Today I will take you to understand the impact of methoxysilane additives on perovskite solar cells through pictures and texts, hoping to share some knowledge for the solar cell industry.
1. An effective and reproducible multifunctional additive methoxysilane crosslinker (functionalized by different end groups, moderately electron donating -SH, weak electron donating -CH3 or strong electron withdrawing -CN) is reported. The cross-linking agent is added to the PbI2 precursor);
2. After the introduction of (3-mercaptopropyl) trimethoxysilane (MPTS) containing -SH group, due to improved voltage and current density, the power conversion efficiency (PCE) increased from 18.4% to 20.8%, while containing- CN-based 3-cyanopropyltriethoxysilane (CPTS) will reduce the photovoltaic performance of the device;
3. The -SH in MPTS effectively passivates the defects by interacting with the Lewis acid-base of PbI2, resulting in larger grain size and longer carrier lifetime. Since a cross-linked siloxane network is formed on the grain boundary as a protective layer, the thermal stability and moisture stability of the device are significantly improved.
Related work was published on “ACS Energy Letters” (ACS Energy Lett. 2019, 4(9), 2192-2200) with the title “Importance of Functional Groups in Cross-Linking Methoxysilane Additives for High-Efficiency and Stable Perovskite Solar Cells”
Select -SH as the terminal functional group of the target additive (MPTS). At the same time, the effects of other capped functional groups (such as -CH3 and -CN) on device performance were compared and in-depth studied to understand the relationship between the molecular structure of additives and device performance. The TEM image shows the adjacent perovskite grains drawn from the MPTS-added perovskite film and dispersed in an anti-solvent (chlorobenzene) by ultrasonic treatment. It can be found that there are gray shades between the perovskite particles. The MPTS additive perovskite film exhibits a cubic phase, and its corresponding d-spacing value is 3.1 Å. Figure 1f shows the enlarged heterojunction region of the perovskite-methoxysilane crosslinker between adjacent grains. Unlike the regular crystalline perovskite phase, the apparent amorphous phase can be clarified, which originates from the siloxane organic molecules surrounding the perovskite grains.
Compared with the control device, the JSC, VOC, and FF of the device with MTPS have been improved. Therefore, the PCE increased, and the efficiency of the device with CPTS and PTS added decreased. The results show that the cross-linking group cannot improve the PCE of the device, and the end group opposite to the cross-linking group plays an important role in determining the final device performance. The JV curve shows that the PCE of the best control device is 18.4% (17.3%), the JSC is 22.25 (22.23) mA/cm2, the VOC is 1.04 (1.02) V, and the FF is 79.2% (76.4%) reverse (forward) scanning. In contrast, the most MPTS-added device provides a higher PCE of 20.81% (20.0%), JSC of 23.39 (23.46) mA/cm2, VOC of 1.12 (1.11) V and FF of 80.0% (76.9%) ) Reverse (forward) scanning with reduced hysteresis. The JSC of the control device, CPTS, PTS and MPTS device calculated by the IPCE curve are 22.49 mA/cm2, 21.94 mA/cm2, 22.71 mA/cm2 and 23.18 mA/cm2. The steady-state efficiency and steady-state current show that for the control device and the MPTS device, the bias voltage is 0.87 and 0.92 V, and the steady-state efficiency is 18.16% and 20.0%, respectively.
The FTIR curve shows that the C≡N peaks of CPTS liquid and CPTS-PbI2 powder are both located at 2245.5 cm-1, indicating that there is no interaction between CPTS and PbI2. The C-H tensile vibration bands near the 2800 cm-1 region between the C-CH3 and O-CH3 of PTS liquid and PTS-PbI2 powder overlap, so it is difficult to distinguish the interaction with PbI2 from FTIR measurements. However, compared with pure PTS, the tensile vibration band of -CH3 in the PTS-PbI2 sample seems to change slightly, which may be attributed to the partial solvation of PTS. It was observed in MPTS-PbI2 powder that the tensile vibration of -SH in MPTS at 2564.8 cm-1 shifted to a lower wave number of 2550.3 cm-1, which was attributed to the strong interaction of -SH in the powder. The peaks at 142.1 and 137.2 eV in the control film are assigned to Pb2+ 4f7/2 and Pb2+ 4f5 /2, respectively, and will not move when CPTS and PTS crosslinking agents are added, but after MPTS is added, they will move in the direction of lower binding energy. The shift to lower binding energy indicates that the oxidation state of lead is reduced, which is due to the electron donor of sulfur, and is in good agreement with the FTIR results. In contrast to the film, metal Pb was clearly observed in the CPTS and PTS films. The two weak peaks are located at 140.3 and 135.5 eV, respectively. In addition, the metal Pb peak intensity of the CPTS film becomes higher, which may be one of the main reasons for the worst PCE of the CPTS device. In contrast, no metal Pb was observed in the MPTS film, which indicates that the formation of metal Pb was completely suppressed. In the XRD curve, all perovskite films show the same main characteristic peaks at 2θ values of 14.2º, 28.4º and 43.0º, indicating that the crystal structure of perovskite is difficult to change by additives. The diffraction peak at 12.9º is attributed to PbI2, indicating that all films have unreacted PbI2. In addition, the intensity of the (110) peak of MPTS was relatively enhanced, showing the improved crystallinity of MPTS.
SEM images and statistics show that compared with the control film, the average grain size of CPTS is reduced from 658 nm to 490 nm, the average grain size of PTS is reduced to 568 nm, and the average grain size of MPTS is increased to 759 nm . This means that in the case of CPTS and PTS additives, the grain growth of the perovskite is not controlled. The presence of MPTS will increase the grain size, because the strong interaction between MPTS and PbI2 can effectively prevent rapid crystallization.
The PL intensity of the MPTS perovskite film on the glass substrate is much higher than that of the control film, while the PL intensity of the perovskite film of PTS and CPTS is slightly lower and much lower. The TRPL curve shows that τ1 = 6 ns (A1 = 38.9%) and τ2 = 826 ns (A2 = 61.1%) of the control film. After adding MPTS, it increases to τ1 = 8 ns (A1 = 38.9%) and τ2 = 1070 ns (A2 = 61.1%). The lifetime close to 1100 ns is close to the value observed from high-efficiency PSC. The dark state IV curve of the perovskite film of FTO/perovskite/Au, the VTFL of the control device, CPTS, PTS and MPTS devices are 0.633 V, 0.777 V, 0.619 V and 0.401 V, respectively. The calculated defect state density It is 1.08 × 1016 cm-3, 1.38 × 1016 cm-3, 1.13 × 1016 cm-3 and 0.65 × 1016 cm-3, and the defect state density is reduced.
At 85ºC and about 2%RH in the dark, the MPTS device exhibits excellent thermal stability, and after aging in the dark at 85ºC for 500 hours, its initial PCE remains 85.9%, while the performance of the control device decreases by 55% the above. The stability of the perovskite device was tested at room temperature and 25-45% RH in the dark. The MPTS device retained 87.2% of its initial PCE after 500 hours, while the control device degraded rapidly, and the device degraded rapidly after 240 hours. The efficiency drops to zero.
In this work, an effective and reproducible multifunctional additive engineering strategy was proposed, in which crosslinking agents with different functional groups were introduced into the PbI2 precursor solution. Among the additives studied, MPTS with the -SH functional group showed the best performance, and at the same time had the three functions of crystallization and stability improvement and defect passivation. As a result, the PCE of the device with MPTS is as high as 20.8%, which is much higher than 18.4% of the control device. This is mainly due to the improvement of crystallinity and carrier lifetime, which reduces non-radiative recombination loss. In addition, greatly improved thermal stability and moisture stability were observed after the addition of MPTS. The study also indirectly proved that iodine vacancies in the perovskite film are dominant. Since defects in perovskites may be caused by missing anions (such as I-) and/or cations (such as Pb2+), the defect passivation should be carefully designed. Anion defects will cause positive vacancies, and cations will cause negative vacancies. Because the electron donating group shows a positive effect, the surface defects are mainly related to the vacancy of the iodide. This work provides important insights for the realization of PSC additive engineering to achieve high efficiency and long-term stability.
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