Tuning the Structural Isomers of Phenylenediammonium Cations to Afford E cient and Stable Perovskite Solar Cells and Modules


 Organic halide salt passivation is considered to be an essential strategy to reduce defects in state-of-the-art perovskite solar cells (PSCs). This strategy, however, suffers from the inevitable formation of in-plane favored two-dimensional (2D) perovskite layers with impaired charge transport, especially under thermal conditions, impeding photovoltaic performance and device scale-up. To overcome this limitation, we studied the energy barrier of 2D perovskite formation from ortho-, meta- and para-isomers of (phenylene)di(ethylammonium) iodide (PDEAI2) that were designed for tailored defect passivation. Treatment with the most sterically hindered ortho-isomer not only prevents the formation of surficial 2D perovskite film, even at elevated temperatures, but also maximizes the passivation effect on both shallow- and deep-level defects. The ensuing PSCs achieve an efficiency of 23.9% with long-term operational stability (over 1000 hours). Importantly, a record efficiency of 21.4% for the perovskite module with an active area of 26 cm2 was achieved.


Introduction
As the front runner among emerging photovoltaic technologies, perovskite solar cells (PSCs) with certi ed power conversion e ciencies (PCEs) over 25% show great promise for scale-up and future commercialization due to relatively simple and low-cost solution processes [1][2][3][4] . However, the disordered stoichiometric compositions at surfaces, the loss of organic components during thermal annealing, and the heterogeneous polycrystalline nature inevitably generate abundant defects in the solution-processed perovskite lms, particularly at surfaces and grain boundaries [5][6][7][8] . Such defects incur electronic states in the bandgap of the perovskite and behave as nonradiative recombination centers, which shorten the carrier lifetime and limit the photovoltaic performance 9 . Moreover, these defects are responsible for local charge accumulation, accelerated ion migration, and the initial invasion of moisture or oxygen, ultimately causing device instability issues 3 . The defects also hinder the scale-up of PSCs to modules, thus restricting commercialization 10 .
To address this problem, various defect reduction methods have been proposed including: i) composition tuning [11][12][13] , ii) crystal growth regulation [14][15][16] , and iii) surface passivation [17][18][19][20] . To date, sur cial posttreatment by alkylammonium halides is commonly exploited to achieve e cient and stable perovskite devices, and many compounds have been evaluated [21][22][23][24] . Typically, an additional two-dimensional (2D) perovskite layer is formed on top of the primary perovskite absorber after treatment with alkylammonium halides, improving the stability of the devices 8,25,26 . A stubborn in-plane orientation is usually obtained for the sur cial 2D perovskite layer, which potentially suppresses charge transport and draws back the passivation effect, especially when using bulkier spacer cations [27][28][29] . For this reason, recently, You et al. removed the annealing process after post-treatment to enable a thin phenylethylammonium iodide (PEAI) layer instead of a 2D perovskite layer for more effective surface passivation 30 . As a result, they achieved a certi ed PCE of over 23%, bene ting from a combination of promotion of charge transport by the πconjugated phenyl rings, reduction of Pb 2+ interstitials through amine coordination, and lling of iodine vacancies by iodide ions 17 . However, these devices still suffered from a plummet in PCE under higher operating temperatures due to the conversion of PEAI to 2D PEA 2 PbI 4 . Therefore, an alkylammonium halide that can sustain higher temperatures without undergoing 2D perovskite formation is highly desirable to effectively passivate surface defects for e cient and stable PSCs and modules.
Herein, we describe surface passivation of perovskite lms using ortho-(phenylene)di(ethylammonium) iodide (o-PDEAI 2 ) that leads to high-performance PSCs and modules. From an investigation of the ortho-, meta-, and para-isomers for PDEAI 2 , the ortho-isomer effectively increases the energy barrier of the 2D perovskite formation and prevents the bulky organic cations from entering the perovskite lattice even at elevated temperatures. Sur cial o-PDEAI 2 exhibits a comprehensive passivation effect on both shallowand deep-level defects, which suppresses non-radiative recombination and improves interfacial charge extraction, resulting in a PCE of 23.9% in PSCs with enhanced long-term stability at ambient and elevated temperatures, and light-soaking conditions. Furthermore, a o-PDEAI 2 -based perovskite module, with an active area of 26 cm 2 , presents a record e ciency of 21.4%.

Results And Discussion
Isomer construction for suppressing 2D perovskite formation The ortho-, meta-, and para-isomers of PDEAI 2 (Fig. 1a) were prepared and further details are provided in the Supplementary Information. To examine their potential to form 2D perovskites, lms composed of equimolar quantities of PDEAI 2 and PbI 2 were fabricated. The X-ray diffraction (XRD) patterns of the para-PDEAI 2 (p-PDEAI 2 )-based lm (Fig. 1b) exhibits a dominant peak at ∼7.15° associated with (002) lattice re ections of the 2D (p-PDEA)PbI 4 perovskite 31,32 . The meta-PDEAI 2 (m-PDEAI 2 )-based lm contains the same peak, but with a much lower relative intensity, implying that the 2D perovskite is formed, but to a much lower extent. The non-perovskite amorphous XRD pattern observed in the o-PDEAI 2 -based lm suggests that the 2D perovskite cannot be formed. Absorption and photoluminescence (PL) spectra reveal a similar trend (Fig. 1c). The p-PDEAI 2 -based lm shows an exciton absorption around 516 nm and a PL emission around 532 nm, assigned to the characteristic peaks of 2D perovskite with layer thickness n = 1 33 . These optical characteristics were not observed in the m-PDEAI 2 -based sample due to the negligible 2D component in the lm. Similarly, no absorption or emission signal was identi ed in the region 500-600 nm for o-PDEAI 2 -based lm, indicating that the formation of 2D perovskite is disfavored.
Next, PDEAI 2 isopropanol solutions were spin-coated onto the perovskite surface and experiments were performed to determine whether the sur cial 2D perovskite is formed during the post-treatment process, especially under thermal induction. Widely used PEAI was employed for comparison. Figure 1d displays the temperature-dependent PL spectra of perovskite lms under 450 nm excitation. As the temperature increases, the lms deposited with PEAI and p-PDEAI 2 show enhanced emission in the 510-540 nm region, evidencing the growth of 2D perovskite with n = 1 on the lm surface (Fig. 1b). In contrast, post-treatment with m-PDEAI 2 and o-PDEAI 2 did not lead to the formation of sur cial 2D perovskite, even at elevated temperatures. This was further con rmed by the temperature-dependent XRD ( Fig. 1e and Supplementary Fig. 1), where reinforced (002) re ections of 2D perovskite were observed for the p-PDEAI 2 -and PEAI-treated lms whereas no peak was observed for the other two lms. The upper 2D (p-PDEA)PbI 4 perovskite shows a lower diffraction intensity than that of the (PEA) 2 PbI 4 , which is commonly observed for other Dion-Jacobson (DJ) perovskites 31,34 . Grazing-incidence wide-angle X-ray scattering (GIWAXS) was used to probe the crystal orientation of the annealed lms (Fig. 1f). The sur cial (PEA) 2 PbI 4 exhibits a combined feature of diffraction arcs and Bragg spots along the q z direction at 0.39 Å −1 and 0.78 Å −1 , implying a random but predominantly in-plane orientation of the (002) and (004) crystal planes. A similar orientation was observed for (p-PDEA)PbI 4 with the characteristic (002) crystal plane at q z = 0.51 Å −1 . Only diffraction signals corresponding to the three-dimensional (3D) perovskite (q z = 1.00 Å −1 ) and excess PbI 2 (q z = 0.91 Å −1 ) were identi ed for the m-PDEAI 2 -and o-PDEAI 2 -treated lms, which is consistent with the XRD results ( Supplementary Fig. 1). It is evident that both the monoammonium and the para-diammonium substituents on the phenyl ring enable organic halide salts to react with PbI 2 from the underlying 3D perovskite during the post-treatment process and form an in-plane-orientation dominated 2D perovskite on the surface (Fig. 1g). The arbitrary, especially in-plane, orientation will block the charge transport along with the inorganic framework due to the alternant insulating organic layers 35 .
In contrast, m-PDEAI 2 and o-PDEAI 2 form a thin organic halide salt layer instead of 2D perovskite layer on the surface, which may avoid the energy disorder and the e ciency loss at elevated operating temperatures.
Density functional theory (DFT) calculations were used to understand the speci c behavior of each PDEAI 2 isomer. The formation energies of 2D perovskites formed from p-PDEAI 2 , m-PDEAI 2 or o-PDEAI 2 were calculated ( Fig. 2a and Supplementary Table 1). The formation of 2D perovskite with p-PDEAI 2 and m-PDEAI 2 are energetically more favorable than o-PDEAI 2 . We further checked the possibility of the formation 2D perovskite with a larger layer thickness and calculated the reaction thermodynamics of the process with the different isomers. Supplementary Fig. 2 Table 1 indicate that the reactions of p-PDEAI 2 and m-PDEAI 2 processes are more favorable than that of o-PDEAI 2 . This difference is due to the suitable matching between the distance of two -CH 2 -CH 2 -NH 3 + groups of the cations and the distance of adjacent octahedral voids on the perovskite. As can be seen in Supplementary Fig. 3, the lowest mismatching distances are 0.85, 0.15 and 1.85 Å for p-PDEAI 2 , m-PDEAI 2 and o-PDEAI 2 ,

and Supplementary
respectively. The formation of quasi-2D perovskite is also likely to be energetically costly for the o-PDEAI 2 , which is nicely re ected by the reaction energy values (Supplementary Table 1). Thus, the model demonstrates that o-PDEAI 2 possesses the highest formation energy barrier of sur cial 2D perovskite among the PDEAI 2 isomers, in good agreement with the experimental observations.

Passivation effects on photovoltaic performance
In order to investigate the surface passivation effect, adsorption energies were calculated by depositing the p-PDEAI 2 , m-PDEAI 2 and o-PDEAI 2 on the perovskite surface considering both the ethyl ammonium and phenyl ring adsorption modes ( Fig. 2b and Supplementary Table 2). The calculations indicate that all three cations show similar surface passivation if only adsorption of one molecule is considered. However, passivation is achieved by the cumulative effect related to the number of cations that can be adsorbed per unit surface area. The optimized structures were processed to estimate the maximum number of cations that may be adsorbed on a 2 × 2 supercell (17.7 Å 2 ) of perovskite surface, as illustrated in the insets of Fig. 2b. For p-PDEAI 2 only 4 cations can be adsorbed, increasing to 4-6 for m-PDEAI 2 and 6 for o-PDEAI 2 . Therefore, it can be qualitatively argued that o-PDEAI 2 shows a better surface passivation effect due to higher coverage which may reduce the probability of defect formation.
To examine the passivation effect, we incorporated the PDEAI 2 isomers onto the perovskite lm with a sur cial morphology corresponding to that shown in Supplementary Fig. 4, and the photovoltaic devices  Fig. 3b. Also, the statistic histogram reveals good reproductivity with 75% of the devices having PCEs exceeding 22% (Fig. 3c) To exclude the in uence of the built-in electric eld present in solar cells, we have additionally investigated the PL decays in the treated with PDEAI 2 and nontreated perovskite lms covered with spiro-OMeTAD deposited on glass substrates (see Supplementary Fig. 6f for kinetics and Supplementary Table 4 for bi-exponential tting parameters). For comparison, we have also investigated a pure nontreated perovskite lm, which showed a long lifetime of 284 ns indicating a high material quality 38 .
The spiro-OMeTAD layer signi cantly shortens the PL decay due to e cient hole extraction. Passivation with the different PDEAI 2 isomers shows qualitatively similar results as for the complete devices con rming their in uence on the hole extraction rate.
The carrier extraction dynamics were revealed by transient photocurrent measurements. Due to the high capacitance of the devices restricting the time resolution of photocurrent measurements, the carrier extraction investigations were performed in integral mode measuring discharging of the device capacitance by photocurrents, when voltage was applied through a high (1 kΩ) resistor. The signal growth, in this case, corresponds to the cumulated extracted charge. Figure  We further addressed the role of PDEAI 2 passivation by investigating transient photocurrent kinetics in lateral sample geometry. The perovskite lms were formed on an interdigitated comb of Pt electrodes (IDE) with 5 µm interelectrode distances and a spiro-MeOTAD layer was deposited on top of perovskite lms. Due to the large, 5 µm interelectrode distance, the carrier extraction was slow. We also used low excitation intensity, when carrier recombination could be ignored. In these conditions, the photocurrent kinetics on a sub-microsecond time scale was mainly determined by the carrier trapping and by the hole extraction to hole transport layer (HTL) 39 . The control device with HTL shows faster photocurrent decay than that without HTL during initial ~ 50 ns due to the hole extraction (Fig. 3f). The in uence of PDEAI 2 treatment is expected to be bilateral: passivation of surface traps shall cause slower photocurrent decay, and modi cation of the hole extraction rate may cause additional photocurrent changes. Indeed, all passivated samples show slower photocurrent decay indicating reduced carrier trapping. The m-PDEAI 2treated sample shows particularly slow decay, apparently because of reduced carrier trapping and the prevented hole extraction. While the o-PDEAI 2 -treated sample shows similar slow carrier decay during initial ~ 20 ns, due to reduced carrier trapping, but at longer times the photocurrent decays faster attributed to the improved hole extraction. The overall transient PL and photocurrent results provide indications of interfacial trap passivation by the PDEAI 2 isomers and also demonstrate that o-PDEAI 2 treatment reduces the carrier transfer barrier at the perovskite/HTL junction and effectively increases the hole extraction rate contributing to the FF enhancement of PSCs.
Ultraviolet photoelectron spectroscopy (UPS) was used to understand the surface energy band structure with and without o-PDEAI 2 . The work function is determined to be -4.45 eV and − 4.91 eV for the pristine perovskite surface and the o-PDEAI 2 passivated surface, respectively ( Supplementary Fig. 7). This shift is in accordance with the change in surface potential probed by Kelvin probe force microscopy (KPFM) as re ected in Fig. 4a. The o-PDEAI 2 -passivated surface exhibits a lower average electronic chemical potential (0.16 mV) than that of the control (0.33 mV). The downshift suggests a more p-type surface after o-PDEAI 2 treatment, which can improve hole extraction and cause band-bending for higher V OC values 40 . Confocal PL microscopy mapping was applied to probe the homogeneity of perovskite lms. The lm with o-PDEAI 2 presents more uniform PL emission compared to the control lm, indicating less surface defect densities (Fig. 4b). This is also revealed by the cathodoluminescence (CL) intensity maps, where a more homogeneous distribution was demonstrated for the lm with o-PDEAI 2 passivation (Fig.   4c). To complement these data the corresponding CL spectra (Supplementary Fig. 8) con rm that o-PDEAI 2 treatment does not induce 2D perovskite formation on the surface and the CL peaks at around 508 nm may be assigned to the presence of excess PbI 2 in both lms. To con rm the presence of o- can enhance the driving force of carrier injection and reduce the recombination losses at the interface. 10 The tDOS distribution under each energy demarcation was characterized as shown in Fig. 4f. The charging and discharging of trap states within the bandgap contributes to a larger variation in capacitance with the change of frequencies. Thus, the frequency differential capacitance can disclose the defects at certain energy demarcations 43 . It was found that the control device has higher trap densities of 10 19 -10 21 m − 3 eV − 1 over three trap bands. After o-PDEAI 2 treatment, the tDOS was reduced over the entire energy region, to 10 18 -10 20 m − 3 eV − 1 , con rming that the o-PDEAI 2 effectively passivate the defects with both shallow and deep energy levels on the surface of perovskite lm.
We further evaluated the scalability of surface passivation by o-PDEAI 2 by fabricating a perovskite solar module with a total active area of 26.00 cm 2 . (Fig. 5a). The module was completed by laser etching with 9 subcells in series and the interconnection is schematically illustrated in Fig. 5b. Encouragingly, the perovskite solar module based on o-PDEAI 2 passivation exhibits a PCE of 21.36%, with a V OC of 10.30 V, a J SC of 2.71 mA cm − 2 and an FF of 76.40% (Fig. 5c and Supplementary Fig. 10), which is the highest e ciency reported so far for perovskite solar modules on the active area (Supplementary Table 5) 44 . The high module performance originates from the good uniformity of the perovskite layer, the reduced trap density, and suppressed interfacial recombination, con rming the utility of the o-PDEAI 2 passivation strategy for scale-up of PSCs.

Shelf life and operational stability
Long-term stability tests were carried out to study the in uence of o-PDEAI 2 layer on device stability. The PCEs of the unencapsulated devices under a RH of 40-50% were rst tracked over time (Fig. 5d). The control device maintains 58% of the initial PCE (20.82%) after 1008 hours, compared to that of 85% for the o-PDEAI 2 passivated device with an initial e ciency of 22.65%. The thermal stability was also evaluated by heating the perovskite lms and corresponding devices at 85°C in a nitrogen atmosphere. As shown in Fig. 5e, the o-PDEAI 2 passivated device (initial PCE 22.38%) degrades by 25% over 1000 hours, compared to a 49% decrease for the control device (initial PCE 20.32%). Interestingly, an e ciency plummet is not observed at the start of the annealing process. That is, the suppression of converting o-PDEAI 2 into 2D perovskite effectively retains the passivation effect under constant heating, and avoids the PCE drop at the initial stage, which was normally observed in the widely used PEAI passivation 30 .
XRD tracking shows that the thermal degradation of these perovskite lms results from the decomposition of the perovskite material (Fig. 5f). The control lm shows an increase in the intensity of the peaks that correspond to PbI 2 , whereas decomposition is retarded after o-PDEAI 2 passivation, indicating the o-PDEAI 2 passivation layer reduces the release of volatile organic components and enhances the resistance to heat of the perovskite material 45 . The continuous performance of the unencapsulated PSCs was also examined by MPPT under 1 sun illumination in an inert atmosphere. The device passivated with o-PDEAI 2 exhibits enhanced light stability, maintaining > 90% of its original PCE after 1100 h, outperforming the control device, which degrades to > 60% of the initial PCE. The robust ambient and operational stability of the o-PDEAI 2 -based device may be attributed to the hydrophobicity of the phenyl group, the mitigated interfacial charge accumulation, and the suppressed ion migration bene ting from the passivation of defects 46,47 .

Conclusions
We demonstrate that o-PDEAI 2 is an effective passivation agent which affords highly e cient and stable For module fabrication, 6.5 cm × 7 cm FTO substrates were patterned by laser with nine sub-cells connected in series. The lm deposition processes were the same as the normal solar cells as described above. The geometric ll factor (GFF) was calculated to be 90.2%, which was de ned as the active area divided by the designated area.
Characterization. The GIWAXS was measured with a photon energy of 8.03 keV (1.55 Å) at an incident angle of 0.3°. PL decay kinetics were measured using the Edinburgh Instruments time correlated single photon counting (TCSPC) uorescence spectrometer F900. The KPFM images were acquired using an MFP-3D In nity with AC bias modulation at 7.5KHz and 4V amplitude in ambient.