Migration of ions through the perovskite layer in two dimensions

Electrostatic doping has been widely used in low-dimensional materials, including carbon nanotubes (CNTs) and two-dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs). In contrast to conventional lattice doping with impurity atoms, doping in nanomaterials is difficult to achieve due to the limited physical space. Electrostatic doping opens up an efficient path for tuning charge carriers in nanomaterials without introducing impurity atoms, which can perturb the atomic arrangement and degrade the intrinsic electronic properties of nanomaterials.

In a new paper published in E-litea team of scientists led by Professors Sung-Joon Lee and Hung-Chieh Cheng of UCLA developed methylammonium iodide lead perovskite (CH).₃New Hampshire₃PbI₃) / a heterogeneous 2DSC device.

Recently, ionic solids have been explored to create a p-n junction in 2D monolayer materials. Frozen mobile ions provide electrostatic fields to modulate the carrier density of a 2D semiconductor channel. Due to the well-defined shape of ionic solids, the local control of doping on two-dimensional semiconductors (2DSCs) allows diverse designs of solid-state/optoelectronic device integration with minimal crosstalk.

Silver ion treatment in solid-state superoxide silver iodide (AgI) was used to design the carrier type of 2DSCs to realize programmable transistors, diodes, photodiodes and logic gates.

Single-layer TMD systems have been widely adopted in novel optoelectronic applications such as electrically tunable light-emitting diodes (LEDs), gate-controlled p-n junction diodes, and solar cells. However, single-layer TMDs present some intrinsic limitations for high-performance optoelectronic applications. The incorporation of impurities into the atomically thin 2D lattices has been restricted mainly by the physical space in the atomically thin lattices.

It has been an ongoing challenge to tailor the type/density of charge doping in monolayer 2DSCs using selected lattice dopants. Thus, p-n photodiodes made from 2DSCs often suffer from non-ideal contacts on either the p or n side, which limits the achievable open circuit voltage (Fifth_O.C). In addition, the overall light absorption and spectral sensitivity of 2DSCs are mainly limited by their atomically thin geometries. It compromises the photo-carrier generation efficiency and the achievable external quantum efficiency (EQE).

Considerable efforts have been devoted to overcoming these intrinsic limitations through heterogeneous integration with other known optoelectronic materials. For example, interaction with organic dye molecules has been demonstrated as an effective strategy for controlling their photovoltaic properties. Hybrid lead-halide perovskites (LHPs) have received great attention for photovoltaics due to their excellent optoelectronic performance and low manufacturing cost.

Despite its extraordinary potential, “soft lattice” ionic LHPs typically suffer from ion migrations under voltage bias, which leads to poor material stability and significant slowing down of voltage-dependent photocurrents. Migration of positively or negatively charged ions can lead to ion buildup or ionic charge imbalance under applied electric fields. Here, we exploit ionic charge imbalance in LHPs to induce doping of proximal 2DSCs to create high-performance photodiodes.

Methylammonium lead iodide (CH₃New Hampshire₃PbI₃ or MAPbI₃) represents the most striking example of LHPs with excellent optical absorption and photoresponse properties but seriously suffering from ionic mobility. Although undesirable for the stable operation of solar cell applications, ionic charge buildup from ion transport induced bias in MAPbI₃ They can be exploited to selectively dope close proximity of 2DSCs to create perovskite-sensitive 2D photodiodes with high optoelectronic performance.

In this regard, atomically thin 2DSCs are ideally suited for efficient coupling with ionic solids. It acts as a non-covalent stimulant to reversibly effect p-type or n-type stimulants. The tunable doping effect also provides a new class of 2DSC-based photodiodes with switchable polarities. With van der Waals integration of ionic solids with excellent optoelectronic properties, the two-dimensional dimers formed by the ionic doping effect provide an efficient method for the extraction of photogenerated carriers in MAPbI.₃.