Efficient Organic Photovoltaics Research Articles

Phase-engineered two-dimensional MoO3/MoS2 hybrid nanostructures enable efficient indoor organic photovoltaics

Organic photovoltacis (OPVs) with MoO3/MoS2 nanostrucutres demonstrated 27.9% indoor efficiency. The TMSs nanosheets in active layers exhibit improved absorption, denser molecular packing, large mobilities, and reduced trap-assisted recombination.

Over 31% efficient indoor organic photovoltaics enabled by simultaneously reduced trap-assisted recombination and non-radiative recombination voltage loss.

Indoor organic photovoltaics (OPVs) have shown great potential application in driving low-energy-consumption electronics for the Internet of Things. There is still great room for further improving the power conversion efficiency (PCE) of indoor OPVs, considering that the desired morphology of the active layer to reduce trap-assisted recombination and voltage losses and thus simultaneously enhance the fill factor (FF) and open-circuit voltage for efficient indoor OPVs remains obscure. Herein, by optimizing the bulk and interface morphology via a layer-by-layer (LBL) processing strategy, low leakage current and low non-radiative recombination loss can be synergistically achieved in PM6:Y6-O based devices. Detailed characterizations reveal the stronger crystallinity, purer domains and ideal interfacial contacts in the LBL devices compared to their bulk-heterojunction (BHJ) counterparts. The optimized morphology yields a reduced voltage loss and an impressive FF of 81.5%, and thus contributes to a high PCE of 31.2% under a 1000 lux light-emitting diode (LED) illumination in the LBL devices, which is the best reported efficiency for indoor OPVs. Additionally, this LBL strategy exhibits great universality in promoting the performance of indoor OPVs, as exemplified by three other non-fullerene acceptor systems. This work provides guidelines for morphology optimization and synergistically promotes the fast development of efficient indoor OPVs.

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 (Mw ) is introduced to enable highly efficient all-polymer OPV cells featuring excellent mechanical reliability. By incorporating the high-Mw 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.

Semi-transparent organic photovoltaics.

As an economical solar energy conversion technology, organic photovoltaics (OPVs) are regarded as a promising solution to environmental problems and energy challenges. With the highest efficiency of OPVs exceeding 20%, the research focus will shift from efficiency-oriented aspects to commercialization-oriented aspects in the near future. Semi-transparent OPVs (STOPVs) are one of the most possible commercialized forms of OPVs, and have achieved power conversion efficiency over 14% with average visible light transmittance over 20% so far. In this tutorial review, we first systematically summarize the device structures, operating principles and evaluation parameters of STOPVs, and compare them with those of opaque OPVs. Then, strategies to construct high-performance STOPVs by cooperatively optimizing materials and devices are proposed. Methods to realize the scale-up of STOPVs in terms of minimization of electrode and interconnect resistance are summarized. The potential applications of STOPVs in multifunctional windows, agrivoltaics and floating photovoltaics are also discussed. Finally, this review highlights major challenges and research directions that need to be addressed prior to the future commercialization of STOPVs.

Aggregation state tuning via controlling molecular weights of D–A1–A2 type polymer donors for efficient organic photovoltaics

A polymer donor with a D–A1–A2 type structure was developed, whose aggregation state was tuned by molecular weight control, finally leading to over 15% power conversion efficiency.

An n‐n Heterojunction Configuration for Efficient Electron Transport in Organic Photovoltaic Devices

AbstractSelective electron transport and extraction are essential to the operation of photovoltaic devices. Electron transport layer (ETL) is therefore critical to organic photovoltaics (OPV). Herein, an ETL configuration is presented comprising a solution‐processed n‐n organic heterojunction to enhance electron transport and hole blocking, and boost power conversion efficiency (PCE) in OPV. Specifically, the n‐n heterojunction is constructed by stacking a narrow‐band n‐type conjugated polymer layer (PNDIT‐F3N) and a wide‐band n‐type conjugated molecule layer (Phen‐NaDPO). Based on the ultraviolet photoelectron spectroscopy measurement and numerical simulation of current density‐voltage characteristics, the formation of the built‐in potential is investigated. In three OPVs with different active layers, substantial improvements are observed in performance following the introduction of this ETL configuration. The performance enhancement arises from the combination of selective carrier transport properties and reduced recombination. Another contributing factor is the good film‐forming quality of the new ETL configuration, where the surface energies of the related materials are well‐matched. The n‐n organic heterojunction represents a viable and promising ETL construction strategy for efficient OPV devices.

Halogen-Free -Conjugated Polymers Based on Thienobenzobisthiazole for Efficient Nonfullerene Organic Solar Cells: Rational Design for Achieving High Backbone Order and High Solubility.

In -conjugated polymers, a highly ordered backbone structure and solubility are always in a trade-off relationship that must be overcome to realize highly efficient and solution-processable organic photovoltaics (OPVs). Here, it is shown that a -conjugated polymer based on a novel thiazole-fused ring, thieno[2',3':5,6]benzo[1,2-d:4,3-d']bisthiazole (TBTz) achieves both high backbone order and high solubility due to the structural feature of TBTz such as the noncovalent interlocking of the thiazole moiety, the rigid and bent-shaped structure, and the fused alkylthiophene ring. Furthermore, based on the electron-deficient nature of these thiazole-fused rings, the polymer exhibits deep HOMO energy levels, which lead to high open-circuit voltages (VOC s) in OPV cells, even without halogen substituents that are commonly introduced into high-performance polymers. As a result, when the polymer is combined with a typical nonfullerene acceptor Y6, power conversion efficiencies of reaching 16% and VOC s of more than 0.84V are observed, both of which are among the top values reported so far for "halogen-free" polymers. This study will serve as an important reference for designing -conjugated polymers to achieve highly efficient and solution-processable OPVs.

High-performing organic electronics using terpene green solvents from renewable feedstocks

Accelerating the shift towards renewable materials and sustainable processes for printed organic electronic devices is crucial for a green circular economy. Currently, the fabrication of organic devices with competitive performances is linked to toxic petrochemical-based solvents with considerable carbon emissions. Here we show that terpene solvents obtained from renewable feedstocks can replace non-renewable environmentally hazardous solvent counterparts in the production of highly efficient organic photovoltaics (OPVs) light-emitting diodes (OLEDs) and field-effect transistors (OFETs) with on-par performances. Using a Hansen solubility ink formulation framework, we identify various terpene solvent systems and investigate effective film formation and drying mechanisms required for optimal charge transport. This approach is universal for state-of-the-art materials in OPVs, OLEDs and OFETs. We created an interactive library for green solvent selections and made it publicly available through the OMEGALab website. As potential carbon-negative solvents, terpenes open a unique and universal approach towards efficient, large-area and stable organic electronic devices.

Novel structural feature-descriptor platform for machine learning to accelerate the development of organic photovoltaics

Although the evolution of organic photovoltaics (OPVs) has been remarkable, the discovery of new combinations of materials for ideal bulk heterojunction (BHJ) blend systems remains challenging because of their numerous considerations. Aiming to eliminate the typically labor-intensive trial-and-error approaches, machine learning (ML) models coupled with quantitative structure-property relationships are constructed to efficiently predict the photovoltaic parameters. To this end, a unique structural-feature descriptor that fragments a molecular structure into functional group (FG) units is designed. These units are indexed and the indices are combined to translate the molecular structure into a concise matrix of integers. Our novel descriptor allows the ML models to trace the FGs with ease and assign larger weights to those FGs that significantly contribute to the photovoltaic performance. Furthermore, the descriptor enables the expansion of OPV ML to ternary or multicomponent BHJ systems because of its concise expression. The proposed ML model achieves a high correlation coefficient of 0.86 between experimental and predictive power conversion efficiency of OPVs, despite using only the molecular structure information without any additional input data. Combined with the superior prediction capabilities, our novel approach promises to perfectly screen numerous BHJ cadidates, resulting in accelerating the development of the OPV.

Singlet-Triplet Energy Gap as a Critical Molecular Descriptor for Predicting Organic Photovoltaic Efficiency.

In contrast to the inorganic and perovskite solar cells, organic photovoltaics (OPV) depend on a series of charge generation and recombination processes, which complicates molecular design to improve the power conversion efficiencies (PCEs). Herein, we first propose the singlet-triplet energy gap (ΔEST ) as a critical molecular descriptor for predicting the PCE considering that minimizing ΔEST is beneficial to simultaneously reduce voltage loss and triplet recombination. Remarkably, the results from data-driven machine learning verify that the prediction accuracy of the ΔEST (Pearson's correlation coefficient r=0.72) is apparently superior to that of two commonly used molecular descriptors in OPV, i.e., the optical gap (r=0.65) and the driving force (r=0.53). Moreover, an impressive prediction accuracy of r=0.81 is achieved just by combining the three descriptors. This work paves the way toward rapid and precise screening of efficient OPV materials.

Singlet‐Triplet Energy Gap as a Critical Molecular Descriptor for Predicting Organic Photovoltaic Efficiency

AbstractIn contrast to the inorganic and perovskite solar cells, organic photovoltaics (OPV) depend on a series of charge generation and recombination processes, which complicates molecular design to improve the power conversion efficiencies (PCEs). Herein, we first propose the singlet‐triplet energy gap (ΔEST) as a critical molecular descriptor for predicting the PCE considering that minimizing ΔEST is beneficial to simultaneously reduce voltage loss and triplet recombination. Remarkably, the results from data‐driven machine learning verify that the prediction accuracy of the ΔEST (Pearson's correlation coefficient r=0.72) is apparently superior to that of two commonly used molecular descriptors in OPV, i.e., the optical gap (r=0.65) and the driving force (r=0.53). Moreover, an impressive prediction accuracy of r=0.81 is achieved just by combining the three descriptors. This work paves the way toward rapid and precise screening of efficient OPV materials.

A near-infrared acceptor incorporating selenium heterocycles for efficient semi-transparent photovoltaics and sensitive photodetectors

Near infrared (NIR) acceptors are the key components for the construction of efficient organic photovoltaics (OPVs) and organic photodetectors (OPDs). Herein, a near-infrared acceptor named 6TSe-OFIC incorporating selenium heterocycles has been designed, which shows an absorption onset around 1000 nm with a low optical bandgap of 1.29 eV. OPV device based on PCE10: 6TSe-OFIC demonstrates a power conversion efficiency (PCE) of 13.18% with a high and broad EQE response. The semi-transparent OPV device based on PCE10: 6TSe-OFIC shows a PCE of 10.06% with AVT 20%. With the same active layer, the OPD devices achieve the responsivities of 0.54 A W −1 at ∼850 nm. The results highlight the opportunities to careful design of the NIR molecules for efficient OPVs and OPDs. • A NIR acceptor 6TSe-OFIC incorporating Se in the molecular backbone has designed. • The semi-transparent device of PCE10:6TSe-OFIC achieves a PCE of 10.06% with an AVT over 20% and an outstanding CRI of 91. • The self-powered OPD device of PCE10:6TSe-OFIC demonstrates the responsivities of 0.54 A W −1 at ∼850 nm and a D* of over 10 12 jones from 400 to 960 nm. • The design idea can be a useful guideline for developing high efficiency semi-transparent OPV and sensitive OPD.

Multiphase Morphology with Enhanced Carrier Lifetime via Quaternary Strategy Enables High-Efficiency, Thick-Film, and Large-Area Organic Photovoltaics.

With the continuous breakthrough of the efficiency oforganic photovoltaics (OPVs), their practical applications are on the agenda. However, the thickness tolerance and upscaling in recently reported high-efficiency devices remains challenging. In this work, the multiphase morphology and desired carrier behaviors are realized by utilizing a quaternary strategy. Notably, the exciton separation, carrier mobility, and carrier lifetime are enhanced significantly, the carrier recombination and the energy loss (Eloss ) are reduced, thus beneficial for a higher short-circuit density (JSC ), fill factor (FF), and open-circuit voltage (VOC ) of the quaternary system. Moreover, the intermixing-phase size is optimized, which is favorable for constructing the thick-film and large-area devices. Finally, the device with a 110nm-thick active layer shows an outstanding power conversion efficiency (PCE) of 19.32% (certified 19.35%). Furthermore, the large-area (1.05 and 72.25 cm2 ) devices with 110nm thickness present PCEs of 18.25% and 12.20%, and the device with a 305nm-thick film (0.0473 cm2 ) delivers a PCE of 17.55%, which are among the highest values reported. The work demonstrates the potential of the quaternary strategy for large-area and thick-film OPVs and promotes the practical application of OPVs in the future.

Study of low-temperature sol–gel processed In-doped ZnO for organic photovoltaics

This article studies low-temperature sol–gel processed indium (In)-doped ZnO (IZO) for highly efficient organic photovoltaics (OPVs). Contrary to the prior research trends adopting doped sol–gel processed ZnO with an annealing temperature of over 400 °C for the hydrolysis reaction, IZO with an annealing temperature of 200 °C is studied. Similar to the high-temperature solvent system, it is elucidated that low-temperature sol–gel processed IZO effectively improves the performance of OPVs, increasing the power conversion efficiency from 6.80% to 7.35%. For further analyses, the current–voltage (J–V) characteristics and ideality factors (n) are examined as a function of In doping ratios, which revealed that In doping on ZnO effectively reduces trap-assisted recombination within devices.

Manipulating Charge Transfer and Transport via Intermediary Electron Acceptor Channels Enables 19.3% Efficiency Organic Photovoltaics

AbstractBalancing and improving the open‐circuit voltage (Voc) and short‐circuit current density (Jsc) synergistically has always been the critical point for organic photovoltaics (OPVs) to achieve high efficiencies. Here, this work adopts a ternary strategy to regulate the trade‐off between Voc and Jsc by combining the symmetric‐asymmetric non‐fullerene acceptors that differ at terminals and alkyl side chains to build the ternary OPV (TOPV). It is noticed that the reduced energy disorder and the enhanced luminescence efficiency of TOPV enable a mitigated energy loss and a higher Voc. Meanwhile, the third component, which is distributed at the host donor–acceptor interface, acts as the charge transport channel. The prolonged exciton lifetime, the boosted charge mobility, and the depressed charge recombination promote the TOPV to obtain an improved Jsc. Finally, with synergistically improved Voc and Jsc, the TOPV delivers an optimal efficiency of 19.26% (certified as 19.12%), representing one of the highest values reported so far.

Even a little delocalization produces large kinetic enhancements of charge-separation efficiency in organic photovoltaics.

In organic photovoltaics, charges can separate efficiently even if their Coulomb attraction is an order of magnitude greater than the available thermal energy. Delocalization has been suggested to explain this fact, because it could increase the initial separation of charges in the charge-transfer (CT) state, reducing their attraction. However, understanding the mechanism requires a kinetic model of delocalized charge separation, which has proven difficult because it involves tracking the correlated quantum-mechanical motion of the electron and the hole in large simulation boxes required for disordered materials. Here, we report the first three-dimensional simulations of charge-separation dynamics in the presence of disorder, delocalization, and polaron formation, finding that even slight delocalization, across less than two molecules, can substantially enhance the charge-separation efficiency, even starting with thermalized CT states. Delocalization does not enhance efficiency by reducing the Coulomb attraction; instead, the enhancement is a kinetic effect produced by the increased overlap of electronic states.

Achieving and Understanding of Highly Efficient Ternary Organic Photovoltaics: From Morphology and Energy Loss to Working Mechanism.

Ternary strategy, adding an additional donor (D) or acceptor (A) into conventional binary D:A blend, has shown great potential in improving photovoltaic performances of organic photovoltaics (OPVs) for practical applications. Herein, this review is presented on how efficient ternary OPVs are realized from the aspects of morphology, energy loss, and working mechanism. As to morphology, the role of third component on the formation of preferred alloy-like-phase and vertical-phase, which are driven by the miscibility tuning, is discussed. For energy loss, the effect of the third component on the luminescence enhancement and energetic disordering suppression, which lead to favorable increase of voltage, is presented. Regarding working mechanism, dilution effect and relationships between two acceptors or donor/acceptor, which explain the observed device parameters variations, are analyzed. Finally, some future directions concerning ternary OPVs are pointed out. Therefore, this review can provide a comprehensive understanding of working principles and effective routes for high-efficiency ternary systems, advancing the commercialization of OPVs.

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.

The Intrinsic Role of the Fusion Mode and Electron-Deficient Core in Fused-Ring Electron Acceptors for Organic Photovoltaics.

The A-DA'D-A fused-ring electron acceptors with an angular fusion mode and electron-deficient core has significantly boosted organic photovoltaic efficiency. Here, the intrinsic role of the peculiar structure is revealed by comparing representative A-DA'D-A acceptor Y6 with its A-D-A counterparts having different fusion modes. Owing to the more delocalized HOMO and deeper LUMO level, Y6 exhibits stronger and red-shifted absorption relative to the linear and angular fused A-D-A acceptors, respectively. Moreover, the change from linear to angular fusion substantially reduces the electron-vibration couplings, which is responsible for the faster exciton diffusion, exciton dissociation, and electron transport for Y6 than the linear fused A-D-A acceptor. Notably, the electron-vibration coupling for exciton dissociation is further decreased by introducing the electron-deficient core, thus contributing to the efficient charge generation under low driving forces in the Y6-based devices.

The Intrinsic Role of the Fusion Mode and Electron‐Deficient Core in Fused‐Ring Electron Acceptors for Organic Photovoltaics

AbstractThe A‐DA′D‐A fused‐ring electron acceptors with an angular fusion mode and electron‐deficient core has significantly boosted organic photovoltaic efficiency. Here, the intrinsic role of the peculiar structure is revealed by comparing representative A‐DA′D‐A acceptor Y6 with its A‐D‐A counterparts having different fusion modes. Owing to the more delocalized HOMO and deeper LUMO level, Y6 exhibits stronger and red‐shifted absorption relative to the linear and angular fused A‐D‐A acceptors, respectively. Moreover, the change from linear to angular fusion substantially reduces the electron‐vibration couplings, which is responsible for the faster exciton diffusion, exciton dissociation, and electron transport for Y6 than the linear fused A‐D‐A acceptor. Notably, the electron‐vibration coupling for exciton dissociation is further decreased by introducing the electron‐deficient core, thus contributing to the efficient charge generation under low driving forces in the Y6‐based devices.