Forced planarization by covalently fastening adjacent aromatic units in the polymer backbone strengthens the parallel p-orbital interactions to elongate effective conjugation length and facilitate electron delocalization, providing an effective way to reduce the band gap and enhances the intrinsic charge mobility. It is envisaged that the well-defined ladder-type small molecules can function as donor monomers to polymerize with acceptor units, leading to a new class of donor-acceptor semi-ladder copolymers that can be suitably used for solution-processable polymer solar cells. Therefore, elegant designs and synthesis have been devoted towards hybridizing electron-rich aromatic subunits into mutually fused structures in the anticipation of extracting their individual intrinsic advantages. Various accessible electron-rich aromatic and heterocycles building blocks can be chemically assembled to afford a range of fascinating ladder-type conjugated skeletons with tunable properties and functions. A series of donor-acceptor conjugated copolymers and small molecules have been designed and synthesized. The applications of these materials in solar cells and OFET transistors will be discussed.