Stabilizing the donor-acceptor interface of organic solar cells

Published in Chemistry, Materials, and Physics
Stabilizing the donor-acceptor interface of organic solar cells

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Carbon-based π-conjugated molecules and polymers are promising semiconductors for printed, eco-friendly and biocompatible organic optoelectronic devices. Besides finding applications in commercial displays (organic light-emitting diodes; OLEDs), this material class is also important in the development of next-generation solar photovoltaic cells and human-integrated sensors, among other applications.

For organic solar cells (OSCs), a self-assembled thin-film comprising a blend of electron-donating (donor) and electron-accepting (acceptor) materials is used to harvest solar irradiation. The nanoscale morphology of this donor-acceptor blend needs to be carefully engineered to facilitate energy-efficient separation of bound electron-hole pairs (excitons) into free charge carriers, as well as their transport and collection at the electrodes.

Recently, the development of new acceptor molecules (such as Y6 or BTP-4F) have led to a significant advancement in OSC research. When blended with donor polymers, solar power conversion efficiency (PCE) of nearly 20% can now be achieved in single-junction OSC devices, bridging the performance gap that historically separated organic semiconductors from their inorganic and hybrid counterparts. Scaling up of OSC fabrication using layer-by-layer deposition and other printing methods without significantly comprising device PCE has also made significant progress, paving the way towards large-area device manufacturing.

However, today’s efficient OSC devices based on Y6-type molecules generally suffer from fast performance degradation under solar irradiation. This can be attributed to the over purification (de-mixing) of the donor and acceptor phases seemingly required for efficient charge photogeneration in these systems, which causes the blend morphology to become thermodynamically unstable.

In this paper, we demonstrate how this issue can be solved by the polymerization of Y6-type acceptors. Based on a combination of experimental results (synchrontron X-ray scattering) and theoretical simulations (molecular dynamics), we found that the polymerized Y6 acceptors can form an intermixed (percolated) structure with the donor polymer at their interfaces. Such donor-acceptor percolation offers better stability than the over-purified phase separation found in blends made up of Y6-type molecules, leading to an extended device lifetime. 

We also used ultrafast optical spectroscopy to understand the charge photogeneration mechanism in these systems. Our previous work (Nat. Commun. 2020, 11, 1, 5617) first showed that efficient charge photogeneration in OSC occurs via an endothermic, thermally-activated process. This process takes up to ~100 ps to complete at room temperature, and therefore a nanosecond-scale exciton lifetime is needed to ensure efficient charge generation. We first confirmed that such slow, endothermic charge generation is also taking place in the polymerized Y6 acceptor blends. Furthermore, although excitons have insufficient lifetimes to separate in aggregated chains of the acceptor polymers (i.e. in pure films), a significant increase in exciton lifetime is found in these polymer chains are dispersed (i.e. at the mixed donor-acceptor interface of blends), satisfying the condition for efficient interfacial charge generation.

Taken together, our results show that polymerization of Y6 acceptors provides a pathway towards efficient and stable OSCs. Considering that the use of molecular acceptors generally offers higher device PCE, presumably due to better charge transport over high purity domains, a mixture of both molecular and polymerized acceptors may provide a way to optimize both device PCE and stability. Further optimization of the structural, optical and electrical properties of binary/ternary OSC blends involving polymerized Y6 acceptors is likely to lead to record-breaking device performance in the near future. Besides pushing the limit of pure OSC devices, these systems may also find applications in perovskite/organic tandem solar cells. 

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Organic Molecules in Materials Science
Physical Sciences > Materials Science > Soft Materials > Organic Molecules in Materials Science
Optoelectronic Devices
Physical Sciences > Physics and Astronomy > Optics and Photonics > Optoelectronic Devices
Physical Sciences > Materials Science > Soft Materials > Polymers
Solar Cells
Physical Sciences > Chemistry > Physical Chemistry > Photochemistry > Photovoltaics > Solar Cells

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