Theoretical Study On Properties Of Some Small Organic Hole Transport Materials

Pollution and energy shortage are two serious problems in the world.The development and utilization of renewable energy sources are one of the most efficient methods to resolve the two problems.Solar energy is the most ideal energy resource due to its wide distribution,rich reserves,clean and pollution-free.Solar cells are key devices that convert the solar energy into electrical energy by the photovoltaic effect.Therefore,the development of high-efficiency and low-cost solar cells is one of the most efficient pathways to utilize solar energy.The perovskite solar cells（PSCs）have attracted continuous attention,which is attributed to its dramatically improved power conversion efficiency（PCE）since 2009.As one of the most important components of PSCs,the hole transport material（HTM）plays a key role in transporting holes and avoiding charge recombination in PSCs.Therefore,improvement of HTM properties is also beneficial to refining the PCE of perovskite solar cells.2,2’,7,7’-tetrakis[N,N-bis（4-methoxyphenyl）-amino]-9,9’-spirobifluorene（Spiro-OMeTAD）is the most state-of-the-art HTM with the highest PCE for PSCs.Moreover,it would be helpful to improve the stability of PSCs.However,the low hole mobility,complicated synthesis process,expensive price as well as other disadvantages,have prohibited its applications in large-scale.The exploration of the alternative HTMs with comparable device performance and low cost is highly desirable in the development of PSCs.The organic small molecules are a powerful candidate to be HTMs as compared with other HTMs.To select and/or develop suitable and high efficiency one is still a difficult task from numerous organic molecules.It is impossible to synthesize and test one by one in experiment.No doubt,the computer-assisted design would save time and cost as compared with experimental trial and error,which is based on the accurate and reliable structure-activity relationship.In this thesis,the properties of a series of organic molecules are studied by first principles and molecular dynamics.The possibility of them to be HTMs in PSCs is evaluated with the ultimate goal to construct a pathway from theoretical design to experimental synthesis and final application.The following three main contents are included:1.Three new HTMs,CBP-OD,CBP-2OD,and CBP-3OD,are designed on the basis of CBP and2TPA-2-DP with different number of olefinic bonds.Our aim is to explore the influence of differentπ-conjugation levels and different molecular packing models on hole transport property.The frontier molecular orbitals,absorption spectrum,exciton binding energy,hole mobility,and molecular stability are studied by the first principle and molecular dynamics associated with the Marcus theory and Einstein-Smoluchowski relation.The highest occupied molecular orbitals（HOMO）levels of CBP-OD,CBP-2OD and CBP-3OD are between that of 2TPA-2-DP and that of CBP.As compared with CBP,their maximum absorption wavelengths are red-shifted.In contrast,their maximum absorption wavelengths are blue-shifted as compared to 2TPA-2-DP.Their reorganization energies are all larger than that of CBP and2TPA-2-DP,which is not favorable to hole transport.However,they have the smaller exciton binding energies,which is beneficial for the separation of holes and electrons.The hole mobility of CBP-2OD and CBP-3OD is similar or smaller than that of CBP.In contrast,all three molecules have the much larger hole mobility than that of 2TPA-2-DP.And their stability is better than 2TPA-2-DP but slightly worse than CBP.The HOMO level is destabilized and the maximum absorption wavelength is red-shifted with the increase of olefinic bonds.However,the exciton binding energy,hole mobility,and stability all reduce with different degrees.CBP-OD has the suitable HOMO level,maximum hole mobility,and good stability,which could be an excellent HTM.Only elongation ofπ-conjugation extent is not a smart method to improve the hole mobility for HTMs.The suitableπ-conjugation group and packing model is efficient to improve the performance.2.Based on the hole transport materials FDT and PEH-2,a new hole transport material,SNE,is designed.Their properties are investigated by combination of molecular dynamics and first principle calculations.Besides the isolated HTM molecules,the interfacial properties of HTMs adsorbed on CH₃NH₃PbI₃（110）surface are firstly investigated in detail to evaluate the HTM performance.The performance of isolated HTM molecules is evaluated from aspects of frontier molecular orbitals,absorption spectrum,hole mobility,stability,and solubility.The performance of HTM-CH₃NH₃PbI₃ adsorption system is analyzed according to the density of states.Before adsorption,the HOMO energy levels of three molecules are higher than valence band of CH₃NH₃PbI₃ to ensure efficient hole transport.The maximum absorption wavelengths of both FDT and SNE are in the ultraviolet region,which is favorable for the light absorption efficiency of PSCs.The hole mobility of SNE is larger than those of FDT and PEH-2.After adsorption,not only the band gap of CH₃NH₃PbI₃（110）surface but also the energy levels of HTMs are varied.It is necessary to consider the adsorption system to determine the properties of HTMs.The band gap of SNE is decreased leading to the better light harvesting ability after it is adsorbed on CH₃NH₃PbI₃.The energy difference between HOMO energy level of SNE and valence band of CH₃NH₃PbI₃ is smaller than those for FDT and PEH-2.The smaller energy difference is beneficial for improving the open-circuit voltage of PSCs.In addition,SNE has acceptable stability and hydrophobicity as compared with FDT and PEH-2,which would effectively extend the lifetime of PSCs.The lower production cost would be expected if SNE is employed as HTM,since it has good solubility in various common organic solvents.Finally,the proposed synthetic route is recommended for SNE,which would be completed in the benign condition.In general,SNE would be a potential hole transport material with high hole transport efficiency and low synthesis cost.3.Based on the hole transport material X25,three carbazole hole transport materials,H5,H6,and H7,are designed to explore the effect of different substituents on the hole transport properties.The performance of HTMs are evaluated by the frontier molecular orbitals,absorption spectrum,hole mobility,and the interfacial properties between HTM and CH₃NH₃PbI₃（110）.The HOMO energy levels of all four molecules are higher than valence band of CH₃NH₃PbI₃ resulting in the efficient hole transport.As compared with X25,the maximum absorption wavelength of H5 is red-shift and those of H6 and H7 are blue-shift.The smallest reorganization energy of H7 along with the second smallest centriod to centriod distance results in the largest hole mobility.After adsorption,the band gap of H7 is the smallest,which is more conducive to improving light harvesting ability of PSCs.Moreover,the difference between the HOMO level of H7 and the valence band of CH₃NH₃PbI₃（110）surface is more appropriate,which is beneficial for the hole injection.It should be noted that the side groups in H5,H6,and H7 have been employed as side groups to constitute molecules applied in some relevant regions,which would provide an alternative pathway to develop new HTM rather than synthesis of new groups.