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Relieving the Ion Migration and Increasing Superoxide Resistance with Glutathione Incorporation for Efficient and Stable Perovskite Solar Cells

Relieving the Ion Migration and Increasing Superoxide Resistance with Glutathione Incorporation... IntroductionPSCs have made enormous achievements during the past decade, the power conversion efficiency (PCE) skyrockets from 3.8% to 25.7% from its first debut. Though the PSCs own some superior merits, such as high absorption coefficient, long carrier diffusion length and low exciton binding energy, while the notorious stability issue still hinders its further commercialization.[1] It is known that due to the uncontrollable crystallization process and the natural ionic property, defects are inevitable in the perovskite film which would serve as the pathway of moisture/oxygen infiltration.[2] Furthermore, the ion would migrate under various stimulations due to the soft lattice structure to trigger the hysteresis.[3] More seriously, the absorbed molecule on the defect sites would turn to superoxide (O2−$O_2^ - $) under photoexcitation to react with the organic components to destroy the perovskite film.[4]Some passivation intermediates with multi‐functional groups have been investigated to pacify the issues as mentioned above.[5] For example, Yang et al. demonstrated a polymerization‐assisted grain growth strategy, the monomers were added in PbI2 precursor and the annealing process would trigger the polymerization process.[6] The formed bulk polymer would afford a high energy barrier of the followed FAI reaction process thus render the enlarged crystal grains. And Zhao et al. introduced three similar amphiphilic amino acids into perovskite film and revealed that the aminoethyl phosphonic acid could effectively enhance the stability of the fabricated PSCs devices.[7] Park and colleagues introduced the hydrophilic materials by the post‐treatment process which could significantly enhance the activation energies for ion migration in halide perovskites and the device stability.[8] Gao and co‐workers claimed that by the surface and grain boundaries dual‐passivation strategy, the voltage loss was reduced and the superoxide resistance was increased.[9] The above works highlighted the importance of the regulation of crystallization and the passivation of defects, which is essential for reducing defects and enhance the superoxide resistance. At the same time, the ion migration behavior should get more concern. Han et al. demonstrated that the copper–nickel alloy stabilized by in situ grown bifacial graphene would be beneficial for inhibiting the ion migration.[10] Xing et al. summarized the advantages of 2D/3D heterostructure PSCs and indicated that the incorporation of 2D perovskite could effectively inhibit the ion migration.[11]In this context, we incorporated reduced l‐glutathione (GSH) which could bind strongly to lead in biological systems into the perovskite to tailor the crystallization process and inhibit the generation of O2−$O_2^ - $. The multi‐functional groups in GSH such as sulfhydryl (‐SH), carbonyl (‐COOH) could bind with Pb2+ to slow down the crystallization process by enhancing the energy barrier. While the amino (‐NH2) would interact with I− to form hydrogen bond to inhibit ion migration. Furthermore, GSH could react with the O2−$O_2^ - $ to restrict the decomposition of the organic compounds and the oxidation of I− ion. Attributed to the synergistic effects of GSH, the power conversion efficiency (PCE) increased from 21.53% to 22.89% accompanied with the enhanced stability. The unencapsulated PSCs devices could remain 91% of its initial efficiency after storage in ambient environment for 1000 h.Results and DiscussionThe perovskite film was fabricated through the two‐step deposition process, and GSH shown in Figure S1a, Supporting Information, was dissolved in the PbI2 precursor solution with various concentration. We assumed that the Lewis base functional groups would chela with Pb2+, and the ‐NH2 would bind with I− through the hydrogen bond.[12] First, we conducted Fourier transform infrared spectroscopy (FTIR) analysis to confirm this assumption. The full FTIR spectrum was plotted in Figure S1b, Supporting Information. The detailed FTIR spectrum was shown in Figure 1a,b. The wavenumber at around at 3500 cm−1 could be attributed to the NH vibrations, and the characteristic peak at 1643 cm−1 could be assigned to CO, respectively.[13] After the addition of PbI2, the NH characteristic peak moved to higher wavenumber (Figure 1a), while the CO characteristic peak was down shifted to 1626 cm−1 (Figure 1b). The interaction between GSH and PbI2 would influence the electronic cloud, thus the corresponding characteristic peaks were shifted. While we did not observe the ‐SH characteristic peak at ≈2600 cm−1, since it was easily oxidized under ambient environment. We then conducted X‐ray photoelectron spectrum (XPS) to further investigate the interaction between PbI2 and GSH. The full XPS spectrum was shown in Figure S2, Supporting Information, and the S 2p peak at binding energy (BE) of 164 eV could be identified for the GSH‐PbI2 film in Figure S3, Supporting Information, which confirmed that the GSH was successfully incorporated into the PbI2 film.[14] We focused on the Pb 4f and I 3d spectrum to explore the possible interaction mechanism. As shown in Figure 1c for the pristine PbI2 film, the BE of Pb 4f was located at 143.3 and 138.5 eV, which could be assigned to 4f5/2 and 4f7/2 of divalent Pb2+, respectively. After the addition of GSH, the BE of Pb 4f core level shifted to lower energy. At the same time, the two obvious shoulder peaks could be identified, which may be attributed to the formation of the GSH‐PbI2 complex.[15] Furthermore, compared with the pristine PbI2 film, the I 3d were all shifted to higher BE due to the formation of NH…I hydrogen bond in the GSH‐PbI2 film as shown in Figure 1d.[12] The X‐ray diffraction (XRD) measurement of PbI2 film with or without GSH was also conducted and plotted in Figure 1e. The diffraction peaks intensity of GSH‐PbI2 film was strongly decreased under the same annealing time, the slowly crystallization process also confirmed the strong interaction between GSH and PbI2.1FigureFTIR spectrum of a) NH and b) CO characteristic peaks. XPS spectrum of c) Pb 4f and d) I 3d characteristic peaks. e) XRD spectrum of PbI2 film.We then conducted in situ X‐ray diffraction (XRD) measurement to investigate the crystallization process after the addition of GSH as shown in Figure 2a,b. Before the annealing process (0 s), the diffraction peaks at 6.6°, 7.2°, and 8.7° could be identified for the both two samples, which could be assigned to the MAI/FAI‐PbI2‐DMSO intermediate phase, and the peak at 11.8° could be attributed to the formation of δ‐phase FAPbI3 (δ‐FAPbI3). While the diffraction peaks of intermediate phase and δ‐FAPbI3 were effectively inhibited after the addition of GSH (Figure 2b at 0 s), which could be ascribed to the strong interaction between GSH and PbI2 as we have discussed above.[16] With the extension of annealing process to 5 s, the intermediate phase vanished, accompanied the formation of perovskite phase (α‐phase FAPbI3, α‐FAPbI3). It should be noted the detrimental δ‐FAPbI3 fully transformed to α‐FAPbI3 for the GSH‐incorporated perovskite film when annealing process to 10 s. As for the pristine sample, the δ‐phase FAPbI3 fully transformed to perovskite phase and the PbI2 peak emerged at 12.8° when the annealing time was prolonged to 30 s. With the annealing process going on, the peak intensity of the (100) plane increased continuously for the GSH‐incorporated perovskite film, and the PbI2 peak appeared after 3 min annealing time, which confirmed that the incorporation of GSH could effectively retard the crystallization process. The complete perovskite film could be obtained after 10 min annealing time and the XRD patterns of the two sample were shown in Figure S4, Supporting Information. It can be seen that, after introducing GSH, the peak intensity of (100) plane was much stronger than that of the pristine film, which indicated a better crystallographic orientation.[17]2Figurea,b) In situ XRD spectrum, c,d) SEM images, e,f) AFM images of the perovskite film with or without GSH incorporation.Scanning electron microscopy (SEM) was conducted to investigate the variation of the perovskite film morphology after the addition of GSH. As shown in Figure 2c,d, the perovskite film with or without the addition of GSH all demonstrated the dense and smooth morphology. While the GSH incorporated film exhibited a larger grain size (800 nm) compared with the pristine film (300 nm). The cross‐sectional SEM images shown in Figure S5, Supporting Information, also confirmed the superiority with the addition of GSH. The pristine sample demonstrated pinhole and creak among the film, while the large grain across the complete perovskite film could be observed in the GSH incorporated perovskite film, which would be beneficial for decreasing the defects and promoting carrier transport.[18] The surface roughness was also detected by the atomic force microscopy (AFM) and shown in Figure 2e,f and Figure S6, Supporting Information, the surface root‐mean‐square (RMS) decreased from 29.5 to 20.3 nm by introducing GSH as addictive, which agreed well with the SEM images. The slightly enhancement of absorption intensity shown in Figure S7a, Supporting Information, also confirmed the improvement of perovskite film quality without changing the bandgap (Figure S7b, Supporting Information).[16]The space charge limited current (SCLC) measurement was conducted to evaluate then defects density of the perovskite films with or without GSH incorporation. We fabricated electron‐only (E‐only) and hole‐only (H‐only) devices then characterized the J–V curves in the dark. The trap‐state density (Ntrap) could be calculated by the equation:1Ntrap=2ε0εVTFLeL2\[\begin{array}{*{20}{c}}{{N_{{\rm{trap}}}} = \frac{{2{\varepsilon _0}\varepsilon {V_{{\rm{TFL}}}}}}{{e{L^2}}}}\end{array}\]where ε0 was the vacuum permittivity, ε was the relative dielectric constant of perovskite, VTFL was the trap‐filled limit voltage, e is the electron charge and L was the thickness of perovskite.[19] As shown in Figure 3a,b, the VTFL for the pristine and GSH incorporated E‐only devices was located at 0.371 and 0.207 V, and the calculated Ntrap was 6.68 and 3.73 × 1015 cm−3, respectively. As for the H‐only devices, the VTFL was estimated at 0.324 V for the pristine devices and 0.237 V for the GSH incorporated devices. The value of Ntrap was calculated to be 5.82 and 4.26 × 1015 cm−3, respectively. The decreased Ntrap could be attributed to the passivation effect and the improved crystal quality after the addition of GSH. Photoluminescence (PL) mapping and time‐resolved PL (TRPL) measurement was conducted with the device structure of glass/perovskite to investigate the charge recombination behavior. As shown in Figure 3c,d, the PL intensity of the perovskite film was significantly enhanced with the addition of GSH.[20] The TRPL spectrum was plotted in Figure 3e, the results were fitted via a bi‐exponential decay model and the results were summarized in Table S1, Supporting Information. As expected, the average charge lifetime (τave) was greatly prolonged from 359.4 ns for the pristine perovskite film to 524.5 ns for GSH‐incorporated perovskite film. The enhanced PL intensity and charge lifetime of the perovskite film with the addition of GSH was attributed to the suppression of nonradiative recombination caused by trap states. We also did PL measurements on the samples with SnO2 layer. As shown in Figure S8, Supporting Information, the PL intensity of the GSH‐incorporated sample was quenched more significantly, which confirmed that the charge transport between the perovskite and SnO2 layer were also improved.[21] Electrochemical impedance spectroscopy (EIS) measurement was conducted to further investigate the electrical properties of the PSCs devices. The related Nyquist plot was shown in Figure 3f. In general, the semicircle at low frequency corresponded to the recombination resistance (Rrec). Compared with the pristine devices, the GSH‐incorporated devices demonstrated higher Rrec, indicating a decreased recombination rate, which was in accordance with the TRPL results.[22] Mott–Schottky analysis was carried out to evaluate the variations of the build‐in potential (Vbi) in the PSCs devices. As shown in Figure 3g, The Vbi values were improved from 0.79 V for the pristine devices to 0.91 V for the GSH‐incorporated devices. The increased Vbi could not only facilitate the charge separation but also contribute to the enhancement of open voltage (Voc).[23] The dependency of Voc on light intensity was examined to assess the ideality factor. As shown in Figure S9, Supporting Information, the ideality factor decreased from 1.47 to 1.22 with the incorporation of GSH, which implied that the Shockley‐Read‐Hall recombination was effectively suppressed.[24] The trap density of state (tDOS) was further investigated by thermal admittance spectroscopy as shown in Figure 3h, the reduction in the trap density confirmed that GSH incorporation could effectively improve the quality of perovskite film.[25]3FigureSCLC measurements for a) E‐only devices and b) H‐only devices. PL mapping images of the perovskite film c) with and d) without GSH incorporation. e) TRPL spectrum of the perovskite film (structure: glass/perovskite with or without GSH). f) Nyquist plots, g) built‐in potential measurement and h) tDOS calculation of the perovskite devices with or without GSH incorporation.To explore the mechanism for the reduction in tDOS shown in Figure 3h, we then conducted density functional theory (DFT) calculations to evaluate the variation of defects formation energy before and after GSH‐incorporation, the unit cell of FA0.75MA0.25PbI3 was simplified to FAPbI3 to predigest the calculation.[26] We calculated the formation energy of various defects on the surface of perovskite films and the results were shown in Figure 4a,b and Figure S10, Supporting Information. It could be identified that the iodine vacancy (VI) formation energy increased from 0.55 to 0.69 eV after GSH‐incorporation, which would be beneficial for suppressing defect formation. The formation energy of lead vacancy (VPb) and Pb‐I anti‐site (IPb) defects were also enhanced from 3.19 and 3.21 eV to 3.27 and 4.53 eV, respectively. The detailed parameters could be found in Table S2, Supporting Information.[27] The ion migration behavior was investigated by the temperature‐dependent conductivity measurement under dark conditions.[28] The activation energy (Ea) could be extracted by the Nernst–Einstein relation:2σ (T)=σ0T exp(−EakbT)\[\begin{array}{*{20}{c}}{\sigma \;\left( T \right) = \frac{{{\sigma _0}}}{T}\;exp\left( {\frac{{ - {E_{\rm{a}}}}}{{{k_{\rm{b}}}T}}} \right)}\end{array}\]4Figurea,b) The VI formation energy of perovskite film with or without GSH incorporation. c) Temperature dependent conductivity of the perovskite film. The Ea value could be extracted at high temperature region. d) The fluorescence intensity of HE aliquots, representing the yield of O2−$O_2^ - $. XPS spectrum of e) Pb 4f and f) I 3d characteristic peaks of the aged perovskite film with or without GSH incorporation.Where σ(T) is the conductivity as a function of temperature T, kb is the Boltzmann constant, and σ0 is a constant. The Ea could be calculated by a linear fitting from Figure 4c at the high temperature region. The Ea for the perovskite film with GSH incorporation was 0.482 eV, which was substantially higher than that of the pristine sample (0.258 eV), indicating a strong interaction between GSH and defects which would inhibit the ion migration. It has been previously reported that iodide ion was of similar size to the O2−$O_2^ - $ species, thus the VI sites were the preferred location for the formation O2−$O_2^ - $ by direct electron transfer from the perovskite to oxygen.[29] As demonstrated in Figure 4a–c, the defects formation energy and Ea was significantly enhanced with the incorporation of GSH which would inhibit the formation of O2−$O_2^ - $ species.[4,30] Hydroethidine (HE), which demonstrated a characteristic emission peak at 610 nm when exposed to the O2−$O_2^ - $ species, was employed as a molecular fluorescent probe to evaluate the yield of O2−$O_2^ - $.[31] The emission data under various aging time was collected and the rate of increase in emission at 610 nm was shown in Figure 4d. It was evident that the perovskite samples with GSH‐incorporation demonstrated a relatively lower yield of O2−$O_2^ - $ formation. To confirm the generality of the GSH strategy, we also fabricated the FACsPbI3 perovskite and minored the yield of O2−$O_2^ - $ by the molecular fluorescent probe. The result shown in Figure S13, Supporting Information, further verified the universality of GSH strategy. This could be attributed to the following two factors: i) the improved crystal quality and decreased defect sites which could prevent the absorption of oxygen molecular; ii) the reaction between GSH and O2−$O_2^ - $ species which lead to the decreased concentration of O2−$O_2^ - $ in the perovskite film. According to the previous report, the oxygen and light induced decomposition process could be described by the following equation:34CH3NH3PbI3∗+O2−→4PbI2+2I2+2H2O+4CH3NH2\[\begin{array}{*{20}{c}}{4{\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_3}{\rm{PbI}}_3^ * + O_2^ - \to 4{\rm{Pb}}{{\rm{I}}_2} + 2{{\rm{I}}_2} + 2{{\rm{H}}_2}{\rm{O}} + 4{\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2}}\end{array}\]44(NH2)2CHPbI3∗+O2−→4PbI2+2I2+2H2O+4CH4N2\[\begin{array}{*{20}{c}}{4{{(N{H_2})}_2}CHPbI_3^ * + O_2^ - \to 4Pb{I_2} + 2{{\rm{I}}_2} + 2{{\rm{H}}_2}{\rm{O}} + 4C{H_4}{{\rm{N}}_2}}\end{array}\]5PbI2→Pb0+2I0\[\begin{array}{*{20}{c}}{Pb{I_2} \to P{b^0} + 2{{\rm{I}}^0}}\end{array}\]The I0 could not only serve as the non‐radiative recombination center but also initiate the cascade decomposition process of the perovskite film. Since the decomposition product I0 was volatile, we thus conducted XPS analysis of the perovskite film after aging for 200 h to examine the ratio of I/Pb which could demonstrate the iodine evolution indirectly.[32] The full XPS spectrum and S 2p characteristic peak were shown in Figures S11 and S12, Supporting Information, respectively. The Pb 4f and I 3d spectrum for the aged perovskite film was plotted in Figure 4e,f. The calculated I/Pb ratio was 2.92 of the perovskite film with GSH‐incorporation, which was higher than that of the pristine sample (2.78). At the meantime, the distinct Pb0 peak could be indicated for the pristine perovskite film which was absent in the sample with GSH‐incorporation, indicating that the GSH could passivate the no‐coordinated Pb2+ thereby reducing the formation of metallic lead.[33] It has been proved that the main product between GSH and O2−$O_2^ - $ was oxidized glutathione (GSSG) and the schematic diagram was shown in Figure S14, Supporting Information. It could be distinguished that the GSSG could still locate at grain boundary and maintain the ability to passivate the no‐coordinated Pb2+ defects.[29]To evaluate the effect of GSH incorporation on the photovoltaic performance, the complete PSCs device was fabricated based on the perovskite film with various GSH concentration. 15 devices were fabricated to verify the reproducibility and the detailed parameters was shown in Figure S15, Supporting Information. The champion device with the Voc of 1.157 V, short current (Jsc) of 24.98 mA cm−2, fill factor (FF) of 79.2% and PCE of 22.89% was obtained at a concentration of 1 wt%, which was significantly higher than that of the pristine sample with a Voc of 1.124 V, Jsc of 24.63 mA cm−2, FF of 77.8% and PCE of 21.53% as shown in Figure 5a and Table S3, Supporting Information. Meanwhile, the negligible hysteresis (reverse scan: 22.89%, forward scan: 22.55%) was observed in the GSH incorporated PSCs devices, while the pristine devices exhibited stronger hysteresis (reverse scan: 21.53%, forward scan: 20.29%). Steady‐state output (SOP) of the devices at the maximum power point (MPP) was conducted to verify the reliability of the device in the operating condition. As shown in Figure 5b, the stable JSC and PCE could be obtained for the GSH incorporated samples at MPP of 0.98 V for over 500 s, which was significantly higher than that of the pristine samples. The effect of the GSH incorporation on the stability of the perovskite films was investigated. It should be noted that the perovskite film was storage in ambient environment without encapsulation. As shown in Figure 5c, the PbI2 peak intensity of the pristine perovskite evidently enhanced after aging, and the color blenched from minor black to yellow. While for the perovskite film with GSH‐incorporation, we could still observe the dominant perovskite diffraction peak, accompanied by a minor enhancement of the PbI2 peak, which indicated the integrity of the perovskite crystal structure. The irradiance and thermal stability of the PSCs devices were investigated to prove the superiority of the GSH incorporation. As shown in Figure S16a, Supporting Information, the device incorporated with GSH could maintain 88% of its initial PCE after continuous illumination for 400 h, while the pristine device only retained 47% of the initial PCE. For the thermal stability measurement, the devices were kept at 85 °C in the glove box. The device incorporated with GSH could retain 83% of its primary PCE, while the PCE of the reference device dropped to 39%. It could be concluded that with the GSH‐incorporation, the irradiance and thermal stability were enhanced simultaneously due to the defects passivation and suppressed ion migration.[34] Finally, the ambient stability of the unencapsulated devices with or without GSH incorporation was compared. As shown in Figure 5d, the PCE of the pristine PSCs devices rapidly dropped to 52% of its initial efficiency, while the PCSs with GSH‐incorporation could maintain 91% of its original efficiency after storage for 1000 h under ambient environment.5Figurea) J–V curves and b) SOP measurements of the PSCs devices with or without GSH incorporation. Evolution of the c) measured PCE of unencapsulated PSCs devices and d) XRD spectrum of the perovskite film after storage for 1000 h under ambient environment (RH: 35 ± 10%, T: 27 ± 5 °C).ConclusionIn this work, we introduced the reduced GSH into the planar PSCs devices. The incorporation of GSH could not only tailor the crystallization process, passivate the defects, inhibit the ion migration, but also enhance the superoxide resistance. As a result, the champion PCE of 22.89% was achieved, which was higher than that of the pristine devices. Importantly, the device stability was enhanced simultaneously. The unencapsulated PSCs with GSH‐incorporation could maintain 91% of its initial efficiency after storage in ambient environment for 1000 h. This facial and effective strategy would speed up the commercialization of high‐performance PSCs.AcknowledgementsThis work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61974054 and 61675088), the International Science & Technology Cooperation Program of Jilin (Grant No. 20190701023GH), the Scientific and Technological Developing Scheme of Jilin Province (Grant No. 20200401045GX), and the Project of Science and Technology Development Plan of Jilin Province (Grant No. 20190302011G)Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.a) M. Gratzel, Nat. Mater. 2014, 13, 838;b) B. V. Lotsch, Angew. Chem., Int. Ed. 2014, 53, 635;c) N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476.a) G. Niu, X. Guo, L. Wang, J. Mater. Chem. A 2015, 3, 8970;b) P. 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Relieving the Ion Migration and Increasing Superoxide Resistance with Glutathione Incorporation for Efficient and Stable Perovskite Solar Cells

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Abstract

IntroductionPSCs have made enormous achievements during the past decade, the power conversion efficiency (PCE) skyrockets from 3.8% to 25.7% from its first debut. Though the PSCs own some superior merits, such as high absorption coefficient, long carrier diffusion length and low exciton binding energy, while the notorious stability issue still hinders its further commercialization.[1] It is known that due to the uncontrollable crystallization process and the natural ionic property, defects are inevitable in the perovskite film which would serve as the pathway of moisture/oxygen infiltration.[2] Furthermore, the ion would migrate under various stimulations due to the soft lattice structure to trigger the hysteresis.[3] More seriously, the absorbed molecule on the defect sites would turn to superoxide (O2−$O_2^ - $) under photoexcitation to react with the organic components to destroy the perovskite film.[4]Some passivation intermediates with multi‐functional groups have been investigated to pacify the issues as mentioned above.[5] For example, Yang et al. demonstrated a polymerization‐assisted grain growth strategy, the monomers were added in PbI2 precursor and the annealing process would trigger the polymerization process.[6] The formed bulk polymer would afford a high energy barrier of the followed FAI reaction process thus render the enlarged crystal grains. And Zhao et al. introduced three similar amphiphilic amino acids into perovskite film and revealed that the aminoethyl phosphonic acid could effectively enhance the stability of the fabricated PSCs devices.[7] Park and colleagues introduced the hydrophilic materials by the post‐treatment process which could significantly enhance the activation energies for ion migration in halide perovskites and the device stability.[8] Gao and co‐workers claimed that by the surface and grain boundaries dual‐passivation strategy, the voltage loss was reduced and the superoxide resistance was increased.[9] The above works highlighted the importance of the regulation of crystallization and the passivation of defects, which is essential for reducing defects and enhance the superoxide resistance. At the same time, the ion migration behavior should get more concern. Han et al. demonstrated that the copper–nickel alloy stabilized by in situ grown bifacial graphene would be beneficial for inhibiting the ion migration.[10] Xing et al. summarized the advantages of 2D/3D heterostructure PSCs and indicated that the incorporation of 2D perovskite could effectively inhibit the ion migration.[11]In this context, we incorporated reduced l‐glutathione (GSH) which could bind strongly to lead in biological systems into the perovskite to tailor the crystallization process and inhibit the generation of O2−$O_2^ - $. The multi‐functional groups in GSH such as sulfhydryl (‐SH), carbonyl (‐COOH) could bind with Pb2+ to slow down the crystallization process by enhancing the energy barrier. While the amino (‐NH2) would interact with I− to form hydrogen bond to inhibit ion migration. Furthermore, GSH could react with the O2−$O_2^ - $ to restrict the decomposition of the organic compounds and the oxidation of I− ion. Attributed to the synergistic effects of GSH, the power conversion efficiency (PCE) increased from 21.53% to 22.89% accompanied with the enhanced stability. The unencapsulated PSCs devices could remain 91% of its initial efficiency after storage in ambient environment for 1000 h.Results and DiscussionThe perovskite film was fabricated through the two‐step deposition process, and GSH shown in Figure S1a, Supporting Information, was dissolved in the PbI2 precursor solution with various concentration. We assumed that the Lewis base functional groups would chela with Pb2+, and the ‐NH2 would bind with I− through the hydrogen bond.[12] First, we conducted Fourier transform infrared spectroscopy (FTIR) analysis to confirm this assumption. The full FTIR spectrum was plotted in Figure S1b, Supporting Information. The detailed FTIR spectrum was shown in Figure 1a,b. The wavenumber at around at 3500 cm−1 could be attributed to the NH vibrations, and the characteristic peak at 1643 cm−1 could be assigned to CO, respectively.[13] After the addition of PbI2, the NH characteristic peak moved to higher wavenumber (Figure 1a), while the CO characteristic peak was down shifted to 1626 cm−1 (Figure 1b). The interaction between GSH and PbI2 would influence the electronic cloud, thus the corresponding characteristic peaks were shifted. While we did not observe the ‐SH characteristic peak at ≈2600 cm−1, since it was easily oxidized under ambient environment. We then conducted X‐ray photoelectron spectrum (XPS) to further investigate the interaction between PbI2 and GSH. The full XPS spectrum was shown in Figure S2, Supporting Information, and the S 2p peak at binding energy (BE) of 164 eV could be identified for the GSH‐PbI2 film in Figure S3, Supporting Information, which confirmed that the GSH was successfully incorporated into the PbI2 film.[14] We focused on the Pb 4f and I 3d spectrum to explore the possible interaction mechanism. As shown in Figure 1c for the pristine PbI2 film, the BE of Pb 4f was located at 143.3 and 138.5 eV, which could be assigned to 4f5/2 and 4f7/2 of divalent Pb2+, respectively. After the addition of GSH, the BE of Pb 4f core level shifted to lower energy. At the same time, the two obvious shoulder peaks could be identified, which may be attributed to the formation of the GSH‐PbI2 complex.[15] Furthermore, compared with the pristine PbI2 film, the I 3d were all shifted to higher BE due to the formation of NH…I hydrogen bond in the GSH‐PbI2 film as shown in Figure 1d.[12] The X‐ray diffraction (XRD) measurement of PbI2 film with or without GSH was also conducted and plotted in Figure 1e. The diffraction peaks intensity of GSH‐PbI2 film was strongly decreased under the same annealing time, the slowly crystallization process also confirmed the strong interaction between GSH and PbI2.1FigureFTIR spectrum of a) NH and b) CO characteristic peaks. XPS spectrum of c) Pb 4f and d) I 3d characteristic peaks. e) XRD spectrum of PbI2 film.We then conducted in situ X‐ray diffraction (XRD) measurement to investigate the crystallization process after the addition of GSH as shown in Figure 2a,b. Before the annealing process (0 s), the diffraction peaks at 6.6°, 7.2°, and 8.7° could be identified for the both two samples, which could be assigned to the MAI/FAI‐PbI2‐DMSO intermediate phase, and the peak at 11.8° could be attributed to the formation of δ‐phase FAPbI3 (δ‐FAPbI3). While the diffraction peaks of intermediate phase and δ‐FAPbI3 were effectively inhibited after the addition of GSH (Figure 2b at 0 s), which could be ascribed to the strong interaction between GSH and PbI2 as we have discussed above.[16] With the extension of annealing process to 5 s, the intermediate phase vanished, accompanied the formation of perovskite phase (α‐phase FAPbI3, α‐FAPbI3). It should be noted the detrimental δ‐FAPbI3 fully transformed to α‐FAPbI3 for the GSH‐incorporated perovskite film when annealing process to 10 s. As for the pristine sample, the δ‐phase FAPbI3 fully transformed to perovskite phase and the PbI2 peak emerged at 12.8° when the annealing time was prolonged to 30 s. With the annealing process going on, the peak intensity of the (100) plane increased continuously for the GSH‐incorporated perovskite film, and the PbI2 peak appeared after 3 min annealing time, which confirmed that the incorporation of GSH could effectively retard the crystallization process. The complete perovskite film could be obtained after 10 min annealing time and the XRD patterns of the two sample were shown in Figure S4, Supporting Information. It can be seen that, after introducing GSH, the peak intensity of (100) plane was much stronger than that of the pristine film, which indicated a better crystallographic orientation.[17]2Figurea,b) In situ XRD spectrum, c,d) SEM images, e,f) AFM images of the perovskite film with or without GSH incorporation.Scanning electron microscopy (SEM) was conducted to investigate the variation of the perovskite film morphology after the addition of GSH. As shown in Figure 2c,d, the perovskite film with or without the addition of GSH all demonstrated the dense and smooth morphology. While the GSH incorporated film exhibited a larger grain size (800 nm) compared with the pristine film (300 nm). The cross‐sectional SEM images shown in Figure S5, Supporting Information, also confirmed the superiority with the addition of GSH. The pristine sample demonstrated pinhole and creak among the film, while the large grain across the complete perovskite film could be observed in the GSH incorporated perovskite film, which would be beneficial for decreasing the defects and promoting carrier transport.[18] The surface roughness was also detected by the atomic force microscopy (AFM) and shown in Figure 2e,f and Figure S6, Supporting Information, the surface root‐mean‐square (RMS) decreased from 29.5 to 20.3 nm by introducing GSH as addictive, which agreed well with the SEM images. The slightly enhancement of absorption intensity shown in Figure S7a, Supporting Information, also confirmed the improvement of perovskite film quality without changing the bandgap (Figure S7b, Supporting Information).[16]The space charge limited current (SCLC) measurement was conducted to evaluate then defects density of the perovskite films with or without GSH incorporation. We fabricated electron‐only (E‐only) and hole‐only (H‐only) devices then characterized the J–V curves in the dark. The trap‐state density (Ntrap) could be calculated by the equation:1Ntrap=2ε0εVTFLeL2\[\begin{array}{*{20}{c}}{{N_{{\rm{trap}}}} = \frac{{2{\varepsilon _0}\varepsilon {V_{{\rm{TFL}}}}}}{{e{L^2}}}}\end{array}\]where ε0 was the vacuum permittivity, ε was the relative dielectric constant of perovskite, VTFL was the trap‐filled limit voltage, e is the electron charge and L was the thickness of perovskite.[19] As shown in Figure 3a,b, the VTFL for the pristine and GSH incorporated E‐only devices was located at 0.371 and 0.207 V, and the calculated Ntrap was 6.68 and 3.73 × 1015 cm−3, respectively. As for the H‐only devices, the VTFL was estimated at 0.324 V for the pristine devices and 0.237 V for the GSH incorporated devices. The value of Ntrap was calculated to be 5.82 and 4.26 × 1015 cm−3, respectively. The decreased Ntrap could be attributed to the passivation effect and the improved crystal quality after the addition of GSH. Photoluminescence (PL) mapping and time‐resolved PL (TRPL) measurement was conducted with the device structure of glass/perovskite to investigate the charge recombination behavior. As shown in Figure 3c,d, the PL intensity of the perovskite film was significantly enhanced with the addition of GSH.[20] The TRPL spectrum was plotted in Figure 3e, the results were fitted via a bi‐exponential decay model and the results were summarized in Table S1, Supporting Information. As expected, the average charge lifetime (τave) was greatly prolonged from 359.4 ns for the pristine perovskite film to 524.5 ns for GSH‐incorporated perovskite film. The enhanced PL intensity and charge lifetime of the perovskite film with the addition of GSH was attributed to the suppression of nonradiative recombination caused by trap states. We also did PL measurements on the samples with SnO2 layer. As shown in Figure S8, Supporting Information, the PL intensity of the GSH‐incorporated sample was quenched more significantly, which confirmed that the charge transport between the perovskite and SnO2 layer were also improved.[21] Electrochemical impedance spectroscopy (EIS) measurement was conducted to further investigate the electrical properties of the PSCs devices. The related Nyquist plot was shown in Figure 3f. In general, the semicircle at low frequency corresponded to the recombination resistance (Rrec). Compared with the pristine devices, the GSH‐incorporated devices demonstrated higher Rrec, indicating a decreased recombination rate, which was in accordance with the TRPL results.[22] Mott–Schottky analysis was carried out to evaluate the variations of the build‐in potential (Vbi) in the PSCs devices. As shown in Figure 3g, The Vbi values were improved from 0.79 V for the pristine devices to 0.91 V for the GSH‐incorporated devices. The increased Vbi could not only facilitate the charge separation but also contribute to the enhancement of open voltage (Voc).[23] The dependency of Voc on light intensity was examined to assess the ideality factor. As shown in Figure S9, Supporting Information, the ideality factor decreased from 1.47 to 1.22 with the incorporation of GSH, which implied that the Shockley‐Read‐Hall recombination was effectively suppressed.[24] The trap density of state (tDOS) was further investigated by thermal admittance spectroscopy as shown in Figure 3h, the reduction in the trap density confirmed that GSH incorporation could effectively improve the quality of perovskite film.[25]3FigureSCLC measurements for a) E‐only devices and b) H‐only devices. PL mapping images of the perovskite film c) with and d) without GSH incorporation. e) TRPL spectrum of the perovskite film (structure: glass/perovskite with or without GSH). f) Nyquist plots, g) built‐in potential measurement and h) tDOS calculation of the perovskite devices with or without GSH incorporation.To explore the mechanism for the reduction in tDOS shown in Figure 3h, we then conducted density functional theory (DFT) calculations to evaluate the variation of defects formation energy before and after GSH‐incorporation, the unit cell of FA0.75MA0.25PbI3 was simplified to FAPbI3 to predigest the calculation.[26] We calculated the formation energy of various defects on the surface of perovskite films and the results were shown in Figure 4a,b and Figure S10, Supporting Information. It could be identified that the iodine vacancy (VI) formation energy increased from 0.55 to 0.69 eV after GSH‐incorporation, which would be beneficial for suppressing defect formation. The formation energy of lead vacancy (VPb) and Pb‐I anti‐site (IPb) defects were also enhanced from 3.19 and 3.21 eV to 3.27 and 4.53 eV, respectively. The detailed parameters could be found in Table S2, Supporting Information.[27] The ion migration behavior was investigated by the temperature‐dependent conductivity measurement under dark conditions.[28] The activation energy (Ea) could be extracted by the Nernst–Einstein relation:2σ (T)=σ0T exp(−EakbT)\[\begin{array}{*{20}{c}}{\sigma \;\left( T \right) = \frac{{{\sigma _0}}}{T}\;exp\left( {\frac{{ - {E_{\rm{a}}}}}{{{k_{\rm{b}}}T}}} \right)}\end{array}\]4Figurea,b) The VI formation energy of perovskite film with or without GSH incorporation. c) Temperature dependent conductivity of the perovskite film. The Ea value could be extracted at high temperature region. d) The fluorescence intensity of HE aliquots, representing the yield of O2−$O_2^ - $. XPS spectrum of e) Pb 4f and f) I 3d characteristic peaks of the aged perovskite film with or without GSH incorporation.Where σ(T) is the conductivity as a function of temperature T, kb is the Boltzmann constant, and σ0 is a constant. The Ea could be calculated by a linear fitting from Figure 4c at the high temperature region. The Ea for the perovskite film with GSH incorporation was 0.482 eV, which was substantially higher than that of the pristine sample (0.258 eV), indicating a strong interaction between GSH and defects which would inhibit the ion migration. It has been previously reported that iodide ion was of similar size to the O2−$O_2^ - $ species, thus the VI sites were the preferred location for the formation O2−$O_2^ - $ by direct electron transfer from the perovskite to oxygen.[29] As demonstrated in Figure 4a–c, the defects formation energy and Ea was significantly enhanced with the incorporation of GSH which would inhibit the formation of O2−$O_2^ - $ species.[4,30] Hydroethidine (HE), which demonstrated a characteristic emission peak at 610 nm when exposed to the O2−$O_2^ - $ species, was employed as a molecular fluorescent probe to evaluate the yield of O2−$O_2^ - $.[31] The emission data under various aging time was collected and the rate of increase in emission at 610 nm was shown in Figure 4d. It was evident that the perovskite samples with GSH‐incorporation demonstrated a relatively lower yield of O2−$O_2^ - $ formation. To confirm the generality of the GSH strategy, we also fabricated the FACsPbI3 perovskite and minored the yield of O2−$O_2^ - $ by the molecular fluorescent probe. The result shown in Figure S13, Supporting Information, further verified the universality of GSH strategy. This could be attributed to the following two factors: i) the improved crystal quality and decreased defect sites which could prevent the absorption of oxygen molecular; ii) the reaction between GSH and O2−$O_2^ - $ species which lead to the decreased concentration of O2−$O_2^ - $ in the perovskite film. According to the previous report, the oxygen and light induced decomposition process could be described by the following equation:34CH3NH3PbI3∗+O2−→4PbI2+2I2+2H2O+4CH3NH2\[\begin{array}{*{20}{c}}{4{\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_3}{\rm{PbI}}_3^ * + O_2^ - \to 4{\rm{Pb}}{{\rm{I}}_2} + 2{{\rm{I}}_2} + 2{{\rm{H}}_2}{\rm{O}} + 4{\rm{C}}{{\rm{H}}_3}{\rm{N}}{{\rm{H}}_2}}\end{array}\]44(NH2)2CHPbI3∗+O2−→4PbI2+2I2+2H2O+4CH4N2\[\begin{array}{*{20}{c}}{4{{(N{H_2})}_2}CHPbI_3^ * + O_2^ - \to 4Pb{I_2} + 2{{\rm{I}}_2} + 2{{\rm{H}}_2}{\rm{O}} + 4C{H_4}{{\rm{N}}_2}}\end{array}\]5PbI2→Pb0+2I0\[\begin{array}{*{20}{c}}{Pb{I_2} \to P{b^0} + 2{{\rm{I}}^0}}\end{array}\]The I0 could not only serve as the non‐radiative recombination center but also initiate the cascade decomposition process of the perovskite film. Since the decomposition product I0 was volatile, we thus conducted XPS analysis of the perovskite film after aging for 200 h to examine the ratio of I/Pb which could demonstrate the iodine evolution indirectly.[32] The full XPS spectrum and S 2p characteristic peak were shown in Figures S11 and S12, Supporting Information, respectively. The Pb 4f and I 3d spectrum for the aged perovskite film was plotted in Figure 4e,f. The calculated I/Pb ratio was 2.92 of the perovskite film with GSH‐incorporation, which was higher than that of the pristine sample (2.78). At the meantime, the distinct Pb0 peak could be indicated for the pristine perovskite film which was absent in the sample with GSH‐incorporation, indicating that the GSH could passivate the no‐coordinated Pb2+ thereby reducing the formation of metallic lead.[33] It has been proved that the main product between GSH and O2−$O_2^ - $ was oxidized glutathione (GSSG) and the schematic diagram was shown in Figure S14, Supporting Information. It could be distinguished that the GSSG could still locate at grain boundary and maintain the ability to passivate the no‐coordinated Pb2+ defects.[29]To evaluate the effect of GSH incorporation on the photovoltaic performance, the complete PSCs device was fabricated based on the perovskite film with various GSH concentration. 15 devices were fabricated to verify the reproducibility and the detailed parameters was shown in Figure S15, Supporting Information. The champion device with the Voc of 1.157 V, short current (Jsc) of 24.98 mA cm−2, fill factor (FF) of 79.2% and PCE of 22.89% was obtained at a concentration of 1 wt%, which was significantly higher than that of the pristine sample with a Voc of 1.124 V, Jsc of 24.63 mA cm−2, FF of 77.8% and PCE of 21.53% as shown in Figure 5a and Table S3, Supporting Information. Meanwhile, the negligible hysteresis (reverse scan: 22.89%, forward scan: 22.55%) was observed in the GSH incorporated PSCs devices, while the pristine devices exhibited stronger hysteresis (reverse scan: 21.53%, forward scan: 20.29%). Steady‐state output (SOP) of the devices at the maximum power point (MPP) was conducted to verify the reliability of the device in the operating condition. As shown in Figure 5b, the stable JSC and PCE could be obtained for the GSH incorporated samples at MPP of 0.98 V for over 500 s, which was significantly higher than that of the pristine samples. The effect of the GSH incorporation on the stability of the perovskite films was investigated. It should be noted that the perovskite film was storage in ambient environment without encapsulation. As shown in Figure 5c, the PbI2 peak intensity of the pristine perovskite evidently enhanced after aging, and the color blenched from minor black to yellow. While for the perovskite film with GSH‐incorporation, we could still observe the dominant perovskite diffraction peak, accompanied by a minor enhancement of the PbI2 peak, which indicated the integrity of the perovskite crystal structure. The irradiance and thermal stability of the PSCs devices were investigated to prove the superiority of the GSH incorporation. As shown in Figure S16a, Supporting Information, the device incorporated with GSH could maintain 88% of its initial PCE after continuous illumination for 400 h, while the pristine device only retained 47% of the initial PCE. For the thermal stability measurement, the devices were kept at 85 °C in the glove box. The device incorporated with GSH could retain 83% of its primary PCE, while the PCE of the reference device dropped to 39%. It could be concluded that with the GSH‐incorporation, the irradiance and thermal stability were enhanced simultaneously due to the defects passivation and suppressed ion migration.[34] Finally, the ambient stability of the unencapsulated devices with or without GSH incorporation was compared. As shown in Figure 5d, the PCE of the pristine PSCs devices rapidly dropped to 52% of its initial efficiency, while the PCSs with GSH‐incorporation could maintain 91% of its original efficiency after storage for 1000 h under ambient environment.5Figurea) J–V curves and b) SOP measurements of the PSCs devices with or without GSH incorporation. Evolution of the c) measured PCE of unencapsulated PSCs devices and d) XRD spectrum of the perovskite film after storage for 1000 h under ambient environment (RH: 35 ± 10%, T: 27 ± 5 °C).ConclusionIn this work, we introduced the reduced GSH into the planar PSCs devices. The incorporation of GSH could not only tailor the crystallization process, passivate the defects, inhibit the ion migration, but also enhance the superoxide resistance. As a result, the champion PCE of 22.89% was achieved, which was higher than that of the pristine devices. Importantly, the device stability was enhanced simultaneously. The unencapsulated PSCs with GSH‐incorporation could maintain 91% of its initial efficiency after storage in ambient environment for 1000 h. This facial and effective strategy would speed up the commercialization of high‐performance PSCs.AcknowledgementsThis work was financially supported by the National Natural Science Foundation of China (Grant Nos. 61974054 and 61675088), the International Science & Technology Cooperation Program of Jilin (Grant No. 20190701023GH), the Scientific and Technological Developing Scheme of Jilin Province (Grant No. 20200401045GX), and the Project of Science and Technology Development Plan of Jilin Province (Grant No. 20190302011G)Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.a) M. Gratzel, Nat. Mater. 2014, 13, 838;b) B. V. Lotsch, Angew. Chem., Int. Ed. 2014, 53, 635;c) N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476.a) G. Niu, X. Guo, L. Wang, J. Mater. Chem. A 2015, 3, 8970;b) P. 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Journal

Advanced Materials InterfacesWiley

Published: Apr 1, 2023

Keywords: glutathione; ion migration; perovskite solar cells; superoxide resistance

References