Bulk heterojunction (BHJ) organic solar cells (OSCs) offer advantages for flexible and roll-to-roll production via solution processing. Significant progress in PCE has been driven by the development of non-fullerene acceptors (NFAs), with efficiencies exceeding 17-18%. However, BHJ technology faces limitations: (i) complex morphology evolution during donor (D) and acceptor (A) mixing requiring precise control of processing parameters; (ii) reliance on toxic solvents like chlorobenzene and chloroform, hindering scalability and environmental friendliness; (iii) performance degradation with increased thickness due to charge transport limitations; and (iv) difficulty in morphology control during blade coating, a crucial technique for mass production. Inspired by successful gradient junctions in other solar cell technologies like amorphous silicon and GaAs, this study explores the graded BHJ (G-BHJ) concept for improved OSC performance and processability, leveraging sequential deposition (SD) with nonhalogenated solvents for better morphology control and environmental impact.

The literature extensively discusses the advantages and challenges of BHJ OSCs. The development of high-performance NFAs like the Y6 series has significantly improved efficiencies. However, issues related to morphology control, the use of toxic solvents, thickness dependence of efficiency, and the challenges of scalable manufacturing techniques such as blade coating remain key obstacles. Previous studies have investigated sequential deposition methods for fullerene-based OSCs, but their application to NFA-based systems has been less explored, lacking a clear understanding of the morphological picture and solvent selection rules. The successful integration of gradient junctions in other solar cell technologies provides a basis for the proposed G-BHJ approach. Studies on solvent engineering and additive effects in BHJ OSCs also inform the current research, highlighting the potential for improved efficiency and processability.

This study utilizes the PM6 donor and BTP-eC9 NFA system. The G-BHJ films were fabricated via sequential deposition using two solvents: o-xylene (XY) and chlorobenzene (CF). Depth-profiling X-ray photoelectron spectroscopy (DP-XPS) was employed to quantitatively analyze vertical composition distributions, determining polymer weight content at various depths based on O/F atomic ratios. Angle-dependent grazing incidence X-ray diffraction (GI-XRD) was used to visualize the gradient distributions of polymer composition and crystallinity. Time-of-flight secondary-ion mass spectrometry (ToF-SIMS) provided further confirmation of the G-BHJ morphology. Device performance was evaluated via current density-voltage (J-V) curves and external quantum efficiency (EQE) measurements. Charge carrier dynamics were analyzed using photocurrent density (Jph) versus effective voltage (Veff) plots to assess exciton dissociation and charge collection efficiencies. Photoluminescence (PL) quenching efficiency was examined to investigate charge dissociation properties. Space-charge-limited current (SCLC) measurements were performed to determine hole and electron mobilities. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used for morphological characterization. Additionally, in situ UV-vis absorption measurements were employed to investigate the crystallization kinetics of donor and acceptor materials during film formation. Finally, blade coating techniques were used to fabricate G-BHJ OSCs to assess the scalability of the approach.

The study established solvent selection rules for G-BHJ formation. High-boiling-point solvents (like XY) allow for longer interdiffusion times but limited solubility, whereas low-boiling-point solvents (like CF) offer shorter times but higher solubility, both leading to effective G-BHJ formation. DP-XPS revealed a clear graded polymer distribution in G-BHJ films, unlike BHJ films. Spin-coated G-BHJ OSCs from XY and CF achieved outstanding PCEs of 17.48% and 17.54%, respectively, surpassing the performance of corresponding BHJ devices. The XY-based G-BHJ OSC exhibited a remarkable Jsc of 26.65 mA cm⁻² and FF of 0.781. G-BHJ OSCs showed high tolerance to thickness variations (120-500 nm), maintaining PCEs above 14%. A blade-coated G-BHJ OSC using XY achieved a record PCE of 16.77% in open air. GI-XRD revealed enhanced PM6 crystallinity and more ordered BTP-eC9 aggregates in G-BHJ films compared to BHJ films. AFM and TEM observations showed that G-BHJ processing suppressed excessive aggregation, leading to a more balanced phase separation and improved charge transport. Charge carrier dynamics analysis indicated improved exciton dissociation and charge collection efficiency in G-BHJ devices. SCLC measurements confirmed a better balance between electron and hole mobilities in G-BHJ devices. In situ UV-vis measurements showed that G-BHJ processing leads to faster film drying and more balanced crystallization kinetics of donor and acceptor materials, resulting in the improved device performance.

The findings demonstrate the superior performance of G-BHJ OSCs over conventional BHJ devices, attributed to the synergistic effects of graded composition and crystallinity profiles, leading to enhanced charge transport and reduced charge recombination losses. The G-BHJ strategy effectively alleviates the trade-off between Jsc and FF, enabling high-efficiency devices even with thicker active layers. The use of non-halogenated solvents significantly improves the environmental friendliness and scalability potential of the OSCs. The successful blade-coating process highlights the viability of G-BHJ technology for industrial-scale production. These results provide a new approach for optimizing the morphology and performance of OSCs, suggesting a potential pathway for high-efficiency, eco-friendly, and manufacturable organic solar cells.

This study presents a novel G-BHJ strategy for high-performance OSCs. The use of sequential deposition with non-halogenated solvents enables precise control over film morphology and results in significantly enhanced PCEs, particularly in thicker devices and under blade-coating conditions. The findings highlight the potential of G-BHJ architecture for creating high-efficiency, environmentally friendly, and industrially scalable organic solar cells. Future research could focus on exploring diverse solvent combinations, optimizing deposition parameters, and developing new materials to further enhance the efficiency and scalability of G-BHJ OSCs.

The study primarily focuses on the PM6:BTP-eC9 system. Further investigations are needed to determine the generality of this approach with other donor-acceptor combinations. The blade coating process, while successful, may require further optimization for large-scale production. A deeper investigation into the long-term stability of the G-BHJ devices is also warranted to assess their commercial viability.