Effects of fluorination and thermal annealing on charge recombination processes in polymer bulk-heterojunction solar cells

摘要： We investigate the effect of fluorination on the photovoltaic properties of an alternating conjugated polymer composed of 4,8-di-2-thienylbenzo[1,2-b:4,5-b0]dithiophene and 4,7-bis([2,20-bithiophen]-5-yl)-benzo-2-1-3-thiadiazole (4TBT) units in bulk-heterojunction solar cells. The unsubstituted and fluorinated polymers afford very similar open-circuit voltages and fill factor values, but the fluorinated polymer performed better due to enhanced aggregation which provides a higher photocurrent. The photovoltaic performance of both materials improved upon thermal annealing at 150–200 °C as a result of a significantly increased fill factor and open-circuit voltage, counteracted by a slight loss in photocurrent. Detailed studies of the morphology, light intensity dependence, external quantum efficiency and electroluminescence allowed the exploration of the effects of fluorination and thermal annealing on the charge recombination and the nature of the donor–acceptor interfacial charge transfer states in these films.

Photoresponsive Halogen-Bonded Liquid Crystals: The Role of Aromatic Fluorine Substitution

摘要： A new strategy for controlling the liquid crystalline and photophysical properties of supramolecular mesogens assembled via halogen bonding is reported. Changing the degree of fluorination at the halogen-bond donor of the supramolecular liquid crystal allows for the fine-tuning of the halogen bond strength and thereby provides control over the temperature range of the mesophase. At least three fluorine atoms have to be present to ensure efficient polarization of the halogen-bond donor and the formation of a mesophase. In addition, it was found that stilbazole acceptors are superior to their azopyridine counterparts in promoting stable liquid crystalline phases. The halogen-bond-driven supramolecular liquid crystals between fluorinated azobenzenes and stilbazole/azopyridine acceptors show a rich variety of photoinduced processes driven by azobenzene photoisomerization, dictated not only by the photochemical properties of the molecular components but also by the difference between the operation temperature and the clearing point.

Appropriate Molecular Interaction Enabling Perfect Balance Between Induced Crystallinity and Phase Separation for Efficient Photovoltaic Blends

摘要： Fluorination is a promising modification method to adjust the photophysical profiles of organic semiconductors. Notably, the fluorine modification on donor or acceptor materials could impact the molecular interaction, which is strongly related to the morphology of bulk heterojunction (BHJ) blends and the resultant device performance. Therefore, it’s essential to investigate how the molecular interaction affect the morphology of BHJ films. In this study, a new fluorinated polymer PBDB-PSF is synthesized to investigate the molecular interaction in both nonfluorinated (ITIC) and fluorinated (IT-4F) systems. The results reveal that the F-F interaction in PBDB-PSF:IT-4F system could effectively induce the crystallization of IT-4F while retaining ideal phase separation scale, resulting in outstanding charge transport. On the contrary, poor morphology can be observed in PBDB-PSF:ITIC system due to the unbalanced molecular interaction. As a consequence, the PBDB-PSF:IT-4F device delivers an excellent PCE of 13.63%, which greatly exceeds PBDB-PSF:ITIC device (9.84%). These results highlight manipulating the micromorphology in regard of molecular interaction.

Achieving over 21% Efficiency in Inverted Perovskite Solar Cells by Fluorinating a Dopant-Free Hole Transporting Material

摘要： Hole transporting materials (HTMs) play a critical role in ameliorating performance of perovskite solar cells (PSCs). Dedicated HTMs can not only improve the hole extraction and efficiency but also the stability. Herein, PFDT-COOH and fluorinated derivative, PFDT-2F-COOH were introduced as dopant-free HTMs for inverted PSCs. Compared to PFDT-COOH, PFDT-2F-COOH exhibits a deeper the highest occupied molecular orbital (HOMO) level, a higher work function on indium-tin oxide electrode, and an elevated built-in potential in the device. The PFDT-COOH device based on FA1-xMAxPbI3 mixed-cation perovskite exhibits a champion power conversion efficiency (PCE) of 20.64%, while PFDT-2F-COOH device exhibits a champion PCE of 21.68%, which is touching the highest value (21.7%) attained in inverted single-junction PSCs. The elevated efficiency is attributed to reduction of carrier recombination and enhancemnt of carrier extraction via fluorinated strategy. In addition, the two devices also show excellent operational and thermal stabilities. Therefore, our work offers a feasible strategy for high efficiency and stable inverted PSCs.

Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells

摘要： Incorporating fluorine (–F) substituents along the main-chains of polymer donors and acceptors is an effective strategy toward efficient bulk-heterojunction (BHJ) solar cells. Specifically, F-substituted polymers often exhibit planar conformations, leading to favorable packing, and electronic coupling. However, the effects of fluorine substituents on the charge generation and recombination characteristics that determine the overall efficiency of BHJ active layers remain critically important issues to examine. In this report, two PBDT[2X]T polymer analogs –poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-thiophene] [PBDT[2H]T] and its F-substituted counterpart poly[4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-3,4-difluoro-thiophene] [PBDT[2F]T]—are studied to systematically examine how –F substituents impact the blend morphology, charge generation, carrier recombination and extraction in BHJ solar cells. Considering the large efficiency differences between PBDT[2H]T- and PBDT[2F]T-based BHJ devices, significant emphasis is given to characterizing the out-of-plane morphology of the blend films as vertical phase-separation characteristics are known to have dramatic effects on charge transport and carrier extraction in polymer-fullerene BHJ solar cells. Herein, we use electron energy loss spectroscopy (EELS) in tandem with charge transport characterization to examine PBDT[2X]T-fullerene blend films. Our analyses show that PBDT[2H]T and PBDT[2F]T possess very different charge generation, recombination and extraction characteristics, resulting from distinct aggregation, and phase-distribution within the BHJ blend films.

Chlorination <i>vs.</i> fluorination: a study of halogenated benzo[ <i>c</i> ][1,2,5]thiadiazole-based organic semiconducting dots for near-infrared cellular imaging

摘要： Red/near-infrared organic dyes are becoming increasingly widespread in biological applications. However, designing these dyes with long-wavelength emission, large Stokes shifts, and high fluorescence quantum efficiency is still a very challenging task. In this work, five donor-acceptor (D-A) red/near-infrared fluorophores based on different chlorinated/fluorinated benzo[c][1,2,5]thiadiazole units are designed and synthesized. The photophysical, theoretical calculations, and electrochemical properties explored in this study have proved that the introducing of chlorine atoms will lead to a lower HOMO level, stronger steric hindrance, and a relative lower quantum yield in solutions. When the organic dots are fabricated, the chlorinated dots demonstrate much higher fluorescence quantum yield, larger Stokes shift, and better photostability than that of the fluorinated dots. After labeling A549 cells, all the chlorinated/fluorinated dots exhibit high red emission intensities. All these results indicated that the subtle change in the halogen atom of the benzo[c][1,2,5]thiadiazole unit is a unique method to tune the photophysical properties of those materials, which also provides good guidelines to design highly efficient red/near-infrared molecules for cellular imaging applications.

Comparing blends and blocks: Synthesis of partially fluorinated diblock polythiophene copolymers to investigate the thermal stability of optical and morphological properties

摘要： The microstructure of the active blend layer has been shown to be a critically important factor in the performance of organic solar devices. Block copolymers provide a potentially interesting avenue for controlling this active layer microstructure in solar cell blends. Here we explore the impact of backbone fluorination in block copolymers of poly(3-octyl-4-fluorothiophene)s and poly(3-octylthiophene) (F-P3OT-b-P3OT). Two block co-polymers with varying block lengths were prepared via sequential monomer addition under Kumada catalyst transfer polymerisation (KCTP) conditions. We compare the behavior of the block copolymer to that of the corresponding homopolymer blends. In both types of system, we find the fluorinated segments tend to dominate the UV–visible absorption and molecular vibrational spectral features, as well as the thermal behavior. In the block copolymer case, non-fluorinated segments appear to slightly frustrate the aggregation of the more fluorinated block. However, in situ temperature dependent Raman spectroscopy shows that the intramolecular order is more thermally stable in the block copolymer than in the corresponding blend, suggesting that such materials may be interesting for enhanced thermal stability of organic photovoltaic active layers based on similar systems.

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.

Corrigendum: Impact of Polymer Backbone Fluorination on the Charge Generation/Recombination Patterns and Vertical Phase Segregation in Bulk Heterojunction Organic Solar Cells

Positive effects of side-chain fluorination and polymer additive SBS on the enhanced performance of asymmetric-indenothiophene-based polymer solar cells

摘要： Two new D-A type photovoltaic polymers, namely PITPh-DfQx and PITPhf-DfQx, based on asymmetric indenothiophene (IT) donor units with alkoxyphenyl or fluoroalkoxyphenyl substitutes were designed and synthesized. Effects of the fluorine substitution in the asymmetric IT donor units on the electronic structure, ordering structure, photovoltaic properties, and charge generation and recombination dynamics were investigated. It is found that side-chain fluorination in the asymmetric donor units of the D-A polymers endowed the relative polymers with a deeper HOMO level, higher and more balanced charge mobilites, increased charge dissociation efficiency and reduced bimolecular recombination. As a result, the bulk heterojunction solar cell based on the blend film of PITPhf-DfQx and PC71BM demonstrated an efficiency of 6.10%, whereas the cell efficiency based on PITPh-DfQx was only 3.00%. In addition, a triblock copolymer, poly(styrene-block-butadiene-block-styrene) (SBS), was employed for the first time as a polymer additive into the active layers based on PITPh-DfQx/PC71BM and PITPhf-DfQx/PC71BM devices to promote donor crystallization and tune the extent of phase separation between the donor and acceptor. The presence of SBS obviously improved the molecule packing and induced the crystallization of the two polymers, giving rise to a better phase separation due to enhanced aggregation effect of photovoltaic polymers. Therefore, with a small addition of SBS, the optimal PCE was further increased from 6.10% to 6.60% for PITPhf-DfQx based device and from 3.00% to 5.50% for PITPh-DfQx based device. The positive effects of SBS additive on the performance of photovoltaic polymer/fullerene BHJ solar cells provide a new strategy for developing high performance polymer solar cells.