Effect Of Solution Processed Cathode Modification Layer On The Performance Of Polymer Solar Cells

Polymer solar cells attract much great interest from both academy and industrybecause of their many advantages, such as the simpleness of structure, light weight, lowcost and solution processable. Especially, solution processing makes it possible toeasilly achieve high efficiency solar cells with large scale. The power conversionefficiency and the stability of the solar cells can be dramatically improved by themodification of electrodes. In this thesis, we study the effects of electrode modificaitonlayers with different solution processing condition on the performance of devices bychosing several solution-processable modification materials.Firstly, dye N719and adsorbents of DPPA and CDCA were empolied to modify thecathode. By spin-coating a cathode modifcaiton layer of N719on the surface ofactive-layer, the efficiency of the devices was increased from1.41%to2.74%. Thereverse saturation leak currents was increased and the charge carrier transport wasimproved while the devices were cathode-modified using the N719.This inidcates thatthe modification layer reduced the interface selectivity to carriers. In order to analyzethe mechanism of this effect, we fabricated devices with different work function metalas electrodes. We found that it didn’t form the ohmic contact since the higher the metal’swork function we chosed, the bigger effect it showed.Secondly, a layer of LiAc, which was prepared by both evaporating andspin-coating process, was used as the modification layers. The device performanceshowed that with the modification layer insertion, short circuit current, open circuitvoltage and fill fator were obvious ehanced, which resultly leaded to the efficiencyincreased from1.41%to3.19%for the evaporated modification layer and to3.24%forthe spin-coated modification layer. We attribute these enhancements to the improvementof the cathode interface, which makes electron easier collected from the cathodeinterface. To further analyze the original mechanism, the UPS and XPS were measured.The results showed that dipole was formed at the interface which made the cathode’swork function reduced so that there was a good mismatch of the work-function of metalelectrode to the LUMO of cceptor’s. SEM and FTIR also were used to analyze thedifferences between the evaporated and spin-coated LiAc layers. The SEM picturesshowed that the film formed by spin coating had good morphology without aggregation.The FTIR showed that the characteristic peak of acetate’s disappeared for the sampleprepared via the evaporating processing compareing with the one via spin-coatingprocess, which indicates the LiAc was decomposited during evaporating processing.