Surpassing 10% Efficiency Benchmark for Nonfullerene Organic Solar Cells by Scalable Coating in Air from Single Nonhalogenated Solvent

Regioisomeric thieno[3,4-d]thiazole-based A-Q-D-Q-A-type NIR acceptors for efficient non-fullerene organic solar cells

This study explores the potential of regioisomeric quinoidal-resonance π-spacers in designing near-infrared (NIR) non-fullerene acceptors (NFAs) for high-performance organic solar cell devices. Adopting thienothiazole as the π-spacer, two new isomeric A-Q-D-Q-A NFAs, TzN-S and TzS-S, are designed and synthesized. Both NFAs demonstrate a broad spectral response extended to the NIR region. However, they exhibit different photovoltaic properties when they were mixed with the PCE10 donor to fabricate respective solar cells. The optimal device of TzS-S achieves a PCE of 10.75%, much higher than that of TzN-S based ones (6.13%). The more favorable energetic offset and better molecular packing contribute to the better charge generation and transport, which explains the relative superiority of TzS-S NFA. This work sheds new light on the regioisomeric effect of component materials for optoelectronic applications.

Solution Sequential Deposition Pseudo-Planar Heterojunction: An Efficient Strategy for State-of-Art Organic Solar Cells

Organic solar cells (OSCs) are considered as a promising new generation of clean energy. Bulk heterojunction (BHJ) structure has been widely employed in the active layer of efficient OSCs. However, precise regulation of morphology in BHJ is still challenging due to the competitive coupling between crystallization and phase separation. Recently, a novel pseudo-planar heterojunction (PPHJ) structure, prepared through solution sequential deposition, has attracted much attention. It is an easy-to-prepare structure in which the phase separation structures, interfaces, and molecular packing can be separately controlled. Employing PPHJ structure, the properties of OSCs, such as power conversion efficiency, stability, transparency, flexibility, and so on, are usually better than its BHJ counterpart. Hence, a comprehensive understanding of the film-forming process, morphology control, and device performance of PPHJ structure should be considered. In terms of the representative works about PPHJ, this review first introduces the fabrication process of active layers based on PPHJ structure. Second, the widely applied morphology control methods in PPHJ structure are summarized. Then, the influences of PPHJ structure on device performance and other property are reviewed, which largely expand its application. Finally, a brief prospect and development tendency of PPHJ devices are discussed with the consideration of their challenges.

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-Solvent-Processed High-Performance Broadband Organic Photodetectors

Solution-processed organic photodetectors with broadband activity have been demonstrated with an environmentally benign solvent, ortho-xylene (o-xylene), as the processing solvent. The organic photodetectors employ a wide band gap polymer donor PBDB-T and a narrow band gap small-molecule non-fullerene acceptor CO1-4F, both dissolvable in o-xylene at a controlled temperature. The o-xylene-processed devices have shown external quantum efficiency of up to 70%, surpassing the counterpart processed with chlorobenzene. With a well-suppressed dark current, the device can also present a high specific detectivity of over 1012 Jones at -2 V within practical operation frequencies and is applicable for photoplethysmography with its fast response. These results further highlight the potential of green-solvent-processed organic photodetectors as a high-performing alternative to their counterparts processed in toxic chlorinated solvents without compromising the excellent photosensing performance.

Asymmetric Polymer Additive for Morphological Regulation and Thermally Stable Organic Solar Cells

High thermal stability is crucial for the commercialization of organic solar cells (OSCs). The thermal stability of OSCs has been improved using the tailoring blend morphology of bulk heterojunctions (BHJs). Herein, we demonstrated thermally stable OSCs in a ternary blended system containing low-crystalline semiconducting polymers (asy-PNDI1FTVT and PTB7-Th) and a non-fullerene acceptor (Y6). The asymmetric n-type semiconducting polymer (asy-PNDI1FTVT) differed from general symmetric semiconducting polymers as it randomly substituted fluorine atoms at the donor moiety (TVT), resulting in significantly lower crystallinity. asy-PNDI1FTVT in PTB7-Th:Y6 exhibited a well-mixed morphology at the BHJ and efficiently facilitated the charge dissociation process with an enhanced fill factor and power conversion efficiency. Furthermore, the ternary system of PTB7-Th:Y6:asy-PNDI1FTVT suppressed phase separation with negligible burn-in loss and performance degradation under thermal stress. The experiments showed that our devices without encapsulation retained over 90% of their initial efficiencies after 100 h at 65 °C. These results show significant potential for the development of thermally stable OSCs with reasonable efficiency.

Quantum modeling of dimethoxyl-indaceno dithiophene based acceptors for the development of semiconducting acceptors with outstanding photovoltaic potential

In the current DFT study, seven dimethoxyl-indaceno dithiophene based semiconducting acceptor molecules (ID1-ID7) are designed computationally by modifying the parent molecule (IDR). Here, based on a DFT exploration at a carefully selected level of theory, we have compiled a list of the optoelectronic properties of ID1-ID7 and IDR. In light of these results, all newly designed molecules, except ID5 have shown a bathochromic shift in their highest absorbance (λ _max). ID1-ID4, ID6 and ID7 molecules have smaller band gap (E _gap) and excitation energy (E _x). IP of ID5 is the smallest and EA of ID1 is the largest among all others. Compared to the parent molecule, ID1-ID3 have increased electron mobility, with ID1 being the most improved in hole mobility. ID4 had the best light harvesting efficiency in this investigation, due to its strongest oscillator. The acceptor molecules' open-circuit voltages (V _OC) were computed after being linked to the PTB7-Th donor molecule. Fill factor (FF) and normalized V _OC of ID1-ID7 were calculated and compared to the parent molecule. Based on the outcomes of this study, the modified acceptors may be further scrutinised for empirical usage in the production of organic solar cells with enhanced photovoltaic capabilities.

Terpolymer Donor with Inside Alkyl Substituents on Thiophene π-Bridges toward Thiazolothiazole A2 -Unit Enables 18.21% Efficiency of Polymer Solar Cells

PM6 is a widely used D-A copolymer donor in the polymer solar cells (PSCs). Incorporating second electron-withdrawing (A₂ ) units into PM6 backbone by ternary D-A₁ -D-A₂ random copolymerization strategy is an effective approach to further improve its photovoltaic performance. Here, the authors synthesize the PM6-based terpolymers by introducing thiazolothiazole as the A₂ units connecting with thiophene π-bridges attaching alkyl substituent towards the A₂ unit (PMT-CT) or towards D-unit (PMT-FT), and study the effect of the alkyl substituent position on the photovoltaic performance of them. Two terpolymers PMT-FT-10 and PMT-CT-10 are obtained by incorporating 10% A₂ units in the terpolymers. The film of PMT-CT-10 shows slightly up-shifted highest occupied molecular orbital (HOMO) energy levels while better co-planar structure than that of PMT-FT-10. Meanwhile, the PMT-CT-10:Y6 blend film exhibits better molecular packing properties, more proper phase separation and more balanced hole and electron mobilities, which are beneficial to more efficient exciton dissociation, efficient charge transport and weaker bimolecular recombination. Consequently, the PMT-CT-10 based PSCs obtain the highest power conversion efficiency of 18.21%. The results indicate that side chain position on the thiophene π-bridges influence the device performance of the terpolymer donors, and PMT-CT-10 is a high efficiency polymer donor for the PSCs.

Fluorinated Alcohol-Processed N-Type Organic Electrochemical Transistor with High Performance and Enhanced Stability

Tuning the film morphology and aggregated structure is a vital means to improve the performance of the mixed ionic-electronic conductors in organic electrochemical transistors (OECTs). Herein, three fluorinated alcohols (FAs), including 2,2,2-trifluoroethanol (TFE), 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), and perfluoro-tert-butanol (PFTB), were employed as the alternative solvents for engineering the n-type small-molecule active layer gNR. Remarkedly, an impressive μC* of 5.12 F V^-1 cm^-1 s^-1 and a normalized transconductance of 1.216 S cm^-1 are achieved from the HFIP-fabricated gNR OECTs, which is three times higher than that of chloroform. The operational stability has been significantly enhanced by the FA-fabricated devices. Such enhancements can be ascribed to the aggregation-induced structural ordering by FAs during spin coating, which optimizes the microstructure of the films for a better mixed ion and electron transport. These results prove the huge research potential of FAs to improve OECT materials' processability, device performance, and stability, therefore promoting practical bio-applications.

High Sensitivity of Non-Fullerene Organic Solar Cells Morphology and Performance to a Processing Additive

Although solvent additives are used to optimize device performance in many novel non-fullerene acceptor (NFA) organic solar cells (OSCs), the effect of processing additives on OSC structures and functionalities can be difficult to predict. Here, two polymer-NFA OSCs with highly sensitive device performance and morphology to the most prevalent solvent additive chloronaphthalene (CN) are presented. Devices with 1% CN additive are found to nearly double device efficiencies to 10%. However, additive concentrations even slightly above optimum significantly hinder device performance due to formation of undesirable morphologies. A comprehensive analysis of device nanostructure shows that CN is critical to increasing crystallinity and optimizing phase separation up to the optimal concentration for suppressing charge recombination and maximizing performance. Here, domain purity and crystallinity are highly correlated with photocurrent and fill factors. However, this effect is in competition with uncontrolled crystallization of NFAs that occur at CN concentrations slightly above optimal. This study highlights how slight variations of solvent additives can impart detrimental effects to morphology and device performance of NFA OSCs. Therefore, successful scale-up processing of NFA-based OSCs will require extreme formulation control, a tuned NFA structure that resists runaway crystallization, or alternative methods such as additive-free fabrication.

Recent advances in non-fullerene organic photovoltaics enabled by green solvent processing

Solution-processed organic photovoltaic (OPV) as a new energy device has attracted much attention due to its huge potential in future commercial manufacturing. However, so far, most of the studies on high-performance OPV have been treated with halogenated solvents. Halogenated solvents not only pollute the environment, but are also harmful to human health, which will negatively affect the large-scale production of OPV in the future. Therefore, it is urgent to develop low-toxic or non-toxic non-halogen solvent-processable OPV. Compared with conventional fullerene OPVs, non-fullerene OPVs exist with stronger absorption, better-matched energy levels and lower energy loss. Processing photoactive layers with non-fullerenes as the acceptor material has broad potential advantages in non-halogenated solvents. This review introduces the research progress of non-fullerene OPV treated by three different kinds of green solvents as the non-halogenated and aromatic solvent, the non-halogenated and non-aromatic solvent, alcohol and water. Furthermore, the effects of different optimization strategies on the photoelectric performance and stability of non-fullerene OPV are analyzed in detail. The current optimization strategy can increase the power conversion efficiency of non-fullerene OPV processed with non-halogen solvents up to 17.33%, which is close to the performance of processing with halogen-containing solvents. Finally, the commercial potential of non-halogen solvent processing OPVs is discussed. The green solvent processing of non-fullerene-based OPVs will become a key development direction for the future of the OPV industry.

Synergetic contribution of fluorinated azide for high EQE and operational stability of top-illuminated, semitransparent, photomultiplication-type organic photodiodes

In this study, it is shown that fluorinated azide, employed as a functional additive to photomultiplication-type organic photodiodes (PM-OPDs), can not only enhance the operational stability by freezing the morphology consisting of matrix polymer/localized acceptor but also stabilize the trapped electron states such that the photomultiplication mechanism can be accelerated further, leading to exceptionally high external quantum efficiency (EQE). The consequent semitransparent OPD consisting of molybdenum oxide (MoO₃)/Au/MoO₃/photoactive layer/polyethyleneimine ethoxylated/indium tin oxide (ITO) rendered a maximum EQE of over 500 000% and 370 000% under bottom and top illumination, respectively. Owing to the remarkably high EQE, high specific detectivity of 5.6 × 10¹³ Jones and low noise-equivalent power of 5.35 × 10^-15 W Hz^-0.5 were also demonstrated. Furthermore, the OPD demonstrated stable performance during 20 h of continuous operation and minimal performance degradation even after the damp heat test. To fully visualize the advantages of the proposed high-EQE, top-illuminated, semitransparent OPD with spectral asymmetry between absorption and detection, a reflection-type fingerprint platform consisting of 1 OPD-1 oxide field-effect transistor complementary metal-oxide-semiconductor backplane (300 ppi) is designed and fabricated. The successful recognition of the fingerprint of one of the authors is demonstrated, which indicates the feasibility of the proposed PM-OPD for sensing weak light intensity.

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.

An N-type Naphthalene Diimide Ionene Polymer as Cathode Interlayer for Organic Solar Cells

Polymer solar cells (PSCs) based on non-fullerene acceptors have the advantages of synthetic versatility, strong absorption ability, and high thermal stability. These characteristics result in impressive power conversion efficiency values, but to further push both the performance and the stability of PSCs, the insertion of appropriate interlayers in the device structure remains mandatory. Herein, a naphthalene diimide-based cathode interlayer (NDI-OH) is synthesized with a facile three-step reaction and used as a cathode interlayer for fullerene and non-fullerene PSCs. This cationic polyelectrolyte exhibited good solubility in alcohol solvents, transparency in the visible range, self-doping behavior, and good film forming ability. All these characteristics allowed the increase in the devices’ power conversion efficiencies (PCE) both for fullerene and non-fullerene-based PSCs. The successful results make NDI-OH a promising cathode interlayer to apply in PSCs.

The Role of Demixing and Crystallization Kinetics on the Stability of Non-Fullerene Organic Solar Cells

With power conversion efficiency now over 17%, a long operational lifetime is essential for the successful application of organic solar cells. However, most non-fullerene acceptors can crystallize and destroy devices, yet the fundamental underlying thermodynamic and kinetic aspects of acceptor crystallization have received limited attention. Here, room-temperature (RT) diffusion coefficients of 3.4 × 10^-23 and 2.0 × 10^-22 are measured for ITIC-2Cl and ITIC-2F, two state-of-the-art non-fullerene acceptors. The low coefficients are enough to provide for kinetic stabilization of the morphology against demixing at RT. Additionally profound differences in crystallization characteristics are discovered between ITIC-2F and ITIC-2Cl. The differences as observed by secondary-ion mass spectrometry, differential scanning calorimetry (DSC), grazing-incidence wide-angle X-ray scattering, and microscopy can be related directly to device degradation and are attributed to the significantly different nucleation and growth rates, with a difference in the growth rate of a factor of 12 at RT. ITIC-4F and ITIC-4Cl exhibit similar characteristics. The results reveal the importance of diffusion coefficients and melting enthalpies in controlling the growth rates, and that differences in halogenation can drastically change crystallization kinetics and device stability. It is furthermore delineated how low nucleation density and large growth rates can be inferred from DSC and microscopy experiments which could be used to guide molecular design for stability.

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.

Large-Area Nonfullerene Organic Solar Cell Modules Fabricated by a Temperature-Independent Printing Method

Despite recent breakthroughs in the fabrication of spin-coated small-area devices (≤0.1 cm²) with power conversion efficiencies (PCEs) of more than 17%, printed large-area organic solar cells (OSCs) are significantly less efficient because of the intrinsic differences between the coating dynamics of the two types of OSCs. The PCEs of printed large-area (∼100 cm²) OSCs have typically been decreased compared with those of small-area spin-coated devices. In this work, an efficient low-temperature printing method to fabricate high-efficiency large-area nonfullerene-based OSC modules is successfully demonstrated. A systematic study of the relationship between the concentration of the photoactive solution and the resulting film properties reveals that the large-area modules (85 cm²) produced in this work deliver excellent performance, yielding PCEs of up to 8.18% with a geometric fill factor of 85%. These novel OSC modules are ∼87% as efficient as small-area printed single cells (cell PCE ∼9.43% with 1 cm²).

A molecular tandem cell for efficient solar water splitting

Artificial photosynthesis provides a way to store solar energy in chemical bonds. Achieving water splitting without an applied external potential bias provides the key to artificial photosynthetic devices. We describe here a tandem photoelectrochemical cell design that combines a dye-sensitized photoelectrosynthesis cell (DSPEC) and an organic solar cell (OSC) in a photoanode for water oxidation. When combined with a Pt electrode for H2 evolution, the electrode becomes part of a combined electrochemical cell for water splitting, 2H2O → O2 + 2H2, by increasing the voltage of the photoanode sufficiently to drive bias-free reduction of H+ to H2 The combined electrode gave a 1.5% solar conversion efficiency for water splitting with no external applied bias, providing a mimic for the tandem cell configuration of PSII in natural photosynthesis. The electrode provided sustained water splitting in the molecular photoelectrode with sustained photocurrent densities of 1.24 mA/cm2 for 1 h under 1-sun illumination with no applied bias.

Liquid-liquid equilibrium in polymer-fullerene mixtures; an in situ neutron reflectivity study

The composition profiles of a series of model polystyrene/fullerene bilayers are measured, before, during and after thermal annealing, using in situ neutron reflectometry. In combination with grazing-incidence X-ray diffraction measurements, these experiments, which quantify layer compositions as a function of molecular weight using changes in both scattering length density and layer thickness, extend and corroborate recent measurements on ex situ annealed samples and demonstrate that the composition profiles rapidly formed in these systems correspond to two co-existing liquid-liquid phases in thermodynamic equilibrium. The measurements also demonstrate a clear and systematic onset temperature for diffusion of the fullerenes into the PS layer that correlates with the known glass-transition temperatures of both the polymer (as a function of molecular weight) and the fullerene, revealing that the molecular mobility of the fullerenes in these systems is controlled by the intrinsic mobility of the fullerenes themselves and the ability of the polymer to plasticise the fullerenes at the interface. Over the temperature range investigated (up to 145 °C), measurements of equilibrated composition profiles as a function of temperature (during gradual cooling) reveal no significant changes in composition profile, other than those associated with the known thermal expansion/contraction of polystyrene thin-films.

Nonfullerene Ternary Organic Solar Cell with Effective Charge Transfer between Two Acceptors

High power conversion efficiency can be realized by using a ternary bulk heterojunction with complementary absorption spectra in organic solar cells. However, as the development of nonfullerene acceptors with a broad absorption spectrum makes the absorption efficiency of the photovoltaic devices close to optimal, such a strategy needs modifying. In particular, charge transfer between the two acceptors is necessary to be considered. Herein, we purposely design a ternary system based on PTB7-Th:CO_i8DFIC:ITIC-4F. Though the presence of ITIC-4F in PTB7-Th:CO_i8DFIC could not broaden the absorption spectrum obviously, the formed cascade-energy-level alignment is beneficial for promoting and balancing exciton separation and charge transport between the donor and two acceptors and even between the acceptors. Insights into the charge transport route in the completed system are provided via using the techniques including photoluminescence spectroscopy and pump-probe photoconductivity spectroscopy. This work provides a new idea for designing highly efficient ternary organic solar cells.

Millimeter wave direct-current transmission and reflection spectral data of some organic photo-responsive materials

Voltage data acquired after probe signal transmitted through the organic film and reflected off the film surface as a function of 0.36 mW millimeter wave signal frequency in the range 110–160 GHz. Five different organic photovoltaic (OPV) materials and one 95:5 blend produced at 2 spin rates are used. These materials are a) fluorinated 2-alkyl-benzol[d] [1–3]triazole (FTAZ), a high hole-mobility polymer used for transistors and photovoltaics, b) diketopyrrolopyrrole (DPP3T), an acceptor polymer used in field-effect transistors (FET), c) Y5(PffBT4T-2OD) film that possesses remarkable temperature controllable morphology, d) a neat conjugated polymer P3HT, Poly(3-(hexylthiophene-2,5diyl) film that is used in optoelectronic devices and as a conductive binder for Li-ion batteries, e) phenyl-C61-butyric acid methyl ester (PCBM) films and its soluble derivatives used as n-type organic semiconductors, and f) excitonic photovoltaic material 95%:5% donor-acceptor blend P3HT:PCBM produced by 2 different spin rates. Measurement of direct-current (dc) transmitted and reflected power (RF voltage signal) are measured using a newly developed continuous wave (CW) D-waveguide band probe (110–160 GHz) apparatus named time-resolved millimeter wave conductivity (TR-mmWC) [1]. Transmission and first surface reflection voltages are captured by a zero-bias Schottky barrier diode (ZBD) and converted to relevant dc voltages. Original voltage signal datasets attached with this can be utilized for photovoltaic, dielectric property estimation, and other semiconductor physics applications. A manually collected dataset of transmission and reflection coefficient at incident probe power level ∼0.9 mW for 95:5 P3HT:PCBM films produced at 2 different spin rates, and one separately only for the neat P3HT film are also presented here in tabular form.

Significance of thermodynamic interaction parameters in guiding the optimization of polymer:nonfullerene solar cells

Polymer solar cells (PSCs) based on polymer donors and nonfullerene small molecule acceptors are a very attractive technology for solar energy conversion, and their performance is heavily determined by film morphology. It is of considerable interest to reveal instructive morphology-performance relationships of these blends. This feature article discusses the recent advances in analysing the morphology formation of nonfullerene PSCs with an effective polymer thermodynamic quantity, i.e., Flory-Huggins interaction parameter χ. In particular, guidelines of high and low χ systems are summarized. The fundamental understanding of χ and its correlations to film morphology and photovoltaic device parameters is of utmost relevance for providing essential material design criteria, establishing suitable morphology processing guidelines, and thus advancing the practical applications of PSCs based on nonfullerene acceptors.

Processing-Friendly Slot-Die-Cast Nonfullerene Organic Solar Cells with Optimized Morphology

The power conversion efficiencies (PCEs) of spin-coated organic solar cells (OSCs) have increased rapidly in recent years. However, spin-coating shows poor reproducibility for large-scale production. Slot-die coating, a lab-scale version of roll-to-roll fabrication, has been considered as the most suitable technique for the production of future large-area commercial devices. For this, the highly efficient slot-die-fabricated devices are required to approach the performance of spin-cast OSCs. We present here, a nonfullerene OSC device utilizing the PBDB-T/i-IEICO-4F blend, fabricated by slot-die coating without post-treatment in the ambient conditions. The device showed an impressive PCE of 12.5%, which is one of the highest reported performance for slot-die-coated OSC devices. Compared to the spin-coated and blade-coated films with optimized thermal annealing time, the films fabricated by slot-die coating (without any treatment) exhibit not only the highest degree of crystallinity and face-on orientation but also the smallest domain size and the purest phase toward enhanced and balanced carrier mobilities. An enhanced excited-state charge generation has been attributed to transient charge kinetics using ultrafast spectroscopic signatures. The optimized slot-die-coated devices exhibit excellent tolerance for the increased thickness of the photoactive layer, attributing to favorable molecular packing. We used slot-die coating as a simple fabrication technique, which is capable of yielding highly efficient OSCs.

Large-Area Organic Solar Cells: Material Requirements, Modular Designs, and Printing Methods

The printing of large-area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small-area devices is approaching 15%, whereas the PCE for large-area devices has also surpassed 10% in a single cell with an area of ≈1 cm² . Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large-area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large-area fabrication, including knife coating, slot-die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick-film material systems with efficient modular designs exhibiting low-efficiency losses and employing the right printing methods, the fabrication of large-area OSCs will be successfully realized in the near future.

Computational analysis of the orientation persistence length of the polymer chain orientation

Analyzing and interpreting the nanoscale morphology of semiconducting polymers is one of the key challenges for advancing in organic electronics. The orientation persistence length (OPL) as a tool to analyze orientation maps generated by photoemission electron microscopy (PEEM) - a state of the art tool for nanoscale imaging/spectroscopy - is presented here. The OPL is a way to quantify the chain orientation within the polymer film in a single graph. In this regard, it is a convincing method that will enable additional direct correlations between the chain orientation and electrical or optical parameters. In this report, we provide computational insights into the factors that contribute to the OPL.

A General Approach for Lab-to-Manufacturing Translation on Flexible Organic Solar Cells

The blossoming of organic solar cells (OSCs) has triggered enormous commercial applications, due to their high-efficiency, light weight, and flexibility. However, the lab-to-manufacturing translation of the praisable performance from lab-scale devices to industrial-scale modules is still the Achilles' heel of OSCs. In fact, it is urgent to explore the mechanism of morphological evolution in the bulk heterojunction (BHJ) with different coating/printing methods. Here, a general approach to upscale flexible organic photovoltaics to module scale without obvious efficiency loss is demonstrated. The shear impulse during the coating/printing process is first applied to control the morphology evolution of the BHJ layer for both fullerene and nonfullerene acceptor systems. A quantitative transformation factor of shear impulse between slot-die printing and spin-coating is detected. Compelling results of morphological evolution, molecular stacking, and coarse-grained molecular simulation verify the validity of the impulse translation. Accordingly, the efficiency of flexible devices via slot-die printing achieves 9.10% for PTB7-Th:PC₇₁ BM and 9.77% for PBDB-T:ITIC based on 1.04 cm² . Furthermore, 15 cm² flexible modules with effective efficiency up to 7.58% (PTB7-Th:PC₇₁ BM) and 8.90% (PBDB-T:ITIC) are demonstrated with satisfying mechanical flexibility and operating stability. More importantly, this work outlines the shear impulse translation for organic printing electronics.

Eco-Compatible Solvent-Processed Organic Photovoltaic Cells with Over 16% Efficiency

Recent advances in nonfullerene acceptors (NFAs) have enabled the rapid increase in power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells. However, this progress is achieved using highly toxic solvents, which are not suitable for the scalable large-area processing method, becoming one of the biggest factors hindering the mass production and commercial applications of OPVs. Therefore, it is of great importance to get good eco-compatible processability when designing efficient OPV materials. Here, to achieve high efficiency and good processability of the NFAs in eco-compatible solvents, the flexible alkyl chains of the highly efficient NFA BTP-4F-8 (also known as Y6) are modified and BTP-4F-12 is synthesized. Combining with the polymer donor PBDB-TF, BTP-4F-12 shows the best PCE of 16.4%. Importantly, when the polymer donor PBDB-TF is replaced by T1 with better solubility, various eco-compatible solvents can be applied to fabricate OPV cells. Finally, over 14% efficiency is obtained with tetrahydrofuran (THF) as the processing solvent for 1.07 cm² OPV cells by the blade-coating method. These results indicate that the simple modification of the side chain can be used to tune the processability of active layer materials and thus make it more applicable for the mass production with environmentally benign solvents.

p-Doped Conducting Polyelectrolyte as an Anode Interlayer Enables High Efficiency for 1 cm2 Printed Organic Solar Cells

Manufacturing large-area devices through a low-cost and large batch printing technique is the key to the commercialization of organic solar cells (OSCs). However, the lack of printable anode interlayer (AIL) materials severely impedes the development of high-efficiency printed OSCs. Herein, we synthesize three p-type self-doped conjugated polyelectrolytes (CPEs), namely, PCP-B, PCP-2B, and PCP-3B, as printable AIL materials for fabricating high-performance and large-area OSCs. By increasing the number of benzene units in the polymer backbone, the work function of the CPEs was enhanced from 4.57 to 5.01 eV, and the optical transparency was also improved because of the enlarged polymer band gap. The improved photoelectronic properties as well as a good film-forming capacity make the PCP-3B an ideal AIL material to be processed by the printing technique. By using PCP-3B, a 1 cm² printed device was fabricated in which all the functional layers, including the AIL, active layer, and cathode interlayer were processed by blade-coating, achieving a power conversion efficiency (PCE) of 9.67%. The PCE belongs to the highest efficiency at present for printable large-area OSCs, showing a promising prospect for the OSC mass production.

Recent Progress in Molecular Design of Fused Ring Electron Acceptors for Organic Solar Cells

The quest for sustainable energy sources has led to accelerated growth in research of organic solar cells (OSCs). A solution-processed bulk-heterojunction (BHJ) OSC generally contains a donor and expensive fullerene acceptors (FAs). The last 20 years have been devoted by the OSC community to developing donor materials, specifically low bandgap polymers, to complement FAs in BHJs. The current improvement from ≈2.5% in 2013 to 17.3% in 2018 in OSC performance is primarily credited to novel nonfullerene acceptors (NFA), especially fused ring electron acceptors (FREAs). FREAs offer unique advantages over FAs, like broad absorption of solar radiation, and they can be extensively chemically manipulated to tune optoelectronic and morphological properties. Herein, the current status in FREA-based OSCs is summarized, such as design strategies for both wide and narrow bandgap FREAs for BHJ, all-small-molecule OSCs, semi-transparent OSC, ternary, and tandem solar cells. The photovoltaics parameters for FREAs are summarized and discussed. The focus is on the various FREA structures and their role in optical and morphological tuning. Besides, the advantages and drawbacks of both FAs and NFAs are discussed. Finally, an outlook in the field of FREA-OSCs for future material design and challenges ahead is provided.

Highly Efficient, Stable, and Ductile Ternary Nonfullerene Organic Solar Cells from a Two-Donor Polymer Blend

Organic solar cells (OSCs) are one of the most promising cost-effective options for utilizing solar energy, and, while the field of OSCs has progressed rapidly in device performance in the past few years, the stability of nonfullerene OSCs has received less attention. Developing devices with both high performance and long-term stability remains challenging, particularly if the material choice is restricted by roll-to-roll and benign solvent processing requirements and desirable mechanical durability. Building upon the ink (toluene:FTAZ:IT-M) that broke the 10% benchmark when blade-coated in air, a second donor material (PBDB-T) is introduced to stabilize and enhance performance with power conversion efficiency over 13% while keeping toluene as the solvent. More importantly, the ternary OSCs exhibit excellent thermal stability and storage stability while retaining high ductility. The excellent performance and stability are mainly attributed to the inhibition of the crystallization of nonfullerene small-molecular acceptors (SMAs) by introducing a stiff donor that also shows low miscibility with the nonfullerene SMA and a slightly higher highest occupied molecular orbital (HOMO) than the host polymer. The study indicates that improved stability and performance can be achieved in a synergistic way without significant embrittlement, which will accelerate the future development and application of nonfullerene OSCs.

Sequential Deposition of Organic Films with Eco-Compatible Solvents Improves Performance and Enables Over 12%-Efficiency Nonfullerene Solar Cells

Casting of a donor:acceptor bulk-heterojunction structure from a single ink has been the predominant fabrication method of organic photovoltaics (OPVs). Despite the success of such bulk heterojunctions, the task ofcontrolling the microstructure in a single casting process has been arduous and alternative approaches are desired. To achieve OPVs with a desirable microstructure, a facile and eco-compatible sequential deposition approach is demonstrated for polymer/small-molecule pairs. Using a nominally amorphous polymer as the model material, the profound influence of casting solvent is shown on the molecular ordering of the film, and thus the device performance and mesoscale morphology of sequentially deposited OPVs can be tuned. Static and in situ X-ray scattering indicate that applying (R)-(+)-limonene is able to greatly promote the molecular order of weakly crystalline polymers and form the largest domain spacing exclusively, which correlates well with the best efficiency of 12.5% in sequentially deposited devices. The sequentially cast device generally outperforms its control device based on traditional single-ink bulk-heterojunction structure. More crucially, a simple polymer:solvent interaction parameter χ is positively correlated with domain spacing in these sequentially deposited devices. These findings shed light on innovative approaches to rationally create environmentally friendly and highly efficient electronics.

A review of non-fullerene polymer solar cells: from device physics to morphology control

The rise in power conversion efficiency of organic photovoltaic (OPV) devices over the last few years has been driven by the emergence of new organic semiconductors and the growing understanding of morphological control at both the molecular and aggregation scales. Non-fullerene OPVs adopting p-type conjugated polymers as the donor and n-type small molecules as the acceptor have exhibited steady progress, outperforming PCBM-based solar cells and reaching efficiencies of over 15% in 2019. This review starts with a refreshed discussion of charge separation, recombination, and V _OC loss in non-fullerene OPVs, followed by a review of work undertaken to develop favorable molecular configurations required for high device performance. We summarize several key approaches that have been employed to tune the nanoscale morphology in non-fullerene photovoltaic blends, comparing them (where appropriate) to their PCBM-based counterparts. In particular, we discuss issues ranging from materials chemistry to solution processing and post-treatments, showing how this can lead to enhanced photovoltaic properties. Particular attention is given to the control of molecular configuration through solution processing, which can have a pronounced impact on the structure of the solid-state photoactive layer. Key challenges, including green solvent processing, stability and lifetime, burn-in, and thickness-dependence in non-fullerene OPVs are briefly discussed.

Simplified synthetic routes for low cost and high photovoltaic performance n-type organic semiconductor acceptors

The application of polymer solar cells (PSCs) with n-type organic semiconductor as acceptor requires further improving powder conversion efficiency, increasing stability and decreasing cost of the related materials and devices. Here we report a simplified synthetic route for 4,4,9,9-tetrahexyl-4,9-dihydro-s-indaceno [1,2-b:5,6-b'] dithiophene by using the catalyst of amberlyst15. Based on this synthetic route and methoxy substitution, two low cost acceptors with less synthetic steps, simple post-treatment and high yield were synthesized. In addition, the methoxy substitution improves both yield and efficiency. The high efficiency of 13.46% was obtained for the devices with MO-IDIC-2F (3,9-bis(2-methylene-5 or 6-fluoro-(3-(1,1-dicyanomethylene)-indanone)-4,4,9,9-tetrahexyl-5,10-dimethoxyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b'] dithiophene) as acceptor. Based on the cost analysis, the PSCs based on MO-IDIC-2F possess the great advantages of low cost and high photovoltaic performance in comparison with those PSCs reported in literatures. Therefore, MO-IDIC-2F will be a promising low cost acceptor for commercial application of PSCs.

Structural dependence of the optical properties of narrow band gap thiophene–thiadiazoloquinoxaline derivatives and their application in organic photovoltaic cells

The structural causes for NIR absorption bands on new [1,2,5]thiadiazolo[3,4-g]quinoxaline derivatives were determined on the basis of DFT calculations and organic photovoltaic cells incorporating the new compounds were fabricated.

Critical review of the molecular design progress in non-fullerene electron acceptors towards commercially viable organic solar cells

A critical analysis of the molecular design strategies employed in the recent progress of non-fullerene electron acceptors for organic photovoltaics.

Non-Fullerene-Based Printed Organic Photodiodes with High Responsivity and Megahertz Detection Speed

Digitally printed organic photodiodes (OPDs) are of great interest for the cost-efficient additive manufacturing of single and multidevice detection systems with full freedom of design. Recently reported high-performance non-fullerene acceptors (NFAs) can address the crucial demands of future applications in terms of high operational speed, tunable spectral response, and device stability. Here, we present the first demonstration of inkjet and aerosol-jet printed OPDs based on the high-performance NFA, IDTBR, in combination with poly(3-hexylthiophene), exhibiting a spectral response up to the near-infrared (NIR) region. These digitally printed devices reach record responsivities up to 300 mA/W in the visible and NIR spectrum, competing with current commercially available technologies based on Si. Furthermore, their fast dynamic response with cutoff frequencies surpassing 2 MHz outperforms most of the state-of-the-art OPDs. The successful process translation from spin-coating to printing is highlighted by the marginal loss in performance compared to the reference devices, which reach responsivities of 400 mA/W and detection speeds of more than 4 MHz. The achieved high device performance and the industrial relevance of the developed fabrication process provide NFAs with an enormous potential for the development of printed photodetection systems.

Achieving Balanced Crystallinity of Donor and Acceptor by Combining Blade-Coating and Ternary Strategies in Organic Solar Cells

As a prototype tool for slot-die coating, blade-coating exhibits excellent compatibility with large-area roll-to-roll coating. A ternary organic solar cell based on PBDB-T:PTB7-Th:FOIC blends is fabricated by blade-coating and exhibits a power conversion efficiency of 12.02%, which is one of the highest values for the printed organic solar cells in ambient environment. It is demonstrated that blade-coating can enhance crystallization of these three materials, but the degree of induction is different (FOIC > PBDB-T > PTB7-Th). Thus, the blade-coated PBDB-T:FOIC device presents much higher electron mobility than hole mobility due to the very high crystallinity of FOIC. Upon the addition of PTB7-Th into the blade-coated PBDB-T:FOIC blends, the crystallinity of FOIC decreases together with the corresponding electron mobility, due to the better miscibility between PTB7-Th and FOIC. The ternary strategy not only maintains the well-matched crystallinity and mobilities, but also increases the photocurrent with complementary light absorption as well as the Förster resonant energy transfer. Furthermore, small domains with homogeneously distributed nanofibers are observed in favor of the exciton dissociation and charge transport. This combination of blade-coating and ternary strategies exhibits excellent synergistic effect in optimizing morphology, showing great potential in the large-area fabrication of highly efficient organic solar cells.

Across the Board: Antonio Facchetti

In this series of articles, the board members of ChemSusChem discuss recent research articles that they consider of exceptional quality and importance for sustainability. This entry features Prof. Dr. Antonio Facchetti, who discusses the use of organic photovoltaic (OPV) devices as a source of renewable energy, and challenges that must be met for OPVs to serve as a viable fully sustainable technology for future energy production, taking into account the components used in such devices and their stability and durability.

Organic Solar Cell Materials toward Commercialization

Bulk-heterojunction organic solar cells (OSCs) have received considerable attention with significant progress recently and offer a promising outlook for portable energy resources and building-integrated photovoltaics in the future. Now, it is urgent to promote the research of OSCs toward their commercialization. For the commercial application of OSCs, it is of great importance to develop high performance, high stability, and low cost photovoltaic materials. In this review, a comprehensive overview of the fundamental requirements of photoactive layer materials and interface layer materials toward commercialization is provided, mainly focusing on high performance, green manufacturing, simplifying device fabrication processes, stability, and cost issues. Furthermore, the perspectives and opportunities for this emerging field of materials science and engineering are also discussed.

Aggregate-Induced Self-Assembly and Ultrafast Dynamics of Light-Harvesting D-A-A Polymorphs

Organic dipolar molecules are an emerging class of light harvesters useful in electronic applications and have captured new urgency with the design and synthesis of new molecular structures for device testing. However, research has not evolved beyond the cyclical thin film preparation-device testing-chemical structural modification approach. Without an understanding of polymorphism, molecular photophysics at the interface or metastable morphologies that regulate charge carrier dynamics, it is not obvious a priori if a new molecular structure will produce a suitable thin film morphology for superior device performance without developing structure-function relationships that consider morphology and photophysics. Dipolar, light harvesting molecules are synthesized with a covalent, para-functionalized triphenylamine donor (D) and acceptor (A) in π-conjugated structures, D-A₁ and D-A₁ -A₂ , that have previously achieved 9.6% power conversion efficiency in thermally evaporated organic solar cell devices with C₇₀ . Solution processing and morphological manipulation are hypothesized to reduce ultrafast radiative charge recombination, unique to dipolar structures, that prevents full charge separation to the fullerene. The photophysics of the D-A interface using femtosecond transient absorption spectroscopy is explained, and microscopy data reveal a newly discovered, supramolecular amorphous polymer metastable state presented as a transient absorption assisted strategy for photofunctional polymorph design.