Communication
Solar Cells
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Lead Selenide (PbSe) Colloidal Quantum Dot Solar Cells
with >10% Efficiency
Waqar Ahmad, Jungang He,* Zhitian Liu,* Ke Xu, Zhuang Chen, Xiaokun Yang,
Dengbing Li, Yong Xia, Jianbing Zhang, and Chao Chen*
Among various types of CQDs, lead chalcogenides (PbE, E = S, Se, and Te) are
regarded as excellent active absorber in
a solar cell owing to their size and shape
tunability, effective multiple-exciton generation (MEG), flexible bandgap (Eg), and
modifiable physical properties.[9–12] In
particular, PbS CQDs were widely investigated because of its excellent intrinsic air
stability and thus attracted great attention
in recent years.[13,14] To date, PbS based
CQD solar cells exhibited a certified power
conversion efficiency (PCE) of 12.01% due
to effective surface passivation and device
architecture engineering.[15]
Recently, PbSe CQDs appeared as
a potential absorber material for optoelectronic devices due to their exceptional advantages, paralleled to PbS
CQDs. For instance, PbSe has a larger
exciton Bohr radius (46 nm) in contrast
to PbS (23 nm), which facilitates stronger electronic coupling,
allowing superior charge carrier transport within PbSe CQD
films.[16,17] Furthermore, PbSe CQD has higher MEG as compared to PbS CQD, which could enhance the contribution
of photon absorption to additional exciton generation.[18,19]
These features suggested that PbSe CQD solar cell has the
potential to surpass the Shockley–Queisser efficiency limit
(31%) in single-junction devices.[17,20] In addition, PbSe CQDs
possess the superior infrared tunability (into the short- and
mid-wave infrared), making it potential in infrared optoelectronic device, such as infrared photodetectors and multijunction solar cells. However, despite of all these attractive
properties, the achieved PCE of PbSe CQD solar cells are still
lower than those of PbS CQD devices. It is mainly due to low
air stability and occurrence of traps/defects during film fabrication. These trap/defect states in PbSe cause recombination
loss and hinder charge transport, which eventually limit opencircuit voltage (VOC) and fill factor (FF) of device.[21,22] Based on
these issues, PbSe CQDs remained relatively less optimized for
PV devices compared to PbS CQD.
Halide passivation has been applied to solve the air-stability
issues in PbSe CQD solar cells recently.[23,24] For instance,
researchers at NREL (National Renewable Energy Laboratory)
developed a promising cation exchange strategy to improve
the air stability of PbSe CQDs using cadmium selenide (CdSe)
CQDs and lead chloride (PbCl2).[25] On the basis of this technique, PbSe CQDs achieved a PCE of 6.2% due to protection
Low-cost solution-processed lead chalcogenide colloidal quantum dots
(CQDs) have garnered great attention in photovoltaic (PV) applications.
In particular, lead selenide (PbSe) CQDs are regarded as attractive active
absorbers in solar cells due to their high multiple-exciton generation and large
exciton Bohr radius. However, their low air stability and occurrence of traps/
defects during film formation restrict their further development. Air-stable
PbSe CQDs are first synthesized through a cation exchange technique,
followed by a solution-phase ligand exchange approach, and finally absorber
films are prepared using a one-step spin-coating method. The best PV device
fabricated using PbSe CQD inks exhibits a reproducible power conversion
efficiency of 10.68%, 16% higher than the previous efficiency record (9.2%).
Moreover, the device displays remarkably 40-day storage and 8 h illuminating
stability. This novel strategy could provide an alternative route toward the
use of PbSe CQDs in low-cost and high-performance infrared optoelectronic
devices, such as infrared photodetectors and multijunction solar cells.
Colloidal quantum dots (CQDs) are exceptional sort of nanomaterials being widely explored for light-emitting diodes
(LEDs),[1–3] photodetectors,[4,5] and photovoltaic (PV) devices.[6–8]
Dr. W. Ahmad, Dr. J. He, X. Yang, Dr. D. Li, Dr. C. Chen
Wuhan National Laboratory for Optoelectronics (WNLO)
and School of Engineering Sciences
Huazhong University of Science and Technology
1037 Luoyu Road, Wuhan, 430074 Hubei, P. R. China
E-mail: jungang_hd@163.com; cchen@mail.hust.edu.cn
Dr. J. He, Prof. Z. Liu
School of Materials Science and Engineering
Wuhan Institute of Technology
Wuhan, 430205 Hubei, P. R. China
E-mail: able.ztliu@wit.edu.cn
K. Xu
School of Chemistry
Chemical Engineering and Life Science
State Key Laboratory of Advanced Technology for
Materials Synthesis and Processing
Wuhan University of Technology
122 Luoshi Road, Wuhan, 430070 Hubei, P. R. China
Z. Chen, Y. Xia, Prof. J. Zhang
School of Optical and Electronic Information
Huazhong University of Science and Technology
1037 Luoyu Road, Wuhan, 430074 Hubei, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201900593.
DOI: 10.1002/adma.201900593
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from air oxidation. Furthermore, Halpert and coworkers utilized the same air-stable PbSe CQDs as active absorber in solar
cell and pushed the device PCE to 9.2%, which was the record
till now.[26] Unfortunately, all these attempts were based on
PbSe CQD film produced via conventional layer-by-layer (LBL)
processes, which not only requires tedious fabrication process
but also lead to CQD disorder within thin films, negatively
affecting the final device performance.[27] Therefore, there is
dire need of alternative passivation method to completely avail
PbSe CQD advantages as well as to alleviate solar cell air-stability problem.
In this scenario, one-step deposition process could be a
promising approach to overcome the associated problems
in PbSe CQDs active layer. Thanks to solution-phase ligand
exchange protocol, ligand exchange could be performed completely in solution first and the film could be deposited on substrate by one-step spin-coating method.[28] In above mentioned
route, PbI2 is commonly chosen for surface passivation because
of low formation energy of I− ions with Pb2+ ions, and thus
PbI2-capped CQDs could effectively isolate the CQDs from oxidation.[29] Consequently, solution-phase ligand exchange substantially suppresses recombination centers and simplifies the
manufacturing process. Most recently, Shen and coworkers passivated PbSe CQD solar cell by above protocol and reported 6%
PCE.[30] Evidently, there is still space for further optimization of
PbSe CQD solar cells utilizing simultaneous improvements in
surface passivation and architecture design.
Herein, we attempted to unite the advantages of cation
exchange and solution-phase ligand exchange to construct high
performance PbSe CQD solar cells. Initially, we synthesized
high-quality PbSe CQDs via in situ Cl− and Cd2+ ions passivation. Then compact film was fabricated by using one-step spin
coating via solution ligand exchanged PbSe CQD inks. X-ray
photoelectron spectroscopy (XPS), infrared photoresponse, and
ultrafast transient absorption (TA) characterizations revealed
that the obtained PbSe CQD films had less trap states. Finally,
by constructing ZnO/PbI2-capped PbSe CQDs/EDT- (1,2-ethanedithiol) PbS CQDs configuration, our device showed a reproducible PCE of 10.68%, which is the highest PCE of PbSe CQD
solar cells. Convincingly, our results provide a facile and lowcost approach to engineer highly efficient infrared PbSe CQD
optoelectronic devices such as infrared photodetector and multijunction solar cells.
To fabricate efficient and stable CQD solar cells, high-quality
PbSe CQDs with high monodispersity, good air stability,
and well passivation were initially obtained as illustrated in
Figure 1a. In brief, in situ cadmium (Cd2+) passivation through
a cation exchange reaction was adopted to convert CdSe CQDs
directly into PbSe CQDs.[25] Following, solution-phase ligand
exchange process was used to assist colloidal stabilization with
iodide passivation. For the solution-phase ligand exchange, OA(oleic acid) capped PbSe CQD solution in octane and PbI2 solution in N, N-dimethylformamide (DMF) were mixed together
at room temperature for 10–15 min. Upon vigorous shaking,
PbSe CQDs were transferred to polar DMF solution (comparatively faster than those of PbS CQDs),[29] and then washed carefully three times with octane to completely remove the residual
oleate ligand. After successful phase-transfer, cosolvents of
low boiling-point butylamine (BTA, 78 °C) and high relative
Adv. Mater. 2019, 1900593
dielectric-constant DMF (37.1) were used to achieve complete
dispersability with optimism of electrostatic stabilization.[27,31]
Subsequently, OA-capped PbSe CQD and PbI2-capped PbSe
CQD solution were characterized by UV–vis spectroscopy. Both
of them demonstrated significant exciton absorbance at similar peak position (900 nm, Figure 1b) and no obvious change
was monitored in PbSe CQD absorbance after solution-phase
ligand exchange. Fourier-transform infrared (FT-IR) spectroscopy was employed to further investigate the exchanged PbSe
CQDs. FT-IR absorption spectra measurement confirmed that
OA ligands were successfully replaced by short PbI3−/I− ligands
after ligand exchange, as shown in Figure 1c. These results
were further confirmed by high resolution X-ray photoelectron
spectroscopy characterization. As shown in Figure 1d,e, after
ligand exchange, the decreasing intensity of C-1s peak and
the increase intensity of I-3d peak indicated that most of the
hydroxyl (–OH) and carboxyl (–COO) groups were replaced by
PbI3−/I− ligands. Moreover, PbI2-capped PbSe CQDs exhibited
high colloidal stability in N2 environment even up to two days
(Figure S1, Supporting Information). Transmission electron
microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) images showed that PbSe CQDs
retained their mean lattice constant after the ligand exchange
(Figure S2, Supporting Information), implying that PbI2-capped
PbSe CQD still maintained high quality.[29] All these characterizations indicated that PbSe CQDs are well exchanged and are
ready for the fabrication of solar cells.
Before assembling the complete solar cell device, we first
studied the PbSe CQD films fabricated by the as-obtained highquality CQD solution. PbSe CQD active layer were deposited via
one-step spin-coating ligand-exchanged PbSe CQD inks. The
conventional layer-by-layer technique was also used to synthesize
benchmark control films utilizing PbI2-treated and TBAI-treated
(tetrabutylammonium iodide, TBAI) PbSe CQDs (see the details
in the Experimental Section).[32,33] As is obvious from the scanning electron microscope (SEM) image (Figure 2a), films deposited by one-step approach on ZnO coated ITO (indium tin oxide)
substrate exhibited a uniform and compact morphology. On the
other hand, conventional LBL technique induced nonuniform
or cracks during the solvent evaporation and organic ligand
removal (Figure 2d,g), which could potentially result in shunt
path and poor device performance.[34] Atomic force microscope
(AFM) results showed that roughness of film obtained via onestep method (0.9 nm) was far less than that obtained via LBL
method (1.6 nm for PbI2-treated LBL films and 2.5 nm for
TBAI-treated LBL films), as shown in Figure 2b,e,h, suggesting
remarkable controllability of film roughness compared to multi­
steps LBL technique. Moreover, XPS characterization was carried out to investigate defect categories of the three kinds of
deposited films. It is well known that O-1s spectrum can provide
the information about oxygen-containing organic groups (such
as –OH and –COO groups) bound to (111) facet of PbS CQDs,
which are considered as the major source of trap states.[35] Convincingly, deconvolution of these O-1s spectra demonstrated that
Pb–O (529.3 eV) and Pb–OH (531.5) groups were substantially
suppressed in one-step deposited PbSe CQD film,[36] revealing
more effective removal of OA ligand by the solution-ligand
exchange and less oxygen contamination during the one-step
film deposition, as shown in Figure 2c,f,i.
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Figure 1. a) Sketch of PbI2-capped PbSe CQD inks using cation exchange and solution-phase ligand exchange with Cd2+ and metal halide precursors,
respectively. b) UV–vis absorption spectra of CQD solution before and after ligand exchange in octane and DMF, respectively. c) FT-IR spectra of CQD
solution before and after ligand exchange. No peak of organic ligands (i.e., CH, CO, and CC) was observed after ligand exchange (PbI2-capped
PbSe film), indicating complete OA ligand removal after the solution-phase ligand exchange. The subscript “as” and “s” stand for symmetrical and
asymmetrical vibration. d,e) XPS spectra of C-1s and I-3d in pristine and PbI2-capped PbSe CQD films.
It is well-acknowledged that surface oxidation is responsible
for harmful defects.[37] So we believe one-step PbI2-capped
PbSe CQD film may have less defects compared to LBL film.
According to the extrinsic photoconductive effect of semiconductor physics, photoresponse excited by photons with energy
lower than bandgap is an easy and effective strategy to investigate the electrical defects, because the photocurrent originates
from defect state absorption rather than band edge absorption.
The photoresponse of PbSe CQD photoconductive detectors was
characterized under infrared light (1450 nm) at an applied bias
voltage (5 V) (Figure 3a). It was observed that PbSe CQD film
deposited by one-step method had much lower photo­current
than that obtained via conventional LBL approach, suggesting
less oxidation defects in one-step PbI2-capped PbSe CQD film.
The larger dark conductivity and infrared photo­
conductivity
means higher concentration of traps/defects, which could
hinder the transport efficient of carriers and potentially degrade
Adv. Mater. 2019, 1900593
the device performance. Furthermore, to explore energy disorder in both CQD films, Urbach behavior was investigated by
tail state absorption (Figure 3b). The sharp band tails in 1.0 to
1.25 eV range were observed for PbI2-capped PbSe CQD film
deposited via one-step technique and exhibited lower Urbach
energy (28 meV). However, Urbach energy in PbI2-treated
and TBAI-treated PbSe CQD LBL films were 43 and 83 meV,
respectively (Figure 3b). The Urbach energy results showed
that one-step deposition method had lower trap density and tail
states as compared to LBL approach, which is in line with
the observed infrared photoresponse (Figure 3a).[38] Ultrafast
transient absorption spectroscopy was further used to explore
carrier kinetics in one-step deposited and LBL films. Figure 3c
illustrates the TA decay of the corresponding films. The lifetime (τ) of PbI2-capped one-step as well as PbI2-treated and
TBAI-treated LBL PbSe CQD films was measured as ≈857, 572,
and 58 ps by double exponential fitting (Table S1, Supporting
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Figure 2. a,d,g) The top-view SEM and b,e,h) topographical AFM images of PbI2-capped PbSe CQD one-step films as well as PbI2-treated and TBAItreated PbSe CQD LBL films. The yellow ovals in (g) depict visible crakes in the film prepared by LBL technique. c,f,i) XPS signals of O-1s in various
PbSe CQD films. The red curves represent the oxidation of Pb–O and Pb–OH in PbSe CQD LBL films.
Information), respectively. The longer τ revealed the less defect
recombination in one-step PbSe CQD film, further verifying
the less oxidation defects in one-step PbI2-capped PbSe CQD
film. Similarly, spectro-temporal TA map (Figure 3d) of PbI2capped PbSe CQD film exhibit a narrower bleach peak (1.28–
1.32 eV) at first excitonic peak (1.32 eV) than the reported
TBAI-treated PbSe CQD film,[30] suggesting a small redshift of
the transient bleach peak (13 meV) in PbI2-capped PbSe CQD
film in contrast to the reported value of TBAI-treated PbSe
CQD film (18 meV).[28] The reduced energy tunneling in PbI2capped PbSe CQD film evidences a flatter energy landscape and
reduced tail states, that is benefit for VOC of solar cells. The TA
results of PbI2-treated and TBAI-treated PbSe CQD LBL films
were presented in Figure S3 (Supporting Information). Distinctly, the fusion effect of CQDs in LBL films weakened and
broadened the exciton absorption, therefore, the TA signal was
Adv. Mater. 2019, 1900593
significantly broadened (Figure S3, Supporting Information).
Limited by the upper detection wavelength (1020 nm, corresponding to the photon energy of 1.22 eV) of our TA equipment, we could only obtain partial TA spectra with the photon
energy higher than 1.22 eV on the LBL films. However, we
could combine the partial TA and absorption spectra (Figure 3d
and Figure S3, Supporting Information) to qualitatively explain
why the LBL films were inferior to one-step PbSe CQD film.
The optimum one-step films as well as PbI2-treated and
TBAI-treated PbSe CQD LBL films were then assembled into
photovoltaic devices, employing device architecture of ITO/
ZnO/PbSe active layer/PbS-EDT/Au. As an example, we chose
the one-step PbSe CQD active layer to describe the device fabrication process. ≈50 nm ZnO prepared by sol–gel method was
used as an electron transporting layer (ETL).[39] Subsequently,
≈200 nm PbI2-capped PbSe CQD film was deposited on it by
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Figure 3. a) Photoresponse of PbSe CQD photoconductive detectors under 1450 nm light excitation. The active layers were fabricated by one-step
or LBL methods. b) Urbach energy of PbI2-capped PbSe CQD one-step film, PbI2-treated PbSe CQD LBL film and TBAI-treated PbSe CQD LBL film.
c) Normalized absorption changes (ΔA) as a function of decay time in PbI2-capped PbSe CQD one-step films, PbI2-treated, and TBAI-treated PbSe
CQD LBL films. d) Spectro-temporal TA maps of PbI2-capped PbSe CQD one-step film.
one-step spin-coating technique. Last, ≈40 nm p-type EDT
treated PbS CQD film and ≈80 nm Au film were used as hole
transporting layer (HTL) and electrodes, respectively.[40] The
cross-sectional SEM image (Figure 4a) of our PbSe CQD solar
cell indicated that the thickness of photoactive absorber was
≈200 nm. Combining the ultraviolet photoelectron spectroscopy (Figure S4a–c, Supporting Information) and absorption
spectroscopy (Figure S3a, Supporting Information) measurements, the highest occupied molecular orbital (HOMO), Fermi
energy level (Ef), and lowest unoccupied molecular orbital
(LUMO) energy levels of PbI2-capped PbSe CQD one-step film
was determined as −3.98, −4.40, and −5.36 eV, respectively.
Employing the HOMO, Ef, LUMO of PbS-EDT and ZnO in literature,[41] energy diagram of the device was shown in Figure 4b.
The photogenerated holes and electrons were extracted by HTL
and ETL without transport barrier, confirming the suitable
band alignment. By using the same approach, the energy band
alignment of PbI2-treated and TBAI-treated PbSe CQD LBL
film demonstrated small spike conduction band offset, which
could slightly affect the transport of electrons from absorber
layer to ZnO buffer layer, but it was still suitable for solar cells
(Figure S4 and Table S2, Supporting Information). Figure 4c
displays current density–voltage (J–V) characteristics of PbI2capped PbSe based device under forward and reverse scanning direction with negligible hysteresis. The statistical value
of 45 devices demonstrated (10.68 ± 0.28)% PCE with a VOC of
(0.573 ± 0.012) V, a JSC of (28.1 ± 0.6) mA cm−2 and a FF of
(66.3 ± 1.3)% under forward bias scanning (Figures S5 and S6,
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Supporting Information), absolutely 1.48% and comparatively
16% higher than the previous record (9.2%).[26] Moreover, we
compared the device performance with different fabrication
process (Figures S5 and S6, Supporting Information), and our
results showed that the PCE of PbSe CQD solar cell with active
layer prepared by one-step deposition was better than device
prepared by PbI2-treated (PCE = (8.50 ± 0.18)%) and TBAItreated (PCE = (7.60 ± 0.30)%) LBL spin-coating process. Additionally, VOC of our device is improved as compared to previous
reports engineered through LBL method,[26,42] which could be
credited to low surface defects and flat energy landscape in
accordance with the defect and energy diagram analysis
(Figures 3a and 4b). The representative device portrays the
highest JSC in particular among all previously reported PbSe
CQD solar cells.[16,26,30,42] Figure 4d displays the EQE spectrum
of PbI2-capped PbSe CQD solar cells in the wavelength range
of 300–1100 nm. The increase of EQE in blue region could be
attributed to favorable band alignment of ZnO/PbI2-capped
PbSe and PbI2-capped PbSe/PbS-EDT heterojunction, facilitating an efficient charge separation and consequently collection
in device. The integrated photocurrent from EQE and AM 1.5G
spectra was in line with JSC. The current progress of the PbSe
CQD solar cells inspired us to exploit near infrared absorber
materials with bandgap from 1.03 to 1.41 eV (Figure S7,
Supporting Information). We found that this technique is also
benefited for different bandgap PbSe CQDs. Compared to
1.38 eV PbSe CQDs, the narrow bandgap PbSe CQDs exhibited
inferior device performance due to the poor air stability of large
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Figure 4. a) The cross-sectional SEM image of representative device with an active layer thickness of ≈200 nm. b) Alignment of energy levels between
the layers. c) Typical J–V (forward and reverse scanning) profiles of a standard processed device under simulated AM 1.5G illuminations. d) EQE spectrum and the corresponding integrated photocurrent of PbI2-capped PbSe based CQD solar cells in the wavelength range of 300–1100 nm.
size PbSe CQDs.[43] However, the PbSe solar cell with large
bandgap (1.41 eV) also demonstrated slight lower performance
than that with 1.38 eV PbSe CQDs. It might be attributed to the
low carrier mobility of small size PbSe CQDs.[21] In the end,
although we obtained a record efficiency in PbSe CQD solar
cell, it is still far behind the PbS CQD solar cells (12.01%).[34]
The low efficiency in PbSe CQD solar cells could be due to the
complicated issue of surface oxidation or simply insufficient
optimization.
To investigate the improved VOC and JSC in representative
device, device physical characterizations such as capacitance–
voltage (C–V) and transient photovoltage (TPV) measurements
were performed. Here we should stress that these characterizations were not carried out on LBL PbSe CQD device because
of the poor diode quality and device performance. In p–n junction, Vbi was extracted from the intercept of C−2–V curve on
horizontal axis (Figure 5a). The extracted Vbi (0.65 V) of representative device was consistent with potential difference of
ZnO and PbS-EDT (0.62 V), calculated by Fermi level of ZnO
and PbS-EDT as reported in the literature.[41] It should be mentioned that EDT treated PbSe CQD was also used as HTL in
PbI2-capped PbSe based device and the related device PCE and
Vbi were 8.27% and 0.496 V (Figure S8a, Supporting Information), absolutely 2.41% and 0.077 V lower than PbS-EDT
HTL device, respectively. The low Vbi might originate from
the Cl− presence at PbSe CQD surface during cation exchange
synthesis (see the Experimental Section and Figure S8b,c, Supporting Information). The Cl− could bond with Pb2+ terminator
Adv. Mater. 2019, 1900593
and provide electrons to CQDs, resulting in an upper shift of
the Fermi level.[44] So the Fermi level difference between ZnO
and EDT treated PbSe (or say Vbi) was reduced. This leaded us
not to choose more similar PbSe as HTL in our device.
Light bias TPV measurements were further carried out to
examine the recombination lifetime in PbI2-capped PbSe CQD
solar cells. Experimental and exponentially fitted TPV results
are shown in Figure 5b. The normalized TPV decay curve
showed that passivated PbSe CQD solar cell could show an
increased charge carrier recombination lifetime (τrec) of 1.86 µs,
as illustrated in Figure 5b. A prolonged exciton lifetime indicated that nonradiative recombination centers were suppressed,
which was in accordance with high VOC and FF in our device.
Furthermore, dark J–V curve of representative device (PbI2capped PbSe) was recorded and plotted semi-logarithmically
(Figure S9, Supporting Information). It showed that reverse saturation current was suppressed in reverse bias region, leading
to a high shunt resistance (3100 Ω cm−2). On the other side,
enhanced dark current in forward bias region exhibits lower
series resistance (0.6 Ω cm−2). Moreover, calculated diode ideality factor n for PbI2-capped PbSe CQD was 1.54. These results
demonstrated that high diode quality and suppressed recombination in our PV device.
Apart from excellent PV parameters, better passivation on
CQDs promotes long term air stability. The un-encapsulated
devices were therefore stored in ambient air and then tested
their performances over time as shown in Figure 6a. PbI2capped PbSe devices retained about 94% of their initial PCE
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Figure 5. a) C−2–V plot, where A (0.09 cm−2) is the active area of our PbSe CQD solar cell. b) Normalized TPV curves. The fitted recombination lifetime
is 1.86 µs.
after 1000 h (nominally reduced from 10.68% to 10.04%) and
was in good agreement with reference.[42] In addition, illumination stability was also considered as an essential criterion
for comprehensive stability assessment of PbI2-capped PbSe
CQD solar cells. After incessant 8 h soaking under AM 1.5G
irradiation, engineered device exhibited ≈96% of initial PCE
(Figure 6b), comparable with PbS CQDs based devices.[35]
These all-remarkable results justify that one-step passivation
method could improve distinctive features of the active layer of
the device, which agree well with our suppositions.
In summary, we reported a record PCE of 10.68% for
PbSe CQD solar cells with a high FF of 66.30% and a JSC of
28.11 mA cm−2 using cation exchange synthesis and one-step
film fabrication technique. First, we produced air-stable and
monodisperse PbSe CQDs using cation exchange approach.
Then the high quality films with less surface defects was fabricated by one-step deposition technique using PbSe CQD inks
derived from solution-phase ligand exchange. Consequently,
the suppressed nonradiative recombination and long carrier
lifetime led improved PCE of solar cells, consistent with our
photoresponse and TA results. Moreover, the PbI2-capped PbSe
CQDs devices also demonstrated excellent air storage (40 days)
and illumination stability (8 h). We are very optimistic that this
strategy explored here could offer an effective platform to architect highly efficient and stable PbSe CQDs photovoltaics as well
as other optoelectronic devices in future.
Experimental Section
CdSe (480 nm) Synthesis: For Cd precursor, 0.512 g cadmium oxide
(CdO) was dispersed in a three-neck round bottom flask containing
4 mL oleic acid (OA), and 30 mL 1-octadecene (ODE).[25] The system
was initially stirred and degassed for 2 h at ambient temperature, and
then temperature increased up to 100 °C using a standard Schlenk line
technique. Temperature of reaction mixture was further increased up
to 225 °C. When mixture became transparent under N2 environment,
0.16 g Se in 5 mL ODE was quickly added to it. After 5 min interval,
crude solution was naturally cooled down to 175 °C, and then switched
to water bath for rapid cooling. The obtained CdSe CQDs were purified
with chloroform/ethanol and precipitated via centrifugation. To obtain
purified CdSe CQDs, above washing process was repeated several
times. Finally, CdSe CQDs were dried in vacuum oven and dispersed in
degassed ODE at 40 mg mL−1 concentration.
PbSe (900 nm) CQDs Synthesis: 1.7 g lead chloride (PbCl2) and 20 mL
oleylamine (OLA) were added in a three-neck round bottom flask and
Figure 6. a) Long-term air-storage stability assessment and b) continuous operational stability of PbI2-capped PbSe CQD solar cells under AM 1.5G
illumination. The devices were stored in air and measured at room temperature under indoor relative humidity of >50% without encapsulation. Day 0
denotes measurements performed after Au anode evaporation.
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degassed at 60 °C for 2 h.[25] Subsequently, reaction temperature was
increased up to 115 °C. For cation exchange, purified 10 mL CdSe CQD
solution was rapidly added in above Pb precursor solution. Then, the
reacting precursor reflected dark color indicating onset of nucleation
phase. After 30 s, flask was quickly immersed in a water bath. Later
on, 20 mL hexane and 16 mL OA were poured in at 75 and 40 °C
temperatures, respectively. As-obtained product was tempestuously
stirred at 40 °C for another 30 min, and then naturally cooled at ambient
temperature. For PbSe CQDs purification, hexane/ethanol solution was
added in CQDs solution for repeated washing and centrifugation. To
remove residual solvents, PbSe CQDs were dried in a vacuum oven.
Finally, as-obtained PbSe CQDs were dispersed in octane at 20 mg mL−1
for further use. PbS CQDs (880 nm) used for HTL were synthesized by
previously reported method.[41]
Solution-Phase Ligand Exchange: 5 mL oleate-capped PbSe CQDs
(20 mg mL−1 in octane) and 0.35 m PbI2 in 6 mL DMF were intermingled
together at ambient temperature. Upon vigorous shaking, the OA ligand
on the surface of PbSe CQDs were moved by PbI3−/I− ligand and then
transferred from nonpolar phase to the polar DMF phase. After ligand
exchange, the octane layer became transparent then was decanted.
The PbSe CQDs/DMF solution was thoroughly washed three times
with octane to completely remove the oleate ligands. Subsequently,
PbI2-capped PbSe CQDs were centrifuged at 6000 rpm for 5 min and
then desiccated in vacuum oven for 20 min to get dry product. For film
deposition, the PbI2-capped PbSe CQDs were suspended in DMF and
BTA (7:3 v/v) with concentration of 550 mg mL−1.
Device Fabrication: ZnO sol–gel precursor was synthesized according
to previously reported literature.[39] For compact film, ZnO suspension
was layered onto precleaned ITO/glass substrate and the ZnO film was
prepared after spin-coating process at 4000 rpm for 30 s, followed by
thermal annealing at 320 °C for 15 min. The process was repeated 2 to
3 times to obtain ≈50 nm thick film. After successive ZnO layers, ETL
substrate was moved to N2 filled glove box for active layer deposition.
PbI2-capped PbSe CQD active absorber was then spin coated onto
ZnO layer using one-step spinning speed 2500 rpm for 30 s (the
corresponding thickness of PbSe CQD film was ≈200 nm), followed
by rapid annealing at 80 °C for 3 min on hotplate in glove box. Then,
another two layers of EDT (0.02% volume EDT-acetonitrile solution)
exchanged PbS CQDs as HTL were deposited on active layers in a fume
hood, followed by washing using acetonitrile.[40] At last, ≈80 nm thick
Au top contact was fabricated via thermal evaporation. A benchmark
controlled sample (TBAI-treated and PbI2-treated PbSe CQD LBL film)
was fabricated in glove box using solid-state ligand exchange method
according to recently reported method.[32,33] For TBAI-treated PbSe CQD
LBL film deposition, several drops of PbSe CQD solution (50 mg mL−1
in octane) was dropped onto ZnO/ITO substrate and immediately spunt
at 2500 rpm for 30 s. After one layer of CQDs deposited, TBAI solution
(10 mg mL−1 in methanol) was used for OA removal, following by being
immersed in neat methanol for residual TBAI removal. For PbI2-treated
PbSe, the approach was similar as the process of TBAI-treated PbSe.
Commonly, PbI2 solution (10 × 10−3 m in DMF) was chosen for ligand
exchange, then neat acetonitrile (ACN) was applied to remove residual
DMF. After ligand exchange, the as-prepared films were in need of drying
by using N2 stream. In order to investigate the photoresponse property
of PbSe CQD film, the active absorber with same process was deposited
on glass substrate, following with ≈80 nm thick Au electrodes for planar
photoconductor fabrication.
Material Characterization: UV–vis spectrophotometer (PerkinElmer
instruments, Lambda 950 using integrating sphere) was applied to
study the optical absorption of PbSe CQDs. In order to characterize the
absorbance property of pristine and PbI2-capped PbSe CQDs, quartz
cell was used for container. For film characterization, PbSe CQDs was
deposited onto quartz by using the standard solar cell fabrication
process. Fourier transform infrared spectroscopy was performed on
a Thermo Fisher FTIR6700 Fourier Transform Infrared Spectrometer
in transmission and attenuated total reflection (ATR) mode, to study
the effect of ion exchange treatment on PbSe CQD surface. The film
preparation process for FT-IR characteristic was same as UV–vis
Adv. Mater. 2019, 1900593
characteristic. The film surface and roughness were measured by
field emission scanning electron microscopy (FE-SEM, FEI NOVA
NanoSEM 450) and atomic force microscopy (SPM9700), respectively.
In order to acquire the surface morphology information, PbSe CQD
films were deposited onto ZnO/ITO substrates. Transmission electron
microscopy (FEI Tecnai G2 20 UTwin) was used to investigate the
structural morphologies of the samples, the samples were prepared
by dropping PbSe CQDs onto Formvar/Carbon film on copper grids
(300 mesh) and dried naturally in air. X-ray photoelectron spectroscopy
using Al Kα excitation (EDAX Inc. Genesis, 300W) was applied to analyze
the chemical compositions of pristine and PbI2-capped PbSe CQDs.
Ultraviolet photoelectron spectroscopy (UPS) measurement (Specs
UVLS) conducted in ultrahigh vacuum photoemission spectroscopy
system with a He I excitation, was used to study the energy levels of
samples, and 21.2 eV was referenced to the Fermi edge of argon etched
gold. The samples for XPS and UPS characteristics were deposited on
ITO glass. For transient absorption measurement, the laser source was
a YAG crystal laser (Spirit 1040-8-SHG, Newport Corporation) with a
wavelength of 1040 nm, a pulse width of <400 fs, a repetition rate of
200 kHz. Because the transmittance model was used for TA test, so
the films were deposited onto quartz and all samples were pumped
at 400 nm (replication, insufficient knowledge of TA characterization).
The pump-probe delay was controlled by a mechanical delay stage.
The excitation fluence in each measurement was ≈60 µJ cm−2. Infrared
photoresponse performance was measured in an optically and
electrically sealed box, the extrinsic photoconductor was constructed by
depositing ≈200 nm PbSe CQDs on quartz then evaporating ≈80 nm Au
electrodes. The configuration of the device was 400 µm in length, 5 µm
in spacing gap, and ≈200 nm in height (the height value was same as the
thickness of the PbSe CQD film). Then a LED with 1450 nm wavelength
(Thorlabs M1450L2) was chosen for light source. In order to obtain
periodic photoresponse, the LED was controlled by a function generator
(Agilent 33600A Series) with frequency of 0.1 Hz. The corresponding
signal was extracted by Agilent B1500A.
Device Characterization: The J–V characteristics were measured by
using Keithley 2400 digital source meter under simulated AM 1.5G
(100 mW cm−2) illumination in air at room temperature. A 450 W xenon
light source (Oriel, Model 9119, Newport) was used to the light source.
The light intensity was calibrated with a standard Si solar cell (Oriel,
Model 91 150 V, Newport). The sweep rate, step voltage, and delay time
were set at 60 mV s−1, 6 mV, and, 10 ms respectively. The device was
covered by a metal mask with an effective aperture area of 0.09 cm2
during efficiency measurement. EQE measurements were performed
on a home-made setup containing a Keithley 2400 Source Measure
unit and light source was generated by a 300 W xenon lamp (Oriel,
69 911, Newport) and then split into monochromatic wavelength
using Newport oriel cornerstone 130 1/8 Monochromator (Oriel,
model 74 004). The spectra were calibrated by a reference standard
silicon solar cell. Illumination aging for 8 h of high performance PbSe
CQD solar cells under the maximum power point tracking by a white
LED lamp was conducted at room temperature. The corresponding J–V
characteristic of as-made solar cell was measured after 0.5 h intervals.
TPV measurements were performed by using the same method in the
previous work.[45] C–V measurements were conducted using Agilent
4200A at a frequency of 10 kHz and an AC signal of 50 mV, scanning
from −1 to +1 V, with a step of 50 mV.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
W.A. and J.H. contributed equally to this work. This work was
financially supported by the National Natural Science Foundation
1900593 (8 of 9)
© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.advancedsciencenews.com
www.advmat.de
of China (61725401, 61804061, and 51602114), the Major State
Basic Research Development Program of China (2016YFA0204000),
and the Fundamental Research Funds for the Central Universities
(2017KFXKJC0020) and the China Postdoctoral Science Foundation
Funded Project (2017M622416). The authors thank the facility support
of “the Center for Nanoscale Characterization & Devices (CNCD),
WNLO of HUST” and “the Analytical and Testing Center of HUST.”
Conflict of Interest
The authors declare no conflict of interest.
Keywords
cation exchange, quantum dots, solar cells, solution-phase ligand
exchange
Received: January 24, 2019
Revised: April 15, 2019
Published online:
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1900593 (9 of 9)
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