of various substrates. An additional feature of
LHPs is their much slower cooling of photogenerated hot carriers (at ~1 to 10 meV/ps) than
that in conventional semiconductors (for example,
up to 1 eV/ps in GaAs) (52). This slower cooling
may open new possibilities to harness the energy
of hot carriers for efficient PV and other applications. The peculiarities of the exciton fine structure
of LHP NCs, such as bright triplet excitons—leading
to ~20 and ~1000 times faster emission than any
other semiconductor NCs at room and cryogenic
temperatures, respectively—become the focus of
theoretical studies (53). Last, a topic that remains
completely unexplored is the rational control of
charge transport in densely packed assemblies of
LHP NCs. Beyond APbX3-type (3D) perovskites,
an extremely active area of research is in 2D
perovskites, such as Ruddlesden-Popper phases,
(RNH3)2(MA)n–1PbnX3n+1 (R = C4H9, C9H19–,
or Ph–CH2CH2– and X– = Br– or I–) (54, 55), in
which the potential library of compositions and
structures is believed to be much greater. The
synthesis of 2D perovskites in the form of colloidal NCs becomes an additional exciting opportunity (56–58).
There is an urgent need to explore alternative
metal halide compounds that comprise environ-
mentally friendly elements instead of Pb. The
success of LHPs in PV has naturally led to an ex-
tensive experimental and computational search
for new compounds with similar defect-tolerant
photophysics. However, faithful optical and elec-
tronic analogs of LHPs remain elusive. Some of
the major difficulties encountered thus far have
been in the oxidative instabilities of Sn and Ge
analogs; the inability of Sb and Bi halides to
form 3D extended frameworks; and in so-called
double perovskites of composition A2M+M3+X6
(M+ = Ag+ or Cu+ and M3+ = In3+, Sb3+, or Bi3+,
the structural analogs of 3D-APbX3), the prohib-
itively large or indirect band gaps, oxidative in-
stability (for M+ = In+), or difficulty in synthesis
because of competition with more thermody-
namically stable ternary phases (such as Cs3Bi2I9).
Another obstacle is that the predictive power
of high-throughput computational screening
is generally limited by the inability of density
functional theory–based methods to discover
metastable phases. However, most inorganic
compounds are actually metastable, which leaves
ample opportunity for future experimental seren-
dipity in the discovery of new LHP-like materials.
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M.V.K. is very grateful to his former and present co-workers and
collaborators, whose names can be found on joint publications.
This work was financially supported by the European Research
Council (ERC) under the European Union’s Seventh Framework
Program (grant agreement 306733, ERC Starting Grant
“NANOSOLID”). M.I.B. acknowledges the Swiss National Science
Foundation (SNF Ambizione Energy grant PZENP2_154287). We thank
N. Stadie for reading the manuscript; N. Schwitz for providing
photographs of colloidal LHP NCs; and F. Bertolotti and I. Infante for
the help in preparing Figs. 3 and 4B, respectively.
750 10 NOVEMBER 2017 • VOL 358 ISSUE 6364 sciencemag.org SCIENCE
Fig. 5. Toward applications of LHP NCs in television displays and LEDs. (A) PL spectra of
CsPbX3 NCs plotted on CIE chromaticity coordinates (black points) compared with common color
standards (LCD television, dashed white line, and NTSC television, solid white line), reaching 140%
of the NTSC color standard (solid black line) (7). [Reproduced with permission from (7)] (B) Operation
principle of a QD LCD display, showing blue emission from standard InGaN LEDs transmitted by the
diffuser into a polymer film containing LHP NCs, undergoing partial conversion into green and red PL. The
mixture of colors is then incident upon a standard LCD matrix, containing liquid crystals and color filters
to define the mixing ratios of the three primary colors so as to achieve any color within the color gamut.
Green and red LHP NCs are proposed to be separated into different polymer layers or beads in order
to avoid inter-NC anion exchange. (C) Schematic of a three-color LED pixel with LHP NCs as the emissive
layer. The hole and electron injecting materials can be inorganic (such as conductive oxides or metals)
or organic (such as small molecules or conductive polymers). LEDs have fewer layers in their device
architecture than LCDs and can therefore afford thinner devices and make more efficient use of the light.