Organic solar cells are on the dawn of the next era. The change of focus toward non-fullerene acceptors has introduced an enormous amount of organic n-type materials and has drastically increased the power conversion efficiencies of organic photovoltaics, now exceeding 18%, a value that was believed to be unreachable some years ago. In this Review, we summarize the recent progress in the design of ladder-type fused-ring non-fullerene acceptors in the years 2018–2020. We thereby concentrate on single layer heterojunction solar cells and omit tandem architectures as well as ternary solar cells. By analyzing more than 700 structures, we highlight the basic design principles and their influence on the optical and electrical structure of the acceptor molecules and review their photovoltaic performance obtained so far. This Review should give an extensive overview of the plenitude of acceptor motifs but will also help to understand which structures and strategies are beneficial for designing materials for highly efficient non-fullerene organic solar cells.

No abstract

A series of non‐fullerene acceptors based on perylene monoimides coupled in the peri position through phenylene linkers were synthesized via Suzuki‐coupling reactions. Various substitution patterns were investigated using density functional theory (DFT) calculations in combination with experimental data to elucidate the geometry and their optical and electrochemical properties. Further investigations of the bulk properties with grazing incidence wide angle X‐ray scattering (GIWAXS) gave insight into the stacking behavior of the acceptor thin films. Electrochemical and morphological properties correlate with the photovoltaic performance of devices with the polymeric donor PBDB‐T and a maximum efficiency of 3.17 % was reached. The study gives detailed information about structure–property relationships of perylene‐linker‐perylene compounds.

New methoxylated oligosilyl-substituted metallocenes were synthesized by the reaction of two oligosilanides with different metallocene dichlorides (M = Ti, Zr, and Hf). The first investigated tris(trimethoxysilyl)silanide [(MeO) 3 Si] 3 SiK ( 1 ) underwent a selective monosubstitution to the respective oligosilyl-decorated metallocenes [(MeO) 3 Si] 3 SiMClCp 2 ( 2 – 4 ). Surprisingly, the attempted disilylation with this silanide was not possible. However, in the case of titanocene dichloride, a stable radical [(MeO) 3 Si] 3 SiTiCp 2 ( 5 ) was formed. The unsuccessful isolation of bisilylated metallocenes encouraged us to investigate the reactivity of another silanide. Therefore, we synthesized a hitherto unknown disilanide K[(MeO) 3 Si] 2 Si(SiMe 2 ) 2 Si[(MeO) 3 Si] 2 K ( 8 ), which was accessible in good yields. The reaction of compound 8 and different metallocene dichlorides (M = Ti, Zr, and Hf) gave rise to the formation of heterocyclic compounds 9 – 11 in good yields.

Quinoxaline has recently gained interest as monomer in conjugated copolymers because of its easy synthetic accessibility and successful use in highly efficient organic solar cells. In this contribution, we introduce a quinoxaline–fluorene-co-polymer, PFQ10, synthesized by copolymerization of 5,8-dibromo-6,7-difluoro-2-[(2-hexyldecyl)oxy]quinoxaline and 9,9-dioctyl-9H-9-fluorene-2,7-bis(boronic acid pinacol ester) using the Suzuki–Miyaura reaction. By optimization of the reaction conditions, polymers with molecular weights up to 17.2 kDa and a low dispersity of 1.3 were obtained. PFQ10 showed blue photoluminescence with an emission maximum at 459 nm and a relative fluorescence quantum yield of 0.37. As proof of principle, PFQ10 was employed in organic light-emitting diodes and showed a blue–green electroluminescence. Graphical abstract

Organic solar cells have been continuously studied and developed through the last decades. A major step in their development was the introduction of fused‐ring non‐fullerene electron acceptors. Yet, beside their high efficiency, they suffer from complex synthesis and stability issues. Perylene‐based non‐fullerene acceptors, in contrast, can be prepared in only a few synthesis steps and display good photochemical and thermal stability. Herein, we introduce four monomeric perylene diimide acceptors obtained in a three‐step synthesis. In these molecules, the semi‐metals silicon and germanium were added in the bay position, on one or both sides of the molecules, resulting in asymmetric and symmetric compounds with a red‐shifted absorption compared to non‐substituted perylene diimide. Introducing two germanium atoms improved the crystallinity and charge carrier mobility in the blend with the conjugated polymer PM6. In addition, charge carrier separation is significantly influenced by the high crystallinity of this blend, as shown by transient absorption spectroscopy. As a result, the solar cells reached a power conversion efficiency of 5.38%, which is one of the highest efficiencies of monomeric perylene diimide‐based solar cells recorded to date.