Research Achievements and Highlights

• Novel DMAs Development: This research aims to improve the performance of organic solar cells (OSCs) based on dimer acceptors (DMAs), leading to the design and synthesis of two novel DMAs: DC9-HD and DYSe-3.
• Molecular Structure: DC9-HD and DYSe-3 possess nearly identical conjugated backbones, enabling excellent compatibility when blended and promoting efficient charge generation.
• Record Efficiency: The incorporation of DYSe-3 into the PM6 binary mixture ultimately achieved a power conversion efficiency (PCE) of 19.4%, representing the highest performance to date for single-junction dimer acceptor-based OSCs.
• Key Parameters: The study demonstrates that this ternary blend exhibits an open-circuit voltage (Voc) of 0.898 V, short-circuit current density (Jsc) of 27.4 mA cm-2, and a fill factor (FF) approaching 79%.
• Stability Performance: All binary and ternary OSCs maintained over 80% of their original efficiency after approximately 800 hours of storage at 65°C, demonstrating excellent thermal stability. 

Research Team

This research was conducted under the correspondence of Professor Yongsheng Chen, Director of the Nanoscience and Technology Research Center at Nankai University, and Professor Bin Kan from the School of Materials Science and Engineering.

Research Background

Prior to this research, significant progress had been made in the field of organic solar cells (OSCs), with single-junction OSCs surpassing 20% power conversion efficiency (PCE). However, due to non-ideal open-circuit voltage (VOC) losses in OSCs, the current best PCE values remain far below the ideal PCE predicted by the thermodynamic Shockley-Queisser limit theory.

Based on the excellent Y-series acceptors, researchers proposed combining two Y6 monomers into a “quasi-polymer” molecule with a defined chemical structure and excellent film-forming properties, showing great potential for manufacturing roll-to-roll large-area OSCs with low voltage loss and high morphological stability.

The research team developed a series of directly connected dimer acceptor (DMA) materials that not only demonstrated higher PCE than small molecule and polymer acceptors but also exhibited superior device stability. Building on this, researchers further developed oligomerized small molecule acceptors (OSMAs) composed of two or more SMA wing structures, breaking through 19% PCE.

Key Advantages of OSMA Materials

• Structural Benefits: Defined molecular structures and good batch reproducibility
• Energy Properties: Low reorganization energy and low diffusion coefficients
• Performance Potential: Ideal materials for developing high-performance, high-stability organic solar cells

Research Strategy and Previous Studies

• Strategy Implementation: The team utilized the ternary strategy common in SMA-based organic solar cells to enhance dimer acceptor device performance
• Previous Results: Adding SMA (Y6) to D18:DYF-TF binary system improved efficiency from 18.26% to 18.73%
• Further Advancement: Introduction of trimer acceptor into PM6:L8-BO-X binary system approached 20% efficiency

Expected Advantages

• Absorption Enhancement: Extended red-shifted absorption range
• Voltage Optimization: Reduced open-circuit voltage loss
• Stability Improvement: Enhanced device stability

Solution

Design and Synthesis of Novel Dimer Acceptors

• DC9-HD Development:
  • Based on DC9 as the foundation material
  • Shortened the 2-octyl dodecyl on the pyrrole unit to 2-hexyl decyl
  • Paired with wide-bandgap polymer donor PM6, achieving 18.7% PCE and approximately 80% fill factor
• DYSe-3 Development:
  • Based on multi-selenophene-substituted dimer acceptor DYSe-1
  • Replaced external undecyl side chains with nonyl side chains
  • Achieved high short-circuit current density of 27.5 mA cm⁻² when paired with PM6

Ternary Device Development Strategy

• DYSe-3 Selection Criteria:
  • Similar conjugated backbone to DC9-HD, ensuring good compatibility
  • Optimizes morphology when blended with main binary mixture
  • Features red-shifted near-infrared absorption
  • Enhances charge transport performance
• Performance Optimization:
  • Optimized material ratio (PM6:DC9-HD:DYSe-3 = 1:1:0.2)
  • Achieved highest photovoltaic conversion efficiency of 19.4%
  • Improved morphological characteristics and charge dynamics

Experimental Process and Steps

• Material Synthesis: The synthesis routes for DC9-HD and DYSe-3 are shown in Figure S1, with starting compounds 1-1 and 1-2 synthesized according to previously reported methods.
  
• Device Fabrication Process:
  • Substrate Treatment:
    • ITO glass substrates underwent standard cleaning procedures
    • UV-ozone treatment for 20 minutes for surface modification
  • Device Construction:
    • Spin-coating Br-2PACz hole transport layer (0.25 mg/ml, 3000 rpm)
    • Active layer solution preparation with specified ratios
    • Spin-coating active layer (2000 rpm) and thermal annealing (90°C, 10 minutes)
    • Spin-coating PNDIT-F3N electron transport layer
    • Vacuum deposition of 150 nm silver electrode
• Device Parameters:
  • Active area: 4 mm²
  • Test mask area: 3.24 mm²
• Characterization Methods:
  • Current density-voltage (J-V) curves recorded using Keithley 2400
  • Light source: Enlitech’s SS-F5-3A solar simulator (AM1.5 G)
  • EQE measurements: Enlitech’s QE-R spectral response system
  • Active layer thickness measured using profilometer

Research Characterization Results

Device Performance Characterization

1. Current Density-Voltage (J-V) Curves Analysis:
   • Figure 3a results:
     
     
     • PM6:DC9-HD and PM6:DYSe-3 binary devices achieved optimal PCE values of 18.7% and 18.6% respectively
     • Ternary device JSC increased from 26.2 mA cm−2 to 27.4 mA cm−2
     • VOC reached 0.898 V with FF approaching 79%
     • PM6:DC9-HD:DYSe-3 ternary device achieved 19.4% PCE at 1:1:0.2 ratio
   • Figure S6a
     
     demonstrates J-V curves comparison between PM6:DC9 and PM6:DC9-HD devices
     
     
     
     
2. External Quantum Efficiency (EQE) Analysis:
   • This study utilized Enlitech’s QE-R solar cell spectral response measurement system and FTPS-EQE instrument for EQE measurements
     
     
   • Figure 3b findings:
     
     
     • PM6:DC9-HD:DYSe-3 ternary device showed enhanced EQE values in 600-800 nm range
     • Integrated JSC reached 26.5 mA cm−2, 0.8 mA cm−2 higher than PM6:DC9-HD binary device (25.7 mA cm−2)
   • Figure S6b
     
     shows EQE spectra comparison between PM6:DC9 and PM6:DC9-HD devices
     
     
   • Figure S7
     
     presents normalized FTPS-EQE spectra for all three device types
     
     
     
3. Large-Area Device Performance:
   • Devices fabricated with 1 cm2 effective area using PM6:DC9-HD:DYSe-3 ternary blend
     
   • Figure 3e shows J-V curves achieving 16.4% PCE, demonstrating potential for large-area device manufacturing
4. Thermal Stability Testing:
   • Figure 3f results:
     
     
     • All devices maintained 80% of original efficiency after 800 hours at 65°C
     • Demonstrates excellent thermal stability of DMA-based OSCs
5. Photo-aging Testing:
   • Figure S8 results after 200 hours MPP tracking:
     
     
     • PM6:DC9-HD: retained 70% of original PCE
     • PM6:DYSe-3: retained 63% of original PCE
     • PM6:DC9-HD:DYSe-3: retained 71% of original PCE

Charge Dynamics Analysis

• Exciton Diffusion Length (LD):
  
  
  • Measured through pump-energy-dependent transient absorption spectroscopy
  • DC9-HD film: 46.4 nm
  • DYSe-3 film: 38.2 nm
  • Results shown in Figure 4a,b
    
    
• Charge Separation and Collection Probability:
  • Figure 4c findings:
    
    
    • PM6:DC9-HD and ternary devices showed higher Pdiss (98%)
    • Collection probability (Pcoll) reached 90% and 89% respectively
• JSC-Light Intensity Relationship:
  • Figure 4d shows ternary device achieved highest α value (0.99)
    
  • Indicates suppressed bimolecular recombination
• Transient Photocurrent (TPC) Measurements:
  • Figure 4e charge extraction times:
    
    
    • PM6:DC9-HD: 0.28 μs
    • PM6:DYSe-3: 0.25 μs
    • PM6:DC9-HD:DYSe-3: 0.22 μs (fastest extraction)
• Charge Transport Performance:
  • Evaluated using SCLC method (Figure S9)
    
  • Results shown in Figure 4f
    
    
  • Ternary device achieved highest and most balanced hole and electron mobility

Morphology Analysis

• Atomic Force Microscopy (AFM):
  
  
  • Figure 5a findings:
    • PM6:DYSe-3 binary film showed smaller fiber diameter due to high miscibility
    • DC9-HD’s longer exciton diffusion length promoted effective exciton dissociation
• Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS):
  
  
  • Figure 5b results show enhanced crystallinity of both donor and acceptor in ternary blend films
  • Improved charge transport performance
  • Enhanced EQE response

Research Achievements

• Device Performance:
  • Binary OSCs based on DC9-HD and DYSe-3 achieved over 18.5% PCE
  • Ternary device achieved 19.4% PCE with:
    • VOC: 0.898 V
    • JSC: 27.4 mA cm−2
    • FF: approaching 79%
• Stability Performance:
  • All devices retained over 80% of original efficiency after 800 hours at 65°C
  • Demonstrates excellent thermal stability of OSMA-based OSCs
• Research Impact:
  • Best performance to date for OSMA-based devices
  • Highlights potential of OSMA-based OSCs for high-efficiency, stable devices

Original publication: “Rational Design of Two Well-Compatible Dimeric Acceptors Through Regulating Chalcogen-Substituted Conjugated Backbone Enable Ternary Organic Solar Cells with 19.4% Efficiency”, Advanced Energy Materials, Published: November 2024, DOI: 10.1002/aenm.202404062