Organic solar cells exhibit potential to provide light-weight and low-cost solar energy on flexible substrates. However, current efficiency is still low for applications. New materials and device designs are needed to increase cell efficiency and make this technology available for large-scale applications. The dependence of double junction solar cell efficiency on polymer bandgaps in top and bottom subcells are presented, which provides guidance for engineering new conjugated polymers for efficient photovoltaic device development. The achievable cell efficiency can be beyond 16% with the bandgap of the bottom subcell at similar to 1.6 eV (similar to 775 nm) and that of the top subcell at similar to 1 eV (similar to 1240 nm). In addition, the LUMO and HOMO energy levels of the donor polymers are provided depending on various acceptor materials such as PCBM, TiO2, ZnO and CdSe. The interfacial layers between the subcells in double junction organic devices are also discussed. (C) 2012 Elsevier B.V. All rights reserved.