The effects of interfacial dipole caused by annealing-free Al-doped NiOx in efficient perovskite solar cells
Recent researches show that nickel oxide (NiOx) thin films can be utilized as an efficient and stable hole transport layer (HTL) in inverted planar perovskite solar cells (PSCs) replacing costly and chemically unstable organic materials. Nevertheless, thermal annealing process is necessary for NiOx made by conventional methods with metallic dopants, which is not applicable to flexible substrates with low melting points or temperature-sensitive materials. In this work, annealing-free Al-doped NiOx nano-particles (NPs) were synthesized and employed as a HTL in PSCs. Without using any Dimethyl sulfoxide (DMSO) additive in devices, the electrical conductivity, morphology, external quantum efficiency, and power conversion efficiency were all significantly enhanced after 2% Al doping in NiOx. The Ultraviolet Photoemission Spectroscopy (UPS) was conducted to explore the mechanisms behind the device improvements. An interfacial dipole of 0.3 eV was found between the ITO electrode and the NiOx layer directing away from the active layer, boosting the hole extraction efficiency at ITO/MAPbI3 (MA = methylamine) interface. This work not only provides a deep insight of the ITO/NiOx NPs/perovskite interface but also proposes a new strategy for further development of highly efficient and reliable PSCs.
Solar Energy, 233, 345-352 (2022)
StaStabilization of hybrid perovskite CH3NH3PbI3 thin films by graphene passivation
In summary, we have demonstrated the long-term (at least up to 3 months) stability of the extremely water-sensitive hybrid perovskite CH3NH3PbI3 thin films by passivating them with high-quality PECVD-grown monolayer graphene and verifying the invariance of their Raman, XPS, UPS and optical absorbance spectra with time. The successful passivation was enabled by our development of a new water- and polymer-free method that can transfer high-quality PECVD-grown graphene onto various target materials, including commonly used substrates (such as a-SiO2/Si) and even extremely water-sensitive hybrid perovskite samples. This method not only keeps the graphene quality and size fully intact but also excludes organic residues and water that are known to seriously compromise the performance of graphene-incorporated electronic and optoelectronic devices. Therefore, our water- and polymer-free graphene transfer method is promising for advancing graphene-based optoelectronic applications. Moreover, our successful demonstration of the graphene-enabled passivation and long-term stability of CH3NH3PbI3 thin films opens up a new pathway towards realizing practical photovoltaic applications of hybrid perovskites.