Apurba
Mahapatra
a,
Rashmi
Runjhun
b,
Jan
Nawrocki
b,
Janusz
Lewiński
bc,
Abul
Kalam
d,
Pawan
Kumar
a,
Suverna
Trivedi
e,
Mohammad Mahdi
Tavakoli
f,
Daniel
Prochowicz
*b and
Pankaj
Yadav
*g
aDepartment of Physics & Astronomy, National Institute of Technology, Rourkela, 769008, India
bInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. E-mail: dprochowicz@ichf.edu.pl
cFaculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
dDepartment of Chemistry, Faculty of Science, King Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
eDepartment of Chemical Engineering, National Institute of Technology, Rourkela, 769008, India
fDepartment of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
gDepartment of Solar Energy, School of Technology, Pandit Deendayal Petroleum University, Gandhinagar-382 007, Gujarat, India. E-mail: Pankajphd11@gmail.com
First published on 15th April 2020
Ion migration plays a significant role in the overall stability and power conversion efficiency of perovskite solar cells (PSCs). This process was found to be influenced by the compositional engineering of the A-site cation in the perovskite crystal structure. However, the effect of partial A-site cation substitution in a methylammonium lead iodide (MAPbI3) perovskite on the ion migration process and its activation energy is not fully understood. Here we study the effect of a guanidinium (GUA) cation on the ion transport dynamics in the single crystalline GUAxMA1−xPbI3 perovskite composition using temperature-dependent electrochemical impedance spectroscopy (EIS). We find that the small substitution of MA with GUA decreases the activation energy for iodide ion migration in comparison to pristine MAPbI3. The presence of a large GUA cation in the 3D perovskite structure induces lattice enlargement, which perturbs the atomic interactions within the perovskite lattice. Consequently, the GUAxMA1−xPbI3 crystal exhibits a higher degree of hysteresis during current–voltage (J–V) measurements than the single-crystalline MAPbI3 counterpart. Our results provide the fundamental understanding of hysteresis, which is commonly observed in GUA-based PSCs and a general protocol for in-depth electrical characterization of perovskite single crystals.
The migration process of ions in perovskite films has been intensively studied using photo-thermal induced resonance (PTIR) microscopy,18 temperature-dependent impedance spectroscopy,19,20 Muon spin relaxation (μSR),19 defect spectroscopy,21 current–potential curves22,23 and Kelvin probe force microscopy (c-KFM).24 There are several ions that act as migrating ions in the prototypical MAPbI3 perovskite i.e., (i) methylammonium (MA), (ii) iodide and (iii) proton or hydrogen ions (H+).15,25,26 The H+ ions play a considerably minor role in comparison with other ions due to their low concentration in PSCs.25,27 Moreover, Pb2+ migration within the perovskite structure is hindered due to its high activation energy.28,29 Thus, MA and iodide ions are primarily migrating ions that affects the stability and hysteresis of PV devices due to their low activation energy to move through the perovskite structure (0.5–0.8 eV for MA and 0.2–0.7 eV for iodide ions).20,23,28,30–33 Moreover, the observed hysteresis in perovskite solar cells (PSCs) is ascribed to the iodide ion because it has a diffusion coefficient of 10−12 cm2 s−1, which is 5 times larger than that of MA cations (10−16 cm2 s−1).14,32 By controlling the chemical composition and surface passivation of the perovskite film, the ion motion through the perovskite lattice can be retarded, leading to improved stability and suppressed J–V hysteresis.31,34 For example, the performance, and stability of PSCs are improved, when a part of iodide ions is replaced by smaller bromide ions, which show stronger hydrogen-bonding interactions with the organic ammonium cations35,36 due to higher electronegativity (Mulliken, bromide: 6.76, iodide: 4.24) or hardness (bromide: 4.24, iodide: 3.70).35,37 On the other hand, the stability, and efficiency of PSCs can be enhanced by substitution of MA cations with formamidinium (FA), azetidinium (Az) or guanidinium (GUA) cations.19,38–40 As theoretically studied, GUA cations show potential to solve the hysteresis issue in the PSCs due to its zero dipole moment and larger size of 278 pm as compared to the MA cation (217 pm).41,42 Surprisingly, it was found that the incorporation of GUA in the MAPbI3 perovskite structure enhances the hysteresis effect in comparison to the pristine MAPbI3 based PSCs.38,43 Initially, Yang et al. assumed that GUA is not confined within the perovskite lattice but rather is weakly hydrogen-bonded to the under-coordinated ionic species mainly at grain boundaries.38 They proposed that mobile GUA cations can migrate to the interfaces and screen the applied electric field leading to enhanced hysteresis in the device. However, more recent studies revealed that GUA is able to replace MA cations forming a pure phase of the GUAxMA1−xPbI3-type perovskite.40,44 Under these circumstances, the determination of the precise effect of GUA incorporation into the MAPbI3 perovskite lattice on the ionic migration needs further investigations. Notably, the mentioned studies concerned the investigation of completed devices, where the ion motions can be influenced by the grain boundaries in the perovskite active layer, interfaces of the absorber layer with the electron and hole transporting layers and aging of the solar cells. The presence of grain boundaries provides fast paths for the migration of ions leading to a higher hysteresis and lower stability. Thus, the elucidation of the exact role of the cations and halides on the ion migration process by investigating the polycrystalline perovskite films has not been achieved.
In this work, we study the effect of GUA on ion transport in mixed-cation GUAxMA1−xPbI3 perovskite single crystals using temperature-dependent electrochemical impedance spectroscopy (EIS). The investigation of single crystals allows elimination of the effect of grain boundaries and interfaces on the kinetics of ion migration, and it is crucial to study the intrinsic behavior of perovskite semiconductors for a better understanding of the charge transport and recombination phenomena. We show that the GUAxMA1−xPbI3 perovskite single crystals exhibit lower activation energy for the iodide ion migration than MAPbI3 single crystals. The GUAxMA1−xPbI3 single crystals also exhibit a higher degree of hysteresis during current–voltage (J–V) measurements than the single-crystalline MAPbI3 counterparts, demonstrating higher ionic motion in the GUAxMA1−xPbI3 perovskite system. These results provide a basic insight into the precise effect of the GUA incorporation into the MAPbI3 perovskite lattice on the ion migration process.
In order to calculate the activation energy of ions, we performed electrochemical impedance spectroscopy (EIS) measurements as a function of temperature in the frequency range from 1 MHz to 1 Hz at an AC perturbation voltage of 20 mV under dark conditions. Fig. 1a and b show the EIS Nyquist spectra of both MAPbI3 and GUAMA single crystals at different temperatures. To check the thermal stability of the investigated crystals, the EIS measurements as a function of decreasing temperature were also measured (Fig. S4, ESI†). We found that the EIS spectra recorded during both measurement cycles exhibit a similar behavior, which signifies that the perovskite crystals are thermally stable in the temperature range of 303–363 K. Notably, the obtained Nyquist spectra of both crystals show a single semicircle, which is consistent with our recent report.47 However, two or three semicircles corresponding to the high, mid and low-frequency regions are observed for PSCs. The interpretation of these semicircles is usually explained with the help of an electrical equivalent circuit, which is still controversial in the literature. Nevertheless, it is generally acceptable to ascribe the mid or low frequency (from 1 to 103 Hz) response to ion motion. In the present work, Nyquist spectra of both crystals decrease with increasing temperature due to the increase in the conductivity. The rate of change in the real part of Nyquist spectra is found to be higher for MAPbI3 than that of GUAxMA1−xPbI3 crystals (Fig. S5, ESI†). The higher rate of change in real part of Nyquist spectra for MAPbI3 crystal signifies its lower temperature stability.
Fig. S6 (ESI†) shows the plots of the complex part of impedance as functions of frequency and temperature. The peaks in the complex impedance spectra at frequencies ranging from 101 to 103 Hz and 101 to 102 Hz are clearly visible for MAPbI3 and GUAxMA1−xPbI3 crystals, respectively. The observed frequency ranges and corresponding time constant is on the same order as the low frequency response in PSCs, which suggests that the net response of perovskite crystals is due to the ion migration. In our previous reports on GUAxMA1−xPbI3 PSCs, we observed a complex impedance peak between 105 and 106 Hz and the corresponding time constant depicts the recombination kinetics in the cell, which is significantly suppressed upon GUA incorporation.43,48 However, such high-frequency response is not observed in the present study. In turn, we observed that with incorporation of a small amount of the GUA cation in the MAPbI3 crystal structure, the rate of change in the apparent time constant is significantly suppressed with a change in temperature. The higher apparent time constant is found to be associated with the hysteresis phenomenon in PSCs.49 The activation energies were extracted from the apparent time constant for both crystals as a function of increasing and decreasing temperature and plotted in Fig. 1c and d. The activation energy of 0.53 and 0.36 eV is found for MAPbI3 and GUAMA, respectively, which is consistent with our47 and other group's results.38 The lower activation energy of the GUAMA crystal signifies that ion migration occurred more easily in this crystal in comparison with the pristine MAPbI3. The activation energy of ions calculated for both MAPbI3 and GUAxMA1−xPbI3 thin films using EIS and temperature-dependent ion conductivity are summarized in Table 1.
Activation energy of ions for MAPbI3 and GUAxMA1−xPbI3 | |||
---|---|---|---|
Activation Energy | Measurements | Ref. | |
MAPbI3 | 0.624 eV (I−) (single crystal) | Temperature-dependent ion conductivity | 50 |
0.82 eV (thin film) | Temperature-dependent ion conductivity | 51 | |
0.53 eV (I−) | Temperature-dependent electrochemical impedance spectroscopy (EIS) | 30 | |
0.51 eV (I−) | Temperature-dependent ion conductivity from Warburg impedance | 30 | |
1.05 eV (single crystal) | Temperature-dependent ion conductivity | 31 | |
0.5 eV (thin film) | |||
0.43 eV (I−) (pellets) | Temperature-dependent ion conductivity | 52 | |
MAPbI3 | 0.0211 eV (thin film) | AdmittanceSpectroscopy | 38 |
MA0.95GA0.05PbI3 | 0.0148 eV | ||
MAPbI3 | 0.29 eV (thin film) | Temperature-dependent electrochemical impedance spectroscopy (EIS) | 19 |
MA0.95GA0.05PbI3 | 0.38 eV | ||
Mid frequency (1 Hz–105 Hz) | |||
MAPbI3 | 0.4 eV | ||
MA0.95GA0.05PbI3 | N/A | ||
Low frequency (>1 Hz) |
Most of the values for MAPbI3 and GUAxMA1−xPbI3 thin films are found in the range of 0.3–0.6 eV and 0.1 to 0.7 eV, respectively. For example, Yang et al., have calculated an activation energy of 21.11 meV for MAPbI3 PSC and a significantly lower activation energy of 14.86 meV for GUAxMA1−xPbI3 devices.38 This indicates that the recombination within the bulk of the perovskite film has been successfully suppressed. In turn, Ferdani et al., calculated an activation energy of 0.44 eV for the MAPbI3 PSC by plotting the EIS low-frequency time constant as a function of temperature.19 The obtained activation energy is close to our calculation and is mainly found to be associated with the diffusion of iodide ions. The activation energy for the GUA0.05MA0.95PbI3 PSC has not been derived from EIS, however, the value of 0.78 eV was obtained from Ab initio simulation, which is comparatively higher with respect to our values and the reported values by Yang et al.38
To further confirm the values of activation energies derived from EIS measurements, the conductivity of perovskite crystals was measured under dark conditions (Fig. S7, ESI†) and plotted in the Nerst–Einstien formalism (Fig. 2). It is well-established for mixed conductors that ionic conduction is dominant in the high-temperature region, while electronic conduction is dominant in the low-temperature region.53 From the obtained plot, the activation energy is derived using the expression of , where kB is the Boltzman constant, T is the absolute temperature and σ0 is a constant.54 By using the Arrhenius plot, the activation energies of 0.52 and 0.36 eV are obtained for MAPbI3 and GUAMA crystals, respectively. The obtained activation energies are in good agreement with our EIS findings. For metal halide perovskites, iodide vacancies have been found to have a more detrimental influence on the charge trapping and non-radiative recombination because of their deep energy levels.
Fig. 2 Temperature-dependent conductivity of (a) MAPbI3 and (b) GUA0.015MA0.985PbI3 single crystals (Arrhenius plot). |
The ion activation energy is also defined as a result of the interaction between the ion and the perovskite lattice. This interaction can be easily tuned by the substitution of the halide or A-site cations in the perovskite structure. In our previous works, using solid-state NMR, we showed that when MA with a radius of 2.17 Å is substituted by a cation with larger ionic radius, i.e., FA (2.53 Å)55 or GUA (2.78 Å),40 these organic groups exhibit a higher degree of rotation. This behavior can cause an additional strain and weaken the atomic interaction inside the lattice and consequently decrease the ion activation energy (see Fig. 3).56 However, the substitution of the A-site cation with a lower radius cation, e.g. Rb (1.52 Å) or Cs (1.67 Å), compresses the perovskite lattice and suppresses the ion motion.57 For instance, Zhou et al., have shown that Cs-based perovskites possess higher ion activation energy as compared to MA-based ones, which led to better light stability.58
Fig. 3 Schematic representation of the introduction of the GUA cation into the MAPbI3 crystal lattice. |
The suppression of ion migration with increasing ion activation energy can be easily observed during current–voltage (I–V) measurement at different scan rates. Fig. S8 (ESI†) shows the hysteresis behavior of both crystals at different scan rates. A higher hysteresis effect is observed for the GUAMA crystal, which confirms the higher ionic motion in the GUAxMA1−xPbI3 perovskite systems.19,42,44
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cp01119c |
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