Non-centrosymmetric materials, lacking structural spatial inversion symmetry, exhibit various properties including ferroelectricity [
1,
2], piezoelectricity [
3,
4], pyroelectricity [
3,
5,
6], and frequency-doubling effect [
7]. Particularly, asymmetric single-phase compounds can also generate stable photocurrents and above-bandgap photovoltages under continuous uniform illumination [
8,
9], known as the bulk anomalous photovoltaic (BAPV) effect, which is anticipated to achieve a high-performance self-powered photoelectric response [
10–
12]. Currently, the exploration of BAPV has largely focused on a few insulating inorganic oxides like LiNbO
3 [
13,
14], BaTiO
3 [
15–
17], and BiFeO
3 [
18,
19]. However, the majority of these devices suffer from poor electronic properties which limit their application in the field of photovoltaics. Fortunately, the proposition of the pyro-phototronic effect aims to enhance photoelectric performance by naturally connecting the photo-induced pyroelectric effect with semiconductor properties [
20–
23]. The charges generated by photo-induced pyroelectric polarization can efficiently modify carrier separation and transportation. Upon light illumination, the transient temperature rises in these materials can generate current signals in the polarization direction to enhance the light response. Therefore, the exploration of novel non-centrosymmetric materials with BAPV effect accompanied by pyro-phototronic effect is desirable.
Organic−inorganic halide perovskites have become mainstream semiconducting materials with wide-ranging applications, including photovoltaics [
24], light-emitting devices [
25], and photodetectors [
26,
27]. Furthermore, the flexibility of the perovskite structures enables the incorporation of chiral ligands, giving rise to a novel concept known as chiral hybrid perovskites (CHPs) [
28,
29]. As a consequence, by combining the advantages of the perovskite structure with inherent chiral features, CHPs are widely used in chiral optoelectronic devices, such as spintronic devices, nonlinear optics devices and circularly polarized light detectors [
30,
31]. Given the unique properties, introducing chiral organic cations disrupts spatial inversion symmetry, opening avenues for high-performance photovoltaic applications. For instance, previously reported chiral perovskites, such as (
R/
S-1-phenylpropylamine)
2BiI
5 (0.63 V) [
32], (
R/
S-1-phenylpropylamine)EAPbCl
4 (0.5 V) [
33], and (
R-
β-methylphenethylammonium)
4AgBiI
8 (0.36 V) [
34], have fully exhibited their fascinating bulk photovoltaic effect. On the other hand, the introduction of chiral cations renders CHPs more likely to crystallize in five chiral-polar point groups, namely
C1,
C2,
C3,
C4, and
C6 [
28,
35,
36]. These chiral-polar perovskites with inherent polarization will readily generate the pyro-phototronic effect [
37,
38].
Herein, we synthesized a pair of zero-dimensional (0D) bismuth-based CHPs (S-α-MBA)4Bi2I10 and (R-α-MBA)4Bi2I10 (1S and 1R), by assembling chiral methylbenzylamine (S/R-α-MBA) cations. These compounds crystallized in the chiral-polar point group (P21), induced by the organic chiral ligands. Remarkably, the intrinsic asymmetric structure of these compounds gives rise to a notable BAPV effect, characterized by a steady-state photovoltage (Voc) reaching an impressive 15 V. The synergism of BAPV and pyro-phototronic effects can further modulate the optoelectronic processes, enabling highly enhanced self-powered detection based on CHPs. These merits enable 1S and 1R to exhibit a broadband response range that surpasses the inherent bandgap limitation, spanning from 405 nm to 785 nm. The photodetector can realize simultaneous multi-signal sensing and effectively utilize the electric energy generated by photo-thermal coupling, opening up possibilities for the design and development of prominent self-powered materials.
Single crystals of
1S and
1R with a size up to 10 × 5 × 5 mm
3 have been successfully grown using the temperature-lowering crystallization method.
Scheme 1 provides schematic illustrations for the crystal growth process. Powder X-ray diffraction (XRD) analysis confirmed the high purity by closely matching the simulation results (Fig. S1 in Supporting information). Furthermore, the thermogravimetric analysis revealed no mass loss from the crystals up to 495 K, disclosing its high thermostability (Fig. S2 in Supporting information). The crystallization of
1S and
1R is largely dependent on the addition of chiral components. These crystals adopted a chiral-polar space group of
P2
1 (detailed crystallographic data are provided in Table S1 in Supporting information). Their chiral-polar characteristic was confirmed through the second harmonic generation signals (Fig. S3 in Supporting information). Given the similarity between the crystallographic structural parameters of the two enantiomers, the crystal structure of
1R will be described in detail.
Fig. 1a illustrates the structural arrangement, comprising four
R-MBA
+ cations and one [Bi
2I
10]
4− dimer as the fundamental structural unit. Each [Bi
2I
10]
4− anionic dimer is formed by BiI
6 octahedra sharing their edges through two iodine atoms. The chiral organic molecules are firmly anchored to iodine atoms of dimers
via N-H···I hydrogen bonds. Moreover, the Bi-I bond lengths and I-Bi-I bond angles are 2.92−3.33 Å and 84.56°−177.30°, indicating the tilts and distortion of the inorganic skeleton (
Fig. 1b, Tables S2 and S3 in Supporting information). The electric polarization (
Ps) of
1R primarily arises from the arrangement of the electric dipole moments of the organic cation [
28]. The dipole moment of the
S/
R-α-MBA ion is estimated to be 7.75 Debye by DFT (Fig. S4 in Supporting information). Then, the calculated
Ps along the
b-axis was found to be 0.4 µC/cm
2 using the point charge model (Tables S4 and S5 in Supporting information). Thus, this chiral polarity feature empowers
1S and
1R with considerable potential to display the BAVE effect for self-driven detection.
The optical properties of
1S and
1R were investigated first. The ultraviolet-visible absorption spectra reveal they present a sharp absorption edge at around 610 nm (
Fig. 2a and Fig. S5 in Supporting information). By fitting the Tauc curve, the optical bandgap (
Eg) was calculated to be 2.0 eV [
39]. This value is comparable to that of other 0D binuclear bismuth halide perovskites, such as (
R/
S-1-phenylpropylamine)
2BiI
5 (2.17 eV) and (
R-1-(4-fluoro)-phenylethylammonium)
4Bi
2I
10 (2.09 eV) [
32,
40]. Such a relatively narrow bandgap holds the potential for achieving broadband optoelectronic detection. To verify the intrinsic chirality of
1S and
1R crystals, circular dichroism (CD) signals were recorded for the thin films (
Fig. 2b). The prepared films also show a preferred orientation of the (100) plane (Fig. S6 in Supporting information). Notably, the CD spectra exhibit that the CD signals of
1S and
1R are mirror images of each other and correspond to the optical intrinsic absorption of perovskites. These results provide compelling evidence of the intrinsic chirality feature of the crystals, further confirming the efficient transfer of chirality from organic molecules to inorganic parts.
Subsequently, we fabricated photodetectors using both
1S and
1R chiral crystals to investigate the photovoltaic effect. Under 405 nm laser illumination, the current-voltage (
I–
V) curves clearly showed the BAPV effect when the electrode direction was aligned parallel to the polar
b-axis, as depicted in
Fig. 3a. In this configuration, the
Voc impressively reached a maximum of 15 V, approximately 7.5 times larger than its optical bandgap. Particularly, the
VOC of
1R increases with incident light intensity (Fig. S7 in Supporting information). Such a huge
Voc should be the first demonstration in the CHPs, much higher than that of double perovskite (4-bromobenzylammonium)
2CsAgBiBr
7 (3.2 V) and hybrid perovskite ferroelectrics (
n-hexylammonium)
2(ethylammonium)
2Pb
3Br
10 (7.4 V) [
10,
11]. Crucially, the BAPV effect is intricately connected to the
Ps of the crystals. The
Voc and short-circuit photocurrent (
Isc) are observed for the opposite
Ps direction:
Isc < 0 and
Voc > 0 for
1R (
b-axis//[010]) and
Isc < 0,
Voc > 0 for
1S (
b-axis//[0
0]). These opposing electric polarizations between enantiomers result from the crystallographic constraints imposed by the chiral polar system. To verify polarity dependency, the crystal was rotated 180° around the
b-axis. This resulted in a reversal of the
Voc and
Isc signs (Fig. S8 in Supporting information):
Isc > 0,
Voc < 0 for
1R (
b-axis//[0
10]) and
Isc > 0,
Voc < 0 for
1S (
b-axis//[010]). Because of the chirality remained unchanged, the sign change of
Isc may be explained by the change of
Ps direction.
To clarify the correlation between
Ps and the BAPV effect, we measured photocurrents perpendicular to the
b-axis. As expected, there are no
Isc and
V0 signals (
Fig. 3b). This result strongly suggests that the BAPV effect is intricately linked to polarity along the
b-axis. In other words, the photovoltaic effect, as manifested solely along the
b-axis, reaffirms its origin in the chiral-polar character of these crystals.
Fig. 3c and Fig. S9 (Supporting information) illustrate the photovoltaic-time (
V-
t) curve under continuous irradiation, exhibiting above-bandgap
Voc = 15 V in
1S and
1R. Additionally, the
V-
t curve measured at zero bias during the light switching on/off cycles verified the good stability of the BAPV effect (
Fig. 3d). The remarkable BAPV effect, independent of bandgap, highlights
1S and
1R as promising candidates for prominent pyro-phototronic detection.
In pyro-phototronic devices, the simultaneous operation of pyroelectric and photovoltaic effects synergistically promotes the transport of photogenerated carriers and reduces the recombination rate of electron-hole pairs [
22,
41,
42]. The principle is illustrated in
Fig. 4a. When exposed to light, charge carriers are generated within the chiral-polar structure, while the increased temperature of the
1R crystal induces pyroelectric polarized potential. The transient current and four-stage behavior reveal the pyro-phototronic effect (
Fig. 4b) [
43]. In stage ‘Ⅰ’, the pyroelectric material undergoes a transient current peak characterized by a sudden increase in temperature (d
T/d
t > 0). This current comprises both the output current from the pyroelectric and photovoltaic effects, represented as
Ipyro+photo. Under continuous illumination, the temperature remains constant, leading to a stable plateau in the output current without the effect of pyroelectric, represented as
Iphoto (d
T/d
t = 0, stage ‘Ⅱ’). However, when the light is turned off, a reverse pyroelectric peak
Ipyro’ is generated (d
T/d
t < 0, Stage Ⅲ). In stage ‘Ⅳ’, the pyroelectric current gradually disappears as the temperature returns to room temperature (d
T/d
t = 0).
Fig. 4c depicts the
I–
t curves of
1R within the spectral range from 405 nm to 785 nm, indicating the capability of a broadband response, which breaks the limit of optical bandgap. The curves reveal two distinct stages: initial sharp transient current peaks (
Ipyro+photo), followed by a steady plateau (
Iphoto) without pyroelectric effect as the temperature remains constant. The current peak, a result of the combined BAPV and pyroelectric effects, shows a clear wavelength dependence and a high enhancement. Moreover, at zero bias, the peak pyroelectric current monotonically increases with increasing incident power (Fig. S10 in Supporting information). Since the temperature gradient increases with stronger light, it leads to higher densities of electrons and holes. These findings show that
1R not only generates stable photovoltaic
Iphoto directly but also amplifies the response
via the pyro-phototronic effect. The rising and falling components of the response time of
1R inferred from single switching cycle are approximately 0.5 s and 3.4 s, respectively (
Fig. 4d). Furthermore, the photostability of
1R devices under 520 nm irradiation is depicted in
Fig. 4e, demonstrating that the transient photoinduced pyroelectric current remains consistently stable without obvious degradation. These findings highlight the synergistic interaction between the BAPV and pyroelectric effects, offering a promising path toward advanced broadband self-powered photoelectric detection.
In summary, a pair of CHPs (S-α-MBA)4Bi2I10 and (R-α-MBA)4Bi2I10 have been synthesized, which crystallize in the chiral-polar space group of P21. This chiral-polar structure induces a superior bulk anomalous photovoltaic effect (BAPV), which exhibits an above-bandgap photovoltage extraordinary 15 V. As far as we know, this should be the first discovery of the BAPV effect in lead-free chiral hybrid perovskites. In addition, such BAPV effect gives rise to a distinctive pyro-phototronic effect, which further enables a highly enhanced self-powered detection with a broad response from 405 nm to 785 nm. Such photo-thermal coupling is capable of simultaneous multi-signal sensing and effectively enhances photoelectric response. These unique features establish a potential platform for optoelectronic applications, particularly in the field of photovoltaics.
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