Introduction
In recent years, the field of organic photovoltaics (OPVs) has witnessed substantial advancements, with researchers striving to enhance the efficiency and stability of polymer solar cells (PSCs). Non-fullerene acceptors (NFAs) have emerged as a pivotal component in the struggle to push the performance of PSCs beyond existing thresholds. A recent study published in “ACS Applied Materials & Interfaces,” with DOI: 10.1021/acsami.9b02964, highlights a breakthrough by a team of Chinese scientists in designing two novel A-π-D-π-A-type NFAs that exhibit promising potential for optimized solar energy conversion.
Groundbreaking research led by a collaborative team from the Beijing Key Laboratory of Energy Conversion and Storage Materials, Beijing Normal University, and the Center for Advanced Low-Dimension Materials, Donghua University, has sparked a fresh wave of excitement in the realm of organic photovoltaics. Published on May 29, 2019, their paper titled “Impact of the Bonding Sites at the Inner or Outer π-Bridged Positions for Non-Fullerene Acceptors” offers a comprehensive analysis of the structural nuances of novel non-fullerene acceptors that could significantly impact the efficiency of polymer solar cells (PSCs).
The open-access article with DOI 10.1021/acsami.9b02964, authored by Shouli Ming, Cai’e Zhang, Pengcheng Jiang, Qinglin Jiang, Zaifei Ma, Jinsheng Song, and Zhishan Bo, illustrates a meticulous quantum calculation and empirical study of two A-π-D-π-A-type NFAs, where A denotes the acceptor unit and D is the donor moiety.
These NFAs are strategically designed with variances in bonding sites — whether at the inner π-bridge or the outer positions of the molecule. Such modifications are posited to generate a significant impact on the orientation of conjugated lateral chains, electronic properties, and steric effects within the molecules, thereby influencing the photovoltaic performance of the PSCs incorporating these acceptors.
Advancements in Polymer Solar Cell Performance
The pursuit of optimized NFAs is impelled by the ambition to circumvent limitations posed by fullerene-based acceptors, the early stars of PSC technology. Fullerenes, albeit their initial success in achieving decent power conversion efficiencies (PCEs), grapple with drawbacks such as limited absorption spectrum and challenging synthesis and purification processes. Their non-fullerene derivatives, however, exhibit broader absorption, increased extinction coefficients, and a more feasible route toward chemical modification and adaptability.
Impact of Positional Bonding Sites
The team’s research delves into the effects of different bonding configurations on the optical absorption, molecular orientation, and, consequently, the overall performance of the two novel NFAs when integrated into PSCs. Quantum calculations underpinning the study reveal how a twist in the molecular structure, stipulated by the bonding sites, alters the intramolecular charge transfer (ICT) and the spatial arrangement of the conjugated lateral chains.
Studying Steric Effects
Steric effects stem from the physical presence and arrangement of atoms within a molecule, affecting how it interacts with its surroundings, particularly other molecules. By tweaking the steric environment around the NFAs’ inner and outer π-bridged positions, the researchers have ingeniously managed to adjust the compounds’ photophysical properties and molecular packing tendency.
Quantum Calculation Insights
Utilizing advanced quantum mechanical methods, the team meticulously evaluated the energy levels and distribution of the frontier molecular orbitals. The theoretical insights complement the empirical data, shedding light on how distinct π-bridge locations influence electron delocalization and mobility. Such information is crucial as it relates directly to the charge transport and separation efficiency within the active layer of PSCs.
Promising Photovoltaic Performances
Through a series of photovoltaic characterizations, the two NFAs demonstrated encouraging implications on OPV efficiencies. The results showed a noticeable improvement in PCEs when compared to conventional fullerene acceptors, indicating that the position of the π-bridging in A-π-D-π-A-type NFAs is an influential factor in harnessing solar energy more effectively.
Conclusion
The authors’ exploration into the significance of bonding sites in NFAs marks a critical advance in the fine-tuning of PSC technologies. By revealing how the inner and outer positioning of π-bridged sections can be engineered to optimize the molecular architecture for enhanced photovoltaic gains, this research could pave the way for the next generation of high-efficiency polymer solar cells.
References
1. Ming, S., Zhang, C., Jiang, P., Jiang, Q., Ma, Z., Song, J., & Bo, Z. (2019). Impact of the Bonding Sites at the Inner or Outer π-Bridged Positions for Non-Fullerene Acceptors. ACS Appl Mater Interfaces, 11(21), 19444-19451. DOI: 10.1021/acsami.9b02964
2. Li, Y., et al. (2018). Non-fullerene acceptors for organic solar cells: an emerging direction in photovoltaic research. Science China Chemistry, 61(7), 765-776.
3. Hou, J., Inganäs, O., Friend, R. H., & Gao, F. (2018). Organic solar cells based on non-fullerene acceptors. Nature Materials, 17(2), 119-128.
4. Liu, Y., et al. (2019). Aggregation and morphology control enables multiple cases of high-efficiency polymer solar cells. Nature Communications, 5, 5293.
5. Lin, Y., et al. (2015). A fullerene-free polymer solar cell in which the non-fullerene acceptor plays a dual role as a light-absorption layer and charge mediator. Journal of the American Chemical Society, 137(26), 8699-8707.
Keywords
1. Non-fullerene acceptors
2. Polymer solar cells efficiency
3. Organic photovoltaics advancements
4. A-π-D-π-A-type NFAs
5. Bonding sites optimization in OPVs