RESEARCH ARTICLE
www.afm-journal.deHigh-Performance Nanogap Photodetectors Based on 2D
Halide Perovskites with a Novel Spacer Cation
Yi Shen, Linqu Luo, Yuxuan Zhang, You Meng, Yan Yan, Pengshan Xie, Dengji Li, Yu Ji,
Siliang Hu, SenPo Yip, Zhengxun Lai,* Thomas D. Anthopoulos, and Johnny C. Ho*

2D Ruddlesden─Popper (RP) halide perovskites are attracting increasing
research interest due to their enhanced stability compared to 3D perovskites.
However, the quantum conﬁnement eﬀect of bulk organic spacers
hinders the separation and transport of photo-generated carriers. Here, amultiple aromatic ring spacer, 3-benzothiophene methylammonium (BTMA),
is developed for a new 2D RP perovskite. The BTMA spacer is demonstrated,with a signiﬁcant dipole moment, can impair the inﬂuence of the quantum
conﬁnement eﬀect, and the presence of S atoms or thiophene is favorable
for enhancing the interaction between organic spacers and inorganic sheets,
improving the stability of perovskites. The perovskite photodetector with
BTMA as spacers displays higher device performance than the control sample
with 1-naphthalene methylammonium (NMA) as spacers. Importantly,
the outstanding stability of BTMA-based perovskite ﬁlms and devices is also
conﬁrmed under moisture, heat, and illumination conditions. Combining theasymmetric coplanar nanogap electrode architecture, the photodetectors’ enhanced responsivity, detectivity, and external quantum eﬃciency of 314 A W−1 ,3.4 × 1013 Jones, and 865%, respectively, are demonstrated. Importantly,
the nanogap photodetectors display promising self-power characteristics,
which makes them attractive for numerous energy-eﬃcient applications.
The work highlights a new route toward developing high-performance
2D RP perovskite-based photodetectors with excellent long-term stability.

Y. Shen, Y. Zhang, Y. Meng, Y. Yan, P. Xie, D. Li, S. Hu, J. C. Ho
Department of Materials Science and Engineering
City University of Hong Kong
Kowloon, Hong Kong SAR 999077, China
E-mail: johnnyho@cityu.edu.hk
L. Luo, T. D. Anthopoulos
Physical Science and Engineering Division
King Abdullah University of Science and Technology
Thuwal 23955-6900, Saudi Arabia

The ORCID identiﬁcation number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.202403746© 2024 The Author(s). Advanced Functional Materials published by
Wiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial-NoDerivs License,
which permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial and no modiﬁcations
or adaptations are made.

DOI: 10.1002/adfm.202403746

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1. Introduction
In recent years, halide perovskites (ABX3 ,A is Cs+ or CH3 NH3+ ; B is Pb2+ or Sn2+ ;X is halogen) have been garnering increasing attention due to their exceptional photovoltaic conversion capabilities, which rival or even exceed those of silicon-based
devices.[1–6] However, the further commercialization of the halide perovskites is hindered by their poor stability.[7,8] A promising strategy to enhance their stability is using large-volume hydrophobic amine ionsto transform the 3D halide perovskites intoa 2D structure.[9–11]
Ruddlesden─Popper (RP) halide perovskites (R2 BX4 , R is a bulky monoamine)
are a signiﬁcant category in 2D perovskites
and have been extensively researched.[12–14]
However, the energy level of these inserted organic spacer layers is misalignedwith that of the inorganic octahedral
layers, which results in a diminishedinterlayer transport capability for photogenerated electrons and holes in 2D halide
perovskites.[15,16] Developing spacers withhigh dipole moments and suitable volume

Y. Ji
School of Optoelectronic Science and Engineering
University of Electronic Science and Technology of China (UESTC)
Chengdu 610054, China
S. YipInstitute for Materials Chemistry and Engineering
Kyushu University
Fukuoka 816-8580, Japan
Z. LaiCollege of Semiconductors (College of Integrated Circuits)
Hunan University
Changsha 410082, China
E-mail: zxlai3-c@my.cityu.edu.hk
T. D. Anthopoulos
Henry Royce Institute
Photon Science InstituteDepartment of Electrical and Electronic Engineering
The University of Manchester
Manchester M13 9PL, UK
J. C. Ho
State Key Laboratory of Terahertz and Millimeter Waves
City University of Hong Kong
Kowloon, Hong Kong SAR 999077, China© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

www.afm-journal.deis considered an eﬀective strategy to enhance the performanceof 2D perovskite devices.[17] So far, the reported monoamine
cations are mainly single aromatic ring cations and aliphatic
cations, such as propylammonium (PA),[18] isobutylammonium(iBA)[19] and phenylethylammonium (PEA).[11] Aromatic spacers always have more signiﬁcant polarity and conjugation thanaliphatic spacers, which leads to their high electrical conductivity and weakened quantum conﬁnement eﬀect in 2D RP
perovskites.[14] Moreover, multiple aromatic ring spacers withextensive 𝜋-electron delocalization in the conjugated framework can further promote the carrier transport capability in 2D
perovskites.[20–24] In previous reports, Passarelli et al. incorporated naphthalene-, pyrene-, and perylene-based spacers into 2D
perovskites with the n value of 1 and found their excellent outof-plane conductivity.[21] Xu et al. used 1-naphthalene methylammonium and 9-anthracene methylammonium as spacer cationsof 2D perovskites and achieved impressive power conversion
eﬃciency (PCE) for corresponding solar cells with long-term
stability.[23] Subsequently, 1-naphthylamine (1-NA) was developed as spacers by Jin et al.[24] They pointed out that the strong
𝜋-𝜋 interaction of naphthylamine can stabilize the perovskite
lattices and enhance the quality of perovskite ﬁlms simultaneously. Notably, the commonly used multiple aromatic ring spacers in photoelectric devices are mainly naphthyl-based, especiallyin light-emitting diodes. The study for other multiple aromaticring spacers remains unexplored.
Apart from the inherent properties of perovskite materials,
the inﬂuence of the device structure on the performance isalso signiﬁcant. For photodetectors, the typical structure is avertical type, but it has a special requirement for selecting
transparent electrodes. The other coplanar structure is convenient to fabricate photodetectors. Furthermore, for this structure, the reduction of inter-electrode distance can achieve a signiﬁcant enhancement in device performance. Also, in recent
years, a novel adhesion lithography technique was developed toprepare nanogap coplanar electrodes, which have been widelyused in diodes, ﬁeld-eﬀect transistors, and photodetectors owing to their low cost and high device performance.[25,26] However, few studies have been reported on photodetectors withultra-narrow nanogap between electrodes based on layered 2D
perovskites.
Therefore, in this work, we designed and synthesized a novel
multiple aromatic ring spacer, 3-benzothiophene methylammonium (BTMA), and used it to conﬁgure a new kind of layered
perovskite. Compared with a 1-naphthalene methylammonium(NMA) spacer, introducing an S atom or single thiophene increases the molecule’s polarity. It reinforces the interaction between organic spacers and inorganic PbI6 sheets in 2D perovskites. The device performance of BTMA-based perovskite photodetector is demonstrated to be superior to that of the NMAbased perovskite photodetectors while exhibiting better stability under moisture, heat, and illumination conditions. Furthermore, the utilization of nanogap electrodes in BTMA-based perovskite photodetectors signiﬁcantly promotes the properties ofdevices, and the responsivity, detectivity, and external quantum eﬃciency can reach high values of 314 A W−1 , 3.4 × 1013
Jones and 865%, respectively. This work oﬀers a strategy forpromoting the optoelectronic properties of 2D RP perovskite
photodetectors.

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2. Results and Discussion
Previous reports have proved that the S atom or thiophene
can strengthen the electric coupling between organic layers andoctahedral sheets, stabilizing the perovskite lattices.[27–29] Inspired by this character, we intelligently designed and synthesized a new monoamine cation with a long-conjugated length, 3benzothiophene methylammonium (BTMA). For this spacer, its
superiority is mainly reﬂected in the following three aspects, as
shown in Figure 1a: i) The extensive delocalization of 𝜋-electronthat BTMA possesses can contribute to its enhanced conductivity compared with single aromatic ring spacers.[21] ii) The anticipated low symmetry is expected to endow the BTMA spacer withgreater polarity in favor of mitigating the impact of quantum conﬁnement eﬀects in 2D RP perovskites and enhancing the transport of carriers. iii) More importantly, the presence of S atom inthe thiophene ring is expected to further increase the stability of
RP perovskites by reinforcing the interaction of spacers with PbI6
sheets.
To demonstrate the superiority of the BTMA spacer, we used
1-naphthalene methylammonium (NMA) as the control sample.
We prepared the 2D RP perovskites, BTMA2 PbI4 and NMA2 PbI4 ,named BTMA-Pb and NMA-Pb, respectively. Density Functional
Theory (DFT) was employed to study molecules’ structure anddipole moment. Figure 1b shows the chemical structures andelectrostatic potential (ESP) maps of BTMA and NMA spacers.
As expected, the calculated dipole moment of the BTMA spacer
is 1.73 D, more signiﬁcant than that of the NMA spacer (1.43 D).
The higher polarity of the BTMA spacer is believed to increase thedielectric constant, thereby weakening the dielectric mismatch
between organic and inorganic PbI6 layers.[15,30] Also, the increased polarity can potentially reduce the exciton binding energyof 2D RP perovskites and facilitate the separation and transport ofphotogenerated carriers.[30] To prove the regulation eﬀect of the
BTMA spacer, the relative dielectric constants of BTMA-Pb and
NMA-Pb ﬁlms were measured through an impedance analyzer.
The results are shown in Figure S7, Supporting Information,
where the dielectric constants of BTMA-Pb and NMA-Pb ﬁlms
are ≈6.6 and ≈6.1, proving that the introduction of BTMA spacer
can increase the dielectric constant of 2D perovskites. Besides,
compared to the NMA spacer’s benzene ring, the BTMA spacer’s
thiophene on the NH3+ side has a smaller volume, which is beneﬁcial for a tighter binding with inorganic PbI6 octahedrons. Thisis well proved by the calculated perovskite structures for BTMAPb and NMA-Pb. Figure S5, Supporting Information, shows thatthe penetration depth (deﬁned by the distance of nitrogen atomof NH3 + with the plane of I atoms of inorganic PbI6 sheets) of
simulated structure for BTMA-Pb and NMA-Pb are 0.435 and0.405 Å, respectively.[31] The deeper depth suggests that the NH3+of BTMA spacer has a stronger bond with PbI6 octahedrons. To
further investigate the interaction between the spacers and perovskite sheets, X-ray photoelectron spectroscopy (XPS) was conducted on the BTMA-Pb and NMA-Pb perovskite ﬁlms. The results are displayed in Figure 1c, in which the main peaks for
Pb are found at 137.6 and 142.5 eV for BTMA-Pb ﬁlms, while
the corresponding peaks of NMA-Pb ﬁlms are observed at 137.8
and 142.6 eV. These two peaks can be assigned to the Pb 4f7/2
and Pb 4f5/2 , respectively. The apparent shift toward lower binding energy of BTMA-Pb ﬁlms suggests the diﬀerent chemical© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.deFigure 1. a) Schematic diagram for interaction between BTMA spacers and inorganic PbI6 sheets. b) The chemical structure and ESP of NMA and BTMA
spacers. c,d) The XPS spectrum of Pb 4f for the NMA-Pb and BTMA-Pb ﬁlms and S 2p for the BTMA-I powder and the BTMA-Pb ﬁlm.

environment for Pb atoms, possibly caused by more vital interaction between BTMA spacers and PbI6 octahedral sheets compared with the NMA-Pb ﬁlms. Meanwhile, additional tests on
the binding energy (shown in Figure 1d) of S atoms in BTMAPb ﬁlms and BTMA-I powder (3-benzothiophene methylamine
iodide power, which is used to prepare perovskite precursor solution) reveal that the S 2p peaks of the latter have higher binding energy in BTMA-Pb ﬁlms, which also suggests the strong interaction between BTMA spacers and PbI6 octahedrons.[28] Thisenhanced interaction is expected to stabilize the RP perovskite
lattices better.
It is well-known that crystallinity is a crucial factor aﬀecting
the properties of 2D RP perovskite ﬁlms. Therefore, X-ray diﬀraction (XRD) was measured to study the crystallinity of BTMA-Pb
and NMA-Pb perovskite ﬁlms. Figure 2a shows the XRD peaksof BTMA-Pb and NMA-Pb perovskite ﬁlms, and the corresponding simulated XRD curves can be seen in Figure S6, Supporting
Information. Both BTMA-Pb and NMA-Pb ﬁlm samples show
strong XRD intensities, indicating their high crystallinity. Combined with the calculated XRD maps for BTMA-Pb and NMAPb perovskite lattices, all these peaks can be assigned to the(00n) planes, as Figure 2a shows, which suggests highly oriented
growth of these ﬁlms along the direction of [001]. As we know,
it is slow for carriers to transport in organic layers or transport
between organic layers and inorganic layers in 2D perovskite lattices. Therefore, the highly ordered growth direction of BTMAPb and NMA-Pb perovskite lattices can provide the condition forphoto-generated carriers to transport in inorganic PbI6 layers asmuch as possible, thereby promoting the collection of the charges
and enhancing the optoelectronic properties (Figure 2b). Additionally, the UV–vis absorption of BTMA-Pb and NMA-Pb perovskite ﬁlms was also analyzed, and the spectrum is shown in
Figure 2c. Only one sharp absorption edge is observed around
500 nm for BTMA-Pb and NMA-Pb ﬁlms, manifesting their purephase of 2D perovskites. The band gaps for BTMA-Pb and NMAPb perovskite ﬁlms can be determined to be around 2.38 and2.40 eV, respectively, through the analysis for the corresponding

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Tacu plot shown (Figure S9, Supporting Information). Moreover,
the results of the photoluminescence (PL) spectrum for BTMAPb and NMA-Pb ﬁlms under an excitation laser with 450 nm
wavelength are shown in Figure 2d. The main peaks appear
around 525 nm for two samples. As we know, better crystallinity
always leads to a narrower PL peak and is advantageous for carriers’ transport. In this case, it is worth noting that the full-widthat half-maximum (FWHM) of the PL peak of BTMA-Pb ﬁlm isnarrower than that of NMA-Pb ﬁlm, which indicates the more excellent crystallinity for BTMA-Pb ﬁlm. To further study the crystallinity and morphology of BTMA-Pb and NMA-Pb perovskite
ﬁlms, we conducted the measurement of scanning electron microscopy (SEM) and atomic force microscopy (AFM) on these two
ﬁlms. From the SEM images in Figure 2e, it can be observed that
the NMA-Pb perovskite ﬁlm has many cracks, which is detrimental to the transport of charge carriers. However, the BTMA-Pb perovskite ﬁlm has a rugged surface but is very dense and continuous, beneﬁting the carrier transport in perovskite lattices. What’smore, a low value of 2.59 nm is obtained for the root-mean-square(RMS) roughness of BTMA-Pb perovskite ﬁlm, smaller than thatof NMA-Pb perovskite ﬁlm (RMS = 11.9 nm), as shown in AFM
results of Figure 2f. These characterizations demonstrate thatthe BTMA-Pb perovskite possesses superior crystallinity and ﬁlm
quality, potentially providing it with better photoelectric performance.
Subsequently, the BTMA-Pb and NMA-Pb perovskite ﬁlms
were fabricated into photodetectors to explore their photoelectricperformance in detail. The structure of photodetectors is shownin Figure S10, Supporting Information, wherein two gold electrodes are deposited onto the 2D perovskite ﬁlm. The active areaof the photodetectors is stimulated using a laser light source(450 nm). The current–voltage (I–V) curves for these photodetectors were evaluated under varying light intensities, as well as
in the absence of light, as shown in Figure 3a,b. It is noted thatthe x-axis intersection shifts of I–V curves are possibly causedby the ionic migration of perovskites, which always leads to theformation of the internal electric ﬁeld.[32] The on/oﬀ radio for© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.deFigure 2. a) XRD of the BTMA-Pb and NMA-Pb ﬁlms. b) Schematic diagram of the carrier transfer in highly ordered perovskite lattices. c) UV–Vis
absorption spectrum of the BTMA-Pb and NMA-Pb ﬁlms. d) PL spectrum of the BTMA-Pb and NMA-Pb ﬁlms. e) SEM images of the BTMA-Pb and
NMA-Pb ﬁlms. f) AFM images of the BTMA-Pb and NMA-Pb ﬁlms.

BTMA-Pb and NMA-Pb photodetectors can reach a high valueof 103 . Besides, the linear results of I–V curves (Figure S11,
Supporting Information) also indicate nearly ohmic contact between the electrodes and the 2D perovskite ﬁlms, which is advantageous for charge collection. For the BTMA-Pb perovskite
photodetector, the dark current is found to be 1 × 10−13 A under a bias of 1.5 V. But, the NMA-Pb perovskite device shows asmaller dark current, which is as low as 3 × 10−14 A under thesame bias. The lower crystallinity and ﬁlm quality of NMA-Pb
perovskite and the higher quantum conﬁnement eﬀect caused
by NMA spacers with smaller dipole moments hinder the ﬂow ofcharges. The time-dependent photoresponse characteristics werealso studied. The results of current–time (I–t) measurement under a light intensity of 121 mW cm−2 and 1.5 V bias voltage areshown in Figure 3c. The on/oﬀ photoswitching characteristics

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of BTMA-Pb and NMA-Pb perovskite photodetectors are stable
and invertible. Figure 3d displays the curves correlating various
light intensities with the corresponding photocurrent, exhibiting a clear linear relationship,[33] which is a typical phenomenon
in semiconductor-based photodetectors because of the uniqueprocess of generation, trapping, and recombination of electron–
hole pairs. The linear dynamic range (LDR) can be calculated using the equation: LDR = 20log(Pupper /Plower ), where Plower and
Pupper represent the lowest and highest light intensities used inthis work. The BTMA-Pb and NMA-Pb perovskite devices display an obvious linear response from 0.0122 to 121.89 mW cm−2 ,which indicates a large LDR of 80 dB. The responsivity (R) is
an important parameter characterizing the performance of photodetectors and is determined by the formula of R = Ip /ΦS,
where Ip is the photocurrent, Φ is the light intensity, S is the© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.deFigure 3. a,b) I–V curves of photodetectors based on BTMA-Pb and NMA-Pb ﬁlms with varying illumination intensities. c) On-oﬀ switching curves of the
BTMA-Pb and NMA-Pb perovskite-based photodetectors. d–g) The relationship of light intensity with d) photocurrent, e) responsivity, f) detectivity, andg) EQE of the BTMA-Pb and NMA-Pb perovskite photodetectors. Determination of the rise and fall times of the h) BTMA-Pb and i) NMA-Pb perovskite
photodetectors at a bias of 1.5 V.

active area of the photodetector.[30] As illustrated in Figure 3e, the
BTMA-Pb perovskite photodetector exhibits a superior R valueof 122 mA W−1 , which is signiﬁcantly higher compared to the
NMA-Pb perovskite device (15 mA W−1 ) under a light intensityof 0.0121 mW cm−2 . Detectivity (D*) characterizes the weakest
level of light that the device can detect, which is determined by
the responsivity and noise of the photodetector. The D* is calculated by the formula of D* = S1/2 /NEP, where NEP is thenoise equivalent power which can be calculated by the formulaof NEP = In /R (In is the current noise spectral density). Generally, there are three contributions to noise that limit D*: the shotnoise from dark current, Johnson noise, and thermal ﬂuctuation“ﬂicker” noise. For simply estimating the D* value, assumingthat shot noise from the dark current is the major contributor
to the total noise, the shot noise current of the detector can beevaluated by in = (2eIdark Δf)1/2 , where Idark is the dark current,
Δf is the electrical bandwidth and e is the electronic charge.[34]Adv. Funct. Mater. 2024, 2403746

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Therefore, the speciﬁc detectivity (D*) can be calculated by theformula of D* = S1/2 /NEP = RS1/2 /(2eIdark )1/2 . The most signiﬁcant D* values are calculated as 6 × 1011 and 1 × 1011 Jones
at 0.0121 mW cm−2 light intensity for BTMA-Pb and NMA-Pb
perovskite photodetectors, respectively, as shown in Figure 3f.
Moreover, according to the formula of EQE = hcR/e𝜆, where hrepresents the Planck’s constant, c represents the velocity of lightand 𝜆 represents the wavelength of the incident light, the external
quantum eﬃciency (EQE) of photodetectors can also be obtained.
Figure 3g shows that the maximum EQE values of perovskite device samples based on BTMA and NMA spacers can reach 33%
and 4%, respectively. Similar to the R, the D* and EQE values of
BTMA-Pb devices are higher than those of NMA-Pb devices. The
better performance for BTMA-Pb perovskite photodetector canbe attributed to the great crystallinity and high-quality ﬁlm, as
well as the weakened quantum conﬁnement eﬀect in 2D RP perovskite lattices. These factors are in favor of the transport of the© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.dephotogenerated carriers. In addition, a short response time is essential for high-performance photodetectors. Accordingly, highresolution I–t curve testing was employed to study the response
rate of the device samples at the light intensity of 121 mW cm−2 .
Generally, the response time is characterized by two key metrics: the rise time and the decay time, which are deﬁned as theduration required for the current to escalate from 10% to 90%
and diminish from 90% to 10% of the peak value, respectively.
As depicted in Figure 3h,i, the BTMA-Pb perovskite photodetector exhibits a faster rise and decay time of 440 and 530 μs, respectively, in contrast to the NMA-Pb perovskite device, whererise and decay times of 740 and 820 μs are observed. The higher
response speed of BTMA-Pb devices is proﬁting from the increased charge transport ability. Consequently, all these improved
metrics, including responsivity, detectivity, EQE, and response
time, demonstrate the superior performance of the BTMA-Pb
perovskite photodetector device.
In addition, to further study the quality of BTMA-Pb and NMAPb ﬁlms, we performed the space-charge-limited current (SCLC)
measurement as shown in Figure S12, Supporting Information.
It can be seen that with the bias voltage increasing, ohmic, trapﬁlled limit (TLF), and Child’s regions are clearly separated.[35,36]
The VTFL (related to the trap state ntrap ) for BTMA-Pb and NMAPb ﬁlms are 1.9 and 4.38 V, respectively. The ntrap can be calculated from the formula of ntrap = 2𝜖𝜖 0 VTFL /eL2 , where 𝜖 is therelative dielectric constant, 𝜖 0 is the vacuum permittivity, and L
is the length between two electrodes. As a result, the BTMA-Pb
ﬁlm displays a trap density of 3.4 × 1012 cm−3 , which is lower
than that of NMA-Pb ﬁlm (7.2 × 1012 cm−3 ), indicating higher
ﬁlm quality of BTMA-Pb perovskite ﬁlm. Also, by ﬁtting the third
segment (Child’ region) of the SCLC curve with the formula of
J = 9𝜖𝜖 0 μV2 /8L3 (J is the current density), we obtained the carrier mobility, μ, of BTMA-Pb and NMA-Pb ﬁlms being ≈5.16 ×
10−3 and ≈ 3.27 × 10−4 cm2 V−1 s−1 , respectively.
The stability of perovskite materials is critical in determining
their potential for widespread commercialization. Therefore, toexplore the stability of BTMA-Pb and NMA-Pb ﬁlms under various external environmental stimuli, these ﬁlms were exposed
to high humidity and temperature conditions, and their changes
were monitored over time using XRD measurements. For the
moisture stability study, these ﬁlms were placed in an air environment with a constant relative humidity (RH) of 80 ± 5 %at room temperature. The results depicted in Figure 4a revealthat after 48 h, the NMA-Pb ﬁlm changes to be pale, and numerous impurity diﬀraction peaks arise, accompanied by a noticeable
widening of the spacing between the main peaks. After 144 h,
no further changes occurred in the diﬀraction peaks of NMA-Pb
ﬁlms. According to previous reports, these results are possibly
caused by the intrusion of water molecules into the perovskite
lattices.[37] Due to the weak interaction of NMA spacers with inorganic octahedral sheets, water molecules in a humid environment can more easily intrude between the NMA spacers and I
atoms of PbI6 frameworks to form hydrogen bonds with NH3 2+of NMA cations, thereby producing a hydrate phase. In contrast,
BTMA-Pb ﬁlms retain their initial diﬀraction peaks even after 192
hours, indicating their great humidity stability, which can be attributed to the enhanced interaction that has been demonstrated
above between BTMA spacers and PbI6 octahedrons (Figure 4d).
Subsequently, thermal stability was evaluated by placing NMA-

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Pb and BTMA-Pb ﬁlms on a hot plate at 80 °C. The results illustrated in Figure 4b,e manifest that the color of the NMA-Pb ﬁlms
gradually fades to colorless within 27 h, and correspondingly, the
XRD peaks gradually disappear, suggesting that the perovskite
lattices were decomposed. Conversely, no signiﬁcant change wasobserved for
BTMA-Pb ﬁlm after the same period, suggesting its outstanding thermal stability. In addition, long-term on/oﬀ lightswitching stability is also crucial for photodetectors as they are
expected to be operated under persistent illumination. Therefore,
we also investigated the light stability for photodetectors basedon BTMA-Pb and NMA-Pb ﬁlms. The measurement was conducted by continuously recording the I–t curves of the photodetector devices for 5000 s under the laser illumination with a lightintensity of 60 mW cm−2 at a bias voltage of 1.5 V, as shown in
Figure 4c,f. Figure 4f shows that the intensity of the photocurrent for BTMA-Pb photodetector does not exhibit a signiﬁcant decrease after 5000 s. However, for the NMA-Pb device, the current
magnitude drops by two orders of magnitude, indicating its poor
stability. All of these ﬁndings suggest that, compared to NMA-Pb
ﬁlm and its device, BTMA-Pb exhibits superior humidity, heat,and illumination stability, beneﬁting from the enhanced interaction brought by the thiophene group between organic layers
and inorganic layers in 2D perovskite lattices. Although the above
photodetector based on the BTMA-Pb perovskite has achieved
good performance, there is still much room for improvement. In
order to further improve the device performance of the BTMAPb perovskite, we prepared the photodetector using asymmetric aluminum–gold (Al–Au) nanogap electrodes.[25] The SEM images of the Al–Au nanogap electrode interface and the statisticalmean for the gap are shown in Figure 5a and Figure S15, Supporting Information (the diameter of the Au electrode is 600 μm,
and the gap between the Au electrode and the Al electrode is approximately 10 nm). It is noted that the signiﬁcant reduction in
the distance between the electrodes inevitably leads to the promotion of charge collection, which can greatly enhance device performance. Moreover, using asymmetric electrodes can also form
the Schottky barriers (the energy band alignment is shown in
Figure S14, Supporting Information) at the opposite contact between the Al/Au electrodes and the BTMA-Pb perovskiteﬁlm, thereby endowing the photodetector with self-poweredcharacteristics.[38] The nanogap photodetector is illuminated by
a 450 nm laser. Similar to the measurements for photodetector
devices with traditional structures, the I–V curves are studied
under diﬀerent light intensities, as Figure 5b shows. The results
show that the dark current is below 0.04 nA under the bias from
-1 to 1 V. Besides, it is unsurprising that the open circuit voltage (VOC ) displays a noticeable shift of approximately 0.6 V fromthe zero point, which proves the presence of the Schottky barriers at the interfaces between the perovskite ﬁlm and the electrodes. The performance of the nanogap device with bias andwithout bias (self-power) was further studied. The I–t curves of
the BTMA-Pb perovskite nanogap photodetector under an intensity of 13.7 mW cm−2 were recorded and shown in Figure 5c. It
can be seen that under a bias voltage of 1 V, the maximum photocurrent of the nanogap photodetector can reach the nanoampere level. In contrast, under zero bias, the photocurrent is anorder of magnitude smaller. It is noted that at the beginning of
every excitation, the photocurrent rose to a high value and then© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.deFigure 4. a,d) Moisture stability test under 80 ± 5% RH at room temperature monitored using XRD measurements and optical microscopy images forthe a) NMA-Pb and d) BTMA-Pb ﬁlms. b,e) Heat stability test at 80 °C monitored using XRD measurements and optical microscopy images for b) the
NMA-Pb and e) BTMA-Pb ﬁlms. c,f) Long-time operation stability test of c) the NMA-Pb and f) BTMA-Pb perovskite photodetectors.

rapidly decreased to a stable state. The appearance of this sharp
peak for photocurrent is owing to the huge temperature changeof perovskite materials.[39–41] Speciﬁcally, the initial excitation forperovskite ﬁlm results in a signiﬁcant temperature change because the temperature of the material before excitation is low.
However, during subsequent continuous excitation, the material
always remains at a high and stable temperature. Besides, as thelight intensity increases from low to high, the photocurrent of
the nanogap detector exhibits a typical linear change, consistent
with other photoelectric semiconductor detectors (Figure 5d).
The characteristic of the on/oﬀ switching is also distinct and reversible. Furthermore, the R, D*, and EQE of the nanogap photodetectors can also be calculated using the formulas mentioned
above. All these results also present a linear relationship corresponding to the light intensity. Figure 5e–g shows the valuesof R, D*, and EQE of nanogap photodetectors at 1 V bias are

Adv. Funct. Mater. 2024, 2403746

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as high as 314 A W−1 , 3.4 × 1013 Jones, and 865%, respectively,
larger than those of BTMA-Pb perovskite photodetector with traditional device structure. These improvements are attributable
to the nanogap electrodes and their higher charge collection capability. The rise and decay times of the nanogap photodetector
were measured under 13.7 mW cm-2 light intensity and the results are 448 and 424 μs, as shown in Figure 5h. It is worth mentioning that the response speed is close to the traditional BTMAPb perovskite photodetectors, which can be attributed to the
Al2 O3 insulation layer forming on the surface of the Al electrodeand the diﬀerent work functions of Au and Al electrodes.[51,52]
The comparison of the parameters of this nanogap photodetector with those of other halide perovskites-based photodetector devices is also shown in Table 1, demonstrating the superior performance of our nanogap device. Moreover, the nanogap photodetector based on the BTMA-Pb perovskite ﬁlm also displays© 2024 The Author(s). Advanced Functional Materials published by Wiley-VCH GmbH

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www.afm-journal.deFigure 5. a) SEM images of Al–Au nanogap electrodes interface under diﬀerent scale bars. b) I–V curves of the BTMA-Pb nanogap photodetector withvarying illumination intensities. c) On-oﬀ switching curves of the BTMA-Pb based nanogap photodetector with 1 V bias and without bias. d–g) The
relationship of light intensity with d) photocurrent, e) responsivity, f) detectivity, and g) EQE of the BTMA-Pb perovskite nanogap photodetector with
1 V bias and without bias. (h) Determination of the rise and fall times of the BTMA-Pb-based nanogap photodetector at a bias of 1 V.
Table 1. Comparison of diﬀerent ﬁgures of merits for halide perovskites-based photodetectors.
Perovskites

Bias [V]𝜆 [nm]

R [mA W−1 ]D* [Jones]

Rise/decay time [ms]6.5 × 1013 a)

Refs.

CH3 NH3 PbI3 epitaxial ﬁlm

5520

2.07 × 104

0.017/0.018[42](PEA)2 PbI4 crystal

5

Solar light5

1.07 × 1013 a)

N/A

[43]

MAPbI3-x Clx ﬁlm

0632

1.1 × 105

3.53 × 1014 a)

230/380[44]

CsPbCl3 single microplatelet

5

390

450

1.0 × 1011 b)

8/7

[45]

(C4 H9 NH3 )2 (MA)3 Pb4 I13

20

525

4400

5.0 × 1012 a)

0.0036/0.0035[46]

CsPbBr3 QDs

2405

2.4 × 104

8.9 × 1013 a)

4700/2300[47]

MAPbI3 Microwires

10

White light

1.3 × 1045.25 × 1012 a)

0.08/0.24[48](iBA)2 (MA)n −1 Pbn I3 n+1

1.5

532

120

N/A

16/15

[49]

(C6 H5 (CH2 )3 NH3 )3 Pb2 I7

5

515

N/A

1.2 × 1010 a)0.85/0.78[50]

BTMA2 PbI4

1

450

3.14 × 105

3.4 × 1013 a)

0.448/0.424

This worka)Diﬀerent formula for calculating detectivity(D*): D* = RS1/2 /(2eIdark )1/2 , where e is the electronic charge, S is the eﬀective area of the photodetector, R is the responsivity,b)and Idark is the dark current; D* = R(ΔfS)1/2 /(In )1/2 , where S is the eﬀective area of the photodetector, Δf is the electrical bandwidth, R is the responsivity, and in is the noise
current.

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excellent performance in the self-powered mode. Under thismode, the R value of nanogap photodetectors can reach 24 A W−1 ,which is still three orders of magnitude higher than that of the device with a traditional structure using the same 2D perovskite material. The values of D* and EQE also present large values, whichare 1.9 × 1013 Jones and 66%, as shown in Figure 5e–g. Table S1,
Supporting Information, shows the comparison of previous selfpowered halide perovskite photodetectors and our work, also indicating the excellent self-powered characteristics of the BTMAPb nanogap detector. All these data indicate that the BTMA-Pb
perovskite photodetectors fabricated with asymmetric nanogap
electrodes can achieve excellent photoelectric performance while
also endowing the photodetector with great self-powered characteristics.

3. Conclusion
In conclusion, a new multiple aromatic ring spacer BTMA wasdesigned and successfully employed to form one new kind of 2D
halide perovskites. Our results suggest that the BTMA spacer hasa larger dipole moment than the NMA spacer, and the presenceof S atoms or thiophene can enhance theinteraction between
spacer layers and inorganic octahedral layers, thereby stabilizing
the perovskite lattices. Contrasted with NMA-Pb ﬁlm, BTMA-Pb
ﬁlm exhibits a dense surface without observable cracks. Beneﬁting from the weakened quantum conﬁnement eﬀect broughtby the higher polarity of BTMA spacer and enhanced interaction between organic layers and inorganic layers, better photoelectric performance is demonstrated for BTMA-Pb perovskite
photodetector compared with the device based on NMA-Pb perovskite. Meanwhile, excellent stability under moisture, heat andillumination conditions was also observed for the BTMA-Pb ﬁlm
and device. More importantly, BTMA-Pb perovskite photodetector with nanogap electrodes dramatically enhances its performance and self-powered characteristics. The maximum values ofR, D*, and EQE for nanogap devices can reach 314 A W−1 , 3.4
× 1013 Jones, and 865%, respectively. The work creates new research avenues towards more stable and higher-performance 2D
halide perovskite-based optoelectronic devices.

Supporting Information
Supporting Information is available from the Wiley Online Library or fromthe author.

Acknowledgements
Y.S. and L.L. contributed equally to the work. This work was ﬁnancially
supported by the City University of Hong Kong (Project 7020088 and
7005944).

Conﬂict of Interest
The authors declare no conﬂict of interest.

Data Availability Statement
The data that support the ﬁndings of this study are available from thecorresponding author upon reasonable request.

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2403746 (9 of 10)

Keywords
2D halide perovskite, nanogap electrode, photodetector, spacer cation,
stability
Received: March 1, 2024
Revised: June 20, 2024
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