Introduction

Organic photovoltaic cells (OPV) have emerged as a focal point in photovoltaic research due to their tunable light absorption properties, lightweight nature, flexibility, and excellent performance under various lighting conditions. While advances in material design and fabrication techniques have significantly improved OPV performance and driven their industrialization, high-efficiency OPV cells often require complex post-processing techniques such as annealing and additive usage. These requirements not only increase production costs but also limit practical applications. To achieve high-efficiency, post-processing-free fabrication strategies, the development of morphology control technology has become crucial.

In this context, this research breakthrough involves the design and synthesis of a novel fully fused non-fullerene acceptor GS-20, which was introduced as a third component in PBQx-TF-based organic solar cells. This innovative design achieved a high power conversion efficiency (PCE) of 19.0% without any post-processing, making it one of the highest-performing post-processing-free OPV cells reported to date. The introduction of GS-20 accelerated the film deposition process and promoted eC9-2Cl molecular aggregation, improving molecular stacking structure and thereby enhancing charge transport efficiency and thermal stability.

Research Achievements and Highlights

• Novel Material Design: The research designed and synthesized a new fully fused non-fullerene acceptor GS-20, introducing it as a third component in PBQx-TF:eC9-2Cl-based organic solar cells to achieve high-efficiency post-processing-free organic solar cells.
• High Efficiency Achievement: The ternary cell achieved a high PCE of 19.0% without any post-processing, representing one of the highest efficiencies reported for post-processing-free organic solar cells.
• Enhanced Properties: The introduction of GS-20 accelerated film deposition, promoted eC9-2Cl molecular aggregation, and improved molecular stacking.
• Stability Performance: Post-processing-free ternary cells demonstrated excellent thermal stability and morphological stability, maintaining 90% of their initial PCE after 1000 hours of heating at 85°C in a nitrogen glovebox.
• Large-Area Application: The ternary strategy proved applicable for large-area module fabrication, with post-processing-free organic solar modules (23.6 cm2) achieving a PCE of 13.5% using blade coating.
• Industrial Potential: This research provides an effective molecular design strategy using the ternary approach to achieve post-processing-free organic solar cells, paving the way for organic solar cell technology industrialization.

Research Team

This research was completed by Professor Jianhui Hou’s team from the Institute of Chemistry, Chinese Academy of Sciences.

Research Background

Organic solar cells (OPV) show immense potential for various lighting conditions due to their tunable light absorption, lightweight nature, and flexibility. The development of organic semiconductor materials and preparation methods has significantly improved OPV cell photovoltaic performance, attracting considerable attention and research efforts toward OPV industrialization.

OPV cell performance relates to numerous electrical processes, including exciton dissociation, charge transport, and recombination, all of which are influenced by molecular aggregation and phase separation within the active layer. Therefore, morphology control is crucial for obtaining high-efficiency OPV cells during manufacturing. However, since aggregation structures are primarily determined by material structure and processing conditions during film deposition, substantial investment is needed to develop effective and universal morphology control strategies.

Currently, various morphology control strategies have been established, including annealing, processing solvent selection, additives, and ternary strategies. The combination of additives and annealing is the most widely used method for adjusting aggregation structures and phase separation in OPV cells.

For example, volatile solid additives have been successfully developed to adjust the aggregation structure of the active layer, achieving PCEs exceeding 19% in binary cells. On the other hand, the ternary strategy has been widely applied in state-of-the-art OPV cells, pushing PCEs to approximately 20%. The introduction of a third component can regulate phase separation, crystallinity, and molecular stacking through selective interaction with donors or acceptors, promoting charge dissociation and transport, thereby improving open-circuit voltage (Voc) and fill factor (FF). Xie et al. introduced the third component Qx-5Cl into PBDB-TF:BTP-eC9 blends to regulate crystallization kinetics, achieving a PCE of 19.8%. Jiang et al. reported a ternary OPV cell based on D18:Z8:L8-BO with a PCE of 20.2%.

Despite these notable achievements, additional post-processing, such as annealing and additives, is still required for morphology regulation of ternary films. For OPV cell industrialization, post-processing requires complex manufacturing processes and high costs in equipment and time, which presents challenges for industrial manufacturing. Therefore, effective and simple morphology control remains one of the many challenges facing the industrialization of high-efficiency post-processing-free OPV cells.

Solution

To address the aforementioned issues, this research designed and synthesized a novel fully fused non-fullerene acceptor GS-20. Due to its highly planar conjugated backbone, GS-20 exhibits strong crystallinity and aggregation characteristics. The research team introduced GS-20 as a third component into the highly efficient PBQx-TF:eC9-2Cl binary system to prepare ternary OPV cells.

GS-20 shows higher compatibility with eC9-2Cl than with PBQx-TF, enabling it to induce intermolecular aggregation. In terms of film formation kinetics, the PBQx-TF:eC9-2Cl:GS-20 ternary blend demonstrated accelerated film deposition processes and rapid molecular aggregation.

Notably, compared to blade-coated binary films, the ternary films showed enhanced molecular stacking and ordering, as well as smooth film surfaces without any post-processing, which is beneficial for reducing non-radiative energy losses and improving charge transport.

Consequently, the ternary OPV cells achieved a high PCE of 19.0% without using any additives or annealing, representing state-of-the-art performance for post-processing-free OPV cells.

Furthermore, this ternary strategy is feasible for large-area module fabrication. Post-processing-free OPV modules (23.6 cm2) were successfully prepared using blade coating, achieving a PCE of 13.5%.

Experimental Process and Steps

Materials and Device Preparation

The materials used in this study include:

• Active Materials:
  1. Donor material: PBQx-TF
  2. Acceptor materials: eC9-2Cl and GS-20
  3. Interface layer materials: PEDOT:PSS, NDI-Ph, ZnO, MoO3
  4. Electrode materials: ITO, Ag, Al
• Synthesis Process:
  1. The synthesis route for GS-20 is shown in Figure S1
     
     
• Device Structure:
  1. OPV cells were fabricated using the conventional structure of ITO/PEDOT:PSS/BHJ/NDI-Ph/Ag
• Active Layer Preparation:
  1. Active layer materials were prepared with the following ratios:
     – PBQx-TF:GS-20 (D/A=1:1)
     – PBQx-TF:eC9-2Cl (D/A=1:1.2)
     – PBQx-TF:eC9-2Cl:GS-20 (D/A=1:1:0.2)
  2. Materials were dissolved in toluene with a donor concentration of 7.5 mg mL-1
  3. For PBQx-TF:eC9-2Cl-based cells requiring post-processing, 0.5 vol% 1,8-diiodooctane was added to the solution before spin-coating
• Module Fabrication:
  1. OPV modules were fabricated with the structure ITO/ZnO/PBQx-TF:eC9-2Cl:GS-20/MoO3/Al
  2. Active layer material PBQx-TF:eC9-2Cl:GS-20 (D/A=1:1:0.2) was dissolved in toluene at a concentration of 6 mg mL-1

Research Characterization Methods

Material Characterization

• Optical Properties:
  1. UV-visible absorption spectra and photoluminescence spectra were measured for PBQx-TF, eC9-2Cl, and GS-20
  2. Cyclic voltammetry (CV) was used to measure the HOMO and LUMO energy levels of GS-20
  3. Studies focused on light absorption and emission characteristics
• Aggregation and Morphology:
  1. Contact angle measurements and Flory-Huggins parameters (χ) were used to evaluate material miscibility
  2. GIWAXS was employed to study crystallinity and molecular stacking
  3. AFM was used to characterize morphology and surface roughness of blend films

Film Preparation and Device Assembly

• Active Layer Film Preparation:
  1. Different compositions were prepared by spin-coating method:
     – Binary blends: PBQx-TF:eC9-2Cl and PBQx-TF:GS-20
     – Ternary blend: PBQx-TF:eC9-2Cl:GS-20
  2. For binary cells requiring post-processing:
     – 0.5 vol% 1,8-diiodooctane (DIO) was added before spin-coating
     – Active layer films underwent thermal annealing treatment
  3. Complete device structure: ITO/PEDOT:PSS/active layer/NDI-Ph/Ag
• Large-Area Module Fabrication:
  1. Large-area ternary OPV modules were prepared using blade coating method
  2. Device structure: ITO/ZnO/PBQx-TF:eC9-2Cl:GS-20/MoO3/Al

Device Performance Characterization

• Current Density-Voltage (J-V) Characteristics (Figure 4a):
  
  
  1. PBQx-TF:GS-20-based cells:
     – Voc: 0.845 V
     – FF: 74.9%
     – PCE: 9.94%
  2. Post-processing-free PBQx-TF:eC9-2Cl-based cells:
     – PCE: 17.1%
     – Voc: 0.886 V
     – Jsc: 26.7 mA cm-2
     – FF: 72.4%
  3. Post-processed PBQx-TF:eC9-2Cl-based cells:
     – PCE: 17.7%
     – Voc: 0.857 V
     – Jsc: 27.0 mA cm-2
     – FF: 76.6%
  4. Ternary cells with GS-20:
     – Voc: 0.890 V
     – FF: 78.4%
     – PCE: 19.0%
• External Quantum Efficiency (Figure 4b):
  
  
  1. High EQE demonstrated in 300-800 nm wavelength range
  2. Excellent photoelectric conversion capability in visible light range
• Electroluminescence External Quantum Efficiency (EQEEL) Measurement (Figure 4c):
  
  
  1. EQEEL values comparison:
     – Ternary cell: 1.77 × 10−4
     – PBQx-TF:eC9-2Cl-based cell: 9.21 × 10−5
     – Post-processing-free binary cell: 2.26 × 10−4
  2. Results indicate that GS-20 introduction can:
     – Regulate aggregation structure
     – Reduce non-radiative recombination loss
• Light Intensity Dependent Voc Measurement (Figure 4e):
  
  
  1. Ternary cell showed Voc vs. light intensity curve slope of 1.14 kT/q
  2. Results indicate minimal trap-assisted recombination
• Light Intensity Dependent Jsc Measurement (Figure 4f):
  
  
  1. All OPV cells showed exponential factor (α) close to 0.98
  2. Indicates minimal impact of bimolecular recombination on device performance
• Transient Photocurrent (TPC) Measurement (Figure 4g):
  
  
  1. Post-processed PBQx-TF:eC9-2Cl-based cells showed:
     – Weaker trap-assisted recombination compared to untreated cells
     – Improvement attributed to optimized aggregation structure
  2. GS-20 introduction resulted in:
     – Further suppression of trap-assisted recombination
     – Enhanced Voc and FF in ternary cells
• Photo-induced Charge Extraction by Linearly Increasing Voltage (photo-CELIV) Measurement (Figure 4h):
  
  
  1. Ternary cell carrier mobility: 1.90 × 10−4 cm2 V−1 s−1
  2. Moderate mobility compared to PBQx-TF:eC9-2Cl-based binary cells

Additional Characterization Techniques

• UV-Visible Absorption Spectra (Figure 1b):
  
  
  1. GS-20 film characteristics:
     – Maximum absorption peak at 687 nm
     – Absorption onset wavelength at 751 nm
     – Optical bandgap (Eg) of 1.65 eV
  2. Complementary absorption with PBQx-TF:eC9-2Cl blend film
• Cyclic Voltammetry (CV) (Figure S2b):
  
  
  1. GS-20 energy levels:
     – HOMO: −5.89 eV
     – LUMO: −4.14 eV
  2. Deeper energy levels compared to PBQx-TF and eC9-2Cl
• Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) (Figure 1d):
  
  
  1. Both GS-20 and eC9-2Cl showed preferential out-of-plane orientation
  2. Mixing eC9-2Cl with 20% GS-20 resulted in:
     – Enhanced crystallinity
     – Maintained consistent out-of-plane orientation
  3. Ternary blend films showed improvements compared to blade-coated binary films:
     – Better π-π stacking distance (3.78 Å)
     – Enhanced crystalline coherence length (CCL, 18.5 Å)
• Atomic Force Microscopy (AFM) (Figure S8):
  
  
  1. PBQx-TF:GS-20 films showed distinct aggregation and fibrous network morphology
  2. Untreated PBQx-TF:eC9-2Cl films:
     – Low surface roughness (Rq) of 1.34 nm
     – Weak molecular aggregation
  3. Post-treated PBQx-TF:eC9-2Cl films:
     – Enhanced fibrous network morphology
     – More pronounced phase separation
     – Increased Rq value to 2.60 nm
  4. Ternary films characteristics:
     – More uniform morphology
     – Lowest Rq value of 1.30 nm
     – Good phase separation

Module Fabrication and Performance Testing

• Module Fabrication and Performance Testing:
  1. Large-area OPV modules were fabricated
  2. I-V curves and PCE were measured
  3. Verified feasibility of ternary strategy in large-area device fabrication

Research Results

This research successfully designed and synthesized a novel fully fused non-fullerene acceptor GS-20 and introduced it as a third component in PBQx-TF:eC9-2Cl-based organic solar cells to fabricate high-efficiency post-processing-free organic solar cells.

The ternary cell achieved a high PCE of 19.0% without any post-processing, representing one of the highest efficiencies reported for post-processing-free organic solar cells. The introduction of GS-20 accelerated the film deposition process, promoted eC9-2Cl molecular aggregation, and improved molecular stacking, thereby enhancing device Voc and FF.

Furthermore, this ternary strategy proved applicable for large-area module fabrication, with post-processing-free organic solar modules (23.6 cm2) achieving a PCE of 13.5% using blade coating.

Development and Performance of Post-Processing-Free Organic Solar Cells

Researchers designed and synthesized a novel non-fullerene acceptor GS-20 and investigated its effects as a third component added to PBQx-TF:eC9-2Cl-based organic solar cells.

Experimental results showed that the GS-20-added ternary cell achieved a PCE of 19.0% without any post-processing, representing one of the highest efficiencies reported for post-processing-free organic solar cells.

GS-20 Design and Synthesis

GS-20 features a fully fused conjugated backbone with high planarity, providing strong crystallinity and aggregation characteristics. The synthesis route for GS-20 is shown in Figure S1.

GS-20’s Impact on Film Formation Kinetics

The influence of GS-20 on film formation kinetics was studied through in-situ UV-visible absorption spectroscopy measurements. Results showed that the addition of GS-20 accelerated the film deposition process and shortened film formation time.

GS-20’s Impact on Film Morphology

• GIWAXS Measurements: Showed improved π-π stacking distance (3.78 Å) and crystalline coherence length (CCL, 18.5 Å) in ternary blend films, indicating that GS-20 introduction enhanced eC9-2Cl molecular aggregation and regulated molecular stacking.
• AFM Measurements: Showed more uniform ternary films with minimum surface roughness (Rq) of 1.30 nm and good phase separation.

GS-20’s Impact on Device Performance

• The GS-20-added ternary cell achieved Voc of 0.890 V and FF of 78.4%, resulting in a final PCE of 19.0% without any post-processing.
• Compared to binary cells, ternary cells showed lower energy loss, suppressed trap-assisted recombination, and improved carrier mobility.
• The ternary cells also demonstrated excellent thermal stability and morphological stability.

Fabrication of Post-Processing-Free Organic Solar Modules

Post-processing-free organic solar modules based on the PBQx-TF:eC9-2Cl:GS-20 ternary blend system were fabricated using blade coating, achieving a PCE of 13.5% with an effective area of 23.6 cm2, demonstrating the feasibility of this ternary strategy in large-area device fabrication.

This research achieved high-efficiency post-processing-free organic solar cells through molecular design and ternary strategy, providing new insights for the industrial production of organic solar cells.

Original publication: “Post-Processing-Free Organic Solar Cells Achieving High Photovoltaic Performance through Crystallinity Control by a Third Component“, Advanced Energy Materials, Published: 2024, DOI: doi.org/10.1002/aenm.202404482