In the context of low-carbon industrial parks, solar panels serve as the primary source of renewable electricity. However, the photoelectric conversion efficiency of conventional solar panels remains limited due to material constraints. To address this challenge, we focused on developing a wide-bandgap donor material synthesized from 1,3-dibromo-5,5-dimethylhydantoin (DBH) and diketoisoindole, paired with non-fullerene acceptors Y6-BO and PTIC. This study aims to fabricate photovoltaic panel cells with enhanced power conversion efficiency, thereby supporting the intelligent energy collection system for next-generation solar panels. Our work demonstrates that the choice of acceptor material critically influences the performance of solar panels, and we systematically optimized the donor-acceptor ratio and coating thickness. The ultimate objective is to provide a theoretical foundation for efficient solar panels in smart energy systems.
Previous studies have explored perovskite materials for hot-carrier solar panels, highlighting their potential for high efficiency. Others have developed high-performance organic solar cells with improved short-circuit current density and energy conversion, or introduced rare-earth doped quantum dots to enhance optical and electrical properties. However, the use of wide-bandgap materials in combination with non-fullerene acceptors remains an underexplored pathway for solar panels. We therefore synthesized a novel wide-bandgap polymer donor (denoted as A6) and systematically compared two acceptors, aiming to maximize the photoelectric conversion efficiency of solar panels.
Synthesis of the Donor Material (A6)
The synthesis of A6 involved a multi-step procedure. First, we added 2.313 g of monofluoro monoalkoxy compound, 8.988 g of tributyl(3-thienyl)tin, 0.4973 g of tetrakis(triphenylphosphine)palladium, and 45 mL of anhydrous dimethylformamide (DMF) into a Schlenk tube. The mixture was kept at 130°C for 12 hours, then evaporated. The residue was purified with dichloromethane (DCM) to obtain product A1. Second, we dissolved 1 g of A1, 4.713 g of anhydrous iron(III) chloride, and 200 mL of anhydrous DCM in a flask, reacted for 5 hours at 26°C, then added silica gel. After filtration and evaporation, the product was purified with n-hexane to obtain A2. Third, we added 23 mL of chloroform, 230 mg of A2, 320 mg of NBS, and 0.7 mL of concentrated sulfuric acid into a flask, stirred for 30 minutes at 26°C, then added another 76 mg of NBS and 0.2 mL of sulfuric acid. After 30 minutes, a mixture of 20 mL methanol and 1 mL water was added, and the precipitate was filtered to obtain A3. Fourth, we mixed 300 mg of A3, 944 mg of tributyl(4-(2-butyloctyl)thiophen-2-yl)stannane, 82 mg of tetrakis(triphenylphosphine)palladium, 10 mL of toluene, and 2 mL of DMF in a Schlenk tube, reacted at 112°C for 12 hours. After cooling, water and petroleum ether (PE) were added for extraction. The organic layer was evaporated and separated using a 1:2 (v/v) mixture of PE and DCM to obtain A4. Fifth, we dissolved 220 mg of A4 in 25 mL chloroform, added 106.2 mg of NBS, stirred for 2 hours, then added 25 mL of methanol. The precipitate was filtered and purified with PE:DCM (1:2) to obtain A5. Sixth, we added 80 mg of A5, 80.7 mg of 2-ethylhexyloxybenzo[1,2-b:4,5-b’]dithiophene (FBDT-Sn), 2.37 mg of tris(dibenzylideneacetone)dipalladium, 7.84 mg of tri(o-tolyl)phosphine, and 0.8 mL of anhydrous toluene into a Schlenk tube, reacted at 112°C for 16 hours, then added 8 mL of chlorobenzene and stirred for 10 minutes. The mixture was poured into 100 mL of methanol, filtered, and separated into DCM fraction, a 1:1.5 DCM/CF (carbon-fluorine) fraction, and a chloroform fraction. The chloroform fraction was precipitated into 100 mL of methanol, filtered, and dried to obtain A6. This final product served as the donor material for the solar panels.
| Step | Key reagents | Temperature (°C) | Time | Purification method |
|---|---|---|---|---|
| A1 | Monofluoro monoalkoxy, tributyl(3-thienyl)tin, Pd(0) | 130 | 12 h | Evaporation, DCM |
| A2 | A1, FeCl₃ | 26 | 5 h | Silica, n-hexane |
| A3 | A2, NBS, H₂SO₄ | 26 | 1 h | Methanol precipitation |
| A4 | A3, tributyl(4-(2-butyloctyl)thiophen-2-yl)stannane, Pd(0) | 112 | 12 h | PE:DCM column |
| A5 | A4, NBS | 26 | 2 h | PE:DCM (1:2) |
| A6 | A5, FBDT-Sn, Pd₂(dba)₃, P(o-tol)₃ | 112 | 16 h | Methanol precipitation, column |
Fabrication of Solar Panel Cells
We used indium tin oxide (ITO) glass substrates (1.5 cm × 1.5 cm) for the solar panels. The substrates were sequentially ultrasonicated in deionized water, acetone, and isopropanol for 15 min each, then treated with ultraviolet ozone for 15 min. PEDOT:PSS was spin-coated onto the substrates at 4000 rpm, followed by annealing at 150°C for 15 min to form a hole transport layer. The donor A6 was mixed with either Y6-BO or PTIC (acceptor) in designated ratios (1:1, 1:1.5, 1:2 by weight) and stirred in nitrogen for 12 h in chloroform. The blend solution was spin-coated on top of the PEDOT:PSS layer at various speeds to achieve target coating thicknesses (80, 100, or 120 nm), then annealed at 150°C for 15 min. Subsequently, PDIN (2 mg/mL in MeOH/AcOH, 1000:3 v/v) was spin-coated at 5000 rpm as an electron transport layer. Finally, silver electrodes were thermally evaporated onto the active layer at a pressure below 1×10⁻⁴ Pa and a rate of 0.5 Å/s, completing the photovoltaic panel cells.
Optical and Electrochemical Characterization of A6
We measured the absorption spectra of A6 in solution (chloroform) and in thin film using a Shimadzu UV-1800 spectrophotometer. The solution exhibited an absorption maximum at 532 nm, while the film showed a red-shifted peak at 568 nm with a single maximum, indicating strong aggregation. The absorption onset of the film was used to calculate the optical bandgap via:
$$ E_g^{\text{opt}} = \frac{1240}{\lambda_{\text{onset}}} $$
where λₒₙₛₑₜ is the onset wavelength (approximately 590 nm), yielding an optical bandgap of 2.10 eV. Thus, A6 is a wide-bandgap material suitable for solar panels.
| State | λ_max (nm) | λ_onset (nm) | E_g^opt (eV) |
|---|---|---|---|
| Solution | 532 | — | — |
| Film | 568 | 590 | 2.10 |
Cyclic voltammetry (CV) was performed using a CHI620D analyzer with a three-electrode system. The oxidation and reduction potentials of A6 were 0.7 V and -1.98 V (vs. Fc/Fc⁺), respectively. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were calculated via:
$$ E_{\text{HOMO}} = -e(E_{\text{ox}} + 4.8\ \text{V}) $$
$$ E_{\text{LUMO}} = -e(E_{\text{red}} + 4.8\ \text{V}) $$
yielding -5.51 eV and -2.82 eV, respectively. The electrochemical bandgap was 2.69 eV, while the effective LUMO obtained by adding the optical bandgap was -3.46 eV. The introduction of diketoisoindole lowered the HOMO level, which is beneficial for high open-circuit voltage in solar panels.
| Parameter | Value |
|---|---|
| E_ox (V vs Fc/Fc⁺) | 0.70 |
| E_red (V vs Fc/Fc⁺) | -1.98 |
| E_HOMO (eV) | -5.51 |
| E_LUMO (eV) | -2.82 |
| E_g^ec (eV) | 2.69 |
| E_LUMO^eff (eV) | -3.46 |
Optimization of Solar Panel Efficiency
We systematically varied the donor:acceptor mass ratio and coating thickness for solar panels fabricated with A6:Y6-BO and A6:PTIC. The power conversion efficiency (PCE) was measured under standard AM 1.5G illumination (100 mW/cm²). Table 4 summarizes the results.
| Active layer | D:A ratio | Coating thickness (nm) | PCE (%) |
|---|---|---|---|
| A6:Y6-BO | 1:1 | 100 | 11.22 |
| 1:1.5 | 100 | 15.23 | |
| 1:2 | 100 | 13.66 | |
| 1:1.5 | 80 | 13.87 | |
| 1:1.5 | 120 | 13.39 | |
| 1:1.5 | 100 | 15.23 | |
| A6:PTIC | 1:1 | 100 | 9.38 |
| 1:1.5 | 100 | 8.45 | |
| 1:2 | 100 | 11.37 | |
| 1:2 | 80 | 7.22 | |
| 1:2 | 120 | 9.22 | |
| 1:2 | 100 | 11.37 |
The optimal performance for solar panels was achieved with A6:Y6-BO at a 1:1.5 ratio and 100 nm coating thickness, yielding a PCE of 15.23%. For A6:PTIC, the best PCE was 11.37% at the same thickness but with a 1:2 ratio. The Y6-BO based solar panels showed a relative increase of 3.86% in PCE compared to PTIC-based ones, demonstrating the superiority of Y6-BO as an acceptor for wide-bandgap donors in solar panels.
Current-Voltage Characteristics
We measured the J-V curves under illumination for the best-performing solar panels. The A6:Y6-BO device exhibited a short-circuit current density (Jₛc) significantly higher than that of the A6:PTIC device; specifically, the Jₛc increased by 4.38 mA/cm². The open-circuit voltage (Vₒc) and fill factor (FF) also contributed to the enhanced PCE. The improvement is attributed to the broader absorption range of Y6-BO, which allows more photons to be harvested in solar panels.
| Active layer | V_oc (V) | J_sc (mA/cm²) | FF (%) | PCE (%) |
|---|---|---|---|---|
| A6:Y6-BO (1:1.5, 100 nm) | 0.89 | 24.6 | 69.5 | 15.23 |
| A6:PTIC (1:2, 100 nm) | 0.85 | 20.3 | 66.0 | 11.37 |
Charge Carrier Transport Properties
We evaluated the charge carrier mobilities using the space-charge-limited current (SCLC) method. The hole mobility of the pure A6 film was 5.80×10⁻⁴ cm²/(V·s). For the blend films, the hole mobilities were 2.40×10⁻⁴ and 1.83×10⁻⁴ cm²/(V·s) for A6:Y6-BO and A6:PTIC, respectively. The charge transfer rates, estimated from the slope of the SCLC curves, were 1.04×10⁻⁴ and 0.84×10⁻⁴ cm²/(V·s). The higher mobility and charge transfer rate in A6:Y6-BO blends facilitate efficient charge extraction, contributing to the superior performance of solar panels.
| Material | Hole mobility (cm²/V·s) | Charge transfer rate (cm²/V·s) |
|---|---|---|
| A6 (pure film) | 5.80×10⁻⁴ | — |
| A6:Y6-BO (1:1.2) | 2.40×10⁻⁴ | 1.04×10⁻⁴ |
| A6:PTIC (1:1.2) | 1.83×10⁻⁴ | 0.84×10⁻⁴ |
Exciton Dissociation and Bimolecular Recombination
We analyzed the exciton dissociation probability (P₍dᵢₛₛ₎) by measuring the photocurrent density versus effective voltage. Under a high effective voltage (~3 V), the saturation current densities for A6:Y6-BO and A6:PTIC solar panels corresponded to P₍dᵢₛₛ₎ values of 96.22% and 92.38%, respectively. This indicates that the Y6-BO blend more efficiently dissociates excitons into free charge carriers. Furthermore, we examined the dependence of Jₛc on light intensity (I). The power-law exponent α was 0.975 for A6:PTIC and 0.943 for A6:Y6-BO, suggesting that bimolecular recombination is less severe in the Y6-BO based solar panels. These results confirm that the A6:Y6-BO combination reduces charge recombination losses, leading to enhanced fill factor and overall efficiency in solar panels.
Conclusion
In this work, we successfully synthesized a wide-bandgap donor polymer A6 (optical bandgap 2.10 eV, HOMO -5.51 eV) and employed it in photovoltaic panel cells with non-fullerene acceptors. The solar panels fabricated with A6:Y6-BO at a 1:1.5 donor-acceptor ratio and 100 nm coating thickness achieved an optimal power conversion efficiency of 15.23%, which is 3.86% higher than that of A6:PTIC devices. The enhanced performance originates from the broader absorption of Y6-BO, higher charge carrier mobilities (2.40×10⁻⁴ cm²/V·s), improved exciton dissociation (96.22%), and reduced bimolecular recombination. These findings provide a robust material platform for high-efficiency solar panels, supporting the development of intelligent energy collection systems in low-carbon parks. Future work will focus on scaling up these solar panels and integrating them with smart grid technologies to maximize renewable energy utilization.