High-efficiency organic solar cells processed from a real green solvent

Unexpected MoO3/Al Interfacial Reaction Lowering the Performance of Organic Solar Cells upon Thermal Annealing and Methods for Suppression

Understanding the degradation mechanism and improving the thermal stability of organic solar cells are essential for this new photovoltaic technology. In this work, we found that the high-performance polymer solar cells suffer from significant performance decay upon thermal annealing at 150 °C owing to the fast decay of VOC and FF. We demonstrated that the thermal annealing process leads to a severe chemical reaction of MoO3 with Al, forming an Al2O3 barrier layer at the MoO3/Al interface, which lowers the built-in potential (Vbi) of the cells and consequently reduces charge collection efficiency. Inserting a thin C60 interlayer between MoO3/Al slows the chemical reaction of MoO3 with Al, which ensures a high Vbi and charge collection efficiency for the annealed MoO3/C60/Al cells. Such a protection effect of the C60 layer in improving device performance against thermal annealing was also confirmed for cells with different polymer photoactive layers and metal electrodes, demonstrating the generality of the interfacial degradation of the cells and the protection effect of the C60 layer. Finally, we demonstrated that the inverted polymer solar cells with the C60-modified anode showed almost no performance decay upon high-temperature hot-press encapsulation, demonstrating excellent heat tolerance of this new device structure.

Triplet State Suppression for Energy Loss Reduction in 20% Nonhalogenated Solvent Processed Binary Organic Solar Cells

Boosting power conversion efficiency (PCE) of organic solar cells (OSCs) has been restricted by its undesirably high energy loss, especially for those nonhalogenated solvent-processed ones. Here,a dichloro-methoxylated terminal group in an asymmetric small molecular acceptor design, which realizes a significantly reduced non-radiative energy loss (0.179 eV) compared to its symmetric counterpart (0.202 eV), is reported. Consequently, the device efficiency is improved by up to 20% for PM6:BTP-eC9-4ClO, without sacrificing the photon harvest or charge transport ability of the control system PM6:BTP-eC9. Further characterizations reveal the asymmetric acceptor BTP-eC9-4ClO's blend film demonstrates a suppressed triplet state formation, enabled by an enhanced electron delocalization. In addition, the asymmetric BTP-eC9-4ClO is found to be thermally stabler than BTP-eC9, and thus providing an improved device stability, whose T80 value reaches > 7800 h under 80 °C anneal in N2 via linear extrapolation. This work represents state-of-the-art device performance for nonhalogenated solvent-processed binary OSCs with certified results (19.45%).

Small Singlet-Triplet Gap Terpolymer Donor with a Simple Pt Complex Enables Organic Solar Cells with Low Energy Loss and Over 19.2% Efficiency

Suppressing the non-radiative loss in the organic solar cells (OSCs) through molecular design remains a significant challenge. Typically, triplet state of organic semiconductors is lower than the charge transfer (CT) state, contributing to substantial non-radiative loss via the triplet state. Herein, a set of terpolymers is prepared by introducing a simple Pt complex block into the PM6 polymer backbone. These metalated terpolymers exhibit high triplet energy (ET1) and small singlet-triplet energy gap (∆EST), facilitating fast intersystem crossing (ISC) process to generate triplet excitons. Consequently, the metalated terpolymers show enhanced exciton lifetime and diffusion length, and most importantly, effectively suppress the non-radiative recombination via terminal triplet loss channels. Moreover, the Pt complex modifies the molecular aggregation of the polymer, hence optimizing the morphology of the active blends. The PM6-Pt1:L8-BO devices achieve a champion power conversion efficiency (PCE) of 18.54% (certified as 18.32%), the highest reported for metalated terpolymers to date. The PCE is further increased to a record high 19.24% in the PM6-Pt1:PM6:L8-BO (0.8:0.2:1.2, wt/wt/wt) ternary devices. Overall, this work provides a feasible approach to designing terpolymers with high ET1, thereby reducing non-radiative loss in the OSCs.

Fullerene-Hybridized Fused-Ring Electron Acceptor with High Dielectric Constant and Isotropic Charge Transport for Organic Solar Cells

A novel isotropic fullerene-hybridized fused-ring electron acceptor, designated C60-Y, has been synthesized via a mild [4+2] Diels-Alder cycloaddition reaction with fullerene C60 to enhance the performance of organic solar cells (OSCs). Comparative analysis shows that C60-Y significantly outperforms the control acceptor Me-Y, with a notable increase in the relative dielectric constant from 2.79 to 3.95. This improvement enhances exciton dissociation and reduces non-radiative energy losses. Additionally, the isotropic molecular packing of C60-Y, similar to fullerene, facilitates efficient interface formation with donor polymers and improves charge mobility. As a result, incorporating C60-Y as an electron acceptor increases the power conversion efficiency (PCE) of binary OSCs to 15.02 %, surpassing the 13.31 % achieved with Me-Y. Moreover, when integrated into a ternary blend system, an impressive PCE of 19.22 % is achieved, top-performing among reported ternary OSCs utilizing fullerene derivatives as the third component. These results suggest that fullerene-hybridized acceptors like C60-Y hold great potential for advancing high-efficiency OSCs by enhancing exciton dissociation, reducing energy losses, and improving charge mobility.

The Pseudo-Bilayer Bulk Heterojunction Active Layer of Polymer Solar Cells in Green Solvent with 18.48% Efficiency

Planar heterojunction (PHJ) is employed to obtain proper vertical phase separation for highly efficient polymer solar cells (PSCs). However, it heavily relies on the choice of orthogonal solvent in the production process. Here, we fabricated a pseudo-bilayer bulk heterojunction (PBHJ) PSC with cross-distribution in the vertical direction by preparing two layers of PM6 and BTP-eC9 blends in an o-XY solution with different dilution ratios to study the morphological evolution of PBHJ film. We found that the PBHJ film exhibits more uniform and suitable continuous interpenetrating network morphology and proper phase separation in the vertical direction for the formation of p-i-n structure. This provides an effective channel for exciton dissociation and charge transport, which is confirmed by both exciton generation simulations and charge dynamics measurements. The PBHJ devices can effectively inhibit trap recombination and accelerate charge separation and transfer. Based on good active layer morphology and balanced charge mobility, all-green solvent-processed PSCs with champion power conversion efficiencies (PCEs) of 18.48% and 16.83% are obtained in PM6:BTP-eC9 and PTQ10:BTP-eC9 systems, respectively. This work reveals the potential mechanism of morphological evolution induced by the PBHJ structure and provides an alternative approach for developing solution processing PSCs.

Inner Side Chain Modification of Small Molecule Acceptors Enables Lower Energy Loss and High Efficiency of Organic Solar Cells Processed with Non-halogenated Solvents

Organic solar cells (OSCs) processed with non-halogenated solvents usually suffer from excessive self-aggregation of small molecule acceptors (SMAs), severe phase separation and higher energy loss (Eloss), leading to reduced open-circuit voltage (Voc) and power conversion efficiency (PCE). Regulating the intermolecular interaction to disperse the aggregation and further improve the molecular packing order of SMAs would be an effective strategy to solve this problem. Here, we designed and synthesized two SMAs L8-PhF and L8-PhMe by introducing different substituents (fluorine for L8-PhF and methyl for L8-PhMe) on the phenyl end group of the inner side chains of L8-Ph, and investigated the effect of the substituents on the intermolecular interaction of SMAs, Eloss and performance of OSCs processed with non-halogenated solvents. Through single crystal analysis and theoretical calculations, it is found that compared with L8-PhF, which possesses strong and abundant intermolecular interactions but downgraded molecular packing order, L8-PhMe with the methyl substituent possesses more effective non-covalent interactions, which improves the tightness and order of molecular packing. When blending the SMAs with polymer donor PM6, the differences in intermolecular interactions of the SMAs influenced the film formation process and phase separation of the blend films. The L8-PhMe based blend film exhibits shorten film formation and more homogeneous phase separation than those of the L8-PhF and L8-Ph based ones. Especially, the OSCs based on L8-PhMe show reduced non-radiative energy loss and enhanced Voc than the devices based on the other two SMAs. Consequently, the L8-PhMe based device processed with o-xylene (o-XY) and using 2PACz as the hole transport layer (HTL) shows an outstanding PCE of 19.27 %. This study highlights that the Eloss of OSCs processed with non-halogenated solvents could be decreased through regulating the intermolecular interactions of SMAs by inner side chain modification, and also emphasize the importance of effectivity rather than intensity of non-covalent interactions introduced in SMAs on the molecular packing, morphology and PCE of OSCs.

Highly Conductive Alcohol-Processable n-Type Conducting Polymer Enabled by Finely Tuned Electrostatic Interactions for Green Organic Electronics

Solution-processable conducting polymers open up a new era in organic electronics, fundamentally altering the processing methods of electronic devices. P-type conducting polymers, exemplified by aqueous solution-processed poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT : PSS), have been successfully commercialized. However, the performance of electron-transporting (n-type) materials remains considerably poorer. One of the primary challenges lies in striking a balance between conductivity and solvent processability. At present, most n-type conducting polymers necessitate toxic solvents for processing, which contradicts environmentally sustainable principles and impedes their potential for large-scale industrial applications. Herein, we developed an alcohol-processable high-performance n-type conducting polymer, poly(3,7-dihydrobenzo[1,2-b : 4,5-b']difuran-2,6-dione): poly(2-ethyl-2-oxazoline) (PBFDO : PEOx), which utilized electrostatic interactions between PEOx and PBFDO to simultaneously achieve high conductivity and alcohol-processability. The PBFDO : PEOx films exhibited remarkable electrical conductivity exceeding 1000 S cm-1 with outstanding stability even at temperatures up to 250 °C, establishing it as a prominent green solvent-processed n-type conducting polymer that rivals the most advanced p-type counterparts. Various applications including organic thermoelectric, electrochemical transistor, and electrochromic devices were showcased, highlighting the broad potential of PBFDO : PEOx in advancing green organic electronics.

Polymer Based on Asymmetrically Halogenated Benzotriazole Enables High Performance Organic Solar Cells Prepared in Nonhalogenated Solvent

Halogenation on the A unit of the D-π-A-type polymer donor has been proven as an effective strategy to improve the performance of organic solar cells (OSCs). Compared with fluorination, chlorination usually increases the open-circuit voltage because of the downward shift of energy levels, but decreases the charge transport ability due to the large steric hindrance of the chlorine atom. We reported herein a method to balance the energy loss and charge transport through asymmetric halogenation on the benzotriazole (BTA) unit of the polymer. The designed PE3-FCl based on the BTA unit containing fluorine and chlorine atoms rendered the highest power conversion efficiency (PCE) of 17.83% when eC9-2Cl-γ and o-xylene were used as the electron acceptor and solvent, respectively. The performance is obviously higher than that of the polymer PE3 containing a difluorinated BTA unit (16.65%) and polymer PE3-2Cl with dichlorinated BTA (14.65%) due to the manipulated morphology by preaggregation, improved and more balanced charge carrier transport, and reduced recombination loss. Notably, this PCE is a breakthrough for the BTA-based polymers processed by nonhalogenated solvent. This work gives deep insight into the asymmetric halogenation of polymer donors for high-performance green solvent-processed OSCs.

3-Methylthiophene: A Sustainable Solvent for High-Performance Organic Solar Cells

The use of harmful halogenated or aromatic solvents such as chloroform (CF), chlorobenzene (CB), and o-xylene (o-XY) is one of the greatest barriers to the industrial-scale manufacturing of high-performance organic solar cells (OSCs). Therefore, it is necessary to eliminate the effects of these solvents to ensure practical feasibility of OSCs. We found that the anthracene-terminated polymer donor and small-molecule acceptor BO-4Cl had good solubility in 3-methylthiophene (3-MeT). There were no toxicity labels in the SDS and exposure control limits for 3-MeT. An overall power conversion efficiency of 16.87% was achieved by using 3-MeT as the solvent for solar cell fabrication, which was higher than that of the cells made from CF (16.18%) and o-XY (15.69%). The best OSC based on PM6:D18:L8-BO and fabricated with 3-MeT exhibited a high PCE of 18.13%, which is one of the highest values for cells fabricated from halogen-free solvents. These results indicate that 3-MeT is an eco-friendly and low-toxicity solvent for the sustainable fabrication of the OSC active layer.

A Three-in-One Hybrid Strategy for High-Performance Semiconducting Polymers Processed from Anisole

The development of semiconducting polymers with good processability in green solvents and competitive electrical performance is essential for realizing sustainable large-scale manufacturing and commercialization of organic electronics. A major obstacle is the processability-performance dichotomy that is dictated by the lack of ideal building blocks with balanced polarity, solubility, electronic structures, and molecular conformation. Herein, through the integration of donor, quinoid and acceptor units, an unprecedented building block, namely TQBT, is introduced for constructing a serial of conjugated polymers. The TQBT, distinct in non-symmetric structure and high dipole moment, imparts enhanced solubility in anisole-a green solvent-to the polymer TQBT-T. Furthermore, PTQBT-T possess a highly rigid and planar backbone owing to the nearly coplanar geometry and quinoidal nature of TQBT, resulting in strong aggregation in solution and localized aggregates in film. Remarkably, PTQBT-T films spuncast from anisole exhibit a hole mobility of 2.30 cm2 V-1 s-1, which is record high for green solvent-processable semiconducting polymers via spin-coating, together with commendable operational and storage stability. The hybrid building block emerges as a pioneering electroactive unit, shedding light on future design strategies in high-performance semiconducting polymers compatible with green processing and marking a significant stride towards ecofriendly organic electronics.

Photonic Pigments of Polystyrene-block-Polyvinylpyrrolidone Bottlebrush Block Copolymers via Sustainable Organized Spontaneous Emulsification

Prior studies on photonic pigments of amphiphilic bottlebrush block copolymers (BBCPs) through an organized spontaneous emulsification (OSE) mechanism have been limited to using polyethylene glycol (PEG) as the hydrophilic side chains and toluene as the organic phase. Herein, a family of polystyrene-block-polyvinylpyrrolidone (PS-b-PVP) BBCPs are synthesized with PVP as the hydrophilic block. Biocompatible and sustainable anisole is employed for dissolving the obtained BBCPs followed by emulsification of the solutions in water. Subsequent evaporation of oil-in-water emulsion droplets triggers the OSE mechanism, producing thermodynamically stable water-in-oil-in-water (w/o/w) multiple emulsions with uniform and closely packed internal droplet arrays through the assembly of the BBCPs at the w/o interface. Upon solidification, the homogeneous porous structures are formed within the photonic microparticles that exhibit visible structural colors. The pore diameter is widely tunable (150∼314 nm) by changing the degree of polymerization of BBCP (69∼110), resulting in tunable colors across the whole visible spectrum. This work demonstrates useful knowledge that OSE can be generally used in the fabrication of ordered porous materials with tunable internal functional groups, not only for photonic applications, but also offers a potential platform for catalysis, sensing, separation, encapsulation, etc.

Chalcogen Atoms Regulate the Organic Solar Cell Performance of B-N-Based Polymer Donors

Donor polymers play a key role in the development of organic solar cells (OSCs). B-N-based polymer donors, as new types of materials, have attracted a lot of attention due to their special characteristics, such as high E(T1), small ΔEST, and easy synthesis, and they can be processed with real green solvents. However, the relationship between the chemical structure and device performance has not been systematically studied. Herein, chalcogen atoms that regulate the OSCs performance of B-N-based polymer donors were systematically studied. Fortunately, the substitution of a halogen atom did not affect the high E(T1) and small ΔEST character of the B-N-based polymer. The absorption and energy levels of the polymer were systematically regulated by O, S, and Se atom substitution. The PBNT-TAZ:Y6-BO-based OSCs device demonstrated a high power conversion efficiency of 15.36%. Moreover, the layer-by-layer method was applied to further optimize the device performance, and the PBNT-TAZ/Y6-BO-based OSCs device yielded a PCE of 16.34%. Consequently, we have systematically demonstrated how chalcogen atoms modulated the electronic properties of B-N-based polymers. Detailed and systematic structure-performance relationships are important for the development of next-generation B-N-based materials.

Modeling and Fundamental Dynamics of Vacuum, Gas, and Antisolvent Quenching for Scalable Perovskite Processes

Hybrid perovskite photovoltaics (PVs) promise cost-effective fabrication with large-scale solution-based manufacturing processes as well as high power conversion efficiencies. Almost all of today's high-performance solution-processed perovskite absorber films rely on so-called quenching techniques that rapidly increase supersaturation to induce a prompt crystallization. However, to date, there are no metrics for comparing results obtained with different quenching methods. In response, the first quantitative modeling framework for gas quenching, anti-solvent quenching, and vacuum quenching is developed herein. Based on dynamic thickness measurements in a vacuum chamber, previous works on drying dynamics, and commonly known material properties, a detailed analysis of mass transfer dynamics is performed for each quenching technique. The derived models are delivered along with an open-source software framework that is modular and extensible. Thereby, a deep understanding of the impact of each process parameter on mass transfer dynamics is provided. Moreover, the supersaturation rate at critical concentration is proposed as a decisive benchmark of quenching effectiveness, yielding ≈ 10-3 - 10-1s-1 for vacuum quenching, ≈ 10-5 - 10-3s-1 for static gas quenching, ≈ 10-2 - 100s-1 for dynamic gas quenching and ≈ 102s-1 for antisolvent quenching. This benchmark fosters transferability and scalability of hybrid perovskite fabrication, transforming the "art of device making" to well-defined process engineering.

Processability Considerations for Next-Generation Organic Photovoltaic Materials

The evolution of organic semiconductors for organic photovoltaics (OPVs) has resulted in unforeseen outcomes. This has provided substitute choices of photoactive layer materials, which effectively convert sunlight into electricity. Recently developed OPV materials have narrowed down the gaps in efficiency, stability, and cost in devices. Records now show power conversion efficiency in single-junction devices closing to 20%. Despite this, there is still a gap between the currently developed OPV materials and those that meet the requirements of practical applications, especially the solution processability issue widely concerned in the field of OPVs. Based on the general rule that structure determines properties, methodologies to enhance the processability of OPV materials are reviewed and explored from the perspective of material design and views on the further development of processable OPV materials are presented. Considering the current dilemma that the existing evaluation indicators cannot reflect the industrial processability of OPV materials, a more complete set of key performance indicators are proposed for their processability considerations. The purpose of this perspective is to raise awareness of the boundary conditions that exist in industrial OPV manufacturing and to provide guidance for academic research that aspires to contribute to technological advancements.

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.