Efficient and Stable Thin‐Film Luminescent Solar Concentrators Enabled by Near‐Infrared Emission Perovskite Nanocrystals

Abstract

TPP für LSCs: Eine Nachbehandlung mit Triphenylphosphin (TPP) wurde für die Herstellung von hochstabilen, großflächigen lumineszierenden Solarkonzentratoren (LSCs) aus CsPbI3‐Polymer‐Kompositfilmen entwickelt. Die LSCs haben eine hohe optische Konversionseffizienz und sind hoch emittierend (absoluter PLQY ca. 100 %).

Abstract
A novel triphenylphosphine (TPP) treatment strategy was developed to prepare the near‐infrared emission CsPbI3 nanocrystal (NC)‐polymer composite thin‐film luminescent solar concentrators (LSCs) featuring high absolute photoluminescence quantum yield (PLQY), low reabsorption, and high stability. The PL emission of the LSCs is centered at about 700 nm with 99.4±0.4 % PLQY and narrow full width at half maximum (FWHM) of 75 meV (30 nm). Compared with LSCs prepared with classic CsPbI3 NCs, the stability of the LSCs after TPP treatments has been greatly improved, even after long‐term (30 days) immersion in water and strong mercury‐lamp irradiation (50 mW cm⁻²). Owing to the presence of lone‐pair electrons on the phosphorus atom, TPP is also used as a photoinitiator, with higher efficiency than other common photoinitiators. Large‐area (ca. 75 cm²) infrared LSCs were achieved with a high optical conversion efficiency of 3.1 % at a geometric factor of 10.

Full text

Perovskites
Efficient and Stable Thin-Film Luminescent Solar Concentrators
Enabled by Near-Infrared Emission Perovskite Nanocrystals
Jiajing Wu, Jianyu Tong, Yuan Gao, Aifei Wang, Tao Zhang, Hairen Tan, Shuming Nie, and
Zhengtao Deng*
Abstract: A novel triphenylphosphine (TPP) treatment strat-
egy was developed to prepare the near-infrared emission
CsPbI3nanocrystal (NC)-polymer composite thin-film lumi-
nescent solar concentrators (LSCs) featuring high absolute
photoluminescence quantum yield (PLQY), low reabsorption,
and high stability. The PL emission of the LSCs is centered at
about 700 nm with 99.40.4% PLQY and narrow full width
at half maximum (FWHM) of 75 meV (30 nm). Compared
with LSCs prepared with classic CsPbI3NCs, the stability of
the LSCs after TPP treatments has been greatly improved, even
after long-term (30 days) immersion in water and strong
mercury-lamp irradiation (50 mWcm2). Owing to the pres-
ence of lone-pair electrons on the phosphorus atom, TPP is
also used as a photoinitiator, with higher efficiency than other
common photoinitiators. Large-area (ca. 75 cm2) infrared
LSCs were achieved with a high optical conversion efficiency
of 3.1% at a geometric factor of 10.
Colloidal nanocrystals (NCs) are of great interest for
a variety of high-performance optical and optoelectronic
applications, such as bioimaging,[1] solar cells,[2] light-emitting
diodes,[3] photodetectors,[4] and luminescent solar concentra-
tors (LSCs).[5,12] In particular, compared with traditional
organic dyes, NCs are being widely used to develop large-area
LSCs owing to their advantages of high PLQY, tunable
absorption/emission spectra, and good stability.[5a,d,6] At
present, these NCs mainly include C-dots,[7] Si,[8] CdSe/
CdS,[12] CdSe/CdZnS,[5d] CdSe/CdPbS,[5c] Mn2+-doped ZnSe/
ZnS,[9] CuInS2/ZnS,[5e] CuInSSe/ZnS,[10] CuInSeS/ZnS,[11] PbS/
CdS.[5b] Among them, the optical conversion efficiency of
2.86% has been reported based on Si NCs.[8] Optical
conversion efficiency of LSC-based giant CdSe/CdS NCs
could reach 48% under single-wavelength excitation.[12]
However, it is a major challenge to fabricate NC-based
LSCs with high PLQY and long-term stability under outdoor
conditions.
In recent years, lead halide perovskites (APbX3A=MA+,
FA+or Cs+;X=I,Br
,Cl
) have attracted extensive
attention owing to their excellent performance and low
processing cost.[13] An attractive feature of these materials is
a tunable band gap between 1.48 eV and 2.3 eV, which is
required for LSCs.[13b,14] Recently, some researchers have
begun to prepare LSCs from lead halide perovskite.[14,15] For
example, Meinardi et al. demonstrate Mn-doped CsPbCl3
NCs as reabsorption-free emitters for large-area LSCs.[16]
Wei et al. fabricated low-loss large-area LSCs based on
green emission layered perovskite nanoplatelets, achieving
optical quantum efficiency of 26 % and external optical
quantum efficiency of 0.87%.[17] Our group prepared
FAPbBr3NC–polymer slabs with high PLQY ( 92 5%)
and stable green emission for LSCs.[18] With a direct band gap
of 1.73 eV, CsPbI3is very suitable for LSCs.[19] However,
traditional CsPbI3NCs have serious phase-transition prob-
lems under ambient conditions, which limit their practical
applications. To resolve this phase instability problem, many
strategies have been proposed, such as doping, surface ligand
engineering, and providing protective shells.[19,20] Several
group have reported that trioctylphosphine could enhanced
stabilization of CsPbI3quantum dots in solution.[19] Recently,
Bi et al. demonstrated a surface ligand modification of CsPbI3
by using a 2-aminoethanethiol (AET) treatment to improve
the stability of NCs both in solution and as films. However,
AET-CsPbI3NCs dispersed in solution have relatively low
PLQY of 51% with broad FWHM of 41 nm; the AET-CsPbI3
NC-films have low PLQY of only 33 %.[21] Equally important,
the long-term stability of CsPbI3NC–polymer composite
films remains a major challenge. Therefore, the development
of a more effective method for preparing CsPbI3NC–polymer
composite films is of great significance for LSCs.
In this study, for the first time, we prepared stable and
bright near-infrared emissive thin-film LSCs by UV-polymer-
ization of acrylic resin containing TPP treated CsPbI3NCs.
Our TPP treatment strategy has several advantages: 1) the
PLQY of the CsPbI3NC–polymer composite films increased
significantly from 52 % to 99.4 0.4 %; 2) the stability of the
films has been greatly improved, even after long-term
(30 day) immersion in water and strong mercury lamp
irradiation (50 mW cm2); 3) owing to the presence of lone-
pair electrons on the phosphorus atom,[22] TPP is also used as
a photoinitiator, which is more efficient than other common
initiators. Therefore, we used the TPP-treated CsPbI3NCs to
produce near-infrared LSCs devices, with an optical conver-
sion efficiency of 3.1% at the geometric factor of 10 and good
[*] J. Wu, Dr. J. Tong, Dr. Y. Gao, Dr. A. Wang, Prof. T. Zhang, Prof. H. Tan,
Prof. Z. Deng
College of Engineering and Applied Sciences, Nanjing National
Laboratory of Microstructures, Nanjing University
Nanjing, Jiangsu, 210023 (P. R. China)
E-mail: dengz@nju.edu.cn
Prof. S. Nie
Departments of Bioengineering, Chemistry, Electrical and Computer
Engineering, and Materials Science and Engineering
University of Illinois at Urbana-Champaign
Urbana, IL 61801 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201911638.
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International Edition: doi.org/10.1002/anie.201911638
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environmental stability. In the absence of encapsulation, the
light efficiency of the LSCs can remain 93.6% after two
months of storage in the atmosphere. Therefore, we believe
that the TPP post-treatment method will be a promising
strategy for designing efficient and stable thin-film LSCs.
Near-infrared emission CsPbI3perovskite NCs were
prepared using an improved hot-injection procedure (see
the Supporting Information for details). Figure 1 a,b shows
photographs of the fresh untreated CsPbI3NCs under room
light and UV light, which is bright red when excited by 365 nm
UV light. Transmission electron microscopy (TEM) and high-
resolution TEM (HRTEM) images (Figure 1 c,d) show the
cubic shape of the as-synthesized NCs, with an average edge
length of 12.6 nm (Supporting Information, Figure S1). Fig-
ure 1e shows the powder X-ray diffraction (XRD) pattern of
CsPbI3NCs, which is a cubic-phase structure.
Figure 1f shows the typical UV/Vis absorption and PL
emission spectra of CsPbI3NCs in the toluene solution. The
UV/Vis absorption spectrum showed that there was an
absorption tail between 700 nm and 800 nm, indicating the
presence of NCs aggregates in the toluene solution. The PL
peak is located at 695 nm, and the full-width at half-maximum
(FWHM) is 30 nm.
To apply these colloidal NCs to high-performance LSCs, it
is often necessary to process them into highly transparent thin
films on the substrate. Here, we prepared CsPbI3NC–
polymer films by photopolymerization in the atmosphere
for LSCs. Using the ratio of the emission spectrum of the
TPP-CsPbI3film to the blank spectrum (Figure 2 a), the
absolute PLQY of a typical TPP-treated NC–polymer film
was calculated, and near-unity PLQY was obtained. As shown
in the Supporting Information, Figure S2, the PLQY meas-
urements were repeated 16 times, and the average value was
99.4 0.4%. Figure 2b shows the non-normalized absorption
and emission spectra of NC–polymer films. Compared to the
CsPbI3NCs in solution, the absorption edge of the NC–
polymer films showed the blue-shift without obvious absorp-
tion tails, and the PL peak position is slightly red-shifted to
700 nm with narrow FWHM of 75 meV (that is, 30 nm).
Furthermore, for comparison, we prepared CsPbI3NC–
polymer films without TPP or zinc iodide precursor in the
UV gel. Their UV/Vis absorption and PL emission spectra
were shown in Figure 2b. Compared with untreated NC
polymer films, the PL peak position of TPP treated NC–
polymer films show a slight blue-shift. It is worth noting that
the overlap of the emission and absorption of the TPP treated
NC–polymer films is smaller than that of untreated NC
polymer films (Supporting Information, Figure S4), indicating
that TPP can cause a decrease in reabsorption. The possible
reason is that, on the one hand, because TPP has a large steric
hindrance, it can improve the dispersibility of perovskite NCs
in the polymer matrix and inhibit their agglomeration. On the
other hand, TPP has strong coordination and deoxidation
ability, which can effectively protect NCs and prevent NCs
from being decomposed and regrowth during photopolyme-
rization. These will reduce the absorption tail and thus reduce
reabsorption.
To verify the stability of the TPP treated NC–polymer
composite films, we tracked the PLQYs of these films over
time in different environments. The TPP treated films show
excellent air stability and water stability (Supporting Infor-
mation, Figures S5,S6). Furthermore, photostability is an
important aspect of LSCs applications. We placed the films
in a photochemical reactor under 500 W mercury lamp
(power density 50 mWcm2, acceleration coefficient 10 at
365 nm; Supporting Information, Figure S7). As shown in
Figure 2c, the PLQY of untreated films and ZnI2-treated
Figure 1. a),b) Photographs of the as-prepared CsPbI3NCs in a three-
necked flask under room light (a) and UV light (b). c) TEM and
d) HRTEM images of CsPbI3NCs. e) The XRD pattern of CsPbI3NCs ;
inset: corresponding crystal diagram. f) UV/Vis absorption (black line)
and PL emission spectra (red line) of CsPbI3NCs; inset: photograph
of the NCs in toluene under UV light, which emitted strong red
fluorescence under UV light.
Figure 2. a) The PL emission spectra of blank (without sample, black
line) and TPP-CsPbI3(red line) used for measuring the PLQYs. Inset:
photograph of a TPP-CsPbI3NCs film in a mounting boat. b),c) UV/
Vis absorption spectra, PL emission spectra, and photostability of the
untreated, the ZnI2treated, and the TPP treated NC–polymer compo-
site films. Inset in (c): pictures of the untreated NCs films (1), ZnI2
treated films (2), and TPP treated NCs films (3) at illumination time of
0 h and 100 h.
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films rapidly decreased to 6.4% and 10.3 %, respectively.
However, the TPP treated films still account for 88% of
PLQY under standard outdoor conditions (PLQY drops to
90% of the initial value), which is converted into 1000 h T90
under continuous light conditions. These results indicate that
TPP greatly improves the stability of the CsPbI3NC–polymer
composite films.
Interestingly, apart from being a strong ligand, TPP can
also act as a photoinitiator because there are lone pairs of
electrons on the phosphorus atoms. For comparison, various
NC–polymer composite films were fabricated using other
common initiators, such as TPO, DMPA, HPK, and HMPO.
Their structural formulas are shown in Figure 3a. Compared
to NC–polymer films treated with other initiators, the overlap
between non-normalized absorption and emission spectra of
TPP treated NC–polymer composite films is significantly
reduced (Figure 3b; Supporting Information, Figure S8). To
evaluate the photostability of these films, we tracked the
optical properties of the illumination duration (Supporting
Information, Figures S9–S11). The absorbance values of the
films with TPO, DMPA, HPK, or HMPO as initiators
changed significantly at 685 nm, while the absorbance of
TPP treated films did not change much (Figure 3c). The
PLQYs of the films treated with HMPO decreased signifi-
cantly to 25.8% after photopolymerization. With TPO,
DMPA, and HPK as initiators, PLQYs decreased to 26%,
3.5%, and 7.7% after 40 h of mercury lamp illumination,
respectively (Figure 3d). These results indicate that TPP as
a photoinitiator can improve the photostability of CsPbI3NC–
polymer composite films because TPP can remove oxygen in
polymer gels, reduce chemical damage of NCs during photo-
oxidation, and effectively passivate the surface of NCs.
The TPP-treated CsPbI3NCs have a higher PLQY, good
stability, and lower reabsorption, which is necessary for the
application of LSCs device application in building-integrated
solar windows. A typical LSC consists of a mixture of a plastic
lightguide or a transparent glass plate coated with an emitting
material. The luminophores absorb the sunlight and radiate at
longer wavelengths. It is then guided to the edge by total
internal reflection (TIR) and converted into electricity by
photovoltaic cells mounted on the side of the LSCs. Here,
a layered LSCs device is constructed, coated by a circa 50 mm
layer of the NC–polymer composite onto the high-optical-
quality PMMA glass slab (ca. 2 mm) by using a doctor-blade
deposition technique (Figure 4a,b). It can be clearly seen that
the edge of the slab presents bright emission under weak UV
illumination. The absorption spectrum in Figure 4c of the
CsPbI3-based LSCs device shows a broadband absorption
from the ultraviolet to the near-infrared region.
We further measured the PL spectra of CsPbI3based
LSCs at different optical paths. The integrated PL emission of
the TPP-treated NC–polymer films was maintained at 49 % at
an optical length of 6 cm, while the integrated PL emission of
untreated NC–polymer films decreases to 27% at d=6cm
owing to the larger reabsorption (Figure 4 d). It is well-known
that the optical efficiency (hopt), a key figure of merit (FOM)
for LSCs, is the ratio of the output power (Pout) from the edges
of LSCs devices and the input power (Pin) through the top
surface of the LSCs. When the LSC is coupled with a silicon
PV-cell, the (hopt) can be easily calculated by the following
equation [Eq. (1)]:[12, 14]
Figure 3. a) The molecular structure of several photoinitiators. b) UV/
Vis absorption and PL emission spectra of the different initiator
treated NC–polymer films at illumination time of 0 h. c) The absorb-
ance values at 685 nm of films with TPO, DMPA, HPK, or HMPO as
initiators at different illumination time. d) Time-dependent PLQYs of
different photoinitiators treated NC–polymer films under the strong
mercury lamp without inert gas protection.
Figure 4. a) Preparation of the thin-film LSCs using the doctor-blade
method. b) Photograph of a 5 15 cm CsPbI3NCs-based LSC under
ultraviolet light. c) AM 1.5 G spectrum, UV/Vis absorption, and PL
emission spectra of CsPbI3NCs-based LSCs. d) Integrated PL emission
of the untreated and TPP-treated LSCs at different optical paths.
e) The optical efficiency of the LSCs coupled with silicon cell as
a function of geometric factor (Gfactor). f ) The I–Vcurves of the Si
solar cell coupled with the edges of an LSC before and after storage
for 2 months in ambient air.
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hopt ¼Ilsc=ðIsc GÞð1Þ
where ILSC is the short circuit current from the Si cell coupled
with LSC, Isc is the short circuit current from Si cell under
direct illumination, and Gis the geometric factor, defined as
the ratio of the top area and the edge area.
The LSCs with different sizes were made to test their
optical efficiency. During the course of the experiments, these
LSCs were illuminated by a 1.5 AM global solar simulator
(100 mWcm2). A commercial silicon cell was coupled with
the edge of LSCs. As shown in Figure 4 f, the hopt decreased
exponentially with the increasing of the Gfactors of LSCs
devices, which is also seen in previous reports.[5b,9] The
maximum optical efficiency is 4.9% with a Gfactor of 5.
The optical quantum efficiency hint, which is defined as the
ratio of number of photons emitted from the edge to the
number of photons absorbed by the LSC, is 36% for a 2 
2 cm LSCs with G=10. To further evaluate the stability of the
device, we measured the optical efficiency of a 2  2 cm LSCs
before and after 2 months of storage in ambient air (Fig-
ure 4 f). After 2 months of storage, the optical efficiency of
devices only decreased from 3.1% to 2.9 %, still maintaining
93.5% optical efficiency, indicating that these LSCs have
excellent stability.
In summary, we have demonstrated a novel TPP post-
treatment method for efficient and stable thin-film LSCs with
bright and low reabsorption near-infrared emissive perovskite
CsPbI3NC–polymer composite films. These LSCs exhibited
a high absolute PL quantum yield of 99.4 0.4% and long-
term air, water, and light stability. These thin-film LSCs
exhibit an impressive optical conversion efficiency of 3.1%
and optical quantum efficiency of 36 % at a geometric factor
of 10. This work may lay a foundation for efficient and stable
LSCs based on low-cost perovskite NCs.
Acknowledgements
This work was supported by Natural Science Foundation of
Jiangsu Province (Grant No. BZ2018008) and Shuangchuang
Program of Jiangsu Province.
Conflict of interest
The authors declare no conflict of interest.
Keywords: lead halides · luminescent solar concentrators ·
nanocrystals · perovskites · polymer composites
How to cite: Angew. Chem. Int. Ed. 2020,59, 7738–7742
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Manuscript received: September 11, 2019
Revised manuscript received: November 18, 2019
Accepted manuscript online: January 30, 2020
Version of record online: March 6, 2020
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