Application Of D-A-D Type Organic Small Molecules In Perovskite Solar Cells

To reduce the cost of solar systems,researchers have been working hard to improve the power conversion efficiency(PCE)of solar cells.Perovskite solar cells(PSCs),as a "new star" in the photovoltaic industry,have received much attention in recent years.In just more than one decade,they have been extensively studied,and their PCE has rapidly increased from the initial 3.8%to 25.7%.However,there are still many factors hindering the commercialization of PSCs.According to the Shockley-Queisser limit theory,the PCE of PSCs still has a large range for improvement.At the same time,the stability of PSCs is not enough to meet their commercial application.In this thesis,on the basis of n-i-p structure PSCs,D-A-D type small molecules are introduced to modify the perovskite active layer.On the one hand,the electron-rich central core in small molecules can passivate defects in perovskite and inhibit non-radiative recombination.On the other hand,the introduction of small molecules with appropriate energy levels can lead to a more appropriate alignment of energy levels between layers,thus reducing the energy level gradient and energy loss.Further discussing the regulation mechanism provides a way to prepare efficient and stable PSCs.The research content mainly includes the following aspects.One organic molecule coded as 4-BA with appropriate energy level and hole transport property was synthesized with bipyridine as the central core and methoxy triphenylamine as the terminal group.And it was used as a buffer layer between the perovskite layer and hole transport layer(HTL).Calculation based on the density functional density and energy level test shows that the 4-BA molecule has a suitable energy level and appropriate passivation site.While passivating the perovskite layer,the energy level alignment can be constructed between perovskite layers,and the benzene ring forms π-π stack with Spiro-OMeTAD,which improves the carrier transport characteristics of the perovskite and thus improves the device performance of PSCs.When 4-BA is covered on the surface of perovskite,it can reduce the surface roughness of perovskite and promote light absorption.Moreover,the electron-rich N atoms in 4-BA molecules can interact with the uncoordinated Pb2+ in perovskite,thus playing a role in passivation of defects.At the optimized concentration,the maximum efficiency is 22.4%,which is significantly higher than that of pristine perovskite devices(20.2%),and the open-circuit voltage and filling factor(FF)is significantly improved.The improvement of device performance is attributed to effective defect passivation and reduction of energy level barrier.After aging,the efficiency of the devices containing 4-BA remains at 81%of its initial value,while the pristine PSCs remains at 53%of its initial value.The coating of 4-BA on perovskite film can increase the hydrophobicity of the film.A molecule coded as DBT is synthesized by a two-step reaction with sulfone as the central group and methyoxy triphenylamine as the terminal group.The theoretical calculation results show that the electron-rich region of the DBT is mainly concentrated on the oxygen atom of the sulfone,and it has a large dipole moment(μ=12.49 Debye)and a suitable energy level.When it is introduced into the perovskite/HTL,on the one hand,DBT with a large dipole moment is arranged in order,thus enhancing the internal potential;On the other hand,the introduction of the DBT can reduce the energy level barrier and form a suitable energy level alignment,thus promoting the effective carrier transport.The introduction of DBT molecule can reduce the surface roughness and form a better surface morphology.At the same time,the DBT can interact with uncoordinated Pb2+in perovskite,passivating lead ion defects and inhibiting the production of Pb0 on the perovskite surface.The molecule can also inhibit the conversion of perovskite from black phase to yellow phase.The efficiency of the DBT applied to perovskite device is 21.6%,higher than that of original perovskite(19.70%).After aging,the stability was improved from 61%to 85%.The perovskite/DBT film has a large water contact angle,which can resist water erosion,thus improving the stability of the device.