Metal‐Free Organic Halide Perovskite: A New Class for Next Optoelectronic Generation Devices

Abstract

Pb and Sn; X = halogen) was shown to be feasible as sensitizers for photovoltaic cells, [2] and 2012, when conversion efficiencies around 10% were first demonstrated for solid-state cells, [3,4] that the field exploded. Since then, the power conversion efficiency (PCE) of perovskite solar cells has increased to 25.5%, surpassing Cu(In,Ga)Se 2 (CIGS) and cadmium telluride (CdTe), and approaching that of single crystal silicon solar cells. [5] The structural formula of halide perovskite materials is generally ABX 3 , where A and B are two different cations and X is a halide anion (Cl − , Br − , or I −). [6] Currently metal-based organic (usually) halide perovskites have taken a dominant position for optoelectronics due to confluence of several factors, including extremely high optical absorption, long charge lifetimes/diffusion lengths, dominant point defects that only generate shallow levels, and grain boundaries that are essentially benign. [4,7,8] However, these metal-based perovskites do have some intrinsic problems. 1) The toxic solvents involved in fabrication including N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO-not particularly toxic by itself but can transport potentially toxic dissolved species rapidly through the skin), gamma-butyrolactone (GBL), and chlorobenzene. 2) Metal-based perovskites may become available to humans through leaching and transport through water, air, and soil. Toxins like Pb accumulate in the human body and can cause several brain-related symptoms such as memory problems and intellectual disability. [9,10] These toxic materials are not compatible with wearable devices. 3) In general, Pb-based perovskites are prone to attack by moisture, which causes material (and device) degradation. [11,12] Bi-based perovskites are more stable but have relatively poor chargetransport properties. [13] Only recently, metal-free halide organic perovskites, a novel member of the perovskite family, have emerged. [14] Metal-free perovskites take advantage not only of the ABX 3 perovskite structure, but also of the tunability, diversity, and lightweight nature of organic materials. Meanwhile, these materials can be fabricated from aqueous solution and are fully degradable, which hold promise for eco-friendly fabrication and application. Despite the initial discovery of metalfree perovskites in 2002 by Bremner et al., [14] and a similar paper on perchlorate perovskites in 2011, [15] advancements have been reported only during the past 2 years, including Even though the metal-halide perovskites are attracting ever-increasing interest for their breakthrough efficiencies in photovoltaics, light-emitting diodes (LED), X-ray imaging, and general optoelectronics, it was not until very recently that metal-free halide perovskites become recognized, not only for their good optoelectronic performance, but also for their wide chemical diversity, tunability, lightweight, mechanical flexibility, and ecofriendly processability. The community is turning their attention to these lightweight semiconductors, and p...

Flexible and Wearable Optoelectronic Devices Based on Perovskites

“…The instability of Pbbased perovskite to water also makes it easy to decompose and produce toxic substances, while the abundance of lead in the human body can cause serious damage to the blood system and nervous system. [212] When Pb-based perovskites are used for wearable devices, the potential safety risk is greatly increased. Some achievements on non-toxic and low-toxic wearable perovskite devices have been reported, [51] but the performance of the flexible devices still needs to be improved.…”

Prospects of Metal‐Free Perovskites for Piezoelectric Applications

“…[ 58 , 59 , 60 ] The fundamental chemical design offers an efficient route by considering the ionic radius and formal charge neutrality, and replacing toxic elements with low/non‐toxic elements. [ 60 , 61 , 62 , 63 , 64 ] For instance, Pb 2+ in lead‐halide perovskite was first replaced by divalent Sn 2+ and Ge 2+ cations due to their adequate charge balance and coordination. [ 65 , 66 ] Interestingly, first‐principle studies and machine‐learning methods (ML) have been recently employed to predict and provide the accurate prerequisite and rational design of the materials for structurally stable and low‐toxic metal‐free perovskites.…”

Molecular solid solutions for advanced materials – homeomorphic or heteromorphic

“…B-Site is NH 4 + , H 3 O + , or alkali metal ion, and X site is a halide and molecular ions such as BF 4 − . [77][78][79][80][81][82][83][84][85] The size evolutions from inorganic perovskite to molecular perovskite via organic-inorganic perovskite are summarised in Table 1. B-Site ions of the molecular perovskite are more friendly to both human health and the earth environment.…”

“…77 After the breakthrough by the discovery of ferroelectric behaviours by introducing non-centrosymmetric A-site molecular cation, 78 optoelectronic and energy-related applications are of great interest. 79 5.4.2 Solid solution of heteromorphic molecules. Both inorganic and organic-inorganic hybrid perovskites are known to be highly miscible with similarly sized metal ions or organic molecules such as CH 3 NH 3 + and NH 2 NH 3 + .…”