Thursday, January 23, 2025

Synthesis and Characterisation of Non-Fullerene based Photovoltaics

 

96.1) Introduction

Organic photovoltaic cells (OPVs) are a scalable way of gathering solar energy that uses carbon-based materials to create electricity from sunlight via the photovoltaic effect. The potential advantages of OPVs over conventional silicon-based cells include lower cost, weight, flexibility, and large-area fabrication. Organic photovoltaic cells (OPVs) are a scalable way of gathering solar energy that uses carbon-based materials to create electricity from sunlight via the photovoltaic effect. The potential advantages of OPVs over conventional silicon-based cells include lower cost, weight, flexibility, and large-area fabrication. In an OPV cell's photoactive layer1, an n-type absorbs electrons, and a p-type emits them. PC61BM and PC71BM Fullerenes were used as acceptor materials because of their high charge carrier capacity and electron affinity. Fullerenes have inherent restrictions, such as high synthesis costs, restricted electrical flexibility, and morphological instability due to heating. Non-fullerene acceptors (NFAs) are developed for practical reasons. Non-fullerene acceptors (NFAs) are developed for practical reasons. Perylene diimide (PDI), diketopyrrolopyrrole (DPP), and the A-D-A family2 (Fig 1) have been the most successful and thoroughly researched NFAs. PCEs for PDI3, DPP, and A-D-A-based4 non-fullerene OPVs5 have been reported to be over 11%, 13%, and 19%, respectively. This review article focuses on compounds with high PCEs from three classes.


96.2) Discussion

PDI is a well-known and extensively researched NFA in OPVs due to its high electron affinity and mobility, variable energy levels, and excellent chemical, thermal, and photochemical stability. Tang and coworkers reported the first PDI-based acceptor in 1986, using bilayer heterojunction OPVs. Currently, the best PCE for PDI monomer-based OPV devices is 3.7% lower than fullerene-based OPVs. PDI's intrinsic planarity and intermolecular solid interactions lead to undesirable micrometer scale crystallinity. Large crystalline domains in the polymer blend restrict exciton splitting, resulting in decreased photocurrent and poor device performance. Investigations have concentrated on functionalizing the modifiable locations of the backbone to lower molecular crystallinity and generate better NFAs.

DPPs are a versatile dye that exhibits high absorption in the visible region and is photochemically stable. DPPs' high backbone planarity and strong intermolecular π-π stacking make them ideal for creating charge transfer systems. DPP6 molecules have strong electron affinity, high electron mobility, and low LUMO energy levels. DPP derivatives have been created and tested as NFAs for efficient OPV.

Using A-D-A acceptors has proven to be a very successful approach to NFAs. A-D-A-type NFAs have an electron-rich core (D) and two electron-deficient terminals (A). The D and A components can be adjusted independently to adjust the energy levels, bandgap, molecule packing, and other features. ITIC, the Zhan group reported that was one of the first examples of this type, and more recently, high-performing Y67 are typical A-D-A type NFAs, with fused ladder-type arene as the backbone and electron-drawing units as the flanking arms. The planar skeletal structure8, organized π-π stacking, and improved optical absorption to the NIR range through the push-pull effect provide these molecules with high charge mobility. In order to create new A-D-A type NFAs, π-conjugated spacers, solvent-soluble sidechains9, electron-donating cores, and electron-drawing end groups must be made.

The A-D-A type structure of M310 is made up of two 2-(5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene) malononitrile (IC-2F) end groups and a benzo [1,2-b:4,5 b’] bis(4-H-di-thieno[3,2-b:20,30-d] pyrrole) (BDTPT) core with four side chains branching to the p-conjugated central unit. To create the brominated compound C1, the end groups of the M3 acceptor are altered by replacing the IC-2F end group with the mono-brominated IC-Br (Fig 1). Compound C2 is then created by replacing the IC-Br with 3-ethyl rhodanine end groups (Fig 2). Solar cells made using the mono-brominated acceptor C1 and the conjugated polymer PM6 demonstrated power conversion efficiencies of up to 11.6%, whereas solar cells based on the latter NFA only had very low efficiencies. The normalized absorption spectra of C1, C2, and the reference molecule M3 are displayed in Fig. 3. The absorbance peaks' maxima in solution were discovered to be 603 nm, 742 nm, and 744 nm, respectively. The three peaks in the solid state were moved toward longer wavelengths, peaking at 779 nm for M3, 797 nm for C1, and 624 nm for C2 thin films. When comparing M3 and C1, the steeper absorption edge of C1 suggests that the clean C1 film's molecular packing is more ordered. The significantly steeper absorption edge of C1, when compared to M311,12, suggests that the pristine C1 film's molecular packing is more ordered.


Fig 15 The chemical structures of fullerenes (PC61BM and PC71BM) and non-fullerene acceptors from the PDI, DPP, and A-D-A families (ITIC and Y6).

Fig 210 The structures of (b) the reference acceptor M3, (c) the donors PM6 and PBDB-T, and (a) the synthetic pathway for the acceptor molecules C1 and C2.

Fig 310 Normalized absorption spectra on glass substrates in thin films and solutions

OPV NFA can be discovered by integrating generative and predictive ML models13. In a generative machine learning model to learn chemical patterns and produce new molecules, the NFA chemical Target Synthesis method (Fig 4) starts with training the model using a dataset of roughly 50,000 NFA candidates from Lopez et al. Through the iterative incorporation of chemical laws, domain expertise was utilized to enhance the molecular structure via the generative models. In order to forecast HOMO/LUMO energy levels, the produced compounds were encoded as SMILES strings and fed into a predictive machine learning model that was trained using the same data set. A virtual screening criterion for identifying NFA candidates for synthesis was the computed PCE based on the anticipated HOMO/LUMO energy levels. Following multiple rounds of molecule synthesis and virtual screening, the molecular motif was determined using the chemical structure of the produced compounds as well as the computed PCE MS. Practical factors, including the availability of precursors and stricter synthetic requirements, were taken into account when manually creating molecules based on the molecular motif. We synthesized seven NFA candidates with different computed PCE MS. In order to validate the computed PCE MS, the HOMO and LUMO energy levels of the NFA candidates were lastly measured via CV and UV-vis measurements.


Fig 413 Schematic of the NFA Molecular Target Generation and Synthesis Workflow

TTE-PDI4 has a highly twisted molecular shape due to the free rotation of PDIs and nearby thiophene units. TTE-PDI4 undergoes ring fusion to produce FTTE-PDI4 (Fig 5), a more rigid molecule with more intramolecular stacking. Interestingly, TTE-PDI4 and FTTE-PDI4 have comparable energy levels, but their UV-Vis absorptions differ significantly. The latter exhibits high broad-band absorption with several abrupt peaks in the 300–600 nm range. TTE-PDI4 exhibits lesser absorption at long wavelengths even if its energy absorption start is lower. When combined with the polymer donor PFBDB-T, FTTE-PDI4 exhibits a larger photocurrent and, consequently, a higher power conversion efficiency (PCE) of 6.6% than blends based on TTE-PDI4 (PCE of 3.8%). This is because of its higher absorption and enhanced stiffness. The blend devices' high fill factor is probably a result of FTTE-PDI4's increased stiffness. It is determined that there is room for improvement by lowering voltage losses.

Fullerene-free organic solar cells (OSCs) have emerged as leaders in the photovoltaic field due to their superior optical and electrical properties. The quantum chemical study centered on developing pentacyclic aromatic bislactam-based chromophores14 for extremely efficient OSCs. Eight molecules (PCLMD1-PCLMD8) were designed from a reference compound (PCLMR) with an A2-π-A1-π-D-π-A1-π-A2 configuration through end group redistribution with benzothiophene acceptors. A UV-Vis comparison of simulated and experimental PCLMR values led to selecting the MPW1PW916-31G(d,p) functional for the DFT approach. The photovoltaic properties of the chromophores were investigated using several techniques, including UV-Vis, FMOs, TDM, Voc, and DOS. Modification of peripheral acceptors resulted in considerable modifications in charge-transfer characteristics. The changes led to a lower exciton binding energy of 2.277 to 2.087 eV, a larger maximum absorption wavelength of 829 to 882 nm in the solvent phase, and a smaller bandgap of 1.746 to 1.868 eV.


Fig 515 Synthetic route to TTE-PDI4 and FTTE-PDI4.

The results were compared with PCLMR, which had an exciton binding energy of 2.443 eV, a bandgap of 1.895 eV, and a maximum absorption wavelength of 813 nm. Significant charge dispersion between HOMO and LUMO was discovered in the hypothesized chromophores by the FMO research. All benzothiophene acceptor-based compounds (PCLMD1-PCLMD8) exhibited higher open-circuit voltage and electron and hole mobility rates compared to PCLMR. Benzothiophene acceptors with electron-drawing groups enhance charge transfer to acceptor components in organic solar cells (OSCs), improving the JSC and Voc values. The process broadens the absorbance spectrum as lowest unoccupied molecular orbital's (LUMO) energy level falls, while the highest occupied molecular orbital (HOMO) normally remains unchanged. Molecular engineering with benzothiophene acceptor moieties can improve the solar efficiency of NF-based materials.

96.3) Conclusion

This review focuses on the structure-property connection, synthesis, and characterization of PDI, DPP, and A-D-A derivatives used to improve non-fullerene OPV performance16. In OPV devices, excessive PDI molecule self-aggregation reduces blend shape and efficiency. To minimize intermolecular packing, structural changes were made to the nitrogen, bay, and ortho positions of the PDI monomer. To prevent the over-aggregation17 of PDI derivatives in mixed films, consider creating twisted or star-shaped NFAs with several PDI monomers. Designing new twisted and star-shaped PDI derivatives requires considering the trade-off between electron transport and nanometer-sized phase-separated domains in PDI-based NFAs. The strong electron mobility and NIR light-absorbing capabilities of DPP-based NFAs attract non-fullerene OPVs. DPP-based small molecules can be further classified as DPP-cored acceptors or DPP-terminated acceptors based on their different molecular design strategies. DPP-based polymers18 in organic transistors have significant hole and electron mobilities and absorb near-infrared light like naphthalene diimide-based polymers, such as N2200.Consequently, non-fullerene OPVs may likewise benefit from the use of DPP polymers. The A-D-A type19 NFAs' solubility, crystallinity, and miscibility can be further adjusted by utilizing various side chains, end groups, and π-spacers. The current development of A-D-A NFAs still faces certain obstacles. Simple synthetic pathways should be developed to support economical and scalable materials. Because most A-D-A acceptors have narrow bandgaps and NIR absorption, designing wide bandgap donors with complementary absorption, well-matched energy levels, and the ideal blend shape is also necessary. Solar cells17,20,21 with PM6:C1 absorber layer blends achieved up to 11.6% efficiency and a higher open circuit voltage of 0.914 V compared to PM6:M3-based solar cells (0.894 V), owing to the upshifted LUMO level of C1. PM6:M3 and PM6:C1 devices had similar high exciton dissociation probabilities. However, the PM6:C1 sample had reduced charge collection efficiency compared to the reference system PM6:M3. Computational methods using DFT and ML models plays crucial roles in predicting more efficient photovoltaic devices, which is experimentally proven. So, modern technology helps improve ongoing research.


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Adarsh Tiwari

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