Organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) have witnessed significant advancements, with power conversion efficiencies (PCEs) exceeding 18%. However, a considerable PCE gap remains compared to perovskite solar cells, primarily due to high non-radiative recombination losses in OSCs. Improving the quality of the active layer, specifically the distribution and molecular stacking of the donor:acceptor (D:A) BHJ blend, is crucial for reducing these losses without hindering charge separation and transport. Previous research on fullerene-based OSCs demonstrated that controlling solvent evaporation rates can induce highly ordered polymer crystallinity. Similar morphology-modifying techniques in NFAs, such as solvent additives (e.g., DIO), have been used, but these often lead to excessive NFA aggregation and increased non-radiative recombination. The research aimed to develop new morphology-regulating techniques that could simultaneously optimize D:A self-organization and minimize non-radiative recombination. The use of 1,3,5-trichlorobenzene (TCB) as a crystallization regulator to achieve a non-monotonic intermediate state manipulation (ISM) strategy was explored, aiming to optimize the self-organization process of the D:A blend, thereby enhancing efficiency and reducing non-radiative recombination losses. The high volatility of TCB allows its removal during spin-coating, preventing the excessive aggregation issues associated with less volatile additives like DIO.

The literature review highlights the significant progress in OSCs using NFAs, with reported PCEs exceeding 18%. However, the persistent challenge of high non-radiative recombination loss compared to perovskite solar cells (25.7% PCE) is emphasized. The importance of active layer morphology, specifically the D:A BHJ blend distribution and molecular stacking, is discussed. Previous work on fullerene-based OSCs, involving techniques like solvent annealing and additive strategies, is reviewed, showing the link between BHJ active layer drying, crystallization kinetics, and device performance. The limitations of existing morphology-modifying techniques in NFA OSCs, such as molecule optimization and ternary strategies, are discussed, highlighting the need for new approaches. The use of solvent additives like DIO, while effective in increasing NFA crystallinity, is shown to lead to excessive aggregation and increased non-radiative recombination losses. The use of volatile solid additives is mentioned as a potential improvement over traditional solvent additives, but with limited overall PCE enhancement. This sets the stage for the proposed ISM strategy using TCB.

The study employed several techniques to investigate the interaction between TCB and the active materials and to evaluate the impact on OSC performance. Differential scanning calorimetry (DSC) was used to identify the formation of new phases in mixtures of TCB with PM6, Y6, and their blend. Density functional theory (DFT) simulations were performed to understand the molecular-level interactions, revealing hydrogen bond formation between TCB and the carbonyl/cyano groups in PM6 and Y6. OSCs with a standard sandwich structure (ITO/PEDOT:PSS/PM6:Y6/PFN-Br/Ag) were fabricated using TCB and the benchmark additive DIO for comparison. Current density-voltage (J-V) curves and external quantum efficiency (EQE) measurements were conducted to assess device performance. Space charge limited current (SCLC) methods were used to investigate charge carrier transport properties, while transient photovoltage (TPV) measurements probed charge recombination dynamics. The dependence of Voc on light intensity was also analyzed. Atomic force microscopy (AFM) and grazing incidence wide-angle X-ray scattering (GIWAXS) were used to characterize the nanostructure and crystalline ordering of the D:A blends. In situ GIWAXS and time-resolved UV-vis reflectance spectroscopy were used to monitor the spin-coating process in real time, providing insights into the drying and crystallization dynamics. Electroluminescence quantum efficiency (EQEEL) measurements were performed to quantitatively assess non-radiative recombination losses. The methodology included the fabrication and characterization of OSCs using various material combinations (PM6:Y6, PM1:BTP-eC9, PM6:BTP-eC9, etc.), employing both DIO and TCB as additives. The analysis of energy losses (Eloss) using different calculation methods (from J-V characteristics and EQEEL) was also carried out. Finally, operational stability tests were conducted to assess the long-term performance under continuous illumination.

The DSC and DFT results confirmed interactions between TCB and both donor and acceptor materials. The TCB-treated devices showed significantly improved performance compared to DIO-treated devices, reaching a PCE of 18.06% for PM6:Y6 (compared to 16.83% with DIO). The EQE measurements validated the Jsc values from the J-V tests. The SCLC measurements revealed similar electron and slightly faster hole mobility in the TCB-processed device, attributed to the more ordered molecular stacking. TPV measurements indicated that TCB treatment was more effective in suppressing charge carrier recombination than DIO. Light intensity dependence of Voc analysis showed less trap-assisted recombination in TCB-treated devices, due to TCB's higher volatility leading to less residue in the active layer. The analysis of energy loss (Eloss) revealed lower non-radiative recombination losses (ΔE3) in TCB-treated devices (0.214 eV vs. 0.233 eV for DIO in PM6:Y6). EQEEL measurements further confirmed the lower non-radiative recombination in TCB-treated devices (3.4 × 10⁻² vs. 1.7 × 10⁻² for DIO). AFM and GIWAXS revealed more ordered molecular stacking and higher crystallinity in TCB-processed films, resulting in enhanced charge transport. In situ UV-vis spectroscopy showed a non-monotonic intermediate state transition in TCB-treated samples (first redshift, then blueshift), indicating a two-step process of enhancing and then relaxing molecular aggregation. This non-monotonic behavior was not observed in DIO-treated samples. The versatility of the TCB-ISM strategy was demonstrated in five additional OSC systems, with consistent performance improvements. In PM1:BTP-eC9 and PM6:BTP-eC9 systems, the TCB-ISM strategy achieved PCEs of 19.10% and 19.31% (18.93% certified), respectively, representing the highest efficiency for binary OSCs to date. In these high-efficiency systems, the TCB-ISM strategy resulted in record-low non-radiative recombination energy loss (0.168 eV in PM1:BTP-eC9 and 0.190 eV in PM6:BTP-eC9), as determined by J-V characteristics. EQEEL measurements further support these findings. Operational stability tests showed that the TCB-treated devices had better long-term stability than DIO-treated devices under continuous illumination.

The findings demonstrate that the non-monotonic intermediate state manipulation strategy using TCB is highly effective in optimizing the morphology of BHJ OSCs. The carefully controlled crystallization process and the subsequent relaxation of molecular aggregation, enabled by TCB's volatility, lead to a significant reduction in non-radiative recombination loss. This contributes to a remarkable increase in the efficiency of binary OSCs. The achievement of a record PCE of 19.31% and exceptionally low non-radiative recombination loss highlights the potential of the ISM strategy. The versatility demonstrated across multiple OSC systems underscores the generality and applicability of this approach. The superior stability of the TCB-treated devices suggests that the ISM strategy not only enhances efficiency but also improves the long-term operational stability of OSCs. The results provide a significant step towards developing high-performance and stable OSCs, and open new avenues for exploring emerging non-fullerene materials.

This study successfully developed a non-monotonic intermediate state transition strategy using TCB to enhance the performance of organic solar cells. The strategy leads to a record-high efficiency of 19.31% (18.93% certified) with remarkably low non-radiative recombination losses, demonstrated across multiple material systems. This approach offers a promising avenue for further advancements in OSC technology by optimizing morphology and reducing energy losses. Future research could explore the use of other volatile additives with similar or improved properties, and further optimize the ISM process for even higher efficiencies and stability.

While the study demonstrates remarkable success, limitations exist. The in situ GIWAXS measurements were limited by the crystallinity of the organic film, hindering the complete understanding of the film formation process. The study primarily focuses on specific material combinations, and further investigation is needed to determine the general applicability of the ISM strategy across a broader range of materials. While improved stability is observed, long-term stability tests under various environmental conditions should be conducted for comprehensive evaluation.