Studies On Preparation Of Optoelectronic Polymer Nanofibers And Organic Solar Cells

Organic photovoltaics?OPVs?have gained considerable attention due to their light weight,low-cost processing and adaptability to roll-to-roll large area fabrication.Their short-circuit current densities are primarily determined by the quality of charge transport in the active layers.It is well known that the nanofibers of optoelectronic polymers are useful in building effective charge transport pathways.Therefore,the introduction of nanofibers into the active layer would increase OPV efficiency.However,organic optoelectronic nanofibers reported to date are mostly hard to solution-reprocess because of their low solvent resistance.This limits their further development in the field of organic photovoltaics.Taking the advantage of electrospinning technology in preparation of nanofibers with a coaxial core-shell structure,this thesis work proposes a new strategy for preparation of organic optoelectronic nanofibers that can keep fiber structure stable in solution processing and re-disperse in solution.The strategy utilizes crosslinkable optoelectronic polymers as core layer of the fabricated coaxial core-shell nanofibers and inert polymers as shell layer to prevent crosslinking between nanofibers.After crosslinkable optoelectronic polymers are subjected to crosslinking reaction and the inert protecting polymer shell is removed,the desired nanofibers would be produced.In the first part work,we verified this strategy.Firstly,a series of crosslinkable optoelectronic conjugated polymers,PTB7-V_n,were synthesized by introducing a certain amount of terminal vinyl unit into part side chains of PTB7,an outstanding donor material in organic photovoltaics.We found that the material thermal,optical and electrochemical properties were not significantly changed by such modification,and the crosslinking degree of polymer PTB7-V_0.05 was higher than 90% after UV irradiation for 8 h.So we started to prepare PTB7 nanofibers via single needle electrospinning with PTB7-V_0.05.The spinning solution was prepared to dissolve PTB7-V_0.05 in chloroform.Poly?ethylene oxide??PEO?was added as a protecting shell polymer and to increase solution viscosity.Polar solvents,AcOH and DMF,were added to increase the conductivity of spinning solutions.After optimization on electrospinning parameters,the PTB7-V_0.05/PEO nanofibers with a PTB7-V_0.05 core and a PEO shell were obtained.After UV irradiation to crosslink inner PTB7-V_0.05 polymer and wash away PEO polymer with isopropanol,the pure inner-crosslinked PTB7-V_0.05 nanofibers with diameters less than 100 nm were finally achieved.Experiments proved that these fibers can be stably dispersed in a good solvent.In the second part work,we studied the preparation of OPV devices based on PTB7-V_n in detail and the effect of crosslinking on device performance.We found that trimethylolpropane tris?3-mercaptopropionate??TTMP?can significantly promote UV crosslinking.It only cost 30 minutes to achieve a crosslinking degree of 63.41% for PTB7-V_0.05 polymer when 0.01% TTMP was added.Moreover,the addition of TTMP also increased short-circuit current densities of solar cells so prepared.Finally,when the PTB7-V_0.05 device was fabricated using chlorobenzene,diphenyl ether?DPE?and 0.01% TTMP mixed solvent,device power conversion efficiency reached 6.78%.After UV crosslinking,the device can retain 85% of its initial efficiency.Now,the device stability testing is in progress.In the third part of this thesis,we were endeavored to increase the solubility of a BDT-DTBT-based small molecular photovoltaic material.It was found that the integration of two additional alkyl side chains greatly enhanced its solubility not only in chlorinated solvents but also in non-chlorinated solvents.Furthermore,the functionalization position exhibited a significant effect on photovoltaic performance of the resulted material,in which the integration at inner part displayed a better performance than the terminal functionalization.Moreover,we studied on their OPV preparation with non-chlorinated solvents and achieved comparable device performance using toluene as compared with chloroform.Since non-chlorinated solvents are more environmentally friendly than conventional chlorinated solvents,such studies are of significance for future manufacture of PSCs.