Flexible fiber-shaped solar cells (FSCs) offer a unique combination of power generation and weavability, making them ideal candidates for powering wearable electronics. Several types of FSCs exist, including fiber-shaped dye-sensitized solar cells (FDSSCs), fiber-shaped organic solar cells (FOSCs), and fiber-shaped perovskite solar cells (FPSCs). FDSSCs and FPSCs have achieved efficiencies exceeding 10%, demonstrating their potential to meet the energy demands of various applications. However, FOSCs have lagged significantly, with efficiencies remaining around 2% for over a decade. Despite this limitation, FOSCs remain a promising choice due to their all-solid-state design, simple fabrication process, environmental compatibility, and good material stability. They have even been successfully integrated into organic photovoltaic textiles, showcasing their potential in wearable technology. The performance of FOSCs is heavily dependent on the properties of organic semiconductors. Traditional FOSCs utilize polymer-based donors and fullerene-based acceptors, like P3HT:PC61BM and PTB7:PC71BM. These materials have limited light absorption in the visible range (<700 nm), restricting efficiencies below 10%. Recently, non-fullerene acceptor (NFA) materials have emerged as a significant advancement in organic photovoltaics (OPVs), extending the absorption range beyond 1000 nm and pushing efficiencies of planar OPVs to over 18%. These NFAs are also solution-processable, making them suitable for flexible substrates. However, they haven't been integrated into FSCs until now. This study aims to address the low efficiency of FOSCs by incorporating NFA-based organic semiconductors as light-harvesting materials. Low-cost stainless-steel wire serves as the substrate and core electrode, with carbon nanotube (CNT) yarn or silver wire used as the counter electrode. A perylene diimide derivative acts as the electron transfer layer. The researchers aim to demonstrate improved efficiency and systematically study various factors influencing device performance.

The existing literature extensively covers advancements in flexible fiber-shaped solar cells, particularly FDSSCs and FPSCs, which have shown considerable progress in achieving high power conversion efficiencies exceeding 10%. Various strategies, including the development of novel photovoltaic materials and optimization of device architectures, have contributed to these improvements. In contrast, the literature reveals a significant performance gap in FOSCs, with efficiencies stagnating around 2% for the past decade. While several studies have explored the fabrication and basic characteristics of FOSCs, significant advancements in efficiency have been lacking. This performance bottleneck is largely attributed to the limitations of traditional fullerene-based active layers with narrow light absorption spectra. The recent surge in research on non-fullerene acceptors (NFAs) in planar organic solar cells, leading to significantly improved efficiencies exceeding 18%, has not yet been translated into the fiber-shaped solar cell architecture. This gap provides the impetus for the current research, focusing on leveraging the advantages of NFAs to enhance the performance of FOSCs.

To fabricate the FOSCs, the researchers employed a homemade programmable slide-coating system to ensure reproducibility and reduce the variability associated with manual coating techniques. Industry-grade stainless-steel wire, chosen for its low cost, flexibility, and tensile strength, was used as the substrate and core electrode. The functional layers were sequentially coated onto the wire. The process began with the electron transport layer (ETL), followed by the ternary active layer (a blend of PM6:Y6:PC71BM), and finally the hole transport layer (HTL). Carbon nanotube (CNT) yarn was used as the counter electrode, twined around the primary electrode to ensure good electrical contact. (N,N-dimethyl-ammonium N-oxide) propyl perylene diimide (PDINO) served as the ETL, leveraging its high electron affinity. The active layer consisted of a ternary blend of PM6:Y6:PC71BM, chosen for its NFA-based enhanced light absorption. PEDOT:PSS (AL4083 and PH1000) were used as HTLs. The slide-coating system allowed for the fabrication of FOSCs over one meter in length. Scanning electron microscopy (SEM) was utilized to characterize the morphology of the various layers. Cross-sectional SEM images, obtained using a Helium ion microscope and focused ion beam (HIM-FIB) system, revealed the layer-by-layer structure and the thickness of each layer. To optimize device performance, a systematic investigation was conducted by varying the thickness of the ETL (PDINO), active layer, and HTL. The influence of counter electrode pitch was also examined. The photovoltaic performance of the FOSCs was characterized under AM 1.5 G standard illumination conditions using a Keithley 2400 system and an Oriel solar simulator. External quantum efficiency (EQE) measurements were performed to validate the short-circuit current density (Jsc) values obtained from J-V curves. The effect of humidity on device performance was studied by recording photovoltaic parameters over time and correlating them with environmental humidity levels. SEM was used to analyze the morphology of active layers prepared under various humidity conditions. Electrical impedance spectroscopy (EIS) was employed to investigate charge transfer and recombination processes. The study also explored the use of alternative ETL materials (ZnO nanoparticles and nanocrystals) and active layer compositions (PM6:Y6 binary blend). The flexible and uniform properties of the FOSCs, along with their long-term stability, were also assessed. Finally, the researchers demonstrated applications of the FOSCs in wearable electronics, including powering small electrical devices and charging a smartwatch by weaving the FOSCs into a watchband.

This study achieved significant advancements in the efficiency of fiber-shaped organic solar cells (FOSCs). Key findings include: 1. **High Efficiency:** The optimized FOSCs achieved a power conversion efficiency (PCE) of up to 9.40% under AM 1.5 G standard illumination conditions, representing a substantial improvement over previously reported values. This efficiency was characterized by a Voc of 0.708 V, a Jsc of 25.4 mA/cm², and a fill factor (FF) of 52.5%. The integrated EQE yielded a Jsc of 25.1 mA/cm², consistent with the J-V measurement. 2. **Systematic Optimization:** The researchers systematically optimized various device parameters, including the thickness of the electron transport layer (ETL), active layer, and hole transport layer (HTL), as well as the pitch of the counter electrode. This led to a significant enhancement in device performance. 3. **Humidity Effect:** A crucial finding was the strong influence of environmental humidity on the device efficiency. Lower humidity during fabrication resulted in significantly higher efficiencies. SEM analysis revealed that high humidity during active layer deposition led to the formation of holes in the film, which increased charge recombination and reduced device performance. The use of chlorobenzene as the solvent, which has lower water absorption than chloroform, mitigated this issue, resulting in better performance under high-humidity conditions. 4. **Material Flexibility:** The researchers demonstrated the effectiveness of using both carbon nanotube (CNT) yarn and silver (Ag) wire as counter electrodes, indicating flexibility in material selection. 5. **Device Uniformity:** The FOSCs exhibited excellent radial and axial uniformity, with minimal variation in performance along the fiber length. The use of a reflector significantly enhanced the power output of the devices by nearly doubling the output power. 6. **Scalability and Practical Applications:** The use of a programmable slide-coating system enabled the scalable fabrication of long FOSCs (over one meter). Multiple FOSCs were connected in series and parallel to achieve higher output power, sufficient to drive various small electrical devices, such as a mini-fan and a red LED. The FOSCs were successfully integrated into a watchband and used to charge a smartwatch, demonstrating their potential for practical wearable applications.

The findings of this study address the long-standing challenge of low efficiency in fiber-shaped organic solar cells. The use of NFA-based active layers, coupled with a systematic optimization of device parameters and processing conditions, has resulted in a significant breakthrough in performance. The achievement of a PCE exceeding 9% represents a considerable advancement in the field and brings FOSCs closer to practical applications in wearable electronics and smart textiles. The significant impact of humidity on device performance highlights the importance of controlled environmental conditions during fabrication. The identification of chlorobenzene as a suitable alternative solvent for high-humidity environments provides a practical solution for maintaining high device performance in various climates. The demonstration of successful power generation and application in wearable devices underscores the potential of FOSCs to become a viable power source for a wide range of applications. The study's methodology and findings could inspire further research into optimizing the design and fabrication of fiber-shaped solar cells, potentially leading to even higher efficiencies and broader applicability.

This research successfully demonstrated highly efficient fiber-shaped organic solar cells (FOSCs) with power conversion efficiencies exceeding 9%, surpassing previous limitations. The study highlighted the critical role of humidity in device fabrication and introduced strategies for mitigating its negative effects. The successful integration of FOSCs into wearable devices, such as a watchband capable of charging a smartwatch, underscores their significant potential for practical applications. Future research could focus on further enhancing efficiency, improving long-term stability, and exploring new materials and device architectures to expand the applications of FOSCs in the growing field of flexible electronics.

While this study achieved significant progress in FOSC efficiency, several limitations exist. The long-term stability of unencapsulated devices is a concern, with significant degradation observed over time in ambient conditions. Although encapsulation improves stability, further investigation into long-term stability under various environmental conditions is needed. The study primarily focused on a specific set of materials and device architectures; further exploration of other materials and designs could lead to further performance enhancements. The influence of humidity on device performance remains a significant challenge, requiring continued effort to develop robust fabrication processes that are less sensitive to environmental conditions. Lastly, scaling up the production of FOSCs for large-scale applications requires further research and development.