Investigation on Electronic Structures of Charge-Transporting Layer/Perovskite Interfaces Using in-situ Ultraviolet Photoemission Spectroscopy

Description

In organic optoelectronic devices including organic light-emitting diodes and solar cells, charge-transporting layers (CTLs) are widely used for enhancing device performance. By applying a CTL between an electrode and a light-emitting/absorbing layer, a suitable energy-level alignment can form and dramatically improve charge injection/extraction (Nat. Mater. 17, 329, 2018). In the past 30 years, interfaces of CTLs in organic optoelectronic devices have been broadly investigated. Specifically, for organic/organic interfaces, some models have been proposed to explain the energy-level alignments at the interfaces. (Nat. Mater. Rev. 4, 627, 2019) For metal oxide/organic interfaces, a universal rule has been reported by Luet. al.in 2009. (Nat. Mater. 11, 76, 2012) These understandings on the interfacial energy level alignment greatly facilitate the development of organic optoelectronic devices. Perovskite is emerging as a novel optoelectronic material. Benefiting from device knowledge accumulated in organic optoelectronic devices, perovskite solar cells have achieved very high efficiency in the past decay. Light-emitting diodes with perovskites as emission layers also draw much attention. Among these perovskite devices, traditional CTLs in organic optoelectronic devices, like organic hole/electronic-transporting materials and metal oxides, are still used. Compared with organics, however, perovskites show more complex interfaces, mainly due to their various compositions and ionic bonding features. It is thus reasonable to expect that perovskite-based devices may have different interfacial electronic structures from their organic counterparts. Some results on CTL/perovskite interfaces have been reported. However, the energy-level alignment rule is still not clear. Further understanding of the electronic structures and energy-level alignments of CTL/perovskite interface is highly desired. To gain deeper insight on CTL/perovskite interfaces, the PI has carried out preliminary experiments using in-situ ultraviolet photoemission spectroscopy (UPS), which is a powerful tool to probe interfacial electronic structures, on a series of organic CTL/perovskite combinations. In these preliminary studies, we found that organic CTL/perovskite interfaces show a different energy-level alignment trend with those in CTL/organic interfaces, indicating a high necessity for further investigation on CTL/perovskite interfaces. Therefore, in this project, we propose to apply the in-situ UPS and other surface analysis techniques to study a broader spectrum of CTL/perovskite configurations and explore their interfacial electronic structures and energy-level alignments. With such knowledge, we aim to develop an effective CTL selection strategy for interfacial engineering, which can be applied for further improving the performance of perovskite optoelectronic devices.