Theoretical Study On Electron Transporting Performance Of Some Small Organic Materials

As emerging optoelectronic materials,organic-inorganic metal perovskites have excellent performance in thin film solar cells due to their advantages,such as long carrier diffusion length,broad light absorption,high absorption efficiency and low exciton binding energy.In fact,perovskite solar cells(PSCs)are being considered as possible next-generation solar cells for practical applications owing to their simple solution processing,low cost and high power conversion efficiency(PCE).Electron transport materials(ETMs)are one of the important components in highly-efficient PSCs.They play a significant role in extracting and transporting photogenerated electrons,and serve as a hole blocking layer to suppress charge recombination.Generally,the device architecture of PSCs can be classified into normal(n-i-p)or inverted(p-i-n)type devices in terms of what type of materials are placed on the transparent conducting electrode(TCE).For the n-i-p type perovskite photovoltaics,metal oxides(such as Ti O2)are commonly used as ETMs.However,high-temperature annealing is required to prepare metal oxides,which has become a major problem to fabricate solar cells on a large scale for practical applications.In order to solve this problem,inverted PSCs without metal-oxides layer have developed.In the architecture of inverted PSCs,organic ETMs such as phenyl-C61-butyric acid methyl ester(PCBM)were employed.Its morphological instability,however,can significantly reduce the performance of PSCs under annealing conditions.Thus,researchers are committed to developing efficient and low-cost non-fullerene ETMs.Compared to non-fullerene polymeric ETMs,n-type small organic molecules have become the focus of current research due to their advantages of simple structure,high electron mobility and reliable synthetic reproducibility,etc.It has been an issue of general concern in the academic and industrial communities to select the suitable ETMs for commcialization from numerous ones.Occasionally,it often takes a lot of manpower and financial resources to prepare the PSCs,but its PCE may not reach the researchers' expectations.If the structure can be designed theoretically,and then its performance is evaluated,the experimental time would be greatly shortened,thus saving the preparation cost.In this thesis,density functional theory and semi-classical Marcus transfer theory were used to investigate the electron transport properties of a series of small organic molecules.In addition,the interface interaction between ETMs and perovskite surface was also explored by the molecular dynamics simulations.The main research contents are as follows:1.Inverted PSCs have attracted much attention due to their low-temperature and solution-based process.Electron transport layers are important components in inverted PSCs.Compared to fullerene PCBM,non-fullerene n-type organic small molecules seem to be more attractive as electron transport layers,because their structures are easy to be synthesized and modified.In this work,density functional theory and semi-classical Marcus electron transfer theory were used to explore the electron transport properties in three azaacene derivatives,including one experimentally reported molecule,1,4,9,16-tetrakis((triisopropylsilyl)ethynyl)quinoxalino[2''',3''':4'',5'']cyclopenta[1'',2'',3'':5',6']acenaphtho[1',2':5,6]pyrazino[2,3-b]phenazine(1),and two theoretically designed molecules(2 and 3).Compound 2 is formed by substituting i-Pr groups in compound 1 with H atoms,which is designed to evaluate the effect of i-Pr groups on the electron transport properties.Compound 3 is designed by adding one more benzopyrazine group to the conjugation structure of compound 1.It shows that i-Pr group can increase HOMO and LUMO energy levels and improve solubility in organic solvent and hydrophobicity.Enlarging conjugation can not only decrease LUMO energy level and electron reorganization energy,but also can increase solubility and electron mobility.So our designed compound 3 is expected to be a potential ETM in inverted PSCs.2.Density functional theory and Marcus electron transfer theory were used to explore the electron transporting properties of a series of mono-halogenated perylene diimides X-PDI(X = F,Cl,Br),as well as the parent compound H-PDI.Besides studying the electronic structures,absorption spectra,electron mobility,hydrophobicity and soluability of these molecules,the adsorption property of Br-PDI adsorbed on CH3NH3 Pb I3(110)surface system was also considered.It is found that the theoretically simulated absorption spectra reproduce very well the experimentally available data.The predicted electron mobilities follow the order of F-PDI < H-PDI < Cl-PDI ? Br-PDI,which is in accordance with the experimental trend.Furthermore,our theoretically designed molecule Cl-PDI exhibits good hydrophobicity,stability and solubility,as well as similar electron mobility as Br-PDI.Considering that Br-PDI has been observed to be a good ETM in inverted PSCs,Cl-PDI is also expected to be a potential ETM.3.Density functional theory and semi-classical Marcus electron transfer theory were employed to investigate the electron transport properties of a sulfur-containing azaacene molecule 10,14-bis(5-(2-ethylhexyl)thiophen-2-yl)dipyrido[3,2-a:20,30-c][1,2,5]thiadiazolo[3,4-i]phenazine(TDTP),which has been proved to have high performance as an ETM in inverted PSCs in previous experiments.By changing thiophene rings in both side chains to thiazole/benzene rings,two new compounds TDTP-I and TDTP-II were designed.The electron transport properties of these three molecules were evaluated in terms of the frontier molecular orbitals,absorption spectra,electron mobility,soluability and stablity.It has been found that both new compounds have lower HOMO and LUMO energy levels and higher electron mobility than TDTP.In particular,TDTP-II has the highest electron mobility among the three.Furthermore,the adsorption properties of TDTP/TDTP-II adsorbed on CH3NH3 Pb I3(110)surface systems were also studied.It has been shown that TDTP-II has a more negative adsorption energy than TDTP.Different from the above two systems,we also calculated the Hansen solubility parameters and compared the solubility of the stuied molecules in chlorobenzene.It shows that the newly designed molecule TDTP-II has the highest solubility.So our designed compound TDTP-II is expected to be a potential electron transport material in inverted PSCs and changing the side chains could be a feasible way to improve the electron transport properties.