An ultraviolet, self-powered, and large area photodetector based on a n-SnO₂/p-spiro-OMeTAD organic–inorganic heterojunction

Abstract

Self-powered photodetectors are of importance for practical applications. However, inorganic heterojunctions for photodetectors often contain defect states due to lattice mismatch. Herein, an ultraviolet photodetector based on a n-SnO₂/p-spiro-OMeTAD organic–inorganic heterojunction (OIH) was fabricated by spin-coating. On the basis of spectral data (absorption, reflection, and ultraviolet photoemission spectra) and the Hall effect measurements, energy level alignment at the SnO₂/spiro-OMeTAD interface is unveiled. The results of linear sweep voltammetry indicate that an ohmic contact is formed at the FTO/SnO₂ interface. The responsivity and detectivity of the SnO₂/spiro-OMeTAD photodetector are 1.42 mA W⁻¹ and 3.10 × 10⁷ Jones at 386 nm with 30 W m⁻², respectively. In addition, the performance of the device has no degradation for nearly 1000 hours. The working mechanism of the device based on SnO₂/spiro-OMeTAD OIH is that when the device is irradiated with UV light, electrons transfer from the SnO₂ layer to FTO, and holes transfer from the spiro-OMeTAD layer to Au. Finally, large area devices with 10 × 10 cm (active area 52 cm²) were prepared by the spraying method. In the rotating state, large area devices can effectively detect ultraviolet light. The device has potential to be applied as a high-performance, large-area ultraviolet photodetector.

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1 Introduction

Self-powered photodetectors and photosensors play an important role in communication, military, health, and space exploration applications.^1–5 SnO₂ is an outstanding n-type semiconductor with high carrier mobility, low-temperature crystallinity, and ultraviolet (UV) sensitivity.^6,7 Therefore, it is widely used as an electron transport layer in photovoltaics, photodetectors, diodes, etc.^8,9 Photodetectors based on SnO₂ inorganic heterojunctions (IHs) have been widely studied.^10–12 A self-powered photodetector on the basis of a n-SnO₂ microwire/p-InGaN IH was constructed, and it responded to ultraviolet-visible (330–530 nm) light irradiation.¹³ In addition, a photodetector based on SnO₂/NiO IH was fabricated by depositing NiO using magnetron sputtering and SnO₂ using the electron beam evaporation technique.¹⁴ Introducing an In₂O₃ interlayer into SnO₂/SnS₂ IH was also designed.² A SnO₂/MoS₂ IH was fabricated by Sn sputtering followed by oxidation and depositing a MoS₂ film by pulsed laser deposition.¹⁵ A SnO₂ quantum dots/β-Bi₂O₃ nanofiber IH-based self-powered UV photodetector and an optical UV filter were also fabricated.¹⁶ A core–shell SnO₂ microwire/CuZnS IH was constructed by simply coating a p-type (CuS)_0.35:(ZnS)_0.65 nanocomposite film (CuZnS) on a n-type SnO₂ microwire.¹⁷ However, IHs often contain lattice mismatches at the interface, which reduce the performance and stability of the photodetector.^18,19

In this study, we fabricated an ultraviolet photodetector based on a n-SnO₂/p-spiro-OMeTAD organic–inorganic heterojunction (OIH) by spin-coating. The absorption, reflection, and ultraviolet photoemission spectra were obtained, the Hall effects were studied and the energy level alignment at the SnO₂/spiro-OMeTAD interface was unveiled. Characterization using interface resistance, AFM, SEM, and cross-sectional SEM was also carried out. UV photodetectors were fabricated in a configuration of FTO/SnO₂/spiro-OMeTAD/Au, with two different device areas; smaller with 0.35 cm × 0.4 cm and larger with 10 × 10 cm. Their optoelectronic properties and stability were studied.

2 Experimental

2.1 Fabrication of FTO/SnO₂/spiro-OMeTAD/Au devices

The FTO conductive glass was cleaned in an ultrasonic cleaning machine for 15 min in the order of detergent, deionized water, and anhydrous ethanol. The electron transport layer was prepared on the cleaned FTO conductive glass. The solution obtained by diluting the SnO₂ solution with deionized water 1 : 6 was spin-coated on the FTO substrate for 30 s at a speed of 3000 rpm, and then annealed at 180 °C in an air environment for 30 min to form a dense layer of SnO₂. 72.3 mg of spiro-MeOTAD was dissolved in 1 mL chlorobenzene solution. Then, 18 μL lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI) acetonitrile solution (520 mg mL⁻¹), 20 μL tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris(bis(trifluoromethylsulfonyl)imide) (FK209 Co(III)-TFSI) acetonitrile solution (300 mg mL⁻¹) and 29 μL 4-tert-butylpyridine (TBP) were added to the above solution and stirred at room temperature for 1 h to obtain spiro-MeOTAD solution. The solution was spin-coated on a SnO₂ film at a speed of 2500 rpm by the dynamic spin coating method to prepare the hole transport layer. The device was oxidized in dry air for 12 h. Finally, the device was prepared by depositing a gold (Au) layer on the hole transport layer in a high vacuum of 10⁻⁴ Pa using resistive coating equipment (KC-ZD400).

2.2 Preparation of large area FTO/SnO₂/spiro-OMeTAD/Au devices

The SnO₂ layer was formed by spraying SnO₂ solution diluted with deionized water on 10 × 10 cm FTO conductive glass after cleaning and annealing at 180 °C for 30 min. Then, spiro-OMeTAD solution was sprayed on the SnO₂ layer in an air environment to prepare the hole transport layer. Finally, a gold (Au) layer was deposited over the hole transport layer by thermal evaporation.

2.3 Characterization

The SEM analysis of devices was carried out using a scanning electron microscope (Thermo Fisher Scientific FIB-SEM GX4, USA). XRD analysis was carried out using a high-resolution diffractometer (SmartLab 9 kW, Rigaku, Japan). The Hall coefficient, conductivity, carrier concentration, carrier mobility, and carrier type of SnO₂ films and spiro-OMeTAD films were measured using a Hall effect measurement system (MMR K2500, USA). The surface roughness of the device was tested using atomic force microscopy (MultiMode 8, Bruker Nano, USA). The contact angle of water droplets on FTO and SnO₂ films was measured using a contact angle meter (Shanghai Zhongchen Digital Technology Apparatus Co., Ltd, JC2000C1, China). X-Ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy of films were carried out using an X-ray photoelectron spectrometer (Thermo Fisher Escalab Xi+, USA). As the excitation source, a helium Iα radiation light (21.22 eV) was used for UPS. A photoelectrochemical workstation system (ZAHNER CIMPS-2 pro, Germany) was used to investigate the photocurrent density–time performance and LSV of devices. The UV absorption and reflection spectra were obtained using a micro-spectrometer (ATP5020P, Optosky, China).

3 Results and discussion

3.1 SnO₂/spiro-OMeTAD devices

3.1.1 Band gap and interfacial properties. To determine the band gap (E_g) of the SnO₂ powder (SnO₂-P) and spiro-OMeTAD powder (spiro-OMeTAD-P), their ultraviolet-visible (UV-Vis) absorbance and reflection spectra were obtained. The UV-Vis absorption and reflection spectra of SnO₂-P and spiro-OMeTAD-P are shown in Fig. S1a–d (ESI†), respectively. According to the Tauc plot method, the band gaps of SnO₂ and spiro-OMeTAD are calculated to be 4.13 (Fig. S1e, ESI†) and 2.63 eV (Fig. S1f, ESI†), respectively. To obtain the valence band maximum (VBM), ultraviolet photoelectron spectra (UPS) of SnO₂, spiro-OMeTAD, and Au film were measured (Fig. 1a). The binding energy cut-off (E_cutoff) values of SnO₂, spiro-OMeTAD, and Au are 16.89, 17.73, and 17.13 eV, respectively, and the binding energy starting (E_onset) values are 2.78, 1.43, and 0.00 eV, respectively. According to VBM = −hν − (E_cutoff − E_onset)/W_F = −hν − (E_cutoff − E_onset), where W_F represents the work function, the VBM of SnO₂ and spiro-OMeTAD are −7.11 and −4.49 eV, respectively, and the work functions of FTO and Au are −4.60 and −4.09 eV, respectively.^20,21 Thus, the extrapolated conduction band minimum (CBM) values of SnO₂ and spiro-OMeTAD are −2.98 and −1.86 eV, respectively. Using the UPS analysis results, the energy level diagrams of FTO/SnO₂/spiro-OMeTAD/Au are drawn as shown in Fig. 1b. To measure the Hall effect of SnO₂ thin films (SnO₂-TFs) and spiro-OMeTAD-TFs, they were spin-coated on non-conductive glass, and their four corners were covered by a copper electrode using an evaporation method, as shown in Fig. 1c. The schematic diagram of the Hall effect is shown in Fig. 1d. The mechanism of the Hall effect to determine the type of semiconductor is as follows: when the applied current passes through the semiconductor as shown in the following figure, the carriers in the semiconductor (n-type semiconductors are electrons, p-type semiconductors are holes) are deflected by the Lorentz force. Therefore, electrons and holes are concentrated on both sides of the semiconductor, resulting in the formation of a potential between the two sides of the semiconductor. Therefore, p-type and n-type semiconductors can be determined based on the positive and negative potentials. As shown in Tables 1 and 2, the mobility of SnO₂-TFs is −407.8 cm² V⁻¹ s⁻¹, which indicates that SnO₂-TFs is an n-type semiconductor. The mobility of spiro-OMeTAD-TFs is 432.66 cm² V⁻¹ s⁻¹, which indicates that spiro-OMeTAD-TFs is a p-type semiconductor. It is speculated that a p–n heterojunction is formed at the interface between p-type spiro-OMeTAD and n-type SnO₂.^22–24 Its built-in field is 0.43 V as shown in Mott–Schottky curves (Fig. S2, ESI†). We compared the linear sweep voltammetry (LSV) curves of the different device structures of FTO/Au (Fig. 1e), FTO/SnO₂/Au (Fig. 1f), FTO/spiro-OMeTAD/Au (Fig. 1g), and FTO/SnO₂/spiro-OMeTAD/Au (Fig. 1h). The LSV curves of FTO/Au and FTO/SnO₂/Au are almost coincident, indicating that SnO₂ has good conductivity and ohmic contact at the interface between FTO, SnO₂, and Au. The current of FTO/spiro-OMeTAD/Au and FTO/SnO₂/spiro-OMeTAD/Au is close, which further indicates ohmic contact at the interface of FTO and SnO₂. Due to the good conductivity of FTO, a Schottky junction is formed at the interfaces between FTO and the SnO₂ thin film.

Fig. 1 (a) UPS spectra of SnO₂, spiro-OMeTAD, and Au: secondary electrons’ cut-off edge and valence band spectrum. (b) Energy level diagrams of FTO, SnO₂, spiro-OMeTAD, and Au. (c) Photographs of copper plating on four corners of SnO₂-TFs by evaporation. (d) Schematic diagram of the Hall effect. Device structures of FTO/Au (e), FTO/SnO₂/Au (f), FTO/spiro-OMeTAD/Au (g), and FTO/SnO₂/spiro-OMeTAD/Au (h). (i) LSV curves of FTO/Au and FTO/SnO₂/Au. (j) LSV curves of FTO/spiro-OMeTAD/Au and FTO/SnO₂/spiro-OMeTAD/Au.

Table 1 Hall parameters of the SnO₂ film at 297.0 K (8000 G field intensity)

Description	Mobility (cm^2 V^−1 s^−1)	Density (cm^−3)	Hall coeff. (cm^−3 coul^−1)	Carrier type
SnO2	−407.80	−6.42 × 10^12	−1.00 × 10^6	Electrons

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Table 2 Hall parameters of the spiro-OMeTAD film at 294.0 K (8000 G field intensity)

Description	Mobility (cm^2 V^−1 s^−1)	Density (cm^−3)	Hall coeff. (cm^−3 coul^−1)	Carrier type
Spiro-OMeTAD	432.66	−2.19 × 10^11	−2.85 × 10^7	Holes

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3.1.2 Optical, phase, and morphological analyses. The photodetector was fabricated by spin-coating in a configuration of FTO/SnO₂/spiro-OMeTAD/Au, as shown in Fig. 2a. First, the optical properties of SnO₂-TFs were investigated. We collected the UV-Vis absorbance spectra of FTO and SnO₂-TFs deposited on FTO substrates (Fig. 2b). The peaks at 26.61, 33.89, and 51.78 degrees of the SnO₂ powder obtained by volatilizing SnO₂ solution at 180 °C correspond to the (110), (101), and (211) crystal planes, respectively, matching with the JCPDS 41-1445 card (Fig. S3a, ESI†).²⁵ A large number of diffraction peaks can be observed in the XRD pattern of the spiro-OMeTAD powder, which indicates that the spiro-OMeTAD molecules are capable of self-assembling into a crystalline material (Fig. S3b, ESI†). No strong peaks are observed for SnO₂ (Fig. S3c, ESI†) and spiro-OMeTAD films (Fig. S3d, ESI†). Compared with FTO-TFs, SnO₂-TFs exhibit enhanced absorption in the wavelength range of 400–800 nm. The crystallinity of SnO₂-TFs was analyzed by powder X-ray diffraction (PXRD) and the PXRD patterns are shown in Fig. 2c. Compared with that of FTO-TFs, no new diffraction peaks appeared. The covering of SnO₂-TFs leads to a weakening of the diffraction intensity of FTO. The diffraction peaks of FTO originate from the tetragonal rutile SnO₂ structure (JCPDS no. 41-1445).²⁵ The peaks at 26.37, 33.63, 37.70, 52.48, 54.54, 61.53, 65.53, and 78.3 degrees correspond to the (110), (101), (200), (211), (220), (310), (301), and (321) crystal planes, respectively. To characterize the surface morphology of FTO and SnO₂-TFs, the roughness (Fig. 2d and e) of FTO and SnO₂-TFs was measured using an atomic force microscope (AFM). The root mean square roughness (R_q) values of the FTO substrate and SnO₂-TFs are 11.7 and 10.2 nm, respectively. The average roughness (R_a) of the FTO substrate and SnO₂-TFs is 9.29 and 8.13 nm, respectively. The reason for the low R_q and R_a values of SnO₂-TFs is that the coverage of the film reduces the undulation of the FTO surface. The scanning electron microscopy (SEM) images of FTO (Fig. 2f), SnO₂-TFs (Fig. 2g), and spiro-OMeTAD-TFs (Fig. 2h) were analyzed. It is found that the FTO surface is composed of several polygonal crystals. The coverage of SnO₂-TFs caused the SEM image of the FTO to become blurred. The cross-sectional SEM image of the device (Fig. 2i) shows that the thicknesses of the SnO₂-TFs, spiro-OMeTAD-TFs, and Au layers are 18, 160, and 50 nm, respectively.

Fig. 2 (a) Schematic diagram of the preparation process of the FTO/SnO₂/spiro-OMeTAD/Au device. (b) UV-Vis absorption spectra of FTO and SnO₂-TFs. (c) XRD patterns of FTO and SnO₂-TFs. AFM images of FTO (d) and SnO₂-TFs (e). SEM images of FTO (f), FTO/SnO₂-TFs (g), and FTO/SnO₂/spiro-OMeTAD-TFs (h). (i) SEM images of the cross-section of the FTO/SnO₂/spiro-OMeTAD/Au device.

3.1.3 Photoelectronic properties. A schematic diagram of the FTO/SnO₂/spiro-OMeTAD/Au device is shown in Fig. 3a. The photoelectronic properties of the FTO/SnO₂/spiro-OMeTAD/Au devices were analysed. The J–t curves of the device were measured at different wavelengths (386, 397, 405, 433, and 454 nm) with a light intensity of 30 W m⁻² (Fig. 3b). The device exhibits the best photocurrent response at 386 nm. The photocurrent response gradually decreases with increasing wavelength, and becomes weak beyond 454 nm, indicating that the SnO₂/spiro-OMeTAD device has a high photocurrent response in the ultraviolet region. The working wavelength of SnO₂/spiro-OMeTAD OIH is 386–433 nm. The photocurrent density of the device reaches 4.25 μA cm⁻² at 386 nm with 30 W m⁻². To further investigate the photoelectric sensitivity of the device, we measured the photocurrent response at 386 nm with different light intensities (Fig. 3c). The results show that the device is capable of producing a regular response to a low intensity of 1 W m⁻². In addition, the device can produce a linear response (R² = 0.99) to different light intensities (0.1 to 50 W m⁻²), as shown in Fig. 3d, which facilitates the estimation of light and electrical signals. Fig. 3e shows an enlarged view of the device's response period at 386–433 nm. At 386 nm, the response time (τ_res) and recovery time (τ_rec) are 42 and 43 ms, respectively.²⁶ In addition, we will investigate whether there is a charge trapping mechanism in a future work. Responsivity (R) is one of the important performance indicators of photodetectors, R = (I_PC − I_Dark)/S × P, where I_PC is the photocurrent, I_Dark is the dark-current, S is the illumination area and P is the illumination intensity.²⁷ The detectivity (D*) is also another key parameter, D* = R/(2eJ_Dark)^0.5, where e is the electron charge and J_Dark is the dark current density.²⁸ Based on the above equations, the R and D* values calculated for the device at 386 nm with 30 W m⁻² are 1.42 mA W⁻¹ and 3.10 × 10⁷ Jones, respectively. A comparison of the performance of our detectors and related detectors that have been published is shown in Table 3. The energy band bending diagram of the photodetector based on SnO₂/spiro-OMeTAD OIH is shown in Fig. 3f. The device of FTO/spiro-OMeTAD/Au has no photoelectric response, indicating that the photons absorbed by spiro-OMeTAD cannot be converted into electrons (Fig. S4, ESI†). Thus, the working mechanism of the photodetector based on SnO₂/spiro-OMeTAD OIH is as follows: when the device is irradiated with UV light, electrons transfer from the SnO₂ layer to FTO, and holes transfer from the spiro-OMeTAD layer to Au. In addition, the stability of the photodetector was investigated, and the normalization curve is shown in Fig. 3g. The results show that the performance of the device has no degradation for nearly 1000 hours, which indicates the device exhibits good stability. The device is prepared by the solution method, and its stability may be improved due to self-adjustment between the interfaces. In addition, the oxidation of spiro-OMeTAD is also a reason for the gradual increase in performance.

Fig. 3 The performance of the photodetector based on SnO₂/spiro-OMeTAD OIH. (a) Schematic diagram of J–t measurement. (b) J–t curves at different wavelengths with 30 W m⁻². (c) J–t curve at 386 nm with different light intensities. (d) Normalized curves of photocurrent density under different light intensities. (e) Enlarged image of the single response period of the device shown in Fig. 3(c) at wavelengths of 386–433 nm to obtain its response time and recovery time. (f) Band bending diagram. (g) The normalized curve for the stability of the devices.

Table 3 Performance of self-powered photodetectors based on SnO₂/spiro-OMeTAD OIH

Photodetector	Bias (V)	Responsivity (R/mA W^−1)	Detectivity (D*/Jones)	Applied conditions	Ref.
SnO2/Mg:SnO2	0	1.55	9.8 × 10^10	290 nm, 0.1177 W m^−2	29
SnO2 microwire/InGaN	0	152.5	3.77 × 10^12	360 nm	13
F-SnO2/ZnS	0	24	15.10 × 10^10	UV	30
SnO2/In2O3/SnS2	0	2.9	5.9 × 10^7	1.4 mW cm^−2	2
NiO TF/SnO2 TF	0	0.141	2.8 × 10^8	365 nm	14
SnO2/MoS2	0	350	1.25 × 10^11	900 nm, 0.025 mW cm^−2	15
SnO2/CuZnS core–shell microwire	0	1.6	—	315 nm	17
SnO2/spiro-OMeTAD	0	1.42	3.10 × 10^7	386 nm, 30 W m^−2	This work

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3.2 Large area devices

Large area devices (FTO/SnO₂/spiro-OMeTAD/Au) with 10 × 10 cm (active area 52 cm²) were prepared by the spraying method, and the preparation process is shown schematically in Fig. 4a. Subsequently, 9 positions of the large-area device were selected (Fig. 4b) to measure their performance at 386 nm with 30 W m⁻². It is found that the photocurrents generated at different positions of the device do not differ significantly and are relatively uniform (Fig. 4c and d). To evaluate the all-around detection performance of the device similar to a radar in actual work, the device is rotated, and the UV lamp is fixed at a fixed angle to simulate the UV signal. A schematic of the rotation testing of the device is shown in Fig. 4e. The J–t performance of the device in the rotating state under UV illumination (white light as a reference) is shown in Fig. 4f. The spectral profiles of UV and white light are presented in Fig. S5a and b (ESI†). The device has no significant response to white light as shown in Fig. 4f. Thus, it is visible blind. In contrast, there is a regular response when the device is irradiated with UV light. Fig. 4f is a zoomed in image from 476 to 485 s. The J–t curve detected during rotation presents a triangular profile (the rising curve represents the UV light, and the falling curve represents the reverse UV light), as shown in Fig. 4g.

Fig. 4 The performance of the large area photodetector based on SnO₂/spiro-OMeTAD OIH. (a) Schematic diagram of the preparation process of a 10 × 10 cm large area device. (b) Photographs of large area devices. (c) J–t curves at 9 positions of the device. (d) Current distribution diagram at 9 positions of the device. (e) Schematic diagram of rotation testing for the large-scale device. (f) J–t curve for rotation measurement. (g) Enlarged image of the J–t curve.

4 Conclusions

In summary, in this study, we used SnO₂ and spiro-OMeTAD thin films to form an OIH self-powered ultraviolet photodetector. The results indicate that the device exhibits a high photocurrent response in the ultraviolet region. The device exhibits good stability and large-area devices can be prepared, which indicates that our work provides a simple and effective method for preparing large-area high-performance optoelectronic devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Shandong Province Natural Science Foundation (Grant No. ZR2019MB021).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3tc03474g

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