Donor–Acceptor-Type Organic-Small-Molecule-Based Solar-Energy-Absorbing Material for Highly Efficient Water Evaporation and Thermoelectric Power Generation

Article abstract of DOI:10.1002/adfm.202106247

Recently, owing to the great structural tunability, excellent photothermal property, and strong photobleaching resistance, organic-small-molecule photothermal materials are proposed as promising solar absorbent materials. Herein, through fusing two strong

Full text of DOI:10.1002/adfm.202106247

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Donor–Acceptor-Type Organic-Small-Molecule-Based
Solar-Energy-Absorbing Material for Highly Efficient Water
Evaporation and Thermoelectric Power Generation
Yuanyuan Cui, Jing Liu, Zhiqiang Li, Mingyang Ji, Meng Zhao, Meihua Shen, Xue Han,
Tao Jia,* Chenglong Li,* and Yue Wang
1. Introduction
Recently, owing to the great structural tunability, excellent photothermal prop-
erty, and strong photobleaching resistance, organic-small-molecule photo-
As an alternative to fossil fuels, solar
energy has been considered more envi-
thermal materials are proposed as promising solar absorbent materials. Herein,
ronmentally benign and renewable,[1] thus
through fusing two strong electron-withdrawing units dibenzo[f,h]quinoxaline
has aroused widespread research interest
and anthraquinone units, a rigid planar acceptor dibenzo[a,c]naphtho[2,3-h]
phenazine-8,13-dione (PDN) with stronger electron-withdrawing ability is obtained
and used to construct donor–acceptor-type organic-small-molecule solar-energy-
absorbing material, 2,17-bis(diphenylamino)dibenzo[a,c]naphtho[2,3-h]phenazine-
8,13-dione (DDPA-PDN). The new compound exhibits a strong intramolecular
charge transfer character and conjugates rigid plane skeleton, endowing it with
a broadband optical absorption from 300 to 850 nm in the solid state, favorable
photothermal properties, high photothermal conversion ability, and good photo-
bleaching resistance. Under laser irradiation at 655 nm, the solid photothermal
conversion efficiency of the resulting DDPA-PDN molecule reaches 56.23%. Addi-
tionally, DDPA-PDN-loaded cellulose papers equipped with abundant microchan-
nels for water flow are integrated with thermoelectric devices, thus achieving an
evaporation rate and voltage as high as 1.07 kg m−2 h−1 and 83 mV under 1 kW m−2
solar irradiation, respectively. This study demonstrates the application of photo-
thermal organic-small-molecules in water evaporation and power generation,
therefore offering a valuable prospect of their utilization in solar energy harvesting.
among scientists in photochemical,
photovoltaic and photothermal fields.[2–4]
The excessive growth of population and
concomitant demands on higher living
standard lead to unprecedented require-
ments on fresh water, which render the tra-
ditional water purification systems relying
upon fossil fuel consumption unsustain-
able. Therefore, it is necessary to develop
eco-friendly technologies to achieve
sustainable
Photothermal
development
conversion
goals.[5–10]
of solar
energy has been considered as a renew-
able, green and pollution-free way for
sustainable development, thus exerting
usage in water evaporation and electricity
generation without fossil fuel consump-
tion.[11–13] In the process of photothermal
conversion, the approach of water evapo-
ration critically affects the photothermal
conversion efficiency and evaporation rate. In recent years,
interfacial evaporation becomes the primary structural guid-
ance to construct the solar-driven water evaporation system,
which consists of solar energy absorption layer (photothermal
material), steam escape channel, water transportation channel
and heat insulation layer.[14–17] Accordingly, the selective heating
system surface in the form of interfacial evaporation gives rise
to a challenge on the feasibility of thermoelectric conversion
in resolving the energy crisis. At present, it is impendent to
develop efficient photothermal materials for the construction of
solar energy driven water and electricity integration device.
Currently the common photothermal materials mainly
involve carbon-based inorganic materials,[18–20] metal-based
inorganic materials,[21,22] organic–polymer,[23–25] organic–inor-
ganic hybrid materials,[26,27] organic cocrystals materials,[28]
and purely organic-small-molecule materials.[29] Among them,
photothermal materials comprised of organic-small-molecules
have attracted growing attention due to their unique advantages
in processing feasibility, structural diversity and fine-tuned
properties.[29,30–36] As solar-energy-absorbing material, organic-
small-molecule photothermal material can collect waste heat to
generate electricity during water evaporation, which promises
Y. Cui, C. Li, Y. Wang
State Key Laboratory of Supramolecular Structure and Materials
College of Chemistry
Jilin University
2699 Qianjin Street, Changchun 130012, P. R. China
E-mail: chenglongli@jlu.edu.cn
J. Liu, M. Zhao, M. Shen, X. Han, T. Jia
Key Laboratory of Forest Plant Ecology
Ministry of Education
Engineering Research Center of Forest Bio-Preparation
College of Chemistry
Chemical Engineering and Resource Utilization
Northeast Forestry University
26 Hexing Road, Harbin 150040, P. R. China
E-mail: jiataopolychem@nefu.edu.cn
Z. Li
Jihua Laboratory
No. 28 Island Ring South Road, Foshan 528200, P. R. China
M. Ji
Research School of Polymeric Materials
School of Materials Science and Engineering
Jiangsu University
Zhenjiang 212013, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202106247.
DOI: 10.1002/adfm.202106247
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access to multifunctional devices capable of photothermal
water evaporation and thermoelectric power generation simul-
taneously.[18] Despite the promising prospect of organic photo-
thermal small molecules in solar energy harvesting and con-
version, further exploration is still required to broaden their
absorption spectra and promote their photothermal conversion,
therefore enable their utilization in high-performance water
evaporation/thermoelectric devices.[29,35] In general, organic
conjugated molecules with strong electron donor–acceptor
(D–A) structure can largely redshift absorption spectra by effec-
tive electron delocalization. In addition, the conjugate rigid
plane structure contributes to increase the π–π stacking in the
aggregate state, which can further widen absorption spectra.
Even more interestingly, according to the energy-gap law,[37–39]
the resultant small energy gap could promote nonradiative
decay to generate heat. Therefore, the development of organic-
small-molecules with strong D–A structure and conjugate rigid
plane skeleton is of crucial importance to efficiently convert
solar energy to heat for high-performance water evaporation/
thermoelectric devices.
gap, their electron-withdrawing abilities and conjugate rigid
plane skeleton are not sufficient enough. Herein, by fusing
two strong dibenzo[f,h]quinoxaline and anthraquinone units
together, a new stronger electron-accepting core, dibenzo[a,c]
naphtho[2,3-h]phenazine-8,13-dione (PDN) with conjugate
rigid planar skeleton, was developed. A novel organic-small-
molecule,
2,17-bis(diphenylamino)dibenzo[a,c]naphtho[2,3-h]
phenazine-8,13-dione (DDPA-PDN), featuring PDN as the
acceptor and two diphenylamine (DPA) as the donors with
strong D–A structure and conjugate rigid plane skeleton was
synthesized (Figure 1a). DDPA-PDN presented a broad absorp-
tion spectrum from 300 to 850 nm in the solid state, contrib-
uting to efficient solar energy harvesting. As expected, DDPA-
PDN exhibited highly efficient photothermal conversion and
achieved a conversion efficiency as high as 56.23% under laser
irradiation at 655 nm. In addition, under a 1 kW m−2 solar irra-
diation, this organic photothermal material with D–A structure
was tested as solar absorber for water evaporation to achieve
an evaporation rate and efficiency as 1.07 kg m−2 h−1 and
73.98%, respectively. Given the efficient solar energy harvesting
and conversion of DDPA-PDN molecule, it was employed
as the solar absorber to coat the surface of thermoelectric
devices. The resulting photothermal–electric power genera-
tion device, under 5 kW m−2 solar irradiation and floating on
water, produced a voltage up to 148 mV that could drive the
rotation of a microfan. Finally, to maximize the utilization
Dibenzo[f,h]quinoxaline and anthraquinone are common
electron-withdrawing groups used for constructing long-wave-
length organic light-emitting materials due to their planarity,
highly conjugated backbones and strong electron-withdrawing
abilities.[40,41] However, to construct organic photothermal
materials with broad absorption spectra and narrow band
Figure 1. a) Chemical structure of DDPA-PDN. b) Optimized ground-state geometry of DDPA-PDN. c) The calculated LUMO and HOMO for DDPA-PDN.
d) Normalized absorption spectra of DDPA-PDN in THF solution (10–5 m, calibrated with blank THF) and solid powder (calibrated with baritite). Insets
show the photographs of DDPA-PDN in solid powder and THF solution taken under daylight. e) Emission spectra of DDPA-PDN in THF solution
(10–5 m) and solid powder at room temperature. Excitation wavelength (λex) is 470 nm.
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Scheme 1. Schematic diagram of water evaporation and cogeneration system.
of heat energy, a multifunctional device capable of simulta-
neous water evaporation and thermoelectric power generation
was constructed by attaching a DDPA-PDN-loaded cellulose
paper on a thermoelectric Peltier plate. With the presence of
interface water evaporation within the DDPA-PDN-loaded cel-
lulose paper, this proposed device realized a water evapora-
tion rate of 0.89 kg m−2 h−1 and produced a stable voltage up
to 43 mV as well under 1 kW m−2 solar irradiation (working
diagram is shown in Scheme 1). This organic-photothermal-
small-molecule with D–A structure is used in solar-driven inte-
grated production of water and electricity combined absorber,
which will be thermal evaporation and waste heat power gen-
eration technology seamless connection for practical outdoor
solar desalination and electricity generation to make bold
assumptions.
2.2. Thermal and Electrochemical Properties
The compound is thermodynamically and electrochemically
stable. The thermal data operated by thermogravimetric analysis
(TGA) and differential scanning calorimetric (DSC) measure-
ment is collected in Table S1, Supporting Information. The TGA
measurement indicates that DDPA-PDN has excellent thermal
stability with high decomposition temperature (Td, corre-
sponding to 5% weight loss) of 447 °C. The glass transition tem-
perature (Tg) was observed at 172 °C from the DSC measurement
(Figure S4–S5, Supporting Information). The stable thermal
properties reveal that this compound is suitable as photothermal
material. The electrochemical property of DDPA-PDN was per-
formed by cyclic voltammetry (CV) measurement. As shown in
Figure S6 (Supporting Information), the quasireversible oxida-
tion and reduction processes observed in dichloromethane solu-
tion are assigned to the oxidation of DPA unit and the reduction
of PDN unit. Cyclic voltammogram of DDPA-PDN (Table S1,
Supporting Information) shows that the highest occupied mole-
cular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) values are −5.40 and −3.64 eV respectively, and an
ideal narrow band gap value of 1.76 eV is calculated. The small
band-gap makes DDPA-PDN have the ability to absorb solar
energy from the full spectral range. Furthermore, the HOMO of
DDPA-PDN measured by ultraviolet photoelectron spectroscopy
is −5.47 eV and the LUMO level estimated from optical band
gap and HOMO level is −3.67 eV, which is in agree with elec-
trochemical measurement (Figure S6, Supporting Information).
2. Results and Discussion
2.1. Synthesis and Characterization
The synthetic procedure of the new compound is depicted in
Scheme S1 (Supporting Information). The compound was syn-
thesized through a two-step reaction. First, the intermediate
2,17-dibromodibenzo[a,c]naphtho[2,3-h]phenazine-8,13-dione
(DDPA-PD) was obtained by Pd-catalyzed Buchwald–Hartwig
crossing-coupling reaction using 3,6-dibromophenanthrene-
9,10-dione and DPA. Second, the target molecule DDPA-PDN
was synthesized by cyclization reactions with good yield of 50%.
The target product was further purified by column chromatog-
2.3. Density Functional Theory (DFT) Calculations
1
raphy and the chemical structure was fully characterized by H
and 13C NMR spectroscopy, mass spectrometry and elemental
analysis. (Figures S1–S3, Supporting Information).
First of all, in order to further understand the geometrical
and electronic properties of DDPA-PDN, the ground-state
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geometry of the compound and the related acceptor units
were optimized in the gas phase using Gaussian 09 program
package at the B3LYP/6-31g(d) level starting from the geometry
obtained from the initial geometry. As shown in Figure 1b,
DDPA-PDN exhibits an almost plane molecular structure,
which is advantageous to increase the π–π stacking in the
aggregate state, and thus further widen absorption spectra.
The optimized molecular structure and the density of state
distributions of HOMO and LUMO are displayed in Figure 1c.
The LUMO of DDPA-PDN is mainly localized on the acceptor
core while the HOMO delocalizes across the whole molecule
except the anthraquinone part. The energy levels of HOMO
and LUMO of the compound based on the theoretical calcu-
lations are calculated to be −5.03 and −2.87 eV and a small
calculated band-gap of 2.16 eV is obtained, which is in agree
with electrochemical band-gap. Additionally, the LUMO of
the fused acceptor PDN is calculated to be shallower than its
composition dibenzo[f,h]quinoxaline and anthraquinone units
(Figure S7, Supporting Information), proving PDN stronger
electron-accepting ability.
powder was evaluated by the rapid temperature change under
the IR camera. The IR camera was applied to quickly and
accurately capture the photos of the DDPA-PDN material under
0.8 W cm–2 illumination density, so as to better observe the
temperature change of the material under 655 nm laser irra-
diation. In Figure 2a, it shows clearly that the temperature
of DDPA-PDN material rises rapidly to about 144 °C within
20 s. When the temperature stable at 146 °C in 30 s the laser
is turned off immediately, and then the temperature drops
dramatically to room temperature with time. In Figure 2b, the
maximum stable temperature of the 5 mg solid powder gradu-
ally increases with the increase of power density under the irra-
diation of 655 nm laser. When the power density is lower than
0.6 W cm–2, the temperature change is slight and negligible.
This is because little heat can be generated when the power
density is below 0.6 W cm–2, and it is easy to be dissipated
through thermal conduction, thermal radiation and thermal
convection processes. Therefore, it is difficult for the IR camera
to capture this tiny temperature change. When the power den-
sity is 0.9 W cm–2, the temperature can even reach as high as
201 °C. The excellent photothermal performance of DDPA-PDN
has been obviously proved, but it is far from enough. Notably,
the organic photothermal materials used for solar energy con-
version can not only lay emphasis on unexceptionable thermal
stability and photothermal performance, but also understand
the photobleaching resistance of the materials, so as to obtain
the possibility of stable and long-term photothermal conversion.
The 5 mg DDPA-PDN powder was irradiated under 655 nm
laser with a power density of 0.8 W cm–2, and the optical sta-
bility was observed through the difference of temperature
rise and temperature drop in five periods (Movie S1, Sup-
porting Information). As depicted in Figure 2c, there is still
about 146 °C under the irradiation of 0.8 W cm−2 laser for 2 h
(Figure S8, Supporting Information), which manifests it has
excellent photobleaching resistance. In addition, under the
irradiation of 655 nm laser, the photothermal conversion effi-
ciency of DDPA-PDN can be as high as 56.23% (Figure S9,
Supporting Information).[42] In a word, DDPA-PDN molecule
has broad absorption spectrum from 300 to 850 nm, admi-
rable solar energy conversion ability, excellent thermal
stability, and photobleaching resistance, which is better
compared with other traditional organic photothermal small
molecules and expected to be a potential candidate material
for solar absorbent in the process of solar–thermal conversion
technology.
2.4. Photophysical Properties
The ultraviolet–visible absorption spectra of DDPA-PDN in tet-
rahydrofuran (THF) solution (10–5 m) and solid state at room
temperature is shown in Figure 1d. The absorption in THF at
lower wavelength region below 450 nm can be attributed to
n–π* and π–π* transitions of the DPA, PDN moieties, while the
broad low-energy absorption band from 430 to 685 nm origi-
nates from intramolecular charge transfer (ICT) transition from
the donor to the acceptor. Noticeably, the aggregated state of the
photothermal molecule exhibits a wide absorption spectrum
from 300 to 850 nm with strong absorption peak that lays a good
foundation for the efficient use of solar energy. Compared with
the solution state, the absorption spectrum of the solid state
behaves a significant red shift and stronger absorption intensity,
which is attributed to strong intermolecular π–π stacking led by
highly conjugated rigid planar skeleton.[41] On the other hand,
the emission spectra of DDPA-PDN at room temperature were
captured (Figure 1e). No emission spectra in solid state and
extremely weak emission spectra in THF solution (10–5 m) can
be collected, and corresponding fluorescence quantum yields
are only 0.29% and 0.08%, which indicates that the emission of
DDPA-PDN can be seriously quenched and finally leads to the
enhancement of nonradiative decay of molecules to generate
heat. Overall, DDPA-PDN mainly leads to strong intramolecular
charge transfer through the D–A structure, resulting in large
electron delocalization and small band gap. Furthermore, the
conjugate rigid plane structure of PDN acceptor core contrib-
utes to increase the π–π stacking in the aggregate state, which
can promote exciton to be quenched and further widen absorp-
tion spectra. Thus, the energy of the excited state of DDPA-PDN
is more tended to generate heat through nonradiation decay.
2.6. Photothermal Water Evaporation Capacity of DDPA-PDN
Based on the excellent photothermal property of DDPA-PDN,
it is proposed in this work that DDPA-PDN can be combined
with cellulose paper to produce a highly efficient solar absorber
for interfacial heating and evaporating water. Cellulose paper
used as floating carrier and interface evaporation of water is
adopted because of its excellent light absorption performance,
high water transportation performance, good heat insulation
performance, high energy conversion efficiency.[43–45] The spe-
cific creation method of cellulose paper is shown in Figure S10
(Supporting Information). Insight into their structure through
2.5. Photothermal Performance of DDPA-PDN
Inspired by a wide absorption spectrum and good thermal
stability, the photothermal performance of DDPA-PDN solid
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Figure 2. a) IR thermal photos of DDPA-PDN powder (5 mg) under 655 nm laser irradiation (0.8 W cm−2) and then turned off at 30 s. b) Photo-
thermal conversion behavior of DDPA-PDN powder (5 mg) under 655 nm laser irradiation at different laser powers (0.6, 0.7, 0.8, and 0.9 W cm−2).
c) Photobleaching resistance property of DDPA-PDN powder (5 mg) during five cycles of on–off processes.
scanning electron microscopy (SEM; Figure 3a) it can be
observed that the surface of DDPA-PDN cellulose paper is
coarser than that of blank cellulose paper, indicating that the
material can be well attached to the surface of the cellulose
paper, and the ideal pore structure is more conducive to the
effective collection of sunlight inside the cellulose paper and
the floating surface evaporation water.[46]
the IR thermal photos manifest that DDPA-PDN has the
rapid heating process and the ideal equilibrium temperature.
The special structural characteristic of DDPA-PDN makes the
highest temperature of the DDPA-PDN cellulose paper almost
independent of the quality (Figure S14, Supporting Information).
In other words, compared to other organic photothermal small
molecules, a little mass of the material required can reach the
desired temperature. Therefore, this cellulose paper is feasible
in water evaporation because of excellent photothermal conver-
sion behavior, the characteristics of saving materials and easy
synthesis.
In the next moment, the DDPA-PDN cellulose paper was
evaporated by water test to verify the evaporation effect, with
the basic design rationale illustrated in Figure 3d. The DDPA-
PDN cellulose paper was placed on top of a small glass bottle
filled with water and irradiated with a sunlight beam for 1 h.
At the same time, the temperature change was recorded by IR
thermal camera, and the mass loss of water was paid attention
to at all times. As can be seen from the insets in Figure 3e,
the photograph of DDPA-PDN cellulose paper floating on the
top of water proves that at the interface, the heat transfer of
internal water and cellulose paper is reduced and the evapora-
tion efficiency is enhanced. The surface temperature of DDPA-
PDN cellulose paper floating on the water is remarkably higher
than blank cellulose paper, the former reaches 35.6 °C while
Under normal air conditions, the load DDPA-PDN cellu-
lose paper has absorption in the range of 300–850 nm, which
is much higher than the blank cellulose paper (Figure 3b). As
shown in Figure S11 (Supporting Information), the little dif-
ference of the absorption spectrum between the powder and
DDPA-PDN cellulose paper is on account of the change of
sample morphology. In order to assess the photothermal con-
version ability of the solar absorber (DDPA-PDN cellulose
paper), the temperature change process of the paper under
one sunlight intensity for 10 min was quickly recorded. It can
be seen that the temperature of DDPA-PDN cellulose paper
can reach up to 62 °C contrasting with blank cellulose paper
only 37.2 °C under the same condition (Figure 3c). After long-
term evaporation and impregnation, although the surface has
slight change, it can still reach the temperature in the dry state
(Figure S12, Supporting Information) and it remains relatively
stable under 2 h of sunlight irradiation (Figure S13, Supporting
Information). In addition, as shown in the insets (Figure 3c),
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Figure 3. a) Digital photos and SEM images of blank cellulose paper and DDPA-PDN-loading cellulose paper (DDPA-PDN cellulose paper). The
amount of DDPA-PDN used in preparing DDPA-PDN cellulose paper is 5 mg. b) Absorption of blank cellulose paper (calibrated with baritite) and the
DDPA-PDN cellulose paper (calibrated with baritite), and the solar spectral irradiance (gray, AM 1.5 G). c) Photothermal conversion behavior of blank
cellulose paper and the DDPA-PDN cellulose paper under 1 kW m−2 solar irradiation. Insets show IR thermal photos of blank cellulose paper and the
DDPA-PDN cellulose paper under 1 kW m−2 solar irradiation. d) Solar energy driven water evaporation diagram. Room temperature is about 24 °C.
e) Temperature variation with time of water evaporation process for blank cellulose paper and DDPA-PDN cellulose paper under 1 kW m−2 solar irradia-
tion. The inset shows an infrared thermal image of 1 kW m−2 solar irradiation. f) Mass loss curves of pure water evaporation (only water) and blank
cellulose paper and DDPA-PDN cellulose paper within 1 h under simulated sunlight of 1 kW m−2 solar irradiation.
the latter only 28 °C after an hour. With the data recorded at
real time, the relationship curve between water mass loss and
time in 1 h shows that the evaporation efficiency of impreg-
nated cellulose paper is much higher than that of blank water
and cellulose paper (Figure 3f). Through calculation, the evap-
oration rate of the DDPA-PDN cellulose paper is as high as
1.07 kg m−2 h−1 compared to pure water only 0.44 kg m−2 h−1
and blank cellulose paper 0.62 kg m−2 h−1. Furthermore, the
efficiency of solar-driven water evaporation (η) for DDPA-PDN
cellulose paper was calculated to be as high as 73.98%. In gen-
eral, the heat loss of a steam generator is divided into conduc-
tion loss, radiation loss and convection loss. In this work, we
calculated the heat loss in the evaporation process according to
the previous report.[29,47] The conduction, radiation, and convec-
tion losses are estimated to be 8.5%, 2.3%, and 2.1%, respec-
tively (Supporting Information). Due to its high photothermal
conversion efficiency and excellent water evaporation rate, such
solar distillation technology can provide potential opportunities
for fresh water.
wasted in the evaporation process. Hence, a combination system
of photothermal evaporation water and afterheat utilization for
power generation is envisaged, and the feasibility of thermoelec-
tric power generation is verified by recording corresponding data.
Thermoelectric devices collect electrical energy through the See-
beck effect triggered by the static temperature difference between
the solar absorber and water. Photothermal water evaporation
material DDPA-PDN is combined with thermoelectric device
design, the heat energy can be converted into electric energy
under the irradiation of sunlight and the experimental schematic
design scheme is shown in Figure 4a. 20 mg of DDPA-PDN
solid powder and a small amount of thermal conductive adhesive
were evenly coated on the surface of the thermoelectric device to
obtain the hot end temperature under simulated sunlight irra-
diation, while the lower part of the thermoelectric device used
flowing water to obtain the cold end temperature.
The electromotive force is generated when the tempera-
ture difference between hot and cold ends is produced, and
the relation of thermoelectric device output voltage with time
under different solar intensity is shown in Figure 4c. Com-
paring blank device and the device loading DDPA-PDN solid
powder in dark, it can be clearly seen that the corresponding
stable voltage is higher with the increase of light density. The
voltage is 83 mV under 1 kW m−2 solar irradiation while the
stable voltage can even reach 148 mV under the irradiation of
2.7. Photothermal–Electric Conversion Ability of DDPA-PDN
As a solar absorber, the DDPA-PDN cellulose paper can be
applied to water evaporation. However, there will still be afterheat
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Figure 4. a) Generating design of thermoelectric device. On the right is a comparison diagram of the actual blank thermoelectric device and the ther-
moelectric device loaded with DDPA-PDN powder (20 mg). b) Temperature difference between the surface of a thermoelectric device and the circulating
water under different light intensity (dark, 1, 2, and 5 kW m−2). c) Thermoelectric conversion capability of DDPA-PDN powder (20 mg) that is attached
to the surface of the thermoelectric device under different light intensities (dark, 1, 2, and 5 kW m−2). d) Stabilizing thermoelectric generating property
of DDPA-PDN powder (20 mg) during three cycles of on–off processes.
5 kW m−2 solar irradiation. Thus, it is possible to use DDPA-
PDN for thermoelectric power generation. Simultaneously,
we tested the voltage stability under different light intensities
in three cycles (Figure 4d). The results reveal that the output
voltage has certain regularity and the thermoelectric conversion
is relatively stable under different illumination densities. When
we simulate sunlight exposure at a high density, the device
can drive a small fan to rotate quickly (Movie S2, Supporting
Information). Figure 4b and Figure S15 (Supporting Informa-
tion) respectively show the curves of temperature difference in
the process of thermoelectric generation and cold and hot end
temperature comparison diagram under different light inten-
sities. In a word, the possibility of the photothermal material
being used for thermoelectric conversion is proved and pave
the way for the development of the synergistic system of photo-
thermal evaporation and thermoelectric power generation.
power with waste heat. Its design diagram and actual device
are shown in Figure 5a. A polystyrene foam frame is used to
fix the thermoelectric device, and the lower part of the cel-
lulose paper is immersed in water. The fiber structure of the
cellulose paper becomes an efficient water pump to transport
water and forms a water evaporation layer above the thermo-
electric device. When the DDPA-PDN cellulose paper is illu-
minated by sunlight, the upper part of the thermoelectric
device reaches a higher temperature, forming a temperature
difference with the lower static water as the power generation
core, which can convert the waste heat into electric energy. In
this way, it provides theoretical support for the combination
of thermoelectric devices and photothermal materials to realize
the combined generation of water and electricity. The syner-
gistic system of evaporated water and power generation could
be comprehensive evaluated by IR camera.
As shown in Figure 5c, the evaporation rate of water under
coproduction can reach 0.89 kg m−2 h−1 and the evaporation effi-
ciency is 61.06% under 1 kW m−2 solar irradiation,[48,49] which is
slightly lower than that of single evaporated water. Figure 5b
shows the stable temperature difference of blank cellulose
paper and DDPA-PDN cellulose paper by thermal imaging
instrument under different solar light intensity. At 1 kW m−2
solar irradiation intensity, the temperature difference (ΔT) of
DDPA-PDN cellulose paper is significantly higher than that of
blank cellulose paper and the stable temperature difference also
2.8. The Ability to Coproduce Water and Electricity
of DDPA-PDN
The possibility of photothermal evaporated water and thermo-
electric power generation has been confirmed and we boldly
assumed the synergistic system of photothermal evaporated
water and thermoelectric power generation, which can gen-
erate water vapor under the irradiation of sunlight and generate
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Figure 5. a) Integrated design drawing of water evaporation and thermoelectric generation. The photos on the right are the top and side views of the
actual device. Among them, polystyrene foam is used as a carrying float. b) IR images of thermoelectric device surface and water under different light
intensities (1, 2, and 5 kW m−2). c) Mass change diagram of integrated unit in 1 h. d) Temperature difference between the surface of the DDPA-PDN
cellulose paper and the water under different light intensity (1, 2, and 5 kW m−2). e) Thermoelectric conversion capability of DDPA-PDN powder (20 mg)
that is attached to the surface of cellulose paper under different light intensities (1, 2 and 5 kW m−2).
gradually increases with the increase of light intensity. The tem-
perature difference causes a current to be generated in the ther-
moelectric device, and the current increases with the increase
of the temperature difference. Figure 5d,e shows the tempera-
ture difference changes and voltage of the integrated system
of water evaporation and thermoelectric generation under dif-
ferent light intensity, and the integrated device can reach 43 mV
under 1 kW m−2 solar irradiation. To sum up, DDPA-PDN cellu-
lose paper has broad application prospects in water evaporation
and waste heat utilization for power generation.
energy conversion. Using cellulose paper as the carrier, the
photothermal water evaporation experiment was carried out,
and DDPA-PDN exhibits stable photothermal behaviors with
a high efficiency of 73.98% under 1 kW m−2 solar irradiation.
At the same time, thermoelectric devices were employed as car-
rier to make full use of waste heat for power generation. The
voltages reach up to 83 mV for flowing water under the light
of 1 kW m−2 solar irradiation and it even drive a small fan at
high solar exposure density. Moreover, the integrated device of
water evaporation and waste heat power generation designed
by us can reach 0.89 kg m−2 h−1 under 1 kW m−2 solar irradia-
tion. In short, the admirable performance of the material will
contribute to the efficient use of solar thermal conversion and
become a new step for the combination of water evaporation
and power generation synergies.
3. Conclusion
In summary, through fusing two strong electron-withdrawing
units dibenzo[f,h]quinoxaline and anthraquinone units together,
the rigid planar acceptor PDN with stronger electron-with-
drawing ability was obtained and used to construct an organic-
small-molecule solar-energy-absorbing material, DDPA-PDN.
Strong ICT character and the conjugate rigid plane skeleton
endow DDPA-PDN with a broadband optical absorption from
300 to 850 nm in the solid state, favorable photothermal
property, high photothermal conversion ability, and good
photobleaching resistance. In addition, DDPA-PDN exhibits
excellent and stable photothermal behavior under 655 nm
laser irradiation, with an efficiency of 56.23%. These excellent
properties make the material an ideal solar absorber for solar
4. Experimental Section
All reagents and solvents were obtained from Energy Chemical Co. and
J&K scientific Ltd. Co. and used as received without further purification.
All reactions were carried out using Schlenk technique under nitrogen
atmosphere.
Synthesis
of
3,6-Bis(diphenylamino)phenanthrene-9,10-dione
(DDPA-PD): A mixture of 3,6-dibromophenanthrene-9,10-dione (3.66 g,
10 mmol), diphenylamine (4.22 g, 25 mmol), tris(dibenzylideneacetone)
dipalladium(0) (Pd2(dba)3) (275 mg, 0.3 mmol), cesium carbonate
(Cs2CO3) (13.04 g, 40 mmol), 1.0 m solution of tri-tert-butyl phosphine (P(t-
Bu)3) in toluene (3 mL) and toluene (100 mL) was stirred at 110 °C for 24 h
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under nitrogen atmosphere. The mixture was cooled to room temperature
and distilled water (20 mL) was added to quench the reaction. The mixture
was then extracted with dichloromethane, and the combined organic
solvent was dried over anhydrous sodium sulfate and removed under
reduced pressure. The residue was purified by column chromatography
using dichloromethane as the eluent to give a deep-red solid (2.71 g).
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Yield: 50%. H NMR (500 MHz, DMSO-d6) δ 7.86 (d, J = 8.7 Hz, 2H),
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4
20 mmol) and acetic acid (30 mL) was stirred at 120 °C for 12 h under
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1
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139.34, 135.38, 134.96, 134.74, 134.41, 134.14, 133.49, 133.33, 132.27, 129.67,
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125.57, 124.72, 124.54, 123.83, 122.73, 122.17, 121.30, 113.53. ESI-MS [M]+
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Acknowledgements
Y.C. and J.L. contributed equally to this work. This work was supported
by the National Natural Science Foundation of China (51803069),
Heilongjiang Province Returnees Merit Subsidize Project (2017QD0052),
China Postdoctoral Science Foundation (2018M631892), and the Science
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Conflict of Interest
The authors declare no conflict of interest.
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Data Availability Statement
Research data are not shared.
Keywords
donor–acceptor structures, organic-small-molecule photothermal
materials, solar–thermal conversion, thermoelectric power generation,
water evaporation
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Received: June 29, 2021
Revised: August 16, 2021
Published online:
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