Non-fullerene Organic Solar Cells Research Articles

A 3,3'-Difluoro-2,2'-Bithiophene Based Donor Polymer Realizing High Efficiency (>17%) Single Junction Binary Organic Solar Cells.

In this study, two novel donor-acceptor (D-A) copolymers are designed and synthesized, DTBT-2T and DTBT-2T2F with 2,2'-bithiophene or 3,3'-difluoro-2,2'-bithiophene as the donor unit and dithienobenzothiadiazole as the acceptor unit, and used them as donor materials in non-fullerene organic solar cells (OSCs). Due to enhanced planarity of polymer chains resulted by the intramolecular F···S noncovalent interactions, the incorporation of 3,3'-difluoro-2,2'-bithiophene unit instead of 2,2'-bithiophene into the polymers can enhance their molecular packing, crystallinity and hole mobility. The DTBT-2T:L8-BO based binary OSCs deliver a power conversion efficiency (PCE) of only 9.71% with a Voc of 0.78V, a Jsc of 20.69mAcm-2 , and an FF of 59.67%. Moreover, the introduction of fluoro atoms can lower the highest occupied molecular orbital levels. As a result, DTBT-2T2F:L8-BO based single-junction binary OSCs exhibited less recombination loss, more balanced charge mobility, and more favorable morphology, resulting in an impressive PCE of 17.03% with a higher Voc of 0.89V, a Jsc of 25.40mAcm-2, and an FF of 75.74%. These results indicate that 3,3'-difluoro-2,2'-bithiophene unit can be used as an effective building block to synthesize high performance polymer donor materials. This work greatly expands the selection range of donor units for constructing high-performance polymers.

Balancing Solvent Polarity and Drying‐Kinetics for Processing Nonfullerene Acceptors Toward High‐Performance Organic Solar Cells

In recent years, the emergence of high‐efficiency nonfullerene acceptors (NFAs) has pushed the power conversion efficiency (PCE) beyond 19% in organic solar cells (OSCs). For solution processing of the photoactive layers, the study of solvent properties on device performance has become a hot research topic toward up‐scaling. Herein, the key roles of balancing solvent polarity and drying kinetics for processing of small molecular NFAs toward high‐performance OSCs are revealed. It is demonstrated that the synergistic effect of solvent polarity and drying kinetics significantly affects the multilength scale morphology of solid films and photovoltaic performance. Furthermore, the self‐assembly of NFAs in solution and the resulting thin film microstructures induced by electrostatic interactions with polarized additives are even more sensitive to solvent polarity and drying kinetics. Finally, the optimized devices achieve an outstanding PCE of 18.54% by fine‐tuning the microstructure with polar solvent and fast‐drying kinetics to form improved molecular packing and structure size. The work provides a novel view and deeper insights into the general selection rules of solvents toward the solution processing of high‐performance nonfullerene OSCs.

Critical role of the end-group acceptor in enhancing the efficiency of indacenodithiophene-benzothiadiazole-linked nonfullerene organic solar cells through morphology optimization

Solution-processed organic solar cells (OSCs) have great promise for next-generation solar energy conversion technologies. Innovative acceptors are constantly being designed to improve device efficiency. In this work, we designed and synthesized a series of A–π–D–π–A type non-fullerene acceptor (NFA) molecules, named IB-TOT, IB-TOM, IB-IOO, and IB-IOM for their applications in OSCs. All molecules showed high thermal stability, broad photoabsorption, and relatively lower highest occupied molecular orbital (HOMO) energy levels from −5.48 eV to −5.58 eV. The optimized OSCs based on these NFAs and PTB7-Th donor deliver power conversion efficiencies (PCEs) of 8.19%, 5.50%, 2.42%, and 3.10% for IB-TOT, IB-TOM, IB-IOO, and IB-IOM molecules, respectively. The higher PCE of IB-TOT-based OSCs is attributed to their excellent miscibility and superior interpenetrating network morphology compared to the other three compounds. These results demonstrate that using the 3-ethylrhodanine acceptor unit to construct the NFA molecule is a successful approach for outstanding achievement in OSCs.

CPE-Na-Based Hole Transport Layers for Improving the Stability in Nonfullerene Organic Solar Cells: A Comprehensive Study.

Organic photovoltaic (OPV) cells have experienced significant development in the last decades after the introduction of nonfullerene acceptor molecules with top power conversion efficiencies reported over 19% and considerable versatility, for example, with application in transparent/semitransparent and flexible photovoltaics. Yet, the optimization of the operational stability continues to be a challenge. This study presents a comprehensive investigation of the use of a conjugated polyelectrolyte polymer (CPE-Na) as a hole layer (HTL) to improve the performance and longevity of OPV cells. Two different fabrication approaches were adopted: integrating CPE-Na with PEDOT:PSS to create a composite HTL and using CPE-Na as a stand-alone bilayer deposited beneath PEDOT:PSS on the ITO substrate. These configurations were compared against a reference device employing PEDOT:PSS alone, as the HTL increased efficiency and fill factor. The instruments with CPE-Na also demonstrated increased stability in the dark and under simulated operational conditions. Device-based PEDOT:PSS as an HTL reached T80 after 2500 h while involving CPE-Na in the device kept at T90 in the same period, evidenced by a reduced degradation rate. Furthermore, the impedance spectroscopy and photoinduced transient methods suggest optimized charge transfer and reduced charge carrier recombination. These findings collectively highlight the potential of CPE-Na as a HTL optimizer material for nonfluorine OPV cells.

Crossbreeding Effect of Chalcogenation and Iodination on Benzene Additives Enables Optimized Morphology and 19.68% Efficiency of Organic Solar Cells.

Volatile solid additives have attracted increasing attention in optimizing the morphology and improving the performance of currently dominated non-fullerene acceptor-based organic solar cells (OSCs). However, the underlying principles governing the rational design of volatile solid additives remain elusive. Herein, a series of efficient volatile solid additives are successfully developed by the crossbreeding effect of chalcogenation and iodination for optimizing the morphology and improving the photovoltaic performances of OSCs. Five benzene derivatives of 1,4-dimethoxybenzene (DOB), 1-iodo-4-methoxybenzene (OIB), 1-iodo-4-methylthiobenzene (SIB), 1,4-dimethylthiobenzene (DSB) and 1,4-diiodobenzene (DIB) are systematically studied, where the widely used DIB is used as the reference. The effect of chalcogenation and iodination on the overall property is comprehensively investigated, which indicates that the versatile functional groups provided various types of noncovalent interactions with the host materials for modulating the morphology. Among them, SIB with the combination of sulphuration and iodination enabled more appropriate interactions with the host blend, giving rise to a highly ordered molecular packing and more favorable morphology. As a result, the binary OSCs based on PM6:L8-BO and PBTz-F:L8-BO as well as the ternary OSCs based on PBTz-F:PM6:L8-BO achieved impressive high PCEs of 18.87%, 18.81% and 19.68%, respectively, which are among the highest values for OSCs.

Improving photostability of non-fullerene acceptor-based inverted organic solar cells using Ga-doped ZnO electron transport layer

Organic solar cells (OSCs) are gaining attention for powering Internet of Things devices due to their impressive power conversion efficiency, flexibility, and slim designs suitable for both indoor and outdoor. The focus on inverted-structured OSCs is increasing due to their superior stability compared to conventional OSCs. This study investigates the use of inverted OSCs with electron transport layers (ETL) made from ZnO and Ga-doped ZnO for light harvesting. OSCs were fabricated using PM6:ITIC-4F as the active layer, and the impact of ZnO doping (Ga) concentration was examined under 1-sun and 1000 lx halogen. Optimizing the doping concentration significantly improved device performance, with PCE values of 10.96 % and 4.95 % for optimally doped ZnO under 1-sun conditions and halogen lamps, respectively. A comparison of photostability under 1-sun and 1000 lx halogen, with varying light soaking durations (0–240 min), showed that Ga-doping in the ZnO ETL improved photostability by reducing oxygen vacancies and slowing down their formation during continuous light exposure. This decrease in oxygen vacancies and enhancement of grain boundaries positively affected charge transport, leading to higher PCE. In summary, Ga-doped ZnO ETL in inverted OSCs demonstrates improved performance and photostability, making them promising for various light harvesting applications.

Enhancing efficiency and stability of non-fullerene organic solar cells via volatilizable solid-liquid-mixed additive system

While traditional high-boiling-point liquid additives like 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN) et al. are commonly used, here we explore the potential of solid additives to optimize organic solar cells (OSCs) performance. Thieno[3,2-b]thiophene (TT), a volatilizable solid additive (VSA), is introduced into the PM6:Y6 OSCs system. TT's similarity to the Y6 backbone allows it high miscibility with Y6 molecules. In light of the fully volatile property, TT induces more uniform and condensed intermolecular packing of the active layer. This refinement increases the power conversion efficiency (PCE) from 14.45% to 15.31%. To further optimize morphology, TT is combined with the liquid additive CN, creating a binary liquid-solid additive system. This combination promotes favorable nanoscale phase separation within the PM6:Y6 active layer. Overall, the OSCs processed with TT+CN achieve an impressive efficiency of 16.43%, attributed to improved charge transport and reduced exciton recombination losses from the dual additives' synergistic effects. Importantly, the OSCs processed with TT demonstrate enhanced long-term stability, retaining 89.5% of their initial performance after 30 days, outperforming devices without additives (87.3%) and those processed with CN alone (80.5%). In summary, this research introduces TT as a promising volatilizable solid additive and a rational additive strategy to manipulate active layer film morphology. These advances hold significant potential for developing high-efficiency and stable OSCs, contributing to sustainable energy solutions.

Selenium substitution for dielectric constant improvement and hole-transfer acceleration in non-fullerene organic solar cells

Dielectric constant of non-fullerene acceptors plays a critical role in organic solar cells in terms of exciton dissociation and charge recombination. Current acceptors feature a dielectric constant of 3-4, correlating to relatively high recombination loss. We demonstrate that selenium substitution on acceptor central core can effectively modify molecule dielectric constant. The corresponding blend film presents faster hole-transfer of ~5 ps compared to the sulfur-based derivative (~10 ps). However, the blends with Se-acceptor also show faster charge recombination after 100 ps upon optical pumping, which is explained by the relatively disordered stacking of the Se-acceptor. Encouragingly, dispersing the Se-acceptor in an optimized organic solar cell system can interrupt the disordered aggregation while still retain high dielectric constant. With the improved dielectric constant and optimized fibril morphology, the ternary device exhibits an obvious reduction of non-radiative recombination to 0.221 eV and high efficiency of 19.0%. This work unveils heteroatom-substitution induced dielectric constant improvement, and the associated exciton dynamics and morphology manipulation, which finally contributes to better material/device design and improved device performance.

Dissociation of singlet excitons dominates photocurrent improvement in high-efficiency non-fullerene organic solar cells

Dissociation of singlet excitons dominates photocurrent improvement in high-efficiency non-fullerene organic solar cells

Non-Covalent Interaction Enhancement on Active/Interfacial Layers via Two-Dimensional Vermiculite Doping for Efficient Organic Solar Cells.

Interface modification plays an important role in improving the power conversion efficiency (PCE) of organic solar cells (OSCs). However, the low non-covalent interaction between the cathode interface layer (CIL) and nonfullerene acceptor (NFA) directly affects the charge collection of OSCs. Here, the non-covalent interaction between the CIL and NFA is enhanced by introducing the 2D vermiculite (VML) in the poly(9,9-bis(3'-(N,N-dimethyl)-Nethylammonium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) dibromide (PFN-Br) interface layer to form an efficient electron transport channel. As a result, the electron extraction efficiency from the active layer to the CIL is increased, and the PCE of OSCs based on PBDB-T:ITIC is boosted from 10.87% to 12.89%. In addition, the strategy of CIL doping VML is proven to be universal in different CIL materials, for which the PCE is boosted from 10.21% to 11.57% for OSCs based on PDINN and from 9.82% to 11.27% for OSCs based on PNDIT-F3N. The results provide a viable option for designing efficient CIL for high-performance non-fullerene OSCs, which may promote the commercialization of OSCs.

Intramolecular Lock Conjugated Polymer Electrolytes as the Cathode Interfacial Layer for Nonfullerene Organic Solar Cells

Intramolecular Lock Conjugated Polymer Electrolytes as the Cathode Interfacial Layer for Nonfullerene Organic Solar Cells

Efficient triisopropylsilylethynyl-substituted benzo[1,2-b:4,5-b′]dithiophene-based random terpolymers enable non-fullerene organic solar cells with enhanced efficiency

Recent work has highlighted the pivotal role of ternary random polymers in the development of efficient high-performance polymer donors for organic solar cells (OSCs) and most of these D-A copolymer donors are so far constituted of an equal proportion of D and A-units. In this contribution, we report series of PM6-based ternary random terpolymers with unequal D- and A-units, namely, PM-TIPS10, PM-TIPS20 and PM-TIPS20, respectively by adding the 10%, 20% and 30% of simple and low-cost triisopropylsilylethynyl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDT-TIPS) unit as second donor (D1) unit in polymer backbone based on D1-A-D2-A molecular design. Ascribed from the structural similarity of the two building blocks in the terpolymer backbone, these polymers showed orderly molecular packing, broadened light absorption, and the face-on orientation of the active layer to facilitate charge transport. Additionally, the new polymers displayed much deeper highest occupied molecular orbital (HOMO) energy levels than host PM6 polymer, which helped in enhancing the open circuit voltage (Voc) in the PM-TIPS-based OSCs. As a result, when combined with BTP-eC9 acceptor unit, PM-TIPS10 and PM-TIPS20 displayed best efficiency of 16.7 and 16.5% with a small energy loss of 0.484 and 0.473 eV, producing overall superior device parameters compared to PM6-based devices tested parallelly (PCE of 15.7%). This study reveals the correlation between introduction of the BDT-TIPS unit on polymer properties and photovoltaic performance and thus contributes to the design of high-PCE polymer donors by adapting a ternary random copolymerization strategy.

Isomerization of Benzothiadiazole Yields a Promising Polymer Donor and Organic Solar Cells with Efficiency of 19.0.

The exploration of high-performance and low-cost wide-bandgap polymer donors remains critical to achieve high-efficiency nonfullerene organic solar cells (OSCs) beyond current thresholds. Herein, the 1,2,3-benzothiadiazole (iBT), which is an isomer of 2,1,3-benzothiadiazole (BT), is used to design wide-bandgap polymer donor PiBT. The PiBT-based solar cells reach efficiency of 19.0%, which is one of the highest efficiencies in binary OSCs. Systemic studies show that isomerization of BT to iBT can finely regulate the polymers' photoelectric properties including i) increasing the extinction coefficient and photon harvest, ii) downshifting the highest occupied molecular orbital energy levels, iii) improving the coplanarity of polymer backbones, iv) offering good thermodynamic miscibility with acceptors. Consequently, the PiBT:Y6 bulk heterojunction (BHJ) device simultaneously reaches advantageous nanoscale morphology, efficient exciton generation and dissociation, fast charge transportation, and suppressed charge recombination, leading to larger VOC of 0.87V, higher JSC of 28.2 mA cm-2, greater fill factor of 77.3%, and thus higher efficiency of 19.0%, while the analog-PBT-based OSCs reach efficiency of only 12.9%. Moreover, the key intermediate iBT can be easily afforded from industry chemicals via two-step procedure. Overall, this contribution highlights that iBT is a promising motif for designing high-performance polymer donors.

Fluorinated Polymer Donors for Nonfullerene Organic Solar Cells.

The rapid development of narrow-bandgap nonfullerene acceptors (NFAs) has boosted the efficiency of organic solar cells (OSCs) over 19 %. The new features of high-performance NFAs, such as visible-NIR light absorption, moderate the highest occupied molecular orbitals (HOMO), and high crystallinity, require polymer donors with matching physical properties. This emphasizes the importance of methods that can effectively tune the physical properties of polymers. Owning to very small atom size and strongest electronegativity, the fluorination has been proved the most efficient strategy to regulate the physical properties of polymer donors, including frontier energy level, absorption coefficient, dielectric constant, crystallinity and charge transport. Owing to the success of fluorination strategy, the vast majority of high-performance polymer donors possess one or more fluorine atoms. In this review, the fluorination synthetic methods, the synthetic route of well-known fluorinated building blocks, the fluorinated polymers which are categorized by the type of donor or acceptor units, and the relationships between the polymer structures, properties, and photovoltaic performances are comprehensively surveyed. We hope this review could provide the readers a deeper insight into fluorination strategy and lay a strong foundation for future innovation of fluorinated polymers.

Enhancing the performance of non-fullerene organic solar cells through side-chain engineering of asymmetrical non-fused-ring electron acceptors

In organic solar cells (OSCs), the intricate synthesis and lower yield of ladder-type fused ring small molecule receptors (FR-SMAs) have prompted the quest for simpler, higher yielding non-fused ring (NFR)-SMAs.

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.

Designing thickness-insensitive cathode interlayers via constructing noncovalently conformational locks for highly efficient non-fullerene organic solar cells

A notable PCE of 15.31% was achieved for a thick cathode interlayer (53 nm) based on PF-BDT with “Noncovalent conformational lock”, which stands out as one of the highest reported PCEs for organic cathode interlayer with a thickness exceeding 50 nm.

Investigation of the open-circuit voltage of non-fullerene acceptors-based ternary organic solar cells based on interpretable machine-learning approach and chemically inspired descriptors

Ternary strategy to enhance the photovoltaic performances (e.g., open-circuit voltage (Voc), current density (Jsc), fill factor (FF), and power conversion efficiency (η)) of non-fullerene acceptors (NFAs)-based organic solar cells (OSCs) has exhibited alluring potential for next generation photovoltaic technology. However, the prediction and optimization of Voc in NFA-based ternary OSCs through expensive and time-consuming experiments or a theoretical perspective (e.g., an empirical rule for Voc prediction of the donor–acceptor binary system) is still an open challenge, mainly because of the complicated ternary photoactive layers. In this study, the prominent predictive performance (R2 > 0.7) of Voc is obtained by utilizing the technique of machine-learning combined with specific descriptors (e.g., frontier molecular orbital theory or electrophilicity index (ɷ)), for the first time forNFA-based ternary OSCs. Furthermore, the Shapley additive explanation (SHAP) model is employed to provide both global and local interpretable explanations for exploring the interpretability of the (eXtreme Gradient Boosting (XGBoost) model prediction and extracting the complex correlation between the specific descriptors and the Voc of NFA-based ternary OSCs. The proposed interpretable machine-learning strategy can contribute to the predictive modeling of highly efficient ternary OSCs based on multicomponent photoactive layers.

Highly Efficient Nonfullerene Organic Solar Cells: Morphology Control and Characterizations

Nonfullerene acceptors (NFAs) are currently a major research focus in the development of organic solar cells (OSCs) because of their readily tunable optical and electronic properties, enabling bulk heterojunction (BHJ) NFA‐based OSCs to achieve photovoltaic efficiencies exceeding 19%. Significant efforts have been made to yield the optimal nanoscale morphology, enabling the achievement of highly efficient NFA‐based OSCs. This review discusses the structural characteristics of NFAs and their relationship with morphology. Subsequently, the correlation between the morphology and photovoltaic parameters is introduced, which provides a fundamental basis for morphology modulation. This review then points out the major challenges of morphological characterization of NFA‐based blend films while using some conventional real‐space techniques due to low phase contrast and summarizes recently emerging characterization techniques capable of characterizing high‐contrast phase morphologies at multiple length scales. Finally, strategies for targeted morphological optimization through materials design, processing solvents, posttreatment, and ternary strategies are presented, although it is challenging to obtain an ideal morphology (e.g., appropriate phase separation and ideal donor/acceptor molecular interactions) due to the anisotropic structural characteristics of NFAs in the as‐cast films. This review is expected to provide guidance to continuously advancing the success of NFA‐based OSCs from the morphology perspective in the future.

N-Type Small Molecule Electron Transport Layers with Excellent Surface Energy and Moisture Resistance Siloxane for Non-Fullerene Organic Solar Cells.

Electron transport layers (ETLs) generally contain polar groups for enhancing performance and reducing the work function. Nevertheless, the polar group with high surface energy may cause inferior interfacial compatibility, which challenges the ETLsto balance stability and performance. Here, two conjugated small molecules of ETLs with low surface energy siloxane, namely PDI-Si and PDIN-Si, are synthesized. The siloxane with low surface energy not only enhances the interfacial compatibility between ETLs and active layers but also improves the moisture-proof stability of the device. Impressively, the amine-functionalized PDIN-Si can simultaneously exhibit conspicuous n-type self-doping properties and outstanding moisture-proof stability. The optimization of interfacial contact and morphology enables the PM6:Y6-based OSC with PDIN-Si to achieve a power conversion efficiency (PCE) of 15.87%, which is slightly superior to that of classical ETL PDINO devices (15.27%), and when the PDIN-Si film thickness reaches 28nm, the PCE remains at 13.19% (≈83%), which indicates that PDIN-Si has satisfactory thickness insensitivity to facilitate roll-to-roll processing. Excitingly, after 120 h of storage in an environment with humidity above 45%, the unencapsulated device with PDIN-Si as ETL remains at 75% of the initial PCE value, while the device with PDINO as ETL is only 50%.