RESEARCH ARTICLE

Enhancing UV Stability of Perovskite Solar Cells

with Transparent Fluorinated Polyimide Citation: Niu G, Luan Y, Wang J,
Guosheng Niu1,2,3, Yigang Luan2, Jizheng Wang2,3, and Haixia Yang1,3* Yang H. Enhancing UV Stability
of Perovskite Solar Cells with
1
Key Laboratory of Science and Technology on High-Tech Polymer Materials, Institute of Chemistry, Chinese Transparent Fluorinated Polyimide.
Adv. Devices Instrum. 2024;5:Article
Academy of Sciences, Beijing 100190, China. 2Beijing National Laboratory for Molecular Sciences, CAS Key 0039. https://doi.org/10.34133/
Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. adi.0039
3
University of Chinese Academy of Sciences, Beijing 100049, China. Submitted 20 September 2023
Accepted 10 November 2023
*Address correspondence to: yanghx@iccas.ac.cn Published 9 January 2024

Copyright © 2024 Guosheng Niu et al.

Ultraviolet (UV)-induced degradation is one of the major problems in the field of perovskite solar cells
Exclusive licensee Beijing Institute
(PSCs). Therefore, exploring materials and techniques to prevent UV light from penetrating into the device of Aerospace Control Devices. No
is urgently necessary. Here, we developed a special transparent fluorinated polyimide (FPI) resin, which claim to original U.S. Government
can be directly spin-coated on the front side of conventional indium-doped tin oxide substrates (glass/ Works. Distributed under a Creative

Downloaded from https://spj.science.org on July 23, 2024

ITO). Most aromatic polyimides strongly absorb visible light and are colored. The FPI we designed and Commons Attribution License 4.0
synthesized bears electron-acceptor CF3- groups, which reduces the intra-/intermolecular charge-transfer (CC BY 4.0).
(CT) effect, enabling FPI to possess high transmittance in the visible range while completely blocking UV
light. As a result, the FPI coating slightly pulls down the initial power conversion efficiency (PCE) (21.02%
to 20.19%). Remarkably, the coating significantly improves the PSC UV stability. Upon an 8-h enhanced
UV aging test in air, the FPI/glass/ITO-based PSC is able to retain 85.0% of its initial PCE. In contrast, the
control device (glass/ITO-based PSC) only keeps 40.9% of its initial PCE. The protective effect of FPI is
even more prominent in current popular 3D/2D high-performance PSCs because UV light can seriously
damage the 2D layer. The unencapsulated 3D/2D device based on FPI/glass/ITO substrate has a very high
PCE retention of up to 80% after 12-h enhanced UV aging test in air, comparing to 36% for the control
3D/2D device without FPI. This work demonstrates that FPI and its possible derivatives could provide a
feasible avenue to handle UV-induced degradation for PSCs effectively.

Introduction substrate of PSCs to block UV irradiation. Their performances

are listed in Table S1, Supplementary Materials. Interfacial modi-
In recent years, perovskite solar cells (PSCs) have received a lot of fication is also introduced to enhance device UV stability. CsBr
interest due to their outstanding power conversion efficiency was used as an interfacial modifier between the perovskite layer
(PCE) and inexpensive cost of manufacturing [1–4]. Unfortunately, and the electron transport layer to improve device UV stability,
they are generally susceptible to oxygen, moisture, heat, and ultra- and the PSC remained 70% of its initial PCE after 20 min of
violet (UV) light under atmospheric conditions and thus fail to 523 mW cm−2 UV illumination [34]. However, these approaches
achieve satisfactory operational stability [5–8]. Although numer- generally considerably increase the complexity of the fabrication
ous strategies such as encapsulation, interface modification, and process and meanwhile lead to either relatively large PCE loss or
component engineering have been proven efficient in improving insignificant improvement of the UV stability. In this paper, we
device stability [9–11], UV-induced degradation has been barely designed and synthesized a fluorinated polyimide (FPI) resin with
addressed. UV can easily cause perovskite decomposition. Even strong UV absorption ability, which can be directly spin-coated
worse, state-of-the-art n-i-p PSCs typically use TiO2 and SnO2 as on the front side of the conventional indium-doped tin oxide
electron transport material [12–15], which can seriously accelerate substrate (glass/ITO) used for PSCs. The FPI/glass/ITO substrate
the decomposition via active photocatalytic reaction [16–19]. is able to effectively block the “damaging” UV light and allow the
Several strategies have been reported to deal with the UV-induced “useful” visible light to pass through. As a result, for the FPI/glass/
degradation [20,21], including adopting UV filters [22–25], down- ITO-based device (FPI device), only a slight PCE of 3.9% is sacr­
conversion materials (such as Ce- and Eu-based complexes) ificed, but the UV stability is greatly enhanced: the FPI device
[26–33], and interfacial modification [16,34–38]. For example, (control device) is able to remain 85.0% (40.9%) of its initial PCE
some substrate-front-side surface coating materials, such as UV-­ after an 8-h high-intensity UV (365 nm, 100 mW cm−2) aging
234 [24], V570 [26], TiO2 nanoparticles [23], CsPbCl3:Mn-based test in air. Moreover, the FPI was also applied to 3-dimensional
quantum dots (QDs) [29], N-graphene QDs [30], ZnSe QDs [31], (3D)/2D PSCs to protect the 2D passivation layer, and a high-
CdSe/CdS [32], and Eu-complex [33], have been applied to the efficiency PSC with robust UV stability is obtained.

Niu et al. 2024 | https://doi.org/10.34133/adi.0039 1

Fig. 1. The chemical structure of FPI.

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Fig. 2. The properties of FPI. (A) Fourier transform infrared spectroscopy spectrum of the FPI film. (B) TGA curves of the FPI films. (C) Transmittance spectra of the glass/ITO
and FPI/glass/ITO substrates. (D) PL and UV–visible absorption spectra of the FPI coating. a.u., arbitrary units.

Niu et al. 2024 | https://doi.org/10.34133/adi.0039 2

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Fig. 3. Device structures, performance, and UV stability. Schematic architectures of the (A) FPI device and (B) control device. (C) J-V character of the FPI and control devices.
(D) UV stability tests of the 2 devices.

Materials and Methods using thermogravimetry analysis (TGA). The TGA curves and
corresponding derivative weight curves are displayed in Fig. 2B
The materials and methods can be found in the Supplementary and Fig. S1, Supplementary Materials, which indicated a robust
Materials. thermal stability of FPI films after imidization. Besides, UV–­
visible spectra (Fig. S2) indicates that the complete imidization
Results and Discussion is important for realizing a suitable transmittance of the as-
prepared FPI films.
The FPI was copolymerized from 2 units with different optical It is well established that the coloration of polyimide is derived
properties: 6FDA-TFDB with an absorption edge of ~350 nm from the formation of charge-transfer (CT) complex, which is
[39] and BPDA-3,4′-ODA with an absorption edge of ~415 nm caused by intramolecular CT between the diamine (doner) and
[40] (the detailed molecular structure and synthesis can be seen dianhydride (acceptor) moieties [41,42]. However, the trifluo-
in Supplementary Materials). Figure 1 provides their chemical romethyl (–CF3) group in the TFDB monomer can reduce the
structures. By modulating the ratio of the 2 units, the as-­fabricated electron density of the diamine fragment, thus prohibiting the
FPI features a moderate absorption edge of 388 nm. As shown CT effect and contributing to the high visible light transmittance
in Fig. 2A, the molecular structure of the FPI was characterized of FPI [43]. Meanwhile, the intensity of CT interaction is closely
by Fourier transform infrared spectroscopy to demonstrate the related to fluorescence emission. Alleviated intramolecular CT
complete imidization after 300 °C annealing. The appearance between the occupied molecular orbitals on the diamine moi-
of absorption peaks at 1,770 cm−1 (C=O asymmetrical stretch- eties and unoccupied molecular orbitals on the dianhydride
ing of imide groups), 1,720 cm−1 (C=O symmetrical stretching moieties is conducive to high fluorescence quantum yield, which
of imide groups), 1,350 cm−1 (C–N stretching), and 735 cm−1 is responsible for the later observed down-conversion effect in
(C=O bending of imide rings) and disappearance of the peak FPI [43,44].
at 3,400 to 2,500 cm−1 (–OH stretching in –COOH) indicate The optical properties of the FPI coating were further studied
that imidization has been totally completed. The thermal stabil- by UV–visible spectra, and the transmittance of substrates with
ity of the FPI film before and after annealing was investigated and without FPI coating are depicted in Fig. 2C. FPI/glass/ITO

Niu et al. 2024 | https://doi.org/10.34133/adi.0039 3

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Fig. 4. XRD and scanning electron microscopy of perovskite films with and without FPI. XRD patterns (A to C) and scanning electron microscopy images (D to F) of the reference
and aged perovskite films on glass/ITO/SnO2 and FPI/glass/ITO/SnO2.

substrate exhibits high transmittance of over 80% in the visible that there exists UV-to-visible down-conversion in the FPI coat-
range, which is very close to that of the glass/ITO substrate. Due ing during UV illumination, and the active absorber-perovskite
to the optimal absorption edge of the FPI resin, the coating film can make good use of the converted light. This down-­
almost entirely blocks the high-energy UV below 388 nm, the conversion process saves back a certain portion of the energy of
main UV part of the sunlight. Figure 2D gives the photolumi- UV light that would be lost due to the blocking. All-aromatic
nescent (PL) (red line) and UV–visible absorption spectra (black polyimide has 2 different UV–visible absorption bands: “locally
line) of the FPI coating. It can be seen that the absorption edge excited (LE)” transition between the partially occupied and unoc-
of the FPI coating locates at 388 nm, and the PL emission spec- cupied orbitals of diamine or dianhydride, and “charge transfer
trum covers a wide range of 390 to 700 nm. Figure S3, displays (CT)” transition of the charge transfer complex formed between
optical photographs of the substrates illuminated indoor light the electron-donating group (diamine) and the electron-accepting
and UV light. Both substrates are transparent, and FPI/glass/ITO group (dianhydride) [45]. The relaxation of all-aromatic poly-
substrate shows clear fluorescence under UV light. This indicates imide is from CT(π-π*) transition, LE(π-π*) transition, and

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Fig. 5. The performance and UV stability of 3D/2D device and FPI 3D/2D device. (A) J-V character of the control and FPI 3D/2D devices. (B) UV stability tests of the 2 devices.
(C to E) XRD patterns of the reference and aged 3D/2D perovskite films on glass/ITO/SnO2 and FPI/glass/ITO/SnO2.

LE(n-π*) transitions, accompanied by fluorescence emission. The PCE of the FPI device is 20.19%, slightly less than that
Among them, the oscillator strength of LE(π-π*) transition is (21.02%) of the control device. Then, we also tried traditional
much larger than others [46]. We assume that the modified FPI Kapton-type polyimide (with an absorption edge of 410 nm
that bears ether (-O-) linkages in the dianhydride moiety and [42]) to act as a UV-blocking coating, and a large photocurrent
CF3- groups, could make the LE(π-π*) energy level lower than and hence PCE losses were observed due to its relatively larger
the LE(n-π*) energy level, thus obtaining higher fluorescence absorption edge (Fig. S4). Furthermore, External quantum
quantum yield [46,47]. efficiency measurements was conducted to inspect the down
PSCs were then fabricated on the FPI/glass/ITO (the FPI conversion of the FPI device. As shown in Fig. S5, an enhanced
device) and glass/ITO substrates (the control device), and the signal below 350 nm is shown, demonstrating the down-­
corresponding device configurations are shown in Fig. 3A and conversion effect of FPI film.
B. SnO2 was used as ETL in this study for the purpose of ensur- A UV radiation source with a wavelength of 365 nm and a
ing the repeatability and reliability experiment, because SnO2 high intensity of 100 mW cm−2 was used to conduct the
is not susceptible to UV light compared to its TiO2 counterpart. UV-aging test. Considering that solar cells need to work for a
And the perovskite used is (FAPbI3)1−x(MAPbBr3–yCly)x. The very long time under sunlight in practical application, an 8-h
corresponding solution preparation and fabrication process are high intensity of UV light illumination (about 40 times higher
described in the Supplementary Materials, and more details than AM1.5 solar intensity) was used to achieve an accelerated
can be found in our previous reports [4,48]. aging test. The unencapsulated devices were aged in ambient
The current density-voltage (J-V) character of the FPI and air. Figure 3D gives the normalized PCE during the aging test.
control devices are displayed in Fig. 3C, where the device Obviously, the FPI PSC exhibits greatly enhanced UV stability.
parameters, including open circuit voltage (VOC), short-circuit After the aging, the FPI device sustains over 85.0% of its origi-
current (JSC), fill factor (FF), and PCE, are also listed. Obviously, nal PCE, while the PCE of the control PSC sharply drops
the coating mainly impacts the photocurrent. The FPI device (maintains only 40.9% of its original PCE). Figure S6 gives the
obtains a JSC of 22.98 mA cm−2. It is slightly lower than that detailed J-V change of both devices. It is observed that the
(24.60 mAcm−2) of the control device induced by the light loss. performance of the control device (black line and dash line)

Niu et al. 2024 | https://doi.org/10.34133/adi.0039 5

declines dramatically compared to that of the FPI device (red As a result, the FPI coating slightly reduces the initial PCE from
line and dash line). 21.02% to 20.19%, but significantly improves the device UV
To visually demonstrate that damage of the perovskite film stability: after 8 h of high-intensity UV (100 mW cm−2, 365 nm)
by UV irradiation, we directly carried out the UV aging mea- aging in air. The FPI device maintains 85.0% of its initial peak
surement of the perovskite films on the 2 substrates, and x-ray PCE, while only 40.9% is kept for the control device. In addi-
diffraction (XRD) patterns of the perovskite films are displayed tion, FPI was further introduced to 3D/2D devices. It is proved
in Fig. 4A to C. The peak at 14.1° is from perovskite (110), and to be effective in preventing UV damage to the 2D passivation
the peak at 12.7° is from PbI2 (001). The peak intensity ratio of layer. As a result, the unencapsulated FPI 3D/2D PSC obtains
PbI2 (001)/perovskite (110) can be used as an index to characterize a PCE of 21.7% with a high PCE retention rate of up to 80%
the degree of perovskite degradation [49]. For the reference film after the 12 h of high-intensity UV aging test. Our results here
(glass/ITO/SnO2/perovskite) without the aging test, the ratio is demonstrate that FPI and its potential derivatives could have
0.748. For the aged perovskite film on the glass/ITO/SnO2, the broad applications in the PV field as UV blockers.
ratio reaches up to 1.170. The higher ratio indicates severe
perovskite degradation. The ratio of the perovskite film protected Acknowledgments
by FPI coating (FPI/glass/ITO/SnO2/perovskite sample) is 0.768,
which is very close to that of the reference sample. This indicates Funding: This work was supported by the National Natural
that the perovskite layer is well protected by the FPI coating from Science Foundation of China (NSFC grant 51973225).
UV illumination and shows negligible degradation. Moreover, Author contributions: G.N. conducted all the experiments
the scanning electron microscopy test was conducted for these and wrote the main manuscript text. All the authors reviewed
perovskite films to inspect the surface morphology. As shown in and revised the manuscript. H.Y. supervised the work.
Fig. 4D to F, the perovskite film based on the glass/ITO substrate Competing interests: The authors declare that there is no con-
was degraded seriously with a large amount of lead iodide flict of interest regarding the publication of this article.
appearing. However, there is no obvious decomposition for the

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film based on the FPI/glass/ITO substrate. Data Availability
Encouraged by the outstanding UV stability of the FPI device,
we applied the FPI/glass/ITO substrates to 3D/2D PSCs (FPI The data are available from the corresponding author on rea-
3D/2D device) with the p-methoxyphenethylammonium iodide sonable request.
(CH3O-PEAI) interfacial passivator. The J-V curves of the best
3D/2D PSC and FPI 3D/2D PSC are plotted in Fig. 5A with the Supplementary Materials
PCE, VOC, JCS, and FF listed inside. The best FPI 3D/2D device
shows a PCE of 21.7%, a VOC of 1.20 V, a JSC of 22.3 mA cm−2, Materials and Methods
and a FF of 81.18%, with only 4.4% PCE lost compared to the Figs. S1 to S6
control 3D/2D device. UV stability of the 3D/2D devices was Table S1
investigated using the same aging test conditions as that for the
3D PSCs (Fig. 5B). The control 3D/2D device shows a 64% drop References
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