Adv. Mater. 25, 2932–2936 (2013)

An efficient and convenient means of converting solar energy into chemical energy (known as artificial photosynthesis) is required for a 'hydrogen economy' to become a realistic prospect. A promising way of achieving this is using polymer solar cells to generate the electricity required to split water into hydrogen and oxygen. To make the process as efficient as possible, both tandem and triple-junction cells are currently being investigated. Now, René Janssen and colleagues at the Eindhoven University of Technology in the Netherlands report a triple-junction polymer solar cell that simultaneously achieves a high optimized efficiency of 5.3% (a solar-to-hydrogen conversion efficiency of 3.1%) and, importantly, a maximum power point at around 1.70 V, which is sufficiently high to split water. Their triple junction consists of one wide-bandgap front cell and two identical narrow-bandgap middle and back cells (known as the '1 + 2 type configuration'). They demonstrate that this new design gives better performance than the corresponding tandem solar cell, and expect to reduce the water-splitting voltage in future work by using better hydrogen- and oxygen-evolving catalysts.