn-halide bond and higher ion migration activation energy (Ea ) in THPs, which remain nearly constant under illumination. We further figured out the threshold Ea for the absence of ion migration to be around 0.65 eV using the CsSny Pb1-y (I0.6 Br0.4 )3 system whose Ea varies with Sn ratios. Our work shows that ion migration does not necessarily exist in all perovskites and suggests metallic doping to be a promising way of stopping ion migration and improving the intrinsic stability of perovskites.

Dual Effect of Superhalogen Ionic Liquids Ensures Efficient Carrier Transport for Highly Efficient and Stable Perovskite Solar Cells.

Defect accumulation and nonradiative recombination at the interface of the electron-transport layer (ETL) and the photosensitive layer are inevitable obstacles to efficient and stable perovskite solar cells (PSCs). Herein, we reported a dual-effect interface modification strategy that employs potassium tetrafluoroborate (KBF4) molecules for the simultaneous passivation of the SnO2/perovskite interface and perovskite grain boundaries. The introduced highly electronegative BF4- enriched at the SnO2 surface and the chemical bond interaction between them can effectively reduce the hydroxyl (-OH) group defects on the surface of SnO2, improve electron mobility, and reduce nonradiative recombination. Meanwhile, partial K+ diffuses into the grain boundaries, causing the halogen ions to be uniformly distributed in the perovskite film and resulting in better crystallinity. Therefore, the performance of the experimental device was improved from 20.34 to 22.90% compared with the reference device, with a high electrical performance (JSC = 25.1 mA cm-2, VOC = 1.137 V). In particular, the unencapsulated target PSCs retained 85% of their original PCE after aging for 1000 h under ambient conditions (70 ± 10% RH) in the dark.

A Selective Targeting Anchor Strategy Affords Efficient and Stable Ideal-Bandgap Perovskite Solar Cells.

Mixed lead-tin perovskite solar cells (LTPSCs) with an ideal bandgap are demonstrated as a promising candidate to reach higher power conversion efficiency (PCE) than their Pb-counterparts. Herein, a Br-free mixed lead-tin perovskite material, FA0.8 MA0.2 Pb0.8 Sn0.2 I3 , with a bandgap of 1.33 eV, as a perovskite absorber, is selected. Through density functional theory calculations and optoelectronic techniques, it is demonstrated that both Pb- and Sn-related A-site vacancies are pushed into deeper energetic depth, causing severe nonradiative recombination. Hence, a selective targeting anchor strategy that employs phenethylammonium iodide and ethylenediamine diiodide as co-modifiers to selectively anchor with Pb- and Sn-related active sites and passivate bimetallic traps, respectively, is established. Furthermore, the selectivity of the molecular oriented anchor passivation is demonstrated through energetic depth specificity of Pb- and Sn-related traps. As a result, a substantially enhanced open-circuit voltage (VOC ) from 0.79 to 0.90 V for the LTPSCs is achieved, yielding a champion PCE of 22.51%, which is the highest PCE among the reported ideal-bandgap PSCs. The VOC loss is reduced to 0.43 V.

Doped Bilayer Tin(IV) Oxide Electron Transport Layer for High Open-Circuit Voltage Planar Perovskite Solar Cells with Reduced Hysteresis.

Tin oxide (SnO2 ) is an emerging electron transport layer (ETL) material in halide perovskite solar cells (PSCs). Among current limitations, open-circuit voltage (VOC ) loss is one of the major factors to be addressed for further improvement. Here a bilayer ETL consisting of two SnO2 nanoparticle layers doped with different amounts of ammonium chloride is proposed. As demonstrated by photoelectron spectroscopy and photophysical studies, the main effect of the novel ETL is to modify the energy level alignment at the SnO2 /perovskite interface, which leads to decreased carrier recombination, enhanced electron transfer, and reduced voltage loss. Moreover, X-ray diffraction reveals reduced strain in perovskite layers grown on bilayer ETLs with respect to single-layer ETLs, further contributing to a decrease of carrier recombination processes. Finally, the bilayer approach enables the more reproducible preparation of smooth and pinhole-free ETLs as compared to single-step deposition ETLs. PSCs with the doped bilayer SnO2 ETL demonstrate strongly increased VOC values of up to 1.21 V with a power conversion efficiency of 21.75% while showing negligible hysteresis and enhanced stability. Moreover, the SnO2 bilayer can be processed at low temperature (70 °C), and has therefore a high potential for use in tandem devices or flexible PSCs.

Enhanced Performance and Stability of Perovskite Solar Cells Using NH4I Interfacial Modifier.

Despite organic-inorganic hybrid perovskite solar cells have rapid advances in power conversion efficiency in recent years, the serious instability of the device under practical working conditions is the current main challenge for commercialization. In this study, we have successfully inserted NH4I as an interfacial modifier between the TiO2 electron transport layer and perovskite layer. The result shows that it can significantly improve the quality of the perovskite films and electron extraction efficiency between the perovskite and electron transport layer. The devices with NH4I are obtained an improved power conversion efficiency of 18.31% under AM 1.5G illumination (100 mW cm-2). More importantly, the humidity and UV light stability of the devices are greatly improved after adding NH4I layer. The uncoated devices only decrease by less than 15% of its original efficiency during 700-h stability tests in a humidity chamber (with a relative humidity of 80%) and the efficiency almost maintains 70% of its initial value over 20 h under UV light stress tests. This work provides a potential way by interfacial modification to significantly improve photovoltaic performance and stability of perovskite solar cells.

High-efficiency perovskite solar cells prepared by using a sandwich structure MAI-PbI2-MAI precursor film.

Two-step deposition has been widely used in the perovskite layer preparation for perovskite solar cells due to its attractive morphology controllability. However, the limited diffusivity of CH3NH3I (MAI) might cause some PbI2 to remain in the perovskite film. The residual PbI2 in the perovskite film would lead to inferior performance of devices, such as, low power conversion efficiency (PCE), poor reproducibility and weak air stability. In this work, we developed a sandwich structure MAI-PbI2-MAI precursor film to prepare a PbI2-free CH3NH3PbI3 perovskite film. In comparison to the two-step approach, the MAI-PbI2-MAI precursor film with a typical sandwich structure formed a uniform and pinhole-free perovskite film without any PbI2 residue, which could significantly improve the performance of the devices. Moreover, the bottom MAI layer of the MAI-PbI2-MAI precursor film could improve the interfacial contact of the porous TiO2 layer, leading to the promotion of the charge transfer and reduction of the recombination rate. Therefore, the devices fabricated from the sandwich structure MAI-PbI2-MAI precursor films showed dramatic improvements of open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF) and PCE. As a result, a promising PCE of 17.8% with good long-term air stability was achieved for the MAI-PbI2-MAI precursor film based PSC, which is better than that prepared by a two-step approach.

High Consistency Perovskite Solar Cell with a Consecutive Compact and Mesoporous TiO2 Film by One-Step Spin-Coating.

Generally, in classic mesoscopic perovskite solar cells (PSCs), the compact blocking layer and mesoporous scaffold layer prepared by two steps or more will inevitably form an interface between them. It is undoubted that the interface contact is not conducive to electron transport and would increase the recombination in the device, resulting in the inferior performance of PSCs. In this work, we constructed a consecutive compact and mesoporous (CCM) TiO2 film to substitute the compact blocking layer and scaffold layer for mesoscopic PSCs. The bottom of the CCM TiO2 film was dense and the top was mesoporous with large uniform macropores. The two parts of the film were consecutive, which could promote the electron transport rate and decrease the charge recombination effectively. Moreover, due to the existence of macropores in the CCM TiO2 film, it was propitious to the deposition of perovskite and charge separation for the perovskite layer. Over 15.0% of average power conversion efficiency (PCE) with high consistency photovoltaic performances was achieved for the CCM TiO2 film based mesoscopic PSCs, which is higher than that with a classic mesoporous structure.

A Hybrid Tandem Solar Cell Combining a Dye-Sensitized and a Polymer Solar Cell.

A hybrid tandem solar cell was assambled by connecting a dye sensitized solar cell and a polymer solar cell in series. A N719 sensitized TiO2 was used as photocathode in dye-sensitized subcell, and a MEH-PPV/PCBM composite was used as active layer in the polymer subcell. The polymer subcell fabricated on the counter electrode of the dye sensitized solar cell. A solution processed TiO(x) layer was used as electron collection layer of the polymer sub cell and the charge recombination layer. The effects of the TiO(x) interlayer and the spectral overlap between the two sub cells have been studied and optimized. The results shows that a proper thickness of the TiO(x) layer is needed for tandem solar cells. Thick TiO(x) will enhance the series resistance, but too thin TiO(x), layer will damage the hole blocking effect and its hydrophilic. The resulting optimized tandem solar cells exhibited a power conversion efficiency of 1.28% with a V(oc) of 0.95 V under simulated 100 mW cm(-2) AM 1.5 illumination.

Mesoporous BaSnO3 layer based perovskite solar cells.

One of the limitations of TiO2 based perovskite solar cells is the poor electron mobility of TiO2. Here, perovskite oxide BaSnO3 is used as a replacement. It has a higher electron mobility and the same perovskite structure as the light harvesting materials. After optimization, devices based on BaSnO3 showed the best performance of 12.3% vs. 11.1% for TiO2.