X-rays from synchrotron radiation enable incisive spectroscopic techniques which speed up the discovery of new materials for photovoltaics and photoelectrochemistry. A particularly useful method is X-ray absorption spectroscopy (XAS), which probes empty electronic states. XAS is element- and bond-specific, with the additional capability of determining the bond orientation. Close feedback from density functional calculations makes it possible to discover and exploit systematic trends in the electronic properties. Case studies are presented, such as solar cells that combine an absorber with an electron donor and an acceptor in one molecular complex and nanowire arrays serving as photoanodes for water splitting. In addition to the energy levels the lifetimes of the charge carriers play an essential role in device performance. A new generation of laser-like X-ray sources will make it possible to follow the fate of excited charge carriers traveling across a molecular complex or through a device structure in real time.
We have developed phosphor-free InGaN/GaN/AlGaN dot-in-a-wire core-shell white light emitting diodes, which can break the carrier injection efficiency bottleneck of conventional nanowire white light emitting diodes, leading to a dramatic enhancement of the output power. Additionally, such phosphor-free nanowire white light emitting diodes can deliver a very high color rendering index (CRI) of ~92-98.