<p>Polymer solar cells, which include a blend of electron-donating conjugated polymers and electron-accepting molecules in the photovoltaic layer, have been widely studied as next-generation solar cells. To improve photocurrent generation, it is necessary to harvest as many photons as possible in solar light, which distributes over a wide wavelength including the ultraviolet, visible, and near-infrared (near-IR) regions. However, covering such a wide solar spectrum by using binary blend polymer solar cells is inherently difficult because most organic materials (e.g., conjugated polymers) have a narrow absorption bandwidth (less than 200 nm). Ternary blend polymer solar cells can overcome this limitation by combining near-IR light-harvesting materials with the electron-donor conjugated polymer and the electron-acceptor molecule. In this review, recent progress in the development of polymer solar cells is briefly overviewed, followed by a detailed description of ternary blend polymer solar cells.</p>