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How to cite: Angew. Chem. Int. Ed. 2023, e202307395
Solar Cells Hot Paper doi.org/10.1002/anie.202307395
Managing Excess Lead Iodide with Functionalized Oxo-Graphene
Nanosheets for Stable Perovskite Solar Cells
Guixiang Li+, Yalei Hu+, Meng Li,* Ying Tang, Zuhong Zhang, Artem Musiienko, Qing Cao,
Fatima Akhundova, Jinzhao Li, Karunanantharajah Prashanthan, Fengjiu Yang,
Patryk Janasik, Augustine N. S. Appiah, Sergei Trofimov, Nikolaos Livakas, Shengnan Zuo,
Luyan Wu, Luyao Wang, Yuqian Yang, Benjamin Agyei-Tuffour, Rowan W. MacQueen,
Boris Naydenov, Thomas Unold, Eva Unger, Ece Aktas,* Siegfried Eigler,* and
Antonio Abate*
Abstract: Stability issues could prevent lead halide perovskite solar cells (PSCs) from commercialization despite it having
a comparable power conversion efficiency (PCE) to silicon solar cells. Overcoming drawbacks affecting their long-term
stability is gaining incremental importance. Excess lead iodide (PbI2) causes perovskite degradation, although it aids in
crystal growth and defect passivation. Herein, we synthesized functionalized oxo-graphene nanosheets (Dec-oxoG NSs)
to effectively manage the excess PbI2. Dec-oxoG NSs provide anchoring sites to bind the excess PbI2 and passivate
perovskite grain boundaries, thereby reducing charge recombination loss and significantly boosting the extraction of free
electrons. The inclusion of Dec-oxoG NSs leads to a PCE of 23.7% in inverted (p-i-n) PSCs. The devices retain 93.8%
of their initial efficiency after 1,000 hours of tracking at maximum power points under continuous one-sun illumination
and exhibit high stability under thermal and ambient conditions.
[*] Dr. G. Li,+ Prof. Dr. M. Li, Y. Tang, Z. Zhang K. Prashanthan
Key Lab for Special Functional Materials of Ministry of Education, Department of Physics, University of Jaffna
National & Local Joint Engineering Research Center for High- Jaffna 40000 (Sri Lanka)
efficiency Display and Lighting Technology, School of Materials P. Janasik, A. N. S. Appiah
Science and Engineering, Collaborative Innovation Center of Nano Silesian University of Technology
Functional Materials and Applications, Henan University 44-100 Gliwice (Poland)
Kaifeng 475004 (China)
E-mail: mengli@henu.edu.cn N. Livakas
Nanochemistry Department, Istituto Italiano di Tecnologia
Dr. G. Li,+ Prof. Dr. M. Li, Dr. A. Musiienko, F. Akhundova, J. Li, Via Morego 30, 16163 Genova (Italy)
K. Prashanthan, Dr. F. Yang, S. Trofimov, S. Zuo, L. Wu, L. Wang, and
Dr. Y. Yang, Dr. B. Agyei-Tuffour, Dr. R. W. MacQueen, Department of Chemistry and Industrial Chemistry, Universitàdegli
Dr. B. Naydenov, Dr. T. Unold, Prof. Dr. E. Unger, Prof. Dr. A. Abate Studi di Genova
Helmholtz-Zentrum Berlin für Materialien und Energie GmbH Via Dodecaneso 31, 16146 Genova (Italy)
Hahn-Meitner-Platz 1, 14109 Berlin (Germany)
E-mail: antonio.abate@helmholtz-berlin.de L. Wu
Department of Physics, Università di Cagliari
Dr. G. Li,+ Prof. Dr. A. Abate Cittadella Universitaria, 09042 Monserrato (Italy)
Department of Chemistry, Bielefeld University
Universitätsstraße 25, 33615 Bielefeld (Germany) Dr. B. Agyei-Tuffour
Department of Materials Science and Engineering, School of
Dr. G. Li+ Engineering Sciences, College of Basic and Applied Sciences,
Present address: Institute of Chemical Sciences and Engineering, University of Ghana
École Polytechnique Fédérale de Lausanne (EPFL) Legon, GA-521-1966 Accra (Ghana)
1015 Lausanne (Switzerland)
Dr. E. Aktas, Prof. Dr. A. Abate
Dr. Y. Hu,+ Q. Cao, Prof. Dr. S. Eigler Department of Chemical, Materials and Production Engineering,
Institute of Chemistry and Biochemistry, Freie Universität Berlin University of Naples Federico II. Naples
Altensteinstraße 23a, 14195 Berlin (Germany) pzz.le Vincenzo Tecchio 80, 80125 Naples (Italy)
E-mail: siegfried.eigler@fu-berlin.de E-mail: ece.aktas@unina.it
Dr. Y. Hu+ [+] These authors contributed equally to this work.
CNRS, Immunology, Immunopathology and Therapeutic Chemistry,
UPR3572, University of Strasbourg, ISIS © 2023 The Authors. Angewandte Chemie International Edition
67000 Strasbourg (France) published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Angew. Chem. Int. Ed. 2023, e202307395 (1 of 10) © 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
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Introduction sheets (Dec-oxoG NSs) using hydroxyl radicals prepared via
wet-chemical etching. We show that Dec-oxoG NSs stabilize
Lead halide perovskite solar cells (PSCs) have received excess PbI2 release during the crystallization of perovskite
widespread attention as a promising contender in emerging films. PSCs prepared with Dec-oxoG NSs showed signifi-
photovoltaics (PVs). During the past decade, the PSCs have cantly better charge extraction enabling a champion power
achieved incredibly rapid progress in power conversion conversion efficiency of 23.7% in p-i-n architecture. The
efficiency (PCE) from 3.8% to 25.7%.[1] What stands out in Dec-oxoG-based device retained 93.8% of its initial PCE
PSCs is the brilliant optoelectronic properties of perovskites, after 1,000 h tracking at maximum power points (MPP)
for instance, ultrahigh absorption coefficient,[2] tunable band under continuous one sun illumination, as well as high
gaps,[3] superior defect tolerance,[4] and long carrier diffusion stabilities under 85 °C test and ambient conditions.
length,[5] as well as low manufacturing costs.[6] Notwithstand-
ing having remarkable optoelectronic properties and sur-
passing the efficiency of silicon solar cells, the lack of long- Results and Discussion
term stability is one of the main roadblocks towards its
industrialization.[7] In 2016, Saliba et al. reported that to use Tailoring material functionalization
a multi-cation and anion composited perovskite absorber
layer can be an alternative solution for their long-term Oxo-graphene nanosheets (oxoG NSs) were prepared using
stability issue.[8] Since then, cesium ion (Cs+) has been wet chemical synthesis.[19] Hereafter, we prepared function-
incorporated in the perovskite composition to achieve more alized oxoG NSs (Dec-oxoG NSs) by etching oxoG with
reproducible devices with high PCE and long-term hydroxyl radicals (HO*), which were generated by the
stability.[9] In principle, adding more inorganic elements into photolysis of hydrogen peroxide with UV light under the
the perovskite precursor increases mixing entropy and catalysis of Fe2+ (Figure 1a).[20] During this process, HO*
replaces volatile organic cations that can generally improve oxidizes the hydroxyl groups to carbonyl groups and
the stability of devices.[10] carboxylic acids by attacking the unsaturated aromatic ring
However, undesirable excess halide phases within the and phenol-like groups of oxoG via electrophilic addition
perovskite film will eventually lead to irreversible long-term and oxidation reactions.[20]
stability issues.[11] Likewise, as reported in the previous The morphological properties of oxoG and as-prepared
studies, the light-induced phase segregation process can be Dec-oxoG NSs were presented by atomic force microscope
observed while exposing the multiple halide-based perov- (AFM). As expected, the pristine oxoG NSs were flat, and
skite device to constant illumination.[12] Nevertheless, using no pores were observed (Figure 1b). After etching, the
excess lead iodide (PbI2) in perovskite precursor ensures surface of Dec-oxoG NSs became rough, and some pores
larger grain, reduces halide vacancies, and promotes ori- appeared across the Dec-oxoG NSs surface (Figures 1c and
ented α-phase perovskite crystal growth.[13] Additionally, S1, ESI†). From AFM images, we can also observe the thin-
excess or residual PbI2 plays a role in passivating grain layer structure of Dec-oxoG NSs. Raman spectroscopy was
boundary defects.[13a,14] Nonetheless, PbI2 is readily decom- then used to give more information about the structural
posed into metallic lead (Pb0) by releasing iodine (I2) vapor characteristics. As shown in Figure 1d, the intensity ratio of
under continuous light illumination and heating. These disordered/defect-activated D band and graphitized G bands
species generate nonradiative recombination centers and (ID/IG) was increased from 1.15 for oxoG NSs to 1.18 for
promote ion migration, thereby accelerating the attenuation Dec-oxoG NSs. Associated with that the broad full width at
of the device performance due to chemical chain half maximum (FWHM) of the 2D peak (>100 cm 1), we
reactions.[15] Consequently, stabilizing excess PbI2 is essential can learn Dec-oxoG NSs retaining a high functionalization
to improve the long-term operational stability of PSCs. degree. Fourier transform infrared spectroscopy (FTIR)
Graphene derivatives have been utilized as two-dimen- spectra were utilized to study the functional groups,
sional (2D) network structural materials in solar cells due to observing stretching vibrations of O H at about 3330 cm 1,
having high electrical conductivity, chemical stability, and stretching vibrations of C=O at around 1720 cm 1, deforma-
superior semiconductor properties.[16] Lin et al. successfully tion vibrations of the O H at about 1405 cm 1, and
employed graphene on both sides of Cu Ni alloy electrodes stretching vibrations of C O at around 1155 cm 1 for both
in PSCs to stabilize the electrodes by suppressing light- and oxoG and Dec-oxoG (Figure S2, ESI†).[21] To further inves-
heat-induced component migration.[17] In our previous work, tigate the chemical composition of oxoG and Dec-oxoG
dodecylamine-based ultrathin 2D graphene network was NSs, X-ray photoelectron spectroscopy (XPS) and solid-
used to control ion migration in triple-cation perovskites state nuclear magnetic resonance (ssNMR) spectra were
against external environmental factors.[18] However, no work performed. As shown in Figure 1e, the high-resolution C 1s
has completely stabilized excess PbI2 via 2D (graphene) spectrum showed typical features for oxoG with the peaks
network materials, which still limits the operational stability assigned to C C, C OH/C O C, and C=O/COOH bonds.
of PSCs. The proportion of higher oxidised atoms (11.0%) in Dec-
In this work, we modified the functionalization of oxoG NSs, such as carbonyl or carboxyl, was more than that
graphene nanosheets for managing excess PbI2 in perov- (5.3%) in oxoG NSs. Also, the ratio of C=O/COOH to
skites, resulting in more efficient and stable devices. C O C/C OH increased from 0.13 to 0.39 after etching. In
Specifically, we prepared functionalized oxo-graphene nano- the ssNMR spectra of oxoG and Dec-oxoG NSs (Figure S3,
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Research Articles AngewandteChemie
Figure 1. (a) Reaction schematic for the conversion of functional groups in the basal plane of oxoG. AFM images of (b) oxoG and (c) Dec-oxoG
NSs. (d) Average Raman spectra, (e) high-resolution C1s XPS spectra for oxoG and Dec-oxoG NSs.
ESI†), the signal of graphitic sp2 atoms, tertiary alcohols and Origins of enhancing crystal structure
epoxides were found, and the ratio of tertiary alcohols to
epoxides increased from 0.17 to 0.64 after etching.[22] These After demonstrating the functionalized structure of the Dec-
results indicated that the proportion of oxo-groups like oxoG NSs, we investigated its interaction with perovskite
hydroxyl, carbonyl, and carboxyl groups at the rims of using FTIR spectra. The FTIR study confirms the presence
generated pores increased for Dec-oxoG NSs. of Dec-oxoG in the perovskite film and reveals their
interaction. The green curve in Figure 2a labelled “Pure
Dec-oxoG” represents freshly prepared Dec-oxoG NSs
without perovskite layer. The pink curve’s “Dec-oxoG”
Figure 2. (a) FTIR spectra, (b) XRD patterns of the reference and Dec-oxoG NSs perovskite films. Top-view SEM images of the (c) reference film
and (d) film optimized with Dec-oxoG. 3D AFM images of the surface morphologies of (e) reference and (f) Dec-oxoG perovskite films.
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depicts the perovskite film treated with Dec-oxoG NSs, and lower the energy barrier at the perovskite-contact interface,
“Reference” marked by the dark blue curve represents thereby improving the open-circuit voltage (Voc) and fill
pristine perovskite sample without Dec-oxoG NSs (see the factor (FF) of solar cells.[29] As the incident angle increases
method section in Supporting Information for details). We (from 0.1° to 0.5°), the q’s shift decreases (Figure 3b),
noticed that the stretching vibration of ν(O H) in pure Dec- suggesting that the structural effect mainly exists on the
oxoG NSs appeared at �3300 cm 1, while it slightly presents upper surface of the perovskite film. The initial weak
in Dec-oxoG NSs treated perovskite film and is not visible diffraction intensity at low angles (0.1–0.2 degrees) origi-
in Reference film (Figure 2a). The typical peaks of ν(C=O), nates from the highly defective Dec-oxoG NSs coverage.
ν(C=C) and ν(C OH) vibrations in pure Dec-oxoG NSs are We observed a weak PbI2 peak in the reference perovskite
observed at 1729 cm 1, 1615 cm 1, and 1415 cm 1, respec- (Figure 3c). Strikingly, the PbI2 phase in the Dec-oxoG NSs-
tively. We note that the stretching vibration of the double treated perovskite increased gradually with the incidence
bond has shifted due to having lower bond strength between angle (Figure 3d). Diffraction patterns at 2 degree were
sulphur and oxygen due to the adduct formation.[23] Like- presented in Figure S6, ESI†, with the diffraction signal of
wise, these peak shifts to the lower wavenumber of labelled PbI2 in the Dec-oxoG NSs based perovskite. The
1703 cm 1 for ν(C=O), 1524 cm 1 for ν(C=C), and 1350 cm 1 GIWAXS profile with the grazing incidence angle was
for ν(C OH), mainly owing to Lewis base interaction with presented in Figure S7, ESI†, which gave the signal evolution
Lewis acid of Pb2+ and Cs+/FA+/MA+ ions.[24] of PbI2. These results indicate that the treatment of Dec-
X-ray diffraction (XRD), scanning electron microscope oxoG NSs allows additional PbI2 to be released from
(SEM), AFM and grazing incidence X-ray diffraction perovskite by restructuring perovskites. Additionally, the
(GIXRD) measurements were utilized to investigate further faint peak located around 11.7° in Figure 3c corresponds to
the alteration of the perovskite film’s crystal structure and the hexagonal phase, which could be greatly suppressed by
morphological properties with and without Dec-oxoG NSs. Dec-oxoG NSs (Figure 3d).[7] To further investigate the
The perovskite layer’s XRD pattern (Figure 2b) showed structural properties of perovskites influenced by Dec-oxoG
predominant crystallization along the (001) plane that was NSs, we aged the unencapsulated films in air for one week.
consistent between the samples under investigation. Yet, As a result, the reference perovskite film underwent
both perovskites of the patterns have the peak relating to degradation, manifested by enhanced PbI2 content (Fig-
the intentionally used excess PbI2 at �12.6°, although the ure 3e) and increased grain boundaries (Figure S8a, ESI
†).
perovskite film with Dec-oxoG NSs currently has more On the contrary, the morphology and structure of Dec-
dominance.[25] This is attributed to that restructuring per- oxoG NSs treated perovskites are almost retained, exhibit-
ovskite by Dec-oxoG NSs releases more PbI2. Going further, ing excellent film stability (Figures 3f and S8b, ESI
†).
we found that the Dec-oxoG-treated perovskite showed a Although more PbI2 was detected in the new Dec-oxoG
reduced full width at half maximum (FWHM), correspond- NSs-treated perovskites, PbI2 content is not further in-
ing to improved film crystallinity. creased, indicating that the perovskite retains structure
We subsequently elucidate the impact of the Dec-oxoG stability during ageing.
NSs on perovskite film morphology. As is depicted in the Providing these results, we demonstrated that the Dec-
SEM images (Figure 2c,d), the perovskite film with Dec- oxoG NSs treatment effectively stabilized excess/unreacted
oxoG NSs shows a more compact film with large crystals PbI2 left after the crystallization (Figure 3g,h). The excess
(Figure S4, ESI†), resulting in reduced grain boundaries PbI2 at the grain boundaries is expelled to allow the smaller
where defects mainly exist.[26] Increased grain size generally crystal to fuse to larger domains in an Ostwald ripening
benefits in light absorption and charge transfer in perovskite process.[26a,30] Also, the bound PbI2 can prevent self-decom-
films.[27] The white crystals on the film surface indicate position that induces perovskite degradation (Figure S9,
excessive/residual PbI in previous reports.[28]2 Moreover, the ESI
†).[13a,15a,31] The benefited perovskite stability is expected
root-mean-square surface roughness (Rrms) of the perovskite to enhance the operational durability of PV device.
films is estimated from AFM images (1×1 μm2) to be
10.5 nm after covering Dec-oxoG NSs onto perovskite film
(Figure 2e,f) while that for reference perovskite film is Boosted charge carrier dynamics
12.2 nm. The decreased Rrms is expected to improve the
coverage of the electron transport layer.[7b] For further understanding the impact of Dec-oxoG NSs on
To investigate the effect of Dec-oxoG NSs on perovskite the charge recombination kinetics in the perovskite layers,
structure, we performed grazing incidence wide-angle X-ray we performed steady-state photoluminescence (PL), time-
scattering (GIWAXS) measurements on the films. The one- resolved photoluminescence (TRPL) and PL mapping. The
dimensional (1D) GIWAXS patterns were integrated from maximum PL at the same wavelength for perovskite films
corresponding 2D images. The estimated penetration depth were compared. Zhu et al. demonstrated that the improve-
with different incident angles was presented in Figure S5, ment in PL intensity and carrier lifetime could be achieved
ESI†. The introduction of Dec-oxoG NSs caused the by additive engineering. In their study, the  COOH-based
scattering vector q peak within low incident angles to move ligand molecule altered the crystal growth of the perovskite
to a smaller angle (Figure 3a,b). This indicates the extended layer and passivated defects from uncoordinated Pb2+.[32]
lattice spacing due to the interaction between Dec-oxoG Similarly, Dec-oxoG NSs incorporated perovskite film
NSs and perovskites. This structural change is believed to presented stronger PL emission with a longer carrier lifetime
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Figure 3. Integrated GIWAXS patterns on (001) peaks of (a) reference and (b) Dec-oxoG NSs perovskite films with incident 0.1 degree to 0.8
degree. Integrated GIWAXS patterns of (c) reference and (d) Dec-oxoG NSs perovskite films with incident 0.1 degree to 0.3 degree. XRD patterns
of the (e) reference and (f) Dec-oxoG perovskite films undergoing ageing in the air for one week (ambient oxygen, 24% humidity and light).
Schematics of forming (g) reference and (h) Dec-oxoG perovskite films.
(over 3 μs) than reference perovskite film on the quartz, as the panel describes charge separation mechanisms in the
indicated in Figures S10 and S11, as well as Table S1, ESI†. thin film of perovskite without transport layers. Dec-oxoG
Enhanced PL intensity and carrier lifetime means the NSs treated perovskite film showed the increase in the
passivation effect of the -COOH-based Dec-oxoG NSs in negative SPV signal, indicating the dominance of free
perovskite film. The increased quasi-Fermi level splitting electrons near the top surface. The result for ITO/Pero/Dec-
(QFLS) value also accounts for the reduced nonradiative oxoG interface demonstrates that Dec-oxoG NSs boost the
recombination of carriers (Figure S12, ESI†). Figure 4a–d concertation of electrons near the perovskite surface due to
shows hyperspectral PL and QFLS maps measured at the the suppression of non-radiative recombination. We then
exact location with a large-size area (around 2.2*2.2 cm2) for studied the effect of Dec-oxoG NSs on charge extraction by
typical samples with a stack of indium tin oxide (ITO) constructing the device layer by layer and measuring trSPV.
substrate/[2-(3,6-dimethoxy-9H-carbazol-9- In Figure 4e, a self-assembled monolayer (SAM) of
yl)ethyl]phosphonic acid (MeO-2PACz)/perovskite. The ex- MeO 2PACz as a hole-selective contact (HSC) is incorpo-
citation source is 455 nm LED, and the photon flux was rated between the ITO substrate and perovskite layer.
1.31 e21 (m 2 s 1) under one sun condition. The reference Surprisingly, we found that Dec-oxoG NSs significantly
sample has PL emission inhomogeneities on the bottom part boost the extraction rate of free holes, which is, to the best
of the substrate, which can be observed from PL peak and of your knowledge, the first observation of such a significant
QFLS images. The Dec-oxoG NSs treated film shows a effect of a top layer on the bottom-hole extraction process.
uniform emission distribution. Also, the Dec-oxoG NSs This effect is possibly induced by larger charge carrier
based film has higher PL intensity and QFLS values. It mobility in Dec-oxoG NSs treated perovskite films due to
means that the Dec-oxoG NSs treatment doesn’t only make better crystallinity, reduced trap concentration, and larger
the film more homogeneous but decreases the nonradiative perovskite grains. This hypothesis is supported by photo-
recombination centers. We show that Dec-oxoG NSs can be luminescence maps showing much wider bright regions in
successfully used for passivating film defects. Dec-oxoG NSs treated samples. Further, we deposited
We subsequently investigated the charge carrier dynam- electron-selective contacts (ESC) on the perovskite surface
ics caused by Dec-oxoG NSs using the transient surface to monitor the effect of Dec-oxoG NSs on interfacial
photovoltage (trSPV) measurements.[33] In Figure S13, ESI†, electron extraction. As depicted in Figure 4f, the trSPV
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Figure 4. Photoluminescence maps of (a) reference and (b) Dec-oxoG perovskite films. Quasi-Fermi level splitting maps for (c) reference and (d)
Dec-oxoG perovskite films. Imaging analysis measurements were prepared on the stack of glass/ITO/MeO-2PACz/perovskite. The excitation
source is 455 nm LED, and the photon flux was 1.31 e21m 2 s 1 (one sun condition). Surface photovoltage transients of (e) ITO/MeO-2PACz/
perovskite and (f) ITO/perovskite/C60 and ITO/perovskite/C60/BCP, where both treated with Dec-oxoG NSs and untreated perovskites were
studied. Schematic models representing photoexcitation by a nanosecond laser pulse, charge generation, and transport to ESC or HSC are
provided on the right.
amplitude and signal rise time of ITO/perovskite with Dec- Photovoltaic performance in p-i-n perovskite solar cells
oxoG NSs/C60 sample are greater than that of ITO/
perovskite/C60 sample. It suggests that Dec-oxoG NSs also The passivating effect of the Dec-oxoG NSs in the
boost electron extraction in the perovskite/C60 interface. The completed perovskite device performance is investigated by
bathocuproine (BCP) layer further boosts electron extrac- comparing with the reference device. The state-of-the-art
tion across the device (Figure 4f). The trSPV results are device’s architecture is based on the inverted p-i-n config-
consistent in the visible spectrum range (1.5 to 3 eV). We uration of glass/ITO/MeO-2PACz/perovskite/C60/BCP/silver
also observed passivating capabilities of the Dec-oxoG NSs (Ag), as shown in the cross-sectional SEM image (Fig-
layer on perovskites (Figure S14, ESI†). ure 5a). Herein, the light absorber layer is passivated with
The insights into the mechanism of the charge carrier Dec-oxoG NSs, and the thin thickness of these nanosheets
recombination process in PV devices are also investigated. prevents their detection. The device treated with 0.2 mg/mL
The minimized nonradiative recombination losses in the Dec-oxoG NSs exhibited champion efficiency compared to
device can be demonstrated from the dark current density- concentration (Figure S19 and Table S3, ESI†). The uniform
voltage (J–V) curves (Figure S15 and Table S2, ESI†), and pinhole-free perovskite film, after incorporating Dec-
implying higher Voc in the device.
[34] Additionally, the oxoG NSs, is worth noting that covers the MeO-2PACz
increased resistance in Nyquist plots implicates better molecule surface. Herein, the MeO-2PACz molecule was
interfacial contact and favorable electron transfer at the preferred as a hole-selective contact because its phosphonic
perovskite/ETL interface for PSCs with Dec-oxoG NSs acid anchoring group quickly formed a condensation reac-
treatment (Figure S16, ESI†).[35] The increased build-in tion with the hydroxyl groups of the metal oxide by a spin-
potential of Dec-oxoG NSs based device was verified from coating method.[37] The new device performance statistically
Mott–Schottky’s analysis (Figure S17, ESI†), attributing to surpassed an average of 20.5% with the MeO-2PACz
improving the V .[36]oc The space-charge limited current molecule in Figure 5b, in agreement with the literature work
(SCLC) model for electron-only devices was measured to using the p-i-n architecture with triple-cation perovskite as
understand the enhanced conductivity (Figure S18, ESI†). an absorber.[37–38] Dec-oxoG NSs optimized device efficiency
The reduced electron trap density could be attributed to the exceeds 22% average. The best performance from the
fact that Dec-oxoG NSs passivate the trap states (e.g., reference device exhibited a Voc of 1.121 V, a FF of 81.5%, a
uncoordinated Pb2+) by forming Lewis adducts. short-circuit current density (Jsc) of 24.1 mA/cm
2, and a PCE
of 22.0% (Figure 5c). In contrast, the device with modified
Dec-oxoG NSs gave a Voc of 1.147 V, a FF of 82.9%, a Jsc of
24.3 mA/cm2, and a PCE of 23.1%. This boosting effect by
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Figure 5. (a) Device architecture with cross-sectional SEM image of the PSCs. (b) The box charts of statistical PCEs from 19 devices that were
performed forward sweep and reverse sweep, (c) the best-performing J–V curves and (d) EQE curves of new reference and Dec-oxoG-based PSCs.
(e) Self-healing optimal device efficiency evolution and (f) champion J–V curves for reference and Dec-oxoG-based PSCs stored in the dark for one
month. (g) The stabilized power outputs with MPP as a function of time. (h) The long-term stability of unencapsulated Dec-oxoG-based device
under continuous MPP tracking in N2 atmosphere. The data were normalized to the initial PCE value, presented after 50 h burning in.
Dec-oxoG NSs was also significantly greater than that of the the PCE of the control device increased to 22.4%, having
oxoG-based device (Figure S20 and Table S4, ESI†). The J– Voc of 1.133 V, FF of 82.3% and Jsc of 24.0 mA/cm
2
V curves with the reverse and forward scans of the best- (Figure 5f). The Dec-oxoG NSs device delivered a remark-
performing reference and Dec-oxoG NSs modified devices able PCE of 23.7%, with an increased Voc of 1.163 V, FF of
showed negligible hysteresis, respectively 1.8% and 1.3% 84.1%, and a Jsc of 24.2 mA/cm
2. The reverse- and forward-
(Figure S21 and Table S5, ESI†). As shown in the statistical scanned J–V curves are displayed in Figure S23 and
distribution of box charts (Figure S22, ESI†), Voc and FF Table S6, ESI
†. The improved device performance under
parameters are significantly increased, thanks to the effec- storage conditions is contributed by the introduction of Dec-
tive passivation of Dec-oxoG NSs. To passivate the ionic oxoG NSs enhancing the self-healing ability of perovskite to
defects at the grain boundaries and interface of perovskite minimize nonradiative recombination losses (Figures S24–
films are crucial for reaching high solar cell efficiency as S26, ESI†). Additionally, we measured the power outputs
discussed above.[15c,39] Figure 5d displays the external quan- shown with photocurrent under maximum power points
tum efficiency (EQE) spectra for the champion PSCs. (MPP) tracking. As shown in Figure 5g, the Dec-oxoG PSCs
Integrated Jsc values present a negligible difference ( possessed a more stable power output than reference PSCs,
�0.4 mA/cm2) with the J values gained from the J–V scans. showing higher photostability (Figures S27, ESI†sc ). For this
Subsequently, we monitored the PV performance evolution reason, we further measured the long-term stability under
of devices stored in the dark for one month. As storage time continuous one-sun illumination at MPP for Dec-oxoG-
increases to 15 days, we can observe a gradual increase in based PSCs to certify the reliability. As shown in Figure 5h,
device efficiency (Figure 5e). The efficiency then showed the Dec-oxoG-based device retained 93.8% efficiency after
relatively stable values from 15 to 28 days. In this process, 1,000 h tracking, reflecting a significant device stability by
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Research Articles AngewandteChemie
Dec-oxoG NSs optimization. In addition, the thermal measurements. HySPRINT Helmholtz Innovation Lab that
stability of the device that tested at 85 °C is also well provides the infrastructure and Prof. Dr. Steve Albrecht’s
improved (Figure S28, ESI†). The solar cell also shows team for lab setup support are acknowledged. K.P. acknowl-
enhanced stability under ambient condition, exposing to edges the Deutscher Akademischer Austauschdienst
oxygen and humidity of air (Figure S29, ESI†). The im- (DAAD) for funding via the Research Grants - Doctoral
proved efficiency of PSCs is attributed to the Dec-oxoG Programmes in Germany, 2018/19 (57381412). B.A.-T.
NSs-optimized perovskite film quality, and further managing appreciates the support from the TWAS-DFG Programme.
PbI2 enhances the device’s operational stability. This work’s The authors gratefully acknowledge the facility of the HZB
resultant PV performance (PCEs and stability) is superior to SPV lab led by Thomas Dittrich. The authors thank mySpot
those previously reported that applied low dimensional (2D) (BESSY II, HZB, Germany) for providing the synchrotron
materials, such as MXene,[40] black phosphorous,[41] boron radiation beamtime. The authors acknowledge Park Systems
nitrides,[42] transitional metal dichalcogenides,[43] and gra- for support. This project has received funding from the
phene and its derivatives,[44] to improve PSCs’ stability or European Union’s Framework Programme for Research
efficiency. and Innovation HORIZON EUROPE (2021–2027) under
the Marie Skłodowska-Curie Action Postdoctoral Fellow-
ships (European Fellowship) 101061809 HyPerGreen. This
Conclusion work has received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020
Functional two-dimensional materials can meet specific research and innovation programme (Grant agreement No.
needs by introducing functional groups for multi-functional- 804519). Open Access funding enabled and organized by
ization. As the Dec-oxoG NSs we designed in this work, Projekt DEAL.
defect passivation and crystal stabilization are achieved
simultaneously. Perovskite films and integrated devices with
Dec-oxoG NSs modulation show reduced defect density, Conflict of Interest
facilitated electron and hole transfer, and enhanced struc-
tural stability. We propose a mechanism of Dec-oxoG NSs The authors declare no conflict of interest.
in strengthening and stabilizing PSCs performance: (i)
introducing Dec-oxoG NSs triggers Ostwald ripening, lead-
ing to grain-enlarged and flatter perovskite films, releasing Data Availability Statement
additional PbI2 to be captured by Dec-oxoG NSs (managing
PbI2 to fabricate high-quality perovskites); (ii) Dec-oxoG Research data are not shared.
NSs prevents the degradation of perovskite polycrystalline
and even the further generation of PbI2 from films (stabiliz- Keywords: Excess Lead Iodide · Operational Stability ·
ing perovskites); (iii) Dec-oxoG NSs stabilize residual PbI2 Oxo-Graphene Nanosheets · Perovskite Photovoltaics · Solar
with solid bonds and then reduce perovskite crystal structure Cells
damage (preventing PbI2 damage to perovskites).
As a proof of concept, by employing functional-rich
Dec-oxoG NSs to passivate and stabilize the perovskite
structure, we reported a PCE of up to 23.7% in p-i-n PSCs. [1] a) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am.
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operational stability, which retained 93.8% of its initial Cell Efficiency Chart, https://www.nrel.gov/pv/cell-efficiency.
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G. Li, Y. Hu, M. Li,* Y. Tang, Z. Zhang,
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K. Prashanthan, F. Yang, P. Janasik,
A. N. S. Appiah, S. Trofimov, N. Livakas,
S. Zuo, L. Wu, L. Wang, Y. Yang, B. Agyei- Effectively managing excess lead iodide This leads to a significant boost in
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