P3HT Nanowire Complex - The

Article pubs.acs.org/JPCC

Multiferroic Nanohybrid MAPbI3/P3HT Nanowire Complex Tian Xie†,‡ and Wei Qin*,‡ †

Department of Physics, Fudan University, Shanghai, 200433, China College of physics and optoelectronics, Taiyuan University of Technology, Taiyuan, 030024, China

‡

S Supporting Information *

ABSTRACT: Room-temperature multiferroic effects in organic materials have brought more attention in the area of organic spintronics and organic electronics in recent years. In this work, through fabricating MAPbI3/P3HT nanowire nanohybrid complex, the mechanisms of room-temperature ferromagnetism and magnetoelectric coupling are studied. The MAPbI3 layer provides substantial photogenerated carriers charging into the nw-P3HT layer, where the P3HT nanowire structure induces carrier spins aligning to generate room temperature ferromagnetism. Through controlling the strength of external electric field, magnetization of the MAPbI3/P3HT nanowire complex is modified due to the electric field driven carrier spins redistribution. Additionally, photon spin of circularly polarized light could interact with electron spin in organic multiferroics to enhance saturation magnetization at room temperature. interaction from organized π stacking tends to align the spins in nw-P3HT, which results in a net magnetic moment to generate room temperature ferromagnetism. However, in this type of fullerene based charge transfer complex, merohedral distribution of neighbored fullerene molecules induced π electrons spins alignment could also result in a formation of magnetic domain, which may be another origin of organic ferromagnetism.9,11,27 Additionally, excitonic ferromagnetism17,28 is also studied as the third mechanism to induce room temperature magnetism in nw-P3HT/fullerene multiferroics.29 Therefore, in this growing organic multiferroic field, although multiferroic effect is realized in an organic nw-P3HT/ fullerene system, concerns remain regarding which mechanism is the dominant one to induce room temperature organic ferromagnetism; i.e., what is the mechanism behind magnetoelectric coupling effects?

1. INTRODUCTION Multifunctional materials have attracted much interest due to their broad technological applications such as optoelectronics,1−3 thermoelectrics,4 and sensors.5 Known from individual ferromagnetic or ferroelectric materials,6−11 magnetoelectric (ME) coupling of multiferroics, showing dual electrical and magnetic degrees of freedom, has drawn increasing interest because of their tailored magnetic permeability and dielectric permittivity for potential applications.12−15 Interestingly, the coexistence of ferroelectricity, ferromagnetism in polymeric multiferroics16−19 open up a host of new functionalities that are absent in each of the individual components.17,18,20−24 Such as in 2010, ferroelectricity was observed in a one-dimensional organic charge transfer quantum magnet TTF-BA (tetrathiafulvalene-p-bromanil) below 53 K.21 In the TTF-BA charge transfer complex, spin−peierls instability plays an important role to induce magnetic field dependent variations in the electric polarization, which means converse ME coupling is realized. In contrast to the well-known spin driven ME coupling mechanism, in 2012, electric-dipole driven magnetism in an organic charge-transfer salt [k-(BEDTTTF)2CuTN(CN)2UC), below 26 K] is suggestive to present another type of ME coupling, direct ME coupling.18 Recently, room temperature multiferroic effects and magnetoelectric coupling are studied in P3HT nanowire (nw-P3HT)/fullerene charge transfer complex,17,22−26 where strong exchange © XXXX American Chemical Society

2. EXPERIMENTAL SECTION 2.1. P3HT Nanowire Fabrication. First, P3HT (bought from Sigma) was dissolved by 1,2-dichlorobenzene (1,2-DCB) (15 mg/mL) in a glovebox. Then acetonitrile (ACN) was added into the P3HT solution (usually adding 10% ACN into P3HT solution) at room temperature followed by 5 min of Received: October 6, 2016

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DOI: 10.1021/acs.jpcc.6b09891 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) AFM image of nw-P3HT. (b) Photoluminescence of MAPbI3 thin film with and without nw-P3HT layer, blue line is PL quenching induced by dropping nw-P3HT on MAPbI3 thin film surface. (c) M-H loops of MAPbI3/nw-P3HT complex under dark and 700 nm 100 mW/cm2 light luminescence. The inset of panel c is the light intensity-dependent saturation magnetization of the MAPbI3/nw-P3HT complex.

response on magnetization are studied in the nanohybrid MAPbI3/nw-P3HT complex. Figure 1a shows the atomic force microscopy (AFM) morphologies of nw-P3HT. Optical properties of nw-P3HT is characterized through absorption (Figure S1). Pronounced absorption around 610 nm is observed in nw-P3HT due to strong π−π stacking, which is unsighted in amorphous P3HT. Through studying optical properties of the MAPbI3 (Figure S1b), it is noted that the absorption onset is 790 nm, which is consistent with the fact that MAPbI3 is known to have a bandgap of 1.55 eV.34−36 From Figure 1b, the PL of MAPbI3 is quenched once the nw-P3HT solution (solvent is 1,2Dichlorobenzene) is coated on the MAPbI3 surface. However, dropping solvent (1,2-dichlorobenzene) on the MAPbI3 surface could not induce PL quenching, as shown in Figure S2b. PL quenching ensures that the photogenerated substantial holes are charged into the nw-P3HT layer. In Figure 1c, it is clearly noted that, no ferromagnetism is observed in MAPbI3/nwP3HT under dark environment. However, once light is illuminated (700 nm, 100 mW/cm2) on this complex, roomtemperature ferromagnetism is generated at room temperature. In addition, MAPbI3/amorphous P3HT complex does not have this character. In previous works, three mechanisms are used to explain the origins of ferromagnetism in nw-P3HT/fullerene charge transfer complex: (i) spin alignment of charged holes in nwP3HT;22 (ii) merohedral distribution of neighbored fullerene molecules-induced electron spin alignment in fullerene;9,11,27 (iii) excitonic ferromagnetism.17,28 In organic nw-P3HT/ fullerene multiferroic complex, because charge transfer excitons, holes in nw-P3HT, and merohedral disorder of neighbored fullerene molecules coexist, it is unrealistic to distinguish which one is the dominated mechanism to induce room temperature ferromagnetism.29 Under 700 nm light illumination (P3HT nanowire do not have absorption on 700 nm), photogenerated excitons in perovskite layer experience a full ionization to result in extremely quick spontaneous free carrier generation following light absorption30−33 (only 1−2 ps32,33). However, in P3HT nanowire/PCBM charge transfer complex, the lifetime of photogenerated excitons in P3HT nanowire is around 0.6 ns.15 Therefore, exciton density is very small in MAPbI3/nw-P3HT complex comparing with nw-P3HT/PCBM complex. It is tenable that MAPbI3/nw-P3HT multiferroic complex can exclude the effect of excitonic effect on the origin of ferromagnetism. Thus, spin alignment in the P3HT nanowire structure is the main cause of room-temperature organic ferromagnetism. As shown in the inset of Figure 1c, with increasing incident light intensity, more photoexcited holes are charged into the nw-P3HT layer, which leads to a promotion of saturation magnetization in the MAPbI3/nw-

stirring. Finally, the solution was placed into the glovebox overnight at room temperature. 2.2. Perovskite Device Fabrication. Methylammonium iodide (MAI) with 10 mg/mL in IPA and lead(II) iodide (PbI2) with 462 mg/mL in DMF were prepared. These solutions were heated at 100 °C for 10 min before use. Titanium diisopropoxide bis(acetylacetonate) was spin-coated on previously cleaned ITO substrates at 6000 rpm for 30 s in N2 atmosphere, followed by annealing at 450 °C for 30 min. The PbI2 solution was spin-coated on a combined TiO2 layer at 6000 RMP for 35 s and dried at 70 °C for 3 min then at 100 °C for 5 min. 200 μL of MAI solution (perovskite nanowire: through doping 3% DMF in volume into MAI solution) was then spin-coated on the PbI2 layer 6000 RMP for 35 s. After that, the ITO/TiO2/MAPbI3 substrates were dried at 100 °C for 1 h (if nanowire MAPbI3 is prepared, substrate is dried at 100 °C for 10 min). Furthermore, the nw-P3HT was spincoated on top of the MAPbI3 layer at 2000 rpm for 60 s. Finally, the Au was evaporated as an anode with a thickness of 50 nm. 2.3. Experimental Measurements. A linearly polarized laser beam with 2 mm dimeter was combined with a 1/4 wave plate to generate switchable linearly and circularly polarized photoexcitation with identical intensity when the 1/4 wave plate was rotated with 0° and 45° relative to the direction of linear polarization of laser beam. Before time-dependent PL measurements under linear and circular polarization, devices were illuminated for 30 min to enable more stable PL measurement. M-H loop and electric field-dependent magnetization were studied through LakeShore VSM (7404).

3. RESULTS AND DISCUSSION Here, we choose the broad absorption ferroelectric organometal halide perovskites (MAPbI3) material fabricating the MAPbI3/nw-P3HT system to figure out the misgivings in organic multiferroics. MAPbI3 is a critical photovoltaic material with strong optical absorption and quick spontaneous charge dissociation due to extremely small exciton binding energy.30−33 Accordingly, using light to excite the MAPbI3/nwP3HT multiferroic complex, the MAPbI3 layer provides substantial photogenerated holes charging into the nw-P3HT layer to induce room-temperature ferromagnetism. Additionally, the dominant mechanism of inducing room-temperature ferromagnetism in the nw-P3HT based complex is distinguished. Comparing with natural light, circularly polarized light could more effectively increase the polarized spin state in the MAPbI3 layer to enhance saturation magnetization. Overall, through involving MAPbI3, the origin of organic ferromagnetism, magnetoelectric coupling, and circular polarization B


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Figure 2. (a) P-E loops of MAPbI3/nw-P3HT complex under external magnetic field. The inset shows external magnetic field-dependent electric polarization. (b) Under 100 mW/cm2 and 50 mW/cm2 light illumination, the tunability of magnetization by external electric field (104 V/cm) in MAPbI3/nw-P3HT complex. ON/OFF means external electric field is tuned on/off. (c) Under 100 mW/cm2 light illumination, electric fielddependent magnetization change ΔM = M(E) − M(E = 0). The set shows that, under different light intensity, electric field ON/OFF (100 mW/ cm2) induced the change of magnetization.

Figure 3. (a) Scheme of MAPbI3/nw-P3HT complex under circularly and linearly polarized light excitation. (b) Diagram of spin evolution in MAPbI3/nw-P3HT complex under circular and linear polarization excitation. (c) M-H loops of MAPbI3/nw-P3HT complex under circular and linear excitation with identical intensity. Circular polarization could enhance the magnetization of MAPbI3/nw-P3HT complex. (d) Light intensity dependent saturation magnetization difference between circular and linear polarization in MAPbI3/nw-P3HT complex and nw-P3HT/Fullerene complex (ΔMs = Ms(Circular)-Ms(Linear)). Circularly polarized light has no effect to tune the magnetization in nw-P3HT/Fullerene complex. (e) Circular and linear polarization dependent photoluminescence with identical incident light intensity.

the magnetization of MAPbI3/nw-P3HT complex under light illumination. With increasing the incident light intensity, the tunability of magnetization by electric filed is further enhanced. The presence of the electric field could rearrange energy levels through the Stark effect, which may induce the redistribution of spin-up and spin-down electrons to influence the magnetization (Figure 2b). Following the increase of external electric field, Stark effect becomes more pronounced, resulting in an enhancement in magnetization, as shown in Figure 2c. Additionally, with larger incident light intensity, more photogenerated holes are charged into nw-P3HT, where electric field could more effectively tune magnetization (the inset of Figure 2c). In MAPbI3/nw-P3HT complex, the tunability of magnetization by circular and linear polarization excitation are studied. The device scheme is presented in Figure 3a. Using the circular polarization to excite MAPbI3/nw-P3HT based devices, electron spin was flipped through spin angular moment conservation43,44 (Figure 3b). Linearly polarized light is a superposition of right and left-handed circular polarizations, which induces an equal number of excited spin up and down states. Therefore, under circularly polarized photoexcitation, the probability of forming triplet spin states is larger than that under linear polarization excitation with identical light intensity. Theoretically, if incident light is a linear polarization and

P3HT complex. However, switching from light illumination to dark environment induces a disappearance of ferromagnetism. In addition, it should be noted that individual MAPbI3 or nwP3HT material does not show any ferromagnetism signal, even under light illumination. Through involving ferroelectric materials MAPbI3, the ferroelectricity of MAPbI3/nw-P3HT complex can reach around 2 μC/cm2, as presented in Figure 2a, which is more pronounced than that in nw-P3HT/fullerene complex (less than 0.1 μC/cm2).37,38 In addition, positive up negative down (PUND) measurement is also studied to further confirm the ferroelectricity of MAPbI3, as shown in Figure S3. Based on P-E loops analysis, residual polarization is Pr1 = 1.2 μC/cm2 (Figure 2a), while Pr2 = 0.65 μC/cm2 (Figure S3) through PUND measurement. So the losses ratio presented in the P-E loop is (Pr1−Pr2)/ Pr1 = 45.8%. In MAPbI3 materials, although lone pair electrons of lead could induce MAPbI3 ferroelectricity,39 dipole alignment of organic polar cation MA+ mainly contributes to the ferroelectricity of perovskite.40−42 Magnetic field has no effect on the tunability of ferroelectric polarization, even applying 2 T external magnetic field (the inset of Figure 2a) in complex, ruling out the spin-driven mechanism of magnetoelectric coupling. However, charge-induced magnetoelectric coupling is observed in MAPbI3/nw-P3HT complex, as shown in Figure 2b. External electric field could effectively tune C


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conclude the dominant mechanism of room-temperature ferromagnetism is P3HT nanowire structure-induced spin alignment. In this multiferroic complex, magnetoelectric coupling results from electric field-driven spin redistribution in nw-P3HT. Circularly polarized light could more effectively tune spin state in the MAPbI3 layer to enhance saturation magnetization of this complex at room temperature.

perpendicular to the wave plate, rotating the 1/4 wave plate with 0° and 45° can switch the photoexcitation between linear and circular polarizations, which can not change the light intensity. Furthermore, light intensities of circular and linear polarization have been further confirmed experimentally. It should be noted that the spin relaxation and carrier dissociation times can essentially determine whether circular polarization-dependent spin effects can be observed on the improvement of magnetization in MAPbI3/nw-P3HT complex. In perovskite MAPbI3, switching the photoexcitation from linear to circular polarization can shift the spin population to triplet states through momentum conservation. If the charge dissociation time is comparable to spin relaxation time, shifting spin population toward parallel states can increase the density of dissociated carriers based on the fact that parallel pairs can more readily dissociate due to the forbidden recombination. However, in an inorganic semiconductor with significant spin− orbit coupling (strong spin relaxation), spin relaxation time will be shorter than dissociation time. Before charge dissociation, spins will fully relaxed although circular polarization excitation increase triplet density initially. Therefore, charge dissociation rates are nearly the same under circular and linear polarization excitation if materials are of significant spin−orbit coupling, because 1−2 ps32,33 electron−hole pair dissociation time in perovskites is comparable to a spin relaxation time of 1 ps.45 So before dissociating, substantial electrons and holes still keep their initial spin states. With circular polarization excitation, more triplet electron−hole pairs will be excited, which will be difficult to recombine (ground states are singlet states). Thus, recombination probability is larger under linear polarization (rL) than that under circular polarization (rC) before dissociating: rL > rC. After dissociating, electrons and hole will experience a long extraction time, from 10 to 100 ns. During this time period, spins in perovskites will be fully relaxed due to 1 ps spin relaxation time, which will induce same spin state distribution of linear and circular polarization excitations. So bimolecular recombination probability (rB) in perovskite is the same under linear and circular polarization. Therefore, considering photogenerated electron−hole experiencing monomolecular and bimolecular recombination processes, under circular polarization excitation, total dissociation probability in perovskite is (1 − rC)·(1 − rB), which is larger than that under linear polarization excitation: (1 − rC)·(1 − rB) > (1 − rL)·(1 − rB). Thus, under continuous circular polarization excitation on perovskite, more photoexcited holes can be charged into nw-P3HT to enhance magnetization, as shown in Figure 3c. Associating with the increase of incident light intensity, ΔMs becomes more pronounced (Figure 3d), ΔMs = Ms(C) − Ms(L), where Ms(C) and Ms(L) are the saturation magnetization under circular and linear polarization, respectively. However, circularly polarized light has no effect to tune the magnetization in organic nw-P3HT/fullerene multiferroics. Due to smaller charge recombination under circular polarization excitation, photoluminescence of MAPbI3 is weaker under circular polarization than that under linear polarization, as shown in Figure 3e.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09891. Description of MAPbI3 Abs., PL measurements, and MH hysteresis loop (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +86-351-6014672. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS W.Q. acknowledges the funding support of the Natural Science Foundation of China (11504257). REFERENCES

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P3HT Nanowire Complex - The

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