While
research on derivatives of both bulk and low-dimensional
metal halide perovskite (MHP) semiconductors has grown exponentially
over the past decade, the understanding and intentional applications
of electronic doping have lagged behind. In this Focus Review, we
take a critical look at these challenges by considering the different
potential doping routes, the advantages and pitfalls of each route,
and the unique properties of MHP systems that may contribute to the
inherent difficulties of realizing successful electronic doping. We
specifically consider low-dimensional MHP derivatives as a case study,
given that the mechanistic understanding of how defect chemistry affects
electronic doping has been studied less extensively in these systems
than in their three-dimensional counterparts, but we also consider
lessons learned from the prototypical bulk methylammonium lead iodide
perovskite semiconductor to inform our discussion. We discuss the
potential roles that the partially ionic nature of the chemical bonds
and the soft, polarizable nature of the lattice may play in the realization
of doping in MHPs, with an emphasis on defect chemistry, redox side
reactions, and polaronic stabilization. Informed by relevant case
studies, we illustrate lessons taken from the literature and our own
experience in an effort to provide a foundation for successful electronic
doping of MHPs. We conclude that the successful realization of doped
MHPs will likely hinge upon careful consideration and application
of doping strategies and mechanisms that have been established in
both the inorganic and organic semiconductor fields over the past
several decades.
The development of metal halide perovskite/perovskite heterostructures is hindered by rapid interfacial halide diffusion leading to mixed alloys rather than sharp interfaces. To circumvent this outcome, we developed an ion-blocking layer consisting of single-layer graphene (SLG) deposited between the metal halide perovskite layers and demonstrated that it effectively blocks anion diffusion in a CsPbBr 3 /SLG/CsPbI 3 heterostructure. Spatially resolved elemental analysis and spectroscopic measurements demonstrate the halides do not diffuse across the interface, whereas control samples without the SLG show rapid homogenization of the halides and loss of the sharp interface. Ultraviolet photoelectron spectroscopy, DFT calculations, and transient absorbance spectroscopy indicate the SLG has little electronic impact on the individual semiconductors. In the CsPbBr 3 / SLG/CsPbI 3 , we find a type I band alignment that supports transfer of photogenerated carriers across the heterointerface. Lightemitting diodes (LEDs) show electroluminescence from both the CsPbBr 3 and CsPbI 3 layers with no evidence of ion diffusion during operation. Our approach provides opportunities to design novel all-perovskite heterostructures to facilitate the control of charge and light in optoelectronic applications.
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