Tuning Magneto-Photocurrent Between Positive and Negative

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Tuning Magneto-Photocurrent Between Positive and Negative Polarities in Perovskite Solar Cells Wenbin Li, Sijian Yuan, Yiqiang Zhan, and Bao-Fu Ding J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00571 • Publication Date (Web): 10 Apr 2017 Downloaded from http://pubs.acs.org on April 10, 2017

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The Journal of Physical Chemistry

Tuning Magneto-photocurrent between Positive and Negative Polarities in Perovskite Solar Cells Wenbin Li,† Sijian Yuan,‡ Yiqiang Zhan,∗,‡ and Baofu Ding∗,¶ †Department of Materials Science, Fudan University, Shanghai 200433, China ‡State Key Laboratory of ASIC and System, SIST, Fudan University, Shanghai 200433, China ¶Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen, Guangdong 518055, China E-mail: [email protected]; [email protected]

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Abstract Magneto-photocurrent, namely magnetic-field-modulated photocurrent, with a single polarity has been observed in perovskite solar cells, and attracted great interest, due to its potential application as a non-contact approach to characterizing and adjusting electro-optic characteristics of perovskite solar cells. Here, we demonstrate that magneto-photocurrent polarity can be tuned between positive and negative ones by adjusting the size of perovskite crystal domain in perovskite film. The experimental results show that 1) magnetic-field-enhanced intersystem crossing between singlets and triplets, 2) dissociation of the singlets and 3) diffusion of the triplets have a combined impact on the magneto-photocurrent polarity. Further study reveals that the magnetic-field-induced increase of average dissociation rate and decrease of average diffusion rate diffusion lead to negative and positive magneto-photocurrent, respectively. As a result, our investigations on magnetic field effects of perovskite solar cells provide a penetrating insight on the spin-correlated photon-to-charge dynamic process in all perovskite based optoelectronic devices.

Introduction The lead halide hybrid perovskite have become extremely attractive due to its many potential applications in light emitting devices 1 and high-efficiency solar cells. 2–6 In the past several years, the power conversion efficiency of perovskite solar cells has undergone continuous breakthroughs via material processing and device engineering efforts. 3,5,7,8 Since the deposition of the lead halide hybrid perovskites is performed in room temperature, such as printing, spin-coating and dipping coating based solution-processed deposition, perovskite solar cells (PSCs) have been considered as promising alternatives of conventional silicon solar cells due to their high power conversion efficiency and low power-consumption fabrication. 4,5,8–10 It is well known that the morphology of perovskite film, including the size of crystal domain and surface roughness, can be precisely controlled by the preparing processes, which has 2 ACS Paragon Plus Environment

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been demonstrated to determine the performance of perovskite solar cell. 4,10–16 However, the impact of the size of crystal domain on spin dependent photon-to-charge dynamic process are not yet revealed. 17–19 Generally, a perovskite film has many naturally formed micrometerscale crystalline domains, in or between which the photon-excited electron-hole (e-h) pairs can be formed. 20,21 The subsequent evolution of these e-h pairs, including dissociation, diffusion and recombination, determines the final detectable photocurrent. Since the e-h pairs are in the given spin configuration of singlet or triplet state, magnetically characterizing the evolution of these e-h pairs will shed light on the vague understanding of spin dependent photon-to-charge dynamic process. This is also essential for controlling the photovoltaic actions, and thus provides strategy to further improve the performance of PSCs. Recently, magnetic field effect (MFE) has been employed as a simple but powerful tool for investigating the above mentioned spin-dependent photon-to-charge dynamic processes in organic optoelectronics. The output signals of optoelectronic devices, such as the resistance, photocurrent, electroluminescence and photoluminescence, could be modulated by an external magnetic field, B, which are named as magnetoresistance, magneto-photocurrent (MPC), magneto-electroluminescence and magneto-photoluminescence (MPL), respectively. In 2015, MFEs were observed in PSCs at room temperature. Zhang et al., found an MPC and an MPL responses in hybrid perovskite devices which have the same negative polarities. 17 At the same time, Hsiao et al., reported a positive MPC and a negative MPL at room temperature, under a photoexcitation over a threshold intensity. 18 Some basic consensuses have been reached in the two reported literatures, including 1) MFE technique provides an essential tool to characterize the spin-dependent photon-to-charge interconversion in PSCs; 2) both the monomolecular and bimolecular e-h pairs play a significant role in determining the final MPC; 3) the ratio of singlets/triplets decreases with the external B. These findings lay an important foundation for understanding the spin-dependent photon-to-charge interconversion in perovskite based optoelectronic devices. In view of Pauli Exclusion Principle, only singlet excitons evolved from singlet e-h pairs



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have high radiative transition rate to the ground state, whereupon a magnetic-field induced decrease in the singlet e-h pairs leads to a weakened PL intensity, namely, a negative MPL. Both papers have reported the negative MPLs. 17,18 It is important to note that, in addition to these consensuses, some other questions are left and open to researchers in different fields. Among these questions, a fundamental one is related to the way to control the MPC polarity. Despite the negative-polarity 17 and positive-polarity MPCs 18 have been reported separately, a lack of simultaneous observation of both polarities makes some of spin-dependent photophysical processes, including dissociation and diffusion of e-h pairs, still unclear. In this work, relying on the structure design, we successfully fabricated two types of PSCs, which have similar performances but different morphologies. The morphology control enables the device to be a dissociation-limit one (device A) or a diffusion-limit one (device B). Meanwhile, the similar performance for the two PSCs is attained to mitigate the influence of other factors on the MPC polarity, such as light absorption and generated charge densities. Finally, the negative and positive MPCs are observed in device A and B, respectively. Further analysis and experimental results show that the dissociation-limit device A prefers more singlets formation in the active layer, while the diffusion-limit device B favors more triplets. Our investigations give a deep understanding on a fundamental issue of spin-dependent dissociation and diffusion processes, and pave the new way to further improve performance of PSCs by applying different spin modulating methods on the given type of PSCs.

Experimental Material Lead chloride (PbCl2 ) and lead iodide PbI were both purchased from Sigma-Aldrich. Methylammonium iodide (MAI) was purchased from Materwin. All the materials were used as received without further treatment. Indium-Tin Oxide (ITO) substrates were purchased from 4 ACS Paragon Plus Environment

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Shenzhen Nanbo Co. Ltd., China.

Methods Perovskite precursor solution preparation: To generate the precursor solution A, MAI and PbCl2 were dissolved in anhydrous N,N-Dimethyformamideat a 3:1 molar ratio with the concentration of 40 wt %. The precursor solution B was prepared by adding MAI and PbI at a 10:1 molar ratio into 1-Butanol with a concentration of 30 mg/ml, and then gamma-butyrolactone was added with a volume ratio of 1:6. The suspension was stirred. Finally, the supernatant of the solution was used as the precursor solution B. Both the prepared precursor solution A and B were ready to use in the subsequent device fabrication. Device fabrication: ITO substrates with ∼10 Ω/ resistance were cleaned with 1) mild detergent, 2) ethanol, and 3) distilled water in an ultrasonic bath. Finally, they were dried in vacuum. All the devices in this work have the same structure of ITO / PEDOT:PSS / CH3 NH3 PbI3 / PCBM / C60 / BCP / Ag. PEDOT:PSS, PCBM BCP are abbreviations of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, phenyl-C61 -butyric acid methyl ester and bathocuproine, respectively. PEDOT:PSS was spincoated on ITO at a speed of 4000 rpm for 30 s from an aqueous solution (CLEVIOS P VP Al 4083), followed by thermal annealing at 140 ◦ C for 30 min in vacuum. The perovskite layers were formed by spincoating the prepared precursor solution A onto the substrate at 1800 rpm, and annealed at 100 ◦ C for 90 min under a high-purity nitrogen environment. Once cooled, device B was dipped in the precursor solution B on a hot plate at 100 ◦ C for 2 minutes, and then cleaned by isopropyl alcohol. After that the solution with a weight ratio of 20 mg/ml PCBM in chlorobenzene was spincoated at 1500 rpm for 60 s. Finally, all samples were transferred into a vacuum chamber, where C60 layer (20 nm), BCP layer (8 nm) and Ag electrode (100 nm) were thermally evaporated at rates of 0.8, 0.4 and 2 ˚ A/s, respectively, at a base pressure of around 10−5 Pa. The device area, defined by the overlap between the ITO and



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Ag electrodes, was 6 mm2 .

Magnetic effect measurement: The fabricated devices were transferred into a vacuum chamber for the subsequent magnetic-optical-electrical property measurement. The photocurrent was generated by putting PSCs under the illumination of blue light beam with a wavelength of 470 nm and a light intensity of 10 mW/cm2 , which was emitted from a light emitting diode. The MPC measurements were performed at room temperature by placing the devices between two opposite magnetic poles of an electromagnet, and measured by Keithley 2602B, when the external magnetic field was switched on and off for several times to improve the signal/noise ratio. Here, the MPC is defined as ∆J/J = (JB − J0 )/J0 , where JB and J0 are the current with and without external magnetic field B, respectively. To ensure the experimental reliability of measured MPC, the average results were given after several-round measurements. The maximum strength of magnetic field generated by the electromagnet was 500 mT.

Other Characterization: The J-V curves were measured by ZAHNER CIMPS electrochemical workstation, under irradiance of simulated AM 1.5 sun with an intensity of 100 mW/cm2 , which was generated by a class AAA sun simulator (SF300-A Sciencetech-Inc., Canada). A desktop scanning electron microscope (SEM) (Phenom Pro) was used to take images of surface morphologies. The X-ray diffraction were obtained from samples of perovskite deposited on ITO/PEDOT:PSS substrate using an X-ray diffractometer (Bruker-AXS D8).

Results Figure 1a shows the structure of ITO / PEDOT:PSS / CH3 NH3 PbI3 / PCBM / C60 / BCP / Ag. J-V characteristics of device A and B (the difference in fabrication between two devices has been introduced in the experimental section) are shown in figure 1b. Device A has a power conversion efficiency (PCE) of 7.3%. The short circuit current density (Jsc ), open 6 ACS Paragon Plus Environment

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Figure 1: (a) Schematic view of the device structure. (b)J-V curves of device A and B

circuit voltage (Voc ), and fill factor (FF) are 15.2 mA/cm2 , 0.9 V and 53.6 %, respectively. The PCE of device B is 8.1%, and the measured Jsc , Voc , and FF are 14.1 mA/cm2 , 0.86 V and 66.7 %, respectively. Both EQE and absorption spectra (not shown here) for device A and B are almost same, implying the similar Jsc for two devices. Therefore, device A and B have the similar performance. Generally, the performance of PSC is closely dependent on the morphology of perovskite film. Some reports have shown that the dissociation rate, recombination rate and diffusion length of e-h pairs can be modulated by selecting proper fabrication methods via their modulation on the morphology of perovskite film. 6,22,23 For example, the boundaries of crystal domains formed in the active perovskite film provide interfaces for both photo-generated e-h pair dissociation and charge scattering. 6,22,23 Therefore, the effective control over the size and distribution of crystal domains is crucial for obtaining a dissociation-limit or a diffusionlimit PSC, which allows the concentrated investigation of the impact of magnetic field on the dissociation or diffusion dynamic process. SEM images of the perovskite film A and B, are compared in Figs 2 (a and b). For film A, larger and less domains with an average size in the range of 1.5-2 µm was displayed. The



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Figure 2: SEM images of perovskite film (a) A and (b) B. The perovskite layers were deposited on ITO/ PEDOT:PSS substrates. The preparation process of perovskite film A and B are the same as that of device A and B, respectively. The scale bars are 3 µm.

sizes for some domains are even larger than 3 µm. While for film B, the size and number of domains are smaller and more, respectively, in comparison with those of film A. For example, the average domain size for film B is in the range of 0.5-1 µm. Generally, film A is the moisture-assisted perovskite film, which is annealed in air, whereupon new nucleations are hardly formed in the relatively high-moisture environment. Accordingly, the finite nucleations and full growth of crystal result in the less domain formation and larger domains size, 13 as shown in figure 2a. While for film B, which was specially annealed in precursor solution B, undergoes fast nucleation at the spin-coating stage, followed by the modest crystal growth during annealing in precursor solution B. The supersaturation consequently happens during fast evaporation of solvent, which leads to the formation of new nucleated perovskite crystallines. 24 Therefore, as shown in figure 2b, film B comprises many densely packed small crystal domains with the clear boundaries, which are only in the scale of several hundred nanometers. The large-scale and interconnected domains as shown in figure 2a can penetrate through the perovskite film, and function as charge transporting channels. The generated free charges can almost freely transport or diffuse inside the channels toward the electrodes, experiencing the minimum scatter from boundary. At the meantime, the boundary is propi8 ACS Paragon Plus Environment

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tious for the e-h pair or exciton dissociation. Therefore, despite the diffusion distance is long in film A, the finite boundary possibly restricts the averaged dissociation rate. Device A is consequently regarded as a diffusion-efficient but dissociation-limit one. Conversely, film B has much larger area of domain boundary as shown in figure 2b. Thus, the dissociation of e-h pair is efficient, but the charge or e-h pair during transport or diffuse through the perovskite film will encounter the enhanced scatter from the boundary. As a consequence, device B is considered as a dissociation-efficient but diffusion-limit one.

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Figure 3: Magneto-photocurrent of device A under 470 nm light illumination. The magnetophotocurrent results are measured under a) 0.1 V and b) -0.2 V external bias, respectively. The black lines through the data points are Lorentzian function fit.

Figure 3 shows the MPCs of device A under the 470 nm excitation light with an intensity of 10 mW/cm2 . The MPC response as a function of external magnetic field at the applied external bias of 0.1 V and -0.2 V are similar with each other as shown in figure 3a and b, respectively. Note that, to confirm that MPC is only from the perovskite film, a control device without the perovskite film was fabricated: ITO / PEDOT:PSS / PCBM / C60 / BCP / Ag). No MPC (not shown here) can be detected in the control device, indicating that MPC is an intrinsic phenomenon occurred in the perovskite film. As shown in figure 3a, the MPC polarity is negative. A fast increasing region of 0 - 150 mT and a nearly saturate region of >150 mT constitute the whole MPC curve, which can be well fitted with Lorentzian functions 18 as plotted by black lines in Figs 3a and b. Such MPC response including its negative polarity and Lorentzian shape, is fully consistent with those observed by Zhang et 9 ACS Paragon Plus Environment


al. 17 Meanwhile, MPC value in the saturate region is almost the same as reported one. For example, MPC is ∼ 0.02% at 100 mT as shown in figure 3a, while the reported MPC for the high-efficiency device at the same magnetic field is around 0.03%. The consistence of the polarity, shape and value of MPC response with the reported ones indicates the high reliability of experimental results in this work.

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Figure 4: Magneto-photocurrent of device A under 625 nm light illumination and 0.1 V external bias. The black lines through the data points are Lorentzian function fit.

To investigate the relation between MPC response and the wavelength of excited light, another monochromatic light with a wavelength of 625 nm is employed to excite device A. Figure 4 shows MPC response of device A under the illumination of 625 nm light and 0.1 V bias. The difference between MPC response in figure 4 and figure 3a can be negligible. Therefore, the applied voltage and light wavelength are excluded to have influence on MPC response as evidenced by almost the same MPC responses in Figs. 3a, 3b and 4. Figure 5 shows MPC response of device B under the same test condition in figure 3a. Noise in figure 5 is quite high due to the much weakened MPC signal. After several-round test, the positive MPC response in figure 5 can be repeated, which can be well fitted by the Lorentzian function. Note that, for device A and B, their PCE and Jsc are almost same as shown in figure 1. However, MPC polarities for device A and B are opposite with each other. As mentioned above, the impact of external factors, including the applied voltage, light wavelength, light absorption and charge/e-h pair density, have negligible influence on MPC response. Therefore, the opposite polarities for device A and B mainly result from the 10 ACS Paragon Plus Environment

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Figure 5: Magneto-photocurrent of device B under 470 nm light illumination and -0.1 V external bias. The black lines through the data points are Lorentzian function fit.

apparent difference in the morphology, achieved by the control of fabrication condition. ͣͤ ͣͤ

Figure 6: X-ray diffraction spectra. The perovskite layers were deposited on ITO/ PEDOT:PSS substrates. The black line and red line are the normalized spectra of layer A and B, respectively. The preparation process of perovskite film A and B are same as that of device A and B, respectively. The inset shows the zoom-in of XRD curves around 28◦ .

In the end, to investigate the impact of crystallinity on the polarity of magnetophotocurrent, XRD patterns of perovskite films A and B are measured and shown in figure 2. The curves with black color and red color present the normalized XRD spectra of films A and B, respectively. The peak around 14◦ is observed in both films A and B, which is indexed to the (110) plane of perovskite crystal. For the spectrum around 28◦ , which is amplified and shown in the inset of figure 2, two peaks, located at 28.1◦ and 28.3◦ , can be found for film A, which are indexed to the (004) and (220) planes for the tetragonal I4/mcm phase, respectively, whereas the (220)-plane dominated peak is detected for film B. It is important 11 ACS Paragon Plus Environment


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to note that except for the observed three peaks, which originate from the crystal structure of perovskite film, none of the other diffraction peaks was observed, indicating that both perovskite films A and B are fully-grown crystals. Therefore, it can be concluded that in the work, the magneto-photocurrent polarity is mainly dominated by the crystal domain size rather than crystal crystallinity.

Discussion

Figure 7: Diagram to schematically show the generation of negative and positive magnetophotocurrent via the magnetic field-induced change of singlet/triplet ratio.

In a formed e-h pair, the difference in g factors, namely ∆g, between electron and hole (spin = 1/2) can lead to different precession frequencies of their respective spins in an external magnetic field. Generally, in the ordinary organic semiconductors, ∆g is very small and in the order of 10−3 . 25 However, the ∆g in perovskite film has been characterized to be as surprisingly large as ∼0.65 in the recent reported work. 17 In this case, the difference in spin precession frequency between the electron and hole is quite large, which can be expressed as:

∆ω = µB ∆gB/~


(1)

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where ω, µB and ~ are precession frequency, Bohr magneton and Planck constant. The large ∆ω can induce the strong intersystem crossing between singlets, S, to triplets, T0 . Meanwhile, the dissociation rates and diffusion rates for S and T0 e-h pairs or excitons are not equal to each other. The magnetic field-induced modulation in the number of S and T0 e-h pairs or excitons subsequently changes the averaged dissociation and diffusion rates in the perovskite film, which directly determines the final photocurrent. This is the intrinsic physical process to generate the so-called MPC in this work. In general, due to the Spin Conservation Law, the photogenerated e-h pair excitons in the perovskite film are all in the singlet states. Therefore, in the presence of B, triplets are generated and increases via the strong intersystem crossing from the originally formed singlets (upper part in figure 7). For the dissociation process, due to their lower binding energy, singlet excitons in perovskite film can efficiently dissociate into free carriers. In other words, the dissociation rate of singlets dS is larger than that of T0 exciton dT (dS > dT ). Therefore, it can be inferred that an external magnetic field can decrease the generation of free charge carriers due to the reduced singlets via the field-dependent ISC process. As a result, the reduced dissociation rate of excited excitons essentially leads to a negative MPC. For the transport or diffusion process, the generation of free carriers from other interactions becomes possible, such as the triplets-trapped-charges interaction and triplets-electrode interaction. Typically, triplets have longer lifetime, indicating a larger diffusion rate, in comparison with that of singlets. This implies that an increased number of triplets, via the magnetic-field-enhanced intersystem crossing process, can boost the triplet-based free charge generation during diffusion process, which corresponds to the positive MPC. In short, with the increase of triplets, the averaged dissociation rate decreases while the diffusion rate increased, which dominate the negative and positive MPC components, respectively. Therefore, all the observed MPCs in this work comprise both the negative and positive components, i.e., -MPCS from the reduced dissociation rate and +MPCT from the increased diffusion in perovskite film (figure 7).



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Finally, as discussed on figure 2 and evidenced by figure 6, device A is regarded as a diffusion-efficient but dissociation-limit one, while device B is a dissociation-efficient but diffusion-limit one. Therefore, for device A, the photocurrent is more sensitive to the dissociation rate. Consequently, the component of -MPCS dominates the total MPC, which results in a negative polarity of MPC response as shown in figure 3 and 4. While for device B, the photocurrent is mainly dependent on the diffusion rate. The component of +MPCT resultantly dominates the total MPC, and thus leading to a positive polarity of MPC response as shown in figure 5. The findings in this work clearly show that the control over the crystallinity can be used as a powerful method to determine which type of PSC could be, namely, a dissociation-limit one or a diffusion-limit one. More importantly, the magnetic field effect observed in this work can function as a useful tool to identify which factor of diffusion and dissociation severely restricts the performance of a given PSC.

Summary We have demonstrated that the polarity of magneto-photocurrent can be modulated between positive and negative ones by adjusting the size of perovskite crystal domain in pervskite film. The experimental results have shown that magnetic-field-enhanced intersystem crossing between singlets and triplets, the change in averaged dissociation and diffusion rates have a combined impact on the final magneto-photocurrent polarity. It has been displayed that the reduced average dissociation rates and the increased average diffusion rate, in the presence of an external magnetic field, have opposite impacts on the magneto-photocurrent polarity, and lead to negative and positive ones, respectively. As a result, our investigations provide a penetrating insight on the spin-correlated photon-to-charge dynamic process in the perovskite film based optoelectronic devices, and also a method to identify intrinsic factors that limit the performance of perovskite solar cells.


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Acknowledgement This work was supported by the Ministry of Science and Technology of China and the National Natural Science Foundation of China (NSFC).

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