Composite Interlayer Consisting of Alcohol-Soluble Polyfluorene and Carbon Nanotubes for Efficient Polymer Solar Cells

Device Stability and Photoelectric Conversion Evolution of PM6 Solar Cells Based on Different Acceptors

To understand the effects of acceptors on morphology aging, photoelectric conversion evolution, and stability of PM6-based solar cells, multiple characterization techniques, including morphology, transient absorption, and electrical characterizations, were conducted to analyze the correlation among morphology aging, photoelectric conversion evolution, and performance degradation of devices. The results demonstrated that the morphology features of PM6:Y6 and PM6:PC71BM active layers would change with time due to their unstable bulk heterojunction structures. The unstable active layers determined the evolution of photoelectric conversion and the stability of the devices. Furthermore, morphology aging was responsible for the increase of charge recombination. Compared with PM6:PC71BM, more delocalized and localized polarons were generated in PM6:Y6 solar cells, and the increased probability of charge recombination with morphology aging was relatively smaller. Therefore, the PM6:Y6 device showed a higher efficiency and better stability than the PM6:PC71BM device.

Hole/Electron Transporting Materials for Nonfullerene Organic Solar Cells

Nonfullerene acceptor based organic solar cells (NF-OSCs) have witnessed rapid progress over the past few years owing to the intensive research efforts on novel electron donor and nonfullerene acceptor (NFA) materials, interfacial engineering, and device processing techniques. Interfacial layers including electron transporting layers (ETL) and hole transporting layers (HTLs) are crucially important in the OSCs for facilitating electron and hole extraction from the photoactive blend to the respective electrodes. In this review, the lates progress in both ETLs and HTLs for the currently prevailing NF-OSCs are discussed, in which the ETLs are summarized from the categories of metal oxides, metal chelates, non-conjugated electrolytes and conjugated electrolytes, and the HTLs are summarized from the categories of inorganic and organic materials. In addition, some bifunctional interlayer materials served as both ETLs and HTLs are also introduced. Finally, the prospects of ETL/HTL materials for NF-OSCs are provided.

Resolving Atomic-Scale Interactions in Nonfullerene Acceptor Organic Solar Cells with Solid-State NMR Spectroscopy, Crystallographic Modelling, and Molecular Dynamics Simulations

Fused-ring core nonfullerene acceptors (NFAs), designated "Y-series," have enabled high-performance organic solar cells (OSCs) achieving over 18% power conversion efficiency (PCE). Since the introduction of these NFAs, much effort has been expended to understand the reasons for their exceptional performance. While several studies have identified key optoelectronic properties that govern high PCEs, little is known about the molecular level origins of large variations in performance, spanning from 5% to 18% PCE, for example, in the case of PM6:Y6 OSCs. Here, a combined solid-state NMR, crystallography, and molecular modeling approach to elucidate the atomic-scale interactions in Y6 crystals, thin films, and PM6:Y6 bulk heterojunction (BHJ) blends is introduced. It is shown that the Y6 morphologies in BHJ blends are not governed by the morphology in neat films or single crystals. Notably, PM6:Y6 blends processed from different solvents self-assemble into different structures and morphologies, whereby the relative orientations of the sidechains and end groups of the Y6 molecules to their fused-ring cores play a crucial role in determining the resulting morphology and overall performance of the solar cells. The molecular-level understanding of BHJs enabled by this approach will guide the engineering of next-generation NFAs for stable and efficient OSCs.

Recent Applications of Carbon Nanotubes in Organic Solar Cells

In recent years, carbon-based materials, particularly carbon nanotubes (CNTs), have gained intensive research attention in the fabrication of organic solar cells (OSCs) due to their outstanding physicochemical properties, low-cost, environmental friendliness and the natural abundance of carbon. In this regard, the low sheet resistance and high optical transmittance of CNTs enables their application as alternative anodes to the widely used indium tin oxide (ITO), which is toxic, expensive and scarce. Also, the synergy between the large specific surface area and high electrical conductivity of CNTs provides both large donor-acceptor interfaces and conductive interpenetrating networks for exciton dissociation and charge carrier transport. Furthermore, the facile tunability of the energy levels of CNTs provides proper energy level alignment between the active layer and electrodes for effective extraction and transportation of charge carriers. In addition, the hydrophobic nature and high thermal conductivity of CNTs enables them to form protective layers that improve the moisture and thermal stability of OSCs, thereby prolonging the devices' lifetime. Recently, the introduction of CNTs into OSCs produced a substantial increase in efficiency from ∼0.68 to above 14.00%. Thus, further optimization of the optoelectronic properties of CNTs can conceivably help OSCs to compete with silicon solar cells that have been commercialized. Therefore, this study presents the recent breakthroughs in efficiency and stability of OSCs, achieved mainly over 2018-2021 by incorporating CNTs into electrodes, active layers and charge transport layers. The challenges, advantages and recommendations for the fabrication of low-cost, highly efficient and sustainable next-generation OSCs are also discussed, to open up avenues for commercialization.

Chromatic Conductive Polymer Nanocomposites of Poly (p-Phenylene Ethynylene)s and Single-Walled Carbon Nanotubes

We report on dispersions and thin films of chromatic conductive nanocomposites of poly(p-phenylene ethynylene)s (PPEs) and single-walled carbon nanotubes (SWNTs) generated via solution mixing. The linear, conjugated PPEs with dialkyl- and dialkyloxy-side chain groups are shown to debundle and disperse high concentration (up to 2.5 mg/mL) SWNTs in various organic solvents. The solubilization of SWNTs and PPE wrapping is accompanied with the change in the solution color. Ultraviolet visible absorption spectra of nanocomposite solutions demonstrate a new absorption peak at a higher wavelength, supporting the observed chromatism. Fluorescence spectra of nanocomposite solutions display significant quenching of the fluorescence intensity and the Stern–Volmer model is used to analyze fluorescence quenching. Electron microscopy of the chromatic solid films of high mass fraction PPE/SWNT nanocomposites obtained by vacuum filtration reveals the debundled SWNTs in the PPE matrix. The tensile strength and Young’s modulus of these PPE/SWNT nanocomposite films are as high as 150 MPa and 15 GPa, respectively. The composite films exhibit remarkably high conductivities, ranging from ~1000 S/m to ~10,000 S/m for 10 wt% and 60 wt% SWNT nanocomposites, respectively.

Orthogonal Printable Reduced Graphene Oxide 2D Materials as Hole Transport Layers for High-Performance Inverted Polymer Solar Cells: Sheet Size Effect on Photovoltaic Properties

Creating an orthogonal printable hole-transporting layer (HTL) without damaging the underlying layer is still a major challenge in fabricating large-area printed inverted polymer solar cells (PSCs). In this study, we prepared orthogonal-processable fluorine-functionalized reduced graphene oxide (FrGO) series with various two-dimensional sheet sizes such as large-sized FrGO (1.1 μm), medium-sized FrGO (0.7 μm), and small-sized FrGO (0.3 μm) and systematically investigated the size effect of FrGOs on the hole transport properties of PSCs. The FrGOs exhibit highly stable dispersion without change over 90 days in 2-propanol solvent, indicating very high dispersion stability. Decreasing the sheet size of FrGOs enhanced hole-transporting properties, resulting in power conversion efficiencies (PCEs) of 9.27 and 9.02% for PTB7-Th:EH-IDTBR- and PTB7-Th:PC₇₁BM-based PSCs, respectively. Compared to devices with solution-processed poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS), a 14% enhancement of PCEs was achieved. Interestingly, the PCEs of devices with the smallest FrGO sheet are higher than the PCE of 8.77% of a device with vacuum-deposited MoO₃. The enhancement in the performance of PSCs is attributed to the enhanced charge collection efficiency, decreased leakage current, internal resistance, and minimized charge recombination. Finally, small-sized FrGO HTLs were successfully coated on the photoactive layer using the spray coating method, and they also exhibited PCEs of 9.22 and 13.26% for PTB7-Th:EH-IDTBR- and PM6:Y6-based inverted PSCs, respectively.

Polymer wrapping-induced dispersion of single walled carbon nanotubes in ethylene glycol under mild sonication

SWCNTs were individually dispersed in ethylne glycol (EG) via mild bath-type sonication using quaternized poly(furfuryl methacrylate)-co-(2-(dimethylamino)ethyl methacrylate) p(FMA-co-QDMAEMA) as a dispersing agent. QDMAEMA, which has alkyl groups, was more favorable to the dispersion ability of single walled carbon nanotubes (SWCNTs). The dispersion mechanism of SWCNTs in EG via helical wrapping of polymer chains along their sidewalls was suggested based on transmission electron microscopic observation.

The dispersion of bundled SWCNTs via helical wrapping of polymer chains along their sidewalls.