Applications Of Multi-functional Small Molecule Organic Semiconductors In Perovskite Solar Cells

Perovskite solar cells（PSCs）have attracted much attention due to the outstanding features of perovskite materials,such as long carrier diffusion length,low exciton binding energy,and good solution processability.Since 2009,the power conversion efficiency（PCE）of PSCs has dramatically increased from the initial 3.8%to the current highest value of 25.7%.The rapid improvement of PCE makes PSCs have the potential to be commercialized,but the energy loss and performance degradation of the devices hinder the process of commercialization.In order to accelerate the commercialization process of PSCs,this paper attempts to provide easy and effective solutions to some problems existing in devices.As is known to all,uncoordinated Pb²⁺in perovskite is a defect that seriously affects the device performance,and the non-radiative recombination of carrier caused by it is the internal reason of the device efficiency is not ideal.Therefore,the passivation of Pb²⁺defects is the main research content of this paper.It has been reported that uncoordinated Pb²⁺is mainly formed on the surface of perovskite and at grain boundaries.In addition,Li⁺migration in spiro-OMe TAD dopant Li TFSI is also a factor leading to device performance degradation and hysteresis effect,so the inhibition of Li⁺migration is also part of our research content.Based on the sources of Li⁺and uncoordinated Pb²⁺,interface engineering and dopant engineering will be the most direct and effective methods.For passivation Pb²⁺defects and inhibition of Li⁺migration,we through the literature research attention to the crown ether ring compounds.It has been reported that there are interactions between crown ether and metal ions such as Pb²⁺,and the appropriate size of crown ether has complexation with Li⁺.Therefore,selecting appropriate crown ether to introduce into perovskite surface and doping into HTL at the same time will be an effective way to passivate Pb²⁺defects and inhibit Li⁺migration.In addition,the interaction between crown ether and Pb²⁺can be further enhanced by introducing heteroatoms into crown Ether Ring,which is the basis of further strengthening Pb²⁺defect passivation.However,due to its low freezing point at room temperature and the fact that the molecule itself is not Organic semiconductor,crown ethers may bring some negative effects when applied to PSCs.Therefore,we combine crown ether with carbazole diphenylamine to make the whole molecule an organic semiconductor,which can avoid the potential adverse effects of crown ether on the device and promote the extraction and transmission of holes.Finally,a series of small molecule organic semiconductors designed and synthesized were used in PSCs,effectively improving the performance and stability of the devices.The results are as follows:（1）According to reports,crown ethers can interact with Pb²⁺,and the 12-crown-4 in crown ethers can also chelate with Li⁺due to its suitable size,aiming to simultaneously passivate Pb²⁺defects and inhibit Li⁺migration.We designed and synthesized organic semiconductor CDTs containing 12-crown-4 and applied them to PSCs.Structurally,a12-crown-4 is attached to the carbazole diphenylamine to obtain the CDT molecule.Subsequently,CDT was introduced into the perovskite surface by anti-solvent method and doped into HTL.Through investigation,it was found that CDT plays multifunctional roles in both perovskite and HTL.Firstly,the introduction of12-crown-4 in perovskite is able to improve the quality of perovskite crystal and meanwhile to passivate the under coordinated Pb²⁺defect through the Lewis acid–base interaction.In HTL,12-Crown-4 has a strong“host–guest”interaction with Li⁺through the ion–dipole interaction,in this case,the crowned Li⁺be difficult to migrate in PSCs.In addition,the carbazole diphenylamine group in the CDT molecule can enhance the hole transport performance of HTL.Finally,the hydrophobicity of CDT endows HTL and perovskite films with good moisture stability,which is conducive to device stability.Due to these improvements,the introduction of CDT into perovskite and HTL improves the PCE and long-term stability of the device.After CDT treatment,the device achieved a high efficiency of 22.88%,and maintained an initial efficiency of 95.2%after 1000hours.（2）Through the first work of this thesis,we found that the passivation effect of12-crown-4 on Pb²⁺defect is not strong.Therefore,this work aims to enhance the passivation of Pb²⁺defects and maximize device performance and stability.By investigating the literature,we found that introducing heteroatoms into crown ethers and enlarging the ring size of crown ethers can increase the interaction with Pb²⁺.To sum up,in this work,we designed and synthesized Organic semiconductor CDT2 and CDT3containing 15-crown-5 heterocyclic crown ethers and introduced them into the perovskite layer by anti-solvent method,the defect is passivated by the interaction of heterocyclic crown ethers on the Organic semiconductor with uncoordinated Pb²⁺.Among them,CDT2 containing heterocyclic crown ether of S atom interacts more strongly with Pb²⁺than CDT3 molecule,which makes CDT2 have stronger defect passivation ability than CDT3.Less uncoordinated Pb²⁺means less carrier non-radiative recombination,which is beneficial to the device to obtain higher efficiency.The results of PL and TRPL tests show that the perovskite films containing CDT2 have higher PL intensity and longer average carrier lifetime,which is consistent with the trap density test.In addition,CDT2 and CDT3 as organic semiconductors,increase the water contact angle of the perovskite film after being covered.Based on these improvements,the efficiency of devices containing CDT2 and CDT3 is 23.05%and 21.97%,respectively,while the efficiency of control devices is only 21.17%.In addition,device containing CDT2 maintained initial efficiencies of 90.5%after 1000 hours in an air environment（25℃,RH≈30%）,showing good long-term stability.