Abstract <p>Due to their ability to combine exceptional electronic and optical properties such as tunable energy levels, high transmittance in the visible to near-infrared (NIR) region, high conductivity, and strong thermal and chemical stability, π-conjugated polymers and molecules have shown great promise as donor and acceptor materials in organic solar cells (OSCs). Indeed, the emergence of these materials offers a promising opportunity to develop high-performance OSCs with outstanding power conversion efficiency (PCE) of over than 20%. These excellent results are achieved through optimization of the active material design, the donor/acceptor interface and device engineering. In this regard, we present in this review a comprehensive summary of the properties of π-conjugated materials, including their electronic band structure and charge transfer mechanisms and their applications as donor and acceptor materials in OSCs. The different OSC architectures, operating principles and charge carrier transport mechanisms are reviewed. This review also highlights recent advances in the use of organic materials, including small molecular and polymeric donors, fullerene and non-fullerene acceptors in OSCs, to illustrate structure-performance correlations. Despite the unprecedented progress in increasing PCE of OSCs, commercialization and large-scale production of these cells remains a challenge due to the shortcomings in the stability of conjugated systems.</p>