Stable Triple Cation Perovskite Precursor for Highly Efficient Perovskite Solar Cells Enabled by Interaction with 18C6 Stabilizer

www.afm-journal.de

Stable Triple Cation Perovskite Precursor for Highly Efficient Perovskite Solar Cells Enabled by Interaction with 18C6 Stabilizer

Xiayan Wu, Yue Jiang,* Cong Chen, Jiali Guo, Xiangyu Kong, Yancong Feng,*

Sujuan Wu, Xingsen Gao, Xubing Lu, Qianming Wang, Guofu Zhou, Yiwang Chen, Jun-Ming Liu, Krzysztof Kempa, and Jinwei Gao*

Triple cation perovskites (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃) have received lots of attention owing to the excellent stability and photovoltaic performance.

However, the development toward efficient solar cells has been significantly impeded by its intrinsic precursor instability, as well as defective crystal surface. Herein, a strategy for introducing the additive of 1,4,7,10,13,16-hex- aoxacyclooctadecane (18C6) in the precursor solution, rendering an excellent stability of more than one month, and the defect passivation effect on the crystal surface are demonstrated. In those perovskite solar cells, a power con- version efficiency of 20.73% has been achieved with a substantially improved open-circuit voltage and fill factor. As evidenced by the density functional theory calculations, the fundamental reason relating to the enhanced perfor- mance is found to be the interaction effect between the 18C6 and cations, and in particular the formation of the 18C6/Pb complex. This finding repre- sents an alternative strategy for achieving a stable precursor solution and efficient perovskite solar cells.

DOI: 10.1002/adfm.201908613

properties such as remarkably high absorp- tion, tunable direct bandgap, excellent bipolar carrier transport, and long charge diffusion length.^[1–4] Based on this excep- tional material, the perovskite solar cells (PSCs) have experienced a rapid develop- ment with power conversion efficiency (PCE) increasing from 3.8%^[5] in 2009 to the current record of 25.2%.^[6] Despite the widely adopted formamidinium lead iodide (FAPbI₃) gives higher PCE as com- pared to with methylammonium lead iodide (MAPbI₃), it suffers from thermal and structural instabilities.^[7] Thus, aiming at the stable and high-efficiency PSCs, by incorporating multiple cations and anion, the mixed triple cation perovskites, such as Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃, have been well developed.^[8–11]

Processing of the perovskite films has been realized via convenient solution- based procedures, including stable colloidal size modulated by the temperature, additive, adding sequence of materials, etc. Among those, the state of precursor solution has been identified as a key role for high stable and efficient perovskite

1. Introduction

Hybrid organic–inorganic perovskites have attracted intense research interest in the last few years owing to the exceptional

X. Wu, Dr. Y. Jiang, C. Chen, J. Guo, X. Kong, Prof. S. Wu, Prof. X. Gao, Prof. X. Lu, Prof. J.-M. Liu, Prof. K. Kempa, Prof. J. Gao

Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Optical Information Materials and Technology South China Academy of Advanced Optoelectronics

South China Normal University Guangzhou 510006, China

E-mail: yuejiang@m.scnu.edu.cn; gaojinwei@m.scnu.edu.cn Dr. Y. Feng, Prof. G. Zhou

Guangdong Provincial Key Laboratory of Optical Information Materials and Technology & Institute of Electronic Paper Displays

South China Academy of Advanced Optoelectronics South China Normal University

Guangzhou 510006, China

E-mail: fengyancong@m.scnu.edu.cn

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201908613.

Prof. Q. Wang

Key Laboratory of Theoretical Chemistry of Environment Ministry of Education

School of Chemistry

South China Normal University Guangzhou 510006, P. R. China Prof. Y. Chen

Institute of Polymers and Energy Chemistry College of Chemistry

Nanchang University Nanchang 330031, China Prof. J.-M. Liu

Laboratory of Solid State Microstructures Nanjing University

Nanjing 210093, China Prof. K. Kempa Department of Physics Boston College

Chestnut Hill, MA 02467, USA

solar cells. Recently, Yan and Xu claimed that the perovskite precursor solution is a colloidal dispersion of lead polyhalide frameworks, and by adding excess MA cations, its coordination degree as well as the colloidal size could be finely controlled, further influencing the final perovskite grains.^[12] Besides, Snaith and co-workers confirmed the function of colloids as nucleation sites in the crystallization process, and by intro- ducing hydrohalic acids they achieved an optimized mixed- perovskite morphology.^[13] However, until now, few attention has been paid on the stability of the precursor solution in spite that it suffers from the degeneration during aging. For example, we found that PbI₂ and δ-phase perovskite in the perovskite film could be distinguished using the aged (5 and 30 days) precursor solution. Although recent studies implied that incorporating an organic molecule ITIC-Th which stabilized CH₃NH₃⁺ (MA) cation in the perovskite crystal could improve the precursor stability to a long term, the origin of such impact still remains unclear and need more deep study.^[14]

Another important obstacle for high-efficiency perovskite solar cells is the crystal surface defect that effects the carrier dynamics and deteriorates the efficiency of cells.^[15] According to the theory calculation results, Frenkel defects, such as Pb, I, and MA vacancies, are the most likely formed defects in MAPbI₃-based perovskite crystals.^[16,17] Passivation engineering, including additives or interfacial materials, is the general method to reduce surface carrier recombination and enhance the efficiencies of PSCs.^[18] For example, You and co-workers used an organic halide salt phenethylammonium iodide (PEAI) on mixed perovskite films for surface defect passiva- tion, leading to a certified PCE of 23.32%.^[19] Kim, Noh, Seok and co-workers introduced I₃⁻ ions into the organic cation solu- tion which decreased the concentration of deep-level defects.^[20]

By adding fluoride into the perovskite precursor and forming chemical bonds, Zhou and co-workers reported the simulta- neously passivated anion and cation vacancies.^[21] While, Yang and Han reduced the loss of decomposed components from soft perovskites by forming strong PbCl and PbO bonds between Pb-rich perovskite surface and chlorinated graphene oxide.^[22] However, the influence of additives in precursor was rarely discussed.

It is well known that 18C6 shows a strong electronegativity and a special cavity, which can selectively complex with various metal cations and organic cations due to the existence of lone- pair electrons, widely used as complexing reagent and phase transfer catalyst.^[23] Mhaisalkar and co-workers presented that, by employing crown ethers, a complete dissolution of the CsBr pre- cursor and core–shell quantum dots (QDs) was obtained, which allows for the synthesis of CsPbBr₃ QDs under low-temperature conditions and achieving efficient and bright LEDs.^[24] Recently, Sun, Sirringhaus, Deschler and co-workers demonstrated that organometal halide perovskites crystallite distribution and phase separation can be precisely controlled by incorporating small concentration of 18C6, leading to an improvement in the photoluminescence quantum yield up to ≈70% and an enhanced external quantum efficiency (EQE) of 15.5%.^[25] In this paper, we demonstrate a more efficient way to stabilize triple cation perovskite (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃) precursor for solar cells, and simultaneously passivate the sur- face defects of the perovskite crystals, by introducing a small

amount of 18C6 (≈0.2 mg mL⁻¹ in precursor solution). Due to the existence of six O atoms, 18C6 could complex with metal cations and organic cations. In particular, via interacting with Pb, 18C6 prohibited the rapid crystallization of PbI₂ and the formation of δ-phase (nonperovskite), leading to the stable precursor solution over one month. Moreover, the presence of 18C6 in perovskite films significantly passivated the crystal surface defects through saturating Pb and reducing I vacancies, leading to the enhanced V_OC and fill factor (FF) (1.17 V and 74.8%), yielding a PCE up to 20.71% in perovskite solar cells.

2. Results and Discussion

2.1. The Role of 18C6 in Perovskite Precursor Solution and Films

The precursor solution of perovskite was prepared as described in the Methods in the Supporting Information. After optimiza- tion, when 18C6 at the concentration of 10 mg mL⁻¹ in DMF solution (≈0.2 mg mL⁻¹ of 18C6 in precursor solution), the pre- cursor gave the best performance (see Figure S1, Supporting Information). We denote this precursor solution and its formed film as PC-10, and take the blank (PC-0) as a reference.

The prominent enhancement of the perovskite precursor sta- bility was firstly observed when monitored by X-ray diffraction (XRD) for perovskite films prepared from the fresh and aged (5, 15, 30 days in glovebox) PC-0 and PC-10 solution, shown in Figure 1a (also see Figure S2, Supporting Information). After 5 days, PC-0 film started to show the diffraction peak at ≈12.6°

due to the formation of PbI₂, which is further strengthened with longer aging time. Further, 30 days aging led to the appearance of photoinactive hexagonal δ-phase of FAPbI3 (≈11.7°) in PC-0, which could significantly deteriorate the light absorption ability and device performance.^[26,27] Apparently, the aging process barely changed the PC-10 film and in which PbI₂ phase is free.

Due to the O atoms in 18C6, we speculate that the introduction of 18C6 could form complex with Pb as well as other organic cations in precursor solution. Based on this hypothesis, the colloidal size in precursor solution was investigated by dynamic light scattering (DLS). The colloidal size of PC-10 is reduced when 18C6 was added, demonstrating that 18C6 has disrupted the meso-stability in PC-10 system (see Figure S3, Supporting Information).

The interaction between cations and 18C6 has been theoreti- cally speculated by density functional theory (DFT) calculation.

The 18C6 molecule has six electronegative oxygen atoms, thus expected to show strong interaction with these cations. The structure where 18C6 complexed with Pb is the most stable, with binding energy equaling to −1.12 eV and the distance of O1 to O4 reduced from 5.83 to 5.76 Å (see Figure S4, Sup- porting Information). To analyze the PbO bond, we calculated the electron density of states. Figure 1b shows that both the local density of states of Pb and O locate at the same position, indicating the outermost electron of the Pb can overlap with the outermost electron of O, i.e., the covalent bonds form between the Pb and O. From the charge density difference of 18C6/Pb complex displayed in Figure 1b, it can be seen that the electrons accumulate between the Pb and O atoms, further proving the

existence of PbO covalent bond. For the case of 18C6/MA, the strong interaction of multihydrogen bonds (NH···O) make the N atom of MA close to the 18C6 molecule (further optimi- zation and discussions see Figures S5–S7, Supporting Informa- tion). However, its binding energy equaling to −0.74 eV is still comparatively weaker than that of 18C6/Pb. While for Cs and FA, their binding energies with 18C6 are −0.70 and −0.42 eV, separately. Accordingly, 18C6 is more prone to interact with Pb, and form the stable complex of 18C6/Pb.

The X-ray photoelectron spectroscopy (XPS) of PC-0 and PC-10 films is shown in Figure 1c. Except the presence of the same Pb 4f_7/2 and Pb 4f_5/2 peak at binding energy of 137.5 and 142.3 eV for both PC-0 and PC-10, two other small peaks at 135.7 and 140.5 eV are shown for PC-0 sample, which is attrib- uted to the unsaturated Pb atoms.^[14,28,29] This, again, proves that the existence of 18C6 is beneficial to suppressing the for- mation of unsaturated Pb and stabilizing the Pb atom, thus inhibiting the rapid crystallization of PbI₂ and suppressing the formation of nonperovskite δ-phase.

On the other hand, the existence of 18C6 in PC-10 film was examined by Fourier-transform infrared (FTIR) spectrometer, as shown in Figure 1d. In the spectrum of PC-0, the peak at 785 cm⁻¹ corresponds to the bend vibration of CH bonds;

the strong broad absorption peak at 983 cm⁻¹ corresponds to the stretching vibration of the CN bonds in FA and MA cations. For PC-10, except the peak of CH bend vibration

slightly bathochromic shifts, no obvious change could be found, possibly due to the low concentration of 18C6. Thereby, we increased its concentration to 200 mg mL⁻¹ in DMF (PC-200), which leads to the obvious CH peak shift to 778 cm⁻¹ and the CN peak to ≈959 cm⁻¹, together with a new peak at 1092 cm⁻¹ which is ascribed to the COC stretching vibration from 18C6.^[30] Apart from these differences, three spectra are all identical (see Figure S8, Supporting Information). Thus, the presence of 18C6 in PC-10 film was confirmed.

The possible location of 18C6 in PC-10 film was then investigated by DFT calculations, shown in Figure 2. The cal- culation models are based on the typical crystal structures of FAPbI₃ and MAPbI₃ (see Figure S9, Supporting Infor- mation). In order to imitate the real crystalline structure of Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ and simplify the calcula- tion, we built and optimize the bulk model of MA_0.125FA_0.875PbI₃. Bulk model of cubic FAPbI₃ consisting of (2 × 2 × 2) cells was built and a FA was replaced by a MA, i.e., the ratio of FA to MA is 0.875:0.125 which is close to the experimental ratio (0.83:0.17). The optimized bulk structures (both side and top view) of MA_0.125FA_0.875PbI₃ are shown in Figure 2a (also see Figure S10, Supporting Information). The bulk structures of MA_0.125FA_0.875PbI₃ with different directions of CN (from N atom to C) bond of MA are shown in Figures S10–S13 (Sup- porting Information). Apparently, the CN direction of [010] in MA is most stable (see Figure S14, Supporting Information).

Figure 1. a) XRD spectra of PC-0 and PC-10 films fabricated from the fresh and aged precursor solutions. b) Electron density of states (DOS) of 18C6/

Pb complex. The inset are the charge density differences (∆ρ = ρ(18C6/Pb) − ρ(18C6) − ρ(Pb)) of 18C6/Pb complex, with isosurface level equaling to 0.001 e Bohr⁻³. The yellow and cyan regions represent electron accumulation and depletion, respectively. The Pb atom locates at the middle of 18C6 molecule. c) XPS spectra in Pb 4f region. d) FTIR spectra.

We found that the replacement by MA destroyed the original regularity of FA, and forced the CN bonds in FA to rotate, so as to form the stable state. Notably, in this MA_0.125FA_0.875PbI₃ crystal, FA are not totally irregular, that in the same FAMAI layer (010), the two neighboring FA are either on the (001) plane symmetry or in the same direction.

To investigate the surface structure of MA_0.125FA_0.875PbI₃, we built three slab models, topC and topN model with FAMAI- and PbI-terminated surfaces (see Figure S15, Supporting Infor- mation). The structure of topC model (the CH3 group of MA orienting toward vacuum) is more stable. When the free 18C6 was absorbed on the FAMAI- or PbI-terminated surfaces of MA_0.125FA_0.875PbI₃, we found that their binding energies are rather weak, ≈−0.17 and −0.01 eV, respectively (for details see Figures S16 and S17, Supporting Information). Meanwhile, due to the strong binding energy between 18C6 and Pb (−1.12 eV), it is difficult for the stable 18C6/Pb complex formed in pre- cursor solution to dissociate. Thus, the slab models with 18C6/

Pb complex absorbed at FAMAI- or PbI-terminated surfaces were built (see Figures S18 and S19, Supporting Information), with the corresponding binding energies of −0.75 and −2.00 eV, respectively (summarized in Figure 2b). As a result, the 18C6 additive has more possibility to form the 18C6/Pb complex and be absorbed at the crystal surface. Figure 2c,d presents the charge density differences of structures with 18C6/Pb complex absorbed at FAMAI- or PbI-terminated surfaces, respectively.

Prominent electron transfer between the 18C6/Pb complex and PbI-terminated surface further proves the more probability of the absorbed position. Furthermore, the Pb at crystal surface

was, therefore, saturated by 18C6, agreeing well with the XPS results (Figure 1c).

Considering the XPS result that the unsaturated Pb was detected, it is indicated that the perovskite was mostly fabri- cated with a surface rich in Pb.^[22] Moreover, the higher binding energy between PbI-terminated crystal with 18C6/Pb also sug- gested that the 18C6/Pb has more possibility to locate on the PbI surface. Based on this structure, the calculated vibrational energies of C and N atoms in the first FAMAI layer with and without 18C6/Pb absorbed on the FAMAI- and PbI-terminated surfaces were reduced (see Figure S20, Supporting Informa- tion), which is consistent with FTIR result. Furthermore, the formation energy of I vacancies (V_I) at surface was found increased for both PbI-terminated or FAMAI-terminated sur- face (Figure 2e and Figure S21, Supporting Information), which is consistent with the experimental result that the Pb:I ratio was improved from 1:2.62 of PC-0 to 1:2.83 of PC-10 according to the Pb 4f and I 3d core-level energy spectra (see Figure S22, Supporting Information).

2.2. Electronic Properties and Morphology

The morphologies of PC-10, PC-0, and PC-200 were investigated by scanning electron microscopy (SEM) (see Figure S23a–c, Supporting Information). Basically, the morphology of perov- skite film has no change when the additive concentration is low. But, the grain size of perovskite crystal begins to decrease when the concentration of 18C6 increases to 200 mg mL⁻¹. Figure 2. a) Optimized bulk structure of MA_0.125FA_0.875PbI₃ crystal. The side view and top view of FAMAI layer is displayed. b) Binding energy between a free 18C6 or 18C6/Pb complex and the FAMAI- or PbI-terminated perovskite surfaces. c,d) The charge density differences (∆ρ = ρ(crystal/18C6/Pb) − ρ(crystal) − ρ(18C6/Pb)) of structures with 18C6/Pb complex absorbed at FAMAI- or PbI-terminated surfaces. The yellow and cyan regions represent electron accumulation and depletion, respectively. e) Formation energy of a surface I vacancy in the FAMAI- or PbI-terminated perovskite surfaces with 18C6 or 18C6/Pb complex.

The cross-sectional SEM images of PC-0 and PC-10 are also measured, showing no difference (see Figure S23d,e, Sup- porting Information). Further, the influence of 18C6 on the electronic properties was studied with ultraviolet photoelec- tron spectroscopy (UPS). The valence band maximum (E_v) was determined from the following equation

v cutoff HOS

E =

(

E −

)

−hv (1)

where E_cutoff and highest occupied states (HOS) positions are indicated in Figure S24 (Supporting Information), respectively.

As expected, the E_v of PC-10 and PC-0 are −5.48 and −5.43 eV, suggesting the limited influence of 18C6 on electronic proper- ties of perovskite films.

2.3. Performance of Perovskite Solar Cells

To investigate the photovoltaic performance of PC-10, we fabri- cated PSCs based on device architecture: FTO/SnO₂/perovskite active layer/spiro-OMeTAD/Ag, where the perovskite active layers were prepared from the fresh precursor solution (PC-10 and PC-0). The thickness of different layer has been shown in the Experimental Section. Figure 3a shows the current density–

voltage (J–V) curves of the champion devices for PC-10 and PC-0, recorded under AM 1.5 simulated solar illumination.

The control device (PC-0) gives an overall PCE of 18.12% with a V_OC of 1.15 V, a J_SC of 22.94 mA cm⁻², and a FF of 68.98%.

By contrast, the PSCs with PC-10 exhibit an improved perfor- mance with a V_OC of 1.17 V, a J_SC of 23.63 mA cm⁻², and a FF

of 74.80%, yielding a PCE of 20.71%. Figure 3b gives the cor- responding EQE and the integrated current density. The PSC based on PC-10 presents a higher response at the wavelength from 340 to 755 nm than that of control device, leading to an increased J_SC from 19.47 to 20.34 mA cm⁻². And the steady- state outputs of PC-0 and PC-10 were measured at bias voltages of 0.92 and 0.96 V, respectively. The PCEs of devices based on PC-0 and PC-10 stabilized at 16.65% and 19.07% with photo- current densities of 18.10 and 19.86 mA cm⁻², respectively, agrees well with the J–V curves (Figure 3c).

In addition, 24 independent devices with the same fabrica- tion process were prepared, and the statistical distribution of photovoltaic parameter, including PCE, V_OC, FF, and J_SC, are exhibited in Figure 3d (also see Figure S25, Supporting Information). The detailed parameters are summarized in Table S1 (Supporting Information). PSCs with PC-10 give an average PCE of 19.257 ± 0.388%, VOC of 1.181 ± 0.007 V, JSC

of 22.852 ± 0.448 mA cm⁻², and FF of 71.383 ± 1.676%, higher than those of PSCs with PC-0 (PCE of 17.079 ± 0.750%, VOC

of 1.141 ± 0.008 V, JSC of 22.436 ± 0.321 mA cm⁻², and FF of 66.829 ± 2.616%). It is noted that the higher PCE is mainly arisen from the better V_OC and FF. Moreover, the small devia- tion indicates an excellent reproducibility of PSCs based on PC-10.

2.4. Mechanism of the Enhanced Efficiency and Stability

To investigate the charge transfer dynamics, the electrical impedance spectroscopy (EIS) of PSCs in dark was measured,

Figure 3. a) J–V curves. b) EQE spectra. c) Steady-state current density and PCE of the champion devices for PC-0 and PC-10. d) PCE distributions for the corresponding devices.

and the Nyquist plots are shown in Figure 4a. The PSCs based on PC-10 show a smaller transfer resistance (R_tr) of 410 Ω and a larger recombination resistance (R_rec) of 1715 Ω compared to the control device (463 and 873 Ω, separately), which suggests a more efficient charge transport process and a decreased charge recombination rate in PC-10. Further, the steady-state photoluminescence (PL) and the time-resolved photolumines- cence (TRPL) spectra of PC-10 and PC-0 films were character- ized. In Figure 4b,c, taking PC-0 as reference, an enhanced PL intensity of Cs_0.05(MA_0.17FA_0.83)_0.95Pb(I_0.83Br_0.17)₃ at 776 nm is obvious for PC-10. The improvement of the PL intensity suggested the decrease in traps density, which is responsible for nonradiative charge recombination of the polycrystalline

perovskite films. The TRPL spectra exhibit a biexponential decay with lifetime τ1 and τ2 of 10.1 and 673.5 ns for PC-10, and 9.3 and 523.6 ns for PC-0. τ1 is considered related to trap- assisted recombination and τ2 represents free carrier recom- bination.^[31] These PL and TRPL results demonstrate the significant suppression of traps and carrier recombination in PC-10,^[32] which benefits the charge transport process, as proved by the EIS results.

Specifically, the hole-trap density and electron-trap density of PC-10 and PC-0 films were assessed based on the hole-only and electron-only devices (see the Experimental Section) by space- charge-limited current (SCLC) method (Figure 5a,b). The defect (trap) density (n_traps) is reported linearly proportional to the Figure 4. a) Nyquist plots of the cells based on PC-0 and PC-10, measured in dark. The inset shows the equivalent circuit. b) PL spectra and c) TRPL decay curves for PC-0 and PC-10.

Figure 5. a,b) The dark current–voltage curves for hole-only devices and electron-only devices. c) Arrhenius plot of the characteristic transition frequency and d) trap state density (NT) of the perovskite solar cells measured at 300 K.

onset voltage of the trap-filled limit (V_TFL).^[33] In our case, PC-10 presents both the lower hole-trap density (0.69 × 10¹⁶ cm⁻³) and the electron-trap density (0.90 × 10¹⁶ cm⁻³) comparing to those of PC-0, namely, 2.25 × 10¹⁶ and 1.78 × 10¹⁶ cm⁻³, respectively.

In addition, by carrying out the thermal admittance spectro- scopy (TAS) in dark on the PC-0 and PC-10 films (see Figure S26a,b, Supporting Information), the defect activation energies (E_a) of PC-0 and PC-10 are determined to be 0.222 and 0.177 eV, respectively (Figure 5c).^[34] Combined with Mott–Schottky anal- ysis, the trap energy level of PC-10 (0.25 eV) was shallower than that of PC-0 (0.28 eV) (Figure 5d and Figure S26c, Supporting Information).^[35] Furthermore, it was confirmed that the trap density was reduced to ≈80% as the calculated trap density were 1.8 × 10²⁴ and 1.5 × 10²⁴ m⁻³ eV⁻¹, respectively.

Based on our previous experimental and theoretical results, part of the traps could come from the unsaturated Pb sites which were reported acting as the nonradiative recombina- tion centers.^[29] And as illustrated in XPS spectra (Figure 1c), the Pb traps were significantly mitigated by the addition of 18C6. Besides, the calculated enhanced formation energy of I vacancies as well as the interaction between 18C6 and other cations in PC-10, as discussed before, could also contribute to the overall reduction of traps in PC-10.^[36] Hence, the addition of 18C6 mitigates the recombination process and enhances FF and V_OC for PSCs.

The stability of 18C6 in the perovskite films was firstly moni- tored with FTIR after thermal annealing PC-200 films at 100 °C over 5 hours. Impressively, no obvious difference could be distinguished (see Figure S27, Supporting Information), demon - strating the superstability of 18C6 in perovskite films. Fur- thermore, the stability of PSCs based on PC-10 and PC-0 was then examined with the humidity around 30%. The monitored PCE shows the same trend after two weeks in both devices (see Figure S28, Supporting Information), indicating the stability of 18C6 as well.

3. Conclusion

In summary, we have provided an efficient and simple method for facilitating stable and high-efficiency perovskite solar cells.

We have shown that the inclusion of 18C6 in perovskite pre- cursor enables stabilizing the triple cation perovskite precursor solution and simultaneously passivating its crystal surface.

Both experimental and theoretical results proved that the inter- action between cations and 18C6, particularly forming com- plex of 18C6/Pb, prevents the fast crystallization of PbI₂ and the formation of δ-phase, therefore stabilizing the precursor solution for more than one month. In addition, 18C6/Pb was found playing a fundamental role in reducing the trap density, such as unsaturated Pb and I vacancies. As a result, our best device achieved an enhanced PCE of 20.71% with improved V_OC and FF (1.17 V and 74.80%). Our results demonstrated a fundamentally new way to enhance the stability of precursor solution and efficiency in perovskite solar cells. Those concepts are promising for further researches in the solution-processed perovskite photovoltaics and other aspects, which have the advantage of transferring the active ions into destination without loss.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

The authors thank the financial support from NSFC-Guangdong Joint Fund (No. U1801256), National Key R&D Program of China (No. 2016YFA0201002), NSFC (Nos. 51803064, 51571094, 51431006, 51703070, and 51561135014), Guangdong Provincial Foundation (2016KQNCX035), Program for Chang Jiang Scholars and Innovative Research Teams in Universities (No. IRT_17R40), and Guangdong Innovative Research Team Program (No. 2013C102). The authors also thank the support from the Guangdong Provincial Engineering Technology Research Center for Transparent Conductive Materials, National Center for International Research on Green Optoelectronics (IrGO), MOE International Laboratory for Optical Information Technologies, and the 111 Project. K.K. acknowledges partial support by the Guangdong Innovative and Entrepreneurial Team Program (No. 2016ZT06C517).

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

X.W. contributed to the fabrication and characterization of PSCs. Y.F., Y.J., and J.W.G. conceived and co-directed the project. Y.F. designed and performed the DFT calculations. X.W., Y.J., J.W.G., and K.K. contributed to the writing. J.-M.L. and G.Z. participated in the results discussion. All authors commented on the final paper. J.W.G directed the research.

Keywords

18-crown-6, density functional theory, passivation, precursor stability Received: October 18, 2019 Revised: November 12, 2019 Published online: December 9, 2019

[1] G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344.

[2] F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang, M. G. Kanatzidis, Nat. Photonics 2014, 8, 489.

[3] Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao, J. Huang, Science 2015, 347, 967.

[4] J. S. Manser, P. V. Kamat, Nat. Photonics 2014, 8, 737.

[5] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc.

2009, 131, 6050.

[6] https://www.nrel.gov/pv/assets/pdfs/best-research-cell- efficiencies.20190923.pdf (accessed: September 2019).

[7] N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476.

[8] A. Kumar, A. Priyadarshi, S. Shukla, M. Manjappa, L. J. Haur, S. G. Mhaisalkar, R. Singh, J. Appl. Phys. 2018, 124, 215106.

[9] A. M. Soufiani, Z. Yang, T. Young, A. Miyata, A. Surrente, A. Pascoe, K. Galkowski, M. Abdi-Jalebi, R. Brenes, J. Urban, N. Zhang,

V. Bulovic´, O. Portugall, Y.-B. Cheng, R. J. Nicholas, A. Ho-Baillie, M. A. Green, P. Plochocka, S. D. Stranks, Energy Environ. Sci. 2017, 10, 1358.

[10] M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-Baena, M. K. Nazeeruddin, S. M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Energy Environ. Sci. 2016, 9, 1989.

[11] W. Tan, A. R. Bowring, A. C. Meng, M. D. McGehee, P. C. McIntyre, ACS Appl. Mater. Interfaces 2018, 10, 5485.

[12] K. Yan, M. Long, T. Zhang, Z. Wei, H. Chen, S. Yang, J. Xu, J. Am.

Chem. Soc. 2015, 137, 4460.

[13] D. P. McMeekin, Z. Wang, W. Rehman, F. Pulvirenti, J. B. Patel, N. K. Noel, M. B. Johnston, S. R. Marder, L. M. Herz, H. J. Snaith, Adv. Mater. 2017, 29, 1607039.

[14] M. Qin, J. Cao, T. Zhang, J. Mai, T.-K. Lau, S. Zhou, Y. Zhou, J. Wang, Y.-J. Hsu, N. Zhao, J. Xu, X. Zhan, X. Lu, Adv. Energy Mater.

2018, 8, 1703399.

[15] Y. Yang, M. Yang, D. T. Moore, Y. Yan, E. M. Miller, K. Zhu, M. C. Beard, Nat. Energy 2017, 2, 16207.

[16] A. Buin, P. Pietsch, J. Xu, O. Voznyy, A. H. Ip, R. Comin, E. H. Sargent, Nano Lett. 2014, 14, 6281.

[17] J. Kim, S.-H. Lee, J. H. Lee, K.-H. Hong, J. Phys. Chem. Lett. 2014, 5, 1312.

[18] R. Mai, X. Wu, Y. Jiang, Y. Meng, B. Liu, X. Hu, J. Roncali, G. Zhou, J.-M. Liu, K. Kempa, J. Gao, J. Mater. Chem. A 2019, 7, 1539.

[19] Q. Jiang, Y. Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin, J. You, Nat. Photonics 2019, 13, 460.

[20] W. S. Yang, B.-W. Park, E. H. Jung, N. J. Jeon, Y. C. Kim, D. U. Lee, S. S. Shin, J. Seo, E. K. Kim, J. H. Noh, S. I. Seok, Science 2017, 356, 1376.

[21] N. Li, S. Tao, Y. Chen, X. Niu, C. K. Onwudinanti, C. Hu, Z. Qiu, Z. Xu, G. Zheng, L. Wang, Y. Zhang, L. Li, H. Liu, Y. Lun, J. Hong, X. Wang, Y. Liu, H. Xie, Y. Gao, Y. Bai, S. Yang, G. Brocks, Q. Chen, H. Zhou, Nat. Energy 2019, 4, 408.

[22] Y. Wang, T. Wu, J. Barbaud, W. Kong, D. Cui, H. Chen, X. Yang, L. Han, Science 2019, 365, 687.

[23] S. Shirakawa, K. Maruoka, Angew. Chem., Int. Ed. 2013,52, 4312.

[24] S. A. Veldhuis, Y. F. Ng, R. Ahmad, A. Bruno, N. F. Jamaludin, B. Damodaran, N. Mathews, S. G. Mhaisalkar, ACS Energy Lett.

2018, 3, 526.

[25] M. Ban, Y. Zou, J. P. H. Rivett, Y. Yang, T. H. Thomas, Y. Tan, T. Song, X. Gao, D. Credgington, F. Deschler, H. Sirringhaus, B. Sun, Nat. Commun. 2018, 9, 3892.

[26] Y. H. Park, I. Jeong, S. Bae, H. J. Son, P. Lee, J. Lee, C.-H. Lee, M. J. Ko, Adv. Funct. Mater. 2017, 27, 1605988.

[27] S.-H. Turren-Cruz, M. Saliba, M. T. Mayer, H. Juárez-Santiesteban, X. Mathew, L. Nienhaus, W. Tress, M. P. Erodici, M.-J. Sher, M. G. Bawendi, M. Grätzel, A. Abate, A. Hagfeldt, J.-P. Correa-Baena, Energy Environ. Sci. 2018, 11, 78.

[28] D. Bi, X. Li, J. V. Milic´, D. J. Kubicki, N. Pellet, J. Luo, T. LaGrange, P. Mettraux, L. Emsley, S. M. Zakeeruddin, M. Grätzel, Nat.

Commun. 2018, 9, 4482.

[29] W. Zhang, S. Pathak, N. Sakai, T. Stergiopoulos, P. K. Nayak, N. K. Noel, A. A. Haghighirad, V. M. Burlakov, D. W. deQuilettes, A. Sadhanala, W. Li, L. Wang, D. S. Ginger, R. H. Friend, H. J. Snaith, Nat. Commun. 2015, 6, 10030.

[30] Z. Du, M. Zhu, T. Zhang, J. Ma, J. Am. Ceram. Soc. 2010, 93, 4036.

[31] L. M. Herz, Annu. Rev. Phys. Chem. 2016, 67, 65.

[32] S. D. Stranks, V. M. Burlakov, T. Leijtens, J. M. Ball, A. Goriely, H. J. Snaith, Phys. Rev. Appl. 2014, 2, 034007.

[33] D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent, O. M. Bakr, Science 2015, 347, 519.

[34] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Commun. 2014, 5, 5784.

[35] V. Adinolfi, M. Yuan, R. Comin, E. S. Thibau, D. Shi, M. I. Saidaminov, P. Kanjanaboos, D. Kopilovic, S. Hoogland, Z.-H. Lu, O. M. Bakr, E. H. Sargent, Adv. Mater. 2016, 28, 3406.

[36] N. Aristidou, C. Eames, I. Sanchez-Molina, X. Bu, J. Kosco, M. S. Islam, S. A. Haque, Nat. Commun. 2017, 8, 15218.