Rationally designed universal passivator for high-performance single-junction and tandem perovskite solar cells

doi:10.21203/rs.3.rs-4643346/v1

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Rationally designed universal passivator for high-performance single-junction and tandem perovskite solar cells

https://doi.org/10.21203/rs.3.rs-4643346/v1

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Interfacial trap-assisted nonradiative recombination hampers the development of single junction and tandem perovskite solar cells (PSCs). Herein, we report a rationally designed universal passivator to realize highly efficient and stable single junction and tandem PSCs. Multiple defects are simultaneously passivated by the synergistic effect of anion and cation. Moreover, the defect healing effect is precisely modulated by carefully controlling the number of hydrogen atoms on cations and steric hindrance. Due to minimized interfacial energy loss, L-valine benzyl ester 4-toluenesulfonate (VBETS) modified inverted PSCs achieve a power conversion efficiency (PCE) of 25.26% (certified 25.15%) for PSC devices and 21.00% for the modules with an aperture area of 32.144 cm². The efficiency values both are the record PCEs ever reported for the inverted PSCs using vacuum flash technology in ambition conditions. Further, by suppressing carrier recombination, the perovskite/Si tandem solar cells coupled with VBETS passivation deliver a PCE of 30.98%. This work highlights the critical role of the number of hydrogen atoms and steric hindrance in designing molecular modulator to advance the PCE and stability of PSCs.

Physical sciences/Materials science/Materials for energy and catalysis/Solar cells

Physical sciences/Materials science

Presently, several groups have achieved very attractive power conversion efficiency (PCE) exceeding 26% based on emerging metal halide perovskite solar cells (PSCs), which makes it one of the most promising photovoltaic technologies.^1-3 It is encouraging that perovskite/Si tandem solar cells (TSCs) demonstrated a tremendous PCE of 33.9%.⁴ In recent years, inverted PSCs witnessed remarkable advancements in PCE and stability.^5-12 However, the commercial deployment of single junction and tandem PSCs is hindered by their poor long-term operational stability. Soft ionic lattice properties of perovskites make it suffer from poor intrinsic and extrinsic stabilities.^13-14 The extrinsic stabilities can be overcome by developing advanced encapsulation technology.¹⁵ However, the intrinsic instability induced by ion migration and deep-level defects is difficult to be addressed.^{8, 16} The defects within perovskite films can provide pathways for ion migration.¹⁷ It was reported that the defect density at the interface of perovskite films is 1 ~ 2 orders of magnitude larger than that in the bulk of the films.^18-19 Trap-assisted nonradiative recombination, also called Shockley-Read-Hall (SRH), would reduce not only PCE but also long-term durability.^20-21 Moreover, ion migration is much faster at grain boundaries (GBs) and interface than in the bulk of perovskite films.^{19, 22} Therefore, it is of significant importance to enhance PCE and intrinsic stability of PSCs by passivating interfacial defects and inhibiting interfacial ion migration via rational interface engineering.

The interface between the perovskite and electron transport layer (ETL) plays a critical role in realizing high-performance inverted PSCs. The C₆₀ and its derivatives (e.g., PC₆₁BM) are commonly used as ETL in inverted PSCs^23-24. Unfortunately, it was reported that the perovskite/ETL interface in inverted PSCs usually suffers from severe nonradiative recombination losses resulting from minority carriers and trap states.^25-26 At this interface, there are usually various defects simultaneously encompassing positively charged defects (e.g., undercoordinated Pb²⁺ and halide vacancies) and negatively charged defects (such as cation vacancies and PbI₃^-).²⁷ To effectively heal these harmful defects, a variety of interface materials have been developed, mainly involving low-dimensional perovskites,^28-30 Lewis bases^31-33, organic cations,³⁴ and organic salts^{8, 25, 35-37}. Compared with other interface materials, organic salts exhibit great potential in minimizing nonradiative recombination and suppressing ion migration due to their ability to simultaneously passivate positively and negatively charged defects.^{8, 25, 36-37} For example, Sargent et al.²⁵ synergistically used two types of organic ammonium salts propane-1,3-diammonium iodide (PDAI₂) and 3-(methylthio)propylaminehydroiodide (3MTPAI) to passivate the defects at upper interface in inverted PSCs, which achieved a certified quasi-steady-state PCE of 25.1%. To maximize the potential of organic salt modifiers, the rational design of organic anions and cations is of extreme importance. In terms of anions, halide anions (i.e., F^-, Cl^-, Br^- and I^-) are frequently employed.^{25, 36-37} However, nonhalide anions possess the advantages of adjustable structure and wide varieties as compared to halide anions.^{8, 38} The organic anions containing -SO₃^-³⁹ and -COO^-⁸ have been certified to be capable of effectively passivating undercoordinated Pb²⁺ and halide vacancy defects and thus suppressing ion migration. Generally, cations can only passivate negatively charged defects through ionic bonds or electrostatic interaction. Our groups have revealed that multiple active site molecules are more effective in passivating trap states than single active site molecules.^40-43 In order to increase the functions of cations and strengthen their interaction with perovskites, additional effective functional groups should be incorporated to functionalize cations. For instance, the introduction of Lewis base groups (e.g., -C=O and -SH) in organic cations should be able to passivate both negatively charged cation vacancy defects by ionic bond or hydrogen bond and positively charged undercoordinated Pb²⁺ and halide vacancy defects by coordination bond. Finally, the influencing mechanisms of the steric hindrance and the number of hydrogen atoms on cations on defect passivation remain blurry.

In this work, we proposed an effective defect passivation strategy by rational design of hydrogen atoms and steric hindrance of amino acid benzyl ester organic cations in nonhalide ammonium salts. The three amino acid salts, benzyl glycinate p-toluenesulfonate (BGTS), L-valine benzyl ester 4-toluenesulfonate (VBETS), and L-leucine benzyl ester p-toluenesulfonate salt (LBETS), which have the same p-toluenesulfonate (TS^-) nonhalide anion and different amino acid benzyl ester cations, were employed to passivate the upper surface of perovskite films in inverted PSCs. We found that the defect passivation effect of TS^- -anions greatly depended on the steric hindrance induced by cation size. The only difference between the three cations (i.e., BG⁺, VBE⁺, and LBE⁺) is whether there is isopropyl or isobutyl substituent on BG⁺. We revealed that the number of hydrogen atoms on cations exhibited a profound influence on defect passivation. The defect passivation effect of cations was determined by the balance between the number of hydrogen atoms and cation sizes. The multiple different active sites (TS^-, -NH₃⁺, and, -C=O) enabled simultaneous passivation of multiple defects, mainly including undercoordinated Pb²⁺, halide vacancy, and cation vacancy defects, which minimized interfacial nonradiative recombination losses. Through comprehensive consideration, VBETS was the most successful in passivating interfacial defects due to its appropriate number of hydrogen atoms and cation size. We also demonstrated that the VBETS can be used for passivating the defects of conventional and wide band gap (WBG) perovskite films, which confirmed the universality of this passivation strategy. As a result, the maximum PCEs of VBETS-modified inverted PSCs based on conventional and WBG perovskites PSCs with VBETS modification were 25.26% and 21.74%, respectively. Through this passivation strategy, we fabricated highly efficient perovskite/Si tandem solar cells with a peak PCE of 30.98%. This work provides deep insights into improving the PCE and stability of PSCs by developing multisite nonhalide ammonium salts via rationally tailoring the number of hydrogen atoms and steric hindrance.

Theoretical screening of organic cations

Fig. 1a shows the chemical structures and electrostatic potential (ESP) of three sorts of cations and TS^--anions. All organic salts have TS^-, -NH₃⁺, and -C=O active sites, which should be able to simultaneously manage various positively and negatively charged defects. Density functional theory simulation was performed to investigate the defect passivation effect of three organic nonhalide ammonium salts for the surface of perovskite films. As illustrated in Fig. 1b and S1-3, we explored the passivation effect of different organic salts for various defects, including Pb²⁺ substituted I^-(Pb_I), I^- vacancy (V_I), and FA⁺ vacancy (V_FA). It was found that the three salts can passivate these defects but VBETS and LBETS were better than BGTS regardless of defect types, which could be because of the introduction of alkyl substituent. For positively charged Pb_I (Fig. S1) and V_I (Fig. S2), the defect passivation effect increased in the order of BGTS, VBETS, and LBETS, which is consistent with the cation size order of BG⁺＜VBE⁺＜LBE⁺. In our previous work, we have uncovered that the size of imidazolium cations can markedly impact the defect passivation effect of BF₄^- anions.⁴⁴ The large-sized cation would weaken the Coulomb force between the cation and anion due to large steric hindrance, which heightens the interaction of anions with perovskites. In terms of negatively charged V_FA defects (Fig. S3), the defect passivation order of BG⁺＜LBE⁺＜VBE⁺ was observed. It means that incorporating alkyl substituent can promote the defect passivation of cations for the V_FA defects. From the perspective of steric hindrance, the smaller cation size is more beneficial for passivating V_FA defects. However, this is opposite to our experimental results. Therefore, we speculated that the other factor exerts a highly positive role in passivating V_FA defects, which counteracts the negative effect of steric hindrance. Subsequently, we focused on delving into the passivation effect of three different cations for the V_FA defects. Fig. 1c-e presents that the binding energies of BG⁺, VBE⁺, and LBE⁺ cations with FAPbI₃ perovskites containing V_FA defects are -4.62 eV, -5.15 eV, and -4.91 eV, respectively. This indicates that the cations' passivation order agrees with the salt molecule passivation order. Obviously, the VBE⁺ showcased the optimal passivation effect for V_FA. To study the reason behind the phenomenon, we systematically compared the binding energies of NH₄⁺ (An⁺), CH₃NH₃⁺(MA⁺), CH₃CH₂NH₃⁺(EA⁺), BG⁺, VBE⁺ and LBE⁺with FAPbI₃ perovskites containing V_FA defects (Fig. 1f and S4). It was found that the binding energy gradually increased as the number of hydrogen atoms on alkyl increased. We inferred that hydrogen atoms on alkyl had strong electrostatic interaction with V_FA defects and/or [PbI₆]^4- octahedron, which was further verified by the charge density difference of BG⁺, VBE⁺, and LBE⁺ with FAPbI₃ surface encompassing V_FA defects (Fig. 1g-I and S5). It can be concluded that the defect passivation effect of cations was determined by the compromise between the number of hydrogen atoms and steric hindrance. The proper number of hydrogen atoms and steric hindrance are necessary for achieving optimal defect passivation.

Experimental screening of organic cations via investigating interactions

We further experimentally analyzed the chemical interactions of three organic ammonium salts with the perovskite. As shown from the X-ray photoelectron spectroscopy (XPS) in Fig. 2a, the VBETS BGTS, VBETS, and LBETS modified perovskite films exhibited S 2p characteristic peaks, indicating that all modification molecules have been successfully introduced into the upper surface of the perovskite films. At the same time, VBETS exhibited the maximum peak shift, demonstrating the maximum variation in the chemical environment and binding energy of -SO₃^- with the perovskite. As shown in XPS results in Fig. 2b-c, the binding energy of both Pb 4f and I 3d peaks of the modified film by the three organic ammonium salt molecules shifted to the lower binding energy compared to the control film, which is attributed to the interaction of -C=O in cations and -S=O in anions with undercoordinated Pb²⁺ and/or V_I defects. We can see that VBETS exhibited the largest peak shift from 138.7 eV and 143.7 eV to 138.0 eV and 143.0 eV, respectively, followed by LBETS, and then BGTS. This indicates that VBETS had the best passivation effect for Pb²⁺-related defects through the synergistic coordination of -C=O and -S=O in anions and cations due to appropriate cation size and steric hindrance. The shift of the I 3d binding energy peak indicates that hydrogen bonds were formed between -NH₃⁺ in the three organic ammonium salt molecules and I in [PbI₆]⁴^- in perovskite, which is conducive to filling the V_FA vacancy defects in octahedral crystals and stabilizing perovskite crystals⁴⁵. Additionally, the electrostatic interaction of the hydrogen atoms on the alkyl group with I^- on [PbI₆]⁴^- octahedron should also be responsible for the shifted binding energy of I 3d peaks, which has been confirmed by theoretical calculation results. As shown in the Fourier transform infrared spectroscopy (FTIR) in Fig. 2d-f, the C=O peak was shifted from 1751 cm^-1 of BGTS to 1749 cm^-1of BGTS+PbI₂, from 1748 cm^-1 of VBETS to 1735 cm^-1 of VBETS+PbI₂, and from 1746 cm^-1 of LBETS to 1741 cm^-1 of LBETS+PbI₂, suggesting the coordination bonds between C=O and Pb²⁺. In a similar way, the shift of S=O from 1033 cm^-1 of BGTS to 1038 cm^-1of BGTS+PbI₂, from 1038 cm^-1 of VBETS to 1022 cm^-1of VBETS+PbI₂, and from 1035 cm^-1 of LBETS to 1025 cm^-1 of LBETS+PbI₂ affirmed the coordination interaction of S=O with Pb²⁺. In short, combining XPS and FTIR confirmed the interaction of C=O and S=O with Pb²⁺.

Characterization of perovskite films

The above experimental and theoretical results certified the chemical interaction between organic salt modifiers and perovskites as well as their passivation ability for various defects. Then, we conducted a qualitative and quantitative analysis of the defects through spectroscopic and electrical means. We measured the steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) of perovskite films on bare glass. As shown in Fig. 2g, the PL intensities of the samples gradually increased according to this order of the control, BGTS, LBETS, and VBETS-modified perovskite films, indicating the lowest trap density in the VBETS-modified sample. The TRPL results in Fig. 2h show that the average lifetimes of the control, BGTS-, VBETS-, and LBETS-passivated perovskite films were 362.05 ns, 840.99 ns, 1346.85.11 ns, and 1057.46 ns, respectively. Among all samples, the VBETS-modified perovskite film had the longest average carrier lifetime and strongest PL intensity, indicating its excellent defect healing function, which minimized nonradiative recombination losses. Much reduced nonradiative recombination should be conducive to enhancing open-circuit voltage (V_OC) and fill factor (FF) as well as reinforcing device stability.

To further evaluate the passivation effects of different modification molecules from a quantitative perspective, we calculated the trap state density of perovskite films by the space charge limited current (SCLC) method. According to the formula of N_t= 2𝜀₀𝜀_rV_TFL∕qL²,⁴⁶ the value of V_TFL is positively correlated with the value of trap state density. SCLC results based on electron-only devices (ITO/SnO₂/perovskite/PC₆₁BM/Ag) revealed that the V_TFL of VBETS-modified devices decreased from 0.263 V to 0.153 V (Fig. 2i), while the SCLC test results based on hole-only devices (ITO/NiO_x/perovskite/Spiro-OMeTAD/Ag) revealed that the V_TFL decreased from 0.451 V to 0.239 V (Fig. S6). After VBETS passivation, the electron defect density was reduced from 3.37×10¹⁵ cm^-3 to 1.96×10¹⁵ cm^-3, and hole defect density was decreased from 5.78×10¹⁵ cm^-3 to 3.06×10¹⁵ cm^-3, indicating that VBETS can efficiently passivate the multiple defects on the surface of perovskite films.

From the perspective of macroscopic thin film and crystal morphology, the effects of BGTS, VBETS, or LBETS modification on the morphology of perovskite films were analyzed by scanning electron microscopy (SEM) in Fig. 3a. It can be seen that there are many white PbI₂ phase and pores on the surface of the pristine perovskite film, while the white PbI₂ phase and pores were reduced after BGTS, VBETS, and LBETS modification⁴⁷. Among them, the VBETS-modified perovskite film exhibited the flattest surface morphology, which was further confirmed by its lowest roughness, as exhibited in the atomic force microscope (AFM) in Fig. S7. This could be ascribed to the strongest interaction of VBETS with perovskites. The flat and uniform perovskite film will promote electron extraction at the perovskite/PC₆₁BM interface and reduce nonradiative recombination losses⁴⁸.

The defect distribution and density were further characterized by laser beam-induced current (LBIC) imaging technology, which used a laser beam to scan the surface of the PSCs to reveal the photocurrent mapping and generate the internal trap defect distribution of the device on macroscopic scale⁴⁹. As shown in Fig. 3b, the LBIC results show that the control film exhibited very poor film uniformity and low photon current response, while all modified devices presented enhanced film uniformity and current. It is worth noting that the VBETS-modified devices exhibited the highest current, which is due to its best defect passivation effect. Further, the surface current of the control devices decreased significantly after aging at room temperature under relative humidity (RH) of 60±10% for 5 days, while the surface current of the VBETS-modified PSCs decayed slightly. Moreover, the slowest decay was found in the VBETS-modified devices.

Kelvin probe force microscopy (KPFM) measurement was carried out to gain insights into the surface potentials of the perovskite films without and with modification. As shown in Fig. 3c and d, the average surface potential of the control, BGTS, VBETS, and LBETS-modified perovskite films was 196.84 mV, 22.92 mV, -233.22 mV and -53.46 mV, respectively. It was revealed that the VBETS-modified perovskite film exhibited the lowest surface potential with the largest difference up to 430 mV compared with the control perovskite film. As shown from the ultraviolet photoelectron spectroscopy (UPS) (Fig. 3e) and corresponding bandgap structure schematic diagram (Fig. 3f), we can conclude that the Fermi energy levels of the BGTS, VBETS, and LBETS modified perovskite films shifted upward compared with the control perovskite film. Particularly, VBETS-modified perovskite films exhibited a most obviously shifted Femi energy, indicating that VBETS modification induced more n-type characteristics³⁴. This is related to the reason that VBETS passivated the electronic defects on the surface of the perovskite and formed a back surface field with the bulk phase of the perovskite. The formed back surface field is in the same direction as the built-in electric field of the device, thus making the stronger built-in potential (V_bi) of 0.95 V, which is higher than 0.87 V for control devices from the Mott−Schottky plots (Fig. S8), thus promoting electron transport and extraction⁵⁰. Actually, organic cation-induced n-type doping and back electric field at perovskite/ETL interface have been reported to improve the PCE and stability of inverted PSCs.³⁴

Investigation of photovoltaic performance

To assess the photovoltaic performance of PSCs, we fabricated p-i-n type NiO_X-based inverted PSCs device, where BGTS, VBETS, and LBETS were used to modify the upper surface of perovskite films. Specifically, to verify the universality of our surface passivation strategy, we adopted conventional bandgap perovskites of 1.53 eV-Cs_0.05MA_0.05FA_0.9PbI₃ and 1.58 eV-Cs_0.05(FA_0.95MA_0.05)_0.95Pb(I_0.95Br_0.05)₃, as well as a WBG 1.66 eV-Cs_0.05MA_0.15FA_0.8Pb(I_0.76Br_0.24)₃ perovskites as light-harvesting materials by controlling the ratio of precursor materials. The comparative analysis of the optimized concentrations of three passivating agents of BGTS, VBETS, and LBETS for modifying the perovskites is located at 0.5mg/ml in an isopropanol solution (Fig. S9). The cross-sectional SEM image of the VBETS-modified 1.53 eV- Cs_0.05MA_0.05FA_0.9PbI₃-based PSC device is shown in Fig. 4a, revealing the Good crystallinity and clear functional layer interface. Fig. 4b and S10 show statistical data of all photovoltaic parameters (PCE, V_OC, J_SC, and FF)of the devices without and with different modifiers based on Cs_0.05MA_0.05FA_0.9PbI₃. BGTS, VBETS, and LBETS-modified PSCs exhibited higher average PCE values of 23.61%, 24.93%, and 24.37%, respectively, higher than 23.01% for the control devices. Especially, the V_OC of VBETS-modified devices was much increased, leading to the highest PCE. Fig. 4c presents the J–V curves of the best-performing control BGTS, VBETS, and LBETS-modified PSC devices in reverse scan (RS) and forward scan (FS) testing modes. It was revealed that the control, BGTS, VBETS, and LBETS-modified PSCs delivered a champion PCE of 23.44%, 23.82%, 25.26% and 24.77%, respectively. We also achieved an attractive certified PCE of 25.15% (Jsc of 25.68 mA cm⁻², a V_OC of 1.191 V, and an FF of 82.28%) in reverse scan and a certified value of 24.95% (Jsc of 25.75 mA cm⁻², a V_OC of 1.184 V, and an FF of 81.84%) in forward scan (Fig. S11–S17). To the best of our knowledge, our obtained champion PCE with the certified value of 25.15% is the record PCE reported for the inverted PSCs based on the vacuum flash method in ambition condition for perovskite deposition (Table S2). In addition, the hysteresis index (HI) calculated according to the formula of HI = (PCE_Reverse-PCE_Forwad)/PCE_Reverse was 4.39%, 2.73%, 1.15%, and 2.26% for the control, BGTS, VBETS, and LBETS-modified PSCs, respectively (Table S1). The hysteresis was mitigated for all modified devices, and the smallest hysteresis was found for the VBETS-modified PSCs, which resulted from reduced defect density and facilitated interfacial electron extraction and thus suppressed interfacial charge accumulation. The integrated current density from the EQE spectrum of the control, BGTS, VBETS, and LBETS-modified PSCs was 25.24 mA cm⁻², 25.49 mA cm⁻², 25.74 mA cm⁻², and 25.60 mA cm⁻², respectively, which matched with the values from J–V measurements (Fig. S18).

Our developed passivation approach is also effective for the perovskite-based on 1.58 eV-Cs_0.05(FA_0.95MA_0.05)_0.95Pb(I_0.95Br_0.05)₃, with improved V_OC and FF (Fig. S19-S20), demonstrating the universality of our strategy. We then fabricated a large-area module by vacuum ﬂash method, where 14 subcells were connected in series. The champion VBETS-modified PSC module with an aperture area (including dead area) of 32.144 cm² achieved a PCE of 21.00% (Fig. 4d). Calculating by the active area of 30.408 cm² (which accounts for 94.6% of the aperture area), we obtained an efficiency of 22.20%. According to the statistics, these results are all the highest eﬃciency reported for the large-area modules with an area over 30 cm² (Fig. 4e and Table S3).

To investigate the charge carrier recombination lifetime (τ_r) and carrier transport lifetime (τ_t) of PSC devices, transient photovoltage (TPV) and transient photocurrent (TPC) measurements were performed on the control and VBETS-modified devices.^{23, 51} Fig. S21 shows that the VBETS-modified device exhibited a τ_rvalue of 2.59 µs, which is more than twice as long as that of the control device (1.24 µs). In addition, it can be seen that VBETS-modified devices exhibited a τ_t value of 0.50 µs (Fig. S22), which is dramatically shorter than that of 1.66 µs for the control devices. As shown in Fig. S23, the -3d B bandwidth (f_-3dB) of VBETS-modified devices was 0.40 MHz, which is much larger than the 0.10 MHz of the control devices, indicating that VBETS-modified devices have faster charge carrier transport and light response.⁵² After VBETS modification, the reduced defect density and improved energy band alignment should account for promoted carrier transport and extraction. Electrochemical impedance spectroscopy (EIS) measurements (Fig. S24) further revealed the ameliorated charge transport and inhibited nonradiative recombination following VBETS passivation. V_OC as a function of light intensity measurements (Fig. S25) revealed that the ideal factor (0.92) of the VBETS-modified device is closer to 1 than that of the control device (1.22), indicating the reduced Shockley-Read Hall recombination associated with the trap defects.⁵³

To fabricate high-performance TSCs, we further extended the VBETS passivation strategy to WBG PSCs based on 1.66 eV-Cs_0.05MA_0.15FA_0.8Pb(I_0.76Br_0.24)₃. (Fig. 4f), achieving a PCE enhancement from 20.79% to 21.75%, accompanied by increased V_OC and FF together with slightly improved J_SC. This indicates that our surface passivation method is suitable for both conventional bandgap and WBG 1.66 eV-PSCs. The integrated J_SC (Fig. S26) in WBG PSCs are in good accordance with the values obtained from J–V curves. On this basis, we fabricated perovskite/silicon TSCs, where the bottom cell is 1.10 eV crystalline silicon heterojunction (HJT) solar cells and the top cell is WBG PSCs with VBETS passivation. The SEM cross-sectional view and overall structural schematic diagram of the TSCs are shown in Fig. 4g-h. Precisely, the magnetron sputtering method was applied on a heterojunction Si substrate to prepare transparent ITO as the top transparent conductive electrode. As is exhibited in Fig. 4i, The champion efficiency of the VBETS-passivated tandem device was achieved at 30.98% (with a V_OC of 1.890 V, a J_SC of 20.23 mA cm⁻², and an FF of 80.76%) in the reverse scan, and 30.56% (with a V_OC of 1.895 V, a J_SC of 20.23 mA cm⁻² and an FF of 79.73%) in the forward scan. These efficiency values are much higher than that of 28.69 % (with a V_OC of 1.816 V, a J_SC of 20.44 mA cm⁻², and an FF of 77.29%) in reverse scan and 28.01% (with a V_OC of 1.797 V, a J_SC of 20.44 mA cm⁻², and an FF of 76.25%) in forward scan for the control tandem solar cells. The effective enhancement of TSC devices comes from the improved Voc, demonstrating the effective role of VBETS interface defect passivation in suppressing non-radiative recombination. It is worth noting that, to the best of our knowledge, 30.98% is among the highest PCE for the perovskite/HJT tandem solar cells with NiOx as a tunnelling recombination junction between sub-cells so far. The EQE spectra in Fig. 4j revealed that the integrated J_SCs of the VBETS-modified WBG PSCs and filtered SHJ devices were 20.19 mA cm⁻² and 20.40 mA cm⁻², respectively, which are consistent with the J_SC values from J–V curves.

Study of long-term stability

We assessed the operational stability of single-junction PSCs and perovskite/Si TSCs without and with VBETS passivation. As for the 1.58 eV-Cs_0.05(FA_0.95MA_0.05)_0.95Pb(I_0.95Br_0.05)₃-based single-junction PSCs, encapsulation using glass front with UV-curable adhesive encapsulant for sealing was applied. The effect of VBETS modification on the long-term stability of PSCs was investigated systematically. After 1600 h of aging in ambient conditions (Relative humidity=35±5%, Tmeperature=25±5^oC), the control PSCs degraded much more rapidly than the devices modified by VBETS (Fig. S27). We conducted the maximum power point tracking (MPPT) of the control and VBETS-modified devices under continuous 100 mW cm⁻² white LED light irradiation at room temperature of 25±5^oC and stored in a 99.99% nitrogen environment. As is shown in Fig. 4k, after 4000 hours of continuous light exposure, the VBETS-modified device could retain 90.8% of the initial PCE, while the control device dropped to 70.9% under the same conditions. The improved light stability of VBETS-modified devices is mainly associated with reducing trap defects at surface and grain boundaries. The TSCs were then subjected to stability measurements at room temperature and under white light illumination with internal cyclic J–V tests with a scanning interval of 124.8 mins. As illustrated in Fig. 4l, the VBETS-modified TSCs could maintain 94.2% of the initial PCE value after 1030 hours of aging. The overall output characteristics and stability of the device are higher than those of the control TSC device. It was revealed that VBETS modification can enhance the operational stability of single junction and TSCs due to reduced surface defect density by synergistic passivation of nonhalide anion and organic cations.

In summary, we have developed a universal passivation strategy for organic ammonium salt molecules containing nonhalide organic anions, which minimized the energy loss at the upper interface in inverted PSCs through the rational design of the number of hydrogen atoms on cations and steric hindrance. The synergistic effect of anions and cations enabled simultaneous passivation of positively charged and negatively charged defects and the modulation of interface bands. VBETS possessing an appropriate number of hydrogen atoms and cation size exhibited the best effect in defect passivation and energy band modulation. The universality of this passivation strategy was confirmed by using different bandgap perovskites. Finally, the VBETS-modified inverted PSCs based on conventional bandgap perovskite yielded a PCE of 25.15%, which is the record PCE reported for the inverted PSCs using vacuum flach technology up to now. The perovskite/Si TSCs coupled with VBETS passivation demonstrated a promising PCE of 30.98%, which is among the highest PCE ever reported for the perovskite/Si TSCs. This work highlights the critical role of number of hydrogen atoms and steric hindrance upon designing multisite nonhalide ammonium salts to improve the PCE and stability, which lays the groundwork for the development of perovskite photovoltaics.

Materials

Formamidine iodide (FAI), methylammonium iodide (MAI), methylamine hydrochloride (MACl), methylammonium bromide (MABr) were purchased from Greatcell Solar Materials. Lead chloride (PbCl₂ 99.5%) were purchased from Xi'an Bright Optoelectronics Technology Co., Ltd. Lead iodide (PbI₂, 99.999%), cesium iodide (CsI, 99.99%) and lead bromide (PbBr₂ 99.999%) were purchased from Chengdu Alpha Metal Materials Co., Ltd.. N,N-dimethylformamide (DMF, 99.9%), Dimethyl sulfoxide (DMSO) and 1-methy-2-pyrrolidinone (NMP, 99.9%) were purchased from MACKLIN. [2-(9H-Carbazol-9-yl)ethyl]phosphonic Acid(2PACz, >98.0% HPLC) and [4-(3,6-Dimethyl-9H-carbazol-9-yl)butyl]phosphonic Acid(Me-4PACz, >99.0% HPLC) was purchased from TCI; BCP, PC₆₁BM, C₆₀ were purchased from Advanced Election Technology. The three amino acid salts, benzyl glycinate p-toluenesulfonate (BGTS), L-valine benzyl ester 4-toluenesulfonate (VBETS), and L-leucine benzyl ester p-toluenesulfonate salt (LBETS) was obtained from MACKLIN.

Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O) was purchased from Sigma-Aldrich for synthesizing the NiOx nanoparticles. All chemicals were used without further purification. NiOx nanoparticles were synthesized as follows: Firstly, 3 g of Ni(NO₃)₂·6H₂O was dissolved in 50 mL of deionized (DI) water. The solution was stirred at room temperature for 30 minutes. Subsequently, a 1.0 M NaOH solution was slowly added with continuous magnetic stirring until the pH reached approximately 10. After an additional hour of stirring, a green precipitate was obtained via centrifugation at 9000 rpm for 6 minutes. The precipitate was washed three times with DI water. The collected green solid was frozen for 3 hours, then freeze-dried at 5 Pa for 15 hours, and finally calcined at 270 °C for 2 hours to produce NiOx nanoparticles. Prior to use, the NiOx was dispersed in DI water by sonication for 10 minutes at a concentration of 30 mg/mL.

Preparation of perovskite precursor solutions and film deposition

1.58 eV-Cs_0.05(FA_0.95MA_0.05)_0.95Pb(I_0.95Br_0.05)₃: 718.335 mg PbI₂, 28.2 mg PbBr₂, 19.5 mg CsI, 8 mg MABr, 238.2 mg FAI, 15.2 mg MACl, and were dissolved in 1 ml DMF and DMSO mixed solvent with a volume ratio of 4:1 to form 1.4 M stoichiometric solution. For the 1.58 eV perovskite film crystallization, the perovskite precursor solution of 100 μl was rotated for 10 s and 40 s at 2000 rpm and 5000 rpm, respectively, and 160 μl anti-solvent CB was rapidly dropped into the solution at 45 s. The precursor perovskite film was transferred to the hot plate and annealed at 100 ℃ in the glove box for 30 min.

1.53 eV-Cs_0.05MA_0.05FA_0.9PbI₃: The stoichiometric Cs_0.05MA_0.05FA_0.9PbI₃ precursor solution was prepared by dissolving 20.784 mg of CsI, 247.68 mg of FAI, 12.72 mg of MAI, 15 mg of MACl, 11.124 mg of PbCl₂, and 785.55 mg of PbI₂ in a mixed solvent of DMF/DMSO (8:2, by volume). For the 1.53 eV- Cs_0.05MA_0.05FA_0.9PbI₃ film crystallization, the perovskite films were deposited by spin-coating the perovskite precursor solution on glass/ITO/NiOx/Me-4PACz substrate at 1000 rpm for 10 s and 5000 rpm for 30 s. The wet perovskite film was quickly put into a sample chamber connected to vacuum-pumping instrumentation (Internal space is 12 cm × 12 cm × 2.1 cm). By opening the valve connecting the specimen chamber to the pump system, the perovskite film was immediately exposed to low pressure maintained at 10 Pa for 30 s, followed by full pressurization by admitting ambient air into the specimen chamber. Subsequently, the perovskite film was annealed at 100 °C for 15 min in the air (RH=30±10%, Temperature=25±5 ^oC) for full crystallization.

1.66 eV-WBG Cs_0.05MA_0.15FA_0.8Pb(I_0.76Br_0.24)₃: 18.2 mg of CsI, 192.6 mg of FAI, 536 mg of PbI₂(10% of excess), 131 mg of PbBr₂, 23.5 mg MABr were dissolved in 1 ml DMF and DMSO mixed solvent with a volume ratio of 4:1 to form 1.4 M stoichiometric solution. For the 1.66 eV-Cs_0.05MA_0.15FA_0.8Pb(I_0.76Br_0.24)₃ film crystallization, the perovskite solution of 100 μl was rotated for 5 s and 30 s at 1000 rpm and 5000 rpm, respectively, and 200 μl anti-solvent CB was rapidly dropped into the solution at 20 s. The precursor perovskite film was transferred to the hot plate and annealed at 100℃ in the glove box for 15 min.

Device fabrication

Fabrication of small active area PSCs: First, ITO substrate was etched by laser etching technology. Ultrasonic cleaning of ITO glass after etching was carried out with glass cleaning agent, deionized water, and ethanol in sequence, and each step took 15 min. The wettability of the ITO substrate was enhanced by UV-ozone modification for 15 min. Then, the water-based ink of NiO_x nanoparticles (30 mg NiO_x nanoparticles dispersed into 1 ml deionized water) was coated on ITO glass at 4000 rpm for 30 s and annealed at 150 ℃ for 10 min at room temperature. After annealing, the ITO/NiO_x substrate was quickly transferred into an N₂-filled glove box for subsequent preparation. 2PACZ (1 mg/ml dissolved in IPA, ultrasonic bath for 10 min) or Me-4PACz solution (0.5 mg/ml dissolved in IPA, ultrasonic bath for 10 min) was coated at 4000 rpm for 20 s on ITO/NiOx substrate and then annealed at 150 ℃ for 10 min. The preparation and crystallization processes of various types of perovskites are described in the previous experimental section. For BGTS, VBETS, and LBETS-modified perovskite film, spin coating 0.5 mg/ml IPA solution of BGTS, VBETS, and LBETS on the crystallized perovskite film at 5000 rpm for 30 s, and annealing at 100 ℃ for 5 min. Then, for 1.58 eV-Cs_0.05(FA_0.95MA_0.05)_0.95Pb(I_0.95Br_0.05)₃ and WBG 1.66 eV- Cs_0.05MA_0.15FA_0.8Pb(I_0.76Br_0.24)₃-perovskite-based solar cells, spin-coating PC₆₁BM-solution (23 mg PC₆₁BM dissolved in 1 ml CB) for 30 s at a speed of 1500 rpm. For the 1.53 eV-Cs_0.05MA_0.05FA_0.9PbI₃-based solar cells and modules, about 30 nm C₆₀ was thermally evaporated on the perovskite films under a high vacuum 8×10^-4 Pa; Next, spin-coating of BCP solution (5 mg BCP dissolved in l ml IPA; filtered with a 0.22 μm PTFE filter) on PC₆₁BM layer or C₆₀ layer at a speed of 5000 rpm for 30 s. Finally, the 80 nm Cu, Ag, or Au electrode is thermally evaporated at the top. For sealing the devices for MPP stability testing, a UV-curable adhesive (Eversolar AB-341, Everlight Chemical) was applied over the active area of the perovskite solar cells, followed by the placement of a highly transparent glass panel. The device was then compressed and cured under ultraviolet light for 3 minutes.

Fabrication of large-area PSC modules: The preparation process of 1.53 eV-Cs_0.05MA_0.05FA_0.9PbI₃ perovskite-based large-area modules is similar to that of small-area devices. Specifically, it is needed to evenly disperse 1 mL of Cs_0.05MA_0.05FA_0.9PbI₃ precursor solution on the surface of the large area 10 × 10 cm² ITO substrate, after which is needed to rest for 10 s to achieve automatic dispersion of the perovskite precursor solution on the substrate for the following spin-coating process at 2500 rpm for 20 s. The assembly of subcells in the modules involved a three-stage laser etching technique (P1, P2, P3) on a 6*10 cm² ITO substrate, creating 14 subcells. Initially, P1 targets the ITO substrates for etching, achieving a width of 30 ± 3 µm using a 1064 nm, ns laser. This step involves single optical path etching, ensuring that the insulation resistance between adjacent conductive layers post-etch exceeds 10 MΩ. Importantly, the P1 process leaves no residual TCO film within the etched tracks and does not harm the underlying glass. The second etching step, P2, processes layers of ITO/NiOx/Me-4PACz/Perovskite/C₆₀/BCP, etching to a width of 50 ± 5 µm using a 532 nm, ps laser. The P2 laser meticulously avoids penetrating the bottom TCO layer, maintaining a precise 30 µm gap between P1 and P2 etchings. P3, the final stage, is employed for etching Au electrodes, matching the width specifications of P1 (30 ± 3 µm) using a 532 nm, ps laser. Similarly to P2, P3 does not penetrate the bottom TCO layer, preserving a 30 µm interval between the P2 and P3 etchings.

Fabrication of tandem solar cells: The TSC device is built on a HJT silicon substrate. The wafer surface was finely polished for optimal quality. In the fabrication of a tandem device, HJT silicon substrates were initially sectioned into 12 mm × 12 mm squares using a laser. Subsequently, a perovskite sub-cell was constructed atop the HJT Si sub-cell, employing a fabrication technique similar to that used in the creation of perovskite single-junction devices. The thicknesses of the C₆₀/BCP as electron transporting layers and the perovskite absorbers were meticulously adjusted through the spin-coating duration. The standard active area for these tandem devices was maintained at 1.0 cm².

Device characterization

The J–V characteristics were obtained using a Keithley 2400 source meter in a controlled environment simulating AM 1.5 solar irradiance (100 mW cm⁻²), facilitated by a standard xenon lamp solar simulator (7ISO503A, SOFN INSTRUMENTS). These measurements were conducted either via a forward scan, ranging from -0.1 to 1.4 V, or a reverse scan, from 1.4 V down to -0.1 V. The J–V measurements for the solar cells were conducted in ambient air conditions. The external quantum efficiency (EQE) was determined using an EQE measurement system from EnliTech, Taiwan. All assessments were performed at room temperature in an ambient atmosphere, and the solar cells were not encapsulated during these measurements. LBIC mapping measurements were tested by LSD4 of ENLITECH.

Certification of PSCs

The PSC devices were sent to the Tianjin Institute of Metrological Supervision and Testing, China, for official certification. Specifically, we fill the prepared solar cell samples for certification testing with nitrogen, then store them with a vacuum sealer and always place them in a dark sealed box. During the certification test, the certification agency recalibrates the effective active area of our solar cells to ensure accurate evaluation of data such as photocurrent density. The test environment and test methods are recorded in the certification data in Supplementary Figure S11-17 .

Characterization of device stability

The operational MPPT stability of the encapsulated devices was assessed under continuous one-sun conditions using a white LED lamp without a UV filter in a 99.99% nitrogen environment. These tests were conducted using an MPP tracking system (YH-VMPP-16, Yanghua, Suzhou). Specifically, 16 solar cell samples were simultaneously placed in aging equipment. The environment around solar cells is maintained at around 25 ℃ through a refrigeration system. Aging equipment is tracked and tested by continuously scanning and analyzing the maximum power point of the device. The interval between each test is 8.01 hours. White LED light sources have a light intensity energy of approximately 100 mW cm⁻². The cyclic J-V stability of tandem devices is obtained through interval testing of their J-V curves. The scanning interval is 124.8 minutes. The recording was done using the Keithley 2400 digital source meter, and the data was obtained using the Zeal Young software K2400 Experimental Platform-Professional2450V3.5.

Characterization of morphology and crystal structure of perovskite films

SEM measurement was carried out by JEOL JSM7610F SEM. The scanning voltage is 3 kV. X-ray diffraction (XRD) patterns were measured on Rigaku SmartLab X-ray diffractometer using Cu Ka target radiation (λ = 1.5405 A) (measurement power of 2 kW, scan rate of 8° min^-1). Field-emission SEM (JEOL 7610F) was used to scan the surface morphology of the film and the cross-sectional view of the device at 3 kV.

Photoluminescence and ultraviolet-visible absorption measurements

The SSPL and TRPL were measured by a fluorescence spectrometer (FLS1000, Edinburgh Instruments). The ultraviolet-visible absorption spectra of the films were measured by Shimazu UV-1900 spectrophotometer.

Atomic force microscope measurements

The Asylum Research MFP-3D was used to measure AFM morphology and KPFM in tapping mode.

X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy measurements

XPS and UPS spectra were collected through Thermo ESCALAB XI+. Fourier transform infrared (FTIR) spectroscopy was recorded using the FTIR-805 spectrometer of Tianjin Gangdong SCI.&TECH. CO,.LTD.

Mott–Schottky and electrochemical impedance spectroscopy measurements

The Mott-Schottky measurements were performed using the AMETEK VersSTAT 3F electrochemical workstation at a fixed frequency of 1k Hz. The applied bias voltage range is 0 V~1.5 V. EIS measurements are made in the dark, with frequencies ranging from 0.1 Hz to 1,000,000 Hz, using the same instruments as the Mott-Schottky measuring instruments.

Transient photocurrent and photovoltage measurements

TPC/TPV was tested by PD-RS of ENLITECH, Taiwan. The long-term stability was conducted in ambient conditions at a relative humidity (RH) of 35±5% and a temperature of 25±5°C.

Density functional theory calculation

The DFT calculations are performed using a Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. Projector augmented-wave (PAW) pseudopotentials are employed with a plane-wave cutoff energy of 400 eV. The DFT-D3 method is used for describing van der Waals interactions. The FAPbI₃ (001) surface is modelled by a slab consisting of seven atomic layers and a vacuum gap of 40 Å. The bottom two atomic layers in the slab are fixed during structural relaxation. The convergence criteria for the atomic force is set to 0.02 eV/Å. The binding energy of the ligand with the FAPbI₃ surface is defined as E_b=E(slab@ligand)–E(slab)–E(ligand). The electrostatic potentials (φ) of the passivators are calculated using the Gaussian 09 package at the B3LYP/def2TZVP level with DFT-D3.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (62274018, U21A2076), the S&T Program of Hebei (215676146H, 225676163GH), The Science and Technology Development Fund, Macau SAR (No. 0009/2022/AGJ),

National Natural Science Foundation of China (Grant Nos. 22279149), Youth Innovation Promotion Association of the Chinese Academy of Sciences (No. 2022034), and the Xinjiang Construction Corps Key Areas of Science and Technology Research Project (2023AB029).

Competing interests

The authors declare no competing interests.

Data Availability Statement

The data that support the ﬁndings of this study are available from the corresponding author upon reasonable request.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Data availability

All data generated or analysed during this study are included in the published article and its Supplementary Information and Source Data files. The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Source data are provided with this paper.

Author contributions

Y.F., J.C., J.T., and C.C. conceived the ideas of the work and carried out the basic characterization, including J-V and EQE photovoltaic measurement. Y.F., J.D., Q.M. and Z.Z. prepared the small active area perovskite samples and PSC devices, and performed most measurements, including KPFM, AFM, SCLC, LBIC and MPP stability, etc.. W.G. assisted with FTIR and XPS measurements. W.G. and Y.F. performed SEM measurements. B. Z helped to conduct the TRPL and PL measurements and analysis. Y.F., Y.W. Q.M. and C.C. conducted the fabrication of tandem solar cells. Y.F. and J.D. certified the efficiency of the PSCs. Z.Z., H. C. and C.C. conducted the long-term cyclic J-V stability measurements. Y.F., and Y.W. completed the preparation and testing of large-area PSC module devices. Y.F., C.C. and J. Chen wrote the first draft of the manuscript. H. Z. and T. P. were involved in the data analysis and wrote the final version of the manuscript. J.T., J.C. and C.C. supervised this project. All authors analyzed the data and contributed to the discussions. Y.F., C.C. and J. Z. conducted theoretical calculation analysis.

Additional information

Supplementary information is available for this paper. Reprints and permissions information is available at www.nature.com/reprints. Correspondence and requests for materials should be addressed to C.C.

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Rationally designed universal passivator for high-performance single-junction and tandem perovskite solar cells

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