Origin of JâV Hysteresis in Perovskite Solar Cells - American

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Origin of J-V Hysteresis in Perovskite Solar Cells Bo Chen, Mengjin Yang, Shashank Priya, and Kai Zhu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00215 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 18, 2016

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The Journal of Physical Chemistry Letters

Origin of J-V Hysteresis in Perovskite Solar Cells Bo Chen,a,* Mengjin Yang,b Shashank Priya,a Kai Zhu,b,* a

Center for Energy Harvesting Materials and System, Virginia Tech, Blacksburg, Virginia 24061, United States

b

Chemistry and Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States

AUTHOR INFORMATION Corresponding Author * [email protected] (B.C.) * [email protected] (K.Z.)

ACS Paragon Plus Environment

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ABSTRACT. High-performance perovskite solar cells (PSCs) based on organometal halide perovskite have emerged in the past five years as excellent devices for harvesting solar energy. Some remaining challenges should be resolved to continue the momentum in their development. The photocurrent density-voltage (J-V) responses of the PSCs demonstrate anomalous dependence on the voltage scan direction/rate/range, voltage conditioning history, and device configuration. The hysteretic J-V behavior presents a challenge for determining the accurate power conversion efficiency of the PSCs. Here, we review the recent progress on the investigation of the origin(s) of J-V hysteresis behavior in PSCs. We discuss the impact on the hysteresis behavior of slow transient capacitive current, trapping and de-trapping process, ion migrations, and ferroelectric polarization. The remaining issues and future research required toward the understanding of J-V hysteresis in PSCs will also be discussed.

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Perovskite solar cells (PSCs) synthesized by solution-casting organometal halide perovskite as a light absorber have captured great attention within the energy-harvesting community. The power conversion efficiencies (PCE) of PSCs have shown an unprecedented increase from 3.8% to 20.1% over the past five years.1-6 Several outstanding properties of PSCs make them a promising photovoltaic device. First, organometal halide perovskites meet the requirement of optimum bandgap ranging between about 1.2 eV and 2.3 eV as a function of composition.7-10 Second, a superior light absorption coefficient (~105 cm-1) creates a high density of photoexcited charges and a smaller absorption length that requires only a sub-micrometer thickness of perovskite for sufficient light harvesting.11-13 Third, long electron and hole diffusion lengths in thin-film (>1 µm) and single-crystal (>175 µm) perovskite suppress the recombination of photoexcited charges.14-16 Fourth, organometal halide perovskites can achieve a crystalline structure by simply precipitating out of solution followed by low-temperature annealing ( Vpreset and was larger at V < Vpreset after pre-treatment with Vpreset (Figure 10b). Such an S-shape of the J-V curves can be simulated by assuming that the net built-in potential Vbi after the light-soaking is equal to Vpreset (Figure 10c). This indicates that the light-soaked PSCs


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with applied bias voltage can compensate the net built-in potential. Figure 10d shows that this reduced built-in potential of PSCs after being equilibrated at Vpreset before J-V scan can also be observed in dark J-V curves. Therefore, modification of the net built-in electric field by the applied bias in the PSCs is independent of illumination. This transient timescale is characterized in seconds to minutes. Based on these results, they proposed that the migration of mobile ions under an electric field can screen the built-in electric field independent of illumination; thus, the slow process of ion migration results in J-V hysteresis.50

Figure 10. a) Rate-dependence of the J-V hysteresis; b) J-V curves under fast scan rate after keeping PSCs at different starting voltage Vpreset for 30 s; c) Device simulations with net built-in potential equal to Vpreset; d) J-V curves in dark after the PSCs equilibrated at different Vpreset. Reproduced with permission from ref 50. Copyright 2015 The Royal Society of Chemistry. It is worth noting that there are some challenges associated with the ion-migration mechanism with respect to explaining the hysteresis behavior. Reenen et al. tried to replicate the ionmigration-induced J-V hysteresis as reported by Tress et al. in Figure 10 through a numerical drift-diffusion model.69 However, they found that the J-V response is independent of the Vpreset when considering the ion-migration-induced band-bending alone, which is in contrast to the


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experimental result; this challenges the dominant effect of ion migration on induced J-V hysteresis. Moreover, Chen et al. found by dynamic JSC transient processes after poling that the slow redistribution process of mobile ions under an electric field requires several minutes.52 Therefore, ion migration would not be able to respond quickly enough to induce non-steady-state photocurrent and J-V hysteresis. In the future, it is important to identify the timescale for the ion migration. Chen et al. discovered that ion migration can modulate the steady-state photocurrent and influence the J-V response after electric poling.52 After negative poling, the photocurrent value at different applied bias during J-V scanning was significantly decreased compared to the unpoled sample. This explains the different PV performance of PSCs after electric poling or after light soaking under different applied bias. It appears that ion migration plays an important role on the steady-state photocurrent due to band bending, whereas it has a small impact on the nonsteady-state photocurrent due to the slow timescale of ion migration. Band bending due to ferroelectric polarization. Ferroelectric (FE) effect is another possible mechanism for J-V hysteresis in PSCs. If the MAPbI3 thin films have ferroelectric domains, the interface band structure can be engineered to exhibit different polarization character, resulting in different PV performance under forward and reverse scans. For perovskite solar cells based on ferroelectric MAPbI3 with a p-FE-n device configuration, negative poling generates a polarization electric field inside the MAPbI3 film opposite to the built-in electric field, which hinders charge separation and deteriorates the PV performance. On the other hand, positive poling facilitates the separation and collection of photoexcited charges, thereby improving the PV performance. For the forward scan starting from short-circuit condition (or negative bias), the direction of initial polarization electric field offsets, to some degree, the built-in electric field and suppresses the charge extraction. In contrast, the initial polarization electric field can enhance the built-in electric field during reverse scan starting from a large positive bias.59-60 Ferroelectric polarization domains of the organometal perovskite thin films have been investigated through theoretical modeling based on first-principles calculations.77-79 The possibility of ferroelectric domains in organometal perovskite was first simulated considering the orientational alignment of the MA+ dipole. The phase structure of MAPbI3 transforms from an orthorhombic to tetragonal structure at 160 K and further to a cubic phase at 327 K.80-81 Although the cubic phase is centrosymmetric without ferroelectric effect, Frost et al. proposed that the presence of the polar MA+ molecule at the center of the perovskite cage can reduce the


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symmetry and introduce bulk polarization.77 Considering the cubic phase with aligned MA+ dipole, a polarization of 38 µC/cm2 was estimated for MAPbI3 through periodic DFT simulation by Frost et al.77 Experimental X-ray diffraction (XRD) patterns revealed that the MAPbI3 thin films imposed a tetragonal structure with I4/mcm space group at room temperature.26, 82 Zheng et al. calculated the bulk ferroelectric photovoltaic property of MAPbI3 based on a tetragonal inorganic lattice from first principles, and a polarization of 5 µC/cm2 was achieved when all of the net MA+ molecular dipoles aligned along the c-axis of the tetragonal phase.78 Fan et al. reported a polarization of ~8 µC/cm2 by considering three forms of polarization: orientational polarization of the MA+ dipole, ionic polarization due to a shift of MA+ relative to the negative charge center of the PbI3− cage, and ionic off-centering due to the displacement of Pb within the PbI6 octahedra.79 The ferroelectric properties due to the interaction between the orientational order of MA+ dipoles and the structure of the inorganic lattice have recently been investigated.83-85 The distortion of the inorganic lattice causes the I-Pb-I angles to deviate from the ideal 180º, thus yielding uncompensated dipoles and ferroelectric polarization. The aligned orientation of MA+ dipoles can reduce lattice symmetry from I4/mcm to noncentrosymmetric I4cm, which would induce the possibility of ferroelectric behavior.83 Moreover, by using DFT combined with symmetry mode analysis, Stroppa et al. disentangled the impact of the organic MA+ cation (PMA) and PbI3 inorganic lattice (PPbI3) on ferroelectric polarization.84 For the polarization of the tetragonal phase with aligned MA+ dipoles at room temperature, a polarization of 4.42 µC/cm2 was estimated, with 4.47 µC/cm2 arising from PMA and -0.23 µC/cm2 from PPbI3.84 Monte Carlo simulation of the interaction of MA+ with the inorganic lattice demonstrated that the ferroelectric domain can form when all dipoles are aligned together in parallel if the dipole-dipole interactions were screened by the inorganic lattice.85 Recently, several researchers reported their experimental observations on the ferroelectric effect in organometal perovskite. Kutes et al. showed directly the presence of ferroelectric domains of the MAPbI3 thin film by piezoresponse force microscopy (PFM).86 The PFM phase image showed a significant phase contrast, indicating spontaneous polarization of the MAPbI3 thin film. There was no correlation between the polarization domain shape and surface topography, which rules out topographic effect to the observed PFM contrast. Moreover, a reversible switching of the polarization direction was observed by poling with DC bias (Figure


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11). Chen et al. used the locally measured PFM amplitude and phase hysteresis loops to further analyze the ferroelectric effect of MAPbI3 thin films.59 A sharp 180º phase switching in the polarization domain direction, coupled with the dip in the PFM amplitude, occurs at a coercive field with a magnitude of 8 kV/cm.59 The hysteresis loops confirm the switchable ferroelectric domains through an externally applied field. Kim et al. investigated the ferroelectric polarization behavior of MAPbI3 thin films in dark and under illumination.87 They found that spontaneous polarization is commonly present in MAPbI3 with negligible size dependency and it can be tuned by an external electric field. Under illumination, the spontaneous polarization remained the same in comparison to the dark condition, whereas the photoinduced polarization was significantly enhanced in the presence of an external electric field. Furthermore, the ferroelectric polarization was retained for 30–60 min under illumination after removal of the external electric field.87

Figure 11. PFM topography and phase image (2.5×2.5 µm2 area) for the MAPbI3 thin film after electric poling with different bias. Reproduced with permission from ref 86. Copyright 2014 American Chemical Society.


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However, the ferroelectric property of organometal MAPbI3 perovskite is a subject under debate. Xiao et al. reported that that PFM phase image of MAPbI3 thin film had no obvious phase contrast, and local phase hysteresis loops demonstrated no phase switching, thus indicating the absence of a ferroelectric effect in MAPbI3 thin films.73 Although 180º phase switching was observed in the PFM phase hysteresis loop by Coll et al., they found coercive field decay on the order of seconds and polarization retention for very short times.88 Leguy et al. estimated that the timescale of the ferroelectric domain relaxation for perovskite solar cells was around 0.1–1 ms, which was much faster than the timescale of J-V hysteresis.85 Coll et al. also probed the macroscopic polarization by the classical Sawyer-Tower circuit and it did not display the typical ferroelectric P-E hysteresis loops.88 These conflicting reports about ferroelectricity in organometal perovskite may originate from the structure of the perovskite solar cell, such as the presence of the mesoporous TiO2 layer. Therefore, more studies are needed to clear the debate on ferroelectricity. Moreover, if the J-V hysteresis originates from ferroelectric polarization, then the photocurrent can only change when the effective electric field is larger than the coercive field. The ferroelectric effect cannot explain the formation of non-steady-state photocurrent when the applied bias is changed during the stepwise scanning. These observations appear to rule out ferroelectric polarization as the dominant factor on the J-V hysteresis behavior. Future Outlook. Although the slow transient process of capacitive current and band bending have been proposed to explain the J-V hysteresis behavior, many questions remain unresolved. Addressing these questions will help in understanding the formation of J-V hysteresis behavior, but will also facilitate the design of advanced device structures with better characteristics. The link between ion migration and dielectric permittivity/capacitance should be investigated in the future. For example, one area of research should be to establish the direct evidence of the relation between the ion migration and the electrode polarization. Moreover, the impact of the dielectric loss tangent on the permittivity and the corresponding capacitance need more studies. The effect of the unbalanced photoexcited electrons and holes at the interfaces due to inefficient extraction of photoexcited carriers has not been fully examined. These interfacial electrons and holes can enhance the polarization density but also modulate the band structure at the interface. Therefore, a comparison of the impact of unbalanced photoexcited electrons/holes with trapped charges and mobile ions on the capacitive current and band bending should be


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investigated to understand the dominant physical mechanism for capacitive current and band bending. KPFM imaging of the device showed the trapped hole and electron after illumination, and the trap density of states obtained from thermal admittance spectroscopy revealed the reduction of trap density after PCBM passivation. These and other experiments demonstrate the presence of charge traps in perovskite. Identifying the origin of these charge traps—whether from immobile defects or ion migration—is another important research goal for future studies. The timescale for the redistribution of ion migration under the modulated electric field across the perovskite layer is another subject of investigation. However, the reported timescales for ion migration vary by orders of magnitude. Zhang et al. found that the mobile ions relaxed in a timescale of ~10 s after removing the poling bias.61 However, Xiao et al. found that the accumulated charges at the interface due to ion migration can be maintained for months.73 O’Regan et al. reported the double-exponential decay process of the transient photovoltage, which was ascribed to capacitive effect and ion migration, and the timescales for both effects were on the order of microseconds.89 The dynamic photocurrent transient processes after electric poling reveal that ion migration occurs at a timescale on the order of serval minutes.52 To quantify the timescale for the redistribution of ion migration, additional characterizations (e.g., PL and thermal admittance spectroscopies) are needed to understand the dynamic change of charge density. Considering the conflicting reports about the ferroelectric properties of organometal halide perovskite thin film, more experiments are also needed to verify whether organometal halide perovskites exhibit a ferroelectric effect and what its relationship is to hysteresis behavior. Currently, large single-crystal MAPbI3 samples with sizes up to 2 inches have been fabricated; thus, the best way to analyze the ferroelectric effect would be to study the ferroelectric properties of the bulk organometal halide perovskite samples.16,

90-91

Using single crystals, one could

determine the coercive electric field, ferroelectric domain size, piezoelectric coefficient, and Curie temperature. Conclusions. Perovskite solar cells have demonstrated anomalous J-V hysteresis behavior, with the PV performance affected by the voltage scan direction/rate/range, history of voltage conditioning, and device configuration. Four primary mechanisms have been reported for explaining the anomalous J-V hysteresis behavior: a) slow transient capacitive current, b)


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dynamic trapping and de-trapping processes of charge carriers, c) band bending due to ion migration, and d) band bending due to ferroelectric polarization. The stepwise scan and JSC transient process of PSCs clearly revealed the presence of non-steady-state photocurrent. Considering its dependence on voltage variation and its slow decay characteristic, the nonsteady-state photocurrent has been speculated to arise primarily from the capacitive current. The large interfacial polarization and corresponding dielectric constant may be the cause of the enhanced capacitive effect in PSCs. The slow decay process of the capacitive current allows the remnant non-steady-state photocurrent to increase (or decrease) the photoresponse under fast reverse scan (or forward scan), which yields the J-V hysteresis. The passivation of charge traps at the grain boundary and/or surface of perovskite thin films can eliminate the hysteresis, which indicates the influence of charge traps on electron/hole extraction efficiency. However, the timescale of the trapping process is on the order of milliseconds, which is much faster than the timescale of J-V hysteresis behavior; thus, it is not likely to be the dominant factor in J-V hysteresis. Ion migration under an electric field has also been proposed to modulate the band structure of the PSCs due to accumulated mobile charges at the interface. As a result, the ionmigration-induced band bending can modulate the steady-state photocurrent and influence the JV response with electric poling. Moreover, the slow redistribution of ion migration may be responsible for the formation of non-steady-state photocurrent and J-V hysteresis, but the impact of ion migration in terms of the appropriate timescale is still uncertain. If the timescale of ion migration is on the order of several minutes, it is not expected to respond quickly enough to create the J-V hysteresis. More studies are needed to identify the timescale for the redistribution of mobile ions. The discovery of ferroelectric properties in perovskite thin films allows the possibility of interface band engineering by ferroelectric polarization. However, the ferroelectric effect cannot explain the formation of non-steady-state photocurrent when the applied bias is changed during voltage scan. Thus, there are still many unresolved questions about the mechanism that controls the J-V hysteresis, and these need to be addressed to further advance perovskite solar cells.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (B.C.)


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*E-mail: [email protected] (K.Z.).

Notes The authors declare no competing financial interest.

Biographies: Bo Chen is a Research Associate Fellow in the Center for Energy Harvesting Materials and System at Virginia Tech. He received his B.S. degree in Physics from Zhejiang University in 2007 and his Ph.D. degree in Materials Science and Engineering from Virginia Tech in 2012. His recent research focuses on organometal halide perovskite solar cells, dye-sensitized solar cells, and photoelectrochemical water splitting.

Mengjin Yang received his Ph.D. degree in Materials Science from the University of Pittsburgh. He is now a post-doctoral researcher at the National Renewable Energy Laboratory. His research focuses on the development and characterization of hybrid solar cells and other optoelectronics.

Shashank Priya is currently Robert E Hord Jr. Professor in the Department of Mechanical Engineering at Virginia Tech. Prior to that he served as the I/UCRC program director at the National Science Foundation. At Virginia Tech, he has served as the director of the NSF I/UCRC: Center for Energy Harvesting Materials and Systems and associate director of the Center for Intelligent Material Systems and Structures.

Kai Zhu is a senior scientist in the Chemistry and Nanoscience Center at the National Renewable Energy Laboratory. He received his Ph.D. degree in physics from Syracuse University in 2003. His recent research focuses on both basic and applied studies on perovskite solar cells, including material development, device fabrication/characterization, and basic understanding of charge-carrier dynamics in these cells.

ACKNOWLEDGMENT


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S.P. acknowledges the financial support from Office of Naval Research (ONR). M.Y. and K.Z. acknowledge the support at the National Renewable Energy Laboratory by the U.S. Department of Energy SunShot Initiative under the Next Generation Photovoltaics 3 program (DE-FOA0000990) under Contract No. DE-AC36-08-GO28308.

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Quotes to highlight in paper:

Hysteretic J-V behavior presents a challenge for determining the actual power conversion efficiency of perovskite solar cells.

Non-steady-state photocurrents associated with the capacitive effect resulting from electrode polarizations at perovskite/electrode interfaces affect J-V hysteresis behavior.

Enhancing charge extraction/suppressing charge trapping is critical for minimizing the hysteresis behavior.

Ion migration induced adjustment of electric field distribution can influence the separation and collection of photo-generated charges.

Possible ferroelectric polarization provides another approach to modulate the electric field distribution and photovoltaic performance.


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Origin of JâV Hysteresis in Perovskite Solar Cells - American

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