Article Improving Efficiency and Stability of Perovskite Solar Cells Enabled by A Near-Infrared- Absorbing Moisture Barrier A multi-functional interface layer with integrated roles of (1) electron transport, (2) moisture barrier, (3) near-infrared photocurrent enhancement, (4) trap passivation, and (5) ion migration suppression to simultaneously enhance the device efficiency and stability is demonstrated. The narrow-band-gap non-fullerene acceptor, Y6, was screened out to replace the most commonly used PCBM in inverted perovskite solar cells. A significantly improved efficiency of 21.0% was achieved along with the remarkable device stability (up to 1,700 h) without encapsulation upon exposure to moisture, heat, and light. Qin Hu, Wei Chen, Wenqiang Yang, ..., Zhubing He, Rui Zhu, Thomas P. Russell [email protected] (F.L.) [email protected] (Z.H.) [email protected] (R.Z.) [email protected] (T.P.R.) HIGHLIGHTS A new strategy to enhance the device performance while simplifying the device structure New design rules to rationally screen multi-functional interface layer Long-term stability without encapsulation upon exposure to moisture, heat, and light Correlations between molecular orientation or passivation and device performance Hu et al., Joule 4, 1575–1593 July 15, 2020 ª 2020 Elsevier Inc. https://doi.org/10.1016/j.joule.2020.06.007 ll
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Article
Improving Efficiency and Stability of PerovskiteSolar Cells Enabled by A Near-Infrared-Absorbing Moisture Barrier
Miquel Salmeron,2 Aleksandra B. Djuri�si�c,3 Feng Liu,5,* Zhubing He,6,* Rui Zhu,4,11,*
and Thomas P. Russell1,2,13,*
Context & Scale
Perovskite solar cells (PSCs) have
attracted tremendous attention
because of the high efficiencies,
ease of fabrication, and low cost
of production. However, further
enhancement of device efficiency
has been a bottleneck, and the
instability of the PSCs hampers
their commercialization. In this
work, we strategically introduce a
new multi-functional interface
layer that integrates five different
functions to improve the device
efficiency and long-term stability
of PSCs, pushing forward the
development of the PSC
technology. A significantly
improved power conversion
SUMMARY
Simultaneously improving device efficiency and stability is the mostimportant issue in perovskite solar cell (PSC) research. Here, westrategically introduce a multi-functional interface layer (MFIL)with integrated roles of: (1) electron transport, (2) moisture barrier,(3) near-infrared photocurrent enhancement, (4) trap passivation,and (5) ion migration suppression to enhance the device perfor-mance. The narrow-band-gap non-fullerene acceptor, Y6, wasscreened out to replace the most commonly used PCBM in the in-verted PSCs. A significantly improved power conversion efficiencyof 21.0% was achieved, along with a remarkable stability (up to1,700 h) without encapsulation under various external stimuli (light,heat, and moisture). Furthermore, systematic studies of the molec-ular orientation or passivation and the charge carrier dynamics atthe interface between perovskite and MFIL were presented. Theseresults offer deep insights for designing advanced interlayers andestablish the correlations between molecular orientation, interfacemolecular bonding, trap state density, non-radiation recombina-tion, and the device performance.
efficiency of 21.0% was achieved
along with the remarkable stability
(up to 1,700 h) without
encapsulation under various
external stimuli (light, heat, and
moisture). These results open new
avenues to design advanced
interlayers, simplifying the device
structure, and enhancing
efficiency and stability, that can
accelerate the market readiness of
perovskite-based
optoelectronics.
INTRODUCTION
Organic-inorganic hybrid perovskite thin-film solar cells have emerged as an effi-
cient solar-energy technology with excellent power conversion efficiencies (PCEs),
ease of fabrication, and low production cost, proving to be a game changer in pho-
tovoltaics.1–3 Significant effort has been devoted to optimizing device efficiencies4
and to further understand inherent material properties.5 Some strategies, including
crystallization and morphology optimization of perovskite thin films,6 composition-
tunable alloying7 for increasing light absorption, and interface engineering8 within
the device structure, have been adapted to enhance the PCEs. Despite these efforts
to improve device performance, the lifespan of perovskite solar cells (PSCs) is still
too short for practical use.9,10 Thus, simultaneously improving device efficiency
and stability has become the most important issue at present. The inherent ‘‘soft’’
crystal lattice of perovskite solids is one of the key reasons for poor stability, which
makes PSCs vulnerable to aging stresses, such as UV-light, moisture, electric field,
and thermal annealing. In addition, structural defects in the bulk and on the surfaces
of perovskite polycrystalline thin films, ion migration, hygroscopic additives, and the
thermal instability of charge transporting layers, are also contributors to PSCs’
Joule 4, 1575–1593, July 15, 2020 ª 2020 Elsevier Inc. 1575
1Department of Polymer Science andEngineering, University of Massachusetts,Amherst, MA 01003, USA
2Materials Sciences Division, Lawrence BerkeleyNational Laboratory, Berkeley, CA 94720, USA
3Department of Physics, The University of HongKong, Pokfulam, Hong Kong
4State Key Laboratory for Artificial Microstructureand Mesoscopic Physics, School of Physics,Frontiers Science Center forNano-optoelectronics & CollaborativeInnovation Center of Quantum Matter, PekingUniversity, Beijing 100871, China
5Frontiers Science Center for TransformativeMolecules and in situ Center for physicalSciences, School of Chemistry and ChemicalEngineering, Shanghai Jiao Tong University,Shanghai 200240, China
6Department of Electrical and ElectronicEngineering, Shenzhen Key Laboratory of FullSpectral Solar Electricity Generation (FSSEG),Southern University of Science and Technology,No. 1088, Xueyuan Rd., Shenzhen, Guangdong518055, China
7School of Materials Science & Engineering, SunYat-sen University, Guangzhou, Guangdong510275, China
8National Renewable Energy Laboratory, Golden,CO 80401, USA
9Adavanced Light Sources, Lawrence BerkeleyNational Laboratory, Berkeley, CA 94720, USA
10Molecular Foundry, Lawrence BerkeleyNational Laboratory, Berkeley, CA 94720, USA
11Collaborative Innovation Center of ExtremeOptics, Shanxi University, Taiyuan, Shanxi 030006,P.R. China
This study did not generate any unique datasets or code
Materials
Anhydrous solvents (DMF, DMSO, isopropanol [IPA], CB, CF, and 1-chloronaphtha-
lene [1-CN]) were purchased from Sigma-Aldrich and used as received. Lead (II) io-
dide (PbI2), lead (II) bromide (PbBr2), cesium iodide (CsI), and buckminsterfullerene
(C60) were also obtained from Sigma-Adrich and used as received. Methylammo-
nium bromide (MABr) and formamidinium iodide (FAI) were purchased from Great-
Cell Solar Ltd. (Australia). PC61BM was purchased fromMerck Company, and Y6 was
ordered from WEIZU Chemical Company (Shanghai, China).
Device Fabrication
Inverted PSCs were fabricated with a configuration of ITO/PTAA/perovskite/ETL/
C60/BCP/Ag. ITO glass was cleaned by sequentially washing with detergent, deion-
ized water, acetone, and IPA. The substrates were dried with N2 and cleaned by UV
ozone for 20 min. PTAA HTLs were prepared by spin coating the Tol-solution of
PTAA (1.5 mg/mL) at 6,000 rpm for 30 s and annealing at 100�C for 10 min. The
CsFAMA mixed perovskite films were fabricated according to our previously re-
ported one-step antisolvent method.52 Briefly, precursor solution was made by mix-
ing PbI2, PbBr2, FAI, and MAI in DMF/DMSO (v/v 4/1). The mole concentration was
kept at 1.3 M with 0.1 M PbI2 excess. The I/Br and FA/MA mole ratios were main-
tained at 0.85/0.15. Additional 35 mL of CsI (2 M in DMSO) was added into the pre-
cursor solution. The solution was stirred 2 h at 65�C. Perovskite films were prepared
by spin coating the precursor solution (5,000/2,000 rpm, 35 s). In the last 25 s of the
procedure, films were quickly treated with 300 mL CB and annealed at 100�C for
60 min. After the perovskite growth, for the control device, PCBM (2 wt % in CB) films
were spin-coated with 1,000 rpm 30 s, followed by annealing at 100�C for 30min. For
Y6 devices, Y6 was dissolved in CF, CB, and Tol, respectively, with the addition of
0.5% 1-CN as an additive, and stirred on a hotplate in a nitrogen-filled glove box
for 2 h. The blend solution was spin-cast on the top of the perovskite layer
(3,000 rpm, 30 s), and the film was thermally annealed at 80�C for 5 min to optimize
the blend morphology. For Y6/C60 devices, C60 (20 nm) layers were thermal evap-
orated on top of Y6. After that, BCP (0.5 wt % in IPA) was deposited as cathode buffer
layer at 4,000 rpm spinning speed. Finally, an Ag back electrode was deposited by
thermal evaporation at high vacuum. The device active area was 10 mm2 and all de-
vices were measured with the mask area of 8.0 mm2.
Film and Device Characterizations
J-V measurements were carried out using a Keithley 2400 source meter in ambient
environment at �25�C and �60%–65% relative humidity (RH). The devices were
measured both in reverse scan (1.2 /�0.2, step 0.01 V) and forward
scan (�0.2 /1.2, step 0.01 V) with 10 ms delay time. Illumination was provided
by an Oriel Sol3A solar simulator with AM1.5G spectrum and light intensity of
100 mW/cm2 calibrated by a standard KG-5 Si diode. The aperture area of our de-
vice is 8.0 mm2, calibrated with a shadow mask during the measurements. The
UV-vis-NIR spectra were recorded by a Cary 5000 UV-Vis-NIR spectrometer (Agi-
lent). EQE measurements for devices were conducted with an Enli-Tech (Taiwan)
EQE measurement system. A FEI Helios Nanolab 600i dual beam focus ion beam
and field emission gun-scanning electron microscope (FIB/FEGSEM) was used to
1588 Joule 4, 1575–1593, July 15, 2020
llArticle
prepare cross-section for STEM imaging and analysis. FEI Talos transmission elec-
tron microscope (TEM) with Super-X EDX was employed to acquire the STEM image
with high-angle annular dark field (STEM-HAADF) mode. PL and time resolved PL
spectra were carried out by Spectrofluorometer (FS5, Edinburgh instruments).
Xenon lamp and 405 nm pulsed laser were used as excitation sources for the PL
(excited from glass side) and TRPL measurement (excited from Y6 or PCBM side),
respectively. The MS and t-DOS spectra for the devices were measured by a Zahner
IM6e electrochemical station (Zahner, Germany) in ambient environment of 23�Cand 37% RH. KPFMmeasurements were performed in a single-pass frequency-mod-
ulation mode using Cypher ES (Asylum Research) and HF2LI lock-in amplifier (Zurich
Instrument) with PtIr coated tips (PPP-EFM, nanosensors).53 XPS measurement was
conducted on a Thermo Scientific K-alpha XPS apparatus equipped with a mono-
chromatic Al K(alpha) source and food gun for charge compensation. For the XPS
samples of PVSK with Y6 films, diluted Y6 solution (0.5 mg/mL in CF) was spin-cast
on the top of the perovskite layer at 3,000 rpm, 30 s, to control thickness of Y6
film below 10 nm. Film thicknesses were measured with a surface profilometer
Beijing National Laboratory for Molecular Sciences (BNLMS201902). W.C. and
A.B.D. are grateful for support from the Seed Funding for Strategic Interdisciplinary
Research Scheme of the University of Hong Kong and RGCCRF grant 5037/18G. The
authors thank Dr. Tao Liu and Prof. He Yan for the material of IOIC-2Cl and IEICO-4F;
Dr. Xinle Li and Dr. Chongqing Yang for the discussion of XPS results; Prof. Benzhong
Tang, Prof. Lu-tao Weng, and Zaiyu Wang for the ToF-SIMS analysis; and Dr. Wanli
Yang for the discussions of TFY studies. TFY and GIXD were performed at beamline
11.0.1.2 and 7.3.3 at Advanced Light Source, Lawrence Berkeley National Labora-
tory, which was supported by US Department of Energy, Office of Science, and
Office of Basic Energy Sciences. We thank the support for sample preparation and
device fabrication at Molecular Foundry, LBNL. Work at Molecular Foundry was sup-
ported by the Office of Science, Office of Basic Energy Sciences, the US Department
of Energy under contract no. DE-AC02-05CH11231. SSMC and TAS work were sup-
ported by the US Department of Energy, Office of Basic Energy Sciences, Division of
Chemical Sciences, Biosciences, and Geosciences under contract no. DE-AC36-
08GO328308 with the National Renewable Energy Laboratory.
AUTHOR CONTRIBUTION
Q.H., T.P.R., and F.L. conceived the idea; T.P.R., F.L., R.Z., and Z.H. supervised the
project; Q.H., W.C., and W.Y. fabricated and optimized the PSCs as well as the
related device characterizations, including the UV-vis absorption, PL and TRPL,
t-DOS, EQE, STEM and SEM, and so on; Q.H. and Y.L. performed GIWAXS; W.Z.,
Q.H., and C.W. conducted TFY measurements; Y.Z. and J.X. performed the theoret-
ical calculation; Y.-H. L. and M.S. conducted the KPFMmeasurements; Q.H. and L.K.
conducted the XPS experiment; B.W. L., J.C.J., and Q.H. performed the SSMC and
TAS measurements; Q.H. wrote the first draft of the paper; W.C., W.Y, Y.L., Y.Z.,
B.W.L., and J.C.J. revised the paper; all the authors discussed the results and edited
the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: January 16, 2020
Revised: April 10, 2020
Accepted: June 4, 2020
Published: July 1, 2020
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Qin Hu, Wei Chen, Wenqiang Yang, Yu Li, Yecheng Zhou, Bryon W. Larson, Justin C.Johnson, Yi-Hsien Lu, Wenkai Zhong, Jinqiu Xu, Liana Klivansky, Cheng Wang, MiquelSalmeron, Aleksandra B. Djuri�si�c, Feng Liu, Zhubing He, Rui Zhu, and Thomas P. Russell
Figure S1. a) The cross-sectional scanning transmission electron microscopy (STEM) image of inverted perovskite solar cells studies in this work. b) The SEM image of perovskite thin film.
Figure S2. The simulation of single-crystal structure of Y6.
Figure S3. The summary of d-spacing, peak area and the crystalline coherence length (CCL) of a) π-π stacking, b) (110) facet, and c) (11-1) facet scatterings at the in-plane direction.
Figure S4. The topography of Y6 films processed with different solvents. The root mean square (rms) roughness are 5.49 nm (CF), 5.95 nm (Tol) and 8.28 nm (CB). All the scale bars are 500 nm.
Figure S5. The UV-vis spectra of Y6 films processed with different solvents.
Figure S6. The EQE spectra of devices based on Y6 films processed with different solvents.
Figure S7. The film thickness optimization of Y6-CF film for perovskite solar cells.
Figure S8. The stabilized power output (SPO) at maximum power points (MPP) of devices with different electron transport layers.
Figure S9. The XPS analysis of the perovskite/Y6 interface.
Figure S10. Density of states of a PCBM a) and Y6 b) adsorbed on a perovskite surface and corresponding orbitals.
Figure S11. Kinetic profiles corresponding to red and black arrows at spectral positions indicated in Figure 5 for PVSK/Y6 sample photoexcited at a) 550 nm and b) 890 nm, and for PM6/Y6 BHJ sample photoexcited at c) 550 nm and d) 890 nm.
Figure S12. The stability of devices based on different ETLs in ambient atmosphere with the humidity of 60%-65%.
Figure S13. The thermal stability of devices with different ETLs upon 85 °C aging condition.
Figure S14. a) ToF-SIMS results of aged a) PCBM and b) Y6 based whole devices.
Table S1. Summaries of champion device performance based different electron transport layer.
Table S3. The reproductivity of device performance based on different electron transport layer. The standard deviations were calculated with 20 devices.