# Impact of polymer aging on main-chain poly(fullerene)s with halide chain-ends for inverted perovskite solar cells

João V Paulin, Gabriel L Nogueira, Douglas H N Santos, Marco Antônio G B Gomes, Valdemiro P Carvalho-jr, Luiz C Silva Filho, Roger C Hiorns, Carlos F O Graeff

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João V Paulin, Gabriel L Nogueira, Douglas H N Santos, Marco Antônio G B Gomes, Valdemiro P Carvalho-jr, et al.. Impact of polymer aging on main-chain poly(fullerene)s with halide chain-ends for inverted perovskite solar cells. Polymer international, 2025, 10.1002/pi.70054. hal-05371395

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# Impact of polymer aging on main-chain poly(fullerene)s with halide chain-ends for inverted perovskite solar cells

João V. Paulin,ª ID Gabriel L. Nogueira,ª Douglas H. N. Santos,ᵇ

Marco Antônio G. B. Gomes,ᶜ ID Valdemiro P. Carvalho-Jr,ᵇ Luiz C. Silva Filho,ᵈ

Roger C. Hiornsᶜ* ID and Carlos F. O. Graeffª* ID

<figure><img src="image_2.png" /></figure>

## Abstract

As perovskite solar cells (PSCs) advance toward commercialization, the need for stable and easily processable materials becomes increasingly important. In this study, we examine how the aging of polymeric electron transport layers (ETLs) affects the performance of inverted PSCs. Two main-chain poly(fullerene)s, PC₆₀DB(IMP) and PC₆₀DB(ATRAP), which differ only in their terminal halogen groups, were stored under nitrogen, in the dark and at room temperature, for up to 11 months before device fabrication. Devices using aged PC₆₀DB(IMP) showed significant drops in efficiency, while those based on PC₆₀DB(ATRAP) maintained more stable performance and even exhibited reduced hysteresis over time. These contrasting results appear to stem from differences in halogen reactivity, with iodine likely triggering degradation pathways not seen in the bromine-containing counterpart. Altogether, the findings highlight how thoughtful molecular design can improve the long-term stability and practical use of ETL materials in scalable PSC technologies.

© 2025 The Author(s). *Polymer International* published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Supporting information may be found in the online version of this article.

**Keywords:** electron transport layer; perovskite solar cells; poly(fullerene)s; polymers; stability

## INTRODUCTION

The growing concern over climate change has brought urgent attention to the need for cleaner and sustainable energy sources. Global efforts to reduce carbon emissions have pushed the scientific community to explore alternatives to fossil fuels.¹ Among various renewable technologies, solar energy stands out due to its abundance and accessibility.¹ However, to fully realize its potential, the development of efficient, low-cost and scalable photovoltaic materials remains a key challenge.¹,²

In this context, the perovskite solar cell (PSC) has emerged as one of the most promising technologies. Its rapid progress in power conversion efficiency, combined with relatively simple and relatively low-cost fabrication processes, has attracted significant interest in both academic and industrial settings.

A typical PSC consists of multiple functional layers that work together to absorb light, separate charges and transport them to the electrodes.¹⁻⁴ Their layered architecture allows for versatile design and fine-tuning of interfacial processes, which are essential for both device performance and long-term stability. In particular, interfacial transport layers are especially critical. For instance, the electron transport layer (ETL) must efficiently collect photo-generated electrons and block holes to minimize recombination losses.¹⁻⁴

To fulfill these functions, several materials have been explored, with fullerene-based materials, such as C₆₀ and [6,6]-phenyl-C₆₁-butyric acid methyl ester, being among the widely used ETLs due to their suitable energy levels, low recombination energy, high electron affinity and mobility, and solution processability.³,⁴ Their chemical structure also allows for molecular engineering to tune interfacial properties and improve compatibility with

* Correspondence to: CFO Graeff, São Paulo State University (UNESP), School of Sciences, Department of Physics and Meteorology, Bauru/SP, Brazil. E-mail: carlos.graeff@unesp.br; or RC Hiorns, CNRS/Univ Pau & Pays Adour, Institut des Science Analytiques et Physico-Chimie pour l'Environnement et les Materiaux, 64053 Pau, Nouvelle-Aquitaine, France. E-mail: roger-clive.hiorns@cnrs.fr

a Department of Physics and Meteorology, São Paulo State University (UNESP), School of Sciences, Bauru/SP, Brazil

b São Paulo State University (UNESP), School of Technology and Sciences, Presidente Prudente/SP, Brazil

c CNRS/Univ Pau & Pays Adour, Institut des Science Analytiques et Physico-Chimie pour l'Environnement et les Materiaux, Pau, France

d Department of Chemistry, São Paulo State University (UNESP), School of Sciences, Bauru/SP, Brazil
perovskite layers.³,⁴ Despite their advantages, they suffer from
poor solubility in most organic solvents, posing significant chal-
lenges for scalable device fabrication.⁴ Additionally, the long-term
stability of fullerene ETLs and their influence on photovoltaic
parameters remain important areas of investigation.

In response to these challenges, main-chain poly(fullerene)s have recently garnered considerable attention due to their unique physicochemical properties.⁵ These macromolecular systems have demonstrated notable efficacy as functional additives for improving the operational stability and power conversion efficiency of organic photovoltaic devices,⁶ as well as serving as efficient electron-accepting interlayers in PSCs.⁷

A key strategy to address the solubility limitations of fullerene-
based materials involves designing alternating copolymers that
include comonomers which improve solubilities, such as
alkyl- or oligoether-substituted units. These solubilizing segments
effectively disrupt the strong intermolecular interactions and
aggregation tendencies of fullerene moieties. This, coupled with
the linear quality of polymeric chains, enables the formation of
high-quality thin films via solution processing. Among the notable
advances in this field are synthetic methodologies such as atom
transfer radical addition polymerization (ATRAP),⁸⁻¹⁰ sterically
controlled azomethine ylide cycloaddition polymerization,¹¹⁻¹³
benzyl-azide polymerization¹⁴ and iodine-mediated polymeriza-
tion (IMP),¹⁵ which have significantly expanded the structural
diversity and solubility of main-chain poly(fullerene)s.

In the present paper, we focus on the application of poly{fullerene-alt-[1,4-dimethylene-2,5-bis(oxydodecylether)benzene]} (PC₆₀DB; Fig. 1). PC₆₀DB was synthesized by both ATRAP and IMP methodologies to give PC₆₀DB(ATRAP) and its IMP variant PC₆₀DB(IMP), respectively. Note that PC₆₀DB(ATRAP) has also been called HSS-12,¹⁰ but here it is denoted PC₆₀DB(ATRAP) to facilitate reading and comparison. Due to the radical nature of both ATRAP and IMP reactions and the large comonomers, both polymers show relatively well-defined structures. In both cases, a single hexagonal ring of the C₆₀ cage is disrupted via a 1,4-radical addition of comonomers, although occasionally the second addition can occur on an adjacent hexagon.⁸,¹⁵ The large side-chains exclude multiple reactions occurring around the C₆₀ moieties to ensure a linear chain, and also bestow an exceptional solubility. The PCDBs can readily dissolve even in solvents typically incompatible with C₆₀, such as tetrahydrofuran.

The PC₆₀DB(IMP) (68% yield) and PC₆₀DB(ATRAP) (87% yield) poly-
mers showed respective number-average molar masses (M_n) esti-
mated by steric exclusion chromatography of 19.25 and

11.34 kg mol⁻¹, and dispersities (D = M<sub>w</sub>/M<sub>n</sub>) of 1.88 and 1.51 (Figs S1 and S2). The NMR profiles (Figs S3 and S4) are consistent with those reported in our reference article, where PC₆₀DB(IMP) was used in a photovoltaic device.¹⁵

Building on this synthetic foundation, our present study investi-
gated an overlooked aspect of ETL material performance: the effect
of polymer aging before device fabrication. While much research
has focused on the operational degradation of PSCs, less is known
about how storage-related changes in the transport materials them-
selves can impact device efficiency and stability. Therefore, we aim
to shed light on how chemical structure influences long-term pro-
cessability and interfacial behavior in PSCs by comparing the perfor-
mance of PC₆₀DB(IMP) and PC₆₀DB(ATRAP) polymers that were stored in
a nitrogen-filled glovebox in the dark and at room temperature in
three different aging conditions: fresh (ca 20 days old), mid-aged
(ca 6 months old) and aged (ca 11 months old) prior to integration
in the PSC as ETLs. To be doubly clear, we are not testing the aging
of a whole cell, but rather the impact of polymer aging on device effi-
ciency as made. A controlled environment was used to exclude the
presence of moisture and oxygen, allowing us to isolate material-
intrinsic degradation phenomena.

RESULTS AND DISCUSSION

Figure 2(a),(b) shows the current density–voltage (J–V) charac-
teristics of the devices, following use of differently aged poly-
mers, in both forward and reverse scanning directions. In
Fig. 2(a), PSC incorporating PC₆₀DB(IMP) shows an apparent
and gradual decline in photovoltaic performance with the var-
iant fresh and aged polymers. The device with fresh polymer
(green curve) behaves almost like an ideal solar cell, with high
current output and minimal hysteresis between forward and
reverse scans. In the mid-aged state (yellow curve), both
short-circuit current density (J<sub>SC</sub>) and fill factor (FF) drop
noticeably, and the curve becomes less rectangular. This
decline becomes even more pronounced with the aged poly-
mer (red curve), which results in devices with low current, a dis-
torted J–V behavior and signs of significant hysteresis. These
results indicate that the PC₆₀DB(IMP) polymer loses its ability
to form an efficient electron-transporting interface when
stored for long periods, even under an inert atmosphere.

In contrast, Fig. 2(b) shows that devices based on PC₆₀DB(ATRAP) maintain relatively stable J–V characteristics with aging. Since the purpose of this comparison is to evaluate the effects of aging, the fresh PC₆₀DB(IMP) device was used as a reference for both polymer systems. Although performance degrades with time, the transition is less dramatic. Indeed, the J–V curves for mid-aged and aged PC₆₀DB(ATRAP) retain their shape, and J<sub>SC</sub> and FF decrease gradually rather than sharply. The open-circuit voltage (V<sub>OC</sub>) shows smaller changes over time compared to devices with PC₆₀DB(IMP) under forward scanning. Interestingly, the hysteresis appears to be reduced in the aged samples, and its direction is reversed. These results suggest that the polymer may undergo some slow reorganization over time that leads to improved interfacial contact or fewer trap states.

These patterns are quantitatively presented in Table 1 with the
statistical plot in Fig. S5. For PC₆₀DB(IMP), J<sub>SC</sub> drops from around
26 mA cm<sup>-2</sup> in the fresh state to just 12 mA cm<sup>-2</sup> after aging.
Power conversion efficiency (PCE) follows the same trend, falling
from over 17% to under 5%. The FF and V<sub>OC</sub> also decrease,
reflecting less efficient charge extraction and higher recombina-
tion. The hysteresis index (HI), estimated via |P<sub>CERev</sub> - P<sub>CERwd</sub>| / P<sub>CERev</sub> |, rises

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Polymerization scheme. PC₆₀DB(IMP): (a) 18-crown-6 ether, KI, toluene, 140 °C, 22 h; X = I. PC₆₀DB(ATRAP): (b) 2,2'-bipyridine, CuBr, toluene, 140 °C, 22 h; X = Br.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** J-V characteristics of inverted PSCs using (a) PC₆₀DB(IMP) and (b) PC₆₀DB(ATRAP) as ETL under different aging conditions: fresh (green), mid-aged (yellow) and aged (red). Solid lines represent forward scans, while dashed lines indicate reverse scans. EQE spectra and corresponding integrated *J*<sub>SC</sub> for (c) PC₆₀DB(IMP)⁻ and (d) PC₆₀DB(ATRAP)-based devices.</figcaption></figure>

from 0.03 to 0.17 as the devices age, further indicating instability in charge transport or extraction. On the other hand, PC₆₀DB(ATRAP)-based devices show better stability under storage. The PCE decreases from around 12% to about 5.5% mostly due

to a moderate reduction in *J*<sub>SC</sub> and FF. The HI drops from 0.15 to 0.06.

External quantum efficiency (EQE) spectra and their integrated current curves are exhibited in Fig. 2(c),(d). The integrated current

<table><caption>**Table 1.** Photovoltaic parameters of inverted PSCs employing PCDB-based ETLs at different aging stages. Data were obtained from forward (Fwd) and reverse (Rev) scans under simulated AM 1.5G illumination. The table reports the open-circuit voltage (*V*<sub>OC</sub>), short-circuit current density (*J*<sub>SC</sub>), fill factor (FF), power conversion efficiency (PCE) and hysteresis index (HI). For each condition, the best-performing device is highlighted in Fig. S5</caption><thead><tr><th>Sample</th><th>Aging</th><th>Scan</th><th>V<sub>OC</sub> (V)</th><th>J<sub>SC</sub> (mA cm<sup>-2</sup>)</th><th>FF (%)</th><th>PCE (%)</th><th>HI</th></tr></thead><tbody><tr><td rowspan="6">PC<sub>60</sub>DB(IMP)</td><td rowspan="2">Fresh</td><td>Fwd</td><td>1.03 ± 0.01</td><td>26.02 ± 1.42</td><td>62.21 ± 3.90</td><td>16.70 ± 1.42</td><td rowspan="2">0.03 ± 0.06</td></tr><tr><td>Rev</td><td>1.02 ± 0.04</td><td>26.66 ± 1.37</td><td>63.67 ± 6.20</td><td>17.29 ± 2.18</td></tr><tr><td rowspan="2">Mid-aged</td><td>Fwd</td><td>0.96 ± 0.07</td><td>18.76 ± 2.44</td><td>54.99 ± 9.55</td><td>10.17 ± 3.36</td><td rowspan="2">0.14 ± 0.10</td></tr><tr><td>Rev</td><td>0.94 ± 0.11</td><td>19.06 ± 2.31</td><td>63.39 ± 7.95</td><td>11.71 ± 3.40</td></tr><tr><td rowspan="2">Aged</td><td>Fwd</td><td>0.87 ± 0.12</td><td>12.22 ± 3.40</td><td>37.95 ± 6.00</td><td>4.06 ± 1.37</td><td rowspan="2">0.17 ± 0.09</td></tr><tr><td>Rev</td><td>0.82 ± 0.03</td><td>11.95 ± 3.09</td><td>50.19 ± 11.94</td><td>5.01 ± 1.95</td></tr><tr><td rowspan="6">PC<sub>60</sub>DB(ATRAP)</td><td rowspan="2">Mid-aged</td><td>Fwd</td><td>0.97 ± 0.04</td><td>21.26 ± 2.00</td><td>56.18 ± 3.19</td><td>11.67 ± 1.61</td><td rowspan="2">0.15 ± 0.11</td></tr><tr><td>Rev</td><td>0.98 ± 0.04</td><td>21.55 ± 1.91</td><td>64.75 ± 7.24</td><td>13.73 ± 2.57</td></tr><tr><td rowspan="2">Aged</td><td>Fwd</td><td>0.92 ± 0.06</td><td>15.74 ± 1.86</td><td>36.78 ± 5.62</td><td>5.40 ± 1.43</td><td rowspan="2">0.06 ± 0.08</td></tr><tr><td>Rev</td><td>0.85 ± 0.05</td><td>16.12 ± 1.62</td><td>40.79 ± 4.60</td><td>5.62 ± 1.23</td></tr></tbody></table>
densities show good agreement with the J<sub>SC</sub> values obtained from the J-V measurements. Notably, the EQE spectra of the devices exhibit pronounced variation as a function of polymer aging.

The fresh PSC displays a high EQE reaching ca 80%, indicating
efficient photogenerated charge collection at short-circuit condi-
tions. In contrast, the diminished EQE observed in the mid-aged
and aged ones suggests reduced electron extraction and higher
recombination at the perovskite (PVK)/ETL interface, which is
compatible with the relatively low FF.<sup>16</sup>

The PC₆₀DB(IMP)-based EQE spectra show a significant reduction in intensity predominantly within the central visible range (400-650 nm), Fig. 2(c) (red curve), suggesting aging-induced deterioration of charge collection. In contrast, although PC₆₀DB(ATRAP)-based devices exhibit an overall decrease in EQE with polymer aging, the drop is less pronounced at around 350 nm, indicating retained charge collection at higher energies.

Despite pronounced variations in the EQE magnitude and
spectral profiles, the extracted photovoltaic band gap (E<sub>g</sub><sup>PV</sup>)
remains centered around 1.64 eV (Fig. S6), which is consistent
with the expected value for the PVK composition used.<sup>17,18</sup> Fur-
thermore, the aging process induced only a minor shift of less
than 10 meV in the respective E<sub>g</sub><sup>PV</sup> values. Therefore, the
changes in device performance after storage of the ETL mate-
rials cannot be explained by degradation or stoichiometric
alterations of the PVK layer.

Since the PC₆₀DB-based polymers were aged independently in
the solid state under an inert atmosphere and in the dark, and
both the PVK and the polymers were freshly processed for each
device, the observed trends must originate from inherent
changes within the polymer itself and how these changes impact
interfacial interactions upon integration.

In particular, devices incorporating the iodine-terminated poly-
mer (PC₆₀DB(IMP)) show a clear decline in efficiency and increased
hysteresis with aging. This trend may be related to slow chemical
transformations in the carbon–iodine terminal bond, which is
known to be relatively weak and highly polarizable.¹⁹ Over time,
such bonds may undergo cleavage, internal rearrangement or
even promote crosslinking between polymer chains. These subtle
molecular changes, although occurring in the solid state, could
impact how the polymer interacts with the PVK surface upon
device fabrication. Once in contact with the PVK, the polymer
may increase the density of interfacial trap states, forming local
barriers that hinder charge extraction and promote recombina-
tion.²⁰,²¹ Additionally, the possible aggregation of excess C₆₀
domains in iodine-functionalized ETLs may exacerbate charge
trapping and hinder charge extraction.²²

On the other hand, devices based on the bromine-terminated
analogue (PC<sub>60</sub>DB<sub>(ATRAP)</sub>) exhibit a more stable performance and
a reduced hysteresis after aging. The stronger and less polarizable
carbon-bromine bond<sup>19</sup> is more resistant to degradation under
storage, and its presence may contribute to a more stable inter-
face by passivating interfacial iodine vacancies and limiting ion
migration. The gradual decrease in hysteresis may also indicate
that mild structural reorganization during aging leads to
improved interfacial contact.<sup>23</sup>

Although these changes may not be visible at the macroscopic
level, they can still interfere with charge movement across the
interface, ultimately affecting the photovoltaic efficiency. Taken
together, the results show that even relatively small differences
in halogen chemistry can have a meaningful impact on interfacial
behavior, with bromine termination helping to create a more sta-
ble and reliable interface.

These observations point to the significant influence of end-
group chemistry on the functional stability of polymeric ETLs.
Although both materials share a similar backbone, the difference
in halogen termination leads to distinct outcomes after storage.
We hypothesize that, due to the very weak carbon–iodine bond,
the iodine appears to promote gradual degradation, while bro-
mine helps preserve or even slightly improve the material’s inter-
facial performance characteristics.

To gain deeper insight into these effects, further investigations
are being conducted using spectroscopic, morphological and
electrical methods. These studies aim to clarify how molecular
structure affects film formation, interfacial quality and charge
dynamics in PSCs. The results will contribute to the design of more
stable and process-compatible ETLs, which is essential for the
development of scalable and long-lasting PVK technologies.

CONCLUSION

This study sheds light on the often-overlooked influence of pre-
fabrication aging on polymer-based ETLs in PSCs. Even when
stored in inert conditions, subtle differences in polymer structure
can lead to distinct degradation pathways. The pronounced insta-
bility observed in iodine-terminated PC₆₀DB(IMP), compared
to the more stable performance of PC₆₀DB(ATRAP), underscores
the importance of halogen termination in maintaining interfacial
quality over time. Gaining a deeper understanding of these intrin-
sic aging processes will be essential for improving the reliability
and manufacturability of perovskite solar technologies.

ACKNOWLEDGEMENTS

This study was financed, in part, by the São Paulo Research Foun-
dation (FAPESP), Brazil, grant numbers 2020/12356-8, 2022/
10998-8, 2023/01117-0, 2023/12686-6, 2024/02935-1, 2025/
00301-8; and by the Brazilian National Council for Scientific and
Technological Development (CNPq) grant number 303750/2024-
3. JVP is also grateful for the support of the São Paulo State Univer-
sity research office (PROPe) postdoctoral fellowship (05/2024).
CFOG is grateful for the support of CNPq under the Instituto
Nacional de Ciência e Tecnologia de Nanomateriais para a Vida
(NanoVIDA) no. 406079/2022-6 and FINEP (grant: 01.22.0289.00
(0034-21)). MAGBG and RCH received funding from the European
Horizon program under grant agreement HORIZON-CL5-2024-D3-
01-02-EFFECTOR-101172820. The Article Processing Charge for
the publication of this research was funded by the Coordenação
de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES)
(ROR identifier: 00x0ma614).

SUPPORTING INFORMATION

Supporting information may be found in the online version of this
article.

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