End Group Engineering on the Side Chains of Conjugated Polymers toward Efficient Non-Fullerene Organic Solar Cells

Research progress and application of high efficiency organic solar cells based on benzodithiophene donor materials

In recent decades, the demand for clean and renewable energy has grown increasingly urgent due to the irreversible alteration of the global climate change. As a result, organic solar cells (OSCs) have emerged as a promising alternative to address this issue. In this review, we summarize the recent progress in the molecular design strategies of benzodithiophene (BDT)-based polymer and small molecule donor materials since their birth, focusing on the development of main-chain engineering, side-chain engineering and other unique molecular design paths. Up to now, the state-of-the-art power conversion efficiency (PCE) of binary OSCs prepared by BDT-based donor materials has approached 20%. This work discusses the potential relationship between the molecular changes of donor materials and photoelectric performance in corresponding OSC devices in detail, thereby presenting a rational molecular design guidance for stable and efficient donor materials in future.

Recent Advances of Benzodithiophene-Based Donor Materials for Organic Solar Cells

Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has increased dramatically, making a big step toward the industrial application of OSCs. Among numerous OSCs, benzodithiophene (BDT)-based OSCs stand out in achieving efficient PCE. Notably, single-junction OSCs using BDT-based polymers as donor materials have completed a PCE of over 19%, indicating a dramatic potential for preparing high-performance large-scale OSCs. This paper reviews the recent progress of OSCs based on BDT polymer donor materials (PDMs). The development of BDT-based OSCs is concisely summarized. Meanwhile, the relationship between the structure of PDMs and the performance of OSCs is further described in this review. Besides, the development and prospect of single junction OSCs are also discussed.

Effect of Number and Position of Chlorine Atoms on the Photovoltaic Performance of Asymmetric Nonfullerene Acceptors

It has been well proved that the introduction of halogen can effectively modify the optoelectronic properties of classic symmetric nonfullerene acceptors (NFAs). However, the relevant studies for asymmetric NFAs are limited, especially the effect of halogen substitution number and position on the photovoltaic performance is not clear. In this work, four asymmetric NFAs with A-D-A1-A2 structure are developed by tuning the number and position of chlorine atoms on the 1,1-dicyanomethylene-3-indanone end groups, namely, A303, A304, A305, and A306. The related NFAs show progressively deeper energy levels and red-shifted absorption spectra as the degree of chlorination increases. The PM6:A306-constructed organic solar cells (OSCs) give a champion power conversion efficiency (PCE) of 13.03%. This is mainly ascribed to the most efficient exciton dissociation and collection, suppressed charge recombination, and optimal morphology. Moreover, by alternating the substitution position, the PM6:A305-based device yielded a higher PCE of 12.53% than that of PM6:A304 (12.05%). This work offers fresh insights into establishing excellent asymmetric NFAs for OSCs.

N-Annulated Perylene Bisimide-Based Double-Cable Polymers with Open-Circuit Voltage Approaching 1.20 V in Single-Component Organic Solar Cells

In this work, we have introduced single/double-sided N-annulated perylene bisimide (PBI) with deep energy levels into double-cable polymers with poly[1-(5-(4,8-bis(4-chloro-5-(2-ethylhexyl)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b']dithiophen-2-yl)thiophen-2-yl)-5,7-bis(2-ethylhexyl)-3-(5-methylthiophen-2-yl)-4H,8H-benzo[1,2-c:4,5-c']dithiophene-4,8-dione] (PBDB-T-Cl) as a donor backbone, marking as s-PPNR and as-PPNR, according to the molecular symmetry. Both double-cable polymers displayed a high open-circuit voltage approaching 1.20 V in light of high energy level discrepancy between electron-donating and electron-withdrawing parts, which is the highest open-circuit voltage among double-cable-based single-component organic solar cell (SCOSC) devices. Additionally, the asymmetric polymer displayed improved absorption spectra, thereby promoting crystallization and phase separation. Consequently, the as-PPNR-based SCOSCs achieved a power conversion efficiency of 5.05% along with a higher short-circuit current density and fill factor than their s-PPNR-based counterparts. In this work, we have successfully incorporated N-annulated PBI into double-cable polymers and revealed the important effects on structural symmetry and phase separation of double-cable polymers for higher SCOSC performance.

Impact of Aryl End Group Engineering of Donor Polymers on the Morphology and Efficiency of Halogen-Free Solvent-Processed Nonfullerene Organic Solar Cells

End group engineering on the side chain of π-conjugated donor polymers is explored as an effective way to develop efficient photovoltaic devices. In this work, we designed and synthesized three new π-conjugated polymers (PBDT-BZ-1, PBDT-S-BZ, and PBDT-BZ-F) with terminal aryl end groups on the side chain of chlorine-substituted benzo[1,2-b:4,5b']dithiophene (BDT). End group modifications showed notable changes in energy levels, dipole moments, exciton lifetimes, energy losses, and charge transport properties. Remarkably, the three new polymers paired with IT-4F (halogen-free solvent processed/toluene:DPE) displayed high power conversion efficiencies (PCEs) compared to a polymer (PBDT-Al-5) without a terminal end group (PCE of 7.32%). Interestingly, PBDT-S-BZ:IT-4F (PCE of 13.73%) showed a higher PCE than the benchmark PM7:IT-4F. The improved performance of PBDT-S-BZ well correlates with its improved charge mobility, well-interdigitated surface morphology, and high miscibility with a low Flory-Huggins interaction parameter (1.253). Thus, we successfully established a correlation between the end group engineering and bulk properties of the new polymers for realizing the high performance of halogen-free nonfullerene organic solar cells.

Near-Infrared Nonfullerene Acceptors Based on 4H-Cyclopenta[1,2-b:5,4-b']dithiophene for Organic Solar Cells and Organic Field-Effect Transistors

The development of nonfullerene small molecular acceptors (NF-SMAs) has dominated the improvement of efficiencies for organic solar cells and the near-infrared (NIR) absorption is the primary feature of NF-SMAs compared with fullerene derivatives. In this article, a series of acceptor-donor-acceptor-structured NF-SMAs (named CPICs) containing 4H-cyclopenta[1,2-b : 5,4-b']dithiophene (CPDT) electron donor and F-substituted 2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile (2FIC) as electron acceptor were designed and synthesized. With the increase of CPDT units, the elongated conjugations broadened the absorption range of the acceptors and tuned their energy levels sequentially. Therefore, their charge-transporting polarities switched from electron-only type to bipolar mode in organic field-effect transistors. Moreover, these changes also influenced the voltages, current densities, and eventual PCEs of their corresponding cells. When blending with PBDB-T, a champion efficiency of 10.01% was achieved in CPIC-2 based cells. This work demonstrated the importance of absorptions, suitable energy levels and charge transports in improving the efficiencies of organic solar cells.