Modulation of Perovskite Grain Boundaries by Electron Donor-Acceptor Zwitterions R,R-Diphenylamino-phenyl-pyridinium-(CH2) n -sulfonates: All-Round Improvement on the Solar Cell Performance.

Inverted perovskite solar cells (PSCs) have attracted intense attention because of their insignificant hysteresis and low-temperature fabrication process. However, the efficiencies of inverted PSCs are still inferior to those of commercialized silicon solar cells. Also, the poor stability of PSCs is one of the major impedances to commercialization. Herein, we rationally designed and synthesized a new series of electron donor (R,R-diphenylamino) and acceptor (pyridimium-(CH2) n -sulfonates) zwitterions as a boundary modulator and systematically investigated their associated interface properties. Comprehensive physical and optoelectronic studies verify that these zwitterions provide a four-in-one functionality: balancing charge carrier transport, suppressing less-coordinated Pb2+ defects, enhancing moisture resistance, and reducing ion migration. Although each functionality may have been reported by specific passivating molecules, a strategy that simultaneously regulates the charge-transfer balance and three other functionalities has not yet been developed. The results are to make an omnidirectional improvement of PSCs. Among all zwitterions, 4-(4-(4-(di-(4-methoxylphenyl)amino)phenyl)propane-1-ium-1-yl)butane-1-sulfonate (OMeZC3) optimizes the balance hole/electron mobility ratio of perovskite to 0.91, and the corresponding PSCs demonstrate a high power conversion efficiency (PCE) of up to 23.15% free from hysteresis, standing out as one of the champion PSCs with an inverted structure. Importantly, the OMeZC3-modified PSC exhibits excellent long-term stability, maintaining almost its initial PCE after being stored at 80% relative humidity for 35 days.

Introduction

Halide perovskites have been attracting enormous attention in the research community due to their excellent ambipolar charge mobilities,[1,2] high dielectric constant,[3,4] panchromatic absorption,[5] tunable band gaps,[6,7] and relatively simple processing.[8] These unique optical and electrical properties were realized to be excellent for light-absorbing materials of solar cells. Ever since the first inverted (p–i–n) perovskite solar cell (PSC) was reported in 2013,[9] the power conversion efficiency (PCE) of inverted PSCs has rapidly been enhanced from 3.90 to 23.80%[10] in the past decade. Furthermore, inverted PSCs exhibit a less hysteresis phenomenon than regular (n–i–p) PSCs,[11] making them more attractive. Meanwhile, it was found that the performance of PSCs would be significantly limited due to charge recombination and accumulation caused by unbalanced carrier transport in the device.[12] Thus, the optimal ratio of hole to electron mobility for the PSCs should be equal to 1, which can reduce the carrier recombination probability and hence improve the efficiency of the device.[13] Unfortunately, most PSCs possess significantly lower hole mobility compared with its electron mobility.[14,15]

Another obvious obstacle to postponing PSC from commercialization is its poor long-term stability in comparison with its silicon-based counterparts.[16,17] Thus, research regarding perovskite stability has also been widely studied.[18−30] The poor long-term stability of PSCs could be ascribed to several major reasons: (i) perovskite devices will gradually decompose by moisture when they work in an atmospheric condition;[31−33] (ii) solution-processed perovskite films usually generate numerous defects, especially less-coordinated Pb2+ or Pb clusters, at the grain boundaries and/or surfaces.[34,35] These defects will act as the charge recombination centers, hastening degradation induced by moisture or oxygen within PSCs;[36,37]and (iii) ion migration within the device could also cause PSC degradation. On the one hand, mobile iodide anions were observed to migrate through the electron transport layer into the silver cathode in the inverted PSCs, resulting in the formation of insulating silver iodide that inhibits the extraction of charge carriers.[38] On the other hand, ion migration also gives rise to the formation of lattice vacancy in the perovskite layer, leading to the decay of PSCs.[39,40] Therefore, developing an effective strategy to enhance the long-term stability of PSCs by concurrently improving the abovementioned drawbacks is necessary and urgent.

Various passivation agents have been reported to improve the stability and efficiency of PSCs. One strategy is to introduce a large-size hydrophobic group into the passivation molecules to avoid moisture penetration.[41,42] Jiang et al. employed a series of polymers with long-alkyl side chains as a boundary passivation agent to improve the moisture resistance of PSCs.[43] Another effective method is the passivation of less-coordinated defects via Lewis acid–base coordination.[44−47] In this approach, zwitterions turn out to be a better passivation agent because they are able to accommodate both the positive and negative defects. For instance, 3-(decyldimethylammonio)-propane-sulfonate was found to passivate the positive and negative ionic defects at grain boundaries of the as-prepared perovskite films by its negatively charged sulfonate and positively charged ammonium motif.[48] Passivators having a strong chemical interaction with perovskite have been demonstrated as an effective strategy to suppress the ion migration in PSCs. Yang and co-workers utilized caffeine possessing strong interaction with perovskite as a passivation agent to suppress the ion migration at grain boundaries, thereby enhancing the thermal stability of PSCs.[49] In addition to the stability issue, additives were also developed to solve the unbalanced carrier transport within PSCs. For example, Wang et al. mixed perovskite and the alcohol-soluble fullerene derivative to make a bulk-heterojunction (BHJ) PSC with a more balanced charge carrier extraction and enlarged interfacial area.[50] However, most reports on carrier transport balance focused on hole transport materials (HTMs),[15] electron transport materials,[14] or interface engineering,[51] while passivating molecules that modulate hole/electron balance and simultaneously solve various stability issues are obscure.

In this work, by virtue of rationally designing the chemical structure of the passivation additive that incorporates electron donor–acceptor zwitterions, we have developed a promising class of additives for multifunctional molecular modulation at the grain boundaries of halide perovskites. These modulators bring balance between the hole and electron transports within the perovskite layer, giving rise to a significant enhancement of device performance. Also, this modification improves moisture resistance and simultaneously reduces the defect formation and ion migration process within PSCs, leading to long-term stability. Details of results and discussion are elaborated as follows.

Results and Discussion

Synthesis of Zwitterions

In this study, a series of intramolecular zwitterions, including 4-(4-(4-(diphenylamino)phenyl)pyridin-1-ium-1-yl)ethane-1-sulfonate (ZC2), 4-(4-(4-(diphenylamino)phenyl)propane-1-ium-1-yl)butane-1-sulfonate (ZC3), 4-(4-(4-(diphenylamino)phenyl)propane-1-ium-1-yl)butane-1-sulfonate (ZC4), 4-(4-(4-(di(4-methylphenyl)amino)phenyl)propane-1-ium-1-yl)butane-1-sulfonate (MeZC3), and 4-(4-(4-(di-(4-methoxylphenyl)amino)phenyl)propane-1-ium-1-yl)butane-1-sulfonate (OMeZC3), were designed and synthesized. The synthetic route to these zwitterions is depicted in Scheme . In brief, N,N-diphenyl-4-(pyridin-4-yl)aniline was prepared from 4-bromotriphenylamine and 4-pyridylboronic acid via the Suzuki–Miyaura coupling reaction. Subsequently, this chemical further reacts with 1,4-butanesultone, 1,3-propanesultone, and sodium 2-bromoethanesulfonate to form ZC4, ZC3, and ZC2, respectively, as shown in Scheme a. For synthesizing −CH3- and −OMe-functionalized zwitterions (see Scheme b), 4-(4-bromophenyl)pyridine was fabricated with the Suzuki–Miyaura coupling between 1-bromo-4-iodobenzene and 4-pyridylboronic acid. The resultant product was further coupled with either 4-methyl-N-(4-(pyridin-4-yl)phenyl)-N-(p-tolyl)aniline or 4-methoxy-N-(4-methoxyphenyl)-N-(4-(pyridin-4-yl)phenyl)-aniline under the standard Buchwald amination conditions. The corresponding products, respectively, react with 1,3-propanesultone to yield MeZC3 and OMeZC3. The detailed synthetic procedure, purification, and characterization are elaborated in the Supporting Information.

Scheme 1

Synthetic Route of Zwitterions, Where the Corresponding Yields are Shown Inside the Parentheses

Molecular Design Concept

Conceptually, these zwitterions may modulate the properties of perovskite films based on the following multifunctionalities. As shown in Figure a, the triphenylamine group marked in blue endows these molecules with a high degree of hydrophobicity. Moreover, the triphenylamine group serves as an excellent hole-extracting site and has been widely used in HTMs, such as 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) and poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA). We thus expect an increase in hole mobility of the as-prepared perovskite film, attaining a more balanced charge carrier transport (see Figure b). Second, the less-coordinated lead cations at the perovskite surface should be greatly passivated by the sulfonate group (region marked in red in Figure a) because the Pb–O bond (bond dissociation energy of ∼382.4 kJ/mol) is stronger than the Pb–I bond (∼194.0 kJ/mol).[52] The strong binding nature of the sulfonate group also facilitates hole extraction at the grain boundaries within the perovskite film (see Figure b), leading to a BHJ-like structure to increase hole conductivity. Finally, pyridinium (see Figure a), which would strongly fix mobile halide anions through both electrostatic and C–H-anion interactions,[53] was selected to bridge the sulfonate anchoring group and the triphenylamine terminal moiety. The pyridinium positive charge balances the negative sulfonate anion to avoid interference by the presence of extra counter cations. Importantly, the triphenylamine–pyridinium pair undergoes internal charge transfer, leading to a further enhanced hole-extracting ability via virtual triphenylamine radical cation formation (see Figure a).[54] These multifunctions are expected to benefit PSCs in terms of efficiency and stability.

Figure 1

(a) Chemical structure of passivation zwitterions before and after internal charge transfer from the triphenylamine donor to the pyridinium acceptor. Here, the regions marked by blue, green, and red color contribute to the higher stability of the as-prepared perovskite via improving moisture resistance, suppressing surface defects, and reducing ion migration, respectively. (b) Schematic illustration of PSCs with zwitterion treatment to balance the charge carrier transport and to ameliorate the stability. Here, the orange, purple, green, red, yellow, gray, blue, and white balls within the white circle represent the lead, halide, A-site cation, oxygen, sulfur, carbon, nitrogen, and hydrogen atoms, respectively. (c) Procedure for preparing zwitterion-doped perovskite films.

Preparation of Zwitterion-Doped Perovskite Films

The zwitterion-doped perovskite thin films were prepared by the following method, as shown in Figure c. First, we prepared zwitterion solutions at various concentrations. Then, these zwitterion solutions were used to dissolve the ammonium and lead salts to form zwitterion-containing perovskite precursor solutions. After heating, the resulting solutions were utilized to fabricate the perovskite wet films via a spin-coating process. Subsequently, the wet films were annealed at 100 °C for 30 min to generate zwitterion-doped perovskite films. Notably, the perovskite precursor solution containing 0.05 mg of zwitterion per 1 mL was optimum for the performance of PSCs and the study mentioned below was carried out under this optimized condition. Detailed methodology regarding zwitterion doping into the Cs0.05(FA0.95MA0.05)0.95Pb(I0.95Br0.05)3 perovskite is illustrated in the Supporting Information.

Characterization

We then performed comprehensive spectroscopic measurements to explore the location of zwitterions and their influence on the crystalline structure and interaction with the as-prepared films. Herein, we chose OMeZC3 as a prototype because of its superior performance on the optoelectronic properties of the as-prepared perovskite films (vide infra). To verify the binding mode of this zwitterion on the perovskite, Fourier-transform infrared spectroscopy (FTIR) was conducted. As shown in Figure a, the asymmetric stretching vibration of the SO3– group at 1036 cm–1 in the neat zwitterion shifts to 1041 cm–1 in the OMeZC3-doped perovskite sample. The blue-shift in the stretching frequency indicates the coordination between the sulfonate groups and the cationic species,[55] confirming that SO3– actually anchors to the less-coordinated Pb2+ cation. Also, the significant shift of C=N stretching from 1640 to 1632 cm–1 demonstrates a strong interaction between perovskite and the pyridinium ring of OMeZC3. The X-ray diffraction (XRD) results (see Figure b) clearly reveal the same crystal patterns for both pristine and OMeZC3-containing perovskites, indicating that the passivation molecule does not enter the perovskite lattice. Furthermore, two-dimensional grazing-incidence wide-angle X-ray scattering patterns (see Figure S20 in the Supporting Information) reveal that OMeZC3 has no effect on the crystalline orientation of perovskite. The energy-dispersive X-ray spectroscopy (EDS) elemental mapping for the OMeZC3-incorporating sample reveals that elements including S, Pb, and I are uniformly distributed within the perovskite thin film, as demonstrated in Figure c–f. In short, the combination of XRD pattern and EDS elemental mapping suggests that OMeZC3 molecules are most plausibly located at the perovskite grain boundary rather than at the interface with charge transport layers or within the perovskite lattice.

Figure 2

(a) FTIR spectra of pure OMeZC3 (green line) and perovskite film with (purple line) or without (black line) incorporation of OMeZC3 (0.05 mg/1 mL precursor solution). (b) XRD patterns of the corresponding perovskite films. (c) Cross-sectional SEM images of the perovskite thin film with OMeZC3 incorporation and the corresponding EDS elemental mapping for (d) S (red), (e) Pb (blue), and (f) I (green).

The proof of the abovementioned concept is also provided by the hole transporting ability of zwitterions. Figure S21 reveals the plot of dark current density versus voltage, that is, the J–V curves of the hole-only devices (ITO/MoO3/zwitterion/MoO3/Al), where the hole mobility (μ) of various zwitterions can be evaluated by applying the space charge limited current (SCLC) model.[56]where εr, ε0, and L represent the relative permittivity, vacuum permittivity, and thickness of the film, respectively. The hole mobility summarized in Table S2 suggests that these passivated zwitterions could also act as hole transport channels, rather than the recombination centers at the boundaries of perovskites.

Encouraged by the abovementioned results, the devices were then fabricated with a configuration of indium tin oxide (ITO)/PTAA/perovskite/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/C60/bathocuproine (BCP)/Ag to verify the improvement of these zwitterions on the photovoltaic performance. Upon optimization, all solar cells prepared by perovskite precursor solutions containing 0.05 mg of zwitterions per 1 mL exhibit the highest PCE value, except that ZC2 decreases the efficiency of the device in all optimizing parameters (see Table S3 in the Supporting Information). We then analyzed all devices prepared with perovskite precursor solutions of 0.05 mg/mL zwitterions in this research. Figure a and Table show J–V curves and the associated parameters of the PSCs prepared with various passivation zwitterions, that is, the PSC efficiency as a function of modulating alkyl chain length between the pyridinium ring and the sulfonate group. Compared with the pristine device, the PCE value enhances from 20.08 to 22.23% and 21.77% for the ZC3- and ZC4-treated PSCs, respectively. By contrast, the ZC2-doped device reveals a decreased efficiency of 12.62%. The results unambiguously demonstrate that the performance of PSC is affected significantly by the alkyl chain length within the zwitterion additives. Such a discrepancy among ZC2-, ZC3-, and ZC4-incorporated PSCs should not arise from their thermostability. As shown in Figure S22, there is no weight loss occurring for each zwitterion in thermogravimetric analysis until 250 °C, which is significantly higher than the perovskite annealing temperature of 100 °C. To gain more insights into their optoelectronic properties, the corresponding hole-only (ITO/PEDOT/PSS/perovskite/PTAA/Au) and electron-only (ITO/SnO2/perovskite/PCBM/Ag) devices were fabricated to evaluate the trap density and charge carrier mobility of control and zwitterion-treated perovskite samples. J–V curves of hole-only and electron-only devices measured in the dark are shown in Figures S23 and S24, respectively. After the linear ohmic response in the low-field region, the current starts to show a nonlinear increase when the applied voltage exceeds the trap-filled limit voltage (VTFL), where the traps are filled with injected carriers. Therefore, the trap density (Nt) can be evaluated by the following equation[57]

Figure 3

(a) J–V curves of the PSCs prepared by pristine (black line), ZC2- (red line), ZC3- (orange line), ZC4- (green line), MeZC3- (blue line), and OMeZC3-incorporating (purple line) perovskites. SCLC fitting for the J0.5–V curve of (b) hole-only and (c) electron-only devices. (d) Energy diagram of PTAA, passivation zwitterions, and perovskite.

Table 1

Optoelectronic Parameters of PSC Fabricated by Various Perovskite Precursors

PSC	VOC (V)	JSC (mA/cm2)	FF (%)	PCE (%)	hole mobility (10–2 cm2 V–1 s–1)	electron mobility (10–2 cm2 V–1 s–1)	h/e mobility ratio
Pristine	1.116	22.85	78.76	20.08	0.693	3.30	0.21
ZC2	1.058	18.68	63.86	12.62	0.0276	0.26	0.11
ZC3	1.153	23.71	81.31	22.23	1.62	1.39	1.17
ZC4	1.146	23.66	80.29	21.77	1.04	1.86	0.56
MeZC3	1.165	23.58	82.80	22.75	2.22	1.66	1.33
OMeZC3	1.162	23.87	83.43	23.15	4.68	5.12	0.91

The hole Nt of the pristine, ZC2-, ZC3-, and ZC4-treated perovskites are calculated to be 3.35 × 1015, 9.96 × 1015, 1.57 × 1015, and 2.49 × 1015 cm–3, respectively. A decrease in hole Nt confirms that the traps on the grain boundary can be successfully passivated in ZC3- and ZC4-incorporating samples. However, the hole Nt increases in the PSC with ZC2 treatment, showing a poor passivation ability of ZC2. For the electron trap density, a similar trend is observed, where Nt decreases in both ZC3- and ZC4-passivated samples but increases in the ZC2-doped one. In the high-bias region of J–V curves, the hole and electron mobility can be estimated through the SCLC model shown in Figure b,c, respectively. Table demonstrates the improvement of hole mobility from 0.693 × 10–2 to 1.62 × 10–2 and 1.04 × 10–2 cm2 V–1 s–1 after the incorporation of ZC3 and ZC4, respectively, affirming that these passivation zwitterions can serve as a conduit to accelerate hole carriers. By contrast, the hole mobility largely declines in the ZC2-doped PSC. Such a discrepancy in hole mobility could be ascribed to the energy mismatch of the highest occupied molecular orbital (HOMO) of the zwitterion and PTAA and the valence band maximum (VBM) of perovskite.

To support the abovementioned viewpoint, we conducted the differential pulse voltammetry (see Figure S25 in the Supporting Information) and ultraviolet photoelectron spectroscopy experiments (see Figure S26 in the Supporting Information) with an aim to determine the HOMO of all zwitterions and VBM of perovskite. In combination with the band gap obtained from the absorption spectrum (see Figure S27 in the Supporting Information), we then obtained the corresponding energy band diagram depicted in Figure d. For ZC3 and ZC4, the HOMO energy levels are −5.47 and −5.40 eV, respectively, which are located at the region between perovskite’s VBM of −5.79 eV and PTAA’s HOMO of −5.20 eV.[58] Thus, the hole can be transferred from perovskite to zwitterionic conduit and then to PTAA. However, the evaluated HOMO of ZC2 (−4.89 eV) is obviously higher than that of PTAA. Therefore, the hole extracted by zwitterions will not be successfully transported into PTAA, leading to a serious decline in the overall hole mobility.

Further optimization of PSC efficiency was performed by harnessing the electronic properties of the hole-transporting zwitterions. Thus, ZC3 was functionalized on its triphenylamine motif with an electron-donating substituent such as the methyl or methoxyl group. As the substituent becomes more electron-donating, the HOMO of the zwitterion will be lifted to a higher energy level, where the HOMO of MeZC3 and OMeZC3 are located at −5.45 and −5.34 eV, respectively (Figure d). These HOMO energy levels are still lower than those of PTAA, ensuring facile hole transport from zwitterions to PATT. As demonstrated in Figure a and Table , the PCE value is then improved from 22.23% of ZC3 to 22.75% of MeZC3 and then to 23.15% of OMeZC3.

We then plot the PCE as a function of the number of −CH2 bridges, as shown in Figure a. The plot shows a volcano type of PCE, being in the order of ZC2 < ZC4 < ZC3 < MeZC3 < OMeZC3 and that of the pristine device is in between ZC2 and ZC4. To gain more insights into the fundamental significance, the hole/electron (h/e) mobility ratios for the pristine and zwitterion-treated PSCs are deduced from the measured mobility data, as shown in Figure b,c. As a result, the h/e value changes from 0.21 in the pristine PSC to 0.11, 1.17, and 0.56 after the incorporation of ZC2, ZC3, and ZC4, respectively. The h/e value of the ZC3-treated PSC is close to 1, indicating that ZC3 has a better ability to balance charge carrier transport, leading to higher efficiency than that of ZC2- and ZC4-doped ones. Balancing the charge mobility and passivating the boundary defect of perovskite make ZC3-treated PSC performance on the top among ZC2, ZC3, and ZC4. Moreover, the OMeZC3-treated sample possesses a h/e mobility ratio of 0.91 that is closest to 1 among all the PSCs (see Table ). We then make a plot of PCE as a function of % of deviation from the mobility ratio of h/e = 1, defined as D % = |1 – (h/e)| × 100%. The results shown in Figure b clearly reveal a mismatch in the relationship between PCE and h/e, where the OMeZC3-treated sample with smallest D % results in the champion PCE. Note that although the MeZC3-treated sample reveals the lowest hole and electron trap density (see Figures S23 and S24 in the Supporting Information), the unbalanced h/e makes it inferior to the OMeZC3-treated sample in terms of PCE, manifesting the key factor of balancing h/e to enhance the device performance.

Figure 4

(a) Plot of PCE as a function of the number of −CH2 bridges. Also shown is PCE of the pristine PSC (black solid circle, which is placed at the arbitrary x-axis). (b) Plot of PCE as a function of |1 – (h/e)| × 100%, where h and e are the hole and electron mobility, respectively. See text for definition.

Next, the effects of OMeZC3 on the optoelectronic properties of champion PSCs were investigated. The pristine device exhibits an open-circuit voltage (VOC) of 1.116 (1.110) V, a short-circuit current density (JSC) of 22.85 (22.65) mA cm–2, a fill factor (FF) of 78.76% (74.28%), and a PCE value of 20.08% (18.68%) measured by reverse (forward) scans (see Figure a). Conversely, the PCE value of the OMeZC3-treated device increases to 23.15% (22.71%), with a VOC of 1.162 (1.158) V, a JSC of 23.87 (23.79) mA cm–2, and an FF of 83.43% (82.43%) under reverse (forward) scans shown in Figure b. We then calculated the hysteresis factor (H factor = (PCEreverse – PCEforward)/PCEreverse) to evaluate the degree of hysteresis. As demonstrated in Figure a,b, the H factors of the control and OMeZC3-modified devices are calculated to be 6.97 and 1.90%, respectively. The considerable decrease of the H factor reaffirms a good balance in electron and hole mobility within the OMeZC3-treated PSC. Figure c reveals the monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra of PSCs, showing the integrated JSC of 21.96 and 23.05 mA cm–2 for the pristine and OMeZC3-based PSCs, respectively, which match well with the values obtained from the J–V curves. Figure S28 shows the distribution and error bar of VOC, JSC, FF, and PCE for PSCs fabricated with different precursors. As a result, the statistics of PCE distribution for PSCs reveals excellent reproducibility of the device and consistently enhanced performance up to 23% with OMeZC3 incorporation, which, up to this stage, is considered to be among one of the champion PSCs with an inverted structure (see Table S4 for a fair comparison to previously reported devices).

Figure 5

J–V characterization for the PSCs (a) without and (b) with OMeZC3 incorporation. (c) IPCE spectrum and the corresponding integrated JSC calculated from IPCE, (d) Nyquist plots, (e) VOC plots against the logarithm of light intensity, and (f) EQEEL–JSC curve (inset: LED working image) of the control (black line) and OMeZC3-passivated (purple line) PSCs.

Typical optoelectronic spectra were then acquired to gain insights into the charge transfer and recombination processes in PSCs. Nyquist plots of PSCs based on the pristine and OMeZC3-modified perovskites measured in the dark, together with an equivalent circuit, are displayed in Figure d, where the equivalent circuit is composed of the sheet resistance (Rs) of the conductive electrode and the recombination resistance (Rrec). The fitting parameters obtained using the equivalent circuit model are listed in Table S5 of the Supporting Information. The Rs of the OMeZC3-treated PSC is similar to that of its pristine counterpart because the identical device architecture was used. Conversely, the Rrec of the device increases from 7355 to 15,474 Ω, indicating that there are fewer defect-associated traps, and the charge recombination rate is lower after OMeZC3 modification. We then measured the variation of VOC with various light intensities (Figure e) to evaluate the passivation ability of the OMeZC3 on the PSCs. The curves of VOC versus semilogarithmic light intensity should be a linear relationship expressed by eq below[59]where n, T, and I represent ideality factor, temperature, and light intensity, respectively. When n is equal to 1, the device is trap-free. The slope of VOC versus ln(I) reduces from 1.48 kBT/e (reference) to 1.15 kBT/e (OMeZC3-treatment), suggesting that the trap was significantly suppressed by OMeZC3. In addition, we studied the carrier migration behavior by conducting electroluminescence (EL) experiments, where the PSC can operate as a light-emitting diode by applying a bias. As demonstrated in Figure f, the device of OMeZC3-passivated perovskite exhibits a strong EL peak compared to that of the pristine one. More importantly, the VOC loss from nonradiative recombination (ΔVOCnrad) can be calculated from the external quantum efficiency (EQEEL) at the same operation current density as PSC’s JSC with eq (46)

As a result, the ΔVOCnrad values were evaluated as 0.116 and 0.079 V for the reference and OMeZC3-incorporating PSCs, respectively, confirming a successful inhibition of nonradiative recombination after OMeZC3 passivation. In short, the abovementioned experimental results demonstrate an excellent passivation ability of OMeZC3 for the PSC.

Last but not least, we performed experiments to evaluate the impact of OMeZC3 on PSC stability. From the X-ray photoelectron spectroscopy (XPS) spectra of the pristine and OMeZC3-passivated perovskite films, Pb 4f (see Figure a), I 3d, C 1s, and N 1s (see Figure S29 in the Supporting Information) signals shift after the incorporation of the zwitterion, revealing the interaction between perovskite and OMeZC3. More importantly, in the Pb 4f XPS spectrum, except for the two main peaks at 138.3 and 143.2 eV that correspond to Pb2+ 4f7/2 and Pb2+ 4f5/2, respectively, the two minor peaks at 136.5 and 141.4 eV suggest the existence of metallic Pb0 resulting from the less-coordinated Pb2+ defects in the pristine perovskite film.[46] By contrast, these Pb0 signals completely disappear after the addition of OMeZC3, indicating that OMeZC3 can suppress the dangling Pb2+ cations via SO3– coordination. We then monitored the morphology change of perovskite films stored in the presence of air via scanning electron microscopy (SEM). The surface of the pristine perovskite film becomes rugged and uneven after being stored under the atmosphere for 14 days (see Figure b), and some cracks appear after 28 days. In sharp contrast to the pristine film, the perovskite sample with the OMeZC3 additive remains a closely packed morphology even after being exposed to the air for 28 days, as shown in Figure c. Clearly, the OMeZC3-treated perovskite film shows higher air stability according to the SEM image. We further immersed the perovskite films into water to evaluate their water stability. The neat perovskite film turned yellow rapidly within 2 s, whereas, remarkably, the perovskite film incorporated with OMeZC3 sustained a black appearance after soaking for approximately 60 s (see Figure d or Movie S1 in the Supporting Information). We then measured the XRD pattern of samples after soaking them in water for 60 s. As demonstrated in Figure e, most α-phase perovskites remain in the OMeZC3-passivated perovskite, while most α perovskites transfer to photoinactive δ-phase and PbI2 in the pristine sample. With OMeZC3 passivation, this improved stability was also observed in an ambient condition. In yet another approach, as shown in Figure S30, the black OMeZC3-treated film survives upon exposure to air for 2500 h, while the pristine film decomposed by showing a yellow color appearance in the same condition.

Figure 6

(a) XPS spectra of Pb 4f for the pristine and OMeZC3-doped perovskite films. Top-view SEM images of the (b) pristine and (c) OMeZC3-passivated perovskite films. (d) Photographs of the control (right-hand side) and OMeZC3-treated perovskite films (left-hand side) during the water immersion experiment. (e) XRD patterns of perovskite thin films after immersion into water for 1 min. (f) Moisture stability measured under the humidity of 80%. (g) Photostability under continuous 1 sun illumination for 720 h. (h) Thermal stability carried out at 80 °C.

The abovementioned results manifest that the highly hydrophobic OMeZC3 can effectively improve the water resistance of perovskites. The superior moisture stability can be rationalized by the efficacious hydrophobic encapsulation of OMeZC3 on the perovskite film, thereby preventing the penetration of moisture into perovskites. We then conducted long-term stability tests for the corresponding PSCs. The PCE of the reference device drops to 30% of its initial value after 35 days of storage at 80% humidity (Figure f). In sharp contrast, the OMeZC3-modified PSC did not exhibit noticeable device degradation under identical experimental conditions. To gain further understanding of the stability of PSCs under operating conditions, we also measured the PCE evolution of the devices with continuous 1 sun illumination. As illustrated in Figure g, while the PCE value of the pristine device drops to 68% of its original performance after 700 h of continuous illumination, the OMeZC3-treated device maintains 90% of its initial performance under the same condition. The thermal stability test of PSCs was further conducted at 80 °C. Compared with the pristine device, the thermal stability of the OMeZC3-based device is significantly improved (Figure h). Lastly, as shown in Figure S31, the XRD patterns of PSCs stored at 80 °C for 100 h remain unchanged, indicating that OMeZC3 can inhibit AgI formation on the Ag electrode of the thermally degraded PSCs by reducing ion migration. The results clearly prove that the OMeZC3 zwitterion passivation effectively improves the stability of PSCs in the inverted structure via modulating the grain boundary of the perovskites.

Conclusions

In summary, a new series of electron donor–acceptor zwitterions, ZC2, ZC3, ZC4, MeZC3, and OMeZC3, have been designed and synthesized to modulate the boundary of the perovskite, aiming to ameliorate perovskite optoelectronic properties and stability. The passivation of zwitterions on the grain boundary has been unambiguously verified and the zwitterions were endowed with multifunctionalities simultaneously, which balance the charge carrier transport, enhance moisture resistance, inhibit less-coordinated Pb2+ defects, and reduce ion migration of perovskites. PCE is in the order of ZC2 < pristine < ZC4 < ZC3 < MeZC3 < OMeZC3 (see Figure a), where C3 with three −CH2 chain severs as the best composition, among which OMeZC3-modified PSCs demonstrates superb device performance with a PCE value up to 23% and excellent stability due to its optimization on the aforementioned four functionalities. This work thus provides a new strategy to develop efficient and long-term stable PSCs by incorporating rationally and systematically designed electron donor–acceptor zwitterionic organic molecules.