Intermolecular Interaction Control Enables Co-optimization of Efficiency, Deformability, Mechanical and Thermal Stability of Stretchable Organic Solar Cells

Intrinsically stretchable organic photovoltaics by redistributing strain to PEDOT:PSS with enhanced stretchability and interfacial adhesion

Intrinsically stretchable organic photovoltaics have emerged as a prominent candidate for the next-generation wearable power generators regarding their structural design flexibility, omnidirectional stretchability, and in-plane deformability. However, formulating strategies to fabricate intrinsically stretchable organic photovoltaics that exhibit mechanical robustness under both repetitive strain cycles and high tensile strains remains challenging. Herein, we demonstrate high-performance intrinsically stretchable organic photovoltaics with an initial power conversion efficiency of 14.2%, exceptional stretchability (80% of the initial power conversion efficiency maintained at 52% tensile strain), and cyclic mechanical durability (95% of the initial power conversion efficiency retained after 100 strain cycles at 10%). The stretchability is primarily realised by delocalising and redistributing the strain in the active layer to a highly stretchable PEDOT:PSS electrode developed with a straightforward incorporation of ION E, which simultaneously enhances the stretchability of PEDOT:PSS itself and meanwhile reinforces the interfacial adhesion with the polyurethane substrate. Both enhancements are pivotal factors ensuring the excellent mechanical durability of the PEDOT:PSS electrode, which further effectively delays the crack initiation and propagation in the top active layer, and enables the limited performance degradation under high tensile strains and repetitive strain cycles.

Advances in Stretchable Organic Photovoltaics: Flexible Transparent Electrodes and Deformable Active Layer Design

Stretchable organic photovoltaics (OPVs) have attracted significant attention as promising power sources for wearable electronic systems owing to their superior robustness under repetitive tensile strains and their good compatibility. However, reconciling a high power-conversion efficiency and a reasonable flexibility is a tremendous challenge. In addition, the development of stretchable OPVs must be accelerated to satisfy the increasing requirements of niche markets for mechanical robustness. Stretchable OPV devices can be classified as either structurally or intrinsically stretchable. This work reviews recent advances in stretchable OPVs, including the design of mechanically robust transparent electrodes, photovoltaic materials, and devices. Initially, an overview of the characteristics and recent research progress in the areas of structurally and intrinsically stretchable OPVs is provided. Subsequently, research into flexible and stretchable transparent electrodes that directly affect the performances of stretchable OPVs is summarized and analyzed. Overall, this review aims to provide an in-depth understanding of the intrinsic properties of highly efficient and deformable active materials, while also emphasizing advanced strategies for simultaneously improving the photovoltaic performance and mechanical flexibility of the active layer, including material design, multi-component settings, and structural optimization.

Stability of organic solar cells: toward commercial applications

Organic solar cells (OSCs) have attracted a great deal of attention in the field of clean solar energy due to their advantages of transparency, flexibility, low cost and light weight. Introducing them to the market enables seamless integration into buildings and windows, while also supporting wearable, portable electronics and internet-of-things (IoT) devices. With the development of photovoltaic materials and the optimization of fabrication technology, the power conversion efficiencies (PCEs) of OSCs have rapidly improved and now exceed 20%. However, there is a significant lack of focus on material stability and device lifetime, causing a severe hindrance to commercial applications. In this review, we carefully review important strategies employed to improve the stability of OSCs over the past three years from the perspectives of material design and device engineering. Furthermore, we analyze and discuss the current important progress in terms of air, light, thermal and mechanical stability. Finally, we propose the future research directions to overcome the challenges in achieving highly stable OSCs. We expect that this review will contribute to solving the stability problem of OSCs, eventually paving the way for commercial applications in the near future.

Intermolecular Interactions, Morphology, and Photovoltaic Patterns in p-i-n Heterojunction Solar Cells With Fluorine-Substituted Organic Photovoltaic Materials

During the layer-by-layer (LBL) processing of polymer solar cells (PSCs), the swelling and molecule interdiffusion are essential for achieving precise, controllable vertical morphology, and thus efficient PSCs. However, the influencing mechanism of material properties on morphology and correlated device performance has not been paid much attention. Herein, a series of fluorinated/non-fluorinated polymer donors (PBDB-T and PBDB-TF) and non-fullerene acceptors (ITIC, IT-2F, and IT-4F) are employed to investigate the performance of LBL devices. The impacts of fluorine substitution on the repulsion and miscibility between the donor and acceptor, as well as the molecular arrangement of the donor/acceptor and the vertical distribution of the LBL devices are systematically explored by the measurement of donor/acceptor Flory-Huggins interaction parameters, spectroscopic ellipsometry, and neutron reflectivity, respectively. With efficient charge transfer due to the ideal vertical and horizon morphology properties, devices based on PBDB-TF/IT-4F exhibit the highest fill factors (FFs) as well as champion power conversion efficiencies (PCEs). With this guidance, high-performance LBL devices with PCE of 17.2%, 18.5%, and 19.1% are obtained by the fluorinated blend of PBDB-TF/Y6, PBDB-TF/L8-BO, and D18/L8-BO respectively.

Unveiling the Morphological and Physical Mechanism of Burn-in Loss Alleviation by Ternary Matrix Toward Stable and Efficient All-Polymer Solar Cells

All-polymer solar cells (All-PSCs) are considered the most promising candidate in achieving both efficient and stable organic photovoltaic devices, yet the field has rarely presented an in-depth understanding of corresponding device stability while efficiency is continuously boosted via the innovation of polymer acceptors. Herein, a ternary matrix is built for all-PSCs with optimized morphology, improved film ductility and importantly, boosted efficiency and better operational stability than its parental binary counterparts, as a platform to study the underlying mechanism. The target system PQM-Cl:PTQ10:PY-IT (0.8:0.2:1.2) exhibits an alleviated burn-in loss of morphology and efficiency under light soaking, which supports its promoted device lifetime. The comprehensive characterizations of fresh and light-soaked active layers lead to a clear illustration of opposite morphological and physical degradation direction of PQM-Cl and PTQ10, thus resulting in a delicate balance at the optimal ternary system. Specifically, the enlarging tendency of PQM-Cl and shrinking preference of PTQ10 in terms of phase separation leads to a stable morphology in their mixing phase; the hole transfer kinetics of PQM-Cl:PY-IT host is stabilized by incorporating PTQ10. This work succeeds in reaching a deep insight into all-PSC's stability promotion by a rational ternary design, which booms the prospect of gaining high-performance all-PSCs.

Sequentially Coated Wavy Nanowire Composite Transparent Electrode for Stretchable Solar Cells

Recent advances in fabricating stretchable and transparent electrodes have led to various techniques for establishing next-generation form-factor optoelectronic devices. Wavy Ag nanowire networks with large curvature radii are promising platforms as stretchable and transparent electrodes due to their high electrical conductivity and stretchability even at very high transparency. However, there are disadvantages such as intrinsic nonregular conductivity, large surface roughness, and nanowire oxidation in air. Here, we introduce electrically synergistic but mechanically independent composite electrodes by sequentially introducing conducting polymers and ionic liquids into the wavy Ag nanowire network to maintain the superior performance of the stretchable transparent electrode while ensuring overall conductivity, lower roughness, and long-term stability. In particular, plenty of ionic liquids can be incorporated into the uniformly coated conducting polymer so that the elastic modulus can be significantly lowered and sliding can occur at the nanowire interface, thereby obtaining the high mechanical stretchability of the composite electrode. Finally, as a result of applying the composite film as the stretchable transparent electrode of stretchable organic solar cells, the organic solar cell exhibits a high power conversion efficiency of 11.3% and 89% compared to the initial efficiency even at 20% tensile strain, demonstrating excellent stretching stability.

Design and Application of an Asymmetric Naphthalimide-based Molecule with Improved Hydrophobicity for Highly Stable Organic Solar Cells

With the photovoltaic efficiency of organic solar cells (OSCs) exceeding 17%, improving the stability of these systems has become the most important issue for their practical applications. In particular, moisture in the environment may erode the interlayer molecules, which has been proved to be the main reason for the efficiency decay. At present, the development of moisture-resistant interlayer molecules remains a great challenge to the field. Herein, we designed two naphthalene diimide (NDI)-based organic compounds, namely, NDI-M and NDI-S, exhibiting suitable energy level and excellent electron extraction property. In addition to this, NDI-S has extremely low hygroscopicity. An efficiency of 17.27% was achieved for the NDI-S inverted cells, and the long-term stability under continuous illumination conditions was significantly improved with a T80 lifetime (the time required to reach 80% of initial performance) of over 28 000 h. More importantly, we demonstrated that, by using a covalent bond to link the counter ions with the host molecular structure in the zwitterion, the asymmetric molecule NDI-S can transform from amorphous to crystalline hydrate at high humidity and exhibited outstanding non-hygroscopic nature; this could decrease the interaction between the cell and the moisture, obviously improving the device stability under high humidity.

Material Design and Device Fabrication Strategies for Stretchable Organic Solar Cells

Recent advances in the power conversion efficiency (PCE) of organic solar cells (OSCs) have greatly enhanced their commercial viability. Considering the technical standards (e.g., mechanical robustness) required for wearable electronics, which are promising application platforms for OSCs, the development of fully stretchable OSCs (f-SOSCs) should be accelerated. Here, a comprehensive overview of f-SOSCs, which are aimed to reliably operate under various forms of mechanical stress, including bending and multidirectional stretching, is provided. First, the mechanical requirements of f-SOSCs, in terms of tensile and cohesion/adhesion properties, are summarized along with the experimental methods to evaluate those properties. Second, essential studies to make each layer of f-SOSCs stretchable and efficient are discussed, emphasizing strategies to simultaneously enhance the photovoltaic and mechanical properties of the active layer, ranging from material design to fabrication control. Key improvements to the other components/layers (i.e., substrate, electrodes, and interlayers) are also covered. Lastly, considering that f-SOSC research is in its infancy, the current challenges and future prospects are explored.

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.

Ultrathin and Efficient Organic Photovoltaics with Enhanced Air Stability by Suppression of Zinc Element Diffusion

Ultrathin (thickness less than 10 µm) organic photovoltaics (OPVs) can be applied to power soft robotics and wearable electronics. In addition to high power conversion efficiency, stability under various environmental stresses is crucial for the application of ultrathin OPVs. In this study, the authors realize highly air-stable and ultrathin (≈3 µm) OPVs that possess high efficiency (15.8%) and an outstanding power-per-weight ratio of 33.8 W g-1 . Dynamic secondary-ion mass spectrometry is used to identify Zn diffusion from the electron transport layer zinc oxide (ZnO) to the interface of photoactive layer; this diffusion results in the degradation of the ultrathin OPVs in air. The suppression of the Zn diffusion by a chelating strategy results in stable ultrathin OPVs that maintain 89.6% of their initial efficiency after storage for 1574 h in air at room temperature under dark conditions and 92.4% of their initial efficiency after annealing for 172 h at 85 °C in air under dark conditions. The lightweight and stable OPVs also possess excellent deformability with 87.3% retention of the initial performance after 5000 cycles of a compressing-stretching test with 33% compression.

Construction of Laterally Asymmetric Heterojunctions with Sub-Micrometer Resolution by Hierarchical Self-Assembly of Polythiophene Nanofibers

Polymeric semiconductors are crucial candidates for the construction of next-generation flexible and printable electronic devices. By virtue of the successful preparation of monodispersed colloidal solution in orthogonal solvent, poly(3-hexylthiophene) (P3HT) nanofibers are developed into versatile building blocks for nanoelectronics and their compatibilities are verified with photolithographic lift-off technology. Then, the joint efforts from both the bottom-up hierarchical self-assembly and top-down self-alignment technology have led to the realization of lateral asymmetric heterojunctions with resolution better than 1 µm. As a result, planar photovoltaic devices incorporating N,N'-dioctyl-3,4,9,10-perylenedicarboximide and P3HT supramolecular nanowires as active components are constructed with the cathode-to-anode distance being tuned from ≈0.1 to 1-2 µm. Based on such a novel device configuration, an interesting phenomenon of channel-length-dependent photovoltaic efficiency is observed for the first time, strongly suggesting the impact of near-field light intensity on the performance of nanophotonic devices.

Blending Donors with Different Molecular Weights: An Efficient Strategy to Resolve the Conflict between Coherence Length and Intermixed Phase in Polymer/Nonfullerene Solar Cells

Long coherence lengths (CLs) of crystals and proper intermixed phase amount guarantee charge transport and exciton dissociate efficiently, which is crucial for organic solar cells (OSCs) to achieve high device performance. However, extending CLs usually reduces the intermixed phase, leading to an insufficient interface for exciton dissociation. Herein, a strategy using a binary polymer with different molecular weights as donor is employed, that is, poly(3-hexylthiophene-2,5-diyl) (P3HT) with high (P3HT-H) and low (P3HT-L) molecular weight are blended as donor, and (5Z,5'Z)-5,5'-(((4,4,9,9-tetraoctyl-4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene-2,7-diyl)bis(benzo[c][1,2,5]thiadiazole-7,4-diyl))bis(methanylylidene))bis(3-ethyl-2-thioxothiazolidin-4-one) (O-IDTBR) is used as acceptor. In kinetics, the entanglements of P3HT-H are relieved due to the higher molecular diffusivity of P3HT-L. In thermodynamics, the miscibility of P3HT-L/O-IDTBR, P3HT-H/O-IDTBR, and P3HT-L/P3HT-H blends increases in turn. Hence, P3HT forms a more ordered structure with longer CLs after adding P3HT-L, which also drives O-IDTBR dispersed in P3HT crystalline regions diffuse to the O-IDTBR crystalline regions to further self-organize. Consequently, the CLs of both P3HT and O-IDTBR are extended, while keeping the intermixed phase amount proper. The optimized microstructure boosts device performance from 7.03% to 7.80%, which is one of the highest values reported for P3HT/O-IDTBR blends. This is a novel way to solve the conflict mentioned above, which may provide guidance to finely regulating the morphology of the active layer.

Recent Advances of Film–Forming Kinetics in Organic Solar Cells

Solution–processed organic solar cells (OSC) have been explored widely due to their low cost and convenience, and impressive power conversion efficiencies (PCEs) which have surpassed 18%. In particular, the optimization of film morphology, including the phase separation structure and crystallinity degree of donor and acceptor domains, is crucially important to the improvement in PCE. Considering that the film morphology optimization of many blends can be achieved by regulating the film–forming process, it is necessary to take note of the employment of solvents and additives used during film processing, as well as the film–forming conditions. Herein, we summarize the recent investigations about thin films and expect to give some guidance for its prospective progress. The different film morphologies are discussed in detail to reveal the relationship between the morphology and device performance. Then, the principle of morphology regulating is concluded with. Finally, a future controlling of the film morphology and development is briefly outlined, which may provide some guidance for further optimizing the device performance.