Nature Commun. 5, 3586 (2014)

In less than five years, solar cells based on organo-lead trihalide perovskites have surpassed the photovoltaic performance of organic semiconductors and quantum dots, and are reaching the efficiency of silicon solar cells. Some mechanisms leading to this improved power-conversion efficiency have been identified, yet the nature of the photoexcited species generated in this material is still under investigation. Annamaria Petrozza and colleagues now show that, when exposed to normal sunlight intensity, the excitons generated in the perovskite layer readily separate into free charges. They estimate the excitons' binding energy by means of absorption measurements and find that, at room temperature, almost all the excited species are split into electrons and holes that can be directly collected at the electrodes of the solar cell. Because the fraction of bound excitons increases when the perovskite layer is irradiated with higher light intensity, the researchers suggest that this material may also prove effective in optoelectronic applications in which a high number of bound species is required.