Two-Dimensional Conjugation Extended CH-Series Acceptors with a Distinctive A–D–A Character

Abstract

Conspectus As one of the most important indicators for evaluating photovoltaic devices, the power conversion efficiencies (PCEs) for the first-class organic solar cells (OSCs) have reached the level of ∼20%, but they still lag far behind that of over 25% for their inorganic counterparts. With the similar if not better fill factor and short-circuit current, this wide gap of PCEs should be fundamentally attributed to the greatly larger nonradiative energy losses in OSCs, which are usually above 0.2 eV for OSCs but only 0.03–0.04 eV for high-performance inorganic solar cells. Note that the stubbornly severe nonradiative recombination in OSCs is associated with multiple characteristics of organic light-harvesting molecules, such as intrinsically large exciton binding energies and small relative dielectric constants, defective intermolecular packing networks, or more crystal defects caused by the flexibility of large organic molecular skeletons, nonideal nanoscale film morphologies, and so on. All the factors above require that rational design of light-harvesting molecules should be carried out not only at single molecule but also at aggregation levels if further dramatic improvement of PCE is to be achieved for OSCs. In this Account, we will first expound the unique merits of acceptor–donor–acceptor (A–D–A) type light-harvesting materials in frontier orbital distribution, energy level tuning, and intermolecular packings, meanwhile revealing the dominant role of A–D–A type molecules in facilitating charge transfer/transport, suppressing energy loss, and improving photovoltaic performance of OSCs eventually. In light of the conspicuous superiority of A–D–A type molecules, a convincing conclusion can be made that further exploration of novel A–D–A type light-harvesting materials is crucially important to shrink the PCE gap between OSCs and inorganic solar cells. Second, our recent studies for a really exciting A–D–A type molecular platform (CH-series) will be discussed comprehensively, involving various high-performance nonfullerene acceptors (NFAs) with small molecular, dimer-like, and polymerized architectures. Note that the most distinctive feature of CH-series NFAs is two-dimensional (2D) conjugation extension, especially for central units. Therefore, the favorable effects of 2D conjugation extension of these molecules on their fundamental physicochemical properties, intermolecular packing modes, blended film morphologies, photovoltaic parameters, and energy losses of resulting OSCs will be fully discussed. Abiding by the unveiled design rules of high-performance A–D–A type NFAs, the highest PCE of approaching 20% has been achieved for OSCs based on CH-series molecules. The evolution path of previous OSCs is based on traditional materials such as that of PCBM, ITIC, Y6, etc. could be one lesson; CH-series molecules are very likely to offer a great platform capable of achieving record-breaking OSCs along with much decreased energy losses, especially considering their wide and various structural modification possibili...

Biaxially Conjugated Materials for Organic Solar Cells

“…To further promote the OSC efficiency to a renewed milestone, it is important to delicately engineer the coplanarity and intermolecular interactions of NFAs to further facilitate the 3D packing ordering. Biaxially conjugated NFAs have great potential to achieve such a preferable 3D packing pattern owing to their unique conjugated structures . In addition to the advancement of biaxially conjugated p-/n-type polymers as described above, the biaxially conjugated NFAs will be discussed in this section from the perspective of extended conjugation in both the π-core and outward side-group.…”

“…Inspired by this, Chen et al developed a category of NFAs by fusing a halogenated phenyl group on the Qx-core (CH-series, NFA6 - NFA9 ) . These CH-series acceptors show extended conjugation in the normal direction, representing a typical type of biaxially conjugated NFAs.…”

A nonfullerene acceptor as a solid additive realizing a record efficiency of 17.74% in quasi-layered all-polymer solar cells

“…[5][6][7][8] Phase separation and molecular packing regulation are still highly challenging for BHJ conguration all-PSCs due to the strongly tangled polymer chains in mixed donor : acceptor solution. 9,10 Quasi-layered all-polymer solar cells (QLA-PSCs) have aroused attention by employing the sequential spin-coating method to achieve more ideal vertical phase separation for efficient charge transport and collection. [11][12][13] The sequential spin-coating method should offer more opportunities to individually optimize donor and acceptor layers through employing distinct solvents, solvent additives and post thermal or solvent treatment on each layer.…”

An efficient interface modification material for improved efficiency and stability in inverted organic solar cells

“…1–4 Due to the advances in designing and synthesizing new active layer materials, morphology control and necessary interface engineering, the power conversion efficiency (PCE) of organic solar cells has broken the threshold of 19% in recent years. 5–9 To realize the application of OSCs, it is necessary to both improve the efficiency and prolong the device lifespan. 10,11 Besides the development of photoactive layer materials, the progress in transport layer materials, especially electron transport layer (ETL) materials, plays a vital role in improving the photovoltaic performance and longevity of OSCs.…”