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Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering Dae-Yong Son, Seul-Gi Kim, Ja-Young Seo, Seon-Hee Lee, Hyunjung Shin, Donghwa Lee, and Nam-Gyu Park J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10430 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Journal of the American Chemical Society

Universal Approach toward Hysteresis-Free Perovskite Solar Cell via Defect Engineering Dae-Yong Son2†, Seul-Gi Kim1†, Ja-Young Seo1, Seon-Hee Lee2, Hyunjung Shin2, Donghwa Lee3 and Nam-Gyu Park1* 1

School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Korea

2

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea

3

Department of Materials Science and Engineering Pohang University of Science and

Technology (POSTECH), Pohang 37673, Korea

†

These authors equally contributed to this work

*Corresponding author E-mail: [email protected], Tel: +82-31-290-7241

ABSTRACT Organic-inorganic halide perovskite is believed to be potential candidate for high efficiency solar cell because power conversion efficiency (PCE) was certified to be more than 22%. Nevertheless, mismatch of PCE due to current density (J)-voltage (V) hysteresis in perovskite solar cell is obstacle to overcome. There has been much lively debate on the origin of J-V hysteresis, however, effective methodology to solve the hysteric problem has not been developed. Here we report universal approach for hysteresis-free perovskite solar cell via defect engineering. A severe hysteresis observed from the normal mesoscopic structure employing TiO2 and spiro-MeOTAD is almost removed or does not exist upon doping the pure perovskites, CH3NH3PbI3 and HC(NH2)2PbI3, and the mixed cation/anion perovskites, FA0.85MA0.15PbI2.55Br0.45 and FA0.85MA0.1Cs0.05PbI2.7Br0.3, with potassium iodide. Substantial reductions in low-frequency capacitance and bulk trap density are measured from the KI1

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doped perovskite, which is indicative of trap-hysteresis correlation. A series of experiments with alkali metal iodides of LiI, NaI, KI, RbI and CsI reveals that potassium ion is the right element for hysteresis-free perovskite. Theoretical studies suggest that the atomistic origin of the hysteresis of perovskite solar cells is not the migration of iodide vacancy but results from the formation of iodide Frenkel defect. Potassium ion is able to prevent the formation of Frenkel defect since K+ energetically prefers the interstitial site. A complete removal of hysteresis is more pronounced at mixed perovskite system as compared to pure perovskites, which is explained by lower formation energy of K interstitial (-0.65 V for CH3NH3PbI3 vs 1.17 V for mixed perovskite). The developed KI doping methodology is universally adapted for hysteresis-free perovskite regardless of perovskite composition and device structure.

Keywords: doping, hysteresis, perovskite solar cell, potassium iodide, Frenkel defect

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INTRODUCTION Perovskite solar cells (PSCs) based on organic-inorganic halide materials attract a great deal of attention due to superb photovoltaic properties. The first report on solid-state perovskite solar cell with a power conversion efficiency (PCE) of 9.7% and 500 h-long-termstability in 2012,1 following the two reports at the early stage on perovskite-sensitized liquid junction solar cells in 20092 and 2011,3 has an enormous impact. It certainly opens up new vistas of the next-generation photovoltaics with unprecedented performances. PCE was swiftly improved up to 22.1% through well-engineered devices followed by fundamental understanding on perovskite layers.4 Although the certified PCE of PSC is comparable to or even higher than conventional thin film solar cells based on semiconductors of multicrystalline Si (~22%), CIGS (~21.7%) or CdTe (~21%),5 PSC is suffering from anomalous current-voltage hysteresis observed at different voltage scan direction. The hysteresis was first issued by Snaith et al. in 2014,6 where hysteresis is associated with perovskite material itself, selective contact materials, and external scanning parameters such as scanning rate and external electric field. Recently, it was suggested that hysteresis could have an adverse effect on stability.7,8 Thus hysteresis-free technology is of importance in PCSs. It has been proposed that several factors might be related to the origin of hysteresis including (1) ferroelectricity due to the switchable polarization induced by breakdown of centrosymmetry,9,10 (2) modulation of charge transport by filling (trapping) and releasing (detrapping) at deep trap sites created by defects,11 and/or (3) ion migration associated with change in interfacial field and barriers resulted from accumulation of ions at interfaces,12-17 Technical approaches were proposed to suppress the hysteresis: promoting the interfacial charge transfer process, hindering the ion migration, and reducing the amount of defects.18 Growth of grain size19 and grain boundary engineering20,21 were found to reduce hysteresis. 3



Impact of selective contacts on hysteresis was reported, where the normal structure with the electron transporting TiO2 layer and the hole transporting spiro-MeOTAD layer exhibited relatively noticeable hysteresis as compared to the NiO and PCBM based inverted structure with almost no hysteresis.22,23 Although ion migration has been regarded as a dominant factor affecting the hysteresis,18 charge diffusion length and surface recombination were suggested to play more critical role in the hysteresis from the viewpoint that most of highly efficient devices showed small hysteresis.24 Despite much effort to better understand the origin of the hysteresis and minimize the hysteresis, the hysteresis in PSCs is still under lively debate and no effective methodology is discovered. Here, we report a universal method to minimize current-voltage hysteresis observed in the mesoscopic and planar perovskite solar cells having TiO2 electron transporting layers. We have considered that trap states generated by defects are one of potential factors causing hysteresis because the defect deep in the band gap can capture electrons and/or holes.25 In our previous work, we found a strong correlation between trap density and hysteresis in which hysteresis was gradually reduced as trap density decreased as evidenced by reduction in nonradiative recombination.20 However, the surface and/or grain boundary engineering does not seem to be sufficient to remove completely the hysteresis, from which we learn that engineering trap states of bulk perovskite will be more effective way. Since perovskite structure can be described in terms of cubic close packing model forming the basic lattice comprising 75% of X ions and 25% of A ions on the same lattice plane and placing B ions in octahedral sites, Oh, between the lattice planes in ABX3 perovskite,26 energetically favorable point defects viz vacancy, interstitial, and antisite are available.25 Frenkel and Schottky defects are also expected in case that an ion moves into an interstitial site and creates a vacancy and stoichiometric cation-anion vacancy, respectively. Among the point defects, 4


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interstitial of Pb and antisites of MA, Pb and I in MAPbI3 (MA = CH3NH3) are related to deep level being responsible for non-radiative recombination and associated with the hysteresis, which motivated us to develop a universal method to minimize hysteresis by defect engineering. Since doping has been known to be one of promising and rather facile methods for defect engineering, for instance thermoelectric figure-of-merit of tin selenide27 or photovoltaic performance of Cu2ZnSn(S,Se)428 was enhanced by alkali ion doping, we have designed doping of perovskite with alkali iodide (LiI, NaI, KI, RbI and CsI) for engineering defects causing the hysteresis of PSCs and discovered that potassium ion plays important role in controlling the defects.

RESULTS AND DISCUSSION 10 µmol of potassium iodide (KI) was mixed with perovskite precursor solutions, where MAPbI3, FAPbI3 (FA = HC(NH2)2) and mixed cation/anion perovskites of FA0.85MA0.15PbI2.55Br0.45 and FA0.85MA0.1Cs0.05PbI2.7Br0.3 were studied in order to investigate effect of KI on the hysteresis of different compositional perovskites. All the devices contain a blocking TiO2 layer on FTO substrate and a mesoporous TiO2 (diamtere of about 50 nm) layer on the blocking film. Figure 1 shows current density (J)-voltage (V) curves of perovskite solar cells with and without KI doping. Little difference in J-V curves between the reverse scan and the forward scan are obeserved after 10 µmol KI doping, while significant mismatch in J-V curvers are obtained from the pristin perovskites. This emphasizes that KI doping eliminates the hysteresis of perovskite solar cells regardless of compisition of perovskite

materials.

For

the

mixed

FA/MA

and

I/Br

perovskite

case

(FA0.85MA0.15PbI2.55Br0.45) in Figure 1A and B, average short-circuit current density (Jsc), 5



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opne-circuit voltage (Voc), fill fcator (FF) and PCE of the pristine perovskite are 22.04 mA/cm2, 1.114 V, 0.698 and 17.14% at the reverse scan, respectively, and 22.10 mA/cm2, 1.093 V, 0.621 and 15.01% at the forward scan, respectively (see Table S1 in supporting information). Difffernce in FF between the reverse and forward scans (∆FF = [FF(reverse) – FF(forward)]/FF(reverse)) is as large as 11.0%, which is substaintially reduced to 1.9% after KI doping. As a result, the PCE of 17.55% (Jsc = 21.47 mA/cm2, Voc = 1.128 V, FF = 0.725) observed at the reverse scan is almost identical with that of 17.14% (Jsc = 21.49 mA/cm2, Voc =

1.122

V,

FF

=

0.711)

at

the

forward

scan.

For

the

triple

cation

of

FA0.85MA0.1Cs0.05PbI2.7Br0.3 in Figure 1C and D, KI doping shows the same tendency. As listed in Table S1, a significant difference in PCE between the reverse scan (17.99%) and the forward scan (15.08%) is observed for the pristine perovskite due to a large difference in FF (0.730 for reverse vs 0.637 for forward, ∆FF = 12.7%). Hysteresis nearly disappears by KI dopping, where a PCE of 18.20% for the reverse scan is again similar to that of 17.97% for the forward scan because of negliable ∆FF = 0.9% (0.7302 for reverse vs 0.732 for forward). Large hysteresis in MAPbI3 and FAPbI3 (Figure 1E and G) is considerably reduced by KI doping as can be seen in Figure 1F and H. Difference in PCE (∆PCE = [PCE(reverse)PCE(forward)]/PCE(reverse)) is reduced from 26.2% to 14.5% for MAPbI3 and from 43.9% to 14.9% for FAPbI3 after KI doping (Table S1). Besides great reduction in ∆FF after KI doping for all the perovskites, it is interesting to note that the Voc values for the forward scan are increased and merged with the ones for the reverse scan after KI doping. A large difference in Jsc at a given bias volatge is related to time-depedent capacitive current,19 leading to lower Voc at forward scan, in which unextracted current is likely to have infleunce on Voc. Such a capcitive current might be originated from junction interface and/or bulk 6


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perovskite. Improvement of forward scanned Voc after KI doping indicates that all the current at the given bias voltage is extracted at the forward scan. Since neither the device configurations nor selective contact materials were not chnaged, it is valid that capacitive current is dominately atrributed to perovskite itself. Nevetheless, modification of interface between the perovskite and the electrode materials, such as TiO2, cannot be ruled out in case that precipitation of KI happens prior to foming perovskite phase. In this case, interface modification can also take part in reducing hysteresis. When considering the the hysteresis occurs not only in perovskite solar cell but also dye-ensitized solar cell29 and silicon solar cell, 30

latter two cases are related to trapping-detrapping process and internal capacitance at short

scan time, respectively, removal (or significantly reduced) of hysteresis in the postssiumdoped perovskite materials could be related to removal of trapped charges in perovskite. Scan rate-independent hysteresis-free behavior is also observed for the KI-doped mixed cation perovskites (Figure S1 and Table S2 in the supporting infromation) as compared to strong dependence of photovoltaic performance on scan rate for the pristine perovskites without KI.

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Figure 1. Current density (J)-voltage (V) curves of perovskite solar cells employing different perovskite materials FA0.85MA0.15PbI2.55Br0.45, FA0.85MA0.1Cs0.05PbI2.7Br0.3, MAPbI3 and FAPbI3 doped with and without 10 µmol KI, measured at reverse (filled circles) and forward (empty circles) scans at the scan rate of 130 mV/s (= voltage settling time of 200 ms) under AM 1.5G one sun illumination (100 mW/cm2). Aperture mask area was 0.125 cm2.

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We further invetigate effect of alkali metal ions on hysteresis of perovskite solar cells. Perovskite precursor solution with 10 µmol of alkali metal iodide (LiI, NaI, KI, RbI or CsI) was

prepared

to

study

effect

of

alkali

metal

ions

on

hysteresis,

where

(FAPbI3)0.875(CsPbBr3)0.125 was chosen as pristine perovskite composition. We have selected FA-based perovskite with a elemetal substitution with Cs and Br as a control perovskite because incorporation of Cs in FA site improves not only PCE but also moisture- and photostability31 and is suitable for top cell in tandem structure.32 Optimal composition in (FAPbI3)1x(CsPbBr3)x

is determined by the Urbach energy estimated from absorbance and Tauc plot in

Figure S2, where x = 0.125 shows minimum Urbach energy among the studied ratio (x values are varied by considering tolerance factor33). In addiiton, this compisition of x = 0.125 leads to largest grains as confimed by scanning electron microscpic (SEM) images in Figure S3 (grain size can be more exactly determined by electron back scattering technique). Hysteresis is strongley dependent on alkali metal ions as shown in Figure 2A-E. When increasing ionic radius of alkali metal ions from Li+ to K+ (Figure 2A-C), hysteresis tends to decrease and disappears at K+. Hysteresis appears again upon futher increase in ionic radius such as Rb+ and Cs+ (Figure 2 D and E) and is apprent in Cs+. Tiny amount of alkali metal ions is expected to be placed in cation vacancy and/or interstitial. In this case, chemical stability of alkali metal ions can be simply expected from cation/anion radius ratio or estimated by theoretical calculation, which will be discussed in detail. Since it is obvious that possasium ion removes the hysteresis, effect of KI concentration on hysteresis is further examined. In Figure 2 F-I, no significant difference in Jsc and Voc between the reverse and forward scans is observed as KI concentration increases from 2 µmol to 10 µmol, while difference in FF is distinct at low concentration but becomes smaller and eventually negliable 9



at 8-10 µmol. Since hysteresis was reported to be influenced by morphology,19 change in morphology by KI concentration may affect hysteresis, which however can be ruled out because of little change in morphology (Figure S4). We have examined carefully XRD patterns (Figure S5a) and found gradual change in lattice parameter with KI addition (Figure S5b). XRD pattern is analyzed using a freeware of EXPO2014 program,34 where (FAPbI3)0.875(CsPbBr3)0.125 perovskite is able to be indexed as cubic phase with space group of Pm-3m. Peaks sift to lower angle by 0.04 ~ 0.11 degree after doping as compared to w/o KI perovskite. The lattice parameters are calculated to be 6.27700 ±0.002167 Å (w/o KI), 6.29346±0.001334 Å (2 µmol KI), 6.29162±0.000820 Å (4 µmol KI), 6.29856±0.000877 Å (6 µmol KI), 6.30098±0.001249 Å (8 µmol KI), and 6.29306±0.000924 Å (10 µmol KI). It was reported that the lattice parameters were changed by impurity doping.35,36,37 For instance, the lattice parameter of c-Si was changed by 0.0007 Å by boron doping (dopant concentration level was 2.3×1019 per cubic centimeters).37 Variation of lattice parameter in GaAs was about ~10-4 Å by Si doping (dopant concentration level was 6×1018 per cubic centimeters). Thus, a slight but gradual increase in lattice parameter (0.016 ~ 0.024 Å) indicates that potassium ion is crystallized in the perovskite lattice. The increase in lattice parameter also underlines that potassium ion might be placed in interstitial site rather than substitution, which is reasonable when considering ionic size of potassium ion much smaller than FA+ and Cs+. So, it is likely that KI is doped in perovskite lattice and plays crucial role in eliminating the hysteresis. It is noted that KI concetration of 15 µmol degrades photovoltaic performance and higher concentration of 20 µmol generates again the hysteresis, which indicates that KI doping amount for controlling hysteresis seems to be saturated at 10 µmol. J-V curves and the relavent photovoltaic parameters depending on KI concetration are displayed in Figure S6 and listed in Table S3, respectively. 10


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Figure 2. (A)-(E) J-V curves of perovskite solar cells based on (FAPbI3)0.875(CsPbBr3)0.125 doped with 10 µmol of LiI, NaI, KI, RbI and CsI, measured at reverse and forward scans at the scan rate of 130 mV/s under AM 1.5G one sun illumination (100 mW/cm2). The aperture mask area was 0.125 cm2. (F)-(I) Effect of KI concentration on statistical photovoltaic parameters of shor-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) obtained at reverse (green) and forward (pink) scans, respectively.

Jsc as a function of time at maximum power point (steady-state photocurrent) can inform charge extarction kinetics. It takes much longer time to stabilize photocurrent for the pristine perovskite without KI doping, whereas Jsc is immediately stabilized upon KI doping, especially for 10 µmol (Figure S7). This underlines that fast and effective extraction of charges are realized by KI doping. In Figure 3A and B, frequency-dependent capacitance is 11



comapred with and without KI doping. The device with KI regardless of selective contacts shows the saturated plateau (not new peak) of capacitance in the region of 10-100 Hz, where little variation of capacitance with frequency is consistent with no I-V hysteresis after KI doping. The capacitance-frequency profile with plateau for the KI doped perovskite is similar to that of hysteresis-free inverted device.22 In Figure 3B, we prepared the device without any selective contacts (FTO/PSK/Au) to confirm dependence of I-V hysteresis on bulk perovskite rather than interface. Capacitance-frequency behavior of this specific structure shows similar behavior, which supports the I-V hysteresis is closely related to bulk defect. Trap density (nt) is evaluated based on space-charge-limited current (SCLC) and impedance measurements using a device structure of FTO/perovskite/Au (Figure S8) in the dark. The nt values are calculated from the relation of ்ܸி௅ = ݁݊௧ ݀ଶ /2ߝߝ଴ ,38 where VTFL represents the onset voltage of trap filled limit, e is electric charge (1.602 × 10-19 C), d is thickness of the active layer, ε0 is the vacuum permittivity (8.8542 × 10-14 F/cm) and ε is dielectric constant. The dielectric constants are calculated from the capacitance measured in high frequency region (~104 Hz). 39,40 As can be seen in Figure 3C and D along with Table S4, VTFL decreases from 0.788 V for the pristine perovskite to 0.617 V for the 10 µmol KI doping. In addition, ε is similarly decreased from 42.45 to 33.53 upon 10 µmol KI doping (Table S4). Since nt is proportional to VTFL×ε at the given perovskite film thickness of 520 nm (Figure S9), nt decreases from 1.367 × 1016 cm-3 to 0.846 × 1016 cm-3 after 10 µmol KI doping. The estimated nt of ~ 1016 cm-3 is in good agreement with the reported value.41 It is noted that nt increases again to 1.399 × 1016 cm-3 and to 1.560 × 1016 cm-3 upon higher doping level of 15 µmol and 20 µmol, respectively. Change in trap density upon KI doping and doping concentration is well consistent with the tendency of hysteresis, which is indicative of 12


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strong correlation between trap density and hysteresis. Interfacial trap can be shown using a high frequency capacitance-voltage (C-V) curve (Figure S10).42 The increase in charge accumulation (capacitance) at negative bias voltage of mixed (FAPbI3)0.875(CsPbBr3)0.125 perovskite is indicative of p-type semiconducting material. As the bias voltage is applied from +1 V to – 1 V, the C-V curve of pristine perovskite (w/o KI) shifts to negative voltage due to the positive charges trapped on the interface of perovskite layer. The C-V curve of 10 µmol KI doped perovskite shows a reduced interfacial trap state, which indicates that the potassium iodide reduces not only the bulk trap but also interfacial trap of the perovskite. Under the bias voltages (at either -1 or +1 voltage) it is clearly showed that higher current can flow through the perovskite layer with KI doping as shown in Figure S11. No carriers can be further trapped by defects in bulk as well as at the interfaces as shown by experimental results above on trap density (nt) and C-V for interfacial traps. As a result, higher conductance through the layers has been observed by C-AFM. Interestingly, it is noted that no microstructural dependence on the conduction in between without and with KI doping is observed. Both cases under the either – or + biases always show grain boundaries are better conduction pathways.

13



Figure 3. Capacitance-frequency (C-V) plots of (A) FTO/TiO2/(FAPbI3)0.875(CsPbBr3)0.125 / spiro-MeOTAD/Au and (B) FTO/(FAPbI3)0.875(CsPbBr3)0.125/Au under one sun illumination. FTO, PSK and spiro represent fluorine-doped tin oxide, perovskite and spiro-MeOTAD, respectively. Dark current-voltage curve of (C) pristine (w/o KI) and (D) 10 µmol KI doped (FAPbI3)0.875(CsPbBr3)0.125 with the structure of FTO/Perovskite/Au.

In order to understand physical origin of the hysteresis-free KI-doped PSCs, the firstprinciples density function theory (DFT) calculations are employed for MAPbI3 as a model compound. We first perform various geometrical relaxations to identify any structural changes of MAPbI3 during photovoltaic operation. Our DFT study finds that I_Frenkel defect can be formed under more than two excess electrons; although cation Frenkel defect is generally more common, I anion leads to Frenekel defect because of smaller size than MA molecule. Figure 4A shows the schematic view of 2×2 perovskite MAPbI3 structure, where 14


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MA cation and I anion occupy one fourth and three fourth of FCC lattice sites while Pb cation sits on the center interstital site of FCC lattice. Thus, among four Oh interstitial sites (3 edges and 1 center) in FCC lattice, the center of I6 cage is only occupied to form PbI6 octahedron (black octahedron). Figure 4B shows the schematic view after the formation of I_Frenkel defect in MAPbI3. As can be clearly seen, the migration of I ion into the Oh interstitial site (Ii) leaves I vacancy on the lattice site (VI) and leads to the formation of I_Frenkel defect. Figure 4C shows the variation in energetics during the formation of I_Frenkel defect. Initial state (IS) represents the perfect perovskite structure with I ion sitting on lattice site, while final state (FS) indicates the I_Frenkel defect structure in which I ion moves 2.05 Å toward the Oh interstitial site (see Figure 4B). As can be seen from the energetics, the formation of I_Frenkel defect occurs along two steps. The first step is associated with the rotation of MA molecule, while the second step is the result of actual migration of I ion from the lattice to the interstitial site. The molecular rotation is necessary since it can open up space for I migration and stabilize I ion sitting on the interstitial site by forming hydrogen bonds. The energy barrier associated with the molecular rotation is 0.16 eV and the intermediate state is only 0.01 eV energetically higher than the initial ground state. Thus, the reorientation of molecules can easily occur at room temperature and so the molecular rotational step does not restrain the formation of I_Frenkel defect. Instead, the I migration step can limit the formation of I_Frenkel defect. Our DFT calculation predicts that the energy barrier associated with I migration is 0.47 eV and the FS is 0.06 eV energetically higher than the IS with the presence of two excess electrons. Thus, the formation of I_Frenkel defect is limited by the second step associated with the migration of I ion from lattice to interstitial site.

15



We now discuss about the reason why the I_Frenkel defect can be stabilized under two excess electrons, while I ion goes back to its original lattice site spontaneously without two excess electrons. In order to understand the physical origin of the formation of I_Frenkel defect under two excess electrons, we further look at the variation of electronic charge density after the formation of I_Frenkel defect. Figure 4D and E shows the spatial location of two excess electrons before and after I_Frenkel defect is formed. Before I_Frenkel defect has formed, excess electrons are distributed on conduction band (p orbital of Pb) of MAPbI3. With I_Frenkel defect, however, the excess electrons are localized near the VI and Pb, see cyon region in Figure 4E. The localized excess electrons can explain the stabilization of I_Frenkel defect under two excess electrons. Without two excess electrons, the existence of positively charged VI between two Pb cations is energetically highly unfavorable and so the negatively charged Ii will spontaneously go back to the original lattice site. With two excess electrons, however, the charge state of VI is changed from positive to negative by trapping excess electrons. Accordingly, the negatively charged VI attracts two nearby Pb cations and forms ionic bond with them. As a consequence, Pb-VI-Pb arrangement is energetically favored under two excess electrons. The estimated distance between two Pb cations is 3.38 Å which is similar to the bond length of metallic Pb (3.5 Å). Thus, the defective I_Frenkel structure can be stabilized by forming Pb-VI-Pb configuration, so-called Pb dimer. In summary, two excess electrons can stabilize I_Frenkel defect by forming Pb dimer on VI site. Our DFT calculation predicts that the energetic preference of I_Frenkel defect can be further enhanced by increasing the number of excess electrons. With three excess electrons, the I_Frenkel defect structure is even energetically 0.29 eV lower than the perfect perovskite structure. It seems that the I_Frenkel defect structure become energetically more preferred by strengthening the formation of the Pb dimer by trapping more excess electrons. Thus, we 16


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believe that the formation of I_Frenkel defect is inevitable near the cathode at which electrons are accumulated, which is associated with origin of hysteresis in bulk perovskite. The question is why the hysteresis disappears by KI doping. Once light is illuminated on MAPbI3, photo-excited charge carriers will be generated and transported into the opposite side of electrodes. During this process, excess electrons can be accumulated on cathode and lead to the formation of I_Frenkel defect. Since I_Frenkel defects are generated by trapping two excess electrons, it is detrimental for accurate determination of actual PCE due to hysteresis. In addition, the opposite charged Frenkel defect can be separated and affect long term stability of MAPbI3. We note here that previous attempts tried to interconnect the stability and hysteresis of MAPbI3 with the migration of VI may not be correct. It can be possible that the migration of VI luckily explains the hysteresis behavior since the energetics for the formation of I_Frenkel defect is similar to the migration of VI. Thus, we suggest that the atomistic origin of the hysteresis of PSCs correlates with the formation of I_Frenkel defect rather than the migration of VI. Hence, the hysteresis can be removed by preventing the formation of I_Frenkel defect. Since I_Frenkel defect can be formed by electron accumulation, we can minimize the formation of I_Frenkel defect by reducing electron accumulation at the cathode or by preventing I ion migration into the Oh interstitial site. In that sense, the KI-doping is an ideal method, since K ion fits well into the Oh interstitial site and prevents I ion migration. The cation/anion radius ratio between K+ and I- (0.616) shows how well K+ ion fits into the interstial Oh site (Figure 4F). Although Na+ and Rb+ are suitable for doping elements in order to reduce the hysteresis, as observed in Figure 2B and D, they are not as effective as K+ ion because of overalp with 4 coordination and 8 coordination, respectively. Li+ ions are too small so that they can be mobile and cannot completely hinder I 17



migration into interstitial sites; cation/anion ratio is 0.315 for Li+ ion. On the other hand, Cs ions are too large to fit into the interstitial site since the cation/anion ratio is 0.782. Thus, K+ is expected to best element to cure the I_Frenkel defect, which is experimentally proved. More strong effect of KI doping is observed for the mixed cation/anion perovskites than pure perovskite (MAPbI3 and FAPbI3), as observed in Figure 1. In order to understand the effect of the mixed cation anion, formation energy of K interstitial is compared between pure and mixed cation perovskite. For the pure system, the formation energy of K interstitial is -0.65 eV for MAPbI3 and -0.98 eV for FAPbI3. Negative sign represents that K ion energetically prefers the interstitial site. For the mixed cation system (FA0.875 MA0.125PbI2.55Br0.45), the formation energy of K interstitial is lowered to -1.17 eV. The lowered formation energy can be understood from enhanced structural distortion induced by the shape difference between MA and FA. The enhanced structral distortion accommodates K ion into the interstitial site more easily and so the defect formation energy of K interstitial is lowered for the mixed cation anion perovskites. In conclusion, the mixed cation anion system shows better performance for removing hysterisis by accommodating K ion into the interstitial site more readily.

18


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Figure 4. Schematic view of MAPbI3 hybrid perovskite structure (A) for perfect and (B) with I_ Frenkel defect. Different species are shown as Green(C), Yellow(N), Whte(H), Purple(I), and Black(Pb). (C) Relative energy profile of the formation of I_Frenkel defect under two excess electrons; initial state (IS) and final state (FS) represents the structure before and after the formation of I Frenkel defect. Two step processes are shown with molecular rotational barrier (TS1) and I migration barrier (TS2). Spatial location of two excess electrons (D) for perfect and (E) with I_Frenkel defect is shown; Blue and red represent the region where electron density increases and decreases; isosurfaces are plotted at the level of +/-0.03e. (F) Cation/anion radius ratio between alkali metal cation and I- anion.

CONCLUSIONS Addition of tiny amount of potassium iodide in perovskite materials (pure perovskites and mixed cation/anion perovskites) was found to reduce current-voltage hysteresis to great extent or even removed completely the hysteresis in case of the mixed cation/anion perovskites. A series of experiments with alkali metal iodides (LiI, NaI, KI, RbI and CsI) confirmed that K ion is right element for this purpose. Bulk and interfacial trap densities were reduced and low-frequency capacitance was lowered by KI doping. Theoretical calculation revealed that atomistic origin of hysteresis was not related to iodide migration but associated with Frenkel defect. Potassium ion was best for preventing Frenkel defect as compared to 19



other alkali metal cations and KI doping effect was more much more pronounced at mixed cation perovskites because of energetically more stabilized K interstitial. According to the computational simulation combined with experimental observations, we propose that defect engineering via KI doping is universal method to solve the hysteric problem in perovskite solar cells, regardless of perovskite composition and device structure.

ACKNOWLEDGEMENTS This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF2012M3A6A7054861 and NRF-2014M3A6A7060583 (Global Frontier R&D Program on Center for Multiscale Energy System) and NRF-2016M3D1A1027663 and NRF2016M3D1A1027664 (Future Materials Discovery Program). This work was also supported by Basic Science Research Program through the NRF under contact 2016R1A2B3008845 and NRF-2017H1A2A1046990 (NRF-2017-Fostering Core Leaders of the Future Basic Science Program/Global Ph.D. Fellowship Program).

AUTHOR CONTRIBUTIONS N.G.P. conceived of concept and experiments, performed data analysis and prepared the manuscript. D.Y.S. and S.G.K. prepared materials, fabricated devices, and measured optoelectronic properties. J.Y.S performed and analyzed Impedance spectroscopy. H.J.S. and S.H.L. measured C-AFM and wrote the relevant part. D.L. performed DFT calculation and wrote the relevant part. All authors discussed the results and commented on the manuscript.

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20

2

(b)

J (mA/cm )

TOC

I

KI

K+

15 10

0.76 Å

5

Li+

0 0.0 0.2 0.4 0.6 0.8 1.0

20 2

25

20

20

5

10 5

1.38 Å K+

0 0.0 0.2 0.4 0.6 0.8 1.0 20

15 2

J (mA/cm )

10

15

2

2

KI

J (mA/cm )

25

15

J (mA/cm )

I

J (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

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10 5

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2

V (V)

V (V)


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1.67 Å

Cs+

0 0.0 0.2 0.4 0.6 0.8 1.0 V (V)

Universal Approach toward Hysteresis-Free ... - ACS Publications

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