To achieve high-performing organic solar cells (OSCs), the ternary OSCs are a feasible and efficient option. It is important to develop a better 3rd component for efficient ternary OSC. Researchers developed a new molecule for this study and discovered that dimerized small molecule can increase efficiency of ternary OSCs
The new molecule shows complementary absorption along with energy levels that can match PM5 and BTP-eC9. Moreover, it can also control the PM6:BTP-eC9 arrangement. Better exciton dissociation, lesser recombination, and better charge transport takes place by using PM6:DSMD-βV:BTP-eC9 in the ternary device. This further led to a higher power conversion efficiency, around 18.26%. This level surpasses the previous efficiency of binary PM6:BTP-eC9 which was 17.63%.
Aim of the Study: To demonstrate the potential of dimerized small molecule donors for ternary organic solar cells (OSCs).
Dimerized Small Molecule Can Increase Efficiency of Ternary OSCs: How?
Researchers are preferring organic solar cells as a promising new entry in PV technology. It has distinctive advantages like translucency, flexibility, and lightweight design. With recent advancements in non-fullerene acceptor materials and device preparation technology, there have been essential improvements in the PV performance of OSCs.
Also, the adoption of a multiple-component strategy in the ternary approach has significantly increased the power conversion efficiency (OCE) of devices. It is evident that the addition of a 3rd guest component to the host binary system while constructing ternary OSCs has multiple advantages.
The addition of an appropriate 3rd component can maintain the simple machining of the single-junction device. This further helps in achieving better short circuit current density (Jsc). Afterwards, it is possible to control the micromorphology and crystallinity of the active layer. This further facilitates the dissociation of excitons. It enhances outstanding values of JSC, fill factor (FF), and charge transport.
Energy level arrangements and non-radiative recombination are also affected and optimize open-circuit voltage (VOC). It becomes an additional transport channel which can improve charge transfer.
Factors to be considered for an ideal 3rd component.
- Complementary absorption range
- Appropriate arrangements of energy levels
- Active layer morphology optimization
It is important to design and develop a matching 3rd component to effectively realize the above-mentioned factors. Also, to optimize the PV parameters of the OSCs.
Previous Studies’ Observation About Dimerized Small Molecule as 3rd Molecule
Active layer formulation in ternary non-fullerene OSCs involves the addition of 2 different donor materials. They both have a single acceptor and a single donor with 2 different acceptor materials.
The 3rd component, like oligomeric materials or polymers, can play an important role. Researching with oligomeric donor material as the 3rd component is limited even though it can effectively improve the efficiency of the devices. As researched, oligomeric donor materials processed with green solvent are capable of attaining higher PCE in ternary OSCs.
In this research, a dimerized small molecule donor was designed and synthesized by connecting 2 asymmetric small molecule donors with the vinyl group. The dimerized small molecule donor is referred to as DSMD-βV and has the following qualities.
- It has a wide absorption range from 300-900 nm.
- Has the highest occupied molecular orbital (HOMO) energy level, -5.55 eV.
- Strong aggregation ability from solution to film state
Further, researchers adopted PM6:BTP-eC9 system as the binary matrix. Which with complementary absorption of dimerized molecules establishes a favorable foundation as the 3rd component for developing efficient ternary organic solar cells.
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Qualities achieved after adding DSMD-βV
- Enhances phase separation of PM6:BTP-eC9-based film micromorphology
- Improve charge transport and exciton dissociation
- Performance increased to 18.26%.
Results and Discussion: Using Dimerized Small Molecule to Increase Efficiency of Ternary OSCs
Synthesis and Characterization of the New Device
Researchers synthesized an asymmetric small donor with a bromine-substituted end group to create the compound (9). Then they used this compound to synthesize the target product DSMD-βV by coupling it with vinyl using a Stille-coupling reaction.
The researchers used Pd2 (dba) 3 as the catalyst and P (o-tol) 3 as ligand. The target product DSMD-βV is soluble in chloroform (CF) and chlorobenzene (CB) solvents. Plus it is thermodynamically stable for up to 374° C and undergoes very little weight loss of about 5%. These qualities of the target materials are important to meet the processing requirements during device fabrication. The following figure shows the detailed synthetic procedure for the dimerized small molecule donor DSMD-βV.
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Optical and Electrochemical Properties
By using UV-vis absorption spectra, the optical property of DSMD-βV was carried out, as shown in the figure below. The corresponding data is mentioned in the table below.
| Molecule | ε (M−1 cm−1) | λpeak, sol (nm) | λpeak, film (nm) | λfilm, onset (nm) | Eg, opt (eV) | EHOMO (eV) | ELUMO (eV) |
| DSMD-βV | 1.20 × 105 | 447, 659 | 536, 744 | 858 | 1.45 | −5.55 | −3.55 |
Observations
- In the solution state, DSMD-βV shows a wide absorption range from 350-800 nm with 2 characteristic absorption bands.
- The long-wavelength region of the absorption range indicated intramolecular charge transfer processes.
- The distinctive absorption band can be ascribed to the localized π-π* electronic transitions.
- The maximum molar absorption coefficient of DSMD-βV is determined to be 1.20×105 M−1 cm−1 by Lemberger’s law.
- In the film state, the absorption spectrum DSMD-βV showed an 80 nm redshift than in the solution state.
- Also, there was an increase in the intensity of the long wavelength absorption peaks.
- As per the DSMD-βV film absorption edge, the calculated optical gap (Eg, opt) is 1.45 eV.
- A certain aggregation behavior was exhibited in DSMD-βV, and researchers think it can regulate the active layer morphology.
- During wavelengths from 350-570 nm and 640-780 nm, excellent absorption complementarity was observed with PM6 and BTP-eC9.
Quick Note – Excellent absorption complementarity is important to improve short-circuit density of ternary devices.
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Different Methods Used in the Experiment
- Researchers also characterized the temperature-dependent absorption to evaluate the aggregation properties of DSMD-βV in solution.
- Also, with the cyclic voltammetry (CV) method, the energy level of DSMD-βV was determined.
- The HOMO level was around −5.55 eV and was calculated by CV curves on the basis of the onset potential of the initial oxidation process.
- The initial reduction potential was used to derive the lowest unoccupied molecular orbital (LUMO) level, which was estimated at −3.55 eV.
- Further, HOMO and LUMO energy levels were positioned between PM6 and BTP-eC9 which allowed a cascade energy level arrangement in the ternary blend system.
Morphology Analysis
There is a close and direct relation between the morphology of the active layer and the overall performance of the device. It plays an important role in determining the efficiency of the OSCs. To fully comprehend the impact of adding DSMD-βV on morphological properties, researchers conducted a comprehensive investigation of the 3 different blends of films. The main aim behind this was to gain an in-depth understanding of the effects taking place due to the addition of DSMD-βV.
Observations
- According to the results found by atomic force microscope (AFM), the micromorphology property of the active layer could be effectively regulated by the addition of DSMD-βV in the binary system.
- The root-mean-square roughness (RMS) of the three films were: DSMD-βV:BTP-eC9 (1.43), PMT:BTP-eC9 (1.48) and PM6:DSMD-βV:BTP-eC9 (1.28). Quick Note: The small RMS values indicate the compatible blending of DSMD-βV with the host system.
- A uniform surface morphology was observed in The PM6:DSMD-βV:BTP-eC9 based ternary blend film than the PM6:BTP-eC9 based film.
- As per the transmission electron microscope (TEM), obvious phase separation morphology was observed in DSMD-βV:BTP-eC9 based film.
- After introducing the guest molecule DSMD-βV in the host PM6:BTP-eC9 system, there was an enhancement in the phase separation of PM6:DSMD-βV:BTP-eC9 based film.
- There is a possibility of improved exciton dissociation and charge transport for better JSC and FF with enhanced stacking and phase separation.
Thus, on the basis of this information, researchers demonstrated that the guest molecule has the potential to serve as the 3rd component. That too, specifically in regulating micromorphological properties of the active layer.
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Performance of the Photovoltaic After Adding 3rd Molecule
Researchers used different active layers to make and study the organic solar cell devices, namely PM6:DSMD-βV:BTP-eC9, PM6:BTP-eC9, and DSMD-βV:BTP-eC9. The device structure consisted of ITO/PEDOT:PSS/active layer/PDINN/Ag. The following figure represents the characterized PV performance and J-V curves data.
| Devices | VOC (V) | JSC (mA cm−2) | FFsss (%) | PCEa (%) | Jcal (mA cm−2) |
| DSMD-βV:BTP-eC9 | 0.817 | 5.22 | 33.36 | 1.42 (1.31±0.11) | 5.51 |
| PM6:BTP-eC9 | 0.838 | 27.02 | 77.82 | 17.63 (17.53±0.09) | 26.33 |
| PM6:DSMD-βV:BTP-eC9 | 0.846 | 27.46 | 78.59 | 18.26 (18.15±0.06) | 26.54 |
Observations
- With the addition of guest molecule to the PM6:BTP-eC9 system there were improvements in the FF, VOC, and JSC.
- VOC shows improvements due to lower HOMO energy levels.
- Additionally, JSC and FF improved due to the optimization of the structure of active layer and energy levels.
- The PM6:DSMD-βV:BTP-eC9 device’s power conversion efficiency (PCE) reached 18.26%.
- DSMD-βV:BTP-eC9 and PM6:DSMD-βV:BTP-eC9 based devices achieved external quantum efficiency (EQE) curves across a wavelength of 300-1000 nm, as shown in the figure below.
- PM6:DSMD-βV:BTP-eC9 ternary devices EQE curves showed higher curve contours than PM6:BTP-eC9 based devices. This indicates improved photon capture capability of the device which leads to enhanced JSC.
Furthermore, to understand FF and JSC enhancement in ternary OSC, researchers analyzed the device’s physics. Then by correlating VOC and varying Plight, researchers investigated trap-assisted recombination.
- Overall, the introduction of the DSMD-βV effectively decreased trap-assisted recombination to achieve superior JSC.
- Then the charge transport capabilities influenced the performance of the device.
- Higher FF, PCE, and JSC were obtained due to the balanced and satisfactory mobility of the ternary devices.
- There can be more suppression of charge recombination during charge transport with the introduction of DSMD-βV. This could further enhance JSC.
- A long carrier lifetime is observed in the DSMD-βV:BTP-eC9 device but slow carrier extraction ability. This causes recombination and results in poor PV performance.
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Transient Photocurrent (TPC) and Transient Photovoltage (TPV) Tests
Researchers studied the carrier extraction time and carrier lifetime through TPC and TPV tests.
Observations
| Types of films | DSMD-βV:BTP-eC9 | PM6:BTP-eC9 | PM6:DSMD-βV:BTP-eC9 |
| TPC curves – Extraction time (ts) | 0.77 μs | 0.62 μs | 0.66 μs |
| TPV curves – Extraction time (ts) | 0.50 μs | 0.24 μs | 0.23 μs |
Conclusion
So, researchers concluded that the newly developed donor material DSMD-βV developed from the combination of 2 small molecule donors has various features. It has a wide absorption range, strong aggregation ability, and low HOMO energy levels. In comparison to BTP-eC9 and PM6, devices, the newly developed one has complementary absorption and energy level arrangement. Moreover, it leads to an improvement of the performance up to 18.26% which is higher than binary devices. Thus, it can be concluded that the dimeric small molecule donor has the potential to make ternary OSCs efficient.
Source: Dimerized small molecule donor enables efficient ternary organic solar cells
