Hot Hydrocarbon-Solvent Slot-Die Coating Enables High-Efficiency Organic Solar Cells with Temperature-Dependent Aggregation Behavior

Pathways to Upscaling Highly Efficient Organic Solar Cells Using Green Solvents: A Study on Device Photophysics in the Transition from Lab-to-Fab

As the rise of nonfullerene acceptors (NFA) has allowed lab-scale organic solar cells (OSC) to reach 20% efficiency, translating these devices into roll-to-roll compatible fabrication still poses many challenges for researchers. Among these are the use of green solvent solubility for large-scale manufacture, roll-to-roll compatible fabrication, and, not least, information on charge carrier dynamics in each upscaling step, to further understand the gap in performance. In this work, the reproducibility of champion devices using slot-die coating with 14% power conversion efficiency (PCE) is demonstrated, under the condition that the optimal thickness is maintained. It is further shown that for the donor:acceptor (D:A) blend PM6:Y12, the processing solvent has a more significant impact on charge carrier dynamics compared to the deposition technique. It is found that the devices processed with o-xylene feature a 40% decrease in the bimolecular recombination coefficient compared to those processed with CB, as well as a 70% increase in effective mobility. Finally, it is highlighted that blade-coating yields devices with similar carrier dynamics to slot-die coating, making it the optimal choice for lab-scale optimization with no significant loss in translation toward up-scale.

Manipulating Alkyl Inner Side Chain of Acceptor for Efficient As-Cast Organic Solar Cells

As-cast organic solar cells (OSCs) possess tremendous potential for low-cost commercial applications. Herein, five small-molecule acceptors (A1-A5) are designed and synthesized by selectively and elaborately extending the alkyl inner side chain flanking on the pyrrole motif to prepare efficient as-cast devices. As the extension of the alkyl chain, the absorption spectra of the films are gradually blue-shifted from A1 to A5 along with slightly uplifted lowest unoccupied molecular orbital energy levels, which is conducive for optimizing the trade-off between short-circuit current density and open-circuit voltage of the devices. Moreover, a longer alkyl chain improves compatibility between the acceptor and donor. The in situ technique clarifies that good compatibility will prolong molecular assembly time and assist in the preferential formation of the donor phase, where the acceptor precipitates in the framework formed by the donor. The corresponding film-formation dynamics facilitate the realization of favorable film morphology with a suitable fibrillar structure, molecular stacking, and vertical phase separation, resulting in an incremental fill factor from A1 to A5-based devices. Consequently, the A3-based as-cast OSCs achieve a top-ranked efficiency of 18.29%. This work proposes an ingenious strategy to manipulate intermolecular interactions and control the film-formation process for constructing high-performance as-cast devices.

Correlating Aggregation Ability of Polymer Donors with Film Formation Kinetics for Organic Solar Cells with Improved Efficiency and Processability

Film formation kinetics significantly impact molecular processability and power conversion efficiency (PCE) of organic solar cells. Here, two ternary random copolymerization polymers are reported, D18─N-p and D18─N-m, to modulate the aggregation ability of D18 by introducing trifluoromethyl-substituted pyridine unit at para- and meta-positions, respectively. The introduction of pyridine unit significantly reduces material aggregation ability and adjusts the interactions with acceptor L8-BO, thereby leading to largely changed film formation kinetics with earlier phase separation and longer film formation times, which enlarge fiber sizes in blend films and improve carrier generation and transport. As a result, D18─N-p with moderate aggregation ability delivers a high PCE of 18.82% with L8-BO, which is further improved to 19.45% via interface engineering. Despite the slightly inferior small area device performances, D18─N-m shows improved solubility, which inspires to adjust the ratio of meta-trifluoromethyl pyridine carefully and obtain a polymer donor D18─N-m-10 with good solubility in nonhalogenated solvent o-xylene. High PCEs of 13.07% and 12.43% in 1 cm2 device and 43 cm2 module fabricated with slot-die coating method are achieved based on D18─N-m-10:L8-BO blends. This work emphasizes film formation kinetics optimization in device fabrication via aggregation ability modulation of polymer donors for efficient devices.

Non-halogenated Solvent-Processed Organic Solar Cells with Approaching 20 % Efficiency and Improved Photostability

The development of high-efficiency organic solar cells (OSCs) processed from non-halogenated solvents is crucially important for their scale-up industry production. However, owing to the difficulty of regulating molecular aggregation, there is a huge efficiency gap between non-halogenated and halogenated solvent processed OSCs. Herein, we fabricate o-xylene processed OSCs with approaching 20 % efficiency by incorporating a trimeric guest acceptor named Tri-V into the PM6:L8-BO-X host blend. The incorporation of Tri-V effectively restricts the excessive aggregation of L8-BO-X, regulates the molecular packing and optimizes the phase-separation morphology, which leads to mitigated trap density states, reduced energy loss and suppressed charge recombination. Consequently, the PM6:L8-BO-X:Tri-V-based device achieves an efficiency of 19.82 %, representing the highest efficiency for non-halogenated solvent-processed OSCs reported to date. Noticeably, with the addition of Tri-V, the ternary device shows an improved photostability than binary PM6:L8-BO-X-based device, and maintains 80 % of the initial efficiency after continuous illumination for 1380 h. This work provides a feasible approach for fabricating high-efficiency, stable, eco-friendly OSCs, and sheds new light on the large-scale industrial production of OSCs.

Green Printing for Scalable Organic Photovoltaic Modules by Controlling the Gradient Marangoni Flow

Despite the rapid development in the performances of organic solar cells (OSCs), high-performance OSC modules based on green printing are still limited. The severe Coffee-ring effect (CRE) is considered to be the primary reason for the nonuniform distribution of active layer films. To solve this key printing problem, the cosolvent strategy is presented to deposit the active layer films. The guest solvent Mesitylene with a higher boiling point and a lower surface tension is incorporated into the host solvent o-XY to optimize the rheological properties, such as surface tension and viscosity of the active layer solutions. And the synergistic effect of inward Marangoni flow generation and solution thickening caused by the cosolvent strategy can effectively restrain CRE, resulting in highly homogeneous large-area active layer films. In addition, the optimized crystallization and phase separation of active layer films effectively accelerate the charge transport and exciton dissociation of devices. Consequently, based on PM6:BTP-eC9 system, the device prepared with the co-solvent strategy shows the a power conversion efficiency of 17.80%. Moreover, as the effective area scales to 1 and 16.94 cm2, the recorded performances are altered to 16.71% and 14.58%. This study provides a universal pathway for the development of green-printed high-efficiency organic photovoltaics.

What We have Learnt from PM6:Y6

Over the past three years, remarkable advancements in organic solar cells (OSCs) have emerged, propelled by the introduction of Y6-an innovative A-DA'D-A type small molecule non-fullerene acceptor (NFA). This review provides a critical discussion of the current knowledge about the structural and physical properties of the PM6:Y6 material combination in relation to its photovoltaic performance. The design principles of PM6 and Y6 are discussed, covering charge transfer, transport, and recombination mechanisms. Then, the authors delve into blend morphology and degradation mechanisms before considering commercialization. The current state of the art is presented, while also discussing unresolved contentious issues, such as the blend energetics, the pathways of free charge generation, and the role of triplet states in recombination. As such, this review aims to provide a comprehensive understanding of the PM6:Y6 material combination and its potential for further development in the field of organic solar cells. By addressing both the successes and challenges associated with this system, this review contributes to the ongoing research efforts toward achieving more efficient and stable organic solar cells.

Modular slot-die coater for in situ grazing-incidence x-ray scattering experiments on thin films

Multimodal in situ experiments during slot-die coating of thin films pioneer the way to kinetic studies on thin-film formation. They establish a powerful tool to understand and optimize the formation and properties of thin-film devices, e.g., solar cells, sensors, or LED films. Thin-film research benefits from time-resolved grazing-incidence wide- and small-angle x-ray scattering (GIWAXS/GISAXS) with a sub-second resolution to reveal the evolution of crystal structure, texture, and morphology during the deposition process. Simultaneously investigating optical properties by in situ photoluminescence measurements complements in-depth kinetic studies focusing on a comprehensive understanding of the triangular interdependency of processing, structure, and function for a roll-to-roll compatible, scalable thin-film deposition process. Here, we introduce a modular slot-die coater specially designed for in situ GIWAXS/GISAXS measurements and applicable to various ink systems. With a design for quick assembly, the slot-die coater permits the reproducible and comparable fabrication of thin films in the lab and at the synchrotron using the very same hardware components, as demonstrated in this work by experiments performed at Deutsches Elektronen-Synchrotron (DESY). Simultaneous to GIWAXS/GISAXS, photoluminescence measurements probe optoelectronic properties in situ during thin-film formation. An environmental chamber allows to control the atmosphere inside the coater. Modular construction and lightweight design make the coater mobile, easy to transport, quickly extendable, and adaptable to new beamline environments.

Current State and Future Perspectives of Printable Organic and Perovskite Solar Cells

Photovoltaic technology presents a sustainable solution to address the escalating global energy consumption and a reliable strategy for achieving net-zero carbon emissions by 2050. Emerging photovoltaic technologies, especially the printable organic and perovskite solar cells, have attracted extensive attention due to their rapidly transcending power conversion efficiencies and facile processability, providing great potential to revolutionize the global photovoltaic market. To accelerate these technologies to translate from the laboratory scale to the industrial level, it is critical to develop well-defined and scalable protocols to deposit high-quality thin films of photoactive and charge-transporting materials. Herein, the current state of printable organic and perovskite solar cells is summarized and the view regarding the challenges and prospects toward their commercialization is shared. Different printing techniques are first introduced to provide a correlation between material properties and printing mechanisms, and the optimization of ink formulation and film-formation during large-area deposition of different functional layers in devices are then discussed. Engineering perspectives are also discussed to analyze the criteria for module design. Finally, perspectives are provided regarding the future development of these solar cells toward practical commercialization. It is believed that this perspective will provide insight into the development of printable solar cells and other electronic devices.

Role of Charge-Carrier Dynamics Toward the Fabrication of Efficient Air-Processed Organic Solar Cells

Over the past couple of decades, immense research has been carried out to understand the photo-physics of an organic solar cell (OSC) that is important to enhance its efficiency and stability. Since OSCs undergoes complex photophysical phenomenon, studying these factors has led to designing new materials and implementing new strategies to improve efficiency in OSCs. In this regard, the invention of the non-fullerene acceptorshas greatly revolutionized the understanding of the fundamental processes occurring in OSCs. However, such vital fundamental research from device physics perspectives is carried out on glovebox (GB) processed OSCs and there is a scarcity of research on air-processed (AP) OSCs. This review will focus on charge carrier dynamics such as exciton diffusion, exciton dissociation, charge-transfer states, significance of highest occupied molecular orbital-offsets, and hole-transfer efficiencies of GB-OSCs and compare them with the available data from the AP-OSCs. Finally, key requirements for the fabrication of efficient AP-OSCs will be presented from a charge-carrier dynamics perspective. The key aspects from the charge-carrier dynamics view to fabricate efficient OSCs either from GB or air are provided.

Regulating Phase Separation Kinetics for High-Efficiency and Mechanically Robust All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) possess excellent operation stability and mechanical robustness than other types of organic solar cells, thereby attracting considerable attention for wearable flexible electron devices. However, the power conversion efficiencies (PCEs) of all-PSCs are still lagging behind those of small-molecule-acceptor-based systems owing to the limitation of photoactive materials and unsatisfactory blend morphology. In this work, a novel terpolymer, denoted as PBDB-TFCl (poly4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b″]dithiophene-1,3-bis(2-ethylhexyl)-5,7-di(thiophen-2-yl)-4H,8H-benzo[1,2-c:4,5-c″]dithiophene-4,8-dione-4,8-bis(4-chloro-5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene), is used as an electron donor coupled with a ternary strategy to optimize the performance of all-PSCs. The addition of PBDB-TCl unit deepens the highest occupied molecular orbital energy level, reducing voltage losses. Moreover, the introduction of the guest donor (D18-Cl) effectively regulates the phase-transition kinetics of PBDB-TFCl:D18-Cl:PY-IT during the film formation, leading to ideal size of aggregations and enhanced crystallinity. PBDB-TFCl:D18-Cl:PY-IT devices exhibit a PCE of 18.6% (certified as 18.3%), judged as the highest value so far obtained with all-PSCs. Besides, based on the ternary active layer, the manufactured 36 cm2 flexible modules exhibit a PCE of 15.1%. Meanwhile, the ternary PSCs exhibit superior photostability and mechanical stability. In summary, the proposed strategy, based on molecular design and the ternary strategy, allows optimization of the all-polymer blend morphology and improvement of the photovoltaic performance for stable large-scale flexible PSCs.

Introducing a Phenyl End Group in the Inner Side Chains of A-DA'D-A Acceptors Enables High-Efficiency Organic Solar Cells Processed with Nonhalogenated Solvent

Power conversion efficiency (PCE) of organic solar cells (OSCs) processed by nonhalogenated solvents is unsatisfactory due to the unfavorable morphology. Herein, two new small molecule acceptors (SMAs) Y6-Ph and L8-Ph are synthesized by introducing a phenyl end group in the inner side chains of the SMAs of Y6 and L8-BO, respectively, for overcoming the excessive aggregation of SMAs in the long-time film forming processed by nonhalogenated solvents. First, the effect of the film forming time on the aggregation property and photovoltaic performance of Y6, L8-BO, Y6-Ph, and L8-Ph is studied by using the commonly used solvents: chloroform (CF) (rapid film forming process) and chlorobenzene (CB) (slow film forming process). It is found that Y6- and L8-BO-based OSCs exhibit a dramatic drop in PCE from CF- to CB-processed devices owing to the large phase separation, while the Y6-Ph and L8-Ph based OSCs show obviously increased PCEs Furthermore, L8-Ph-based OSCs processed by nonhalogenated solvent o-xylene (o-XY) achieved a high PCE of 18.40% with an FF of 80.11%. The results indicate that introducing a phenyl end group in the inner side chains is an effective strategy to modulate the morphology and improve the photovoltaic performance of the OSCs processed by nonhalogenated solvents.

Progress in organic photovoltaics based on green solvents: from solubility enhancement to morphology optimization

Solution-processed organic photovoltaics (OPVs) is one of the most promising photovoltaic technologies in the energy field, due to their clean and renewable low-cost manufacturing potential. OPV has rapidly developed with the design and synthesis of highly efficient photovoltaic materials and the development of smart device engineering. To date, the majority of advanced OPV devices have been prepared using halogenated solvents, achieving power conversion efficiencies (PCE) exceeding 19% on a laboratory scale. However, for industrial-scale production, less toxic manufacturing processes and environmental sustainability are the key considerations. Therefore, this review summarizes recent advances in green solvent-based approaches for the preparation of OPVs, highlighting material design (including polymer donors and small molecule acceptors) and device engineering (co-solvent methods, additive strategies, post-treatment methods, and regulation of coating method), emphasizing crucial factors for achieving high performance in green solvent-processed OPV devices. This review presents potential future directions for green solvent-based OPVs, which may pave the way for future industrial development.

Green-Processed Non-Fullerene Organic Solar Cells Based on Y-Series Acceptors

The development of environmentally friendly and sustainable processes for the production of high-performance organic solar cells (OSCs) has become a critical research area. Currently, Y-series electron acceptors are widely used in high-performance OSCs, achieving power conversion efficiencies above 19%. However, these acceptors have large fused conjugated backbones that are well-soluble in halogenated solvents, such as chloroform and chlorobenzene, but have poor solubility in non-halogenated green solvents. To overcome this challenge, recent studies have focused on developing green-processed OSCs that use non-chlorinated and non-aromatic solvents to dissolve bulk-heterojunction photoactive layers based on Y-series electron acceptors, enabling environmentally friendly fabrication. In this comprehensive review, an overview of recent progress in green-processed OSCs based on Y-series acceptors is provided, covering the determination of Hansen solubility parameters, the use of non-chlorinated solvents, and the dispersion of conjugated nanoparticles in water/alcohol. It is hoped that the timely review will inspire researchers to develop new ideas and approaches in this important field, ultimately leading to the practical application of OSCs.

A Polycrystalline Polymer Donor as Pre-Aggregate toward Ordered Molecular Aggregation for 19.3% Efficiency Binary Organic Solar Cells

Organic semiconductors are generally featured with low structure order in solid-state films, which leads to low charge-transport mobility and strong charge recombination in their photovoltaic devices. In this work, a "polycrystal-induced aggregation" strategy orders the polymer donor (PM6) and non-fullerene acceptor (L8-BO) molecules during solution casting with the assistance of PM6 polycrystals that are incubated through a vapor diffusion method, toward improved solar cell efficiency with either thin or thick photoactive layers. These PM6 polycrystals are redissolved in chloroform to prepare PM6 pre-aggregates (PM6-PA), and further incorporated into the conventional PM6:L8-BO blend solutions, which is found to prolong the molecular organization process and enhance the aggregation of both the PM6 and the L8-BO components. As the results, with the assistance of 10% PM6-PA, PM6:L8-BO solar cell devices obtain power conversion efficiencies (PCEs) from 18.0% and 16.2% to 19.3% and 17.2% with a 100 nm-thick and 300 nm-thick photoactive layer, respectively.

Air-Knife-Assisted Spray Coating of Organic Solar Cells

The power conversion efficiencies (PCEs) of organic solar cells (OSCs) have risen dramatically since the introduction of the "Y-series" of non-fullerene acceptors. However, the demonstration of rapid scalable deposition techniques to deposit such systems is rare. Here, for the first time, we demonstrate the deposition of a Y-series-based system using ultrasonic spray coating─a technique with the potential for significantly faster deposition speeds than most traditional meniscus-based methods. Through the use of an air-knife to rapidly remove the casting solvent, we can overcome film reticulation, allowing the drying dynamics to be controlled without the use of solvent additives, heating the substrate, or heating the casting solution. The air-knife also facilitates the use of a non-halogenated, low-toxicity solvent, resulting in industrially relevant, spray-coated PM6:DTY6 devices with PCEs of up to 14.1%. We also highlight the obstacles for scalable coating of Y-series-based solar cells, in particular the influence of slower drying times on blend morphology and crystallinity. This work demonstrates the compatibility of ultrasonic spray coating, and use of an air-knife, with high-speed, roll-to-roll OSC manufacturing techniques.

Prospects of glove-box versus air-processed organic solar cells

In the search for alternate green energy sources to offset dependence on fossil fuels, solar energy can certainly meet two needs with one deed: fulfil growing global energy demands due to its non-depletable nature and lower greenhouse gas emissions. As such, third generation thin film photovoltaic technology based organic solar cells (OSCs) can certainly play their role in providing electricity at a competing or lower cost than 1st and 2nd generation solar technologies. As OSCs are still at an early stage of research and development, much focus has been placed on improving power conversion efficiencies (PCEs) inside a controlled environment i.e. a glove-box (GB) filled with an inert gas such as N₂. This was necessary until now, to control and study the local nanomorphology of the spin-coated blend films. For OSCs to compete with other solar energy technologies, OSCs should produce similar or even better morphologies in an open environment i.e. air, such that air-processed OSCs can result in similar PCEs in comparison to their GB-processed counterparts. In this review, we have compared GB- vs. air-processed OSCs from morphological and device physics aspects and underline the key features of efficient OSCs, processed in either GB or air.

Residual Strain Evolution Induced by Crystallization Kinetics During Anti-Solvent Spin Coating in Organic-Inorganic Hybrid Perovskite

Organic-inorganic hybrid perovskite (OIHP) polycrystalline thin films are attractive due to their outstanding photoelectronic properties. The anti-solvent spin coating method is the most widely used to synthesize these thin films, and the residual strain is inevitably originates and evolves during the process. However, this residual strain evolution induced by crystallization kinetics is still poorly understood. In this work, the in situ and ex situ synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) are utilized to characterize the evolution and distribution of the residual strain in the OIHP polycrystalline thin film during the anti-solvent spin coating process. A mechanical model is established and the mechanism of the crystallization kinetics-induced residual strain evolution process is discussed. This work reveals a comprehensive understanding of the residual strain evolution during the anti-solvent spin coating process in the OIHP polycrystalline thin films and provides important guidelines for the residual strain-related strain engineering, morphology control, and performance enhancement.

Preparation of Efficient Organic Solar Cells Based on Terpolymer Donors via a Monomer-Ratio Insensitive Side-Chain Hybridization Strategy

Creating new donor materials is crucial for further advancing organic solar cells. Random terpolymers have been adopted to overcome shortcomings of regular alternating donor-acceptor (D-A) polymers of which the performance is often susceptible to batch-to-batch variations. In general, the properties and performance of efficient D₁ -A-D₂ -A and D-A₁ -D-A₂ terpolymers are sensitive to the D₁ /D₂ or A₁ /A₂ monomer ratios. Side-chain hybridization is a strategy to address this problem. Here, six D₁ -A-D₂ -A-type random terpolymers comprising D₁ and D₂ monomers with the same π-conjugated D unit but with different side chains were synthesized. The side chains, containing either fluorine or trialkylsilyl substituents were chosen to provide near-identical optoelectronic properties but provide a tool to create a better-optimized film morphology when blended with a non-fullerene acceptor. This strategy allows improving the device performance to over 18 %, higher than that obtained with the corresponding D₁ -A or D₂ -A bipolymers (around 17 %). Hence, side-chain hybridization is a promising strategy to design efficient D₁ -A-D₂ -A terpolymer donors that are insensitive to the D₁ /D₂ monomer ratio, which is beneficial for the scaled-up synthesis of high-performance materials.

In Situ Absorption Characterization Guided Slot-Die-Coated High-Performance Large-Area Flexible Organic Solar Cells and Modules

Slot-die coating is recognized as the most compatible method for the roll-to-roll (R2R) processing of large-area flexible organic solar cells (OSCs). However, the photovoltaic performance of large-area flexible OSC lags significantly behind that of traditional spin-coating devices. In this work, two acceptors, Qx-1 and Qx-2, show quite different film-formation kinetics in the slot-die coating process. In situ absorption spectroscopy indicates that the excessive crystallinity of Qx-2 provides early phase separation and early aggregation, resulting in oversized crystal domains. Consequently, the PM6:Qx-1-based 1 cm² flexible device exhibits an excellent power conversion efficiency (PCE) of 13.70%, which is the best performance among the slot-die-coated flexible devices; in contrast, the PM6:Qx-2 blend shows a pretty poor efficiency, which is lower than 1%. Moreover, the 30 cm² modules based on PM6:Qx-1, containing six 5 cm² sub-cells, exhibit a PCE of 12.20%. After being stored in a glove box for over 6000 h, the PCE remains at 103% of its initial values, indicating excellent shelf stability. Therefore, these results show a promising future strategy for the upscaling fabrication of flexible large-area OSCs.

Quasi-Homojunction Organic Nonfullerene Photovoltaics Featuring Fundamentals Distinct from Bulk Heterojunctions

In contrast to classical bulk heterojunction (BHJ) in organic solar cells (OSCs), the quasi-homojunction (QHJ) with extremely low donor content (≤10 wt.%) is unusual and generally yields much lower device efficiency. Here, representative polymer donors and nonfullerene acceptors are selected to fabricate QHJ OSCs, and a complete picture for the operation mechanisms of high-efficiency QHJ devices is illustrated. PTB7-Th:Y6 QHJ devices at donor:acceptor (D:A) ratios of 1:8 or 1:20 can achieve 95% or 64% of the efficiency obtained from its BHJ counterpart at the optimal D:A ratio of 1:1.2, respectively, whereas QHJ devices with other donors or acceptors suffer from rapid roll-off of efficiency when the donors are diluted. Through device physics and photophysics analyses, it is observed that a large portion of free charges can be intrinsically generated in the neat Y6 domains rather than at the D/A interface. Y6 also serves as an ambipolar transport channel, so that hole transport as also mainly through Y6 phase. The key role of PTB7-Th is primarily to reduce charge recombination, likely assisted by enhancing quadrupolar fields within Y6 itself, rather than the previously thought principal roles of light absorption, exciton splitting, and hole transport.

Improving the Molecular Packing Order and Vertical Phase Separation of the P3HT:O-IDTBR Blend by Extending the Crystallization Period of O-IDTBR

The morphology with strong molecular packing order and gradient vertical composition distribution associated with efficient charge transport and collection is critical to achieve high performance in nonfullerene solar cells. However, the rapid solidification process of the active layer upon the fast removal of solvent usually results in a kinetically trapped state with undesired morphology. Herein, we proposed a strategy to extend the crystal growth time of the acceptor via a high-boiling-point additive that selectively dissolved the acceptor. This was enabled by adding dibenzyl ether (DBE) to the poly(3-hexylthiophene) (P3HT):O-IDTBR blend in chlorobenzene (CB) solution. The combination of the kinetic study by time-resolved ultraviolet-visible (UV-vis) absorption spectra and detailed morphological characterization allows us to correlate the crystallization kinetics with the microstructural transition. The results show that the crystal growth time of O-IDTBR increases from 3 to 60 s upon the addition of 0.75% DBE, leading to further evolution of the molecular order of O-IDTBR during the DBE-dominated drying period. Meanwhile, O-IDTBR has more time to migrate toward the substrate owing to the larger surface energy. In addition, the onset of the crystallization process of P3HT is brought forward from 8 to 6 s due to the reduced solvent quality, which favors P3HT to crystallize into a fibril network. As a result, an optimized morphology that features the enhanced molecular packing order of P3HT and O-IDTBR as well as the vertical compositional gradient of O-IDTBR is obtained. Devices based on the optimized blend show more balanced charge transport and suppressed bimolecular recombination, giving rise to an improved power conversion efficiency (PCE) from 4.29 ± 0.04 to 7.30 ± 0.12%.

Recent Advances in Nonfullerene Acceptor-Based Layer-by-Layer Organic Solar Cells Using a Solution Process

Recently, sequential layer-by-layer (LbL) organic solar cells (OSCs) have attracted significant attention owing to their favorable p-i-n vertical phase separation, efficient charge transport/extraction, and potential for lab-to-fab large-scale production, achieving high power conversion efficiencies (PCEs) of over 18%. This review first summarizes recent studies on various approaches to obtain ideal vertical D/A phase separation in nonfullerene acceptor (NFAs)-based LbL OSCs by proper solvent selection, processing additives, protecting solvent treatment, ternary blends, etc. Additionally, the longer exciton diffusion length of NFAs compared with fullerene derivatives, which provides a new scope for further improvement in the performance of LbL OSCs, is been discussed. Large-area device/module production by LbL techniques and device stability issues, including thermal and mechanical stability, are also reviewed. Finally, the current challenges and prospects for further progress toward their eventual commercialization are discussed.

Nonhalogenated Dual-Slot-Die Processing Enables High-Efficiency Organic Solar Cells

Organic solar cells (OSCs) are promising candidates for next-generation photovoltaic technologies, with their power conversion efficiencies (PCEs) reaching 19%. However, the typically used spin-coating method, toxic halogenated processing solvents, and the conventional bulk-heterojunction (BHJ), which causes excessive charge recombination, hamper the commercialization and further efficiency promotion of OSCs. Here, a simple but effective dual-slot-die sequential processing (DSDS) strategy is proposed to address the above issues by achieving a continuous solution supply, avoiding the solubility limit of the nonhalogen solvents, and creating a graded-BHJ morphology. As a result, an excellent PCE of 17.07% is obtained with the device processed with o-xylene in an open-air environment with no post-treatment required, while a PCE of over 14% is preserved in a wide range of active-layer thickness. The unique film-formation mechanism is further identified during the DSDS processing, which suggests the formation of the graded-BHJ morphology by the mutual diffusion between the donor and acceptor and the subsequent progressive aggregation. The graded-BHJ structure leads to improved charge transport, inhibited charge recombination, and thus an excellent PCE. Therefore, the newly developed DSDS approach can effectively contribute to the realm of high-efficiency and eco-friendly OSCs, which can also possibly be generalized to other organic photoelectric devices.

Non-Halogenated Solvents and Layer-by-Layer Blade-Coated Ternary Organic Solar Cells via Cascade Acceptor Adjusting Morphology and Crystallization to Reduce Energy Loss

The power conversion efficiency (PCE) of halogenated solvent spin-coated organic solar cells (OSCs) has been boosted to a high level (>18%) by developing efficient photovoltaic materials and precise morphological control. However, the PCE of OSCs prepared from non-halogenated solvents and with a scalable printing process is far behind, limited by tough morphology manipulation. Herein, we have fabricated ternary OSCs by using layer-by-layer (LBL) blade-coating and a non-halogenated solvent. The ternary OSCs based on the PM6:IT-M(1:0.2)/BTP-eC9 active layer are processed with the hydrocarbon solvent 1,2,4-trimethylbenzene with no need of any additives and post-treatment. The vertical donor/acceptor distribution is optimized by LBL blade-coating within the PM6:IT-M(1:0.2)/BTP-eC9 active layer. The cascade acceptor IT-M blended in PM6 not only attenuates the damage of BTP-eC9 to the PM6 crystallization, leading to a dense nanofiber-like morphology, but also prefers to reside between PM6 and BTP-eC9 to form a cascade energy level alignment for a fast charge-transfer process. Finally, the improved morphology and crystallization lead to a reduced molecular recombination, low energy loss, and high open-circuit voltage. The prepared non-halogenated solvent and LBL blade-coated OSCs achieve a PCE of 17.16%. The work provides an approach to fabricate hydrocarbon solvent-processed high-performance OSCs by employing LBL blade-coating and a ternary strategy.

Tailoring the Morphology's Microevolution for Binary All-Polymer Solar Cells Processed by Aromatic Hydrocarbon Solvent with 16.22% Efficiency

Herein, we report a systematic solvent selection for eco-friendly processed binary all-polymer solar cells (APSCs) with decent power conversion efficiencies (PCEs). Three typical solvents, toluene, o-xylene, and 1,2,4-trimethylbezene, are chosen and compared. The device enabled by o-xylene exhibits the most outstanding PCE of 16.22%, thanks to its favorable morphology, which is to say a well-formed face-on orientation packing motif and a suitable crystallinity and size of phase segregation. Consequently, the solar cell affords sufficient charge generation, as well as efficient and balanced charge transport, which are all positive to pursuing high efficiency. This work offers an understanding of using complete solvent selection as the strategy to realize high-performance devices by sophisticatedly controlling the morphology.

High-Performance Organic Solar Modules via Bilayer-Merged-Annealing Assisted Blade Coating

Although encouraging progress is being made on spin-coated prototype cells, organic solar cells (OSCs) still face significant challenges, yet to be explored, for upscaling the multi-stacked photoactive layers in the construction of large-area modules. Herein, high-performance opaque and semitransparent organic solar modules are developed via a bilayer-merged-annealing (BMA)-assisted blade-coating strategy, achieving impressive efficiencies of 14.79% and 12.01% with respect to active area of 18.73 cm² , which represent the best organic solar minimodules so far. It is revealed that the BMA strategy effectively resolves the de-wetting issues between polar charge transport layer solution and non-polar bulk heterojunction blends, hence improving the film coverage, along with electronic and electric contacts of multi-stacked photoactive layers. As result, organic solar modules coated under ambient conditions successfully retain the high-efficiency of small-area cells upon 312 times area scaling-up. Overall, this work provides a facile and effective method to fabricate high-performance organic solar modules under ambient conditions.

Environmentally Friendly and Roll-Processed Flexible Organic Solar Cells Based on PM6:Y6

Organic Solar Cells (OSCs) have reached the highest efficiencies using lab-scale device manufacturing on active areas far below 0.1 cm2. The most used fabrication technique is spin-coating, which has poor potential for upscaling and substantial material waste. This tends to widen the so-called “lab-to-fab gap”, which is one of the most important challenges to make OSCs competitive. Other techniques such as blade or slot-die coating are much more suitable for roll-to-roll manufacturing, which is one of the advantages the technology presents due to the huge potential for fast and low-cost fabrication of flexible OSCs. However, only a few studies report solar cells using these fabrication techniques, especially applied on a roll-platform. Additionally, for environmentally friendly large area OSCs, inks based on non-hazardous solvent systems are needed. In this work, slot-die coating has been chosen to coat a PM6:Y6 active layer, using o-xylene, a more environmentally friendly alternative than halogenated solvents, and without additives. The optimal coating process is defined through fine-tuning of the coating parameters, such as the drying temperature and solution concentration. Moreover, ternary devices with PCBM, and fully printed devices are also fabricated. Power conversion efficiencies of 6.3% and 7.2% are achieved for binary PM6:Y6 and ternary PM6:Y6:PCBM devices measured with an aperture area of ∼0.4 cm2 (total device area ∼0.8 cm2).

Domain size control in all-polymer solar cells

In all polymer solar cells (all-PSCs), the domain size is critical for device performance. In highly crystalline polymer blends, however, precisely adjusting the domain size remains a significant challenge because of the simultaneous crystallization of both components. Herein, a strategy that promotes acceptor and donor to crystallize separately was proposed. Take PBDB-T/N2200 blends for instance; the solution state and confined crystallization were combined, which induced the crystallization of N2200, and PBDB-T occurred during the film-forming process and at thermal annealing stage. This separated crystallization process lowers the driving force of phase separation without affecting the degree of crystallinity of the blends. Thus, an interpenetrating network with high crystallinity and proper domain size was obtained, which boosted the power conversion efficiency to 7.59%. Importantly, the relation between pre-aggregation and domain size was established, which may be a guide to precisely adjust the active layer’s domain size in all-PSCs.

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This strategy decreases domain size without sacrificing crystallinity

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A phase diagram about solution state and domain size was proposed

The Importance of Nonequilibrium to Equilibrium Transition Pathways for the Efficiency and Stability of Organic Solar Cells

Controlled morphology of solution-processed thin films have realized impressive achievements for non-fullerene acceptor (NFA)-based organic solar cells (OSCs). Given the large set of donor-acceptor pairs, employing various processing conditions to realize optimal morphology for high efficiency and stable OSCs is a strenuous task. Therefore, comprehensive correlations between processing conditions and morphology evolution pathways have to be developed for efficient performance and stability of devices. Within the framework of the blend system, crystallization transitions of NFA molecules are tracked utilizing the first heating scan of differential scanning calorimeter (DSC) measurement correlating with respective morphology evolution of blend films. Real-time dynamics measurements and morphology characterizations are combined to provide optimal morphology transition pathways as NFA molecules are shown to be released from the mixed-phase to form balanced ordered packing with variant processing conditions. Polymer:NFA films are fabricated using blade coating incorporating solvent additive or thermal annealing as processing conditions as a correlation is formulated between performance and stability of solar cells with morphology transition pathways. This work demonstrates the significance of processing condition-controlled transition pathways for the realization of optimal morphology leading to superior OSC devices.

The role of entropy gains in the exciton separation in organic solar cells

In organic solar cell (OSC), the lower dielectric constant of organic semiconductor material induces a strong Coulomb attraction between electron-hole pairs, which leads to a low exciton separation efficiency, especially the charge transfer (CT) state. The CT state formed at the electron-donor (D) and electron-acceptor (A) interface is regarded as an unfavorable property of organic photovoltaic devices. Since the OSC works in a nonzero temperature condition, the entropy effect would be one of the main reasons to overcome the Coulomb energy barrier and must be taken into account. In this review, we review the present understanding of the entropy-driven charge separation and describe how factors such as the dimensionality of the organic semiconductor, energy disorder effect, the morphology of the active layer, and the nonequilibrium effect affect the entropy contribution in compensating the Coulomb dissociation barrier for CT exciton separation and charge generation process. We focus on the investigation of the entropy effect on exciton dissociation mechanism from both theoretical and experimental aspects, which provides pathways for understanding the underlying mechanisms of exciton separation and further enhancing the efficiency of OSCs. This article is protected by copyright. All rights reserved.

Flow-Enhanced Flexible Microcomb Printing of Organic Solar Cells

Scalable and roll-to-roll compatible processing methods have become pressing needs to transfer organic solar cells (OSCs) to realistic energy sources. Herein a new fabrication method of flexible microcomb printing is proposed. The microcomb is based on a PET sheet micromachined into comb teeth by a laser marker. A computational fluid mechanics simulation shows that the fluid flow around the microcomb teeth induces high shear as well as extensional strain rates, which enhance the molecular alignment and lateral mass transport. The PTQ10:Y6-BO OSCs printed by the flexible microcomb demonstrate a substantially increased degree of crystallinity and phase separation with a suitable domain size. Devices printed by the flexible microcomb in air achieve PCEs of up to 15.93%, higher than those of control devices spin-coated in the N₂ glovebox. The flexibility of the PET film makes the microcomb teeth contact directly with the substrate without a suspended liquid meniscus, thus facilitating printing on soft or curved substrates. Printing of flexible OSCs and large-area devices are demonstrated. The flexible OSCs exhibit PCEs of up to 13.62%, which is the highest for flexible OSCs made by scalable printing techniques to date. These results make flexible microcomb printing a feasible and promising strategy toward the manufacture of efficient OSCs.

1-Chloronaphthalene-Induced Donor/Acceptor Vertical Distribution and Carrier Dynamics Changes in Nonfullerene Organic Solar Cells and the Governed Mechanism

Electron donors and acceptors in organic solar cells (OSCs) shall strike a favorable vertical phase separation that acceptors and donors have sufficient contact and gradient accumulation near the cathodes and anodes, respectively. Random mixing of donors/acceptors at surface will result in charge accumulation and severe recombination for low carrier-mobility organic materials. However, it is challenging to tune the vertical distribution in bulk-heterojunction films as they are usually made from a well-mixed donor/acceptor solution. Here, for the first time, it presents with solid evidence that the commonly used 1-chloronaphthalene (CN) additive can tune the donor/acceptor vertical distribution and establish the mechanism. Different from the previous understanding that ascribed the efficiency enhancement brought by CN to the improved molecular stacking/crystallization, it is revealed that the induced vertical distribution is the dominant factor leading to the significantly increased performance. Importantly, the vertical distribution tunability is effective in various hot nonfullerene OSC systems and creates more channels for the collection of dissociated carriers at corresponding organic/electrode interfaces, which contributes the high efficiency of 18.29%. This study of the material vertical distribution and its correlation with molecular stacking offers methods for additives selection and provides insights for the understanding and construction of high-performance OSCs.

A Review of Recent Developments in Preparation Methods for Large-Area Perovskite Solar Cells

The recent rapid development in perovskite solar cells (PSCs) has led to significant research interest due to their notable photovoltaic performance, currently exceeding 25% power conversion efficiency for small-area PSCs. The materials used to fabricate PSCs dominate the current photovoltaic market, especially with the rapid increase in efficiency and performance. The present work reviews recent developments in PSCs’ preparation and fabrication methods, the associated advantages and disadvantages, and methods for improving the efficiency of large-area perovskite films for commercial application. The work is structured in three parts. First is a brief overview of large-area PSCs, followed by a discussion of the preparation methods and methods to improve PSC efficiency, quality, and stability. Envisioned future perspectives on the synthesis and commercialization of large-area PSCs are discussed last. Most of the growth in commercial PSC applications is likely to be in building integrated photovoltaics and electric vehicle battery charging solutions. This review concludes that blade coating, slot-die coating, and ink-jet printing carry the highest potential for the scalable manufacture of large-area PSCs with moderate-to-high PCEs. More research and development are key to improving PSC stability and, in the long-term, closing the chasm in lifespan between PSCs and conventional photovoltaic cells.

Kinetics Manipulation Enables High-Performance Thick Ternary Organic Solar Cells via R2R-Compatible Slot-Die Coating

Power conversion efficiency (PCE) of organic solar cells (OSCs) has crossed the 18% mark for OSCs, which are largely fabricated by spin-coating, and the optimal photoactive thickness is limited to 100 nm. To increase reproducibility of results with industrial roll-to-roll (R2R) processing, slot-die coating coupled with a ternary strategy for optimal performance of large-area, thick OSCs is used. Based on miscibility differences, a highly crystalline molecule, BTR-Cl, is incorporated, and the phase-separation kinetics of the D18:Y6 film is regulated. BTR-Cl provides an early liquid-liquid phase separation and early aggregation of Y6, which slightly improves the molecular crystallinity and vertical phase separation of the ternary blends, resulting in high PCEs of 17.2% and 15.5% for photoactive films with thicknesses of 110 and 300 nm, respectively. The ternary design strategy for large-area and thick films is further used to fabricate high-efficiency flexible devices, which promises reproducibility of the lab results from slot-die coating to industrial R2R manufacturing.

Printing fabrication of large-area non-fullerene organic solar cells

Organic solar cells (OSCs) based on a bulk heterojunction structure exhibit inherent advantages, such as low cost, light weight, mechanical flexibility, and easy processing, and they are emerging as a potential renewable energy technology. However, most studies are focused on lab-scale, small-area (<1 cm²) devices. Large-area (>1 cm²) OSCs still exhibit considerable efficiency loss during upscaling from small-area to large-area, which is a big challenge. In recent years, along with the rapid development of high-performance non-fullerene acceptors, many researchers have focused on developing large-area non-fullerene-based devices and modules. There are three essential issues in upscaling OSCs from small-area to large-area: fabrication technology, equipment development, and device component processing strategy. In this review, the challenges and solutions in fabricating high-performance large-area OSCs are discussed in terms of the abovementioned three aspects. In addition, the recent progress of large-area OSCs based on non-fullerene electron acceptors is summarized.

Organic Photovoltaics Printed via Sheet Electrospray Enabled by Quadrupole Electrodes

Developing manufacturing methods that are scalable and compatible with a roll-to-roll process with low waste of material has become a pressing need to transfer organic photovoltaics (OPVs) to a viable renewable energy source. For this purpose, various spray printing methods have been proposed. Among them, electrospray (ES) is an attractive option due to its negligible material waste, tunable droplet size, and tolerance to the substrate defects and roughness. Conventional ES with a circular spray footprint often makes the droplets well separated and unlikely to merge, giving rise to "coffee rings" which cause a rough and flawed film morphology. Here, a quadrupole electrode is introduced to generate a compressing electric field that squeezes the conical ES profile into the shape of a thin sheet. The numerical simulation and experimental data of the trajectories of sprayed droplets show that the quadrupole apparatus can effectively increase the long axis to short axis ratio of the oval spray footprint and hence bring droplets closer to each other and make the merging more likely for the deposited droplets. By promoting the merging of droplets, individual coffee rings are also suppressed. Thus, the quadrupole ES offers untapped opportunities for effectively reducing voids and improving the flatness of the ES-printed active layer. The devices with a PM6:N3 active layer printed by the sheet ES exhibited the highest power conversion efficiency (PCE) of up to 15.98%, which is a noticeable improvement over that (14.85%) of counterparts fabricated by a conventional conical ES. This is the highest PCE reported for ES-printed OPVs and is one of the most efficient spray-deposited OPVs so far. In addition, the all-spray-printed devices reached a PCE of 14.55%, which is also among the most efficient all-spray-printed OPVs.

Boosting Highly Efficient Hydrocarbon Solvent-Processed All-Polymer-Based Organic Solar Cells by Modulating Thin-Film Morphology

Many highly efficient all-polymer-based organic solar cells (OSCs) have been achieved owing to material design and device engineering. However, most of them were achieved by using halogenated solvents to process the active layers, being not beneficial to its nature of green energy technology. In this work, we compared chloroform- and toluene-processed PM6:PY-IT-based all-polymer devices with the same blend solution recipe, same film formation speed, and same postcast treatment. The film cast from toluene exhibited weaker crystallinity. For device performance, toluene enabled a better power conversion efficiency (PCE) of 15.51%, outperforming that of chloroform (15.00%), and it is the highest value for non-halogenated solvent-cast all-polymer-based OSCs to date. Toluene's morphology tuning effect was characterized to increase and balance the charge transport and then suppress the exciton recombination and improve the charge extraction, considered to be the reason for efficiency enhancement. Besides, the toluene-cast active layer-based devices showed slightly better photostability than the chloroform-driven ones. This work provided a new direction for building low-toxicity solvent-treated all-polymer OSCs with cutting-edge performance.

Additive and High-Temperature Processing Boost the Photovoltaic Performance of Nonfullerene Organic Solar Cells Fabricated with Blade Coating and Nonhalogenated Solvents

Benefitting from narrow band gap nonfullerene acceptors, continually increasing power conversion efficiency (PCE) endows organic solar cells (OSCs) with great potential for commercial application. Fabricating high-performance OSCs with potential for large-scale coating and nonhalogenated solvent processing is a necessity. Herein, we have proposed the use of nonhalogenated solvents combined with high-temperature blade coating to prepare a PM6 (poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione)]):Y6 (2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3'':4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)))blend active layer. The resultant OSCs deliver a PCE of 15.51% when the PM6:Y6 active layer is blade-coated at 90 °C in nonhalogenated o-xylene (o-XY) host solvent containing 1,2-dimethylnaphthalene (DMN) additive. It is found that high-temperature blade coating and nonhalogenated solvent additive DMN can suppress excessive aggregation of Y6 and enhance the crystallinity of PM6 and Y6 by regulating the dynamic process of active layer formation. Finally, an optimized blend morphology with nanofibrous phase separation and enhanced crystallinity are achieved for the PM6:Y6 active layer prepared with high-temperature blade coating and nonhalogenated o-XY:DMN solvents, which not only shortens the film-drying time but also leads to increased charge generation, transport, and collection efficiency. The 1.00 cm² OSCs prepared with high-temperature blade coating and nonhalogenated solvents exhibit a high PCE of 13.87%. This approach shows great potential for large-area fabrication of OSCs.

Preaggregation Matching of Donors and Acceptors in Solution Accounting for Thermally Stable Non-Fullerene Solar Cells

The mechanism of how the solvent type influences photovoltaic performance and thermal stability of non-fullerene organic solar cells remains unexplored. In this article, the well-known PTB7-Th was selected as a donor, while F8IC was used as an acceptor. The PTB7-Th:F8IC processed from chloroform (CF) exhibited a superiorly higher power conversion efficiency (PCE) of 10.5%, in contrast to the specimen processed from chlorobenzene (CB) of 6.8%. In addition, upon thermal annealing at 160 °C for 120 min, the device processed from CF was more stable than that processed from CB. The incorporation of perylene diimide derivative TBDPDI-C₁₁, serving as the third additive, could also obviously improve the PCE value and thermal stability of PTB7-Th:F8IC processed from CB. According to ultraviolet spectroscopy, atomic force microscopy, transmission electron microscopy, and grazing incidence wide-angle X-ray scattering analyses, the enhanced photovoltaic performance and thermal stability are mainly attributed to formation of PTB7-Th nanofibers and appropriate aggregation of F8IC. The interaction free energy calculated using water and diiodomethane contact angles reveals that PTB7-Th well disperses in CB and tends to aggregate in CF, while F8IC aggregates strongly in CB. The preaggregation matching of the donor and acceptor in solution is essential for the optimization of morphology, efficiency, and thermal stability. The findings in this article could provide useful guidelines to fabricate efficient and thermally stable organic solar cells simply by analyzing the surface energy of components processed from different solvents.

Eco-Friendly Polymer Solar Cells: Advances in Green-Solvent Processing and Material Design

Despite the recent breakthroughs of polymer solar cells (PSCs) exhibiting a power conversion efficiency of over 17%, toxic and hazardous organic solvents such as chloroform and chlorobenzene are still commonly used in their fabrication, which impedes the practical application of PSCs. Thus, the development of eco-friendly processing methods suitable for industrial-scale production is now considered an imperative research focus. This Review provides a roadmap for the design of efficient photoactive materials that are compatible with non-halogenated green solvents (e.g., xylenes, toluene, and tetrahydrofuran). We summarize the recent development of green processing solvents and the processing methods to match with the efficient photoactive materials used in non-fullerene solar cells. We further review progress in the use of more eco-friendly solvents (i.e., water or alcohol) for achieving truly sustainable and eco-friendly PSC fabrication. For example, the concept of water- or alcohol-dispersed nanoparticles made of conjugated materials is introduced. Also, recent important progress and strategies to develop water/alcohol-soluble photoactive materials that completely eliminate the use of conventional toxic solvents are discussed. Finally, we provide our perspectives on the challenges facing the current green processing methods and materials, such as large-area coating techniques and long-term stability. We believe this Review will inform the development of PSCs that are truly clean and renewable energy sources.

Role of Torsional Flexibility in the Film Formation Process in Two π-Conjugated Model Oligomers

The performance of solution-processed organic semiconductor devices is heavily influenced by the morphology of the active layer. Film formation is a complex process, with the final morphology being the result of the interplay between processing parameters and molecular properties, which is only poorly understood. Here, we investigate the influence of molecular stiffness by using two model oligomers, TT and CT, which differ only in the rotational flexibility of their central building block. We monitor absorption and emission simultaneously in situ during spin coating. We find that film formation takes place in four similar stages for both compounds. However, the time scales are remarkably different during the third stage, where electronically interacting aggregates are created. While this process is fast for the stiff CT, it takes minutes for the flexible TT. By comparing with previously determined aggregation properties in solution, we conclude that even though aggregate formation concurs with a planarization process, a certain amount of backbone flexibility is beneficial for establishing ordered structures during film formation. Here, the elongated time window in the case of the flexible compound can further allow for better processing control.