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A highly photoconductive composite prepared by incorporating polyoxometalate into perovskite for photodetection application
DOI: 10.1039/x0xx00000x
Yuzhuo Zhang, Ran Tao, Xuemin Zhao, Zhixia Sun*, Yanju Wang and Lin Xu* www.rsc.org/
The photoconductive perovskite-polyoxometalate composite was prepared for the first time by a facile low-temperature and solution-processed method, and this composite exhibited a significantly enhanced photoconductivity and photodetection performance due to introducing polyoxometalate into perovskite for fine energy-level matching and efficient charge transfer. Photoconductive materials are a class of functional solidstate components which can increase the electrical conductivity by absorption of photons, and the intrinsical “photoconductivity” is closely relevant with the absorption of the incident light, exciton generation and separation, carrier transport, and electron-hole recombination. Thus the research on photoconductivity of chemical composite materials is very important due to their potential applications in the areas such as solar cells, photodetectors, optical switching devices and 1-5 gas sensors. Conventional semiconductors like TiO2, GaN, WO3, and ZnO, are the most common photoconductive materials. However, the unsatisfactory properties of relative wide direct band gap, low absorption coefficient as well as high manufacturing cost limit their applications in various 6-9 optoelectronic devices. Recently, the next-generation of photoconductive materials, organic−inorganic hybrid perovskites (CH3NH3PbX3, X=Cl, Br, I), 10 with the advantages of direct band gap, high absorption 11 12 coefficient, ambipolar charge mobility, and long diffusion 13 length, have shown great potential for the fabrication of low14-16 cost and high efficiency solar cells, light emitting 17 18 materials, and photodeteotors, etc. Despite the intense research in the past several years, some problems still remain in the preparation of perovskite-based devices with high efficiency and good reproducibility, because the preparation conditions, the interfacial morphology as well as the measurements protocols could affect the property of
a.
Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, P. R. China. E-mail: [email protected]; Fax: +86-431-85099765; Tel: +86-431-85098760 Electronic Supplementary Information (ESI) available: experimental details, fabrication of photoconductive device and measurements used in this work. See DOI: 10.1039/x0xx00000x
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perovskites. Thus, many optimization processes have been 24 carried out as follows: interface modification, optimization of 25 26 the perovskite precursor solution, crystallization control, 27 28 charge dynamics studies, new deposition processes, 29 minimization of hysteresis effect, and optimizing 30-32 morphology. Among these approaches, various additives have been employed for tuning functional properties. For instance, the performances of the perovskite-based devices could be enhanced by adding CH3NH3Cl, 1, 8-diiodooctane (DIO), or polymers into their corresponding standard precursor 30-35 solutions. Therefore, introducing suitable additives into the perovskite to form perovskite-based composite materials is an effective strategy to improve their optoelectronic performance. Polyoxometalates (POMs), a class of molecular metal-oxo cluster compounds based mainly on Mo, W, and V, have shown superior physicochemical properties and various 36-38 applications in catalysis, materials science and medicine. POMs exhibit many excellent features such as low conduction band, highly tunable electronic properties, and high 39 transparency in the visible spectrum, etc. In particular, POMs generally act as the electron acceptor in chemical reaction. In recent years, our group carried out several researches on the incorporation of POMs into semiconductor films for improving the light-to-electricity conversion performance of 40-42 semiconductors. Also, we have reported an advanced photoconductive device based on the POM/TiO2 composite, which achieved a significantly enhanced photoconduction and 43 gas sensing property. These above-mentioned results suggested that the smart combination of POMs with semiconducting perovskite may create a new class of photoconductive composite materials of high performance. Herein, we report the preparation of perovskite-POM composite (perovskite is CH3NH3PbI3; POM is PW12) and the characterization of photoconductive performance by means of a test device. Compared to the pure perovskite device, the composite device showed a significant enhancement in photoconductive performance. Furthermore, the photodetection performance of these devices for the incident
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Fig. 2 SEM images of CH3NH3PbI3 (a) and CH3NH3PbI3/PW12 composite (b) deposited on ITO substrate.
Fig. 2 shows the SEM top-view images of the CH3NH3PbI3 and CH3NH3PbI3/PW12 composite film deposited on ITO glass.The SEM image of the separate ITO substrate and the energy dispersive X-ray (EDX) were shown in Fig. S1. It is worth noted that the film formation in our work was under ambient air at relative humidity (RH) ≈ 40%. As shown Fig. 2a, the coverage of the CH3NH3PbI3 film was not complete, but the Fig. 1 Schematics of the device structure (a); IR spectra (b), XRD patterns (c) and case was well matched with the previous publication.46 Also, UV-vis diffuse reflectance spectra (d). the similar morphology and coverage of CH3NH3PbI3/PW12 film light of 365nm and 420nm was demonstrated. These results were observed in Fig. 2b, suggesting that the PW12 did not provide valuable information for developing composite affect the perovskite formation. In Fig. S1, the energy materials of perovskite-POM for applications in various dispersive X-ray (EDX) analysis of CH3NH3 PbI3/PW12 composite film revealed the presence of basic elements of CH3NH3PbI3 advanced optoelectronic and PW12. Besides, from the observation of the three element Fig. 1a shows a schematic diagram of the photoconductive mappings of Pb, I and W, we can infer that PW12 is uniformly test device. The relevant detail of experiment and device fabrication is provided in Electronic Supporting Information distributed throughout the CH3NH3PbI3 film. To investigate the photoconductive performance of the (ESI). Fig. 1b displays the IR spectra of pure CH3NH3PbI3 and the CH3NH3PbI3/PW12 composite. In comparison with relevant samples, we carried out a series of measurements of CH3NH3PbI3 curve, the CH3NH3PbI3/PW12 curve exhibits the photoconductive devices under Xe lamp irradiation. Fig. -1 characteristic bands for the POMs at 700–1100 cm . The 3(a) displays the photocurrent responses from both devices of -1 the CH3NH3PbI3 and the CH3NH3PbI3/PW12 composite. Both the characteristic peaks observed at 1080, 983, 890, and 798 cm are assigned to υas (P–O), υas (W=O) and υas (W–O–W), samples show steady and reproducible photocurrent respectively, suggesting that the Keggin structure of the PW12 responses during several on/off cycles under the Xe lamp irradiation, whereas the pure PW12 device does not show the is retained in the composite. photoresponse. Compared to pristine CH3NH3PbI3, the Fig. 1c shows the XRD patterns of the pure CH3NH3PbI3 and composite material of CH3NH3PbI3/PW12 displays a remarkable the CH3NH3PbI3/PW12 composite. The XRD of pure PW12 film deposited on ITO is exhibited in ESI. The intense diffraction enhancement (ca. 3-fold) in the photocurrent. Evidently, the peaks at 14.22°, 28.50°, 32.01°, 40.1°, and 43.26° can be presence of PW12 accelerates the photocurrent saturation assigned to (110), (220), (310), (224), and (314) diffractions, when light irradiation, indicating that the electron-hole respectively. The large number of diffraction peaks consistent recombination can be efficiently suppressed due to both the with the reported data could confirm the tetragonal crystal energy level matching and the “shallow electron trap” function 44 structure of CH3NH3PbI3. Also, a close examination of the of PW12. When the light is turned off, slow photoresponse region around the (110) diffraction peak at 14.12° shows no decay occurs in Fig. 3b. The slow decay of the photoresponse measurable peak at 12.65° for PbI2, indicating a high level of represents the strong tendency of capturing charge carrier 47 phase purity. The same diffraction peaks of perovskite resulting from the “shallow electron trap” of PW12. The appeared in both the pure sample and the composite sample, trapped charges in PW12 could be slowly released later under indicating that the perovskite structure still remains in the bias voltage after stopping light radiation. The dependence of current–voltage (I–V) curves on light composite. No diffraction peaks for PW12 are observed in the composite (Fig. 1c and Fig. S2), which may be due to the its intensity for both the pure CH3NH3PbI3 device and the 45 CH3NH3PbI3/PW12 composite devices has been measured (Fig. amorphous state. 3c and d). Upon illumination, the photocurrent of the devices As displayed in the UV–Vis diffuse reflectance spectra of Fig. increases drastically with the increasing of the external bias 1d, both the pure CH3NH3PbI3 and CH3NH3PbI3/PW12 exhibit high light-harvesting capabilities from the ultraviolet to visible voltage. At the same voltage, the light current increases spectra. Since PW12 is high transparent in the most spectral gradually with the increasing of light intensities. Note that the composite device shows higher range, the incorporation of PW12 does not affect the light CH3NH3PbI3/PW12 photocurrent responses than the pure CH3NH3PbI3 device at adsorption of CH3NH3PbI3. each voltage with various irradiation intensities. Furthermore,
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shown in Fig. 3e. It was well-known that PL originates from the 48 radiative recombination of electrons and holes. As seen from Fig. 3e, both the CH3NH3PbI3-based devices show the emission maxima at 776 nm, which is in agreement with the reported 49 results in the literature. Moreover, a clear decrease of PL intensity for the CH3NH3PbI3/PW12 composite could be observed, and the maximal PL intensity of the CH3NH3PbI3/PW12 composite was quenched by 65% with respect to that of pristine CH3NH3PbI3. Such a PL quenching demonstrates that the electron transfer from perovskite to PW12 surely occurred so as to reduce the probability of 50 electron–hole recombination. Therefore, the trapping photoexcited electrons and afterward effective transfer is the key role of PW12 in the CH3NH3PbI3/PW12 composite system. For the CH3NH3PbI3/PW12 composite system, the proposed mechanism for the enhanced photoconductivity is as follows: Without illumination and bias voltage, the device is in its equilibrium state with a Schottky barrier of ITO/ CH3NH3PbI3 interfaces, and there were no notable carriers transfer because the carriers have to overcome the Schottky barrier during transporting. However, the Schottky barrier height was decreased for facilitating carriers transporting when the free carrier density increased upon illumination and bias voltage. Fig. 3 Current-Time (I-T) curves (a); Photocurrent decay curves (b); CurrentSince the relative energy band alignment of the PW12 Voltage (I-V) curve of CH3NH3PbI3 (c); CH3NH3PbI3/PW12 (d); PL spectra (e); correlates quite well with CH3NH3PbI3 band structure (Fig. 3f), Energy levels and the electron transfer processes (f). the majority carrier of electrons from CH3NH3PbI3 can easily transfer to the conduction band of PW12, resulting in a the photocurrent on different irradiance is shown in Fig. S3. significant enhancement in conductivity. The presence of POM Compared to the pure CH3NH3PbI3 device, the provides a highly effective electron transfer pathway to CH3NH3PbI3/PW12 device shows higher photocurrent response separate the photogenerated excitons as well as to reduce the for different light intensity, which is also indicative of that electron-hole recombination rates, and finally leading to an photoconductivity of CH3NH3PbI3 is virtually enhanced by enhanced photoconductivity. incorporation of POM. To primarily explore the potential application of It is well known that the perovskite material could exhibit CH3NH3PbI3/PW12 composite, we investigated on its different responses if it undergoes an electrical bias prior to photodetection performance. We select the monochromatic 23 the measurement whether in light or dark. Gottesman et al. light of 365nm and 420nm as representative examples. The observed an extremely slow photoconductivity response in photosensitivity (R) of the device is defined as R = ΔI/PS, where CH3NH3PbI3 perovskite in a photoconductivity measurement ΔI (ΔI = Iphoto – Idark) is the difference between the photocurrent 21 system. They proposed that structural changes in the and dark current, P is the incident light intensity, and S is the perovskite under the working conditions might cause the welllight incident area. We measured its photosensitivity known hysteresis effect of solar cell. Therefore, we dependence on light intensity at 365nm light as shown in Fig. investigated the different responses of the CH3NH3PbI3 and −2 S4. Based on the irradiance of 100μW·cm at 365nm, S = 1.5 × CH3NH3PbI3/PW12 samples under applied bias 1V. Prior to the −7 2 10 m , and the bias voltage of 3V, the R of the composite photoconductivity measurements, the devices deposited by −1 device gains the maximum of 1.175A·W , while the R of pure relevant samples were held in dark for 0s, 5s 10s, and 20s −1 CH3NH3PbI3 is 0.646A·W . Compared to the pure CH3NH3PbI3 respectively. As shown in Fig. S6, the CH3NH3PbI3 deposited device, the R of the CH3NH3PbI3/PW12 composite device has an devices demonstrated a slow response in the I-t curves except increase of 88%. The photodetection for 420nm was illustrated the device under applied bias for 0s (Fig. S6a). Moreover, this in Fig. S5, and the maximum of the R value for the behaviour was dependent on the dark time before illumination. −1 CH3NH3PbI3/PW12 composite device is 0.621A·W , while the R As the dark time increased, the response became slower. The −1 value of pure CH3NH3PbI3 device is 0.224A·W . The R of current curves in Figure S6e, Figure S7e were derived from the CH3NH3PbI3/PW12 composite device is 179% larger than that of normalization, without physical meaning. Also, similar slow pure CH3NH3PbI3. These results suggest a great potential of responses were found in the PW12 additive CH3NH3PbI3 developing new type of photodetector based on the perovskite under the same conditions as in Fig. S7. CH3NH3PbI3/PW12 composite. To investigate the role of PW12 in the photoconductivity In summary, we prepared a perovskite-polyoxometalate enhancement, we measured photoluminescence (PL) spectra composite by using a low-temperature and solution-processed for both the CH3NH3PbI3 and CH3NH3PbI3/PW12 samples, as method, and demonstrated for the first time that the
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Park, Small, 2015, 1, 10. M. S. and J. Bisquert, J. Phys. Chem. Lett., 2014, 5, 2357. 17 C. Wehrenfennig, M. Z. Liu, H. J. Snaith, M. B. Johnston, and L. 49 A. Abrusci, S. D. Stranks, P. Docampo, H. L. Yip, A. K. Y. Jen M. Herz, J. Phys. Chem. Lett., 2014, 5, 1300. and H. J. Snaith, Nano Lett., 2013, 13, 3124. 18 L. T. Dou, Y. (M.) Yang, J. Bi. You, Z. R. Hong, W. H. Chang, G. 50 B. B. Xu, M. Lu, J. Kang, D. G. Wang, J. Brown, and Z. H. Peng, Li, Y. Yang, Nat.,Commun., 2014, 5, 5404. Chem. Mater., 2005, 17, 2841. photoconductivity enhancement could be gained by incorporating polyoxometalate (PW12) into perovskite The photoconductivity measurements (CH3NH3PbI3). manifested that the enhanced photocurrent of CH3NH3PbI3/PW12 composite was ca. 3-folds higher than that of the pristine CH3NH3PbI3. In addition, the enhanced photodetection performance of the composite was also revealed to be 82% (365nm) and 177% (420nm) higher than that of the pure perovskite device, respectively. Such enhancements in photoconductivity and photodetection performance should be attributed to the incorporation of POM, which provides a highly effective electron transfer pathway to separate the photogenerated excitons and reduce the electron-hole recombination. These results inspirit us to develop advanced perovskite-POM composite materials which may find further applications in solar cells and optoelectronic devices, etc. The authors are thankful for the financial support from the Natural Science Foundation of China (Grant No. 21273031 and 21401020). This work is also supported by the Fundamental Research Funds for the Central Universities (14QNJJ012).
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A highly photoconductive composite prepared by incorporating polyoxometalate into perovskite for photodetection application
DOI: 10.1039/x0xx00000x
Yuzhuo Zhang, Ran Tao, Xuemin Zhao, Zhixia Sun*, Yanju Wang and Lin Xu* www.rsc.org/
The photoconductive perovskite-polyoxometalate composite was prepared for the first time by a facile low-temperature and solution-processed method, and this composite exhibited a significantly enhanced photoconductivity and photodetection performance due to introducing polyoxometalate into perovskite for fine energy-level matching and efficient charge transfer. Photoconductive materials are a class of functional solidstate components which can increase the electrical conductivity by absorption of photons, and the intrinsical “photoconductivity” is closely relevant with the absorption of the incident light, exciton generation and separation, carrier transport, and electron-hole recombination. Thus the research on photoconductivity of chemical composite materials is very important due to their potential applications in the areas such as solar cells, photodetectors, optical switching devices and 1-5 gas sensors. Conventional semiconductors like TiO2, GaN, WO3, and ZnO, are the most common photoconductive materials. However, the unsatisfactory properties of relative wide direct band gap, low absorption coefficient as well as high manufacturing cost limit their applications in various 6-9 optoelectronic devices. Recently, the next-generation of photoconductive materials, organic−inorganic hybrid perovskites (CH3NH3PbX3, X=Cl, Br, I), 10 with the advantages of direct band gap, high absorption 11 12 coefficient, ambipolar charge mobility, and long diffusion 13 length, have shown great potential for the fabrication of low14-16 cost and high efficiency solar cells, light emitting 17 18 materials, and photodeteotors, etc. Despite the intense research in the past several years, some problems still remain in the preparation of perovskite-based devices with high efficiency and good reproducibility, because the preparation conditions, the interfacial morphology as well as the measurements protocols could affect the property of
a.
Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, P. R. China. E-mail: [email protected]; Fax: +86-431-85099765; Tel: +86-431-85098760 Electronic Supplementary Information (ESI) available: experimental details, fabrication of photoconductive device and measurements used in this work. See DOI: 10.1039/x0xx00000x
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perovskites. Thus, many optimization processes have been 24 carried out as follows: interface modification, optimization of 25 26 the perovskite precursor solution, crystallization control, 27 28 charge dynamics studies, new deposition processes, 29 minimization of hysteresis effect, and optimizing 30-32 morphology. Among these approaches, various additives have been employed for tuning functional properties. For instance, the performances of the perovskite-based devices could be enhanced by adding CH3NH3Cl, 1, 8-diiodooctane (DIO), or polymers into their corresponding standard precursor 30-35 solutions. Therefore, introducing suitable additives into the perovskite to form perovskite-based composite materials is an effective strategy to improve their optoelectronic performance. Polyoxometalates (POMs), a class of molecular metal-oxo cluster compounds based mainly on Mo, W, and V, have shown superior physicochemical properties and various 36-38 applications in catalysis, materials science and medicine. POMs exhibit many excellent features such as low conduction band, highly tunable electronic properties, and high 39 transparency in the visible spectrum, etc. In particular, POMs generally act as the electron acceptor in chemical reaction. In recent years, our group carried out several researches on the incorporation of POMs into semiconductor films for improving the light-to-electricity conversion performance of 40-42 semiconductors. Also, we have reported an advanced photoconductive device based on the POM/TiO2 composite, which achieved a significantly enhanced photoconduction and 43 gas sensing property. These above-mentioned results suggested that the smart combination of POMs with semiconducting perovskite may create a new class of photoconductive composite materials of high performance. Herein, we report the preparation of perovskite-POM composite (perovskite is CH3NH3PbI3; POM is PW12) and the characterization of photoconductive performance by means of a test device. Compared to the pure perovskite device, the composite device showed a significant enhancement in photoconductive performance. Furthermore, the photodetection performance of these devices for the incident
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Fig. 2 SEM images of CH3NH3PbI3 (a) and CH3NH3PbI3/PW12 composite (b) deposited on ITO substrate.
Fig. 2 shows the SEM top-view images of the CH3NH3PbI3 and CH3NH3PbI3/PW12 composite film deposited on ITO glass.The SEM image of the separate ITO substrate and the energy dispersive X-ray (EDX) were shown in Fig. S1. It is worth noted that the film formation in our work was under ambient air at relative humidity (RH) ≈ 40%. As shown Fig. 2a, the coverage of the CH3NH3PbI3 film was not complete, but the Fig. 1 Schematics of the device structure (a); IR spectra (b), XRD patterns (c) and case was well matched with the previous publication.46 Also, UV-vis diffuse reflectance spectra (d). the similar morphology and coverage of CH3NH3PbI3/PW12 film light of 365nm and 420nm was demonstrated. These results were observed in Fig. 2b, suggesting that the PW12 did not provide valuable information for developing composite affect the perovskite formation. In Fig. S1, the energy materials of perovskite-POM for applications in various dispersive X-ray (EDX) analysis of CH3NH3 PbI3/PW12 composite film revealed the presence of basic elements of CH3NH3PbI3 advanced optoelectronic and PW12. Besides, from the observation of the three element Fig. 1a shows a schematic diagram of the photoconductive mappings of Pb, I and W, we can infer that PW12 is uniformly test device. The relevant detail of experiment and device fabrication is provided in Electronic Supporting Information distributed throughout the CH3NH3PbI3 film. To investigate the photoconductive performance of the (ESI). Fig. 1b displays the IR spectra of pure CH3NH3PbI3 and the CH3NH3PbI3/PW12 composite. In comparison with relevant samples, we carried out a series of measurements of CH3NH3PbI3 curve, the CH3NH3PbI3/PW12 curve exhibits the photoconductive devices under Xe lamp irradiation. Fig. -1 characteristic bands for the POMs at 700–1100 cm . The 3(a) displays the photocurrent responses from both devices of -1 the CH3NH3PbI3 and the CH3NH3PbI3/PW12 composite. Both the characteristic peaks observed at 1080, 983, 890, and 798 cm are assigned to υas (P–O), υas (W=O) and υas (W–O–W), samples show steady and reproducible photocurrent respectively, suggesting that the Keggin structure of the PW12 responses during several on/off cycles under the Xe lamp irradiation, whereas the pure PW12 device does not show the is retained in the composite. photoresponse. Compared to pristine CH3NH3PbI3, the Fig. 1c shows the XRD patterns of the pure CH3NH3PbI3 and composite material of CH3NH3PbI3/PW12 displays a remarkable the CH3NH3PbI3/PW12 composite. The XRD of pure PW12 film deposited on ITO is exhibited in ESI. The intense diffraction enhancement (ca. 3-fold) in the photocurrent. Evidently, the peaks at 14.22°, 28.50°, 32.01°, 40.1°, and 43.26° can be presence of PW12 accelerates the photocurrent saturation assigned to (110), (220), (310), (224), and (314) diffractions, when light irradiation, indicating that the electron-hole respectively. The large number of diffraction peaks consistent recombination can be efficiently suppressed due to both the with the reported data could confirm the tetragonal crystal energy level matching and the “shallow electron trap” function 44 structure of CH3NH3PbI3. Also, a close examination of the of PW12. When the light is turned off, slow photoresponse region around the (110) diffraction peak at 14.12° shows no decay occurs in Fig. 3b. The slow decay of the photoresponse measurable peak at 12.65° for PbI2, indicating a high level of represents the strong tendency of capturing charge carrier 47 phase purity. The same diffraction peaks of perovskite resulting from the “shallow electron trap” of PW12. The appeared in both the pure sample and the composite sample, trapped charges in PW12 could be slowly released later under indicating that the perovskite structure still remains in the bias voltage after stopping light radiation. The dependence of current–voltage (I–V) curves on light composite. No diffraction peaks for PW12 are observed in the composite (Fig. 1c and Fig. S2), which may be due to the its intensity for both the pure CH3NH3PbI3 device and the 45 CH3NH3PbI3/PW12 composite devices has been measured (Fig. amorphous state. 3c and d). Upon illumination, the photocurrent of the devices As displayed in the UV–Vis diffuse reflectance spectra of Fig. increases drastically with the increasing of the external bias 1d, both the pure CH3NH3PbI3 and CH3NH3PbI3/PW12 exhibit high light-harvesting capabilities from the ultraviolet to visible voltage. At the same voltage, the light current increases spectra. Since PW12 is high transparent in the most spectral gradually with the increasing of light intensities. Note that the composite device shows higher range, the incorporation of PW12 does not affect the light CH3NH3PbI3/PW12 photocurrent responses than the pure CH3NH3PbI3 device at adsorption of CH3NH3PbI3. each voltage with various irradiation intensities. Furthermore,
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shown in Fig. 3e. It was well-known that PL originates from the 48 radiative recombination of electrons and holes. As seen from Fig. 3e, both the CH3NH3PbI3-based devices show the emission maxima at 776 nm, which is in agreement with the reported 49 results in the literature. Moreover, a clear decrease of PL intensity for the CH3NH3PbI3/PW12 composite could be observed, and the maximal PL intensity of the CH3NH3PbI3/PW12 composite was quenched by 65% with respect to that of pristine CH3NH3PbI3. Such a PL quenching demonstrates that the electron transfer from perovskite to PW12 surely occurred so as to reduce the probability of 50 electron–hole recombination. Therefore, the trapping photoexcited electrons and afterward effective transfer is the key role of PW12 in the CH3NH3PbI3/PW12 composite system. For the CH3NH3PbI3/PW12 composite system, the proposed mechanism for the enhanced photoconductivity is as follows: Without illumination and bias voltage, the device is in its equilibrium state with a Schottky barrier of ITO/ CH3NH3PbI3 interfaces, and there were no notable carriers transfer because the carriers have to overcome the Schottky barrier during transporting. However, the Schottky barrier height was decreased for facilitating carriers transporting when the free carrier density increased upon illumination and bias voltage. Fig. 3 Current-Time (I-T) curves (a); Photocurrent decay curves (b); CurrentSince the relative energy band alignment of the PW12 Voltage (I-V) curve of CH3NH3PbI3 (c); CH3NH3PbI3/PW12 (d); PL spectra (e); correlates quite well with CH3NH3PbI3 band structure (Fig. 3f), Energy levels and the electron transfer processes (f). the majority carrier of electrons from CH3NH3PbI3 can easily transfer to the conduction band of PW12, resulting in a the photocurrent on different irradiance is shown in Fig. S3. significant enhancement in conductivity. The presence of POM Compared to the pure CH3NH3PbI3 device, the provides a highly effective electron transfer pathway to CH3NH3PbI3/PW12 device shows higher photocurrent response separate the photogenerated excitons as well as to reduce the for different light intensity, which is also indicative of that electron-hole recombination rates, and finally leading to an photoconductivity of CH3NH3PbI3 is virtually enhanced by enhanced photoconductivity. incorporation of POM. To primarily explore the potential application of It is well known that the perovskite material could exhibit CH3NH3PbI3/PW12 composite, we investigated on its different responses if it undergoes an electrical bias prior to photodetection performance. We select the monochromatic 23 the measurement whether in light or dark. Gottesman et al. light of 365nm and 420nm as representative examples. The observed an extremely slow photoconductivity response in photosensitivity (R) of the device is defined as R = ΔI/PS, where CH3NH3PbI3 perovskite in a photoconductivity measurement ΔI (ΔI = Iphoto – Idark) is the difference between the photocurrent 21 system. They proposed that structural changes in the and dark current, P is the incident light intensity, and S is the perovskite under the working conditions might cause the welllight incident area. We measured its photosensitivity known hysteresis effect of solar cell. Therefore, we dependence on light intensity at 365nm light as shown in Fig. investigated the different responses of the CH3NH3PbI3 and −2 S4. Based on the irradiance of 100μW·cm at 365nm, S = 1.5 × CH3NH3PbI3/PW12 samples under applied bias 1V. Prior to the −7 2 10 m , and the bias voltage of 3V, the R of the composite photoconductivity measurements, the devices deposited by −1 device gains the maximum of 1.175A·W , while the R of pure relevant samples were held in dark for 0s, 5s 10s, and 20s −1 CH3NH3PbI3 is 0.646A·W . Compared to the pure CH3NH3PbI3 respectively. As shown in Fig. S6, the CH3NH3PbI3 deposited device, the R of the CH3NH3PbI3/PW12 composite device has an devices demonstrated a slow response in the I-t curves except increase of 88%. The photodetection for 420nm was illustrated the device under applied bias for 0s (Fig. S6a). Moreover, this in Fig. S5, and the maximum of the R value for the behaviour was dependent on the dark time before illumination. −1 CH3NH3PbI3/PW12 composite device is 0.621A·W , while the R As the dark time increased, the response became slower. The −1 value of pure CH3NH3PbI3 device is 0.224A·W . The R of current curves in Figure S6e, Figure S7e were derived from the CH3NH3PbI3/PW12 composite device is 179% larger than that of normalization, without physical meaning. Also, similar slow pure CH3NH3PbI3. These results suggest a great potential of responses were found in the PW12 additive CH3NH3PbI3 developing new type of photodetector based on the perovskite under the same conditions as in Fig. S7. CH3NH3PbI3/PW12 composite. To investigate the role of PW12 in the photoconductivity In summary, we prepared a perovskite-polyoxometalate enhancement, we measured photoluminescence (PL) spectra composite by using a low-temperature and solution-processed for both the CH3NH3PbI3 and CH3NH3PbI3/PW12 samples, as method, and demonstrated for the first time that the
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Such enhancements in photoconductivity and photodetection performance should be attributed to the incorporation of POM, which provides a highly effective electron transfer pathway to separate the photogenerated excitons and reduce the electron-hole recombination. These results inspirit us to develop advanced perovskite-POM composite materials which may find further applications in solar cells and optoelectronic devices, etc. The authors are thankful for the financial support from the Natural Science Foundation of China (Grant No. 21273031 and 21401020). This work is also supported by the Fundamental Research Funds for the Central Universities (14QNJJ012).
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