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Organic Photovoltaics: Relating Chemical Structure, Local Morphology, and Electronic Properties
摘要： Substantial enhancements in the efficiencies of bulk-heterojunction (BHJ) organic solar cells (OSCs) have come from largely trial-and-error-based optimizations of the morphology of the active layers. Further improvements, however, require a detailed understanding of the relationships among chemical structure, morphology, electronic properties, and device performance. On the experimental side, characterization of the local (i.e., nanoscale) morphology remains challenging, which has called for the development of robust computational methodologies that can reliably address those aspects. In this review, we describe how a methodology that combines all-atom molecular dynamics (AA-MD) simulations with density functional theory (DFT) calculations allows the establishment of chemical structure–local morphology–electronic properties relationships. We also provide a brief overview of coarse-graining methods in an effort to bridge local to global (i.e., mesoscale to microscale) morphology. Finally, we give a few examples of machine learning (ML) applications that can assist in the discovery of these relationships.
关键词： Machine Learning,Density Functional Theory,Organic Photovoltaics,Organic Solar Cells,Bulk-Heterojunction,Electronic Properties,Coarse-Graining Methods,Local Morphology,Chemical Structure,All-Atom Molecular Dynamics
更新于2025-09-23 15:19:57
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Synthesis and Evaluation of Scalable D-A-D ??-Extended Oligomers as p-Type Organic Materials for Bulk-Heterojunction Solar Cells
摘要： The synthesis and characterization of four novel donor-acceptor-donor π-extended oligomers, incorporating naphtha(1–b)thiophene-4-carboxylate or benzo(b)thieno(3,2-g) benzothiophene-4-carboxylate 2-octyldodecyl esters as end-capping moieties, and two different conjugated core fragments, is reported. The end-capping moieties are obtained via a cascade sequence of sustainable organic reactions, and then coupled to benzo(c)(1,2,5)thiadiazole and its difluoro derivative as the electron-poor π-conjugated cores. The optoelectronic properties of the oligomers are reported. The novel compounds revealed good film forming properties, and when tested in bulk-heterojunction organic photovoltaic cell devices in combination with PC61BM, revealed good fill factors, but low efficiencies, due to their poor absorption profiles.
关键词： organic electronics,Cascade organic reactions,fused aromatic rings,organic photovoltaics,π-conjugated oligomers,bulk-heterojunction organic solar cells,direct arylation
更新于2025-09-23 15:19:57
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Direct Observation of Structure and Dynamics of Photogenerated Charge Carriers in Poly(3-hexylthiophene) Films by Femtosecond Time-Resolved Near-IR Inverse Raman Spectroscopy
摘要： The initial charge separation process of conjugated polymers is one of the key factors for understanding their conductivity. The structure of photogenerated transients in conjugated polymers can be observed by resonance Raman spectroscopy in the near-IR region because they exhibit characteristic low-energy transitions. Here, we investigate the structure and dynamics of photogenerated transients in a regioregular poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blend film, as well as in a pristine P3HT film, using femtosecond time-resolved resonance inverse Raman spectroscopy in the near-IR region. The transient inverse Raman spectrum of the pristine P3HT film at 50 ps suggests coexistence of neutral and charged excitations, whereas that of the P3HT:PCBM blend film at 50 ps suggests formation of positive polarons with a different structure from those in an FeCl3-doped P3HT film. Time-resolved near-IR inverse Raman spectra of the blend film clearly show the absence of charge separation between P3HT and PCBM within the instrument response time of our spectrometer, while they indicate two independent pathways of the polaron formation with time constants of 0.3 and 10 ps.
关键词： polaron,femtosecond inverse Raman spectroscopy,conjugated polymer,bulk heterojunction
更新于2025-09-19 17:15:36
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Elucidation of Donor:Acceptor Phase Separation in Nonfullerene Organic Solar Cells and Its Implications on Device Performance and Charge Carrier Mobility
摘要： In bulk-heterojunction solar cells, the device performance strongly depends on the donor and acceptor properties, the phase separation in the absorber layer, and the formation of a bicontinuous network. While this phase separation is well explored for polymer:fullerene solar cells, only little is known for polymer:nonfullerene acceptor solar cells. The main hurdle in this regard is often the chemical similarity of the conjugated polymer donor and the organic nonfullerene acceptor (NFA), which makes the analysis of the phase separation via atomic force microscopic (AFM) phase images or conventional transmission electron microscopy difficult. In this work, we use the donor polymer PTB7-Th and the small molecule acceptor O-IDTBR as the model system and visualized the phase separation in PTB7-Th:O-IDTBR bulk-heterojunctions with different donor:acceptor ratios via scanning transmission electron microscopy (STEM) high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) based elemental mapping, which resulted in a good contrast between the donor and the acceptor despite very low differences in the chemical composition. AFM as well as grazing-incidence wide-angle X-ray scattering (GIWAXS) investigations support the electron microscopic data. Furthermore, we elucidate the implications of the phase separation on the device performance as well as charge carrier mobilities in the bulk-heterojunction layers, and a high performance of the solar cells was found over a relatively broad range of polymer domain sizes. This can be related to the larger domain sizes of the acceptor phase with higher amounts of O-IDTBR in the blend, while the polymer donor phase still forms continuous pathways to the electrode, which keeps the hole mobility at a relatively constant level.
关键词： nanomorphology,organic photovoltaics,charge carrier mobility,bulk-heterojunction,scanning transmission electron microscopy
更新于2025-09-19 17:13:59
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Mechanistic insights into the effect of polymer regioregularity on thermal stability of polymer solar cells
摘要： Thermal stability is a bottleneck toward commercialization of polymer solar cells (PSCs). The effect of PCBM aggregation in multi-length-scale on the bulk-heterojunction (BHJ) structure, performance, and thermal stability of PSCs is studied here by grazing-incidence small- and wide-angle X-ray scattering. The evolution of hierarchical BHJ structures of the blend film tuned by regioregularity of polymers from the as-cast state to the thermally unstable state was systematically investigated. The thermal stability of PSCs with high polymer regioregularity value can be improved because of the good mutual interaction between polymer crystallites and fullerene aggregation. The insights obtained from this study provide an approach to manipulate the film structure in a multi-length scale and to enhance the thermal stability of P3HT-based PSCs.
关键词： PCBM aggregation,bulk heterojunction,nanostructure,polymer solar cell,GIXRD,thermal stability
更新于2025-09-19 17:13:59
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Pronounced Dependence of Alla??Polymer Solar Cells Photovoltaic Performance on the Alkyl Substituent Patterns in Large Bandgap Polymer Donors
摘要： For all-polymer solar cells which are composed of polymer donors and polymer acceptors, the effect of alkyl side chains on photovoltaic performance is a matter of some debate, and this effect remains difficult to forecast. In this concise contribution, we demonstrate that three alkyls namely branched alkyl 2-butyloctyl (2BO), long linear alkyl n-dodecyl (C12), and double-short linear alkyl n-hexyls (DC6) incorporated into the side chains of large bandgap polymer donor PBDT-TTz can induce considerable, of significance, and different electronic, optical, and morphological parameters. Systematic studies shed light on the critical role of the double-short linear alkyl n-hexyls (DC6) in (i) producing large ionization potential value, (ii) increasing propensity of the polymer to order along the π-stacking direction, (iii) generating polymer crystallites with more preferential “face-on” orientation, consequently, (iv) improvement of carriers transportation, (v) suppression of charge recombination, (vi) reduction of energy loss in all-polymer devices. In parallel, we unearth that the PBDT-TTz with double-short linear alkyl n-hexyls (DC6) represents the highest efficiency of 8.3%, whereas, the other two PBDT-TTz analogues (2BO, C12) yield efficiencies of less than 3% in optimized all-polymer solar cells. Though branched or long linear alkyl side chains (2BO, C12) have been applied to provide the solution processability of conjugated polymers, motifs bearing multiple short linear alkyl substituents (DC6) are proved critical to the development of high performing polymers.
关键词： polymer donor,bulk-heterojunction,alkyl substituent,side chain,polymer solar cells
更新于2025-09-19 17:13:59
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Synergistic Effects of Polymer Donor Backbone Fluorination and Nitrogenation Translate into Efficient Non-Fullerene Bulk-Heterojunction Polymer Solar Cells
摘要： State-of-the-art non-fullerene bulk-heterojunction (BHJ) polymer solar cells outperform the more extensively studied polymer–fullerene BHJ solar cells in terms of efficiency, thermal- and photo-stability. Considering the strong light absorption in near infrared region (600–1000 nm) for most of efficient acceptors, the exploration of high-performing large bandgap (LBG) polymer donors with complementary optical absorption ranging from 400 nm to 700 nm remains critical. In this work, the strategy of concurrently incorporating fluorine (–F) and unsaturated nitrogen (–N) substituents along the polymer backbones is used to develop LBG polymer donor PB[N][F]. Results show that the F– and N–substituted polymer donor PB[N][F] realizes up to 14.4% efficiency in BHJ photovoltaic devices when paired with a benchmark molecule acceptor Y6, which largely outperforms the analogues PB with efficiency of only 3.6% and PB[N] with efficiency of 11.8%. Systematic examinations show that synergistic effects of polymer backbone fluorination and nitrogenation can significantly increase ionization potential values, improve charge transport and reduce bimolecular recombination and trap-assisted recombination in PB[N][F]:Y6 BHJ system. Importantly, our study shows that the F– and N–substituted conjugated polymers are promising electron donor materials for solution-processed non-fullerene BHJ solar cells.
关键词： large-bandgap,fluorination,polymer solar cells,nitrogenation,bulk-heterojunction
更新于2025-09-19 17:13:59
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Extraordinarily long diffusion length in PM6:Y6 organic solar cells
摘要： The PM6:Y6 bulk-heterojunction (BHJ) blend system achieves high short-circuit current (JSC) values in thick photovoltaic junctions. Here we analyse these solar cells to understand the observed independence of the short-circuit current upon photoactive layer thickness. We employ a range of optoelectronic measurements and analyses, including Mott-Schottky analysis, CELIV, photoinduced absorption spectroscopy, Kelvin-probe potential measurements and simulations, to conclude that, for the device series studied, the invariant photocurrent for devices with different active layer thicknesses is associated with the Y6 diffusion length exceeding 300 nm. This is despite an unintentional doping that occurs in PM6 and the associated space-charge effect, which is expected to be even more profound upon photogeneration. This extraordinarily long diffusion length - which is an order of magnitude larger than typical values for organics - dominates transport in the flat-band region of thick junctions. Our work suggests that the performance of the doped PM6:Y6 organic solar cells resembles that of inorganic devices with diffusion transport playing a pivotal role. Ultimately, this is expected to be a key requirement for the fabrication of efficient, high-photocurrent, thick organic solar cells.
关键词： bulk-heterojunction,photovoltaic junctions,organic solar cells,diffusion length,PM6:Y6
更新于2025-09-19 17:13:59
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A High‐Resolution Thin‐Film Fingerprint Sensor Using a Printed Organic Photodetector
摘要： Organic photodetectors (OPDs) have attracted much attention in recent years, due to their promise in large-area light sensing applications. Here, high-resolution slot-die-coated large-area bulk heterojunction organic photodiode (OPD) arrays are reported. The OPD uses a novel electron transport layer, indium gallium zinc oxide in combination with a molybdenum oxide top-electrode. Together, these effectively reduce dark current densities to very low levels of ≈10?7 mA cm?2 at ?2 V. The OPDs show linear behavior in a wide range of light intensities and high detectivity values under reverse bias conditions. When coated on a 508 ppi TFT backplane, a high-quality optical fingerprint scanner capable of imaging in reflection is realized. The optical and electrical properties of the fingerprint sensor are characterized and high-resolution fingerprint images are obtained.
关键词： organic photodiode,fingerprint sensor,bulk heterojunction,organic photodetector,biometric sensor
更新于2025-09-19 17:13:59
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High-Performance Ambient-Condition-Processed Polymer Solar Cells and Organic Thin-Film Transistors
摘要： Large-scale commercial synthesis of bulk-heterojunction (BHJ) solar cell materials is very challenging and both time and energy consuming. Synthesis of π-conjugated polymers (CPs) with uniform batch-to-batch molecular weight and low dispersity is a key requirement for better reproducibility of high-efficiency polymer solar cells. Herein, a conjugated polymer (CP) PTB7-Th, well known for its high performance, has been synthesized with high molecular weight and low dispersity in a closed microwave reactor. The microwave reaction procedure is known to be more controlled and consumes less energy. The precursors were strategically reacted for different reaction time durations to obtain the optimum molecular weight. All different CPs were well characterized using 1H NMR, gel permeation chromatography (GPC), UV?vis, photoluminescence (PL), electron spin resonance (ESR), and Raman spectroscopy, whereas the film morphology was extensively studied via atomic force microscopy (AFM) and grazing incidence X-ray diffraction (GIXRD) techniques. The effect of molecular weight on a conventional BHJ solar cell with PC71BM acceptor was investigated to derive systematic structure?property relationships. The CP obtained after 35 min of reaction time and integrated into BHJ devices under ambient conditions provided the best performance with a power conversion efficiency (PCE) of 8.09%, which was quite similar to the results of CPs synthesized via a thermal route. An enhanced PCE of 8.47% was obtained for the optimized polymer (35 min microwave reaction product) when device fabrication was carried out inside a glovebox. The organic thin-film transistor (OTFT) device with the microwave-synthesized CP displayed better hole mobility (0.137 cm2 V?1 s?1) as compared to that with the thermally synthesized CP. This study also proved that the device stability and reproducibility of the microwave-synthesized CP were much better and more consistent than those of the thermally developed CP.
关键词： microwave reactor,π-conjugated polymers,organic thin-film transistors,polymer solar cells,bulk-heterojunction
更新于2025-09-19 17:13:59