Research On Interfacial Modification And Stability Of Perovskite Solar Cells

Organic-inorganic hybrid halide perovskites have come into prominence as excellent light absorbers for emerging solar cells due to their low exciton binding energy,long carrier diffusion length,wide absorption range and tunable bandgap.During the past few years,along with the persistent optimization of perovskite compositions and device structures,the certified power conversion efficiency(PCE)of halide perovskite solar cells(PSCs)have rocketed from 3.8%to 25.5%,approaching the efficiency record of crystalline silicon solar cells and other inorganic semiconductor solar cells.The interfaces in PSCs are closely associated with interfacial carrier dynamics(including carrier separation,transport,injection,collection,and recombination)and accordingly play a decisive role in the PCE and stability of PSCs.The interface defects,interfacial reactions and imperfect energy level alignment are conductive for interfacial non-radiative recombination loss,which is detrimental to the PCE and stability of devices.Interfacial modification is an effective way to reduce the interfacial non-radiative recombination,including interface defect passivation,suppressing of interface reactions and optimizing of interface energy levels.Herein,this thesis focuses on the modification of the interface associated with perovskite layer,so as to improve the device photovoltaic performance and operational stability.The specific works are listed as follows:(1)Titanium dioxide(TiO₂)is widely used as electron transport layer in regular planar PSCs.However,TiO₂ contains a high surface defect density and intrinsically low mobility.Moreover,in the presence of ultraviolet light,some trap sites are caused by oxygen vacancies at the interface of TiO₂/CH₃NH₃PbI₃.In particular,the ultraviolet light induced holes in TiO₂ can directly interact with halide ions,resulting in the severe decomposition of CH₃NH₃PbI₃ light absorbers,thus limiting the photovoltaic performance and stability of devices.Silicon oxide(SiO_x)was used to modify the interface between TiO₂ and perovskite layer.The SiO_x modification passivated the defect of TiO₂ film,leading to the significantly decreased charge recombination,thereby improving the electron extraction and transport at the interface.Devices with SiO_x coated-TiO₂ layer showed a competitive PCE of 18%,with an open-circuit voltage(V_oc)of 1.06 V,a short-circuit current(J_sc)of24.0 mA cm^-2 and a fill factor(FF)of 71.0%.However,the PCE of the control device with TiO₂ layer.is 15%lower than that of the cell with SiO_x coated-TiO₂ layer.Moreover,the devices with SiO_x modifier showed a significantly improved ultraviolet light soaking stability,owing to the decreased photocatalytic capability of SiO_x modified TiO₂.(2)The solution-processed perovskite films are polycrystalline in nature and inevitably introduce a large amount of defects into the surface and grain boundaries of perovskite films.It is well known that the defects are vulnerable to trap-assisted recombination,thereby resulting in severe energy loss in PSCs.Meanwhile,moisture and oxygen will infiltrate into the perovskite from the defective sites to jeopardize the device stability in ambient environment.By introducing a small molecule of 3-picolinic acid(NA)into perovskite precursor,the perovskite films showed larger grain size and excellent crystallinity.The interaction of carboxyl groups in NA with Pb²⁺is verified by the Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)analysis,as well as supported by the density function theory(DFT)calculations.The introduce of NA could control the crystal growth process and suppress the defects on the surface and grain of perovskite films,thereby suppressing carrier recombination.As a result,niacin-incorporated PSCs yield a competitive PCE of 19.52%,with significantly improved humidity and ultraviolet light soaking stability.(3)A cost-effective and environmental friendly organic molecule(KA)was further used to modify the surface of perovskite films.The results of XPS and FTIR revealed that KA could form coordination bonds with Pb²⁺through Lewis acid-base reaction due to its rich lone pair electrons,thus effectively passivating the surface defects of perovskite film and the defect density decreased from 2.66×10¹⁶ cm^-3 to 2.01×10¹⁶ cm^-3,thereby reducing the carrier non-radiative recombination at the interface.As a result,the PCE of the device modified by KA increased from 18.75%to 20.44%.In addition,the humidity stability of the device is improved due to effectively passivated defects.(4)Sodium p-hydroxyl benzene sulfonate(PHS)was utilized to modify the surface of perovskite film.The XPS results showed the organic functional groups(-SO₃^-/-OH)and alkali metal ions(Na⁺)in PHS could bond with the Pb²⁺and Br^-/I^-halogen ions in perovskite film,respectively.Thus,PHS could simultaneously passivate the positive and negative charge defects on the the surface and grain of perovskite films,which effectively suppressed the non-radiative recombination at the interface and the carrier mobility increased from 0.83×10^-3 cm² V^-1s^-1 to 1.40×10^-3 cm² V^-1s^-1.Finally,the device modified by PHS obtained a PCE of 20.60%,with an open-circuit voltage(V_oc)of1.14 V,a short-circuit current(J_sc)of 23.73 mA cm^-2 and a fill factor(FF)of 76.15%.Moreover,the devices modified by PHS exhibited outstanding thermal stability.