Synthesis Of D-A-D Type Organic Small Molecule Hole Transport Materials And The Application In Perovskite Solar Cells

As the third-generation solar cell technology,the organic-inorganic hybrid perovskite solar cells(PSCs)have great potential to solve the energy shortage issue due to their impressive photovoltaic performance and low production cost.PSCs have gone through an unprecedented evolution over the past decade,and the power conversion efficiency(PCE)increases rapidly from 3.8%to certified 25.5%.Hole transport materials(HTMs)are essential in most of the highly efficient PSCs,which play an important role in promoting hole extraction and hole transporting,meanwhile protecting perovskite from the adverse effects of external factors such as moisture and oxygen.Therefore,the rational design of HTMs is very crucial for boosting the efficiency and longevity of PSCs.At present,2,2?,7,7?-tetrakis(N,N?-di-p-methoxyphenylamine)-9,9?-spirobifluorene(spiro-OMe TAD)is the most popular HTM.However,the complexity in synthesizing and purifying of spiro-OMe TAD is still the main problem,which usually involves a low-temperature reaction(-78 ^oC)with highly active reagent butyllithium.The harsh preparation conditions make spiro-OMe TAD uncompetitive.Meanwhile,due to the poor conductivity of pristine spiro-OMe TAD,dopants such as tert-butylpyridine(t-BP)and lithium bis-(trifluoromethanesulfonyl)imide(Li-TFSI)are usually added to improve its conductivity and hole mobility.However,this will increase the total cost of device fabrication and hinder its further commercialization.Therefore,developing high efficiency and low-cost HTMs has become an important topic for PSCs research.When designing HTMs,hole mobility is one of the important considerations.D(donor)-A(receptor)-D(donor)type HTMs usually have higher dipole moment and strong intermolecular interaction,which is conducive to intermolecular charge transfer,allowing HTMs to obtain high hole mobility.In this dissertation,through the rational design of the molecular structure,a series of D-A-D type organic small molecule HTMs were synthesized via mild reaction conditions and applied in PSCs.Moreover,the relationship between HTMs molecular structure and their performance was explored in the work.The main content of this paper is as follows:1.A new D-A-D type dopant-free HTM F23 was synthesized,which could be processed with the nonhalogenated and environmental-friendly green solvent tetrahydrofuran(THF).To have an insight of the effect that the different position of the peripheral group on core moiety to the HTMs performance,F23 and its isomer F22 were compared and studied.The results show that the carbazol-diphenylamine group was modified on different positions on bipyridine moiety,making F22 and F23 show different molecular configurations,charge delocalization,and energy levels.The results of density functional theory(DFT)calculations indicate,compared with F22,F23 has smaller reorganization energy.The hole mobility of F22 and F23 films was carried out by the space-charge-limited current(SCLC)method,and it was found that F23 films have higher hole mobility(1.18×10^-4 cm² V^-1 s^-1,undoped),which guarantees F23 as a potential dopant-free HTM.More interestingly,the non-halogen green solvent THF is successfully applied to prepare F23 film and more uniform morphology is obtained in comparison with F22 film spin-coated from THF.The steady-state photoluminescence(PL)spectra and time-resolved photoluminescence(TRPL)measurements reveal that F23 has superior hole extraction capacity,meaning the faster hole collection at the perovskite/F23 interface relative to the perovskite/F22 interface.Therefore,the PCE of the device based on F23achieves 17.6%.Besides,the process requires only a small amount of F23(3 mg/m L),thus further lowering the costs in device fabrication.Moreover,the device also exhibits good stability due to the absence of hygroscopic dopants.2.A single aromatic heterocycle pyrazine has been employed as a core group to connect two electron-donating groups to construct new D-A-D type HTMs with twist structures(denoted as 3,6-DMPZ and 2,7-DMPZ),which could be synthesized in mild reaction conditions with high yield(over 60%).Moreover,in terms of molecular structure,the electron-withdrawing property of the pyrazine group could increase molecular polarity,enhance intermolecular interactions,thereby promoting charge transfer,affording high hole mobility.It was found that the highly distorted molecular structure of3,6-DMPZ and 2,7-DMPZ make them exhibit excellent thermal stability.The isomerization of peripheral groups of 3,6-DMPZ and 2,7-DMPZ molecules results in differences in energy levels,surface morphology,and hole transport properties.The SCLC experiments indicate the 2,7-DMPZ film has higher hole mobility in comparison with the 3,6-DMPZ film.The AFM and SEM tests reveal that 2,7-DMPZ films get a smoother and uniform surface morphology.The steady-state PL experiment shows that2,7-DMPZ thin films have better hole extraction ability.The results of transient photocurrent(TPC)decay,transient photovoltage(TPV)decay,and electrochemical impedance spectroscopy(EIS)measurements indicate 2,7-DMPZ devices have a more efficient hole-transporting property,and less carrier recombination compared to the 3,6-DMPZ devices.Therefore,2,7-DMPZ-based devices gain an impressive PCE of 19.61%.