Polymer solar cells (PSCs) have attracted much attention recently because of their high flexibility, lightweight, low cost, cheap processing technique and future promise of higher power conversion efficiency. With the realization of photo-induced electron transfer from a conjugated polymer to fullerene component, the bulk heterojunction PSC has become the most successful device structure developed in the field of organic solar cells till present day. The efficiency of bulk heterojunction (BHJ) solar cells has increased steadily over the last decade, currently reaching over 10%. The bulk heterojunction solar cells consist of a phase-separated blend of organic electron donor and acceptor components, where the donor component is a conductive polymer and the acceptor component is a fullerene derivative. In order to achieve high performance and efficiency in BHJ PSCs, a wide range of visible light wavelength needs to be absorbed by the electron donating conjugated polymer and at the same time, the conjugated polymer must possess a characteristic good hole mobility. Recent studies have shown new ways and control mechanisms to improve the efficiency of organic PSCs. This study reviews the various techniques used to improve polymer solar cell technology, the device operation mechanism, and the material design requirements. Potential future applications of this technology in flexible and portable devices are also discussed.