Achieving Record-High Stretchability and Mechanical Stability in Organic Photovoltaic Blends with a Dilute-absorber Strategy

Organic solar cells (OSCs) have potential for applications in wearable electronics. Except for high power conversion efficiency (PCE), excellent tensile properties and mechanical stability are required for achieving high-performance wearable OSCs, while the present metrics barely meet the stretchable requirements. Herein, this work proposes a facile and low-cost strategy for constructing intrinsically stretchable OSCs by introducing a readily accessible polymer elastomer as a diluent for all-polymer photovoltaic blends. Remarkably, record-high stretchability with a fracture strain of up to 1000% and mechanical stability with elastic recovery >90% under cyclic tensile tests are realized in the OSCs active layers for the first time. Specifically, the tensile properties of best-performing all-polymer photovoltaic blends are increased by up to 250 times after blending. Previously unattainable performance metrics (fracture strain >50% and PCE >10%) are achieved simultaneously for the resulting photovoltaic films. Furthermore, an overall evaluation parameter y is proposed for the efficiency-cost- stretchability balance of photovoltaic blend films. The y value of dilute-absorber system is two orders of magnitude greater than those of prior state-of-the-art systems. Additionally, intrinsically stretchable devices are prepared to showcase the mechanical stability. Overall, this work offers a new avenue for constructing and comprehensively evaluating intrinsically stretchable organic electronic films.

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

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

High-Efficiency and Mechanically Robust All-Polymer Organic Photovoltaic Cells Enabled by Optimized Fibril Network Morphology

All-polymer organic photovoltaic (OPV) cells possessing high photovoltaic performance and mechanical robustness are promising candidates for flexible wearable devices. However, developing photoactive materials with good mechanical properties and photovoltaic performance so far remains challenging. In this work, a polymer donor PBDB-TF with a high weight-average molecular weight (M_w ) is introduced to enable highly efficient all-polymer OPV cells featuring excellent mechanical reliability. By incorporating the high-M_w PBDB-TF as a third component into the PBQx-TF:PY-IT blend, the bulk heterojunction morphology is finely tuned with a more compact π-π stacking distance, affording efficient pathways for charge transport as well as mechanical stress dissipation. Hence, all-polymer OPV cells based on the ternary blend film demonstrate a maximum power conversion efficiency (PCE) of 18.2% with an outstanding fill factor of 0.796. The flexible OPV cell delivers a decent PCE of 16.5% with high mechanical stability. These results present a promising strategy to address the mechanical properties and boost the photovoltaic performance of all-polymer OPV cells.

Organic and quantum dot hybrid photodetectors: towards full-band and fast detection

Photodetectors hold great application potential in many fields such as image sensing, night vision, infrared communication and health monitoring. To date, commercial photodetectors mainly rely on inorganic semiconductors, e.g., monocrystalline silicon, germanium, and indium selenide/gallium with complex and costly fabrication, which are hardly compatible with wearable electronics. In contrast, organic conjugated materials provide great superiority in flexibility and stretchability. In this Highlight, the unique properties of organic and quantum dot photodetectors were firstly discussed to reveal the great complementarity of the two technologies. Subsequently, the recent advance of organic/quantum dot hybrid photodetectors was outlined to highlight their great potential in developing broadband and high-performance photodetectors. Moreover, the multiple functions (e.g., dual-band detection and upconversion detection) of hybrid photodetectors were highlighted for their promising application in image sensing and infrared detection. Lastly, we present a forword-looking discussion on the challenges and our insights for the further advancement of hybrid photodetectors. This work may spark enormous research attention in organic/quantum dot electronics and advance the commercial applications.

Unraveling the Stretch-Induced Microstructural Evolution and Morphology-Stretchability Relationships of High-Performance Ternary Organic Photovoltaic Blends

The stretchability and stretch-induced structural evolution of organic solar cells (OSCs) are pivotal for their collapsible, portable, and wearable applications, and they are mainly affected by the complex morphology of active layers. Herein, a highly ductile conjugated polymer P(NDI2OD-T2) is incorporated into the active layers of high-efficiency OSCs based on nonfullerene small molecule acceptors to simultaneously investigate the morphological, mechanical, and photovoltaic properties and structural evolution under stretching of ternary blend films with various acceptor contents. The structural robustness of the blend films is indicated by their stretch-induced structural evolution, which is monitored in real-time by a combination of in situ wide/small angle X-ray scattering. It is found that adding the soft P(NDI2OD-T2) can enhance the stretchability and structural robustness of ternary blend films by more entangled chains and tie chains to dissipate strain. Furthermore, the stretchability of the ternary blends can be superbly predicted by a 3D equivalent box model. This work provides instructive insight and guidance for designing stretchable electronics and predicting the stretchability of multicomponent blends.

A New Polymer Donor Enables Binary All-Polymer Organic Photovoltaic Cells with 18% Efficiency and Excellent Mechanical Robustness

The development of polymerized small-molecule acceptors has boosted the power conversion efficiencies (PCEs) of all-polymer organic photovoltaic (OPV) cells to 17%. However, the polymer donors suitable for all-polymer OPV cells are still lacking, restricting the further improvement of their PCEs. Herein, a new polymer donor named PQM-Cl is designed and its photovoltaic performance is explored. The negative electrostatic potential and low average local ionization energy distribution of the PQM-Cl surface enable efficient charge generation and transfer process. When blending with a well-used polymer acceptor, PY-IT, the PQM-Cl-based devices deliver an impressive PCE of 18.0% with a superior fill factor of 80.7%, both of which are the highest values for all-polymer OPV cells. The relevant measurements demonstrate that PQM-Cl-based films possess excellent mechanical and flexible properties. As such, PQM-Cl-based flexible photovoltaic cells are fabricated and an excellent PCE of 16.5% with high mechanical stability is displayed. These results demonstrate that PQM-Cl is a potential candidate for all-polymer OPV cells and provide insights into the design of polymer donors for high-efficient all-polymer OPV cells.

Unraveling the Photovoltaic, Mechanical, and Microstructural Properties and Their Correlations in Simple Poly(3-pentylthiophene) Solar Cells

The power conversion efficiency of polythiophene organic solar cells is constantly refreshed. Despite the renewed device efficiency, very few efforts have been devoted to understanding how the type of electron acceptor alters the photovoltaic and mechanical properties of these low-cost solar cells. Herein, we conduct a thorough investigation of photovoltaic and mechanical characteristics of a simple yet less explored polythiophene, namely poly(3-pentylthiophene) (P3PT), in three different types of organic solar cells, where ZY-4Cl, PC₇₁ BM, and N2200 are employed as three representative acceptors, respectively. Compared with the reference P3HT-based solar cells, P3PT-based devices all perform more efficiently. Particularly, the P3PT:ZY-4Cl blend exhibits the highest efficiency (nearly 10%) among the six combinations and outperforms the prior top-performance system P3HT:ZY-4Cl. Furthermore, the blend films based on N2200 exhibit a high crack-onset strain of ∼38% on average, which is approximately 15 and 17 times higher than those of ZY-4Cl and PC₇₁ BM, respectively. The microstructural origins for the above difference are well elucidated by detailed grazing incidence X-ray scattering and microscopy analysis. This work not only underlines the potential of P3PT in prolific solar cell research but also demonstrates the superior tensile properties of polythiophene-based all-polymer blends for the preparation of stretchable solar cells. This article is protected by copyright. All rights reserved.

An Aggregation-Suppressed Polymer Blending Strategy Enables High-Performance Organic and Quantum Dot Hybrid Solar Cells

Solution-processing hybrid solar cells with organics and colloidal quantum dots (CQDs) have drawn substantial attention in the past decade. Nevertheless, hybrid solar cells based on the recently developed directly synthesized CQD inks are still unexplored. Herein, a facile polymer blending strategy is put forward to enable directly synthesized CQD/polymer hybrid solar cells with a champion efficiency of 13%, taking advantage of the conjugated polymer blends with finely optimized aggregation behaviors. The spectroscopic and electrical investigations on carrier transport and recombination indicate that polymer blends can endow fast carrier transport and less recombination over the single counterparts. Moreover, the blending strategy offers a "dilution effect" for top-notch photovoltaic polymers with excessively strong aggregation tendency, resulting in moderate feature domain size and surface roughness, which afford fast hole transport and therefore high photovoltaic performance. The effectiveness of this strategy is successfully validated using two pairs of photovoltaic polymers. Accordingly, the relationships between polymer morphology, carrier transport, and photovoltaic performance are established to advance the progress of CQD/polymer hybrid solar cells. Such progress stresses that the utilization of aggregation-suppressed polymer blends is a facile approach toward the fabrication of high-efficiency organic-inorganic hybrid solar cells.

Thermoplastic Elastomer Tunes Phase Structure and Promotes Stretchability of High-Efficiency Organic Solar Cells

Top-performance organic solar cells (OSCs) consisting of conjugated polymer donors and nonfullerene small molecule acceptors (NF-SMAs) deliver rapid increases in efficiencies. Nevertheless, many of the polymer donors exhibit high stiffness and small molecule acceptors are very brittle, which limit their applications in wearable devices. Here, a simple and effective strategy is reported to improve the stretchability and reduce the stiffness of high-efficiency polymer:NF-SMA blends and simultaneously maintain the high efficiency by incorporating a low-cost commercial thermoplastic elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS). The microstructure, mechanical properties, and photovoltaic performance of PM6:N3 with varied SEBS contents and the molecular weight dependence of SEBS on microstructure and mechanical properties are thoroughly characterized. This strategy for mechanical performance improvement exhibits excellent applicability in some other OSC blend systems, e.g., PBQx-TF:eC9-2Cl and PBDB-T:ITIC. More crucially, the elastic modulus of such complex ternary blends can be nicely predicted by a mechanical model. Therefore, incorporating thermoplastic elastomers is a widely applicable and cost-effective strategy to improve mechanical properties of nonfullerene OSCs and beyond.

Open-Circuit Voltage Loss in Lead Chalcogenide Quantum Dot Solar Cells

Lead chalcogenide colloidal quantum dot solar cells (CQDSCs) have received considerable attention due to their broad and tunable absorption and high stability. Presently, lead chalcogenide CQDSC has achieved a power conversion efficiency of ≈14%. However, the state-of-the-art lead chalcogenide CQDSC still has an open-circuit voltage (V_oc ) loss of ≈0.45 V, which is significantly higher than those of c-Si and perovskite solar cells. Such high V_oc loss severely limits the performance improvement and commercialization of lead chalcogenide CQDSCs. In this review, the V_oc loss is first analyzed via detailed balance theory and the origin of V_oc loss from both solar absorber and interface is summarized. Subsequently, various strategies for improving the V_oc from the solar absorber, including the passivation strategies during the synthesis and ligand exchange are overviewed. The great impact of the ligand exchange process on CQD passivation is highlighted and the corresponding strategies to further reduce the V_oc loss are summarized. Finally, various strategies are discussed to reduce interface V_oc loss from charge transport layers. More importantly, the great potential of achieving performance breakthroughs via various organic hole transport layers is highlighted and the existing challenges toward commercialization are discussed.

Organic photovoltaic cell with 17% efficiency and superior processability

The development of organic photoactive materials, especially the newly emerging non-fullerene electron acceptors (NFAs), has enabled rapid progress in organic photovoltaic (OPV) cells in recent years. Although the power conversion efficiencies (PCEs) of the top-performance OPV cells have surpassed 16%, the devices are usually fabricated via a spin-coating method and are not suitable for large-area production. Here, we demonstrate that the fine-modification of the flexible side chains of NFAs can yield 17% PCE for OPV cells. More crucially, as the optimal NFA has a suitable solubility and thus a desirable morphology, the high efficiencies of spin-coated devices can be maintained when using scalable blade-coating processing technology. Our results suggest that optimization of the chemical structures of the OPV materials can improve device performance. This has great significance in larger-area production technologies that provide important scientific insights for the commercialization of OPV cells.

Single-Junction Organic Photovoltaic Cells with Approaching 18% Efficiency

Optimizing the molecular structures of organic photovoltaic (OPV) materials is one of the most effective methods to boost power conversion efficiencies (PCEs). For an excellent molecular system with a certain conjugated skeleton, fine tuning the alky chains is of considerable significance to fully explore its photovoltaic potential. In this work, the optimization of alkyl chains is performed on a chlorinated nonfullerene acceptor (NFA) named BTP-4Cl-BO (a Y6 derivative) and very impressive photovoltaic parameters in OPV cells are obtained. To get more ordered intermolecular packing, the n-undecyl is shortened at the edge of BTP-eC11 to n-nonyl and n-heptyl. As a result, the NFAs of BTP-eC9 and BTP-eC7 are synthesized. The BTP-eC7 shows relatively poor solubility and thus limits its application in device fabrication. Fortunately, the BTP-eC9 possesses good solubility and, at the same time, enhanced electron transport property than BTP-eC11. Significantly, due to the simultaneously enhanced short-circuit current density and fill factor, the BTP-eC9-based single-junction OPV cells record a maximum PCE of 17.8% and get a certified value of 17.3%. These results demonstrate that minimizing the alkyl chains to get suitable solubility and enhanced intermolecular packing has a great potential in further improving its photovoltaic performance.

Increased conjugated backbone twisting to improve carbonylated-functionalized polymer photovoltaic performance

Two conjugated polymers containing different linkers were synthesized to study their photovoltaic performances.

1 cm2 Organic Photovoltaic Cells for Indoor Application with over 20% Efficiency

Organic photovoltaic (OPV) technologies have the advantages of fabricating larger-area and light-weight solar panels on flexible substrates by low-cost roll-to-toll production. Recently, OPV cells have achieved many significant advances with power conversion efficiency (PCE) increasing rapidly. However, large-scale solar farms using OPV modules still face great challenges, such as device stability. Herein, the applications of OPV cells in indoor light environments are studied. Via optimizing the active layers to have a good match with the indoor light source, 1 cm² OPV cells are fabricated and a top PCE of 22% under 1000 lux light-emitting diode (2700 K) illumination is demonstrated. In this work, the light intensities are measured carefully. Incorporated with the external quantum efficiency and photon flux spectrum, the integral current densities of the cells are calculated to confirm the reliability of the photovoltaic measurement. In addition, the devices show much better stability under continuous indoor light illumination. The results suggest that designing wide-bandgap active materials to meet the requirements for the indoor OPV cells has a great potential in achieving higher photovoltaic performance.

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

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

Over 16% efficiency organic photovoltaic cells enabled by a chlorinated acceptor with increased open-circuit voltages

Broadening the optical absorption of organic photovoltaic (OPV) materials by enhancing the intramolecular push-pull effect is a general and effective method to improve the power conversion efficiencies of OPV cells. However, in terms of the electron acceptors, the most common molecular design strategy of halogenation usually results in down-shifted molecular energy levels, thereby leading to decreased open-circuit voltages in the devices. Herein, we report a chlorinated non-fullerene acceptor, which exhibits an extended optical absorption and meanwhile displays a higher voltage than its fluorinated counterpart in the devices. This unexpected phenomenon can be ascribed to the reduced non-radiative energy loss (0.206 eV). Due to the simultaneously improved short-circuit current density and open-circuit voltage, a high efficiency of 16.5% is achieved. This study demonstrates that finely tuning the OPV materials to reduce the bandgap-voltage offset has great potential for boosting the efficiency.

Halogenation has proved an effective strategy to improve the power conversion efficiencies of organic solar cells but it usually leads to lower open-circuit voltages. Here, Cui et al. unexpectedly obtain higher open-circuit voltages and achieve a record high PCE of 16.5% by chlorination.

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

Ternary blending and copolymerization strategies have proven advantageous in boosting the photovoltaic performance of organic solar cells. Here, 15% efficiency solar cells using copolymerization donors are demonstrated, where the electron-withdrawing unit, ester-substituted thiophene, is incorporated into a PBDB-TF polymer to downshift the molecular energy and broaden the absorption. Copolymer-based solar cells suitable for large-area devices can be fabricated by a blade-coating method from a nonhalogen and nonaromatic solvent mixture. Although ternary solar cells can achieve comparable efficiencies, they are not suitable for environment-friendly processing conditions and show relatively low photostability compared to copolymer-based devices. These results not only demonstrate high-efficiency organic photovoltaic cells via copolymerization strategies but also provide important insights into their applications in practical production.