Guangsheng
Liu
a,
Qianwen
Wei
a,
Guijun
Zhang
a,
Mehri
Ghasemi
b,
Qi
Li
b,
Junlin
Lu
b,
Juan
Wang
a,
Baohua
Jia
*b,
Yu
Yang
*a and
Xiaoming
Wen
*b
aNational Center for International Research on Photoelectric and Energy Materials, College of Materials and Energy, Yunnan University, Kunming 650091, Yunnan province, China. E-mail: yuyang@ynu.edu.cn
bSchool of Science, RMIT University, Melbourne VIC 3000, Australia. E-mail: baohua.jia@rmit.edu.au; xiaoming.wen@rmit.edu.au
First published on 15th May 2024
Defect tolerance plays a crucial role in the outstanding photoelectric performance of hybrid organic–inorganic perovskites (HOIPs). Although the origin of defect tolerance has been extensively studied in the past few years through static density functional theory (DFT) calculations and experiments, the mechanism of photoinduced defect dynamic change has not yet been well revealed. In this study, we first demonstrate that defect tolerance is dynamically responsive to illumination. Continuous illumination can result in an increased defect tolerance, manifested as an increase in average carrier lifetime and photoluminescence (PL) efficiency, and the timescales involved are consistent with the reported ion activities. The results of DFT calculations and non-adiabatic molecular dynamics (NAMD) simulation suggest that the photoinduced dynamic PL enhancement can be attributed to the local structural reorganization caused by the defect trapping/detrapping, resulting in the formation of long-living structures with longer carrier lifetimes. The study reveals that the dynamic defect tolerance in HOIPs is powered by a unique interaction between the soft lattice, phonons, excited carriers, and defect sites. It has significant implications for the development of photostable optoelectronic devices based on HOIPs.
The HOIPs serve as light absorbers in optoelectronic devices, and illumination is a necessary condition for the operation of these devices. Continuous light illumination reduces the ion migration activation energy (from 0.82 to 0.15 eV) and accelerates the ion migration dynamics,11 resulting in a dynamic change in the defect tolerance threshold. The change is manifested in solar cells as an increase or decrease in the PCE.12–14 The notorious negative effects, including IV hysteresis,15,16 phase transitions,17,18 phase separation19,20 and PL quenching,21 have been confirmed by a large number of studies to originate from the photoinduced local halogen ion migration. In direct contradiction, some photoinduced positive effects have also been widely reported under continuous illumination, such as prolonged fluorescence lifetime22 and increased photoluminescence quantum efficiency (PLQE),21 which are closely related to variation in the defect tolerance threshold. The dynamic defect tolerance in HOIPs is a complex multi-body coupling issue involving the coupling of defect states (DS), lattice ions, photo-generated charge carriers, and phonons. Furthermore, many experimental scientists have provided phenomenological explanations, such as ion diffusion-induced self-healing,23,24 lattice stress release13 and trap-assisted charge accumulation.25 However, these hypotheses are based on phenomenological interpretations of their respective experimental results with controversial aspects, which makes it difficult to reach a consensus on the matter of photoinduced dynamic defect tolerance in HOIPs.
Herein, we first demonstrate that the abnormal dynamic defect tolerance is widespread in HOIPs through time-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL), characterized by an abnormal increase in PL intensity and prolonged carrier lifetime under continuous illumination. With the aid of DFT and NAMD calculations, we attribute the photoinduced dynamic PL enhancement to charge-state transitions of halogen point defect, driving the formation of stable structures with longer carrier lifetime. Under continuous illumination, the gradual increase in the component of the newly formed structure induced by illumination triggers dynamic PL enhancement related to illumination time. This finding provides insight into abnormal dynamic defect tolerance and charge carrier recombination dynamics in HOIPs.
Fig. 1 An illustrative diagram of dynamic defect tolerance. With continuous illumination perovskite films exhibit increased PL efficiency and prolonged carrier lifetime. |
We consecutively measure PL spectra under constant excitation intensity using a fluorimeter with an electrothermal cooled charge-coupled device (CCD) detector, as shown in Fig. 2b. At low excitation of 10 mW cm−2, a distinct peak can be observed around 780 nm (1.59 eV), consistent with the results obtained from UV-vis absorption spectroscopy (Fig. S2, ESI†). In addition, PL enhancement was clearly observed with continuous illumination, as shown in Fig. 2. When increasing the fluence to 100 mW cm−2, the PL enhancement effect is found to be faster and tends to saturate after illumination for 200 seconds (Fig. 2d, e and f), which suggests that this PL enhancement is related to illumination intensity. Furthermore, a pronounced redshift phenomenon can be observed, which can be attributed to I-rich phase formation resulting from I-migration,20 as shown in Fig. 2c and f. The PLQE is essentially determined by the competition between the radiative and non-radiative recombination processes. In our measurement, all the excitation and detection conditions remain identical. Under such specific conditions the detected PL intensity is approximately proportional to the PL efficiency and thus can be used for comparing relative PL efficiency. The unique illumination time dependent PL enhancement indicates the dynamic change in the carrier recombination induced by illumination, leading to a decrease in non-radiative recombination and an enhancement in radiative recombination. Moreover, MAPbI3 and FAPbI3 also showed similar PL enhancement results in illumination time-dependent PL experiments, as shown in Fig. S3 (ESI†). These results indicate that under continuous illumination, a significant defect healing effect and dynamic tolerance to defects is widespread in HOIPs. A similar illumination induced PL enhancement has been previously reported with various interpretations.21,30,31
It is necessary to note that many negative effects caused by illumination have also been commonly observed, such as IV hysteresis,11 phase separation,18 and so on. This does not conflict with our observed PL-enhancement effect since there exists a light exposure threshold between positive and negative effects. With illumination at high intensity, it is expected that the ions from fabrication (such as interstitials) can be activated and escape from the lattice, which can result in negative illumination effects such as PL quenching.21 Under low density illumination, illumination induced local lattice or sublattice distortion or deformation can be dominant, which is likely beneficial to the photoelectric performance of HOIPs because the effects of charge and defect are minimized in this case. The threshold also determines the tolerance of perovskites to defects and depends on factors such as the trap densities and energy level of defects introduced during sample preparation.
The numerical simulation method can help us to further quantitatively analyze the charge carrier trapping dynamics induced by illumination (see the ESI† for detailed simulation processes and parameters). The charge carrier decay dynamic in terms of charge trapping and detrapping can be described using eqn (1–2):18,32,35
(1) |
(2) |
Before investigating electron–hole (e–h) recombination dynamics, it is instructive to study the ground state properties of various defect systems at 0 K. Fig. 4a and d show that under I-rich conditions, the formation energies of different charge states of vacancy (Iv) and interstitial (Ii) iodine defects as a function of Fermi levels. The +1 charged state of the Iv dominates with the Fermi level for almost the entire bandgap, while the dominant charged state of Ii is +1 (−1) when the Fermi energy level is lower (higher) than 0.68 eV. In the initial equilibrium state (in the dark), the position of the Fermi level depends on the preparation conditions of the sample. According to previous reports, the Fermi level of HOIPs is pinned near VBM under I-rich conditions, which indicates that Iv and Ii mainly exist in the form of +1 charge states.43,44 For non-equilibrium states with continuous illumination, the Fermi level of defective systems would shift due to the charge-state transition of defects caused by the interaction between the trapped charge carrier and the defect site and consequently result in a notable reorganization of the local structure.49 Under the non-equilibrium condition, the Pb–Pb bond length in Iv would change with the charge state transition. When an electron is bound to I+lv and forms I0v, the Pb–Pb bond length decreases from 6.37 to 6.16 Å, and further decreases to 5.94 Å (Form Pb–Pb dimer) by trapping another electron (Fig. 4b). For Ii systems, I+li exhibits a trimer-iodine configuration, which could realize the conversion from trimer iodine to bridge iodine configuration due to local I migration caused by charge state transition from +1 to 0 or −1. The bridge iodine configuration showed a Pb–I bond length of 3.41 and 3.20 Å for 0 and −1 charge states (Fig. 4e). Furthermore, calculation of the density of states (DOS) shows that no obvious DS in Iv systems are observed in the bandgap because the defect level is above CBM,9 which indicates that the charge state transition of Iv has little effect on the carriers recombination rate (Fig. 4c). In the Ii systems, I+li exhibits a deep DS, but it can transform into a shallow p-type DS with electron trapping, which is conducive to the suppression of defect-mediated non-radiative recombination (Fig. 4f). Considering the calculation results of defect formation energy, we speculate that the photoinduced PL enhancement may be attributed to the charge state transition from +1 to −1 assisted by trapped electrons in the Ii system.
At finite temperature, the electron–phonon interactions of various charge state structures have an important effect on the electronic structure of the frontier and defect energy levels, which reflect the stability of the corresponding structure. Fig. 5 shows the edge bands and DS energy evolution of each defect system along molecular dynamics (MD) trajectories at 300 K. As shown in Fig. 5a–c, defect Iv at three charge states (+1, 0, −1) resulted in shallow levels near the CBM. The strong energy fluctuation of the defect level in the −1 charge state further indicates that the Pb–Pb configuration is metastable. Therefore, the electron detrapping in I−lv is relatively fast, and the Iv structure is more likely to stabilize in the I+lv, which is in good agreement with the result of defect formation energy. For the Ii, the +1 charge state has a defect level that oscillates vigorously along the MD trajectory, while for (0, −1) the charge states show typical shallow n-type defects and their energy fluctuations are small. The energy oscillation of the I+li defect level can be attributed to the strong fluctuation of the I–I wrong bond length at 300 K temperature.45 In addition, the deeper defect energy levels enable I+li to undergo electron trapping quickly. Under continuous illumination, the metastable trimer iodine would be transformed into I0i of the bridge-iodine configuration by electron trapping. Since the neutral Ii is metastable, I0i could be further transformed into stable I−1i by electron trapping. In addition, shallow p-type I−1I is beneficial for inhibiting defect-mediated non-radiative recombination, which further suggests that photoinduced PL enhancement is more likely to result from the structural transformation of I+lv to I−1i under illumination.
Fig. 5 Energy evolution of CBM, VBM and DS along MD trajectories at 300 K. (a)–(c) Iv systems. (d)–(f) Ii systems. The reference energy is set to the initial energy of the VBM. |
With the aid of NAMD simulations we can more intuitively understand the change of carrier dynamics caused by charge state transition in each defect system. After photoexcitation, the excited charge carriers can undergo direct recombination with the holes on VBM, or they can undergo a defect-mediated non-radiative recombination, as shown schematically in Fig. 6a. The population evolution of the excited charge carriers on CBM with time is shown in Fig. 6b and c, and the carrier lifetime can be estimated by fitting these curves with the formula f(t) = exp(−t/τ).50 For the Iv system (Fig. 6b), all charge states exhibit a similar e–h recombination rate, and the corresponding carrier lifetimes are 3.86 ns, 3.57 ns and 3.08 ns, respectively. These results indicate that the stable I+lv configuration has a relatively long carrier lifetime, while the photoinduced transition of the charge states from +1 to 0 or −1 instead accelerate carrier recombination. For the Ii system (Fig. 6c), one can clearly see that I−1i has the slowest recombination rate. After 15 ns, the total recombination rates of I+li, I0i and I−1i are 98%, 93.2% and 81.9%, and the corresponding lifetimes are 3.45 ns, 8.03 ns and 11.60 ns, respectively. These results suggest that the carrier recombination rate of Ii decreases when the interstitial iodine structure changes from a trimer in the +1 charge state to a bridge iodine configuration at (0, −1) charge states. Furthermore, the recombination rate of each defect system through different paths was extracted to study the effect of charge state transition on the defect-mediated recombination rate, as shown in Fig. 6d. For all different systems, one can see that non-radiative recombination mediated by defects predominates. What's more, both Iv and Ii systems with non-0 charge states have a relatively fast direct path recombination rate, which can be attributed to the reduced bandgap caused by local lattice distortion of the charged defect (Fig. 4c and f). In Iv systems, the charge state transition from +1 to 0 or −1 does not result in a significant change in the total recombination rate. However, for the Ii system, the transition of charge states from +1 to 0 or −1 significantly reduces the total recombination rate, primarily due to the substantial suppression of defect-mediated non-radiative recombination rates. These results further indicate that the formation of the photoinduced I−1i configuration well meets the necessary conditions for dynamic PL enhancement: (I) long-living structure configuration; (II) slow carrier recombination rate.
In defective systems, the recombination rate of e–h typically strongly depends on the non-adiabatic coupling (NAC) values between the DS, CBM, and VBM, which determine the probability of electron hopping between different energy levels. Generally, a small NAC value implies slow e–h recombination. The NAC matrix element can be expressed as eqn (3).7,50
(3) |
Furthermore, the NAC variations caused by electron trapping/detrapping can also be seen from the visual orbital spatial distribution of defects in each charge state, as shown in Fig. S5 (ESI†). For I+lv with a large NAC value (11.464 meV) between DS and CBM, the orbital spatial distribution shows a large wave function overlap between DS and CBM. After I+lv captures electrons, the electrons are localized on DS and decrease the wave function overlap with CBM, thereby reducing NAC. For Ii, in the trimer configuration, the electrons are delocalized and distributed on the three I atoms of the DS. After trapping the electron and transforming it into 0 or −1 charge states, the electrons are localized and distributed on the interstitial iodine atom of the DS and VBM, which significantly decreases (increases) the NAC value between DS and CBM (VBM). The large NAC difference between DS-VBM and DS-CBM would result in a significant difference in the trapping rates of electrons and holes by traps and thus suppressing the non-radiative recombination rate in the DS. In addition, the redistribution of electron density, in turn, also affects the energy fluctuations and shifts of the band edges and DS (Fig. 5a–f).
Electron–phonon interaction plays a key role in the stability of different charge-state structures and carrier dynamics. To provide insights into the electron–phonon interaction, we calculate the Fourier transform of the autocorrelation function of the energy gaps in the defective system (pristine system) between CBM and DS (VBM). The intensity of each peak in the spectrum represents the strength of the electron–phonon coupling at a particular phonon frequency. As shown in Fig. 8a, both pristine and defect systems show dominant frequency modes below 200 cm−1, which is consistent with previous studies.51–53 In the pristine system, the lowest frequency mode (M1: ∼25 cm−1) corresponds to the Pb–I octahedral twist, and the lower frequency phonon modes M2 and M3 should be assigned to the Pb–I–Pb bending and stretching vibration modes, which are in good agreement with previous experimental results (68 cm−1 and 99 cm−1).54,55 For Iv systems, I−lv shows two prominent peaks below 25 cm−1, which can be attributed to large variations of Pb nucleus velocity in Pb dimers. The strong electron–phonon coupling causes excited electrons to convert energy into lattice vibration energy, thereby accelerating carrier relaxation and reducing lattice stability. This further proves that the capture of electrons by the defect state cannot lead to long-lived structures in Iv under illumination. For Ii systems, I+li shows two high-intensity phonon modes at about 80 and 94 cm−1, which should be assigned to the weak wrong I–I bond associated phonon modes. When the charge state transitions from +1 to −1, the electro–phonon coupling is significantly reduced due to the formation of the Pb–I bond and thus the NAC value decreases by about 7 times (Fig. 7). This further supports that under illumination, the charge state transition from +1 in the dark to −1 by electron trapping is conducive to improving the structural stability of Ii and prolonging the carrier lifetime.
Based on these results, the dynamic mechanism of the anomalous photoinduced PL-enhancing effect in HOIPs is shown in Fig. 9. Specifically, (1) under the condition of equilibrium state (in dark), the point defects in different charge states have different stable structure configurations and carrier lifetimes. The initial structure depends on the growth conditions of the sample (chemical potential of the element). (2) In non-equilibrium state, illumination would lead to photochemical reaction and form a new stable defect configuration, resulting in significant changes in carrier lifetime. Under continuous illumination, the gradual increase of components that are beneficial for carrier lifetime in the material induces the illumination time-dependent PL enhancement, such as I+li + e → I0i + e → I−li. (3) The structural transformation related to charge states is driven by electron–phonon interaction, and enhanced electron–phonon interaction would accelerate the reorganization of the local structure, which well explains the PL intensity changes related to photon flux. The local lattice distortion of defects is essentially the short-range migration of ions, and the halogen ion related local ion migration has the most notable influence on the carrier dynamic behavior. The soft lattice and ion lattice properties of HOIPs play an important role in the photoinduced dynamic process, which allows the multiple configurations of perovskite defects in multiple charge states and small ion migration energy barriers.
Fig. 9 Physical mechanism diagram of the photoinduced dynamic defect response in HOIPs under continuous illumination. |
Although this interesting light gain phenomenon is a reversible positive effect for perovskite materials, the dynamic change triggered by light is detrimental to the photostability of perovskite-based optoelectronic devices. Therefore, some phenomenology-based and effective strategies have been proposed to suppress this photoinduced dynamic response. (1) Minimizing the defect concentrations. This reduces the photoinduced defect charge state transition and related structural changes.31 (2) Interface engineering. Inhibiting ion migration by anchoring uncoordinated ions at defect sites through Lewis acid or base molecules.56,57 (3) Component engineering. Doping of alkali metal ions at the A site has been widely proven to maximize the migration energy barrier of halogen ions and suppress the local structural distortion induced by light, such as Cs+,58,59 K+,58,60 Rh+. These strategies further confirm that light-induce PL enhancement stems from the fact that local structural reorganization is related to charge state transition. Finally, since the physical understanding of perovskites has significantly lagged in the development of their application technology, more advanced in situ characterization techniques and computational methods are urgently in demand to deeply explore the photoinduced dynamic defect tolerance mechanism in HOIPs, which is of crucial significance for the development of perovskite science and technology.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01578a |
This journal is © The Royal Society of Chemistry 2024 |