Performance Improvement Of Rubrene/C₇₀-based Organic Solar Cells With Electrode Modification

Organic solar cells （OSCs） have multiple advantages such as low-manufacturing cost,compatibility with flexible substrates, easiness of fabrication, large-area fabrication, so on and soforth. Recently, the power-conversion efficiency of tandem OSCs have exceeded12%, whichstill needs further improvements for commercial applications. In this thesis, small moleculeorganic solar cells based on rubrene/C₇₀were fabricated by vacuum thermal evaporation system.The performance of rubrene/C₇₀-based OSCs was investaged by inserting bathocuproine asbuffer layer and V₂O₅as hole-extracting layer, which may be helpful in fabricating an efficientconversion contact layer in tandem OSCs. The work was divided into three parts mainly.1. Enhancement of performance in rubrene/C₇₀-based organic solar cells withbathocuproine （BCP） as buffer layer. OSCs with the architecture ITO/MoO₃/rubrene/C₇₀/BCP（xnm）/Al were fabricated and their performance was investigated. Compared the performance ofcell with and without BCP buffer layer, it was found that BCP layer prohibits excitons diffusingtowards the Al electrode, prevents Al from diffusing into C₇₀film, increases built-in field inOSCs and establishes an Ohmic contact between C₇₀film and the Al electrode, thereforeimproves the performance of rubrene/C₇₀-based OSCs. The influence of BCP thickness on theperformance of OSCs was investigated as well. The results show that the optimum thickness ofBCP is6nm and the short circuit current density(J_SC), open circuit voltage(V_OC), fill factor（FF）,power conversion efficiency（ηP） of the optimum cell were4.518mA·cm^-2,0.901V,44%,1.78%,respectively. Compared with cell without buffer layer, J_SC, V_OC, FF and ηPof rubrene/C₇₀-basedheterojunction solar cell with6nm BCP are enhanced by74.8%,95.4%,55.5%and432.9%,respectively.2. Role of V₂O₅hole-extracting layer in rubrene/C₇₀-based small molecule solar cells. Cellswith the architecture ITO/V₂O₅（x nm）/rubrene/C₇₀/BCP/Al were fabricated and theirperformance was investigated. The AFM images show that ITO covered with3nm V₂O₅layerhas improved morphology. Compared results of cells with and without V₂O₅hole-extractinglayer, it was found that the hole-extracting layer allows for an Ohmic contact between ITO andrubrene layer and the semiconducting properties of V₂O₅layer favor the transport and collectionof charges in OSCs. The thickness of V₂O₅is essential to the performance of the device, with thethickness of V₂O₅layer larger than3nm, the optical transmissivity of V₂O₅and electrical conductivity of device become worse. The impedance responses of the devices were alsoinvestigated and found that charges in OSCs with V₂O₅thickness larger than3nm move slowerto reach the electrode. Compared with device without hole-extracting layer, J_SC, V_OC, FF and ηPof rubrene/C70-based heterojunction solar cells with3nm V₂O₅are enhanced by99%,73%,20%and310%, respectively.3. FF analysis and degradation of rubrene/C70-based small molecule solar cells. Cells withthe architecture ITO/V₂O₅（3nm）/rubrene/C70/BCP （6nm）/Al were fabricated. Compared J-Vcurves of device in the dark and under white light illumination, The FF in rubrene/C70-basedOSC was analysed and found the reasons of lower FF were the field-dependent reverse biasphotocurrent （Jph） and the unbalanced charge transport. The degradation results show that theV_OChardly changed with time, FF had slight decrease with time change. On the contrary, J_SCdeclined rapidly with time change, which contributed to a drop in ηP. The degradation of OSCscan be attributed to the diffusion of water and oxygen into the device, which react with the activematerials.