Preparation Of Functionalized Carbon Nanomaterials And Their Application In Polymer Solar Cells

With the rapid development of global economy,the demand of energy is increasing and the excessive use of fossil fuels has caused great damage to the environment.Therefore,the demand for clean energy is becoming more and more urgent,and a series of renewable clean energy,such as wind energy,water energy and solar energy,have been developed.The application of solar cells have been rapidly developed,the inorganic solar cells have been achieving large-scale production and application.However,the inorganic solar cells have the disadvantages of high cost,high weight and non flexible.On the contrary,the polymer solar cells have the advantages of light weight,solution processed and flexibility,which have broad prospects for development.Recently,polymer solar cells have been improved in fabrication technology,new materials and structural engineering.The highest power conversion efficiency has exceeded 13%.However,the efficiency and stability of polymer solar cells are expected to be improved to meet the needs of commercial production.Graphene and carbon nanotubes have extremely high conductivity,high carriers mobility,strong mechanical properties and chemical stability.They have wide applications in fields of photocatalysis,energy storage,sensors and photovoltaic.In this paper,we functionalized the graphene and carbon nanotubes via chemical method and the functionalized materials will be applied to polymer solar cells,which reduced the cost of large-scale fabrication.Moreover,the optical and electrical properties of graphene and carbon nanotubes can be changed by introducing different functional groups.The results are as following:a.Graphene quantum dots?GQDs?was synthesized by a solvothermal method,and the GQDs was doped into the active layer of polymer cells as an acceptor material.We fabricated the polymer solar cells based on two acceptors of PC₇₁BM and GQDs.The addition of GQDs is proved to enhance the devices performance,and the PCE of devices was increased from 5.55%to 7.04%.The devices were characterized by IPCE,PL,UV-vis absorption and Kelvin probe,we can draw a conclusion that the doping of the GQDs offers a general route to increase the length of optical path and optimize the light absorption owing to scattering and reflecting effects of GQDs.On the other hand,GQDs provided exciton interfaces which greatly increased the efficiency of exciton dissociation,and the GQDs also improved the chanrge transport between PC₇₁BM molecules after dissociation.Furthermore,due to its suitable work function,GQDs provided effective transport path for electrons and effectively suppressed the recombination,which contributed to the high J_sc.The R_s and R_sh of devices were also reduced.Therefore,as an aacceptor material,GQDs directly improved the performance of polymer solar cells and contributed to the improvement of the photoelectric conversion efficiency.b.Boronic acid functionalized MWCNTs?bf-MWCNTs?were synthesized via a facile low temperature process and were doped into PEDOT:PSS as hole transport layers?HTLs?for polymer solar cells.The device with the structure of ITO/PEDOT:PSS:bf-MWCNTs/PCDTBT:PC₇₁BM/LiF/Al showed a maximal PCE of6.953%,increasing by 28%than that of the control device.With the addition of bf-MWCNTs into the PEDOT:PSS,the HTLs presented a significant improvement on conductivity which resuced the transport resistance of devices.Comparing to the devices with PEDOT:PSS:MWCNTs HTLs,the hole extraction and transport of devices with PEDOT:PSS:bf-MWCNTs HTLs were significantly improved,which was beneficial to the hole transport from the active layer to the ITO anode and improved the performance of the device.Furthermore,the result of Kelvin probe characterization showed that the work function of HTLs increased with the bf-MWCNTs doping which reduced energy barrier between active layer and ITO,accelerated holes extraction and transport and increased the V_oc of the devices.Therefore,the boronic acid functional groups on carbon nanotubes played an important role in improving the hole extraction and transport.c.Graphene-MoO₃?G-MoO₃?nanoparticles were synthisized by using graphene and MoO₃ via a hydrothermal method.The G-MoO₃ nanoparticles were used as interfacial layer for polymer solar cells via a solution processed method and the devices with the structure of ITO/G-MoO₃/PCDTBT:PC₇₁BM/Li F/Al were fabricated.The devices with evaporated MoO₃ interfacial layers showed a optimal PCE of5.716%,whereas the devices based on G-MoO₃ interfacial layer achieved a maximal PCE of 7.092%.The G-MoO₃ interfacial layer demonstrated enhanced optical and electrical properties.It not only showed high visible light transmittance,but also improved electrical conductivity.By characterizing PL spectrum and hole mobility of the devices,we found that devices with G-MoO₃ interfacial layers exhibited excellent hole transport and extraction capability which effectively improving the J_sc and FF.Furthermore,we measured the work function of G-MoO₃ with UPS.Comparing to MoO₃,the work function of G-MoO₃ had a small increase,which not only reduced the energy barrier and accelerated holes transport,but also increased the V_oc of devices.