Efficient rigid and flexible perovskite solar cells with 86% fill factor using multifunctional electron-enriched molecular passivation

doi:10.21203/rs.3.rs-5096801/v1

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Efficient rigid and flexible perovskite solar cells with 86% fill factor using multifunctional electron-enriched molecular passivation

https://doi.org/10.21203/rs.3.rs-5096801/v1

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Multiple active sites of additives with strong adsorption energies adsorb on perovskite facets, coordinate with lead ions and reduce halogen vacancy defects, leading to a highly ordered atomic arrangement of grain boundaries for efficient perovskite solar cells (PSCs). Herein, we demonstrate efficient rigid and flexible PSCs via 1,3,5-triazine-2,4-diamine hydrochloride (TDH) molecular passivation. The TDH treatments restrain a formation of small twin crystals and remarkably improve the crystallinity through reducing the aligned point vacancy defects on the perovkite lattices, and the dislocations and distortions across the boundaries in the perovkite crystals. The rigid PSCs yield high PCE of 26.15% with fill factor (FF) as high as 85.94% (maximum: 86.03%). The flexible PSCs yield high PCE of 24.68% with FF as high as 86.07%. Both efficient rigid and flexible PSCs without package exhibit superior operation stability. The work provides an effective passivation strategy and valuable insights for crystalline perovskite materials and high-performance rigid and flexible PSCs.

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

Physical sciences/Energy science and technology/Renewable energy/Solar energy

perovskite solar cell

flexible solar cell

molecular passivation

fill factor

stability

Halide perovskite solar cells (PSCs) have attracted much attention in recent years due to their high power conversion efficiencies (PCEs) both in single-junction and tandem photovoltaic devices.^1–¹⁶ To date, the best rigid single-junction PSCs have yielded the highest PCE of 26.69% through interface passivation engineering.¹⁷ As compared to the normal PSCs with a Spiro-OMeTAD layer on perovskite surfaces, the inverted PSCs have the striking advantages of relatively high stability and being compatible with various tandem photovoltaic fabrications, such as, all-perovskite, perovskite-silicon tandem photovoltaic cells, etc.^2,16However, because of the inherent limited stability of the perovskite with halogen vacancy defects, uncoordinated lead ions (Pb²⁺), energy level alignment, interface contact issues, etc., the efficiency of the inverted PSCs is sacrificed along with a low fill factor (FF) related to the non-radiative recombination. Molecular passivation strategies can reduce the non-radiative recombination and substantially improve the photovoltaic parameters including open circuit voltage (V_OC), short-circuit current density (J_SC) and FF,¹⁷^–²⁰ enabling the inverted PSCs close to commercialization.

Recent effects have been made to improve the photovoltaic parameters of the inverted PSCs. For example, Sargent et al. proposed a strategy of Quasi-2D treatment that increased the layer width of reduced-dimensional perovskites (RDPs) in 2D/3D heterostructures, leading to high V_OCraised from 1.06 V to 1.16 V, high FF increased from 79.9% to 82.5% but lower J_SC.²¹ Jen et al. reported 4-guanidinobenzoic acid hydrochloride (GBAc) treatment that modulated the growth kinetics and induced coherent grain growth through a hydrogen-bond-bridged intermediate phase.²² The control PSCs exhibited V_OC of 1.12 V and FF of 83.01%, whereas the GBAc PSCs yielded V_OC of 1.19 V andFF of 84.78% with almost unchanged J_SC. Huang et al. added the organic molecule entinostat into hole transport materials (HTM) and perovskite to enhance adhesion at the perovskite–substrate interface and restrain a formation of voids.²³ The flexible PSCs delivered PCE of 23.4% with almost unchanged FF of 81.8%, which arose from the J_SC increased by 0.95 mA cm^–2. Chen et al. reported a molecular hybrid at the buried interface that co-assembled the HTM molecules to improve the heterojunction interface.¹⁷ The best rigid PSCs yielded record-high PCE of 26.69% with V_OC of 1.20 V, J_SC of 26.3 mA cm^–2 and FF of 84.57%, whereas the control devices exhibited FF of 79.53% and V_OC of 1.14 V. Hence, the photovoltaic performance of the inverted PSCs can be enhanced by material design and molecular passivation strategies, such as increasing the layer width of RDPs, modulating the kinetics of perovskite growth, enhancing the adhesion at perovskite–substrate interfaces, improving the wettability and agglomeration of self-assembled HTMs, adjusting the energy levels of perovskite, etc.^21–25 Despite the high PCEs achieved in these rigid and flexible inverted PSCs, so far, there are only very few cases that report the efficient rigid PSCs with high FF of 86%, and the highest FF of the flexible PSCs is no higher than 83.57%. In order to promote a true adaptation of efficient inverted PSCs, it is necessary to further raise the FF and other photovoltaic parameters of both the rigid and flexible PSCs.

A highly crystalline perovskite film with strong coordination with Pb²⁺ ions and strong ionic strength of Pb−halogen is critical in determining the achievable FF and PCE of the inverted PSCs. Such a crystalline perovskite is achieved using the molecular passivation of triazine hydrochlorides, because the triazine hydrochlorides with rich lone-pair electrons can induce a strong coordination with the uncoordinated Pb²⁺, a strong bond energy of Pb−halogen, and a favorable hydrogen bonding. The triazine hydrochloride molecules have the rich lone-pair electrons on the nitrogen (N) atoms that divide into three N categories, and especially, the N1 atoms show the highest electrostatic potential (ESP). The resultant adsorption energy (E_ad) induces a strong interaction between the uncoordinated Pb²⁺and the N atoms, thereby leading to fewer uncoordinated metal defects. The introduction of chlorine (Cl) inhibits a formation of iodine (I)-anion vacancy defects at grain boundaries and on the perovskite facets. In contrast to the weak Pb−I bonds, the introduction of Cl provides a better passivation effect due to the bond energy of the Pb−Cl bonds, which forms a strong ionic bonding between Cl⁻ and Pb²⁺. Therefore, the multifunctional electron-enriched molecules substantially improve the crystallinity of the perovskite through reducing the point vacancy defects on the perovkite lattices and the dislocations and distortions across the boundaries in the perovkite crystals.

In this article, both efficient rigid and flexible inverted PSCs are demonstrated through electron-enriched molecular passivation using 1,3,5-triazine-2,4-diamine hydrochloride (TDH). The TDH treatments induce fewer uncoordinated metal and halogen vacancy defects, large grain boundaries, high crystallinity with highly ordered atomic arrangement of grain boundaries, and lattice relaxation. The rigid inverted PSCs yield high PCE of 26.15% along with V_OC of 1.171 V, J_SC of 25.99 mA cm⁻² and FF of 85.94%. The flexible inverted PSCs yield high PCE of 24.68% along with V_OC of 1.159 V, J_SC of 24.74 mA cm⁻² and FF of 86.07%. The rigid and flexible PSCs with the TDH treatments exhibit high operation stability, and retain 92.5% and 95.2% of the initial efficiency in the continuous maximum power output tracking for 1,000 h and 600 h, respectively.

In order to understand the relationship between the electron-enriched TDH additive composed of TD molecules and hydrochloride (HCl) and the perovskite surface from the atomic scales, and the microscopic mechanism behind surface orientation regulation, we employ the DFT to analyze the charge distribution of TD and reasonably explain the physical phenomena observed on the (100) and (110) surfaces. Fig. 1a shows the simulated ESP to identify the charge distributions. This result reveals that the N atoms with lone pairs of electrons for TD can be divided into three categories: N1, N2 and N3, due to differences in their surrounding environments. A higher negative potential reflects a higher degree of electron enrichment, suggesting that TDH molecules are able to provide multiple active sites to coordinate with the Pb²⁺ ions at the perovskite surface through the Lewis acid-base reaction. To quantitatively reveal the rivet strength, we select the (100) facet as a single anchoring-model for TD (Fig. 1b), as detailed in Supplementary Fig. 1, because it has lower surface energy compared to others reported in previous literature.^26–28 In Fig. 1c, it indicates that N1 has the strongest adsorption energy, followed by N2 and N3, as further confirmed by bond length of Pb−N (Supplementary Fig. 1). This demonstrates that when THD is introduced, N1 groups readily bind the unsaturated Pb²⁺ on the surface. Here, we consider the adsorption of N1 on the (110) surface (Fig. 1b). The calculated E_ad shows that TDH prefers to adsorb on the (110) facet because of the stronger adsorption energy of −3.21 eV. The strong selective adsorption of TDH can reduce the surface energy of the (110) facet, and lead to more favorable hydrogen bonding with FA molecules, thereby slowing the growth of the (110) facet and effectively improving the film crystallinity.

In addition, the role of hydrochloride and its influence on the surface stability are investigated next. We construct the Pb-X bond (X = I and Cl) to passivate the unsaturated Pb²⁺ on the (110) facet, as shown in Fig. 1d. By calculating the bond energies and Bader charge transfer of Pb−X (Supplementary Table 1), both bond energy and charge transfer increase from Pb−I to Pb−Cl. This characteristic indicates that the introduction of Cl ions is more favorable for enhancing the ionic strength of Pb−X compared with Pb-I, thereby effectively passivating the iodine vacancy defects on the surface and stabilizing the perovskite surface.

The lone-pair electrons of the TDH molecules strongly bind the uncoordinated Pb²⁺. In the ¹H nuclear magnetic resonance (NMR) spectra (Supplementary Fig. 2), for the pure FAI, ¹H peaks doublets at 8.95 ppm and 7.86 ppm symbolize protonated ammonium and hydrogen-bonded carbon atoms, respectively. After the TDH incorporation, the peaks show a lower chemical shift (Fig. 2a) and the peak at 7.86 ppm split into more distinct peaks, demonstrating a good interaction between TDH and ammonium/hydrogen bonded carbon atoms. As also shown in Fig. 2b, following the introduction of TDH, the peaks around ~7.9 ppm split into two distinct peaks, signifying hydrogen bonding formation²⁹ between TDH and N─H groups of FA⁺. In Fourier transform infrared spectra (FTIR, Supplementary Fig. 3), the interaction between TDH and FAI induces the shifts of the C─N, C═N, and N─H stretching peaks to the lower wavenumbers (Fig. 2c,d), respectively, indicating hydrogen bonding engagement between TDH and organic components.

X-ray photoelectron spectroscopy (XPS) is used to further investigate the interaction between TDH and the perovskite. As shown in Fig. 2e, the Pb 4f peaks (144.42 eV for Pb 4f_5/2 and 138.59 eV for Pb 4f_7/2) for the films largely shift by 0.31 eV and 0.33 eV toward low binding energy after the TDH treatments, suggesting the coordination between TDH and Pb²⁺.The shifts are consistent with the shifts of low binding energy in those high-quality perovskites.³⁰^–³³For the perovskite thin layers with the TDH treatments, the I 3d_3/2 and I 3d_5/2 peaks shift 0.37 eV and 0.40 eV toward low binding energy to 630.37 eV and 618.88 eV, respectively (Fig. 2f), suggesting fewer iodide vacancy defects. These strong interactions between TDH and the perovskites will be favorable for the enhancement in PCE and FF of the inverted PSCs.

The topographic morphology of the pristine and TDH-treated perovskite thin films on glass substrate are shown in Figs. 3a and b, respectively, via scanning electron microscope (SEM). Obviously, plenty of small twin crystals appear among the boundaries in the control perovskite films. With the introduction of TDH, large grain boundaries appear in the perovskite films, which is attributed to the slow crystal growth rate of the perovskite induced by the TDH treatments. The TDH treatments restrain the target formation of small twin crystals among the grain boundaries. In addition, the morphology of the perovskite thin layers on PET thermoplastic substrates (Supplementary Fig. 4) is similar to that of the perovskite thin films on glass substrates.

Furthermore, the TDH treatments diminish the surface roughness of the perovskite films (Figs. 3c-e). The TDH treatments lead to a smooth and physically dense perovskite film with large grain boundaries. The average root-mean-square (RMS) roughness of the pristine and TDH treated perovskite films depends on the different sizes (50×50, 10×10, and 2×2 μm²) of the phonographs. In Fig. 3e, the RMS roughness is 12.1 nm and 8.8 nm for the control and TDH-treated perovskite films, respectively. The smooth and dense samples with large grain boundaries are suitable for a following deposition of electron transport layers on the bottom perovskite, resulting in a physically intimate contact at interfaces. The superior morphology is favorable to enhance the fill factor and efficiency of the inverted PSCs.

Figs. 3f,g show the high-resolution transmission electron microscopy (HRTEM) images of the pristine and TDH-treated perovskite films. The lattice parameter of the perovskite film is determined to be 0.26 nm, 0.28 nm and 0.33 nm, which is indexed by (110), (210) and (211) planes, respectively. As shown in Fig. 3f, aligned point vacancy defects (red circles) on the lattices are present in the pristine perovkite crystals, which is related to the formation of the formation of uncoordinated metal ion and halogen vacancy defects. We also find that the dislocations and distortions across the boundaries are common in the pristine perovkite crystals. The dislocations are highlighted in the white circles in Fig. 3f-3. The dislocations are dissociated in a direction perpendicular to their glide plane. Whereas for the TDH perovskite films, there is little distortion across the boundaries, and straight sections of grain boundaries without the aligned point vacancy defects are observed. Besides, moiré fringes appear at grain boundaries among the crystal grains in the TDH perovskite films (Supplementary Fig. 5). It indicates that TDH causes changes in the crystal structure of the perovskite grain edges. Plenty of moiré fringes demonstrate lattice relaxation of the crystals and better stability of interfacial structures of the crystals.

High crytallinility reflects better growth quality of the perovskite films. TDH can coordinate with the perovskite to regulate the crystal growth and raise the perovskite crystallinity. The X-ray diffraction (XRD) results of the perovskite films are shown in Fig. 4a. The peak of PbI₂ at 12.7^odisappears after the TDH treatments and it suggests a consume of PbI₂.³⁰ The peak at 14.0^ois the (100) plane of the perovskites.^34,35When the perovskite passivation is conducted, the ratio of (110) plane to (100) plane is improved from 0.475 to 0.593, while the ratio of (220) plane to (100) plane is improved from 0.658 to 0.746. Besides, the diffraction angles of the planes increase (Supplementary Fig. 6). The results demonstrate an improved crystallinity of the perovskite films, which is energetically favorable to reduce the charge-carrier recombination for high photovoltaic performance.^36,37 The grazing incidence wide angle X-ray scattering (GIWAXS) patterns of the perovskite films are presented in Figs. 4b,c. The radially integrated intensities of (100), (110) and (210) peaks are relatively high for the TDH samples, further confirming the raised crystallinity of the TDH perovskite films.

The ultraviolet photo-electron spectra (UPS) are used to investigate the E_HOMO and E_LUMO of the perovskite films. Fig. 4d illustrate a difference (E_VBF) between valence-band maximum and Fermi energy levels. Fig. 4e presents the secondary electron spectra with cutoff edges (E_cutoff) .hv is 21.2 eV. E_HOMO of the pristine and TDH perovskite is calculated to be −6.84 eV and −6.45 eV. Fig. 4f shows the band gap (E_g) of the perovskite films (see absorption spectra in Supplementary Fig. S7). The enhanced absorption was achieved in the perovskite with the TDH treatments in the visible and near infrared regions; and the E_g is calculated to be 1.56 eV. Thus, the LUMO of the control and TDH perovskites is −5.28 eV and −4.89 eV, respectively. The matched energy levels among the hole transport layer (HTL, E_HOMO: −5.3 eV), perovskite and C₆₀(E_LUMO: −4.5 eV) are definitely favorable for the charge transport at interfaces.

The schematic diagram of the rigid inverted PSCs is shown in Fig. 5a. The MeO-2PACz thin films are used as HTLs. The TDH is incorporated in the perovskite precursor. As shown in the cross-section morphology (Fig. 5b), the thickness of the perovskite films and the indium tin oxide (ITO) transparent electrodes is 1.25 μm and 170 nm, respectively. The current density−voltage (J−V) characteristics of the control and TDH PSCs are presented in Fig. 5c. The control PSCs exhibit PCE of 24.93% with V_OC of 1.151 V, J_SC of 25.96 mA cm⁻², and FF of 83.42%. The PSCs with the TDH treatments yield a raised PCE of up to 26.15% with V_OC of 1.171 V, J_SC of 25.99 mA cm⁻², and FF of 85.94%. The TDH treatments lead to an effective defect passivation for the achievement of the maximum FF up to 86.03% and PCE of 26.03% (Supplementary Fig. 8). On the basis of the data of 50 devices, the distributions of PCE, FF, J_SC, and V_OCare plotted in Figs. 5d-g. Obviously, the average FF is raised by 2.5%, substantially contributing to the high PCE. The average V_OC slightlyincreases by 1.0%; on the contrary, the average J_SCdeceases by 0.2%. These results indicate that the TDH treatments significantly improve the FF and PCE of the rigid inverted PSCs. The rigid devices show the stabilized photo-currents and power outputs when they are held at a bias of 1.0 V for 300 s (Fig. 5h). The steady-state photo-current and power outputs of the devices were conducted via the maximum power point (MPP) tracking at 50 °C. The TDH devices maintain a stabilized PCE of 25.5%, which is higher than that (24.1%) of the control devices. Fig. 5i shows the external quantum efficiency (EQE) spectrum of the TDH PSCs. The devices demonstrate a high integrated J_SCof 25.10 mA cm⁻². The EQE is consistent with the J_SC values of the PSCs.

The TDH treatments are also desirable for making high-performance flexible PSCs fabricated on thermoplastic flexible substrates. Fig. 6a illustrates the schematic diagram of the flexible PSCs, that is, 125 μm-thick polyethylene terephthalate (PET) substrate/ITO electrode/MeO-2PACz/perovskite with TDH/C₆₀/BCP/Ag. The uniform and physically continuous nanoparticles appear in the flexible ITO thin films with RMS roughness of only 4.1 nm (Fig. 6b). The work function of the ITO thin films is about 4.56 eV based on the UPS measurement (Fig. 6c). The J−V characteristics of the flexible PSCs with and without the TDH treatments are shown in Fig. 6d. Obviously, the control flexible PSCs without the TDH treatments exhibit PCE of 23.46% with V_OC of 1.137 V, J_SC of 24.62 mA cm⁻² and FF of 83.83%. Upon the treatments, the flexible PSCs yield a raised PCE up to 24.68% along with V_OC of 1.159 V, J_SC of 24.74 mA cm⁻² and FF of 86.07%. The distributions of the FF and PCE of the flexible PSCs (no less than 15 devices) are shown in Fig. 6e,f for reference. Notably, the FF (86.07%) of the flexible PSCs is much higher than those (77.68%−83.57%) of the champion flexible PSCs reported before (Fig. 6g and Supplementary Table S2).

The steady-state power output of the flexible devices with stabilized PCEs of 24.2%−23.8% is illustrated in Fig. 6h via the MPP tracking. Fig. 6i shows the EQE spectrum of the flexible TDH PSCs. The flexible devices show a high integrated J_SCof 24.08 mA cm⁻². The integrated J_SC is in a good agreement with the J_SC values of the flexible PSCs.

In order to analyze the trap-state density (n_trap) in the PSCs, we fabricate the hole-only devices with a structure of glass/ITO/HTL/perovskite/MoO₃/Au for the space-charge limited current (SCLC) measurements. n_trap is calculated using the formula: n_trap= (2ε_rε₀V_TFL)/(eL²),³⁸where ε_r is relative permittivity (62.23)³⁹ of the perovskite, and L is the film thickness. As depicted in Fig. 7a, the trap-filled limit voltage (V_TFL) of the control and TDH devices is 0.461 V and 0.288 V, respectively. The n_trap deceases from 2.58×10¹⁵cm⁻³to 1.61×10¹⁵ cm⁻³ after the TDH treatments. These results demonstrate fewer defects of the perovskite and a suppressed trap-assisted recombinationin devices. The hole mobility (μ_h) of the hole-only devices is calculated employing Mott−Gurney square law: J_D= (9ε_rε_oμV²)/(8L³),⁴⁰ where J_D is the dark current density. The J_D^0.5−V characteristics of the hole-only devices are shown in Fig. 7b. Upon the treatments, the μ_h is raised from 1.51×10⁻³cm² V⁻¹ s⁻¹to 1.95×10⁻³ cm² V⁻¹ s⁻¹. The substantial enhancement in μ_h contributes to the high PCE of the TDH PSCs. In the steady photoluminescence (PL) spectra (Fig. 7c), the TDH perovskite films possess a relatively high intensity, suggesting more photogenic charge-carriers in perovskite matrices. The high PL intensity is attributed to the few defects and high crystallinity of the perovskite films. Besides, as presented in the dark current (I_D)–V curves of the photovoltaic devices (Fig. 7d), the relatively low currents at ±1.5−0 V are achieved. It demonstrates a reduced leakage current and a better diode characteristic of the TDH photovoltaic cells for a better FF.

Electrical impedance spectroscopy (EIS) under dark conditions is used to clarify charge-carrier transfer in the devices. As presented in Fig. 7e, as compared to (6.442×10¹⁷Ω) of the control devices, the TDH photovoltaic devices exhibited a lower transfer resistance (R_tr) of 3.406×10⁴ Ω, which demonstrated that the TDH treatments restrained the charge recombination. The TDH devices also exhibit recombination resistances (R_rec,4.211×10⁴ Ω) higher than that (8,526 Ω) of the control devices. The higher recombination resistance is related to the fact that the TDH treatments modify the LUMO of the perovskite and enable an extraction of photogenic electron. Mott–Schottky measurements are conducted to present the built-in potential (V_bi) of solar cells (Fig. 7f). V_bi is obtained from a relation of C⁻² = (2V_bi−V)/A²qɛ_oɛN, where C is a depletion-layer capacitance, A is an area of devices, N is a concentration of carriers.⁴¹ The TDH devices deliver a higher V_bi of 0.97 V, as compared to that (0.82 V) of the control devices, which results from the molecular passivation that raises a driving force for an efficient separation of exciton.

Good operation stability is key for a true adaptation of both rigid and flexible photovoltaic devices. Operation stability of both the rigid and flexible PSCs is measured. Fig. 8a displays the operation stability of the rigid photovoltaics in a continuous MPP tracking under one sun illumination (temperature: ~50 °C). The encapsulation-free rigid PSCs with the TDH treatments maintain 92.5% and 86.2% of the initial PCE in the 1,000 h and 2,000 h continuous illuminations, respectively. However, the control rigid photovoltaic cells retain 51.0% of the initial efficiency in the MPP illumination for 1,000 h. The TDH devices exhibit T₉₅ of 717 h, which is 3.2 times longer than the control devices. Fig. 7b shows the operation stability of the flexible perovskite photovoltaic devices. The flexible TDH PSCs retain 95.2% of the initial efficiency in the illumination for 600 h, whereas the control flexible devices show a sharp decrease in efficiency and fail to operation at the 188 h tracking. Both enhanced operation stabilities of the rigid and flexible devices are mainly due to not only the detect passiviation but also the relaxed residual strains and prolonged carrier lifetimes of the perovskite discussed below.

We further demonstrated the lattice contractions of the control and TDH perovskites (Supplementary Fig. S9). To present the lattice contraction of the control and TDH samples, an incidence angle of X-rays varies from 0.1° to 1°. When an incidence angle of 1° was used, the X-rays could penetrate the matrices of the perovskite.⁴² As illustrated in Figs. 8c,d, the (100) peak position of the control samples is reduced from 0.99268 Å⁻¹ to 0.98915 Å⁻¹with increasing incident angle from 0.1° to 1° (see detailed data in Supplementary Table 3). On the basis of the fitting line of lattice distance-spacing (D-spacing), the slope of 0.028 is obtained. For the TDH samples, the slope of the fitting line is 0.000 (Fig. 8e). Interestingly, the position of the (100) peaks is consistent. As the slope is reduced, the residual strains of the perovskite become smaller, thereby resulting in a weak lattice shrinkage.⁴³ The internal residual strains are substantially released by the TDH, leading to a promoted stability as well as an enhanced efficiency of the solar cells.

The time-resolved PL (TRPL) results (Fig. 8f) revealed a visible raise in the lifetime of the free charges of the perovskite after the perovskite films were stored in ambient air for ten days. τ₁ represents a charge transfer process. τ₂ represents a behavioral attribute of carrier recombination.⁴⁴The TDH perovskite samples possess a much longer carrier lifetime (τ₁: 0.66 μs), as compared to that (τ₁: 0.13 μs) of the control perovskite. The average carrier lifetime (τ_a) is also visibly improved from 1.92 μs to 3.87 μs. These results demonstrate that the TDH treatments are in favor with exciton separation as well as the suppression of the non-radiative recombination for a better photovoltaic cell. The carrier lifetime data are summarized in Supplementary Table 4.

We have demonstrated both efficient rigid and flexible inverted PSCs via TDH incorporation strategy. The rigid and flexible photovoltaic devices exhibit high PCE of 26.15% and 24.68% along with high FF of 86%, respectively. TDH strongly adsorbs on the (110) and (100) facets because of the adsorption energy of N1 as strong as −3.21 and −2.72 eV. The introduction of Cl ions of TDH is favorable for enhancing the ionic strength of Pb-X compared with Pb-I. The bond energy and charge transfer of Pb−Cl is 4.16 eV and 0.28 e, respectively. Interestingly, the TDH treatments reduce the aligned point vacancy defects on the perovkite lattices, and the dislocations and distortions across the boundaries in the perovkite crystals. Thus, the TDH treatments induce fewer uncoordinated metal ion and halogen vacancy defects, large grain boundaries, high crystallinity, relaxed residual strains, etc. Both rigid and flexible PSCs without encapsulation show the substantial enhancement in operation stability. The enhanced stabilities of the photovoltaic devices are due to the defect passivation by the TDH with strong adsorption, the released residual strains as well as the prolonged average carrier lifetimes of the perovskite. The high-performance rigid and flexible inverted PSCs have a promising promise for an adaptation of the perovskite photovoltaics involving wearable and flexible photovoltaic cells.

Materials. Cesium iodide (CsI), methylammonium chloride (MACl), lead iodide (PbI₂), formamidinium iodide (FAI), and methylammonium iodide (MAI) and 2-phenylethylamine hydroiodide (PEAI) were obtained via Advanced Election Technology Co., Ltd. Bathocuproine (BCP) and 2-thiopheneethanamine, hydriodide 2-Thiopheneethylammonium iodide (2-ThEAI) were obtained from the Xi’an Polymer Light Technology Corp. [2-(3,6-Dimethoxy9H-carbazol-9-yl)ethyl] phosphonic acid (MeO-2PACz) and 1,3,5-Triazine-2,4-diamine (TD) were got from TCI. Dimethyl sulfoxide (DMSO) was purchased from Alfa Aesar. N,N-Dimethylformamide (DMF), chlorobenzene (CB) and anisole were purchased from Sigma-Aldrich. C₆₀ was obtained via Solarmer, China. The thermoplastics with ITO electrode were got from Solar Applied Materials Technology Corp.

Preparation of TDH powers. TD and HCl with a molar ratio of 1:2.1 were mixed in a reagent bottle. The solution was stirred for 3 h to fully dissolve the TD followed by a heating at 180 ℃ for 6 h to evaporate the solvent. Finally, the reagent bottle was kept in a heating oven at 70 ℃ for 1 day to obtain the TDH powders.

Preparation of perovskite precursor solutions. Cs_0.05MA_0.15FA_0.80PbI₃ perovskite precursors were dissolved with 5 mol.% MACl and 5 mol.% PbI₂ in a mixed DMF and DMSO solvent (4.5:1 v/v) at a concentration of 1.76 M. The precursor solutions were stirred at 25 ℃ for 1 h, and then filtered using 0.22-μm polytetrafluoroethylene membrane. The TDH additives were added into the precursor. The concentration of TDH is 0.5 mg ml⁻¹.

Rigid PSC fabrications. ITO-coated glass substrates were treated via the ultrasonic treatment in deionized water, acetone and isopropanol (IPA). After the ultraviolet ozone treatments for 25 min, the HTL of MeO-2PACz was prepared via spin-coating the MeO-2PACz solutions (0.5 mg ml⁻¹ in ethanol) at 4,000 rpm for 30 s and heated at 100 ℃ for 10 min. PEAI solution (1 mg ml⁻¹ in IPA) was deposited on the ITO/MeO-2PACZ substrates at 5,000 rpm for 25 s and then heated at 100 ℃ for 2 min. The perovskite solution was spin-coated at 1,000 rpm for 10 s (acceleration rate 200 rpm s⁻¹) and 4,500 rpm for 40 s (acceleration rate 2000 rpm s⁻¹), respectively. At the 25 s of the second step, 160 μL anisole was dropped as the antisolvent. Subsequently, the samples were annealed at 120 ℃ for 20 min. 1 mg ml⁻¹ MAI and 2 mg ml⁻¹ 2-ThEAI were dissolved in the IPA/DMF (150:1 v/v) mixed solvents, and then the 100 μl mixed solution was filtered and dynamically spin-coated on the perovskite films at 5,000 rpm for 30 s, followed by a thermal annealing at 100 ℃ for 10 min. Then, C₆₀ with 30-nm thickness and BCP with 7-nm thickness were evaporated. Ag electrodes with 7-nm thickness were thermally evaporated via shadow masks that showed a hole of 4.00 mm². The active area of the photovoltaic devices is 4.00 mm².

Flexible PSC fabrications. ITO-coated PET thermoplastics were treated by ultrasonic treatment in deionized water, acetone and IPA. Note that the underlying thermoplastics were adhered to glass substrates through employing the polyimide double-sided adhesive tapes. The fabrication procedures of the HTLs, perovskite layers, C₆₀, BCP and Ag top electrodes are same and mentioned above. After the deposition of the Ag top electrodes, the flexible inverted photovoltaic devices were peeled off from the glass substrates. The flexible devices showed an active area of 4.00 mm².

Characterization and measurements. FTIR was measured via a spectrometer instrument (Thermo, Nicolet 6700). Absorption spectra were achieved through employing UV/vis/NIR spectrophotometer (Lambda 950, PerkinElmer). Morphology characteristics were obtained via scanning electron microscope (Hitachi SU-70), scanning probe microscope (Veeco, Dimension 3100) and high-resolution transmission electron microscopy (ThermoFisher, Talos F200X). Molecular interaction and elemental analysis were conducted through using X-ray photoelectron spectroscopy (Axis Ultra DLD, Kratos). GIWAXS measurements were conducted by Two-dimensional small Angle X-ray scattering instrument (Xeuss 3.0 UHR). Energy levels were got through using Ultraviolet photo-electron spectra (Kratos AXIS). Steady-state fluorescence spectra were obtained via fluorescence spectrophotometer (Agilent USA; excitation wavelength: 532 nm). We employed a silicon detector and Newport quantum efficiency system (ORIEL IQE 200TM) for light-intensity calibration of EQE measurement, respectively. J–V characteristics were obtained via Keithley 2440 Source Measure Unit with an Oriel Sol3A solar in the N₂ inert atmosphere under illumination of AM1.5G illumination. The operation stability of the photovoltaic cells was conducted using the MPP tracking with 1 sun illumination (white LED lamp).

Acknowledgements

It is supported by Zhejiang Provincial Natural Science Foundation of China (LGG22F040003), Ningbo natural science foundation (2021J195) and the National Natural Science Foundation of China (12204256).

Author Contributions

X.F. and J.W.C contribute equally to this work. X.F. design the research and write the paper. J.W.C contribute to the photovoltaic fabrications. J.Z.W. conduct the XPS, UPS, and HRTEM. J.W. provide the DFT calculations. J.Z. provide the support for the EIS and Mott-Schottky measurements. J.W.C conduct the other characterization of perovskite properties and photovoltaic performances. F.Y., J.L., J.Z., F.W., S.G., and W.J.S. provide suggestions. W.J.S. and F.Y. give a comment. The experimental results and data are analyzed by X. F.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information The online version contains supplementary material available at https://doi.org/

Correspondence and requests for materials should be addressed to Xi Fan or Weijie Song.

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Efficient rigid and flexible perovskite solar cells with 86% fill factor using multifunctional electron-enriched molecular passivation

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Abstract

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Introduction

Density functional theory (DFT) calculation

Molecular interaction

Film morphology and structure

Crystallinity and energy levels

Photovoltaic performances of PSCs

Charge transfer of devices

Stability of the rigid and flexible PSCs

Conclusion

Methods

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References

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