Significance of thermodynamic interaction parameters in guiding the optimization of polymer:nonfullerene solar cells

Tailoring the Molecular Planarity of Perylene Diimide-Based Third Component toward Efficient Ternary Organic Solar Cells

Incorporating a third component into binary organic solar cells (b-OSCs) has provided a potential platform to boost power conversion efficiency (PCEs). However, gaining control over the non-equilibrium blend morphology via the molecular design of the perylene diimide (PDI)-based third component toward efficient ternary organic solar cells (t-OSCs) still remains challenging. Herein, two novel PDI derivatives are developed with tailored molecular planarity, namely ufBTz-2PDI and fBTz-2PDI, as the third component for t-OSCs. Notably, after performing a cyclization reaction, the twisted ufBTz-2PDI with an amorphous character transferred to the highly planar fBTz-2PDI followed by a semi-crystalline character. When incorporating the semi-crystalline fBTz-2PDI into the D18:L8-BO system, the resultant t-OSC achieved an impressive PCE of 18.56%, surpassing the 17.88% attained in b-OSCs. In comparison, the addition of amorphous ufBTz-2PDI into the binary system facilitates additional charge trap sites and results in a deteriorative PCE of 14.37%. Additionally, The third component fBTz-2PDI possesses a good generality in optimizing the PCEs of several b-OSCs systems are demonstrated. The results not only provided a novel A-DA'D-A motif for further designing efficient third component but also demonstrated the crucial role of modulated crystallinity of the PDI-based third component in optimizing PCEs of t-OSCs.

Hysteresis in phase volumes, compositions and interfacial roughness in model OPV-small-molecule/polymer thin-films

Domain morphology and composition, and the structure of interfaces between domains are key factors in the performance and stability of organic photovoltaics (OPVs) fabricated from polymer/small-molecule blends. In this study, we investigate the evolution of composition, phase-volume and interfacial roughness in model polymer/small-molecule bilayers, in response to thermal annealing. Polystyrene/fullerene mixing is studied as a function of annealing temperature, using in situ neutron reflectivity, in thin-film bilayer samples comprising pure component or mixed layers. Remarkably, we discover that thermal annealing at temperatures around or above the reported glass transition temperatures, Tg, of the components can result in extensive mass-transfer between layers, that has the superficial appearance of equilibration, but leaves the layer compositions, thicknesses, and/or the interfacial composition profile in a non-equilibrium state. This is not merely a case of slow kinetics near Tg, as subsequent heating to higher temperatures, followed by cooling, reveals pronounced hysteresis in these systems. This has important implications for the measurement of equilibrium compositions in polymer/small-molecule mixtures for OPV applications, and for device stability during operation.

Design of a Fully Non-Fused Bulk Heterojunction toward Efficient and Low-Cost Organic Photovoltaics

To modulate the miscibility between donor and acceptor materials both possessing fully non-fused ring structures, a series of electron acceptors (A4T-16, A4T-31 and A4T-32) with different polar functional substituents were synthesized and investigated. The three acceptors show good planarity, high conformational stability, complementary absorption and energy levels with the non-fused polymer donor (PTVT-BT). Among them, A4T-32 possesses the strongest polar functional group and shows the highest surface energy, which facilitates morphological modulation in the bulk heterojunction (BHJ) blend. Benefiting from the proper morphology control method, an impressive power conversion efficiency (PCE) of approaching 16.0 % and a superior fill factor over 0.795 are achieved in the PTVT-BT : A4T-32-based organic photovoltaic cells with superior photoactive materials price advantage, which represent the highest value for the cells based on the non-fused blend films. Notably, this cell maintains ≈84 % of its initial PCE after nearly 2000 h under the continuous simulated 1-sun-illumination. In addition, the flexible PTVT-BT : A4T-32-based cells were fabricated and delivered a decent PCE of 14.6 %. This work provides an effective molecular design strategy for the non-fused non-fullerene acceptors (NFAs) from the aspect of bulk morphology control in fully non-fused BHJ layers, which is crucial for their practical applications.

Wide Bandgap Conjugated Polymers Based on Difluorobenzoxadiazole for Efficient Non-Fullerene Organic Solar Cells

Wide bandgap polymers with a donor-acceptor alternating structure play a key role in constructing high-efficiency organic solar cells (OSCs). However, only a handful of high-performance polymers are available owing to the limited choices of acceptor units. 5,6-Difluorobenzo[c][1,2,5]oxadiazole (ffBX) is a promising acceptor unit with high ionization potential, and can afford high charge carrier mobility and strong aggregation for the resulting polymers. Historically, ffBX was successfully used in constructing high-performance polymer donors for fullerene-based OSCs. However, this unit is far less been explored in non-fullerene OSCs. In this work, we report three ffBX-based wide bandgap polymers (Oc00, Oc25, and Oc50) with varied solubilizing side chain content for application in non-fullerene OSCs. The polymers show matched energy levels and complementary optical absorption with the state-of-the-art non-fullerene acceptor Y6. Moreover, the polymer solubility, solid state packing, and bulk-heterojunction morphology were finely tuned via side chain engineering. Encouragingly, a decent efficiency of 14.25% was realized by the polymer Oc25 when blended with Y6 due to the efficient charge transport and favorable active layer morphology. These results suggest the promising prospect of ffBX in constructing high-performance polymer donors for non-fullerene OSCs. This article is protected by copyright. All rights reserved.

Intrinsically Stretchable Organic Solar Cells with Simultaneously Improved Mechanical Robustness and Morphological Stability Enabled by a Universal Crosslinking Strategy

Developing intrinsically stretchable organic solar cells (OSCs) with excellent mechanical robustness and long-term operation stability is highly demanded for practical applications. Here, the representative PM6/Y6 active layer film, crosslinked by a photo-crosslinkable small molecule 2,6-bis(4-azidobenzylidene)cyclohexanone (BAC) containing azide groups, exhibits a significantly enhanced stretchability of 18% and toughness of 6.94 MJ m^-3 , compared to non-crosslinked film (stretchability of 4.5% and toughness of 0.75 MJ m^-3 ). It is found that controlling the crosslinking density, including crosslinker concentration and crosslinking time, plays a vital impact on the stretchability and mechanical toughness of active layer film. The resulting intrinsically stretchable OSCs achieve a high power conversion efficiency (PCE) of 13.4% and retain 80% of its performance even under the large strain of 20%. To date, this is the highest PCE for intrinsically stretchable OSCs based on small molecular acceptors. Moreover, crosslinking of active layer film suppresses the crystallization of PM6 polymer chains and avoids the excessive aggregation of small molecular acceptors under thermal heating or light illumination, leading to a stabilized film morphology and significantly improved device stability. Overall, these results provide a universal strategy to simultaneously enhance the mechanical properties and stability of OSCs without sacrificing their photovoltaic performance.

The Importance of Nonequilibrium to Equilibrium Transition Pathways for the Efficiency and Stability of Organic Solar Cells

Controlled morphology of solution-processed thin films have realized impressive achievements for non-fullerene acceptor (NFA)-based organic solar cells (OSCs). Given the large set of donor-acceptor pairs, employing various processing conditions to realize optimal morphology for high efficiency and stable OSCs is a strenuous task. Therefore, comprehensive correlations between processing conditions and morphology evolution pathways have to be developed for efficient performance and stability of devices. Within the framework of the blend system, crystallization transitions of NFA molecules are tracked utilizing the first heating scan of differential scanning calorimeter (DSC) measurement correlating with respective morphology evolution of blend films. Real-time dynamics measurements and morphology characterizations are combined to provide optimal morphology transition pathways as NFA molecules are shown to be released from the mixed-phase to form balanced ordered packing with variant processing conditions. Polymer:NFA films are fabricated using blade coating incorporating solvent additive or thermal annealing as processing conditions as a correlation is formulated between performance and stability of solar cells with morphology transition pathways. This work demonstrates the significance of processing condition-controlled transition pathways for the realization of optimal morphology leading to superior OSC devices.

Enhanced photovoltaic performance of donor polymers effected by asymmetric π-bridges

Asymmetric π-bridge-based donor polymers produced via a simple one-pot chemical synthesis method exhibit enhanced photovoltaic performance.

Donor-Acceptor Alternating Copolymer Compatibilizers for Thermally Stable, Mechanically Robust, and High-Performance Organic Solar Cells

Small-molecule acceptor (SMA)-based organic solar cells (OSCs) have achieved high power conversion efficiencies (PCEs), while their long-term stabilities remain to be improved to meet the requirements for real applications. Herein, we demonstrate the use of donor-acceptor alternating copolymer-type compatibilizers (DACCs) in high-performance SMA-based OSCs, enhancing their PCE, thermal stability, and mechanical robustness simultaneously. Detailed experimental and computational studies reveal that the addition of DACCs to polymer donor (P_D)-SMA blends effectively reduces P_D-SMA interfacial tensions and stabilizes the interfaces, preventing the coalescence of the phase-separated domains. As a result, desired morphologies with exceptional thermal stability and mechanical robustness are obtained for the P_D-SMA blends. The addition of 20 wt % DACCs affords OSCs with a PCE of 17.1% and a cohesive fracture energy (G_c) of 0.89 J m^-2, higher than those (PCE = 13.6% and G_c = 0.35 J m^-2) for the control OSCs without DACCs. Moreover, at an elevated temperature of 120 °C, the OSCs with 20 wt % DACC exhibit excellent morphological stability, retaining over 95% of the initial PCE after 300 h. In contrast, the control OSCs without the DACC rapidly degraded to below 60% of the initial PCE after 144 h.

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.

Roles and Impacts of Ancillary Materials for Multi-Component Blend Organic Photovoltaics towards High Efficiency and Stability

Organic photovoltaics (OPVs) are a promising next-generation photovoltaic technology with great potential for wearable and transparent device applications. Over the past decades, remarkable advances in device efficiency close to 20 % have been made for bulk heterojunction (BHJ)-based OPV devices with long-term stability, and room for further improvements still exists. In recent years, ancillary components have been demonstrated as effective in improving the photovoltaic performance of OPVs by controlling the optoelectronic and morphological properties of BHJ blends. Herein, an updated understanding of polymer-based blend OPVs is provided, and the role and impact of ancillary components in various blend systems are categorized and discussed. Lastly, a strategic perspective on the ancillary components of blend-based OPVs for commercialization is provided.

Impact of Electrostatic Interaction on Bulk Morphology in Efficient Donor-Acceptor Photovoltaic Blends

Bulk heterojunctions comprising mixed donor (D) and acceptor (A) materials have proven to be the most efficient device structures for organic photovoltaic (OPV) cells. The bulk morphology of such cells plays a key role in charge generation, recombination, and transport, thus determining the device performance. Although numerous studies have discussed the morphology-performance relationship of these cells, the method of designing OPV materials with the desired morphology remains unclear. Herein, guided by molecular electrostatic potential distributions, we have established a connection between the chemical structure and bulk morphology. We show that the molecular orientation at the D-A interface and the domain purity in the blend can be effectively modulated by modifying the functional groups. Enhancing the D-A interaction is beneficial for charge generation. However, the resulting low domain purity and increased charge transfer ratio in its hybridization with the local excitation states lead to severe charge recombination. Fine-tuning the bulk morphology can give balanced charge generation and recombination, which is crucial for further boosting the efficiency of the OPV cells.

Fluorination Enables Tunable Molecular Interaction and Photovoltaic Performance in Non-Fullerene Solar Cells Based on Ester-Substituted Polythiophene

Owing to the advantages of low synthetic cost and high scalability of synthesis, polythiophene and its derivatives (PTs) have been of interest in the community of organic photovoltaics (OPVs). Nevertheless, the typical efficiency of PT based photovoltaic devices reported so far is much lower than those of the prevailing push-pull type conjugated polymer donors. Recent studies have underscored that the excessively low miscibility between PT and nonfullerene acceptor is the major reason accounting for the unfavorable active layer morphology and the inferior performance of OPVs based on a well-known PT, namely PDCBT-Cl and a non-halogenated nonfullerene acceptor IDIC. How to manipulate the miscibility between PT and acceptor molecule is important for further improving the device efficiency of this class of potentially low-cost blend systems. In this study, we introduced different numbers of F atoms to the end groups of IDIC to tune the intermolecular interaction of the hypo-miscible blend system (PDCBT-Cl:IDIC). Based on calorimetric, microscopic, and scattering characterizations, a clear relationship between the number of F atoms, miscibility, and device performance was established. With the increased number of F atoms in IDIC, the resulting acceptors exhibited enhanced miscibility with PDCBT-Cl, and the domain sizes of the blend films were reduced substantially. As a result, distinctively different photovoltaic performances were achieved for these blend systems. This study demonstrates that varying the number of F atoms in the acceptors is a feasible way to manipulate the molecular interaction and the film morphology toward high-performance polythiophene:nonfullerene based OPVs.