Design and Synthesis of Novel Polythiophene Derivatives for Polymer/perovskite Solar Cells

Author:Jia Xiao E

Supervisor:huang fei


Degree Year:2019





Among the emerging photovoltaic techniques,polymer solar cells and perovskite solar cells have attracted extensive research due to their unique merits such as light weight,lowtemperature solution processing and large-area fabrication,etc.In recent years,benefiting from the new molecular design,interface engineering,and device architecture innovation,the power conversion efficiencies(PCEs)of these two photovoltaic techniques have been improved rapidly.Polythiophene derivatives are widely used in polymer solar cells and perovskite solar cells owing to their simple structure,easy synthesis,excellent hole transport mobility,and compatibility with thick film processing,etc.In this thesis,we designed and synthesized a series of novel polythiophene derivatives,the relationship between chemical structures and photoelectric properties of the resulting polymers were investigated,and further applied them to polymer solar cells and perovskite solar cells.In the second chapter,we designed and synthesized three polythiophene derivatives,two fluorine atoms are introduced onto each repeat unit of the polymers to down-shift the HOMO levels and to tune aggregation properties.Different alkyl side chains were used to tune the solubility and processability of the polymers.Compared with poly(3-hexylthiophene),the HOMO energy levels of the newly synthesized three polythiophene derivatives are reduced by 0.4 eV.P4F2F-HD offered best solubility and processable at room temperature.The new polythiophene derivatives all produced PCE over 5% when blended with fullerene acceptor and PCE over 6% when blended with non-fullerene acceptor.Among the three polymers,P4T2 FHD exhibited the best photovoltaic performance with PCE of 6.3% and 7.0% after blending with fullerene acceptor and non-fullerene acceptor,respectively.In addition,the device based on P4T2F-HD and fullerene acceptor blend shows a desirable independence on the thickness variation of the active layer.In the thickness range from 200 to 320 nm,the average PCEs are almost unchanged,which is conducive to thick film processing.In the third chapter,we designed and synthesized four polythiophene derivatives with various fluorine content by changing the ratios between 3,3’-difluoro-2,2’-bithiophene unit and 2,2’-bithiophene unit,in order to investigate the effect of different backbone fluorination on the photoelectric properties of the resulting polymers.All fluorinated polymers exhibit typical temperature-dependent aggregation behavior compared to non-fluorinated polymer.The crystallinity,absorption coefficient,hole transport mobility,and relative dielectric constant of the polymers are significantly improved with the increasing of fluorine content.After blending with the non-fullerene acceptor,the photoluminescence quenching efficiency,charge carrier mobility,and relative dielectric constant enhancement with the fluorine content.Along with the amount of fluorine atoms in polymer backbone,the charge recombination can be effectively suppressed and the charge carrier lifetime extended remarkably.At the same time,the phase separation of the active layer improved significantly,which promotes the short-circuit current density and fill factor.Finally,the polymer with the highest fluorine content,P6T-F100,achieved the best device performance with a PCE of 7.3%.In the fourth chapter,We designed and synthesized three polythiophene derivatives with ether chains as side chains,and adjusted the HOMO levels of the polymers by changing the relative position of the ether chain to the thiophene backbone.As a result,the HOMO levels of the newly synthesized three polythiophene derivatives are-4.58,-4.88 and-4.74 eV,respectively.Although the shallower HOMO levels of these three polymers are not suitable for the hole transport layer materials in perovskite solar cells,this work still provides a reference for the materials design in the future.In the fifth chapter,we designed and synthesized six polymers with indacenodithieno[3,2-b]thiophene as copolymerization unit,and adjusted the HOMO levels of the polymers by changing the triphenylamine,carbazole and fluorocarbazole unit.As a result,after copolymerization with triphenylamine,carbazole and fluorocarbazole unit,the HOMO levels of the resulting polymers gradually decreased from-5.28 to-5.37 eV.According to the results of density functional theory calculation,the planarity of the polymers copolymerized with the triphenylamine unit,carbazole unit and fluorocarbazole unit are sequentially increased.After fabricating the n-i-p type planar perovskite solar cell devices,the device based on P6 achieved a PCE up to 19.4% with a fill factor of 0.82.