In our pursuit of advancing sustainable energy solutions for low-carbon industrial parks, we focused on enhancing the photoelectric conversion efficiency of modern solar panel systems. The performance of solar panel cells is fundamentally limited by the properties of the active layer materials, particularly the interplay between donor and acceptor components. Traditional fullerene-based acceptors suffer from limited absorption and poor morphological stability, prompting the exploration of non-fullerene alternatives. Our study introduces a novel wide-bandgap polymer donor, synthesized from 1,3-dibromo-5,5-dimethylhydantoin and diketoisoindole derivatives, paired with two distinct non-fullerene acceptors: Y6-BO and PTIC. Through systematic optimization of blend ratios, film thicknesses, and device architectures, we achieved a remarkable power conversion efficiency of 15.23% for the Y6-BO-based solar panel cells, representing a 3.86% improvement over the PTIC counterpart. This work not only demonstrates the potential of our custom donor material but also provides a comprehensive framework for understanding charge generation, transport, and recombination in high-performance solar panel devices.
We begin by describing the synthesis of our donor polymer, which we denote as A6. The synthetic route involved multiple steps, beginning with Stille coupling between monofluoromonoalkoxy and tributyl(3-thienyl)tin catalyzed by tetrakis(triphenylphosphine)palladium in anhydrous dimethylformamide to afford intermediate A1. Subsequent oxidative polymerization with anhydrous ferric chloride in dichloromethane yielded A2. Bromination using N-bromosuccinimide in chloroform with concentrated sulfuric acid as a catalyst produced A3. A further Stille coupling with tributyl(4-(2-butyloctyl)thien-2-yl)stannane generated A4, followed by another bromination step to produce A5. Finally, A5 was copolymerized with 2-ethylhexyloxybenzo[1,2-b:4,5-b’]dithiophene stannane via palladium-catalyzed Stille coupling to afford the target polymer A6. The crude product was purified by sequential extraction using dichloromethane (DCM), a mixture of DCM and carbon fluoride (CF) at a volume ratio of 1:1.5, and chloroform. The chloroform fraction was precipitated into methanol, filtered, and dried to yield pure A6 as a dark solid.
We characterized the optical properties of A6 using ultraviolet-visible absorption spectroscopy. As shown in Table 1, the solution-phase A6 exhibited a maximum absorption peak at 532 nm, while the thin-film state showed a red-shifted peak at 568 nm, indicative of enhanced intermolecular aggregation in the solid state. The optical bandgap was determined from the absorption onset of the film using the equation:
$$E_g^{opt} = \frac{1240}{\lambda_{onset}}$$
where λonset is the absorption onset wavelength. This calculation yielded a value of 2.10 eV, confirming that A6 is a wide-bandgap material suitable for tandem or semi-transparent solar panel applications.
Electrochemical characterization was performed via cyclic voltammetry to evaluate the energy levels of A6. The oxidation and reduction potentials were measured as 0.7 V and -1.98 V versus Fc/Fc+, respectively. Using the following equations:
$$E_{HOMO}^{ec} = -e(E_{ox} + 4.8 V)$$
$$E_{LUMO}^{ec} = -e(E_{red} + 4.8 V)$$
$$E_g^{ec} = E_{LUMO}^{ec} – E_{HOMO}^{ec}$$
we obtained a HOMO of -5.5 eV and a LUMO of -2.8 eV. The electrochemical bandgap was 2.69 eV. To derive the effective LUMO relevant for charge transfer, we used the optical bandgap:
$$E_{LUMO}^{eff} = E_{HOMO}^{ec} + E_g^{opt} = -5.5 eV + 2.10 eV = -3.46 eV$$
This deep HOMO level is advantageous for achieving high open-circuit voltage in solar panel cells, while the relatively high-lying LUMO promotes efficient electron transfer to suitable acceptors.
We fabricated bulk-heterojunction solar panel cells using A6 as the donor and either Y6-BO or PTIC as the acceptor. The device architecture consisted of indium tin oxide (ITO) glass substrate, PEDOT:PSS as the hole transport layer, the active layer, PDIN as the electron transport layer, and a silver top electrode. The active layer was spin-coated from a chloroform solution with varied donor:acceptor weight ratios and annealed at 150 °C for 15 minutes. The optimal processing conditions were determined by screening a matrix of parameters, as summarized in Table 1.
Table 1. Photovoltaic performance optimization for A6:Y6-BO and A6:PTIC solar panel cells
| Material | Donor:Acceptor Ratio (w/w) | Active Layer Thickness (nm) | Power Conversion Efficiency (%) |
|---|---|---|---|
| A6:Y6-BO | 1:1 | 100 | 11.22 |
| 1:1.5 | 100 | 15.23 | |
| 1:2 | 100 | 13.66 | |
| A6:Y6-BO (thickness variation at 1:1.5) | 1:1.5 | 80 | 13.87 |
| 1:1.5 | 100 | 15.23 | |
| 1:1.5 | 120 | 13.39 | |
| A6:PTIC | 1:1 | 100 | 9.38 |
| 1:1.5 | 100 | 8.45 | |
| 1:2 | 100 | 11.37 | |
| A6:PTIC (thickness variation at 1:2) | 1:2 | 80 | 7.22 |
| 1:2 | 100 | 11.37 | |
| 1:2 | 120 | 9.22 |
The data clearly indicate that the A6:Y6-BO system outperformed the A6:PTIC system under all conditions. The best device achieved a PCE of 15.23% with a donor:acceptor ratio of 1:1.5 and an active layer thickness of 100 nm. This represents a 3.86% absolute improvement over the best PTIC-based device (11.37%). We attribute this enhancement to the broader and stronger absorption of Y6-BO in the near-infrared region, which complements the wide-bandgap absorption of A6, leading to improved light harvesting in the solar panel cell.
The current density-voltage (J-V) characteristics of the champion devices are shown in Figure 1 (not reproduced here). The A6:Y6-BO device exhibited a short-circuit current density (JSC) of 24.8 mA cm-2, which was 4.38 mA cm-2 higher than that of the A6:PTIC device (20.42 mA cm-2). The open-circuit voltage (VOC) for the A6:Y6-BO device was 0.86 V, slightly lower than the 0.92 V of the PTIC device, but the fill factor (FF) was markedly improved (71.5% vs. 59.8%). The overall increase in JSC and FF compensated for the minor VOC loss, resulting in superior PCE.
To understand the charge transport properties, we measured the hole and electron mobilities using the space-charge-limited current (SCLC) method. The hole mobilities were extracted from hole-only devices (ITO/PEDOT:PSS/active layer/MoO3/Ag) using the Mott-Gurney law:
$$J = \frac{9}{8} \epsilon_0 \epsilon_r \mu_h \frac{V^2}{d^3}$$
where ε0 is the vacuum permittivity, εr is the relative permittivity (taken as 3.0 for organic semiconductors), μh is the hole mobility, V is the applied voltage, and d is the active layer thickness. Similarly, electron mobilities were obtained from electron-only devices (ITO/ZnO/active layer/PDIN/Ag). The results are summarized in Table 2.
Table 2. Charge carrier mobilities of pure and blend films
| Film | Hole Mobility (cm² V⁻¹ s⁻¹) | Electron Mobility (cm² V⁻¹ s⁻¹) | Charge Transport Ratio (μe/μh) |
|---|---|---|---|
| Pure A6 | 5.80 × 10⁻⁴ | – | – |
| A6:Y6-BO (1:1.5) | 2.40 × 10⁻⁴ | 1.04 × 10⁻⁴ | 0.43 |
| A6:PTIC (1:2) | 1.83 × 10⁻⁴ | 0.84 × 10⁻⁴ | 0.46 |
The pure A6 film exhibited a hole mobility of 5.80 × 10⁻⁴ cm² V⁻¹ s⁻¹. In the blend films, the A6:Y6-BO mixture showed higher hole and electron mobilities (2.40 × 10⁻⁴ and 1.04 × 10⁻⁴ cm² V⁻¹ s⁻¹, respectively) compared to the A6:PTIC blend (1.83 × 10⁻⁴ and 0.84 × 10⁻⁴ cm² V⁻¹ s⁻¹). The charge transport ratio (μe/μh) was slightly lower for the Y6-BO blend (0.43) than for PTIC (0.46), indicating a more balanced charge transport in the latter. However, the absolute mobility values were higher in the Y6-BO system, which contributed to the higher JSC and FF.
We further investigated exciton dissociation and bimolecular recombination by measuring the photocurrent density as a function of effective voltage and light intensity. The exciton dissociation probability, Pdiss, was calculated from the ratio of Jph(V) under short-circuit to Jsat (saturation current at high reverse bias). For the A6:Y6-BO device, Pdiss reached 95.3%, while the A6:PTIC device showed 92.2%. This higher dissociation efficiency indicates that the A6:Y6-BO blend forms a more favorable donor-acceptor interface with reduced geminate recombination.
The dependence of JSC on light intensity (I) follows a power law relationship:
$$J_{SC} \propto I^{\alpha}$$
where α is the power-law exponent. An α value close to unity indicates negligible bimolecular recombination. Our measurements yielded α = 0.975 for the A6:Y6-BO device and α = 0.943 for the A6:PTIC device. The higher α in the Y6-BO system suggests that bimolecular recombination is less severe, consistent with the higher fill factor and overall performance.
In addition to the above results, we performed morphological characterization using atomic force microscopy (AFM) to correlate film roughness with device performance. The A6:Y6-BO blend films exhibited root-mean-square roughness of 1.2 nm, significantly smoother than the A6:PTIC films (2.8 nm). A smoother surface reduces contact resistance and facilitates charge extraction at the electrode interface, further contributing to the enhanced efficiency of the solar panel cell.
We also assessed the long-term stability of the unencapsulated devices under ambient conditions (25 °C, 40% relative humidity). After 500 hours, the A6:Y6-BO solar panel cells retained 78% of their initial PCE, whereas the A6:PTIC cells retained only 62%. The superior stability of the Y6-BO-based system is attributed to the higher crystallinity of Y6-BO and its favorable miscibility with the A6 donor, which suppresses phase separation and degradation pathways.
Comparing our results with recent literature, the PCE of 15.23% achieved in this work is competitive with state-of-the-art wide-bandgap donor-based solar panel cells. For instance, systems using similar diketoisoindole polymers paired with Y6 derivatives have reported efficiencies in the range of 13.5–14.8%. Our improvement stems from the optimized donor-acceptor pairing and processing conditions. The 3.86% enhancement over the PTIC-based device underscores the importance of selecting an acceptor with complementary absorption and energy levels.
We further examined the temperature dependence of the device parameters to gain insights into the recombination mechanisms. From the dark J-V characteristics, the ideality factor (n) was extracted using the Shockley diode equation. The A6:Y6-BO device exhibited n = 1.35, indicating that trap-assisted recombination is relatively suppressed compared to the A6:PTIC device (n = 1.52). This is consistent with the higher fill factor and better performance of the Y6-BO system.
To validate the potential of this technology for practical solar panel deployment in low-carbon industrial parks, we performed a simple energy yield simulation based on the measured device parameters. Assuming a standard AM1.5G spectrum and a module area of 1 m², the A6:Y6-BO solar panel cell could generate approximately 152.3 W under peak sunlight. When integrated into a smart energy acquisition system with maximum power point tracking, the system-level efficiency is projected to be around 14.5% after accounting for wiring losses and inverter efficiency. This makes the developed solar panel cell a promising candidate for building-integrated photovoltaics and decentralized energy generation.
In summary, our work presents a comprehensive study on the design, synthesis, and characterization of a new wide-bandgap donor polymer A6 and its application in high-efficiency solar panel cells. By pairing A6 with the non-fullerene acceptor Y6-BO at an optimal donor:acceptor ratio of 1:1.5 and an active layer thickness of 100 nm, we achieved a power conversion efficiency of 15.23%, which is a significant improvement over the PTIC-based counterpart. The enhanced performance is attributed to the superior light absorption, balanced charge transport, efficient exciton dissociation, and reduced recombination in the A6:Y6-BO blend. These findings provide a solid foundation for the development of next-generation solar panel systems that can effectively convert solar energy into electricity, contributing to the goal of low-carbon and sustainable energy in industrial parks.