ConspectusOrganic photovoltaics (OPVs) with a photoactive layer containing a blend of organic donor and acceptor species are considered to be a promising technology for clean energy owing to their unique flexible form factor and good solution processability that can potentially address the scalability challenges. The delicate designs of both donors and acceptors have significantly enhanced the power conversion efficiency of OPVs to more than 18%. Nonfullerene small-molecule acceptors (NFAs) have played a critical role in enhancing the short-circuit current density (JSC) by efficiently harvesting near-infrared (NIR) sunlight. To take full advantage of the abundant NIR photons, the optical band gap of NFAs should be further reduced to improve the performance of OPVs. Incorporating highly polarizable selenium atoms onto the backbone of organic conjugated materials has been proven to be an effective way to decrease their optical band gap. For example, a selenium-substituted NFA recently developed by our group has attained a JSC of approximate 27.5 mA cm-2 in OPV devices, surpassing those of most emerging photovoltaic systems. Inspired by this advance, we concentrate on the topic of selenium-containing materials in this Account to incite readers' interest in further exploring this series of materials.In this Account, we first compare the differences among chalcogen heterocycles and discuss the influence of fundamental electronic behavior on the collective photoelectrical properties of the resulting materials. The superior features of selenium-substituted materials are summarized as follows: (1) The large covalent radius of selenium can diminish the π-orbital overlap, rendering enhanced quinoidal resonance character and a narrowed optical band gap of resulting materials. (2) The selenium atom is more polarizable than sulfur owing to its larger and looser outermost electron cloud, enabling enhanced intermolecular Se-Se interaction and increased charge carrier mobility of relevant materials in the solid state. We then focus on summarizing the design rules for various categories of selenium-containing materials including polymer donors, small-molecule acceptors, and polymer acceptors, especially those composed of ladder-type polycyclic units. The motivation for incorporating selenium atoms into these materials and the structure-property relationships were thoroughly elucidated. Specifically, we discuss the changes in the optical band gap, charge carrier mobility, and molecular packing induced by selenium substitution and correlate the effects of these changes with the exciton behaviors, energy loss, and nanoscale film morphology of corresponding OPV devices. Furthermore, we point out the intrinsic stability of selenium-containing materials under maximum-power-point tracking and long-term photo- or thermostress and indicate their potential use in semitransparent and tandem solar cells. At the end, the prospect of future research focuses and the possible applications of selenium-containing materials in the OPV field are discussed.