Research On Improving The Performance Of Perovskite Solar Cells By Interface Modification Between Electron Transport Layer And Perovskite Layer

As a representative of new solar cells,the organic-inorganic hybrid perovskite solar cells have attracted much attention in the past decade.The photoelectric conversion efficiency(PCE)of perovskite solar cells(PSCs)has dramatically increased from 3.8%in 2009 to 25.5%in 2020.The rapidly development of PSCs benefits from the excellent photoelectric properties and the convenience of solution processing of perovskite materials.The excellent photoelectric properties of perovskite materials include a long charge-carrier lifetime and diffusion length,dual carrier transmission characteristics,a high absorption coefficient in the visible and near-infrared region,a low exciton binding energy,and high defect tolerance.The development of PSCs is unprecedented,however,the commercial application of PSCs still faces many challenges,such as poor device stability and serious hysteresis effect.In the regular planar-structure PSCs,the heterojunction interface between electron transport layer(ETL)and perovskite layer is related to the extraction,transportation and recombination of charge carriers,which has a significant influence on the performance of PSCs.In this work,we focus on the ETL/perovskite layer interface of regular planar-structure PSCs.The interfacial modification strategy is used to improve the crystal quality of perovskite thin films,passivate the interface defects and reduce the interface carrier recombination.Finally,the interfacial modification strategy significantly improves the PCE and stability of regular planar-structure PSCs and the hysteresis effect is suppressed.The main contents of this thesis includes the following aspects:In order to solve the problem of interface defects induced carrier recombination between the ETL and perovskite layer,the polymethyl methacrylate(PMMA):fullerene derivatives(PCBM)mixed passivation layer are used to passivate the stannic oxide(Sn O₂)ETL/perovskite interface.The passivation/conductivity balance is adjusted by adjusting the ratio of PMMA and PCBM,by utilizing the synergistic effect of PMMA:PCBM to achieve the optimal PCE.The result shows that the mixed PMMA:PCBM passivation layer does not change the crystal structure of perovskite,but can passivate the interface defects effectively,inhibit the interface carrier recombination,thus improving the open circuit voltage(V_oc).The passivation effect is controlled by optimizing the quality ratio of PMMA to PCBM.When the ratio of PMMA:PCBM is1:2(mass ratio),the PSCs exhibit the champion PCE.Compared with the unpassivated devices,the V_oc of PSCs increases from 1.13 V to 1.17 V,the PCE increases from 17.32%to 18.63%,and the hysteresis effect of the passivated devices can be ignored.In addition,the air stability of the PMMA:PCBM passivated device is significantly improved.The PCE of the unencapsulated device can maintain 82%of its initial efficiency after 768 h at room temperature,while the PCE of the unpassivated device decreases significantly after 360 h.The small grain size caused by fast nucleation of perovskite,the low matching degree of energy level between Sn O₂ and perovskite,and the serious carrier recombination are common in PSCs.In this work,the methylammonium chloride(MACl)is used as an interface layer to modificate the Sn O₂/perovskite interface.The results demonstrate that the MACl plays the role of auxiliary crystallization by inducing perovskite nucleation,which increasing the grain size of perovskite,thus reducing the number of grain boundaries and improving the short-circuit current density(J_sc)of PSCs.Meanwhile,the MACl interface layer can adjust the conduction band of Sn O₂,and the energy level of the modified Sn O₂is matching well with perovskite,thus accelerating the transportation of carriers and improving the V_oc of devices.In addition,the Cl^-can effectively passivate the anti-position defect of Sn O₂ ETL/perovskite interface,which can inhibit carrier recombination of the Sn O₂/perovskite interface.The passivation effect is adjusted by optimizing the concentration of MACl.When the MACl concentration is 3 mg/m L,the PSCs exhibit the champion PCE.After modification,the PCE of the PSCs increases from 17.46%to 19.20%with a negligible hysteresis effect.The performance improvement of the PSCs mainly attributes to the significant increase of V_oc and J_sc,the V_oc increases from 1.13 V to 1.19 V,and the J_scincreases from 22.44 m A/cm² to 23.99 m A/cm².Moreover,the air stability of the MACl modified device is significantly increased.Without any encapsulation,the device retained 82%of its initial efficiency after 1000 h in the air.Under the same storage conditions,the PCE of unmodified device decreases rapidly after 720 h,retaining only54%of its initial efficiency.In order to solve the problem of carrier recombination caused by oxygen vacancy defects in Sn O₂ and interface defects,we used ethanolamine(EA)interface layer to passivate the Sn O₂ ETL/perovskite layer interface.The results demonstrate that the uncoordinated Sn atom in Sn O₂ can combine with the hydroxyl group in EA molecule to form the Lewis base adduct to reduce the number of oxygen vacancy defect of Sn O₂surface,thus inhibiting the interface carrier recombination.Moreover,the terminal amine group in EA can passivate the uncoordinated Pb²⁺defects between Sn O₂ and perovskite interface,thus reducing the non-radiative recombination of carriers.At the same time,the conduction band minimum of the EA-modified Sn O₂ downshifts,which can better match the energy level of perovskite layer,thus ensuring more efficient electron transport and minimizing the energy loss at the ETL/perovskite interface.By adjusting the content of EA in 2-methoxy ethanol,we can confirm the optimal passivation effect.When the concentration of EA is 2%,the PSCs exhibit the champion PCE.After EA modification,the V_oc increases from 1.10 V to 1.13 V,the J_sc increases from 22.61 m A/cm² to 23.22 m A/cm²,the FF increases from 73.05%to 77.62%,and the PCE increases from 18.17%to 20.44%.Meanwhile,the hysteresis effect of devices is significant reduced after passivation.In addition,the air stability of EA-modified devices is significantly improved.Without any encapsulation,the device retained 78%of its initial efficiency after 1000 h storage in air.The unmodified device retained only55%of its initial efficiency after 760 h under the same storage conditions.In conclusion,this thesis focuses on the interface of ETL and the perovskite layer.The PMMA:PCBM interface passivation,MACl interface modification and the EA interface passivation strategies are applied to enhance the PCE and stability of the regular planar-structure PSCs.This research provides a new research idea for promoting the development of efficient and stable PSCs.