Tandem Organic Solar Cells

With the development of OSCs technology, the efficiency is increased gradually, but it still can not satisfy the requirement of large area application. Tandem organic solar cell was supposed to solve this problem with its advantage of avoiding the issue that single junction solar cell absorbs limited solar spectrum, so it can utilize the solar spectrum more efficiently and enhance the efficiency.In the thesis, we started our work with inter-connecting layer (ICL), because it was the critical issue in tandem organic solar cell. First we febricated single junction cell with TiO2nano-particles via sol-gel solution, and then used it in tandem solar cell. Then we also fabricated ICLs by thermal evaporation Detail results are:1. We fabricated TiO2nano-particle film by sol-gel solution. We studied the effect of speed and dilution ratio which indicated that200times dilution maked the best efficiency of2.01%. Then we studied the effect of thermal annealing temperature which indicated that the RMS of TiO2film was5.91nm and the work function was-4.863eV, it can provide an effective buffer layer decreasing the contact potential between active layer and Al electrode, which leading an efficiency of2.52%.2. All solution ICL combining TiO2nano-particle and PEDOT:PSS was fabricated, and it turned out that the ICL can not link the two subcells effectively leading a Voc between0.50V and0.60V. The PCE was only1.3%. The morphology study illustrated that the ICL was not robust enough to protect the first subcell.Then we used MoO3to substitute PEDOT:PSS which prepared ICL by thermal evaporation and sol-gel, but unfortunately,the ICL also can not satisfiy the requirment of tandem solar cell, although the Voc can reach0.71V and the PCE can increase to1.60%.3. All thermal evaporation ICL using Ca as cathode buffer layer and MoO3as anode buffer layer was fabricated. We found that the ICL worked effectively with Voc of1.02V and PCE of2.16%when the thickness of Ca was5nm and MoO3was12nm. We can conclude that the two subcells were already connected, but it had a voltage loss. So we attemptted to introduce metals such as Al、Ag and Auto the ICL. After a series of experiments, we found that10nm Al can alleviate the S-shape Ⅰ-Ⅴ curve and the Voc reached1.15V. The PCE also improved to1.92%. When a5nm Au was used, the S-shape Ⅰ-Ⅴ curve was completely disappeared, the Voc was1.06V, the Jsc was4.47mAcm-2, and the FF was increased to67.99%while the PCE reached3.24%. With no doubt, the Ca/Au/MoO3was a very effective inter-connection layer which can improve the efficiency of tandem solar cells.