Polymer-based solar cells have made great progress during the past decade and consequently are now attracting extensive academic and commercial interest because of their potential advantages: lightweight, flexible, low cost, and high-throughput production. On the other hand, the recent progress in analytical tools has profoundly enhanced our understanding of the underlying mechanism of polymer-based solar cells, which can provide valuable guidelines for materials design and device engineering and therefore is essential for further improvement of the device performance. In particular, transient absorption spectroscopy is a powerful tool for directly observing ultrafast fundamental processes in polymer-based solar cells. In this article, we first give a brief overview of the basic mechanism of polymer-based solar cells, and the recent progress in the device performance based on the development of materials. We review the method of assigning charge carriers generated in polymer/fullerene solar cells, the dynamics of fundamental processes, and the efficiency of each photovoltaic conversion process. (C) 2011 Elsevier Ltd. All rights reserved.