Voltage Hysteresis in Mixed Perovskite Solar Cells - Wiley Online Library

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Wolfgang Tress,* Juan Pablo Correa Baena, Michael Saliba, Antonio Abate, and Michael Graetzel Since the presentation of the initial perovskite solar cell, reported efficiency values have been rather snapshots as these devices suffer from instabilities occurring on different time scales. On the long timescale (minutes to months) degradation is caused by decomposition of the perovskite material upon exposure to humidity.[9] This process results in a bleaching of the film, but can be retarded by encapsulating the device.[10,11] On intermediate time scales of minutes, changes in the performance are observed that are not related to the optical properties or a considerable decomposition of the perovskite but to the electronic properties of the device.[12] Those include defects in the absorber, e.g., created by hydration,[13] photo- or heatinduced ion migration,[14] and inefficient charge transport at the contacts. Both lead to higher recombination losses and a reduced charge carrier collection efficiency. As we will show below, changes on intermediate timescales are partially reversible. Instabilities on shorter timescales (ms to s) are completely reversible and attributed to trapping of charges and voltage-induced ionic motion. Naturally, the consequences of the processes on short and intermediate timescales can also lead to irreversible changes extending toward longer timescales. Short-term instabilities give rise to a rate-dependent hysteresis observed in the current–voltage (JV) curve when measured under common voltage sweep rates which are in the range of 100 000 – 1 mV s−1.[12,15–17] In this case, a JV measurement is not suitable to provide a reliable value of the maximum power output and thus of power-conversion efficiency. Understanding the hysteresis is essential to avoid ambiguities in JV measurements and to improve on device stability. Consequently, several studies have been performed, and different reasons such as giant capacitances,[18] ferroelectrics,[19] deep surface traps,[20] ionic motion,[12,21–25] and combinations amongst those[26] have been proposed as reasons for the hysteresis. It was furthermore observed that contact materials influence the occurrence of hysteresis.[20,22] Recent investigations of the transient and ratedependent behavior point toward charge extraction probabilities that depend on prebiasing conditions due to charge accumulated at the electrode(s).[12,21,27–33]

Organic-inorganic metal halide perovskite solar cells show hysteresis in their current–voltage curve measured at a certain voltage sweep rate. Coinciding with a slow transient current response, the hysteresis is attributed to a slow voltage-driven (ionic) charge redistribution in the perovskite solar cell. Thus, the electric field profile and in turn the electron/hole collection efficiency become dependent on the biasing history. Commonly, a positive prebias is beneficial for a high power-conversion efficiency. Fill factor and open-circuit voltage increase because the prebias removes the driving force for charge to pile-up at the electrodes, which screen the electric field. Here, it is shown that the piled-up charge can also be beneficial. It increases the probability for electron extraction in case of extraction barriers due to an enhanced electric field allowing for tunneling or dipole formation at the perovskite/electrode interface. In that case, an inverted hysteresis is observed, resulting in higher performance metrics for a voltage sweep starting at low prebias. This inverted hysteresis is particularly pronounced in mixed-cation mixed-halide systems which comprise a new generation of perovskite solar cells that makes it possible to reach power-conversion efficiencies beyond 20%.

1. Introduction In 2009 organic-inorganic metal halide perovskite was identified as a promising photovoltaic material.[1] During the course of the last five years the power-conversion efficiency of solar cells based on this material has been steadily improved, now exceeding 20%[2–6] with a recent certified record value of 22%.[7] Progress was mainly driven by engineering deposition methods to optimize the morphology of methylammonium-lead-iodide (MAPbI3) perovskite films. Recently, perovskites with mixed organic cations (MA and formamidinium, FA) and with mixed halides (I and Br) took the lead on the highest values.[5,8] Dr. W. Tress, Dr. M. Saliba, Dr. A. Abate, Prof. M. Graetzel Laboratory of Photonics and Interfaces (LPI) École polytechnique fédérale de Lausanne (EPFL) 1015 Lausanne, Switzerland E-mail: [email protected] Dr. J. P. Correa Baena Laboratory of Photomolecular Science (LSPM) École polytechnique fédérale de Lausanne (EPFL) 1015 Lausanne, Switzerland

DOI: 10.1002/aenm.201600396

Adv. Energy Mater. 2016, 1600396

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Inverted Current–Voltage Hysteresis in Mixed Perovskite Solar Cells: Polarization, Energy Barriers, and Defect Recombination


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Figure 1. a) Schematic of the layers composing the device. b) Normal and c) inverted hysteresis for an MAPbI3 and a mixed perovskite solar cell, respectively, voltage scan rate 10 mV s−1. The normal hysteresis shows increased performance for the backward scan compared to the forward scan. This is vice versa for the inverted hysteresis. d) Sketch of conduction and valence band edge (CB and VB) for MAPbI3 and mixed perovskite compared to TiO2.

The JV-hysteresis evolves between backward scan, which is from forward to reverse voltage going through open circuit followed by short circuit, and forward scan, which is performed the other way around. Commonly, a forward prebias followed by a backward scan displays enhanced device performance and in particular large fill factor (FF), whereas reverse or zero bias followed by a forward scan delivers lower values (Figure 1b). This difference was explained by screening effects depending on the prebias, which lead to a modified electric field in the device.[12] In this article, we extend the hysteresis study from MAPbI3 toward mixed-cation mixed-halide perovskite systems deposited on thin mesoporous TiO2. We show that the mixed perovskite based solar cells exhibit “inverted” hysteresis, i.e., a forward prebias followed by a backward scan displays reduced device performance and in particular smaller fill factor (Figure 1c). Comparing monochromatic JV data and device modeling, we provide evidence that the accumulation of ionic charge at the interface with the TiO2 electrode modifies the effective electron extraction barrier, whereas spread ionic charge enhances recombination through defects close to the TiO2/perovskite interface.

2. Results and Discussion 2.1. Inverted Hysteresis for Mixed Perovskite We focus our study mainly on two types of devices, both based on an ≈200 nm thick mesoporous TiO2 layer (Figure 1a). One

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solar cell comprises MAPbI3 with a perovskite capping layer thickness of about 250 nm. The other one – denoted as “mixed” – employs a mixed cation-halide perovskite (≈100 nm capping layer thickness) formed from a 0.6 m precursor solution containing FAI, PbI2, MABr, and PbBr2 in a ratio of 1.0 : 1.1 : 0.2 : 0.22. In both cases, the perovskite was deposited using a modified procedure based on the antisolvent method[34] as pioneered by Jeon et al.[4] We, furthermore, include devices with a thicker mixed perovskite layer (≈500 nm) deposited from a 1.25 m solution, devices without the electron-selective compact TiO2 layer, and devices with an ultrathin Al2O3 shell covering the mesoporous TiO2. All solar cells are fabricated on a glass/fluorine doped tin oxide (FTO)/compact TiO2/ mesoporous TiO2 substrate and finalized by depositing p-doped spiro-MeOTAD (≈200 nm, containing Li-TFSI,[35] FK209, and TBP) as hole transport layer covered by a gold electrode (80 nm). The device area is ≈0.275 cm2, and it is fully illuminated when measuring the current–voltage curves with a potentiostat. A cross section of the thick “mixed” perovskite sample is shown in Figure 2a, indicating a compact perovskite film on top of the mesoscopic scaffold. The corresponding current–voltage curve displayed in Figure 2b indicates that devices based on this architecture can reach a power-conversion efficiency of 20 % measured under illumination with AM1.5g. Figure 3a shows the JV curves for an MAPbI3 device measured under various voltage sweep rates applying a voltage loop starting at 1.2 V, where the device was preconditioned under light for at least 10 s. The series of curves demonstrate a typical rate-dependent hysteresis as commonly observed in perovskite

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Figure 2. a) Scanning electron microscopy (SEM) cross section of a representative mixed perovskite device. b) Current–voltage curve of a mixedperovskite device delivering 20 % power-conversion efficiency with negligible hysteresis measured in a loop starting from forward bias and with a voltage sweep rate of 10 mV s−1.

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Figure 3. Rate-dependent hysteresis in different perovskite devices on mesoporous TiO2 scaffold a) “Normal” hysteresis with lower FF for the forward scan in MAPbI3 perovskite device with thickness of 450 nm. b) “Inverted” hysteresis in mixed perovskite with a thickness of 300 nm: the fill factor is lower for the backward scan, which shows an S-Kink. For high scan rates (>1000 mV s−1), inverted and normal hysteresis overlap leading to a point of intersection between both scans.

solar cells of different architectures and processing conditions[12,36,37]: compared to the backward scan, the forward scan delivers lower photocurrents due to a reduced charge extraction efficiency once the solar cell has been under low bias. This gives rise to a stronger dependence of the photocurrent on voltage and a reduced fill factor for the forward scan. Additionally, the short-circuit current density is reduced for low scan rates due to recombination becoming dominant once the solar cell has experienced low applied bias. This effect can cause a “bump” in the JV curve.[12] The resulting hysteresis has been explained by ionic charge accumulating at the electrodes and, thus, screening the electric field in the perovskite film when the device is kept at low voltages. The charge accumulation at low applied bias is rationalized by the presence of a built-in electric potential driving negative ionic charge toward the FTO and/or positive charge toward the Au electrode.[12] The same measurements are performed for the mixed perovskite and are shown in Figure 3b. This series of curves exhibits considerable qualitative differences compared to those in Figure 3a: for moderate sweep rates ( 20 s

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Figure 8. The open-circuit voltage as a function of illumination intensity for mixed perovskite devices with 500 nm capping layer. a) Transient Voc after switching from 0 V (30 s) to Voc. b) Transient Voc after switching from 1.2 V to Voc. c) Voc versus light intensity for white LED illumination as extracted from the transients. Crosses mark the maxima of the transients in (a), circles the minima, and squares the approximate stabilized Voc of (b). Lines are simulations, where recombination close to the TiO2/perovskite interface is enhanced for the solid line compared to the dashed line as described in the text. d) The same experiment as (c) but with red (red circles, squares, and crosses) and blue (blue dots, diamonds, and plus sign) light. Lines are simulation results where the plot color refers to the illumination color. The inset shows an estimation of the absorption profile for blue and red illumination neglecting mesoporous TiO2.

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in the stack exploiting the different absorption coefficients of the perovskite for different wavelengths. Thus, instead of using white light, we repeat the Voc study under red and blue illumination. The results are shown in Figure 8d, where Voc does not depend on the light source for 0 V prebias and for 1.2 V prebias after stabilization. In contrast, the minimum Voc after 1.2 V prebias is lower for blue illumination than for red illumination. Blue light is mainly absorbed close to the TiO2 electrode due to a higher extinction coefficient (see Figure 6b and inset of Figure 8d). Thus, the lower Voc under blue illumination is evidence for enhanced recombination in the mesoporous TiO2 directly after positive prebias. Remarkably, the drift-diffusion simulations (lines) reproduce this trend. Here, for simplicity we assume that the Beer–Lambert law is applicable and that the absorption coefficient for blue light is three times larger than for red light, which is a conservative estimate considering the optical data shown in Figure 6b. We adjust the SRH rate in the perovskite to 4 × 107 s−1 (25 ns). For 1.2 V prebias we additionally enhance the recombination rate to 2 × 1010 s−1 (0.05 ns) in the region closer than 200 nm to the electron contact. Whereas the simulation data discussed so far have been rather insensitive to the electric field profile in the device and, thus, to changes of built-in potential by some hundreds of meV, the results here depend on how quickly charges are driven away or diffuse toward the 200 nm region of high recombination. Note that at this stage the simulations have to be regarded as semi-quantitative, as absorption profile, charge carrier mobilities, and built-in potential (here 1.1 V) are chosen based on crude assumptions. Furthermore, mesostructure and (unintentional) doping concentrations, which are unknown, are neglected. Nevertheless, the coincidence of experimental and simulated Voc including the dependence on light intensity and illumination color indicate a proper choice of parameters and confirm the hypothesis of different SRH recombination rates close to the TiO2 dependent on prebias. The changes in Voc and the inverted hysteresis can be linked via mobile defects as discussed in the following.

2.4. Explanation Based on Mobile Ionic Charge In line with recent reports on the conventional hysteresis, our explanation for the inverted hysteresis including the changes in Voc is based on the voltage-induced migration of ionic species in the perovskite: at 0 V, prebias ions should accumulate at the interface(s), driven by the built-in potential, which originates from the work function offset between the doped spiro-OMeTAD and the FTO. In contrast, under forward bias, the ionic charge should leave the interface and penetrate the perovskite. Since exact equilibrium distributions of ions are unknown, the statements on ion distributions are relative, just telling that less ionic charge should be at the interfaces for higher bias compared to lower bias. According to our[12] and other[21,28–32] previous work, this effect is supposed to cause the conventional hysteresis where the forward scan is characterized by a reduced charge collection efficiency due to a screened field when the ionic charges are at the interfaces. We illustrate these two situations at short circuit in Figure 9, leading to a low internal field in case of 0 V prebias and a



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In contrast, this coincidence in the transients of MAPbI3 and mixed perovskite is not observed when switching from 1.2 V to open circuit (b): The Voc trace for the MAPbI3 reference shows normal asymptotic behavior toward its value obtained in (a), whereas the Voc for the mixed perovskite passes through a minimum followed by a slow increase to a steady-state value that is significantly below the one reached when having prebiased at 0 V (a). These reduced values in Voc indicate that recombination rates are considerably enhanced directly after 1.2 V prebias and remain large even on intermediate time scales. Figure 8c shows three traces of Voc as a function of illumination intensity extracted from the transient measurements. The crosses denote the maximum Voc when the device has been at 0 V prebias (Figure 8a). The circles are extracted from the minimum of the transients after 1.2 V prebias and the squares are the self-stabilized Voc after ≈20 s (Figure 8b). As indicated by the transients, the minimum Voc is considerably lower (300 mV) for 1.2 V prebias independent of illumination intensity. The slope is around 100 mV per decade for both cases indicative for recombination predominantly via defect states. Thus, the reason for the difference in Voc is not a changed built-in potential accompanied by surface recombination dependent on prebias. This effect would cause a decrease of Voc for the forward scan in case of the normal hysteresis, in particular for nonselective contacts.[12] It would mean a leveling-off for the Voc at higher light intensities once the built-in potential is approached.[43] In contrast, for the inverted hysteresis, the recombination mechanism remains unmodified, but its quantity changes. The selfbiasing effect of Voc leads to a partial reduction of recombination, yielding a higher steady-state value of Voc, in particular for low light intensities (cf. circles to squares) whereas Voc tends to saturate at around 0.95 V for higher light intensities. We use a drift-diffusion model to explain the observed behavior. For this purpose we simulate a 600 nm thick intrinsic perovskite layer in between metal contacts with a built-in potential of 1.3 V and a charge carrier generation rate set to 2.3 × 1021 cm−3 s−1 in order to obtain a short-circuit current density of 22 mA cm−2. The parameters for recombination are adjusted to fit the experimental data where we obtain a good fit for a radiative band-to-band recombination constant of 2 × 1011 cm3 s−1 and a Shockley–Read–Hall (SRH) lifetime of (1.5 × 107 s−1)−1 = 67 ns. These values are in the range of data reported for different lead-halide based perovskite films characterized by transient[8] and THz measurements.[44] Charge carrier mobilities are set to 10 cm2 V−1 s−1.[44] However, they do not have considerable influence on the Voc in the prevailing regime. The results from the simulation are plotted as a dashed line in Figure 8c, which reproduces the intensity dependence very well including a slope of 100 mV per decade. Despite the recombination being more than 99 % of SRH type, the slope does not reach the anticipated 120 mV per decade. This is due to a high mobility in comparison to a low recombination rate giving rise to inhomogeneities of charge carrier concentrations and in turn a preferential recombination zone. When simulating the reduction of Voc under 1.2 V prebias (solid line), the SRH lifetime is decreased by more than two orders of magnitude to (5 × 109 s−1)−1 = 0.2 ns. To further understand the origin of the dramatic change in recombination, we tune the charge carrier generation profile

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get trapped in low-energy states of the TiO2 due to a modified energy level alignment or because the nonconfined ions act as recombination centers themselves. The former is supported by the fact that recombination is not enhanced in MAPbI3, the latter is supported by the strong change in Voc even for the samples with Al2O3 shell. The most likely effect is that the ionic species (assume interstitial I−), not confined to the interface but spread in the perovskite, forms a state with Figure 9. Illustration of the electric field profile as a function of ionic charge distribution (blue). Negative charge from the perovskite accumulates after the device was at low bias, leading to an energy in the band gap of the perovskite a surface dipole or a large electric field at the TiO2 perovskite interface which facilitates charge which acts as a recombination center as precarrier extraction in case of misaligned energy levels or tunneling barriers. Positive bias makes dicted by density functional theory.[50] This these charges move away from the interface, which reduces charge carrier extraction probability would explain why V keeps decreasing for oc and facilitates recombination, in particular if the anions (e.g., interstitial I−) themselves can act both scan direction when doing subsequent as recombination centers. This leads to an inverted hysteresis in the JV curves in case of extracJV loops while holding the device at forward tion barriers and a normal hysteresis in case of charge collection problems. bias. Alternatively, other processes such as voltage-induced structural changes in the mixed perovskite capable of forming electronic defects might larger one in case of 1.2 V prebias. Note that this is a simplified occur. Here, more in-depth studies are required to identify the (though experimentally supported[45,46]) image as we do not kinetics and thermodynamics of charge carriers in the TiO2/ know the exact distribution of the electric field in the perovskite as unintentional doping, imbalanced charge transport, or other perovskite intermixed system and potential structural changes space charge effects can alter the electric field profile. of mixed perovskite triggered by voltage and light. The situation upon 1.2 V prebias compared to 0 V prebias is beneficial for the FF in case of a too low diffusivity of charge carriers within the perovskite. This could be due to small crys2.5. Interplay between Normal and Inverted Hysteresis tallites and many grain boundaries in the perovskite layer. Such a film is supposed to have much lower effective mobilities and We want to strengthen the proposed model investigating the higher recombination rates compared to values reported for rate-dependent hysteresis under red and blue illumination with highly crystalline films.[47] In this case, we expect the occurthe aid of Figure 10. Recall that the charge carrier generation profile is less even for blue illumination, where more light rence of the conventional hysteresis, which can be eliminated is absorbed close to the TiO2. A consequence of a more even by either increasing charge transport or decreasing recombination in the perovskite film, which is most likely the underprofile under red illumination is a higher forward current due lying process of the measures to reduce hysteresis reported in to a more homogeneous reduction of resistance by photoconliterature.[20,48,49] ductivity.[41] The relevance of photoconductivity is further confirmed by the dark curve in Figure 10a, showing lower forward In contrast, if the FF is limited by charge extraction at the currents than under illumination. TiO2 contact, the 0 V prebias situation is more beneficial for Coming back to the curves under illumination, sweeping charge extraction. The piled-up ions, which screen the electric at 100 mV s−1 (a) results in JV curves with inverted hysteresis, field in the perovskite, enhance the electric field at the interface to the TiO2. Assuming sufficient conductivity of the TiO2, where Voc and FF are lower for blue illumination due to the fact electronic charge will leave the TiO2 surface and form an interthat most blue photons are absorbed in the mesoporous structure, where recombination is enhanced. At 1000 mV s−1 (b), face dipole with the negative ionic charge. This dipole can align bands or enhance tunneling through a thin insulating layer. this trend is still maintained. Additionally, the normal hysterThe enhanced field can explain the higher FF for devices with esis occurs for voltages