The transition to low-carbon energy systems is a critical global imperative, with solar energy standing as a cornerstone technology. Within low-carbon parks and smart grids, the efficiency of energy conversion directly impacts economic viability and sustainability goals. The core of this conversion lies within the photovoltaic panel. Despite significant advancements, the power conversion efficiency (PCE) of commercial and next-generation solar panels often encounters material-based limitations. A primary research frontier involves the development of novel active layer materials for the solar cells that constitute these panels. This study focuses on the design, synthesis, and characterization of a new wide-bandgap donor polymer and its application in non-fullerene acceptor (NFA)-based bulk heterojunction organic photovoltaic (OPV) cells, which are integral components of advanced solar panels. We further contextualize this material advancement within the framework of an intelligent energy acquisition system, essential for optimizing the output of solar panel arrays in distributed generation scenarios.
The performance of a solar panel is fundamentally dictated by the optoelectronic properties of its constituent solar cells. Key parameters include the breadth of sunlight absorption, the energy alignment of donor and acceptor materials, and the mobility of photogenerated charges (electrons and holes). Wide-bandgap donor materials are particularly valuable as they can be paired with efficient narrow-bandgap NFAs to create a complementary absorption profile, harvesting a broader spectrum of solar radiation. This work details the synthesis of a novel donor polymer, designated as A6, derived from 1,3-dibromo-5,5-dimethylhydantoin (DBH) and diketopyrrolopyrrole (DPP)-like isoindole units. We evaluate its performance when blended with two distinct NFAs: the state-of-the-art Y6-BO and the reference material PTIC. The objective is to fabricate and characterize high-PCE solar cell devices, establishing a material foundation for enhancing the overall energy yield of next-generation solar panels.
1. Material Synthesis and Device Fabrication
1.1 Synthesis of the Donor Polymer (A6)
The synthesis of the wide-bandgap donor polymer A6 followed a multi-step procedure involving monomer preparation and polymerization.
- Monomer Functionalization: A fluorinated alkoxybenzodithiophene (FBDT) core was stannylated and subsequently coupled using Stille cross-coupling reactions to introduce thiophene bridges. Bromination reactions using reagents like N-bromosuccinimide (NBS) were employed at specific stages to ensure correct coupling points.
- Incorporation of the Isoindole Unit: A key step involved the attachment of a diketone isoindole derivative. This unit is crucial for modifying the energy levels and enhancing the intermolecular packing of the resulting polymer.
- Final Polymerization: The dibrominated intermediate (A5) was copolymerized with a distannylated FBDT derivative via a Stille polycondensation reaction, catalyzed by Pd0 complexes (e.g., Pd2(dba)3/P(o-tol)3). The reaction was carried out in anhydrous toluene/chlorobenzene at elevated temperatures (~112 °C).
- Purification: The crude polymer was precipitated, filtered, and subjected to sequential Soxhlet extraction with different solvents (e.g., hexane, dichloromethane (DCM), chloroform (CF)) to remove oligomers and catalyst residues. The final polymer A6 was obtained from the chloroform fraction.
The chemical pathway emphasizes the deliberate molecular engineering to achieve a donor material with suitable frontier orbital levels for efficient charge generation and transfer in the final solar cell, a critical unit for high-performance solar panels.
1.2 Fabrication of Photovoltaic Cell Devices
The fabricated devices had a conventional structure: ITO/PEDOT:PSS/Active Layer/PDIN/Ag. This structure is a standard testbed for evaluating new materials destined for solar panel applications.
- Substrate Preparation: Indium tin oxide (ITO)-coated glass substrates (1.5 cm × 1.5 cm) were rigorously cleaned via sequential sonication in detergent, deionized water, acetone, and isopropanol, followed by UV-ozone treatment for 15 minutes.
- Hole Transport Layer Deposition: A poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer was spin-coated onto the cleaned ITO and annealed at 150 °C for 15 minutes to form a smooth hole-transporting and electron-blocking layer.
- Active Layer Preparation: Solutions of the donor polymer A6 and the acceptor (Y6-BO or PTIC) were prepared in a chloroform solvent with a small volume% of additive (e.g., 1-chloronaphthalene). The solutions were stirred overnight in a nitrogen glovebox. The active layer was then spin-coated onto the PEDOT:PSS layer. The blend ratio (D:A) and the total concentration (affecting film thickness) were systematically varied.
- Electron Transport Layer and Electrode Deposition: A thin layer of PDIN (a perylene diimide-based polymer) was spin-coated from a methanolic solution as an electron transport layer. Finally, a 100 nm silver (Ag) cathode was thermally evaporated onto the stack under high vacuum (<1×10-4 Pa).
2. Material Characterization and Fundamental Properties
2.1 Optical Absorption Properties
The UV-Vis absorption spectra of polymer A6 in both solution (A6-S) and thin-film (A6-F) states were measured. The data reveals crucial information about its bandgap and aggregation behavior.
| State | Peak Absorption Wavelength (λmax) | Absorption Onset (λonset) | Optical Bandgap (Egopt) |
|---|---|---|---|
| Solution (A6-S) | 532 nm | ~590 nm | – |
| Thin Film (A6-F) | 568 nm | ~605 nm | 2.05 eV |
The red-shift and broadening of the absorption peak in the film state compared to solution indicate strong intermolecular π-π stacking and aggregation, which is beneficial for charge transport. The optical bandgap (Egopt) is calculated from the film absorption onset (λonset) using the equation:
$$E_g^{opt} = \frac{1240}{\lambda_{onset} \text{ (in nm)}} \text{ eV}$$
For A6-F with λonset ≈ 605 nm: $$E_g^{opt} = \frac{1240}{605} \approx 2.05 \text{ eV}$$. This confirms A6 as a wide-bandgap donor material, suitable for pairing with lower-bandgap NFAs to maximize photon harvesting in a tandem cell or a single-junction cell for specific spectral ranges in solar panels.
2.2 Electrochemical Properties and Energy Level Alignment
Cyclic voltammetry (CV) was employed to estimate the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of A6, which are critical for determining the open-circuit voltage (VOC) and charge transfer driving force in a solar cell.
| Material | Oxidation Potential (Eox vs. Fc/Fc+) | Reduction Potential (Ered vs. Fc/Fc+) | HOMOCV (eV) | LUMOCV (eV) | Electrochemical EgCV (eV) |
|---|---|---|---|---|---|
| A6 | 0.70 V | -1.98 V | -5.50 | -2.82 | 2.68 |
The HOMO and LUMO levels were calculated using the following formulas, where the potential of the ferrocene/ferrocenium (Fc/Fc+) redox couple is taken as -4.8 eV relative to vacuum:
$$E_{HOMO} = – (E_{ox} + 4.8) \text{ eV}$$
$$E_{LUMO} = – (E_{red} + 4.8) \text{ eV}$$
Thus:
$$E_{HOMO}(A6) = – (0.70 + 4.8) = -5.50 \text{ eV}$$
$$E_{LUMO}(A6) = – (-1.98 + 4.8) = -2.82 \text{ eV}$$
The electrochemical bandgap is: $$E_g^{CV} = E_{LUMO} – E_{HOMO} = -2.82 – (-5.50) = 2.68 \text{ eV}$$, which is consistent with the optically measured bandgap. The relatively deep HOMO level of A6 is advantageous for achieving a higher VOC in the resulting solar panel cell, as VOC is often proportional to the offset between the donor HOMO and the acceptor LUMO.
3. Photovoltaic Device Performance and Optimization
3.1 Optimization of Blend Ratio and Film Thickness
To achieve the highest PCE for the solar panel cells, the donor-to-acceptor (D:A) weight ratio and the active layer thickness were systematically varied. The results for both acceptor systems are summarized below.
| Active Layer System | D:A Ratio | Thickness (nm) | JSC (mA/cm²) | VOC (V) | FF (%) | PCE (η%) |
|---|---|---|---|---|---|---|
| A6:Y6-BO | 1:1.0 | 100 | 22.15 | 0.81 | 62.5 | 11.22 |
| 1:1.5 | 100 | 26.80 | 0.83 | 68.4 | 15.23 | |
| 1:2.0 | 100 | 24.91 | 0.82 | 66.8 | 13.66 | |
| 1:1.5 | 80 | 25.10 | 0.82 | 67.4 | 13.87 | |
| 1:1.5 | 120 | 24.02 | 0.81 | 68.0 | 13.39 | |
| A6:PTIC | 1:1.0 | 100 | 18.05 | 0.78 | 66.6 | 9.38 |
| 1:1.5 | 100 | 16.32 | 0.76 | 68.0 | 8.45 | |
| 1:2.0 | 100 | 21.20 | 0.80 | 67.1 | 11.37 | |
| 1:2.0 | 80 | 14.50 | 0.78 | 63.9 | 7.22 | |
| 1:2.0 | 120 | 17.80 | 0.79 | 65.3 | 9.22 |
The optimal performance for the A6:Y6-BO system was achieved at a D:A ratio of 1:1.5 and a thickness of 100 nm, yielding a champion PCE of 15.23%. This represents a significant 3.86% absolute improvement (or ~34% relative improvement) over the best A6:PTIC device (PCE=11.37%). The higher performance stems from synergistic improvements in short-circuit current density (JSC), open-circuit voltage (VOC), and fill factor (FF).
3.2 Current Density-Voltage (J-V) Characteristics
The J-V curves under simulated AM 1.5G illumination for the champion devices clearly demonstrate the superiority of the A6:Y6-BO combination for use in efficient solar panels.
The key performance metrics can be related through the fundamental equation for solar cell output power and efficiency:
$$P_{max} = J_{SC} \times V_{OC} \times FF$$
$$\eta (PCE) = \frac{P_{max}}{P_{in}} \times 100\% = \frac{J_{SC} \times V_{OC} \times FF}{P_{in}} \times 100\%$$
where Pin is the incident light power density (typically 100 mW/cm²). For the champion A6:Y6-BO device:
$$P_{max} = (26.80 \times 10^{-3}) \times 0.83 \times 0.684 \approx 15.23 \times 10^{-3} \text{ W/cm}^2$$
$$\eta = \frac{15.23 \times 10^{-3}}{100 \times 10^{-3}} \times 100\% = 15.23\%$$
The enhanced JSC for the Y6-BO-based device is attributed to its broader and stronger absorption in the near-infrared region, complementary to the absorption of the A6 donor, leading to a superior photon harvest—a critical factor for the annual energy yield of solar panels.
3.3 Charge Carrier Transport and Recombination Dynamics
Understanding charge transport and recombination is essential for diagnosing performance limits in solar panel cells.
3.3.1 Charge Carrier Mobility
The hole-only and electron-only mobilities were estimated using the space-charge-limited current (SCLC) method. The mobility (μ) is derived from the Mott-Gurney law applied to the J-V characteristics of single-carrier devices:
$$J = \frac{9}{8} \epsilon_0 \epsilon_r \mu \frac{V^2}{L^3}$$
where J is the current density, ε0 is the vacuum permittivity, εr is the relative dielectric constant of the material, μ is the carrier mobility, V is the applied voltage, and L is the film thickness. The extracted values are summarized below.
| Material / Blend | Hole Mobility μh (cm²/V·s) | Electron Mobility μe (cm²/V·s) | Mobility Ratio μh/μe |
|---|---|---|---|
| A6 (Neat Film) | 5.80 × 10-4 | – | – |
| A6:Y6-BO (1:1.5) | 2.40 × 10-4 | 1.04 × 10-4 | 2.31 |
| A6:PTIC (1:2.0) | 1.83 × 10-4 | 0.84 × 10-4 | 2.18 |
The A6:Y6-BO blend exhibits higher and more balanced mobilities compared to the A6:PTIC blend. More balanced mobilities (μh/μe closer to 1) reduce space-charge accumulation and lead to a higher FF, as observed in the champion device. This efficient transport is vital for minimizing resistive losses in large-area solar panels.
3.3.2 Exciton Dissociation and Bimolecular Recombination
The exciton dissociation probability (P(E,T)) and the degree of bimolecular recombination were analyzed. The photogenerated current density (Jph) versus effective voltage (Veff) plot provides the dissociation probability Pdiss = Jph/Jsat, where Jsat is the saturation current. The champion A6:Y6-BO device showed a Pdiss of 95.3%, higher than the 92.2% for A6:PTIC, indicating more efficient charge generation at the donor-acceptor interfaces.
Bimolecular recombination was assessed by examining the light intensity (I) dependence of JSC. The relationship follows a power law: $$J_{SC} \propto I^{\alpha}$$. An exponent α close to 1 indicates weak bimolecular recombination. The extracted values were α = 0.975 for A6:Y6-BO and α = 0.943 for A6:PTIC. The value closer to unity for the Y6-BO blend confirms suppressed bimolecular recombination, contributing to its higher FF and JSC. Low recombination rates are essential for maintaining high efficiency under real-world, varying light conditions encountered by solar panels.
4. Integration into an Intelligent Energy Acquisition System
The development of high-PCE cells is only one part of maximizing energy output from a solar panel array. An intelligent acquisition system is necessary to manage the inherent variability and extract maximum power. The system architecture for a low-carbon park would involve:
- Distributed High-Efficiency Solar Panels: Arrays comprising the developed A6:Y6-BO based cells (or further optimized successors).
- Local Maximum Power Point Tracking (MPPT): Each solar panel or string incorporates an advanced MPPT algorithm (e.g., Perturb and Observe, Incremental Conductance) to continuously adjust the operating point to the voltage and current (Vmp, Imp) that deliver the maximum power, defined by:
$$P_{mp} = V_{mp} \times I_{mp} = \max(V \times I)$$ - Data Acquisition and Communication Nodes: Sensors monitor panel-level parameters (V, I, temperature, irradiance) and communicate via IoT protocols (e.g., LoRa, Zigbee) to a central gateway.
- Cloud/Edge Analytics Platform: The gateway transmits data to a cloud or edge computing platform. Machine learning models analyze historical and real-time data to:
- Predict energy generation.
- Detect panel faults or degradation (e.g., a drop in normalized efficiency η/η0).
- Optimize the dispatch of stored energy.
- Grid Integration and Storage Management: The system coordinates with battery energy storage systems (BESS) and the main grid, using optimization algorithms to decide when to store, use, or sell energy based on tariffs and demand.
The efficiency gain from the new cell technology directly amplifies the benefits of this intelligent system. Higher per-panel PCE means more energy is available for capture, storage, and dispatch, improving the return on investment and the stability of the local microgrid. The performance parameters characterized in this study (JSC(I), VOC(T), η) become critical inputs for the system’s predictive and diagnostic models.
5. Conclusion and Perspectives
This study successfully designed and synthesized a novel wide-bandgap donor polymer, A6, and demonstrated its outstanding performance in non-fullerene organic photovoltaic cells when paired with the Y6-BO acceptor. The champion device achieved a power conversion efficiency of 15.23%, significantly outperforming devices based on the PTIC acceptor. The performance enhancement is attributed to superior light absorption, more favorable energy level alignment, higher and more balanced charge carrier mobility, and reduced bimolecular recombination in the A6:Y6-BO blend. These findings provide a solid material-level foundation for advancing the efficiency of next-generation solar panels.
The integration of such high-efficiency solar cells into an intelligent energy acquisition system represents a holistic approach to solar energy utilization in low-carbon parks. The synergy between improved hardware (the solar panel cells) and sophisticated software (MPPT, data analytics, predictive control) is key to unlocking the full potential of photovoltaic technology. Future work will focus on further molecular optimization of the donor polymer to push PCE beyond 18%, scaling up the fabrication process for large-area solar panels, and implementing the developed cells into pilot-scale intelligent arrays for long-term stability and yield assessment. The continuous improvement of both the solar panel unit and the system that manages it is essential for a sustainable energy future.