Acceptor Domains Research Articles

Enhancing inter-domain connectivity by reducing fractal dimensions: the key to passivating deep traps in organic photovoltaics

Researchers in CUHK identified isolated non-fullerene acceptor (NFA) domains as the morphological origin of deep electron traps in organic photovoltaics. They showcased effective trap passivation by reducing the fractal dimensions of NFA networks.

Efficient Nanoscale Exciton Transport in Non-Fullerene Organic Solar Cells Enables Reduced Bimolecular Recombination of Free Charges.

The highest-efficiency organic photovoltaic (OPV)-based solar cells, made from blends of electron-donating and electron-accepting organic semiconductors, are often characterized by strongly reduced (non-Langevin) bimolecular recombination. Although the origins of the reduced recombination are debated, mechanisms related to the charge-transfer (CT) state and free-carrier encounter dynamics controlled by the size of donor and acceptor domains are proposed as underlying factors. Here,a novel photoluminescence-based probe is reported to accurately quantify the donor-acceptor domain size in OPV blends. Specifically, the domain size is measured in high-efficiency non-fullerene acceptor (NFA) systems and a comparative conventional fullerene system. It is found that the NFA-based blends form larger domains but that the expected reductions in bimolecular recombination attributed to the enhanced domain sizes are too small to account for the observed reduction factors. Further, it is shown that the reduction of bimolecular recombination is correlated to enhanced exciton dynamics within the NFA domains. This indicates that the processes responsible for efficient exciton transport also enable strongly non-Langevin recombination in high-efficiency NFA-based solar cells with low-energy offsets.

Fluorinated carbon nanotubes as nonvolatile additive to the active layer of polymer/fullerene solar cells

Highly dispersed fluorinated SWCNTs of submicron length were prepared, characterized and tested as nonvolatile additive to the active layer of polymer/fullerene organic solar cells. Two types of initial SWCNT materials – TUBALL® (OCSiAl) and Super purified plasma tubes (Nanointegris Inc.) – were used. The absence of big CNT aggregates was revealed by atomic force microscopy. The absence of metallic SWCNTs is confirmed by optical spectra of fluorinated SWCNT dispersions. Reproducible increase of the main performance parameters of solar cells (short circuit current, open circuit voltage, fill factor and power conversion efficiency) was obtained upon admixing a small amount of fluorinated SWCNTs (less than 1% weight) into the polymer/fullerene active layer, with either P3HT or PCDTBT donor and either PC60BM or PC70BM acceptor for both direct and inverted device architectures. Since effective charge mobility did not change upon fluorinated SWCNT admixture, the improvement of solar cell performance is not related to SWCNT-induced modification of the electronic structure of the active layer components or charge transfer via fluorinated SWCNTs. Presumably, the origin of this improvements is geometric optimization of the active layer morphology (improving connectivity of donor and acceptor domains) by fluorinated SWCNTs due to their high aspect ratio.

A 1,3,5-triazine and benzodithiophene based donor-acceptor type semiconducting conjugated polymer for photocatalytic overall water splitting

Donor–acceptor domains hold the key to tailoring photochemical properties and charge carrier mobility of conjugated microporous polymers (CMPs) for application in photocatalytic water splitting. Herein, a family of electron-withdrawing 1,3,5-triazine and electron-donating fused thiophene based porous polymers are obtained via C–H arylation. The TBT-BDT showed an excellent HER of 4238 ​μmol ​g−1 ​h−1, which is 17-fold and 3-fold higher than that of TBT-Th and TBT-TT, respectively. In addition, TBT-BDT facilitates overall water splitting to produce stoichiometric amounts of H2 (28 ​μmol ​g−1 ​h−1) and O2 (14 ​μmol ​g−1 ​h−1). The significant difference in the photocatalytic performance was mainly ascribed to the band structure change of the polymers, which underlined the importance of the donor unit on the performance of D−A type photocatalysts and indicated that thiophene-contained unit BDT is an excellent donor for high photocatalytic activity.

High‐Performance Organic Solar Cells by Adding Two‐Dimensional GeSe

AbstractIn organic solar cells (OSCs), the crystalline morphologies of the donor and acceptor at the microscale play an important role. 2D GeSe with periodic structure and high carrier mobility is an ideal additive material for active layer of OSCs that can effectively improve the crystallinity and molecular orientation of the acceptor molecule ITIC. Here, 2D GeSe is introduced into the active layer of OSCs to adjust the morphology of active layer for the first time, the results show that the incorporation of 2D GeSe induces the crystallization of acceptor molecule and facilitates the formation of efficient bicontinuous interpenetrating charge transport networks, and effectively reduces carrier recombination in OSCs. The power conversion efficiency and lifetime of OSCs are obviously enhanced. More importantly, the results of first‐principles calculation reveal that the interaction between 2D GeSe and ITIC guides the ITIC molecules to form a continuous conductive network. This study provides a method for the morphology regulation of the donor and acceptor domains in bulk heterojunctions, provides an idea for researching the interaction between 2D materials and organic molecules, and also indicates that 2D GeSe has good application prospects in photovoltaic devices.

Deletion of COM donor and acceptor domains and the interaction between modules in bacillomycin D produced by Bacillus amyloliquefaciens

Bacillomycin D is a cyclic lipopeptide produced by Bacillus amyloliquefaciens fmbJ. At present, no relevant report has described the combinatorial biosynthesis of bacillomycin D. Due to the strong biosynthetic potential of the communication-mediating (COM) domains, its crosstalk between NRPS subunits has been studied to some extent, but the interaction of COM domain between modules is rarely reported. Therefore, in this study, we conducted the combinatorial biosynthesis of bacillomycin D through the deletion of the COM donor and acceptor domains between the modules and elucidated the interaction between the NRPS modules. The results showed that the deletion of the donor domain between modules 2 and 3 did not affect catalysis by upstream modules, but prevented downstream modules from catalysing the extension of the lipopeptide product, ultimately resulting in mutant complexes that could form linear dipeptides with the sequence β-NH2FA-Asn-Tyr. However, the engineered hybrid bacillomycin D NRPSs lacking the donor domains between modules 3 and 4 and modules 6 and 7 could form multiple assembly lines that produced bacillomycin D and its analogs (linear tripeptides, cyclic hexapeptides and linear hexapeptides). In addition, all the acceptor domain deletion strains failed to produce bacillomycin D, only truncated peptides produced by module interruption (except for the acceptor domain deletion strains between modules 3 and 4, which also produced cyclic hexapeptides). In conclusion, deletion of the inter-module donor domains led to a more flexible hybrid biosynthetic system for the production of diverse peptide products; compared with the inter-subunit donor domain deletion strains that could only produce truncated peptides, the former had a greater biosynthetic capacity. Meanwhile, the acceptor domains between modules were an important part of module-module interactions and efficient communication within bacillomycin D synthetase.

Single-junction organic solar cells with over 19% efficiency enabled by a refined double-fibril network morphology.

In organic photovoltaics, morphological control of donor and acceptor domains on the nanoscale is the key for enabling efficient exciton diffusion and dissociation, carrier transport and suppression of recombination losses. To realize this, here, we demonstrated a double-fibril network based on a ternary donor-acceptor morphology with multi-length scales constructed by combining ancillary conjugated polymer crystallizers and a non-fullerene acceptor filament assembly. Using this approach, we achieved an average power conversion efficiency of 19.3% (certified 19.2%). The success lies in the good match between the photoelectric parameters and the morphological characteristic lengths, which utilizes the excitons and free charges efficiently. This strategy leads to an enhanced exciton diffusion length and a reduced recombination rate, hence minimizing photon-to-electron losses in the ternary devices as compared to their binary counterparts. The double-fibril network morphology strategy minimizes losses and maximizes the power output, offering the possibility of 20% power conversion efficiencies in single-junction organic photovoltaics.

Nonalloy Model-Based Ternary Organic Solar Cells.

Ternary blending based on an alloy-like model has been proved as an efficient strategy for high-efficiency organic solar cells (OSCs). However, the third component that possesses excellent miscibility with host materials in the alloy-like model may trigger adverse effects for the active layer, especially at a high doping ratio. In this work, we propose a new concept of nonalloy model for the ternary OSCs in which the third component presents moderate miscibility with the acceptor and distributes at the interspace between donor and acceptor domains. The nonalloy model is constructed based on the PM6:Y6 system, and a Y6 analogue (BTP-MCA) is synthesized as the third component. The BTP-MCA can maintain initial excellent morphology of the active layer and enhance the morphological stability by acting as a frame around the host materials. As a result, ternary OSCs based on the PM6:Y6:BTP-MCA blend exhibit an impressive efficiency of 17.0% with a high open-circuit voltage of 0.87 V. Moreover, the devices present a high doping tolerance (keeping high efficiency with a doping ratio of 50%) and improved stability. This work indicates that the nonalloy model can be a promising method to fabricate efficient and stable ternary OSCs apart from the conventional alloy-like model.

N-doping of nonfullerene bulk-heterojunction organic solar cells strengthens photogeneration and exciton dissociation

N-doping of the bulk-heterojunction active layer to enlarge polymeric donor (D) and nonfullerene acceptor (A) domains, dilate D/A interspace and reduce acceptor exciton binding energy to enhance light harvesting and suppress germinate recombination.

Recent Advances of Film–Forming Kinetics in Organic Solar Cells

Solution–processed organic solar cells (OSC) have been explored widely due to their low cost and convenience, and impressive power conversion efficiencies (PCEs) which have surpassed 18%. In particular, the optimization of film morphology, including the phase separation structure and crystallinity degree of donor and acceptor domains, is crucially important to the improvement in PCE. Considering that the film morphology optimization of many blends can be achieved by regulating the film–forming process, it is necessary to take note of the employment of solvents and additives used during film processing, as well as the film–forming conditions. Herein, we summarize the recent investigations about thin films and expect to give some guidance for its prospective progress. The different film morphologies are discussed in detail to reveal the relationship between the morphology and device performance. Then, the principle of morphology regulating is concluded with. Finally, a future controlling of the film morphology and development is briefly outlined, which may provide some guidance for further optimizing the device performance.

Highly Selective Cu Staining of Sulfur-Containing Polymers Facilitates 3D Nanomorphology Reconstruction of Polymer:Fullerene Blends in Organic Solar Cells by FIB-SEM Tomography.

The distinction of different organic materials in phase mixtures is hampered in electron microscopy because electron scattering does not strongly differ in carbon-based materials that mainly consist of light elements. A successful strategy for contrast enhancement is selective staining where one phase of a material mixture is labeled by heavier elements, but suitable staining agents are not available for all organic materials. This is also the case for bulk-heterojunction (BHJ) absorber layers of organic solar cells, which consist of interpenetrating networks of donor and acceptor domains. The domain structure strongly influences the power conversion efficiency, and nanomorphology optimization often requires real-space information on the sizes and interconnectivity of domains with nanometer resolution. In this work, we have developed an efficient approach to selectively stain sulfur-containing polymers by homogeneous Cu infiltration, which generates strong material contrast in scanning (transmission) electron microscopy (S(T)EM) images of polymer:fullerene BHJ layers. Cross-section lamellae of BHJ layers are prepared for STEM by focused-ion-beam milling and are attached to a Cu lift-out grid as a copper source. After thermal treatment at 200 °C for 3 h in air, sulfur-containing polymers are homogeneously infiltrated by Cu, while the fullerenes are not affected. Selective Cu staining is applied to map the phase distribution in PTB7:PC71BM BHJ layers fabricated with different processing additives to tailor the nanomorphology. The strong contrast between polymer and fullerene domains is the prerequisite for the three-dimensional reconstruction of the domain structure by focused-ion-beam/scanning-electron-microscopy tomography.

Interfacial Electrostatic-Interaction-Enhanced Photomultiplication for Ultrahigh External Quantum Efficiency of Organic Photodiodes.

A photomultiplication-type organic photodiode (PM-OPD), where an electric double layer (EDL) is strategically embedded, is demonstrated, with an exceptionally high external quantum efficiency (EQE) of 2210000%, responsivity of 11200 A W-1 , specific detectivity of 2.11 × 1014 Jones, and gain-bandwidth product of 1.92 × 107 Hz, as well as high reproducibility. A polymer electrolyte, poly(9,9-bis(3'-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene))dibromide is employed as a work-function-modifying layer of indium tin oxide (ITO) to construct an EDL-embedded Schottky junction with p-type polymer semiconductor, poly(3-hexylthiophene-diyl), resulting in not only advantageous tuning of the work function of ITO but also an enhancement of the electron-trapping efficiency due to electrostatic interaction between exposed cations and trapped electrons within isolated acceptor domains. The effects of the EDL on the energetics of the trapped electron states and thus on the gain generation mechanism are confirmed by numerical simulations based on the drift-diffusion approximation of charge carriers. The feasibility of the fabricated high-EQE PM-OPD especially for weak light detection is demonstrated via a pixelated prototype image sensor. It is believed that this new OPD platform opens up the possibility for the ultrahigh-sensitivity organic image sensors, while maintaining the advantageous properties of organics.

General Rules for the Impact of Energetic Disorder and Mobility on Nongeminate Recombination in Phase-Separated Organic Solar Cells

State-of-the-art organic solar cells exhibit power conversion efficiencies of 18% and above. These devices benefit from the suppression of free charge recombination with regard to the Langevin limit of charge encounter in a homogeneous medium. It is recognized that the main cause of suppressed free charge recombination is the reformation and resplitting of charge-transfer (CT) states at the interface between donor and acceptor domains. Here, we use kinetic Monte Carlo simulations to understand the interplay between free charge motion and recombination in an energetically disordered phase-separated donor-acceptor blend. We identify conditions for encounter-dominated and resplitting-dominated recombination. In the former regime, recombination is proportional to mobility for all parameters tested and only slightly reduced with respect to the Langevin limit. In contrast, mobility is not the decisive parameter that determines the nongeminate recombination coefficient, ${k}_{2}$, in the latter case, where ${k}_{2}$ is a sole function of the morphology, CT and charge-separated (CS) energetics, and CT-state decay properties. Our simulations also show that free charge encounter in the phase-separated disordered blend is determined by the average mobility of all carriers, while CT reformation and resplitting involves mostly states near the transport energy. Therefore, charge encounter is more affected by increased disorder than the resplitting of the CT state. As a consequence, for a given mobility, larger energetic disorder, in combination with a higher hopping rate, is preferred. These findings have implications for the understanding of suppressed recombination in solar cells with nonfullerene acceptors, which are known to exhibit lower energetic disorder than that of fullerenes.

Selenium-Substituted Non-Fullerene Acceptors: A Route to Superior Operational Stability for Organic Bulk Heterojunction Solar Cells.

Non-fullerene acceptors (NFAs) for organic solar cells (OSCs) have significantly developed over the past five years with continuous improvements in efficiency now over 18%. However, a key challenge still remains in order to fully realize their commercialization potential: the need to extend device lifetime and to control degradation mechanisms. Herein, we investigate the effect of two different molecular engineering routes on the widely utilized ITIC NFA, to tune its optoelectronic properties and interactions with the donor polymer in photoactive blends. Heavier selenium (Se) atoms substitute sulfur (S) atoms in the NFA core in either outer or inner positions, and methyl chains are attached to the end groups. By investigating the effects of these structural modifications on the long-term operational stability of bulk-heterojunction OSC devices, we identify outer selenation as a powerful strategy to significantly increase device lifetime compared to ITIC. Combining outer selenation and methylation results in an impressive 95% of the initial OSC efficiency being retained after 450 h under operating conditions, with an exceptionally long projected half-lifetime of 5600 h compared to 400 h for ITIC. We find that the heavier and larger Se atoms at outer-core positions rigidify the molecular structure to form highly crystalline films with low conformational energetic disorder. It further enhances charge delocalization over the molecule, promoting strong intermolecular interactions among acceptor molecules. Upon methylation, this strong intermolecular interaction stabilizes acceptor domains in blends to be resilient to light-induced morphological changes, thereby leading to superior device stability. Our results highlight the crucial role of NFA molecular structure for OSC operational stability and provide important NFA design rules via heteroatom position and end-group control.

Morphology modulation of organic photovoltaics with block copolymer additive based on rational design strategies

Organic photovoltaics are a promising alternative to silicon-based solar cells with benefits of low-cost production and large scalability. However, its performance is restricted by a non-equilibrium phase-separated morphology. Additive compositions of block copolymer P3HT-b-PFTBT are most likely to mix up and form donor and acceptor morphologies. The parallel bulk-heterojunction model was proposed to show the characteristic photovoltaic parameters and the effect of the parallel cascading heterojunction formation made up of isolated PCBM acceptor domains. We demonstrate block copolymer-based stretchable solar cells on plastic foil substrates, with good power conversion efficiency. To compare the efficiency and stretchability, organic photovoltaic devices were constructed using P3HT/PC61BM, PTB7/PC71BM and P3HT/P3HT-b-PFTBT/PCBM active layer combinations. We find that through rational design of the component ratio, the block-copolymer-based solar cell can withstand tensile strain up to 37%.

Benefits of direct electron detection and PCA for EELS investigation of organic photovoltaics materials

Electron energy-loss spectroscopy (EELS) is a powerful tool for imaging chemical variations at the nanoscale. Here, we investigate a polymer/organic small molecule-blend used as absorber layer in an organic solar cell and employ EELS for distinguishing polymer donor and small molecule acceptor domains in the nanostructured blend based on elemental maps of light elements, such as nitrogen, sulfur or fluorine. Especially for beam sensitive samples, the electron dose needs to be limited, therefore optimized acquisition and data processing strategies are required. We compare data acquired on a post-column energy filter with a direct electron detection camera to data from a conventional CCD camera on the same filter and we investigate the impact of statistical data processing methods (principal components analysis, PCA) on acquired spectra and elemental maps extracted from spectrum images. Our work shows, that the quality of spectra on a direct electron detection camera is far superior to conventional CCD imaging, and thereby allows clear identification of ionization edges and the fine structure of these edges. For the quality of the elemental maps, the application of PCA is essential to allow a clear separation between the donor and acceptor phase in the bulk heterojunction absorber layer of a non-fullerene organic solar cell.

The Importance of Quantifying the Composition of the Amorphous Intermixed Phase in Organic Solar Cells

The relation of phase morphology and solid-state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase-pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device-relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk-heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology-function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high-performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.

Long-range exciton diffusion in molecular non-fullerene acceptors

The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here, we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations, we are able to rationalize the exciton dynamics and draw basic chemical design rules, particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors.

High‐Performance Ternary Organic Solar Cells with Morphology‐Modulated Hole Transfer and Improved Ultraviolet Photostability

A ternary bulk‐heterojunction (BHJ) strategy that synergistically combines the merits of fullerene and nonfullerene acceptors has been regarded as a promising approach to enhance the power conversion efficiencies (PCEs) of organic solar cells (OSCs). Herein, the fullerene derivative ICBA as the morphology regulator is incorporated into a nonfullerene‐based PBDB‐T‐2F:BTP‐4Cl (PM6:BTP‐4Cl) system to fabricate the high‐performance ternary OSCs. The amorphous ICBA prefers to homogeneously distribute in the BTP‐4Cl phase to form the well‐mixed acceptor domains due to their better miscibility, which distinctly reduces the exciton decay loss driven by the unfavorable phase separation and enhances BHJ morphology stability of ternary blends. The appropriate addition of ICBA induces the efficient long‐range Förster resonance energy transfer to BTP‐4Cl and facilitates the ultrafast hole transfer process from BTP‐4Cl to PM6, thereby contributing to charge carrier generation in the actual devices. Ultimately, the optimal ternary OSCs not only yield an average PCE higher than 16.5% but also show the superior ultraviolet photostability relative to binary control devices due to the increased harvesting of ultraviolet photons, boosted charge transfer, more balanced charge transport, and more stable nanostructural morphology. The results provide new insights to enable the simultaneously improved device performance and ultraviolet durability in state‐of‐the‐art ternary OSCs.

Imaging of polymer:fullerene bulk-heterojunctions in a scanning electron microscope: methodology aspects and nanomorphology by correlative SEM and STEM

Scanning transmission electron microscopy (STEM) at low energies (≤ 30 keV) in a scanning electron microscope is well suited to distinguish weakly scattering materials with similar materials properties and analyze their microstructure. The capabilities of the technique are illustrated in this work to resolve material domains in PTB7:PC71BM bulk-heterojunctions, which are commonly implemented for light-harvesting in organic solar cells. Bright-field (BF-) and high-angle annular dark-field (HAADF-) STEM contrast of pure PTB7 and PC71BM was first systematically analyzed using a wedge-shaped sample with well-known thickness profile. Monte-Carlo simulations are essential for the assignment of material contrast for materials with only slightly different scattering properties. Different scattering cross-sections were tested in Monte-Carlo simulations with screened Rutherford scattering cross-sections yielding best agreement with the experimental data. The STEM intensity also depends on the local specimen thickness, which can be dealt with by correlative STEM and scanning electron microscopy (SEM) imaging of the same specimen region yielding additional topography information. Correlative STEM/SEM was applied to determine the size of donor (PTB7) and acceptor (PC71BM) domains in PTB7:PC71BM absorber layers that were deposited from solution with different contents of the processing additive 1,8-diiodooctane (DIO).