Hybrid organic–inorganic halide perovskite based solar cell technology has passed through a phase of unprecedented growth in the efficiency scale from 3.8% to above 25% within a decade.
With rapid progress in the deployment of metal halide perovskites in various device applications such as solar cells, light-emitting devices, field-effect transistors, photodetectors, etc., the next eminent focus is on the single crystals of these materials. With a lack of grain boundaries and low trap densities, remarkably long charge carrier diffusion lengths, and high ambient and operational stabilities, this class of materials seems greatly promising. Yet, the growing concern for lead toxicity in commercial semiconductor devices has entailed a thrust in the research of alternative lead-free perovskites, including their single crystalline forms. However, there is still no consolidated account of the state-of-the-art in this domain and accordingly, countless feasible systems still remain unexplored. To bridge this gap, we attempt to provide here, an up-to-date overview of lead-free perovskite single crystals with respect to their synthesis methods, structural diversity, stability, photophysical and electrical properties, and device applications. We discuss various approaches to designing, modeling, fabricating, and characterizing new single-crystal systems and conclude with some critical insights for further investigating this field of research.
Metal
halide perovskite single crystals (MHPSCs) are gaining enormous
attention in the energy research community due to their impressive
responses both in optical sensing and in photovoltaics. The switching
from polycrystalline to monocrystalline morphology, not only allows
to maintain the outstanding properties that characterize perovskite
materials, but also enhances them. However, the poor control over
the thickness and size during growing methods leads to considerable
differences between surface and bulk responses. Impedance spectroscopy
(IS) has been revealed as a powerful technique to understand the kinetics
governing polycrystalline perovskite materials. The ionic migration,
trap states, and recombination mechanisms occurring in both bulk and
surface of the MHPSCs, need to be analyzed in depth to exploit their
full potential. Here, we highlight the importance of IS to further
advance our knowledge about monocrystalline perovskite materials,
bringing to the table the relevance of other small perturbation techniques
to complement the IS.
Perovskite single crystals have emerged as potential candidates in the field of optoelectronic devices because of their low trap state densities, long diffusion lengths, and high charge carrier mobilities. However, the presence of lead (Pb) in perovskite causes serious concerns due to lead's high-perceived toxicity and poor environmental stability. Therefore, development of lead-free perovskites as a potential alternative material and an environmentfriendly candidate has received critical attention. Here, we report the synthesis of all-inorganic millimeter-sized lead-free bismuthbased halide perovskite Cs 3 Bi 2 X 9 (X = Cl, Br, I) single crystals and their structural disorderness, optical, and spin relaxation properties in detail. Higher Urbach energy in Cs 3 Bi 2 X 9 single crystals reveals a high degree of local structural disorderness and a short range of crystallinity. We show that structural disorder affects not only the optical properties but also the magnetic and spin relaxation properties. We observe that increased structural disorder leads to enhanced smearing of local energy bands and high spin−orbit coupling. The spin relaxation time is determined in the picoseconds time scale, which corresponds to fast charge carrier dynamics. Our work provides a design strategy and in-depth understanding to develop environment-friendly lead-free and stable perovskite-based optoelectronics and spintronic devices.
Recently,
halide perovskites have emerged as a promising material
for device applications. Lead-based perovskites have been widely explored,
while investigation of the optical properties of lead-free perovskites
remains limited. Lead-halide perovskite single crystals have shown
light-induced positive photoconductivity, and as lead-free perovskites
are optically active, they are expected to demonstrate similar properties.
However, we report here light-induced negative photoconductivity with
slow recovery in lead-free Cs3Bi2Cl9 perovskite. Femtosecond transient reflectance (fs-TR) spectroscopy
studies further reveal that these electronic transport properties
are due to the formation of light-activated metastable trap states
within the perovskite crystal. The figure of merits were calculated
for Cs3Bi2Cl9 single-crystal detectors,
including responsivity (17 mA/W), detectivity (6.23 × 1011 Jones), and the ratio of current in dark to light (∼7160).
These observations for Cs3Bi2Cl9 single
crystals, which were optically active but showed retroactive photocurrent
on irradiation, remain unique for such materials.
Light
exposure usually causes an increase in photoconductivity
in perovskite semiconductors. However, we report here light-induced
negative photoconductivity, followed by slow dark self-recovery in
a lead-free Cs3Bi2Br9 perovskite
single crystal. Femtosecond transient reflection (fs-TR) spectroscopy
studies further reveal hole self-trapping at the Vk center
(Br2
– dimer) in the midband states of
this vacancy-ordered perovskite. Subsequently, these charged defect
states (Vk) trap photogenerated charge carriers and produce
an internal electrical field, which essentially opposes the externally
applied field, leading to negative photoconductivity. A highly sensitive
prototype photodetector was fabricated with figure of merits estimated
as responsivity (6.42 mA/W), detectivity (2.51 × 1012 Jones), and current in a dark to light ratio (∼20). Our observation
of this retrospective photocurrent in optically active perovskite
materials can be applied for developing highly sensitive detectors.
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