Insights Into Preaggregation Control of Y-Series Nonfullerene Acceptors in Liquid State for Highly Efficient Binary Organic Solar Cells.

Leveraging breakthroughs in Y-series nonfullerene acceptors (NFAs), organic solar cells (OSCs) have achieved impressive power conversion efficiencies (PCEs) exceeding 19%. However, progress in advancing OSCs has decelerated due to constraints in realizing the full potential of the Y-series NFAs. Herein, a simple yet effective solid additive-induced preaggregation control method employing 2-chloro-5-iodopyridine (PDCI) is reported to unlock the full potential of the Y-series NFAs. Specifically, PDCI interacts predominantly with Y-series NFAs enabling enhanced and ordered phase-aggregation in solution. This method leads to a notable improvement and a redshifted absorption of the acceptor phase during film formation, along with improved crystallinity. Moreover, the PDCI-induced preaggregation of NFAs in the solution enables ordered molecule packing during the film-formation process through delicate intermediate states transition. Consequently, the PDCI-induced preaggregated significantly improves the PCE of PM6:Y6 OSCs from 16.12% to 18.12%, among the best values reported for PM6:Y6 OSCs. Importantly, this approach is universally applicable to other Y-series NFA-based OSCs, achieving a champion PCE of 19.02% for the PM6:BTP-eC9 system. Thus, the preaggregation control strategy further unlocks the potential of Y-series NFAs, offering a promising avenue for enhancing the photovoltaic performance of Y-series NFA-based OSCs.

Eliminating the Adverse Impact of Composition Modulation in Perovskite Light-Emitting Diodes toward Ultra-High Brightness and Stability.

Excess ammonium halides as composition additives are widely employed in perovskite light-emitting diodes (PeLEDs), aiming to achieve high performance by controlling crystallinity and passivating defects. However, an in-depth understanding of whether excess organoammonium components affect the film physical/electrical properties and the resultant device instability is still lacking. Here, the trade-off between the performance and stability in high-efficiency formamidinium lead iodide (FAPbI3)-based PeLEDs with excess ammonium halides is pointed, and the underlying mechanism is explored. Systematic experimental and theoretical studies reveal that excess halide salt-induced ion-doping largely alters the PeLEDs properties (e.g., carrier injection, field-dependent ion-drifting, defect physics, and phase stability). A surface clean assisted cross-linking strategy is demonstrated to eliminate the adverse impact of composition modulation and boost the operational stability without sacrificing the efficiency, achieving a high efficiency of 23.6%, a high radiance of 964 W sr-1 m-2 (The highest value for FAPbI3 based PeLEDs), and a prolong lifetime of 106.1 h at large direct current density (100 mA cm-2), concurrently. The findings uncovered an important link between excess halide salts and the device performance, providing a guideline for rational design of stable, bright, and high efficiency PeLEDs.

Rational molecular and device design enables organic solar cells approaching 20% efficiency.

For organic solar cells to be competitive, the light-absorbing molecules should simultaneously satisfy multiple key requirements, including weak-absorption charge transfer state, high dielectric constant, suitable surface energy, proper crystallinity, etc. However, the systematic design rule in molecules to achieve the abovementioned goals is rarely studied. In this work, guided by theoretical calculation, we present a rational design of non-fullerene acceptor o-BTP-eC9, with distinct photoelectric properties compared to benchmark BTP-eC9. o-BTP-eC9 based device has uplifted charge transfer state, therefore significantly reducing the energy loss by 41 meV and showing excellent power conversion efficiency of 18.7%. Moreover, the new guest acceptor o-BTP-eC9 has excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, which enables an efficiency of 19.9% (19.5% certified) in PM6:BTP-C9:o-BTP-eC9 based ternary system with enhanced operational stability.

Thiol-Functionalized Conjugated Metal-Organic Frameworks for Stable and Efficient Perovskite Photovoltaics.

Metal-organic frameworks (MOFs) have been investigated recently in perovskite photovoltaics owing to their potential to boost optoelectronic performance and device stability. However, the impact of variations in the MOF side chain on perovskite characteristics and the mechanism of MOF/perovskite film formation remains unclear. In this study, three nanoscale thiol-functionalized UiO-66-type Zr-based MOFs (UiO-66-(SH)2 , UiO-66-MSA, and UiO-66-DMSA) are systematically employed and examined in perovskite solar cells (PSCs). Among these MOFs, UiO-66-(SH)2 , with its rigid organic ligands, exhibited a strong interaction with perovskite materials with more efficient suppression of perovskite vacancy defects. More importantly, A detailed and in-depth discussion is provided on the formation mechanism of UiO-66-(SH)2 -assisted perovskite film upon in situ GIWAXS performed during the annealing process. The incorporation of UiO-66-(SH)2 additives substantially facilitates the conversion of PbI2 into the perovskite phase, prolongs the duration of stage I, and induces a delayed phase transformation pathway. Consequently, the UiO-66-(SH)2 -assisted device demonstrates reduced defect density and superior optoelectronic properties with optimized power conversion efficiency of 24.09% and enhanced long-term stability under ambient environment and continuous light illumination conditions. This study acts as a helpful design guide for desired MOF/perovskite structures, enabling further advancements in MOF/perovskite optoelectronic devices.

19.31% binary organic solar cell and low non-radiative recombination enabled by non-monotonic intermediate state transition.

Non-fullerene acceptors based organic solar cells represent the frontier of the field, owing to both the materials and morphology manipulation innovations. Non-radiative recombination loss suppression and performance boosting are in the center of organic solar cell research. Here, we developed a non-monotonic intermediate state manipulation strategy for state-of-the-art organic solar cells by employing 1,3,5-trichlorobenzene as crystallization regulator, which optimizes the film crystallization process, regulates the self-organization of bulk-heterojunction in a non-monotonic manner, i.e., first enhancing and then relaxing the molecular aggregation. As a result, the excessive aggregation of non-fullerene acceptors is avoided and we have achieved efficient organic solar cells with reduced non-radiative recombination loss. In PM6:BTP-eC9 organic solar cell, our strategy successfully offers a record binary organic solar cell efficiency of 19.31% (18.93% certified) with very low non-radiative recombination loss of 0.190 eV. And lower non-radiative recombination loss of 0.168 eV is further achieved in PM1:BTP-eC9 organic solar cell (19.10% efficiency), giving great promise to future organic solar cell research.

Manipulating Crystallization Kinetics in High-Performance Blade-Coated Perovskite Solar Cells via Cosolvent-Assisted Phase Transition.

Manipulating the perovskite solidification process, including nucleation and crystal growth, plays a critical role in controlling film morphology and thus affects the resultant device performance. In this work, a facile and effective ethyl alcohol (EtOH) cosolvent strategy is demonstrated with the incorporation of EtOH into perovskite ink for high-performance room-temperature blade-coated perovskite solar cells (PSCs) and modules. Systematic real-time perovskite crystallization studies uncover the delicate perovskite structural evolutions and phase-transition pathway. Time-resolved X-ray diffraction and density functional theory calculations both demonstrate that EtOH in the mixed-solvent system significantly promotes the formation of an FA-based precursor solvate (FA2 PbBr4 ·DMSO) during the trace-solvent-assisted transition process, which finely regulates the balance between nucleation and crystal growth to guarantee high-quality perovskite films. This strategy efficiently suppresses nonradiative recombination and improves efficiencies in both 1.54 (23.19%) and 1.60 eV (22.51%) perovskite systems, which represents one of the highest records for blade-coated PSCs in both small-area devices and minimodules. An excellent VOC deficit as low as 335 mV in the 1.54 eV perovskite system, coincident with the measured nonradiative recombination loss of only 77 mV, is achieved. More importantly, significantly enhanced device stability is another signature of this approach.

Room-temperature multiple ligands-tailored SnO2 quantum dots endow in situ dual-interface binding for upscaling efficient perovskite photovoltaics with high VOC.

The benchmark tin oxide (SnO2) electron transporting layers (ETLs) have enabled remarkable progress in planar perovskite solar cell (PSCs). However, the energy loss is still a challenge due to the lack of "hidden interface" control. We report a novel ligand-tailored ultrafine SnO2 quantum dots (QDs) via a facile rapid room temperature synthesis. Importantly, the ligand-tailored SnO2 QDs ETL with multi-functional terminal groups in situ refines the buried interfaces with both the perovskite and transparent electrode via enhanced interface binding and perovskite passivation. These novel ETLs induce synergistic effects of physical and chemical interfacial modulation and preferred perovskite crystallization-directing, delivering reduced interface defects, suppressed non-radiative recombination and elongated charge carrier lifetime. Power conversion efficiency (PCE) of 23.02% (0.04 cm2) and 21.6% (0.98 cm2, VOC loss: 0.336 V) have been achieved for the blade-coated PSCs (1.54 eV Eg) with our new ETLs, representing a record for SnO2 based blade-coated PSCs. Moreover, a substantially enhanced PCE (VOC) from 20.4% (1.15 V) to 22.8% (1.24 V, 90 mV higher VOC, 0.04 cm2 device) in the blade-coated 1.61 eV PSCs system, via replacing the benchmark commercial colloidal SnO2 with our new ETLs.

Graded bulk-heterojunction enables 17% binary organic solar cells via nonhalogenated open air coating.

Graded bulk-heterojunction (G-BHJ) with well-defined vertical phase separation has potential to surpass classical BHJ in organic solar cells (OSCs). In this work, an effective G-BHJ strategy via nonhalogenated solvent sequential deposition is demonstrated using nonfullerene acceptor (NFA) OSCs. Spin-coated G-BHJ OSCs deliver an outstanding 17.48% power conversion efficiency (PCE). Depth-profiling X-ray photoelectron spectroscopy (DP-XPS) and angle-dependent grazing incidence X-ray diffraction (GI-XRD) techniques enable the visualization of polymer/NFA composition and crystallinity gradient distributions, which benefit charge transport, and enable outstanding thick OSC PCEs (16.25% for 300 nm, 14.37% for 500 nm), which are among the highest reported. Moreover, the nonhalogenated solvent enabled G-BHJ OSC via open-air blade coating and achieved a record 16.77% PCE. The blade-coated G-BHJ has drastically different D-A crystallization kinetics, which suppresses the excessive aggregation induced unfavorable phase separation in BHJ. All these make G-BHJ a feasible and promising strategy towards highly efficient, eco- and manufacture friendly OSCs.

Bottom-Up Quasi-Epitaxial Growth of Hybrid Perovskite from Solution Process-Achieving High-Efficiency Solar Cells via Template​-Guided Crystallization.

Epitaxial growth gives the highest-quality crystalline semiconductor thin films for optoelectronic devices. Here, a universal solution-processed bottom-up quasi-epitaxial growth of highly oriented α-formamidinium lead triiodide (α-FAPbI3 ) perovskite film via a two-step method is reported, in which the crystal orientation of α-FAPbI3 film is precisely controlled through the synergetic effect of methylammonium chloride and the large-organic cation butylammonium bromide. In situ GIWAXS visualizes the BA-related intermediate phase formation at the bottom of film, which serves as a guiding template for the bottom-up quasi-epitaxial growth in the subsequent annealing process. The template-guided epitaxially grown BAFAMA perovskite film exhibits increased crystallinity, preferred crystallographic orientation, and reduced defects. Moreover, the BAFAMA perovskite solar cells demonstrate decent stability, maintaining 95% of their initial power conversion efficiency after 2600 h ambient storage, and 4-time operation condition lifetime enhancement.

Perovskite Quantum Wells Formation Mechanism for Stable Efficient Perovskite Photovoltaics-A Real-Time Phase-Transition Study.

The combination of a bulk 3D perovskite layer and a reduced dimensional perovskite layer (perovskite quantum wells (PQWs)) is demonstrated to enhance the performance of perovskite solar cells (PSCs) significantly in terms of stability and efficiency. This perovskite hierarchy has attracted intensive research interest; however, the in-depth formation mechanism of perovskite quantum wells on top of a 3D perovskite layer is not clearly understood and is therefore the focus of this study. Along with ex situ morphology and photophysical characterization, the time-resolved grazing-incidence wide-angle X-ray scattering (TS-GIWAXS) technique performed in this study provides real-time insights on the phase-transition during the organic cation (HTAB ligand molecule) coating and PQWs/3D architecture formation process. A strikingly strong ionic reaction between the 3D perovskite and the long-chain organic cation leads to the quick formation of an ordered intermediate phase within only a few seconds. The optimal PQWs/3D architecture is achieved by controlling the HTAB casting, which is assisted by time-of-flight SIMS characterization. By controlling the second ionic reaction during the long-chain cation coating process, along with the fluorinated poly(triarylamine) (PTAA) as a hole-transport layer, the perovskite solar cells demonstrate efficiencies exceeding 22% along with drastically improved device stability.

Highly Crystalline Near-Infrared Acceptor Enabling Simultaneous Efficiency and Photostability Boosting in High-Performance Ternary Organic Solar Cells.

The near-infrared (NIR) absorbing fused-ring electron acceptor, COi8DFIC, has demonstrated very good photovoltaic performance when combined with PTB7-Th as a donor in binary organic solar cells (OSCs). In this work, the NIR acceptor was added to state-of-the-art PBDBT-2F:IT4F-based solar cells as a third component, leading to (i) an efficiency increase of the ternary devices compared to the binary solar cells in the presence of the highly crystalline COi8DFIC acceptor and (ii) much-improved photostability under 1-sun illumination. The electron transport properties were investigated and revealed the origin of the enhanced device performance. Compared to the binary cells, the optimized ternary PBDBT-2F:COi8DFIC:IT4F blends exhibit improved electron transport properties in the presence of 10% COi8DFIC, which is attributed to improved COi8DFIC molecular packing. Furthermore, transient absorption spectroscopy revealed a slow recombination of charge carriers in the ternary blend. The improved electron transport properties were preserved in the ternary OSC upon aging, while in the binary devices they seriously deteriorated after simulated 1-sun illumination of 240 h. Our work demonstrates a simple approach to enhance both OSC efficiency and photostability.

A Cryogenic Process for Antisolvent-Free High-Performance Perovskite Solar Cells.

A cryogenic process is introduced to control the crystallization of perovskite layers, eliminating the need for the use of environmentally harmful antisolvents. This process enables decoupling of the nucleation and the crystallization phases by inhibiting chemical reactions in as-cast precursor films rapidly cooled down by immersion in liquid nitrogen. The cooling is followed by blow-drying with nitrogen gas, which induces uniform precipitation of precursors due to the supersaturation of precursors in the residual solvents at very low temperature, while at the same time enhancing the evaporation of the residual solvents and preventing the ordered precursors/perovskite from redissolving into the residual solvents. Using the proposed techniques, the crystallization process can be initiated after the formation of a uniform precursor seed layer. The process is generally applicable to improve the performance of solar cells using perovskite films with different compositions, as demonstrated on three different types of mixed halide perovskites. A champion power conversion efficiency (PCE) of 21.4% with open-circuit voltage (VOC ) = 1.14 V, short-circuit current density ( JSC ) = 23.5 mA cm-2 , and fill factor (FF) = 0.80 is achieved using the proposed cryogenic process.

Fabrication of WS2/GaN p-n Junction by Wafer-Scale WS2 Thin Film Transfer.

High quality wafer-scale free-standing WS2 grown by van der Waals rheotaxy (vdWR) using Ni as a texture promoting layer is reported. The microstructure of vdWR grown WS2 was significantly modified from mixture of crystallites with their c-axes both parallel to (type I) and perpendicular to (type II) the substrate to large type II crystallites. Wafer-scale transfer of vdWR grown WS2 onto different substrates by an etching-free technique was demonstrated for the first time that utilized the hydrophobic property of WS2 and hydrophilic property of sapphire. Our results show that vdWR is a reliable technique to obtain type-II textured crystallites in WS2, which is the key factor for the wafer-scale etching-free transfer. The transferred films were found to be free of observable wrinkles, cracks, or polymer residues. High quality p-n junctions fabricated by room-temperature transfer of the p-type WS2 onto an n-type GaN was demonstrated with a small leakage current density of 29.6 µA/cm2 at -1 V which shows superior performances compared to the directly grown WS2/GaN heterojunctions.