Temperature-Dependent Electric Field Poling Effects in CH3NH3PbI3

Mar 16, 2017 - When the same device is cooled under continuous poling, the built-in ... Organic–inorganic hybrid perovskites, such as CH3NH3PbX3 (X ...
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Letter

Temperature-Dependent Electric Field Poling Effects in CHNHPbI Optoelectronic Devices 3

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Chuang Zhang, Dali Sun, Xiaojie Liu, Chuanxiang Sheng, and Zeev Valy Vardeny J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00353 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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Temperature-Dependent Electric Field Poling Effects in CH3NH3PbI3 Optoelectronic Devices Chuang Zhang1, Dali Sun2, Xiaojie Liu1, Chuan-Xiang Sheng1 and Zeev Valy Vardeny1* 1

Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112, USA

2

Department of Physics, North Carolina State University, Raleigh, North Carolina, 27695, USA

*Corresponding Author; e-mail: [email protected].

ABSTRACT: Organo-lead halide perovskites show excellent optoelectronic properties; however, the unexpected inconsistency in forward/backward I-V characteristics remains a problem for fabricating solar panels. Here we have investigated the reasons behind this “hysteresis” by following the changes in photocurrent/photoluminescence under electric field poling in transverse CH3NH3PbI3-based devices from 300 K to 10 K. We found that the hysteresis disappears at cryogenic temperatures, indicating the “freeze-out” of ionic diffusion contribution. When cooling down the same device under continuous poling, the built-in electric field from ion accumulation brings significant photovoltaic effect even at 10 K. From the change of photoluminescence upon polling, we found a second dipolerelated mechanism which enhances radiative recombination upon the alignment of the organic cations. The ionic origin of hysteresis was also verified by applying magnetic field to affect the ion diffusion. These findings reveals the coexistence of ionic and dipole-related mechanisms for the hysteresis in hybrid perovskites.

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The organic-inorganic hybrid perovskites such as CH3NH3PbX3 (X stands for halogen) have been widely explored for solution-processed photovoltaic devices,1-5 as well as light emitting diodes,6,7 field effect transistors,8-10 optically pumped lasers,11,12 and other optoelectronic applications.13-15 However, the I-V response characteristics in most hybrid perovskite devices shows a somewhat unexpected “hysteresis behavior”,16 composed of a marked difference in the I-V response characteristics for forward and backward voltage scans, which has been related to the device structure17 and unspecified changes in the perovskite active layer upon the application of a bias electric field.18 This is one of the major hurdles for evaluating the device performance and fabricating stable solar cell panels based on the hybrid perovskites.19 Various underlying mechanisms have been proposed to explain this “electric-field-driven hysteresis behavior” in CH3NH3PbI3-based optoelectronic devices, including the capacitive effect,17,19,20 ferroelectricity,21-23 and ionic transport.24-26 Although a series of experimental works have been performed to address each of the proposed mechanisms, it is still difficult to identify the exact contribution of the anions migration and the dipole alignment of the organic cations in the “I-V hysteresis behavior” of CH3NH3PbI3-based devices. We note that the in-depth understanding and “cure” of this phenomenon is absolutely essential for improving the device stability, which is imperative for practical applications.27 2 ACS Paragon Plus Environment

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The large dielectric constant of perovskite materials can introduce an additional capacitive current to the I-V responses of solar cells based on the sweeping rate of the voltage scans. Charging/discharging at the interface may occur especially when the transport layers have unbalanced charge extraction capabilities for electrons and holes.19 In this case, charge carriers would accumulate between the perovskite and charge transport layers28 under an applied bias voltage, resulting in an “extrinsically driven” hysteresis behavior. Surprisingly, the I-V hysteresis behavior has been also observed (actually even more pronounced) in devices without built-in interfacial issues; this has been recognized as an “intrinsically driven” hysteresis, so-called giant switchable photovoltaic effect in CH3NH3PbI3 discovered by Huang et al.29 They proposed the “ion migration” model upon the application of an external electric field, where the ions move from one side of the CH3NH3PbI3 layer to the other side, thus forming a p-i-n junction along the field direction.25 Follow-up studies have shown that iodine ion vacancies have relatively small activation energy for site-to-site diffusion within the perovskite structure, which allows for the ionic transport in CH3NH3PbI3 at room temperature;24,30-32 in this case the I-V hysteresis would be suppressed at low temperatures. We also note that the contribution from dipole rotation and subsequent alignment of the organic cations along the external field direction has also been predicted, especially when the dipoles are in a more ordered phase at low temperature.33,34 Giving these variety of processes that can contribute to the I-V hysteresis behavior we propose that the temperature dependence of the I-V hysteresis would be of interest for studying the electric field effect in CH3NH3PbI3-based devices.35,36 Here we adopted the transverse device geometry with large gap (tens of micrometers) between electrode pairs to minimize the influence of extrinsic capacitive effect, as the device capacitance is inversely proportional to the distance between the electrodes.37 The ionic transport could be gradually “frozen” at low temperatures (typically < 250 K), when the thermal energy becomes much smaller than the ion activation energy,31 whereas the preferential orientation of the organic cation dipoles may give rise to a distinct temperature-related hysteresis response. Moreover, the electric field poling dependence of the photoluminescence (PL) could be also used to differentiate the various field effect processes, since the emission intensity should decrease under the influence of an applied 3 ACS Paragon Plus Environment

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voltage due to ionic transport,38,39 but may behave differently if the dipole alignment mechanism is involved.40 In this work we systematically study the changes of the I-V hysteresis and PL properties when a transverse poling electric field is applied to CH3NH3PbI3-based device. We found that the I-V hysteresis is fully suppressed at low temperature, say T30 V. As shown in Figure 5a, when poling the MAPbI3 device at 90 V the PL intensity exhibits a rapid recovery, which is consistent with dipole alignment at larger poling fields. To unravel the origin of this bizarre effect we measured the “PL change after 100 seconds poling” at various bias voltages (see Figure 5b). The maximum field effect poling of the PL intensity was found to be at 50 V, which corresponds to relatively low field strength of F~104 V/cm. A stronger field would result in better alignment of the organic dipoles which governs the reversed poling effect above 50 V. This proves that the dipole mechanism at room temperature under high poling voltage is favorable for dipole rotation but hinders their alignment in the direction of the external field. This provides strong evidence for the coexistence and competition between two the PL poling effects, namely ionic transport and dipole alignment mechanisms, which are both responsible for the hysteresis behavior in MAPbI3 solar cells even at room temperature. To verify the influence of ionic transport on PL measurements at room temperature, we applied an external magnetic field in order to modulate the ions migration under an applied bias. As shown in Figure 5c, we propose that the ion motion from one electrode to the other is considered to be a flow of charges (or current), which can be influenced by the Lorentz force in the perpendicular direction. Consequently the ions would move towards the air/perovskite surface where surface defects may trap them and consequently may dope the MAPbI3 layer. To verify our hypothesis we fabricated a “wide-gap” device, where the MAPbI3 layer is deposited on a single pair of Au electrodes that are 1 mm apart. In this case the self-doping process described above occurs only near to the electrodes; a much smaller PL hysteresis was indeed observed in this case. Under these condition the application of a magnetic field may “extend” the self-doping effect to the entire gap area, and this consequently enhances the PL hysteresis. The r” field dependence (see Figure 5d) can be well fit with a Lorentzian line shape and linear

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background which is probably due to the unbalanced defect densities between the top surface and the bottom interface of the MAPbI3 layer. In summary, we studied the electric field poling effect in a transverse MAPbI3-based device by monitoring the changes in PC and PL intensities upon the application of a bias voltage. The ionic transport process in the MAPbI3 active layer introduces a reversible “self-doping” effect caused by ion accumulation near the electrodes (intrinsic p-i-n junction), which is suppressed at cryogenic temperatures. Another poling effect due to “dipole alignment” becomes the dominant mechanism for the poling effect at T< 250 K. We also found that the p-i-n junction formed by the self-doping process does not disappear at low temperatures if the device is cooled under a constant applied bias; this results in significant photovoltaic performance even at T = 10 K. In addition we also found that the PL intensity increases by dipole alignment; this process was observed to occur at low temperature (when the ionic diffusion is frozen), or under high bias voltages. The influence of self-doping on PL intensity was also measured when using magnetic field to affect the ionic transport process. This shows that the two poling-related processes, namely the ionic transport and dipole alignment coexist in hybrid perovskite devices. Our findings provide a detailed picture of the electric-field effects on the optoelectronic properties of organo-lead halide perovskites, which might be helpful in improving the understanding and resulting performance of perovskite-based devices. AUTHOR INFORMATION Notes C. Zhang and D. Sun contributed equally to this work. The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the Department of Energy Office of Science, grant DESC0014579. The device fabrication facility was supported by the National Science Foundation-Material Science & Engineering Center (NSF-MRSEC) program at the University of Utah, grant DMR 1121252.

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