Achieving Over 15% Efficiency in Organic Photovoltaic Cells via Copolymer Design

Rational molecular and device design enables organic solar cells approaching 20% efficiency

For organic solar cells to be competitive, the light-absorbing molecules should simultaneously satisfy multiple key requirements, including weak-absorption charge transfer state, high dielectric constant, suitable surface energy, proper crystallinity, etc. However, the systematic design rule in molecules to achieve the abovementioned goals is rarely studied. In this work, guided by theoretical calculation, we present a rational design of non-fullerene acceptor o-BTP-eC9, with distinct photoelectric properties compared to benchmark BTP-eC9. o-BTP-eC9 based device has uplifted charge transfer state, therefore significantly reducing the energy loss by 41 meV and showing excellent power conversion efficiency of 18.7%. Moreover, the new guest acceptor o-BTP-eC9 has excellent miscibility, crystallinity, and energy level compatibility with BTP-eC9, which enables an efficiency of 19.9% (19.5% certified) in PM6:BTP-C9:o-BTP-eC9 based ternary system with enhanced operational stability.

High-Efficiency Binary and Ternary Organic Solar Cells Based on Novel Nonfused-Ring Electron Acceptors

In this study, three nonfused-ring electron acceptors (2TT, 2TT-C6-F, and 2TT-C11-F) with the same steric hindrance groups (2,4,6-tripropylbenzene) are designed and synthesized and the impact of electron-withdrawing and lateral alkyl side chains on the performance of binary and ternary organic solar cells (OSCs) is explored. For the binary OSCs, 2TT-C11-F with IC-2F terminal groups and lateral undecyl side chains display a red shifted absorption spectrum and suitable energy levels, and the corresponding blend film exhibits appropriate phase separation and crystallinity. Thus, binary OSCs based on 2TT-C11-F achieve an impressive power conversion efficiency of 13.03%, much higher than the efficiencies of those based on 2TT (9.68%) and 2TT-C6-F (12.11%). In the ternary OSCs, 2TT with CC terminal groups and lateral hexyl side chains exhibit complementary absorption and cascade energy levels with a host binary system (D18:BTP-eC9-4F). Hence, the ternary OSCs based on 2TT achieve a remarkable efficiency of 19.39%, ranking among the highest reported values. The research yields comprehensive 2TT-series nonfused-ring electron acceptors, demonstrating their great potential for the fabrication of high-performance binary and ternary OSCs.

Material and Device Design of Flexible Perovskite Solar Cells for Next-Generation Power Supplies

This review outlines the rapid evolution of flexible perovskite solar cells (f-PSCs) to address the urgent need for alternative energy sources, highlighting their impressive power conversion efficiency, which increases from 2.62% to over 24% within a decade. The unique optoelectronic properties of perovskite materials and their inherent mechanical flexibilities instrumental in the development of f-PSCs are examined. Various strategies proposed for material modification and device optimization significantly enhance efficiency and bending durability. The transition from small-scale devices to large-area photovoltaic modules for diverse applications is discussed in addition to the challenges and innovative solutions related to film uniformity and environmental stability. This review provides succinct yet comprehensive insights into the development of f-PSCs, paving the way for their integration into various applications and highlighting their potential in the renewable energy landscape.

Improved Short-Circuit Current and Fill Factor in PM6:Y6 Organic Solar Cells through D18-Cl Doping

Based on the PM6:Y6 binary system, a novel non-fullerene acceptor material, D18-Cl, was doped into the PM6:Y6 blend to fabricate the active layer. The effects of different doping ratios of D18-Cl on organic solar cells were investigated. The best-performing organic solar cell was achieved when the doping ratio of D18-Cl reached 20 wt%. It exhibited a short-circuit current of 28.13 mA/cm2, a fill factor of 70.25%, an open-circuit voltage (Voc) of 0.81 V, and a power conversion efficiency of 16.08%. The introduction of an appropriate amount of D18-Cl expanded the absorption spectrum of the active layer, improved the morphology of the active layer, reduced large molecular aggregation and defects, minimized bimolecular recombination, and optimized the collection efficiency of charge carriers. These results indicate the critical importance of selecting an appropriate third component in binary systems and optimizing the doping ratio to enhance the performance of ternary organic solar cells.

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.

Design of Furan-Based Acceptors for Organic Photovoltaics

We explore a series of furan-based non-fullerene acceptors and report their optoelectronic properties, solid-state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced band gap and red-shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels-Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to most non-fullerene acceptors, and the photostability depends strongly on the molecular structure. Placement of furans next to the acceptor termini leads to better photostability, well-balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker offers the best photostability and power conversion efficiency (>14 %), outperforming all furan-based acceptors reported to date and all indacenodithiophene-based acceptors. Our findings show the possibility of photostable furan-based alternatives to the currently omnipresent thiophene-based photovoltaic materials.

A Review of Manufacturing Methods for Flexible Devices and Energy Storage Devices

Given the advancements in modern living standards and technological development, conventional smart devices have proven inadequate in meeting the demands for a high-quality lifestyle. Therefore, a revolution is necessary to overcome this impasse and facilitate the emergence of flexible electronics. Specifically, there is a growing focus on health detection, necessitating advanced flexible preparation technology for biosensor-based smart wearable devices. Nowadays, numerous flexible products are available on the market, such as electronic devices with flexible connections, bendable LED light arrays, and flexible radio frequency electronic tags for storing information. The manufacturing process of these devices is relatively straightforward, and their integration is uncomplicated. However, their functionality remains limited. Further research is necessary for the development of more intricate applications, such as intelligent wearables and energy storage systems. Taking smart wear as an example, it is worth noting that the current mainstream products on the market primarily consist of bracelet-type health testing equipment. They exhibit limited flexibility and can only be worn on the wrist for measurement purposes, which greatly limits their application diversity. Flexible energy storage and flexible display also face the same problem, so there is still a lot of room for development in the field of flexible electronics manufacturing. In this review, we provide a brief overview of the developmental history of flexible devices, systematically summarizing representative preparation methods and typical applications, identifying challenges, proposing solutions, and offering prospects for future development.

Influence of azacycle donor moieties on the photovoltaic properties of benzo[c][1,2,5]thiadiazole based organic systems: a DFT study

Fullerene free organic chromophores are widely utilized to improve the efficacy of photovoltaic materials. Herein, we designed D-π-A-π-D form chromophores (TAZD1-TAZD5) via end-capped redistribution of donor moieties by keeping the same π-bridge and central acceptor unit for organic solar cells (OSCs). To analyze the photovoltaic characteristics of these derivatives, DFT estimations were accomplished at B3LYP/6-311 G (d,p) functional. Different investigations like frontier molecular orbital (FMO), absorption spectra (UV-Vis), density of states (DOS), binding energy (Eb), open circuit voltage (Voc), and transition density matrix (TDMs) were performed to examine the optical, photophysical and electronic characteristics of afore-mentioned chromophores. A suitable band gap (∆E = 2.723-2.659 eV) with larger bathochromic shift (λmax = 554.218-543.261 nm in acetonitrile) was seen in TAZD1-TAZD5. An effective charge transference from donor to acceptor via spacer was observed by FMO analysis which further supported by DOS and TDM. Further, lower binding energy values also supported the higher exciton dissociation and greater CT in TAZD1-TAZD5. Among all the designed chromophores, TAZD5 exhibited the narrowest Egap (2.659 eV) and maximum red-shifted absorption in solvent as well as gas phase i.e. 554.218 nm and 533.219 nm, respectively which perhaps as a result of the phenothiazine-based donor group (MPT). In a nutshell, all the tailored chromophores can be considered as efficient compounds for promising OSCs with a good Voc response, interestingly, TAZD5 is found to be excellent chromophores as compared to all these designed compounds.

A 3-Fluoropyridine Manipulating the Aggregation and Fibril Network of Donor Polymers for Eco-Friendly Solution-Processed Versatile Organic Solar Cells

The development of eco-friendly solvent-processed organic solar cells (OSCs) suitable for industrial-scale production should be now considered the imperative research. Herein, asymmetric 3-fluoropyridine (FPy) unit is used to control the aggregation and fibril network of polymer blends. Notably, terpolymer PM6(FPy = 0.2) incorporating 20% FPy in a well-known donor polymer 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)] (PM6) can reduce the regioregularity of the polymer backbone and endow them with much-enhanced solubility in eco-friendly solvents. Accordingly, the excellent adaptability for fabricating versatile devices based on PM6(FPy = 0.2) by toluene processing is demonstrated. The resulting OSCs exhibit a high power conversion efficiency (PCE) of 16.1% (17.0% by processed with chloroform) and low batch-to-batch variation. Moreover, by controlling the donor-to-acceptor weight ratio at 0.5:1.0 and 0.25:1.0, semi-transparent OSCs (ST-OSCs) yield significant light utilization efficiencies of 3.61% and 3.67%, respectively. For large-area (1.0 cm2 ) indoor OSC (I-OSC), a high PCE of 20.6% is achieved with an appropriate energy loss of 0.61 eV under a warm white light-emitting diode (3,000 K) with the illumination of 958 lux. Finally, the long-term stability of the devices is evaluated by investigating their structure-performance-stability relationship. This work provides an effective approach to realizing eco-friendly, efficient, and stable OSCs/ST-OSCs/I-OSCs.

Trifluoromethyl-Substituted Conjugated Random Terpolymers Enable High-Performance Small and Large-Area Organic Solar Cells Using Halogen-Free Solvent

The advancement of non-fullerene acceptors with crescent-shaped geometry has led to the need for polymer donor improvements. Additionally, there is potential to enhance the photovoltaic parameters in high-efficiency organic solar cells (OSCs). The random copolymerization method is a straightforward and effective strategy to further optimize photoactive morphology and enhance device performance. However, finding a suitable third component in terpolymers remains a crucial challenge. In this study, a series of terpolymer donors (PTF3, PTF5, PTF10, PTF20, and PTF50) is synthesized by introducing varying amounts of the trifluoromethyl-substituted unit (CF3) into the PM6 polymer backbone. Even subtle changes in the CF3 content can significantly enhance all the photovoltaic parameters due to the optimized energy levels, molecular aggregation/miscibility, and bulk-heterojunction morphology of the photoactive materials. Thus, the best binary OSC based on the PTF5:Y6-BO achieves an outstanding power conversion efficiency (PCE) of 18.2% in the unit cell and a PCE of 11.6% in the sub-module device (aperture size: 54.45 cm2 ), when using halogen-free solvent o-xylene. This work showcases the remarkable potential of the easily accessible CF3 unit as a key constituent in the construction of terpolymer donors in high-performance OSCs.

Tandem organic solar cells with 20.6% efficiency enabled by reduced voltage losses

Large voltage losses are the main obstacle for achieving high efficiency in organic solar cells (OSCs). Here we construct ternary OSCs by introducing an asymmetric small molecule acceptor AITC into PBDB-TCl : BTP-eC9 system and demonstrate the effectiveness in simultaneously decreasing energy disorder and non-radiative voltage losses. It is found that the introduction of AITC can modify domain size and increase the degree of crystallinity, which enhances open-circuit voltage and power conversion efficiency (19.1%, certified as 18.9%). Inspiringly, an output efficiency of 20.6% of the constructed tandem OSCs based on PBDB-TCl : AITC : BTP-eC9 ternary active layer output a recorded efficiency of 20.6% (certified as 20.3%), which is the highest value in OSCs field to date. This work demonstrates that decreasing the voltage losses by ternary strategy and constructing of tandem architecture are effective approaches towards improving photovoltaic performance.

Eco-Friendly Solvent-Processed Dithienosilicon-Bridged Carbazole-Based Small-Molecule Acceptors Achieved over 25.7% PCE in Ternary Devices under Indoor Conditions

Terminal acceptor atoms and side-chain functionalization play a vital role in the construction of efficient nonfullerene small-molecule acceptors (NF-SMAs) for AM1.5G/indoor organic photovoltaic (OPV) applications. In this work, we report three dithienosilicon-bridged carbazole-based (DTSiC) ladder-type (A-DD'D-A) NF-SMAs for AM1.5G/indoor OPVs. First, we synthesize DTSiC-4F and DTSiC-2M, which are composed of a fused DTSiC-based central core with difluorinated 1,1-dicyanomethylene-3-indanone (2F-IC) and methylated IC (M-IC) end groups, respectively. Then, alkoxy chains are introduced in the fused carbazole backbone of DTSiC-4F to form DTSiCODe-4F. From solution to film absorption, DTSiC-4F exhibits a bathochromic shift with strong π-π interactions, which improves the short-circuit current density (J_sc) and the fill factor (FF). On the other hand, DTSiC-2M and DTSiCODe-4F display up-shifting lowest unoccupied molecular orbital (LUMO) energy levels, which enhances the open-circuit voltage (V_oc). As a result, under both AM1.5G/indoor conditions, the devices based on PM7:DTSiC-4F, PM7:DTSiC-2M, and PM7:DTSiCOCe-4F show power conversion efficiencies (PCEs) of 13.13/21.80%, 8.62/20.02, and 9.41/20.56%, respectively. Furthermore, the addition of a third component to the active layer of binary devices is also a simple and efficient strategy to achieve higher photovoltaic efficiencies. Therefore, the conjugated polymer donor PTO2 is introduced into the PM7:DTSiC-4F active layer because of the hypsochromically shifted complementary absorption, deep highest occupied molecular orbital (HOMO) energy level, good miscibility with PM7 and DTSiC-4F, and optimal film morphology. The resulting ternary OSC device based on PTO2:PM7:DTSiC-4F can improve exciton generation, phase separation, charge transport, and charge extraction. As a consequence, the PTO2:PM7:DTSiC-4F-based ternary device achieves an outstanding PCE of 13.33/25.70% under AM1.5G/indoor conditions. As far as we know, the obtained PCE results under indoor conditions are one of the best binary/ternary-based systems processed from eco-friendly solvents.

Organic Photovoltaics Utilizing Small-Molecule Donors and Y-Series Nonfullerene Acceptors

The emerging Y-series nonfullerene acceptors (Y-NFA) has prompted the rapid progress of power conversion efficiency (PCE) of all-small-molecule organic solar cells (ASM-OSCs) from around 12% to 17%. The excellent PCE improvement benefits from not only the outstanding properties of Y-series acceptors but also the successful development of small-molecule donors. The short-circuit current density, fill factor, and nonradiative recombination are all optimized to the unprecedented values, providing a scenery that is obviously different from the ITIC-series based ASM-OSCs. In this review, OSCs utilizing small-molecule donors and Y-NFA are summarized and classified in order to provide an up-to-date development overview and give an insight on structure-property correlation. Then, the characteristics of bulk-heterojunction (BHJ) formation of ASM-OSCs are discussed and compared with that of polymer-based OSCs. Finally, the challenges and outlook on designing ground-breaking small-molecule donor and forming an ideal BHJ morphology are discussed.

Advances in organic photovoltaic cells: a comprehensive review of materials, technologies, and performance

This paper provides a comprehensive overview of organic photovoltaic (OPV) cells, including their materials, technologies, and performance. In this context, the historical evolution of PV cell technology is explored, and the classification of PV production technologies is presented, along with a comparative analysis of first, second, and third-generation solar cells. A classification and comparison of PV cells based on materials used is also provided. The working principles and device structures of OPV cells are examined, and a brief comparison between device structures is made, highlighting their advantages, disadvantages, and key features. The various parts of OPV cells are discussed, and their performance, efficiency, and electrical characteristics are reviewed. A detailed SWOT analysis is conducted, identifying promising strengths and opportunities, as well as challenges and threats to the technology. The paper indicates that OPV cells have the potential to revolutionize the solar energy industry due to their low production costs, and ability to produce thin, flexible solar cells. However, challenges such as lower efficiency, durability, and technological limitations still exist. Despite these challenges, the tunability and versatility of organic materials offer promise for future success. The paper concludes by suggesting that future research should focus on addressing the identified challenges and developing new materials and technologies that can further improve the performance and efficiency of OPV cells.

Large-Area Nonfullerene Organic Photovoltaic Modules with a High Certified Power Conversion Efficiency

Achieving large-area organic photovoltaic (OPV) modules with reasonable cost and performance is an important step toward commercialization. In this work, solution-processed conventional and inverted OPV modules with an area of 216 cm² were fabricated by the blade coating method. Film uniformity was controlled by adjusting the fabrication parameters of the blade coating procedure. The influence of the concentration of the solutions of the interfacial materials on OPV module performance was investigated. For OPV modules based on the PM6:Y6 photoactive layer, a certificated power conversion efficiency (PCE) of 9.10% was achieved for the conventional OPV modules based on the TASiW-12 interfacial layer while a certificated PCE of 11.27% was achieved for the inverted OPV modules based on the polyethylenimine (PEI) interfacial layer. As for OPV modules based on a commercially available photoactive layer, PV-X Plus, a PCE of 8.52% was achieved in the inverted OPV modules. A halogen-free solvent, o-xylene, was used as the solvent for PV-X Plus, which makes the industrial production much more environmentally friendly.

A Versatile and Low-Cost Polymer Donor Based on 4-Chlorothiazole for Highly Efficient Polymer Solar Cells

Benefiting from the emergence of narrow-band-gap small-molecule acceptors (SMAs), especially "Y" series, the power conversion efficiency (PCE) of polymer solar cells (PSCs) is rapidly improved. However, polymer donors with high efficiency, easy synthesis, and good universality are relatively scarce except PBDB-TF and D18. Herein, two polymer donors are designed and synthesized based on 4-chlorothiazole derivatives with simple structures, namely PTz3Cl and PBTTz3Cl. The OSCs based on PBTTz3Cl with slightly weaker intermolecular forces in comparison to PTz3Cl exhibits a decent PCE of 18.38% in blending with SMA L8-BO, owing to its strong donor/acceptor interaction with L8-BO, which shapes suitable phase separation morphology. Further research finds that PBTTz3Cl can exhibit excellent photovoltaic performances with various SMA materials, highlighting its universality. Based on this, ternary PSCs are designed where BTP-eC9 is introduced as a guest into the PBTTz3Cl:L8-BO host system. Thanks to further optimal blend morphology and more balanced charge transport, the PCE is improved up to 19.12%, which is among the highest values for PSCs. This work provides a new design of low-cost electron-deficient units for constructing highly versatile, high-performance polymer donors.

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.

Renewed Prospects for Organic Photovoltaics

Organic photovoltaics (OPVs) have progressed steadily through three stages of photoactive materials development: (i) use of poly(3-hexylthiophene) and fullerene-based acceptors (FAs) for optimizing bulk heterojunctions; (ii) development of new donors to better match with FAs; (iii) development of non-fullerene acceptors (NFAs). The development and application of NFAs with an A-D-A configuration (where A = acceptor and D = donor) has enabled devices to have efficient charge generation and small energy losses (E_loss < 0.6 eV), resulting in substantially higher power conversion efficiencies (PCEs) than FA-based devices. The discovery of Y6-type acceptors (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))dimalononitrile) with an A-DA' D-A configuration has further propelled the PCEs to go beyond 15% due to smaller E_loss values (∼0.5 eV) and higher external quantum efficiencies. Subsequently, the PCEs of Y6-series single-junction devices have increased to >19% and may soon approach 20%. This review provides an update of recent progress of OPV in the following aspects: developments of novel NFAs and donors, understanding of the structure-property relationships and underlying mechanisms of state-of-the-art OPVs, and tasks underpinning the commercialization of OPVs, such as device stability, module development, potential applications, and high-throughput manufacturing. Finally, an outlook and prospects section summarizes the remaining challenges for the further development of OPV technology.

Versatile Sequential Casting Processing for Highly Efficient and Stable Binary Organic Photovoltaics

Forming an ideal bulk heterojunction (BHJ) morphology is a critical issue governing the photon to electron process in organic solar cells (OSCs). Complementary to the widely-used blend casting (BC) method for BHJ construction, sequential casting (SC) can also enable similar or even better morphology and device performance for OSCs. Here, BC and SC methods on three representative donor:acceptor (D:A) blends are utilized, that is, PM6:PC₇₁ BM, PM6:IT-4F and PM6:L8-BO. Higher power conversion efficiencies (PCEs) in all cases by taking advantage of beneficial morphology from SC processing are achieved, and a champion PCE of 18.86% (certified as 18.44%) based on the PM6:L8-BO blend is reached, representing the record value among binary OSCs. The observations on phase separation and vertical distribution inspire the proposal of the swelling-intercalation phase-separation model to interpret the morphology evolution during SC processing. Further, the vertical phase segregation is found to deliver an improvement of device performance via affecting the charge transport and collection processes, as evidenced by the D:A-ratio-dependent photovoltaic properties. Besides, OSCs based on SC processing show advantages on device photostability and upscale fabrication. This work demonstrates the versatility and efficacy of the SC method for BHJ-based OSCs.

Photovoltaic molecules with ultra-high light energy utilization for near-infrared laser triggered synergetic photodynamic and photothermal therapy

Photovoltaic molecules possess strong absorption in the near-infrared (NIR) region and are suitable for NIR laser-triggered phototherapy. Herein, the star molecule IEICO of organic photovoltaic materials, which has a narrow bandgap and large A-D-A conjugated structure, was prepared into water dispersive nanoparticles (NPs) through a simple self-assembly method. The obtained IEICO NPs showed a strong NIR absorption peak at 800 nm and a high ¹O₂ quantum yield of 11% and photothermal conversion efficiency of 85.4% under 808 nm laser irradiation. The ultra-high light energy utilization efficacy (∼96.4%) of the IEICO NPs enables their excellent phototherapeutic effect on tumors. The present work suggested the huge application potential of organic photovoltaic materials in the biomedical field.

Asymmetric electron acceptor enables highly luminescent organic solar cells with certified efficiency over 18

Enhancing the luminescence property without sacrificing the charge collection is one key to high-performance organic solar cells (OSCs), while limited by the severe non-radiative charge recombination. Here, we demonstrate efficient OSCs with high luminescence via the design and synthesis of an asymmetric non-fullerene acceptor, BO-5Cl. Blending BO-5Cl with the PM6 donor leads to a record-high electroluminescence external quantum efficiency of 0.1%, which results in a low non-radiative voltage loss of 0.178 eV and a power conversion efficiency (PCE) over 15%. Importantly, incorporating BO-5Cl as the third component into a widely-studied donor:acceptor (D:A) blend, PM6:BO-4Cl, allows device displaying a high certified PCE of 18.2%. Our joint experimental and theoretical studies unveil that more diverse D:A interfacial conformations formed by asymmetric acceptor induce optimized blend interfacial energetics, which contributes to the improved device performance via balancing charge generation and recombination.

High-performance organic solar cells call for novel designs of acceptor molecules. Here, He et al. design and synthesize a non-fullerene acceptor with an asymmetric structure for diverse donor:acceptor interfacial conformations and report a certificated power conversion efficiency of 18.2%.

Random Copolymerization Strategy for Host Polymer Donor PM6 Enables Improved Efficiency Both in Binary and Ternary Organic Solar Cells

Although breakthroughs have been made in organic solar cells (OSCs) in recent years, the power conversion efficiency (PCE) of OSCs still lags behind inorganic/perovskite solar cells. In this work, two terpolymers were synthesized by introducing the thieno[3,4-c]pyrrole-4,6-(5H)-dione (TPD) block into the host polymer donor PM6. Owing to the lower highest occupied molecular orbital energy level, wider light absorption, optimal molecular packing, and more desirable aggregation morphology by addition of the TPD, the PM6-TPD-5 % : Y6-based device displayed an improved PCE of 16.3 % with an enhanced open-circuit voltage (V_OC ) of 0.860 V, relative to that of PM6-TPD-10 % : Y6 (PCE=14.8 %) and PM6 : Y6-based device (PCE=15.6 %). Interestingly, the V_OC did not always increase in proportion to the third component. Besides, ternary OSCs based on PM6 : PM6-TPD-5 % : Y6 achieved a superior PCE of 17.1 %. This work demonstrated that random copolymerization is a feasible and effective strategy to further increase device performance, and the two polymers that possess similar structure and absorption in ternary devices can also obtain impressive efficiency.

Recent Progress in Organic Solar Cells: A Review on Materials from Acceptor to Donor

In the last few decades, organic solar cells (OSCs) have drawn broad interest owing to their advantages such as being low cost, flexible, semitransparent, non-toxic, and ideal for roll-to-roll large-scale processing. Significant advances have been made in the field of OSCs containing high-performance active layer materials, electrodes, and interlayers, as well as novel device structures. Particularly, the innovation of active layer materials, including novel acceptors and donors, has contributed significantly to the power conversion efficiency (PCE) improvement in OSCs. In this review, high-performance acceptors, containing fullerene derivatives, small molecular, and polymeric non-fullerene acceptors (NFAs), are discussed in detail. Meanwhile, highly efficient donor materials designed for fullerene- and NFA-based OSCs are also presented. Additionally, motivated by the incessant developments of donor and acceptor materials, recent advances in the field of ternary and tandem OSCs are reviewed as well.

Resist or Oxidize: Identifying Molecular Structure-Photostability Relationships for Conjugated Polymers Used in Organic Solar Cells

Although organic solar cells have started to demonstrate competitive power conversion efficiencies of >18 %, their operational lifetimes remain insufficient for wide practical use and the factors influencing the photostability of absorber materials and completed devices are still not completely understood. A systematic study of two series of structurally similar [XTBT]_n and [XTTBTBTT]_n polymers (16 structures in total) reveals the building blocks that enable the highest material stability towards photooxidation: fluorene, silafluorene, carbazole, diketopyrrolopyrrole, and isoindigo. Furthermore, a direct correlation is evident between the electronic properties of the conjugated polymers and their reactivity towards oxygen. The structures with the lowest highest occupied molecular orbital (HOMO) energies show the highest electrochemical oxidation potentials and appear to be the most resistant towards chemical oxidation. These relationships set important guidelines for the further rational design of new absorber materials for efficient and stable organic photovoltaics.

The steric effect of benzodifuran based polymers via alkyl side chain manipulation: a simple approach for enhancing the photovoltaic performance

A series of narrow band gap conjugated copolymers with different alkyl side chains were synthesized via Stille copolymerization of benzodifuran (BDF) and benzothiadiazole (BT) monomers.

Effects of Halogenation on the Benzotriazole Unit of Non-Fullerene Acceptors in Organic Solar Cells with High Voltages

Non-fullerene acceptors (NFAs) can be simply divided into three categories: A-D-A, A-DA'D-A, and A₂-A₁-D-A₁-A₂ according to their chemical structures. Benefiting from the easily modified 1,1-dicyanomethylene-3-indanone end groups, the halogenation on the first two types of materials has been proved to be very effective to modulate their optoelectronic properties and improve their photovoltaic performance. Hence, in this work, we systematically investigate the effect of halogenation on the classic NFA molecule of BTA3, which has the linear A₂-A₁-D-A₁-A₂-type backbone. After fluorination and chlorination, F-BTA3 and Cl-BTA3 have similar optical band gaps but lower energy levels than BTA3. When blending with a linear copolymer PE25 composed of benzodifuran and chlorinated benzotriazole (BTA) according to "Same-A-Strategy", the corresponding V_OC of the halogenated NFAs gradually decreases (1.13 V for F-BTA3 and 1.09 V for Cl-BTA3), compared with that of the BTA3-based device (V_OC = 1.22 V). This tendency mainly comes from the lower lowest unoccupied molecular orbital energy levels due to the strong electron-withdrawing ability of halogen atoms and the larger nonradiative energy loss. However, the power conversion efficiencies of the halogenated materials are slightly improved, from 9.08% for PE25: BTA3 to 10.45% for PE25: F-BTA3 and 10.75% for PE25: Cl-BTA, with the nonhalogenated solvent tetrahydrofuran as the processing solvent. The improved photovoltaic performance of F-BTA3 and Cl-BTA3 should come from the higher carrier mobility, weaker bimolecular recombination, and higher fluorescence quenching rate. This study illustrates that halogenation on the A₁ unit is a promising strategy for developing novel and effective A₂-A₁-D-A₁-A₂-type NFAs.

Assessing Effects of Different π bridges on Properties of Random Benzodithiophene-thienothiophene Donor and Non-fullerene Acceptor Based Active Layer

This report presents the effect of insertion of four different π bridges, furan, thienothiophene, thiophene, and thiazole, into a random benzodithiophene (BDT)-fluorinated-thienothiophene (TT-F) based donor. Starting from a structure of synthesized donor (D)-acceptor (A) random copolymer with 3:1 ratio, we have designed four D-π-A systems with four different π bridges. Structural, optoelectronic, and charge transport/transfer properties of these donors and donor/NDI (NDI = poly[N,N'-bis(2-hexyldecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)) blends are investigated using DFT and TD-DFT methodologies. Our results show that the thiazole based TzP1 oligomer has the deepest HOMO value resulting in the highest open circuit voltage among all systems. The maximum absorption wavelengths of π-linked systems are red-shifted compared to the parent molecule. Rates of charge transfer and charge recombination are the highest and smallest in case of the thiazole/NDI blend system. In addition, hole mobilities in thiophene, thienothiophene, and thiazole based systems are larger than in the parent system. The results indicate that the thiazole unit among the four π bridge units is the most suitable for active layer construction.

Visible photodetector based on transition metal-doped ZnO NRs/PEDOT:PSS hybrid materials

A hybrid Cu-doped ZnO nanorods (ZnO:Cu NRs)/poly(3,4 ethylene dioxythiophene)-polystyrene sulfonate (PEDOT:PSS)-based photodetector was fabricated using a simple hydrothermal method with pre-patterned silver electrodes. In the hybrid structure, PEDOT:PSS with high mobility acts as a carrier transport layer, while ZnO:Cu NRs with high visible absorption works as an “antenna” material to generate electron–hole pairs under light illumination. As a result, the devices exhibits a high response in visible light at a wavelength of 395 nm. The responsivity and photoconductive gain of the hybrid photodetector reached 0.33 A W⁻¹ and 1.306, respectively, which is 1.36 times higher than those of Cu-doped ZnO NRs-based ones. The response and recovery times are improved, with values of 25.21 s and 42.01 s, respectively. The development of hybrid materials for visible photodetectors enables an innovative approach for future optoelectronic devices, especially optical sensors.

This study reports the fabrication of a hybrid photodetector based on Cu-doped ZnO NRs/PEDOT:PSS, which improves the device's performance and applications.

Designs and understanding of small molecule-based non-fullerene acceptors for realizing commercially viable organic photovoltaics

Organic photovoltaics (OPVs) have emerged as a promising next-generation technology with great potential for portable, wearable, and transparent photovoltaic applications. Over the past few decades, remarkable advances have been made in non-fullerene acceptor (NFA)-based OPVs, with their power conversion efficiency exceeding 18%, which is close to the requirements for commercial realization. Novel molecular NFA designs have emerged and evolved in the progress of understanding the physical features of NFA-based OPVs in relation to their high performance, while there is room for further improvement. In this review, the molecular design of representative NFAs is described, and their blend characteristics are assessed via statistical comparisons. Meanwhile, the current understanding of photocurrent generation is reviewed along with the significant physical features observed in high-performance NFA-based OPVs, while the challenging issues and the strategic perspectives for the commercialization of OPV technology are also discussed.

Exploration of promising optical and electronic properties of (non-polymer) small donor molecules for organic solar cells

Non-fullerene based organic compounds are considered promising materials for the fabrication of modern photovoltaic materials. Non-fullerene-based organic solar cells comprise of good photochemical and thermal stability along with longer device lifetimes as compared to fullerene-based compounds. Five new non-fullerene donor molecules were designed keeping in view the excellent donor properties of 3-bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2ethylhexyl) benzo[1,2-:4,5-c']-dithiophene-4,8-dione thiophene-alkoxy benzene-thiophene indenedione (BDD-IN) by end-capped modifications. Photovoltaic and electronic characteristics of studied molecules were determined by employing density functional theory (DFT) and time dependent density functional theory (TD-DFT). Subsequently, obtained results were compared with the reference molecule BDD-IN. The designed molecules presented lower energy difference (ΔΕ) in the range of 2.17-2.39 eV in comparison to BDD-IN (= 2.72 eV). Moreover, insight from the frontier molecular orbital (FMO) analysis disclosed that central acceptors are responsible for the charge transformation. The designed molecules were found with higher λmax values and lower transition energies than BDD-IN molecule due to stronger end-capped acceptors. Open circuit voltage (Voc) was observed in the higher range (1.54-1.78 V) in accordance with HOMOdonor-LUMOPC61BM by designed compounds when compared with BDD-IN (1.28 V). Similarly, lower reorganization energy values were exhibited by the designed compounds in the range of λe(0.00285-0.00370 Eh) and λh(0.00847-0.00802 Eh) than BDD-IN [λe(0.00700 Eh) and λh(0.00889 Eh)]. These measurements show that the designed compounds are promising candidates for incorporation into solar cell devices, which would benefit from better hole and electron mobility.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

Single-Junction Organic Photovoltaic Cell with 19% Efficiency

Improving power conversion efficiency (PCE) is important for broadening the applications of organic photovoltaic (OPV) cells. Here, a maximum PCE of 19.0% (certified value of 18.7%) is achieved in single-junction OPV cells by combining material design with a ternary blending strategy. An active layer comprising a new wide-bandgap polymer donor named PBQx-TF and a new low-bandgap non-fullerene acceptor (NFA) named eC9-2Cl is rationally designed. With optimized light utilization, the resulting binary cell exhibits a good PCE of 17.7%. An NFA F-BTA3 is then added to the active layer as a third component to simultaneously improve the photovoltaic parameters. The improved light unitization, cascaded energy level alignment, and enhanced intermolecular packing result in open-circuit voltage of 0.879 V, short-circuit current density of 26.7 mA cm^-2 , and fill factor of 0.809. This study demonstrates that further improvement of PCEs of high-performance OPV cells requires fine tuning of the electronic structures and morphologies of the active layers.

Performance and Stability of Polymer : Nonfullerene Solar Cells with 100 °C-Annealed Electron-Collecting Combination Layers

Inverted-type organic solar cells, fabricated with low-temperature-processed combination layers of hybrid electron-collecting buffer layers (ECBLs) consisting of zinc oxide (ZnO) and poly(2-ethyl-2-oxazoline) (PEOz) and additional PEOz interlayers, showed improved performance and stability. The ZnO : PEOz precursor films with various PEOz compositions (0-12 wt %) were prepared and thermally treated at 100 °C, leading to the ECBLs on which the PEOz interlayers were subsequently deposited before coating of polymer : nonfullerene bulk heterojunction layers. Results showed that the power conversion efficiency of solar cells reached approximately 9.38 and 10.11 % (average) in case of the ZnO/PEOz and ZnO : PEOz(6 wt % PEOz)/PEOz combination layers, respectively, despite the low-temperature thermal annealing process. A continuous irradiation test for 12 h under one sun condition (air mass 1.5G, 100 mW cm^-2 ) disclosed that the devices with the ZnO : PEOz(6 wt % PEOz)/PEOz combination layers were more stable than those with the ZnO/PEOz layers.

Indoor Organic Photovoltaics for Self-Sustaining IoT Devices: Progress, Challenges and Practicalization

Indoor photovoltaics (IPVs) have great potential to provide a self-sustaining power source for Internet-of-Things (IoT) devices. The rapid growth in demand for low-power IoT devices for indoor application not only boosts the development of high-performance IPVs, but also promotes the electronics and semiconductor industry for the design and development of ultra-low-power IoT systems. In this Review, the recent progress in IPV technologies, design rules, market trends, and future prospects for highly efficient indoor photovoltaics are discussed. Special attention is given to the progress and development of organic photovoltaics (OPVs), which demonstrate great possibilities for IPVs, owing to their bandgap tunability, high absorbance coefficient, semitransparency, solution processability, and easy large-area manufacturing on flexible substrates. Highly efficient indoor organic photovoltaics (IOPVs) can be realized through designing efficient donor and acceptor absorber materials that have good spectral responses in the visible region and better energy-aligned interfacial layers, and through modulation of optical properties. Interfacial engineering, photovoltage losses, device stability, and large-area organic photovoltaic modules are surveyed to understand the mechanisms of efficient power conversion and challenges for IOPVs under indoor conditions as a self-sustaining power source for IoT devices. Finally, the prospects for further improve in IOPV device performance and practical aspects of integrating IOPVs in low-power IoT devices are discussed.

Ground- and excited-state characteristics in photovoltaic polymer N2200

As a classical polymer acceptor material, N2200 has received extensive attention and research in the field of polymer solar cells (PSCs). However, the intrinsic properties of ground- and excited-states in N2200, which are critical for the application of N2200 in PSCs, remain poorly understood. In this work, the ground- and excited-state properties of N2200 solution and film were studied by steady-state and time-resolved spectroscopies as well as time-dependent density functional theory (TD-DFT) calculations. The transition mechanism of absorption peaks of N2200 was evaluated through the natural transition orbitals (NTOs) and hole-electron population analysis by TD-DFT. Time-resolved photoluminescence (TRPL) study shows that the lifetimes of singlet excitons in N2200 chlorobenzene solution and film are ∼90 ps and ∼60 ps, respectively. Considering the absolute quantum yield of N2200 film, we deduce that the intrinsic lifetime of singlet exciton can be as long as ∼20 ns. By comparing the TRPL and transient absorption (TA) kinetics, we find that the decay of singlet excitons in N2200 solution is dominated by a fast non-radiative decay process, and the component induced by intersystem crossing is less than 5%. Besides that, the annihilation radius, annihilation rate and diffusion length of singlet excitons in N2200 film were evaluated as 3.6 nm, 2.5 × 10^-9 cm³ s^-1 and 4.5 nm, respectively. Our work provides comprehensive information on the excited states of N2200, which is helpful for the application of N2200 in all-PSCs.

Synergistic Engineering of Substituents and Backbones on Donor Polymers: Toward Terpolymer Design of High-Performance Polymer Solar Cells

Design of terpolymers via copolymerization has emerged as a potential strategy for expanding the family of high-performing donor polymers and boosting the photovoltaic performance of non-fullerene polymer solar cells (PSCs). Herein, double-ester-substituted thiophenes and thienothiophenes are incorporated as third building blocks into the donor polymer PBDB-TF, developing two groups of terpolymers with donor-acceptor 1-donor-acceptor 2 (D-A1-D-A2)-type backbones. An optimum 10% concentration of double-ester-substituted thiophene units in PBDB-TF-T10 downshifts the molecular energy and increases the dielectric constant, and delivers proper miscibility and nanostructure in blends with the high-performing acceptor Y6. These characteristics are designed to synergistically enhance the photovoltage, photocurrent, and efficiency of PSCs. The resulting power conversion efficiency (PCE) of 16.4% surpasses the conventional PBDB-TF/Y6 PSCs, and it is among the best-performing PSCs based on PBDB-TF-derived terpolymers. Gratifyingly, PBDB-TF-T10 does not show significant batch-to-batch variation and it retains high PCEs above 16% in a broad range of molecular weights. This work introduces a facile strategy to easily synthesize terpolymers in combination with Y6 for the attainment of high-performing and reproducible non-fullerene PSCs.

Bromination and increasing the molecular conjugation length of the non-fullerene small-molecule acceptor based on benzotriazole for efficient organic photovoltaics

Two novel non-fullerene acceptors, namely BZIC-2Br and Y9-2Br, were synthesized by employing a ladder-type electron-deficient-based fused ring central with a benzotriazole core. Y9-2Br is obtained by extending the conjugate length of BZIC-2Br. Compared with BZIC-2Br, Y9-2Br possesses a lower optical bandgap of 1.32 eV with an absorption edge of 937 nm, exhibiting broader and stronger absorption band from 600 to 900 nm. Moreover, Y9-2Br exhibits excellent photovoltaic properties with V oc of 0.84 V, J sc of 21.38 mA cm-2 and FF of 67.11%, which achieves an impressive PCE of 12.05%. Our study demonstrates that bromination and effective extension of the conjugate length can modulate performance from different aspects to optimize photovoltaic characteristics.