Chen Weijie

The mesoscale domain growth, the molecular conformation, and aggregation are studied in situ during the printing of active layers of polymer donor and small-molecule acceptors out of nonhalogenated solvents at ambient conditions. Five characteristic growth regimes are found and the crystalline part of the final printed film deviates from the corresponding neat films.

Slot-die coating is a promising upscaling fabrication method to promote commercialization in the field of organic solar cells. Herein, the nonfullerene active layer blend of a conjugated polymer PffBT4T-2OD and a small molecule acceptor EH-IDTBR, which is printed out of the nonhalogenated solvent 1,2,4-trimethylbenzene, is studied. The film formation kinetics of the active layer PffBT4T-2OD:EH-IDTBR is probed in terms of the temporal evolutions in morphology as well as molecular conformation and aggregation as revealed by in situ grazing-incidence small angle X-ray scattering and UV–vis spectroscopy during the film printing process. A five-regime mesoscale domain growth process is observed in the active layer from the liquid state to the final dry state. The solvent evaporation-induced domain growth is accompanied with molecular stacking in a distinct J-type aggregation of the acceptor and a slight H-type aggregation of the donor molecules. The printed active layers exhibit an edge-on dominated PffBT4T-2OD and a face-on dominated EH-IDTBR crystallite structure. Compared to the neat PffBT4T-2OD and EH-IDTBR films, in the active layer, the crystallite structure deviates slightly in lattice spacing.

The two-step synthesis of mixed halogenation end groups reduces the synthesis steps of nonfullerene acceptor materials, improves the total yield, reduces costs, and is conducive to commercial applications. The influence of halogenation (F, Cl, Br, I) end groups on organic solar cells is systematically studied, which provides new ideas for the design of efficient and low-cost organic solar cells.

Mixed halogen substitution on the end groups of organic photovoltaic materials has been proven to be an effective way to generate materials with enhanced performance. A systematic comparison of the mixed halogen atom types can provide comprehensive information on this strategy; however, this is usually limited by the lack of a facile synthesis route. Herein, a new class of end groups featuring mixed halogenation, easily obtained via two steps with excellent yields, is reported. The new end groups are applied in synthesizing acceptor materials, and their performance is compared in organic solar cell devices. Combined with the small molecule donor BTR-Cl, the best-performing acceptor BTP-FCl-FCl achieves a power conversion efficiency of 15.27% and a high fill factor (FF) of 75.94%. Systematic studies of the mixed halogenation analogues in devices reveal that the higher FF is consistent with the longer carrier lifetime, faster charge extraction, weaker bimolecular recombination, and is associated with the favorable BTR-Cl:BTP-FC1-FC1 domain size and molecular packing, all of which are derived from the different halogen atoms. The results demonstrate that selecting suitable halogen atoms is vital in achieving high-performance organic solar cells.

Herein, a solvent strategy by using N-methylformamide as the precursor solvent of PbBr₂ is used for improving the perovskite crystallization in the printable mesoscopic structure. The efficiency of perovskite solar cells (PSCs) increases from 7.53% to 8.32%. In addition, PSCs show no obvious attenuation after 1000 h of maximum power point tracking, displaying excellent illumination stability.

The CsPbBr₃ perovskite solar cells (PSCs) display extensive potential due to their good thermal and humidity stability, but the presence of heterogeneous phases severely limits the further improvement of device performance. Phase-pure monoclinic CsPbBr₃ can be stabilized by using the printable mesoscopic device structure. However, it is challenging to obtain high-quality perovskite crystals in such confined space. Herein, a solvent strategy is used for improving the perovskite crystallization in the printable mesoscopic structure. By using N-methylformamide as the precursor solvent, the PbBr₂ exhibits a more uniform and controllable distribution, which benefits the CsPbBr₃ crystallization. As a result, the CsPbBr₃ inside the pores showed obvious orientation at (100) lattice plane and (110) crystal plane. The efficiency of modified PSCs increases from 7.53% to 8.32%. Based on the device with effective area of 1 cm², the PSCs obtain a power conversion efficiency of 5.62%. In addition, PSCs show no obvious attenuation after 1000 h of maximum power point tracking, displaying the excellent illumination stability.

This work demonstrates an universal chloride redistribution and passivation induced by post-treatment. The chloride enrichment on the surface can improve charge transport and the passivation can reduce the recombination. When cyclohexylmethylammonium iodide (CHMAI) post-treated perovskite film, the Cl/I ratio on surface increased from 0.037 to 0.439, which leveraged their roles in charge transport/recombination. Finally, CHMAI perovskite solar cells deliver a champion power conversion efficiency of 24.42%.

Abstract

Post-treatment is an essential passivation step for the state-of-the-art perovskite solar cells (PSCs) but the additional role is not yet exploited. In this work, perovskite film is fabricated under ambient air with wide humidity window and identify that chloride redistribution induced by post-treatment plays an important role in high performance. The chlorine/iodine ratio on the perovskite surface increases from 0.037 to 0.439 after cyclohexylmethylammonium iodide (CHMAI) treatment and the PSCs deliver a champion power conversion efficiency (PCE) of 24.42% (certificated 23.60%). The maximum external quantum efficiency of electroluminescence (EQE_EL) reaches to 10.84% with a radiance of 170 W sr⁻¹ m⁻², forming the reciprocity relation between EQE_EL and nonradiative open-circuit voltage loss (86.0 mV). After thermal annealing, 2D component of perovskite will increase while chloride decline, leading to improved photovoltage but reduced fill factor. Hence, it distinguishes that chloride enrichment can improve charge transport/recombination simultaneously and 2D passivation can suppress the nonradiative recombination. Moreover, CHMAI can leverage their roles in charge transport/recombination for better performance than phenylethylammonium iodide (Cl/I = 0.114, PCE = 23.32%), due to the stronger binding energy of Cl⁻. This work provides the insight that the chloride fixation can improve the photovoltaic performance.

A technique to enable deposition of large-area, freestanding, ultra-thin, conjugated polymer films, which can be transferred to a desired substrate without using solvents, is demonstrated. The quality of these films is assessed by a variety of techniques, and limiting factors and strategies are identified.

Abstract

Conventional processes for depositing thin films of conjugated polymers are restricted to those based on vapor, liquid, and solution-phase precursors. Each of these methods bear some limitations. For example, low-bandgap polymers with alternating donor–acceptor structures cannot be deposited from the vapor phase, and solution-phase deposition is always subject to issues related to the incompatibility of the substrate with the solvent. Here, a technique to enable deposition of large-area, ultra-thin films (≈20 nm or more), which are transferred from the surface of water, is demonstrated. From the water, these pre-solidified films can then be transferred to a desired substrate, circumventing limitations such as solvent orthogonality. The quality of these films is characterized by a variety of imaging and electrochemical measurements. Mechanical toughness is identified as a limiting property of polymer compatibility, along with some strategies to address this limitation. As a demonstration, the films are used as the hole-transport layer in perovskite solar cells, in which their performance is shown to be comparable to controls formed by spin-coating.

In situ synchrotron X-ray scattering and real-time thermal imaging for the first time unraveled the effects of the thermally conductive additive, hexagonal boron nitride nanosheets, on enhancing the crystallization kinetics in organic-inorganic hybrid perovskites and the associated solar cell performance and stability.

Abstract

Controlling crystallization and grain growth is crucial for realizing highly efficient hybrid perovskite solar cells (PSCs). In this work, enhanced PSC photovoltaic performance and stability by accelerating perovskite crystallization and grain growth via 2D hexagonal boron nitride (hBN) nanosheet additives incorporated into the active perovskite layer are demonstrated. In situ X-ray scattering and infrared thermal imaging during the perovskite annealing process revealed the highly thermally conductive hBN nanosheets promoted the phase conversion and grain growth in the perovskite layer by facilitating a more rapid and spatially uniform temperature rise within the perovskite film. Complementary structural, physicochemical, and electrical characterizations further showed that the hBN nanosheets formed a physical barrier at the perovskite grain boundaries and the interfaces with charge transport layers, passivating defects, and retarding ion migration. As a result, the power conversion efficiency of the PSC is improved from 17.4% to 19.8%, along with enhanced device stability, retaining ≈90% of the initial efficiency even after 500 h ambient air storage. The results not only highlight 2D hBN as an effective additive for PSCs but also suggest enhanced thermal transport as one of the pathways for improved PSC performance by 2D material additives in general.

An electron-deficient tetrafluoronaphthodithiophene (FNT) was created to construct a family of PFNT-F/Cl polymers for organic solar cells. The PFNT-F/Cl-based OSCs exhibit impressive FF values of 0.80, and remarkable PCEs of 17.53 % and 18.10 %, suggesting multifluorinated polymers are promising donor materials for non-fullerene OSCs.

Abstract

Creating new electron-deficient unit is highly demanded to develop high-performance polymer donors for non-fullerene organic solar cells (OSCs). Herein, we reported a multifluorinated unit 4,5,6,7-tetrafluoronaphtho[2,1-b : 3,4-b′]dithio-phene (FNT) and its polymers PFNT-F and PFNT-Cl. The advantages of multifluorination: (1) it enables the polymers to exhibit low-lying HOMO (≈−5.5 eV) and wide band gap (≈2.0 eV); (2) the short interactions (F⋅⋅⋅H, F⋅⋅⋅F) endow the polymers with properties of high film crystallinity and efficient hole transport; (3) well miscibility with NFAs that leads to a more well-defined nanofibrous morphology and face-on orientation in the blend films. Therefore, the PFNT-F/Cl : N3 based OSCs exhibit impressive FF values of 0.80, and remarkable PCEs of 17.53 % and 18.10 %, which make them ranked the best donor materials in OSCs. This work offers new insights into the rational design of high-performance polymers by multifluorination strategy.

Adding 4-methylphenethylammonium chloride into a 3D perovskite precursor results in better film quality with larger grains. Quasi-2D perovskites are formed and passivate defects at the grain boundaries. The perovskite surface chemistry is modified, and the surface energies become more favorable for hole extraction. This approach leads to methylammonium-free perovskite solar cells with a champion steady-state efficiency of 23.7% and excellent stability.

Abstract

Methylammonium (MA)-free perovskite solar cells have the potential for better thermal stability than their MA-containing counterparts. However, the efficiency of MA-free perovskite solar cells lags behind due to inferior bulk quality. In this work, 4-methylphenethylammonium chloride (4M-PEACl) is added into a MA-free perovskite precursor, which results in greatly enhanced bulk quality. The perovskite crystal grains are significantly enlarged, and defects are suppressed by a factor of four upon the incorporation of an optimal concentration of 4M-PEACl. Quasi-2D perovskites are formed and passivate defects at the grain boundaries of the perovskite crystals. Furthermore, the perovskite surface chemistry is modified, resulting in surface energies more favorable for hole extraction. This facile approach leads to a steady state efficiency of 23.7% (24.2% in reverse scan, 23.0% in forward scan) for MA-free perovskite solar cells. The devices also show excellent light stability, retaining more than 93% of the initial efficiency after 1000 h of constant illumination in a nitrogen environment. In addition, a four-terminal mechanically stacked perovskite-silicon tandem solar cell with champion efficiency of 30.3% is obtained using this MA-free composition. The encapsulated tandem devices show excellent operational stability, retaining more than 98% of the initial performance after 42 day/night cycles in an ambient atmosphere.

An acyloin ligand (1,2-di(thiophen-2-yl)ethane-1,2-dione (DED)) is developed to construct γ-CsPbI₃ perovskite solar cells (PSCs). Comprehensive characterizations confirm that the carbonyl and thienyl in DED can synergistically bind with Pb²⁺-related defects by forming a chelate (one DED bind with two Pb²⁺ ions), yielding a stable γ-CsPbI₃ film. Consequently, DED-CsPbI₃ yields a champion PCE of 21.15% among the reported CsPbI₃ PSCs to date.

Abstract

Cesium lead triiodide (CsPbI₃) is a promising light-absorbing material for constructing perovskite solar cells (PSCs) owing to its favorable bandgap and thermal tolerance. However, the high density of defects in the CsPbI₃ film not only act as recombination centers, but also facilitate ion migration, leading to lower PCE and inferior stability compared with the state-of-the-art organic–inorganic hybrid PSC counterpart. Theoretical analyses suggest that the effective suppression of defects in CsPbI₃ film is helpful for improving the device performance. Herein, the stable and efficient γ -CsPbI₃ PSCs are demonstrated by developing an acyloin ligand (1,2-di(thiophen-2-yl)ethane-1,2-dione (DED)) as a phase stabilizer and defect passivator. The experiment and calculation results confirm that carbonyl and thienyl in DED can synergistically interact with CsPbI₃ by forming a chelate to effectively passivate Pb-related defects and further suppress ion migration. Consequently, DED-treated CsPbI₃ PSCs yield a champion PCE of 21.15%, which is one of the highest PCE among all the reported CsPbI₃ PSCs to date. In addition, the unencapsulated DED-CsPbI₃ PSC can retain 94.9% of itsinitial PCE when stored under ambient conditions for 1000 h and 92.8% of its initial PCE under constant illumination for 250 h.

It is suggested that symmetry-breaking of the nonfullerene acceptor can improve the versatile processability of organic solar cells (OSCs). In a photoactive blend film, symmetrical Y6 with a strong aggregation tendency shows a markedly different stacking structure depending on the processing solvent, whereas asymmetrical IPC-BEH-IC2F has a solvent-independent aggregation property. The OSC performance corresponds to this morphological tendency.

Despite the outstanding photovoltaic performance of symmetrical Y6, its use is limited in that only chloroform (CF), a highly volatile processing solvent, can induce favorable morphology. This dependence on a particular solvent poses an obstacle to mass production. Here, we investigate the effect of symmetry-breaking of nonfullerene acceptors (NFAs) on device performance and processability by employing chlorobenzene (CB) or CF processing solvents. In organic solar cells (OSCs) based on a symmetrical Y6 acceptor, a significant difference in the power conversion efficiency occurs between the OSCs fabricated using CB and those fabricated using CF. However, in OSCs based on IPC-BEH-IC2F with asymmetrical structure, no difference in photovoltaic performance occurs between the two OSCs. Grazing-incidence wide-angle X-ray scattering measurements indicate that IPC-BEH-IC2F exhibits nearly identical diffraction features in both the CB- and CF-processed photoactive films, whereas Y6 shows markedly different stacking structures. Because of these morphological features, OSCs based on Y6 are associated with a relatively large energy loss difference in CB and CF, whereas OSCs based on IPC-BEH-IC2F show no significant difference in energy loss. The introduction of an asymmetrical structure can therefore be an important strategy to enhance the versatile processability of OSCs based on NFAs for future mass production.

Publication date: 18 January 2023

Source: Joule, Volume 7, Issue 1

Author(s): Rui Sun, Tao Wang, Qunping Fan, Mingjian Wu, Xinrong Yang, Xiaohei Wu, Yue Yu, Xinxin Xia, Fengzhe Cui, Ji Wan, Xinhui Lu, Xiaotao Hao, Alex K.-Y. Jen, Erdmann Spiecker, Jie Min

A novel small-molecule acceptor Y-SeNF with selenophene-fused core and naphthalene-fused end-groups is developed. Introducing NIR-absorbing Y-SeNF as a custom-tailor guest into PM6:L8-BO host system, ternary PSCs achieve a high efficiency of 19.28% and excellent stability.

Abstract

Here, a near-infrared (NIR)-absorbing small-molecule acceptor (SMA) Y-SeNF with strong intermolecular interaction and crystallinity is developed by combining selenophene-fused core with naphthalene-containing end-group, and then as a custom-tailor guest acceptor is incorporated into the binary PM6:L8-BO host system. Y-SeNF shows a 65 nm red-shifted absorption compared to L8-BO. Thanks to the strong crystallinity and intermolecular interaction of Y-SeNF, the morphology of PM6:L8-BO:Y-SeNF can be precisely regulated by introducing Y-SeNF, achieving improved charge-transporting and suppressed non-radiative energy loss. Consequently, ternary polymer solar cells (PSCs) offer an impressive device efficiency of 19.28% with both high photovoltage (0.873 V) and photocurrent (27.88 mA cm⁻²), which is one of the highest efficiencies in reported single-junction PSCs. Notably, ternary PSC has excellent stability under maximum-power-point tracking for even over 200 h, which is better than its parental binary devices. The study provides a novel strategy to construct NIR-absorbing SMA for efficient and stable PSCs toward practical applications.

Photoluminescence (PL) of perovskite can be switched by other light with sub-bandgap photon energy in a broad spectral range with high stability. This effect is controlled by photosensitive defects, in which concentration is modified by the crystal stoichiometry. This finding indicates a new application area of metal halide perovskites in optical switches, transistors, and memory.

Abstract

Controllable optical properties are crucial for the application of light-emitting materials in optical devices. In this work, controllable photoluminescence in metal halide perovskite crystals is realized via photoactivation of their defects. It is found that under continuous excitation, the photoluminescence intensity of a CH₃NH₃PbBr₃ crystal can be fully controlled by sub-bandgap energy photon illumination. Such optically controllable emission behavior is rather general as it is observed also in CsPbBr₃ and other perovskite materials. The switching mechanism is assigned to reversible light-induced activation/deactivation of nonradiative recombination centers, the presence of which relates to an excess of Pb during perovskite synthesis. Given the success of perovskites in photovoltaics and optoelectronics, it is believed that the discovery of green luminescence controlled by red illumination will extend the application scope of perovskites toward optical devices and intelligent control.