Introduction

Organic solar cells (OPV) are emerging as a promising next-generation renewable energy technology, offering advantages such as light weight, tunable energy levels, and adjustable absorption properties. In recent years, multi-component strategies have shown tremendous potential in optimizing OPV photoelectric performance. However, adding additional components to already optimized binary blends typically negatively impacts morphology, leading to decreased device performance. To address this challenge, this study presents a dual-additive strategy that precisely controls complex morphology in multi-component systems through the synergistic effects of liquid additive 1,8-diiodooctane (DIO) and solid additive 1,4-diiodobenzene (DIB).

The key to this strategy lies in utilizing the different effects of DIO and DIB on acceptor and donor solidification kinetics to form ideal hierarchical morphology. Specifically, DIO promotes acceptor crystallization, while DIB facilitates pure phase formation. Through precise control of additive ratios, optimal vertical distribution of acceptors and donors in the film can be achieved. This hierarchical morphology benefits exciton dissociation, charge transport, and reduces charge recombination and energy loss, ultimately achieving significant device efficiency improvements.

Using this dual-additive strategy, this study achieved a high efficiency of 20.52% (certified at 19.92%) in the PM6:D18-Cl:L8-BO:BTP-eC9 quaternary blend system, representing one of the highest certified efficiencies for single-junction OPVs to date. This achievement highlights the importance of morphology control in multi-component OPVs and sets a new benchmark for accelerating their commercialization.

Research Team

This research was conducted by a team led by Professor Hongzheng Chen and Professor Lijian Zuo from Zhejiang University.

Other participating institutions include:

•  Xi’an Jiaotong University
• Donghua University
• Wuhan University of Technology

Research Background

Figure 2a reflects the core objective of the literature: developing high-efficiency organic solar cells. The J-V curve graph intuitively shows the impact of different additives on device performance, particularly how the dual-additive (DIO + DIB) strategy achieved the highest PCE, which is the most significant achievement of this research.

Organic photovoltaic devices (OPVs) are candidates for next-generation renewable energy technology, offering advantages such as light weight and tunable energy levels and absorption. In recent years, with rapid advances in molecular design and morphology control, certified power conversion efficiency (PCE) of OPVs has exceeded 19%. However, OPV performance still lags behind inorganic solar cells due to insufficient photon capture and morphology control complexity.

The key to high-efficiency OPV devices lies in bulk heterojunction blends with balanced crystallinity and fine nanoscale phase separation morphology. However, directly casting precursor solutions into solid films typically struggles to form ideal morphology, which usually requires balancing crystallinity and hierarchical donor:acceptor (D:A) phase separation morphology from nano to hundreds of nanometers. Thus, various control methods have been developed in state-of-the-art OPVs, including additives and post-annealing strategies.

Multi-component strategy has proven to be one of the most effective methods for achieving high performance, as it can simultaneously expand absorption range, optimize morphology, reduce charge recombination, and improve charge transport properties. However, most high-efficiency binary OPVs already have optimized morphology, and mixing additional components typically alters the optimal phase separation and crystallinity or molecular packing in multi-component active layers. The addition of extra components doesn’t necessarily guarantee higher PCE due to overall morphology deterioration. Therefore, striking a delicate balance between multi-component blend advantages and morphology optimization is crucial. A simple manipulation strategy is needed to eliminate negative impacts and adjust complex multi-component morphology, which essentially involves kinetic and thermodynamic control of multi-scale morphology, namely crystallinity, molecular orientation, and desired phase separation.

Solution and Experimental Process

This study proposes a dual-additive strategy using both liquid additive DIO and solid additive DIB to separately optimize crystallization and phase separation characteristics. This strategy aims to achieve optimal hierarchical morphology with balanced crystallinity and desired phase separation in quaternary D:A blends through fine-tuning film formation dynamics.

Experimental Process and Steps:

Material Preparation:

• Active Layer Materials: The study used four active layer materials: polymer donors PM6 and D18-Cl, small molecule acceptors L8-BO and BTP-eC9, and additives DIO and DIB. All materials were purchased from commercial suppliers and used as received.

Device Fabrication:

• OPV Device Structure: OPV devices adopted conventional bulk heterojunction structure, with layers deposited sequentially on ITO glass substrates: ITO/2PACz/active layer/PDINN/Ag.
• Active Layer Preparation: The active layer was prepared by spin-coating a mixed solution of PM6:D18-Cl:L8-BO:BTP-eC9 (1:0.25:0.75:0.75, wt%) and annealing at 100°C for 10 minutes.
• Additive Effects: To study additive effects, DIO, DIB, or DIO+DIB were added to the active layer solution.

Morphology Control:

• Film Formation Dynamics: The influence of additives on film formation dynamics was studied through in-situ time-resolved UV-visible absorption spectroscopy measurements.
• Kinetic Control: It was found that dual additives help extend acceptor solidification dynamics while shortening donor solidification dynamics.
• Morphology Formation: This differentiated kinetic control allows PM6 and D18-Cl to precipitate quickly at the bottom, while BTP-eC9 and L8-BO permeate through donors and/or stack at the top at a moderate rate, forming ideal morphology with self-organized hierarchical distribution, balanced crystallinity, and desired phase separation.

Research Characterization

Various characterization techniques were employed to study the effects of the dual-additive strategy on multi-component OPV device morphology and performance.

J-V Curves and Photovoltaic Parameters

As shown in Figure 2a-c and Table 1. Figure 2a displays J-V curves of devices based on PM6:D18-Cl:L8-BO:BTP-eC9 quaternary blends with different additives. Table 1 lists specific photovoltaic parameters including open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and power conversion efficiency (PCE) under different additive conditions.

The device without additives achieved a PCE of 19.03%, with VOC of 0.901 V, JSC of 27.80 mA cm-2, and FF of 75.79%. After adding single additives DIO or DIB, device efficiency improved, achieving maximum PCEs of 19.42% (VOC = 0.863 V, JSC = 28.43 mA cm-2, FF = 78.77%) and 19.25% (VOC = 0.884 V, JSC = 27.55 mA cm-2, FF = 78.67%) respectively. Remarkably, the dual-additive OPV device achieved a maximum PCE of 20.52%, with VOC of 0.879 V, JSC of 28.55 mA cm-2, and FF of 81.33%.

Enlitech Solar Simulator (SS-X50, Enlitech) was used for current density-voltage (J-V) measurements under AM 1.5G spectrum, with light intensity calibrated to 100 mW cm-2.

EQE Spectra

Enlitech Solar Cell Spectral Response Measurement System (QE-R, Enlitech) was used to obtain external quantum efficiency (EQE) data.

Other Characterizations:

• Atomic Force Microscopy (AFM): Used to study nanoscale surface morphology of active layers.
  
  Figure S9 shows AFM height and phase images of PM6:D18-Cl:L8-BO:BTP-eC9 quaternary blend films with different additive treatments, revealing distinct fibrous network structures beneficial for balanced exciton dissociation and charge transport.
• Infrared AFM (IR-AFM): Used to characterize detailed phase structure.
  
  
  Figure 3a and Figure S11 show IR-AFM images of films with different additive treatments, revealing acceptor-rich top surface in dual-additive films, indicating hierarchical separation morphology.
• Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS): Used to study additive effects on crystallinity and orientation at different depths.
  
  Figure 3b shows GIWAXS 2D diffraction patterns and corresponding line-cut profiles for films with different additive treatments.
• Grazing Incidence Small-Angle X-ray Scattering (GISAXS): Used to examine additive effects on phase separation characteristics.
  
  Figure 3d and Figure S19 show GISAXS patterns for films with different additive treatments.
• Film Depth-Dependent Light Absorption Spectra (FLAS) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS): Used to further characterize hierarchical morphology.
  
• Transient Absorption Spectroscopy (TAS): Used to study exciton dissociation dynamics.
  
  Figure 5a and Figure S26 show TAS measurements for films with different additive treatments.
• Space-Charge-Limited Current (SCLC): Used to measure charge transport.
  
  Figure 5b and Figure S27 show electron and hole mobility for devices with different additive treatments.
• Transient Photovoltage (TPV): Used to study additive effects on carrier lifetime.
  
  Figure 5c shows carrier lifetime for devices with different additive treatments.
• Light Intensity-Dependent VOC: Used to study charge recombination.
  
  Figure S29 shows VOC versus light intensity relationships for devices with different additive treatments.
• Energy Loss Analysis: Revealed lowest non-radiative recombination losses in dual-additive devices, with optimized radiative losses through suppressed electron vibration and extended electroluminescence efficiency.
  

Research Results and Advantages

This study successfully developed a dual-additive strategy for fine-tuning morphology in multi-component organic photovoltaic devices, leading to enhanced photovoltaic performance. Through the combination of liquid additive DIO and solid additive DIB, researchers successfully induced self-organized hierarchical morphology in the active layer, characterized by enhanced crystallinity and optimized phase separation.

Key achievements include:

• Record Efficiency: The most notable achievement is the 20.52% maximum efficiency (certified at 19.92%) for single-junction OPV devices based on PM6:D18-Cl:L8-BO:BTP-eC9 quaternary blends, attributed to the optimized morphology achieved through the dual-additive strategy.
• Mechanism Understanding: Researchers utilized various characterization techniques to deeply investigate the effects of the dual-additive strategy on film formation dynamics and morphology evolution, elucidating its working mechanism.
• Morphology Control Importance: The study emphasizes the significance of morphology control for achieving high-efficiency multi-component OPVs, providing valuable insights and guidance for future research in this field.

Conclusions and Future Implications

This research offers a simple yet effective morphology control strategy, opening new pathways for developing high-efficiency, stable multi-component organic photovoltaic devices. Furthermore, it provides a novel approach to optimize device performance through precise control of film formation dynamics, which has significant implications for advancing the commercialization of organic photovoltaic technology.

The dual-additive strategy exhibits several advantages:

• Synergistic Effects: The combination of DIO and DIB shows superior effects compared to using either additive alone. DIO promotes crystallization while DIB facilitates pure phase formation.
• Film Formation Control: The strategy extends acceptor solidification time while shortening donor solidification time, leading to optimal vertical distribution of components.
• Enhanced Performance: The approach achieves improved exciton dissociation, charge transport, and reduced charge recombination, ultimately leading to the record 20.52% single-junction OPV device efficiency.
• Reduced Energy Loss: The dual-additive strategy effectively suppresses non-radiative recombination losses and decreases Urbach energy, reducing energy losses and improving device open-circuit voltage (VOC).