Study Of Silver Thin Film Electrodes For Organic Solar Cells

With the advantages of low cost, light weight, simple process, flexibility and possibility of roll-to-roll mass production, organic solar cells(OSCs) have been in the focus of research interest over the past decade. With the development of the research, continuous improvement in materials, device architectures and device processing have resulted in considerable advancement in the performance of OSCs, particularly in the power conversion efficiency(PCE). The PCE of OSCs is approaching the requirement of the commercialization, turning out that OSCs are no longer considered as only an interesting area of research but a realistic alternative to established photovoltaic technologies.Owing to its excellent properties in optical transparency and electrical conductivity, indium tin oxide(ITO) sputtered on glass has become the commonly-used transparent anode for OSCs. However, cheap roll-to-roll fabrication of OSCs relies on the flexible electrodes with low cost, which ITO is unsuitable for. For example, the limited indium source and stringent deposition conditions(usually sputter) lead to significant cost for the ITO electrode. Additionally, both the brittleness of ITO and high temperature processes in the deposition of ITO make it incompatible with the flexible substrates in roll-to-roll mass production. Hence, the ITO electrode has already become one of the bottlenecks for commercialization of OSCs, and more and more attention has been paid to searching for alternative transparent conductive materials. Indium-free transparent conductive oxides, conductive polymer, metal nanowires, graphene, carbon nanotubes and metal thin film are successfully employed as transparent electrodes to replace ITO.Among these alternative transparent conductive electrodes, due to the advantages of low cost, simple process, their intrinsic flexibility and high conductivity, metal thin film electrodes attract more and more attention. Metal thin film electrodes are suitable for the application in OSCs roll-to-roll production with the flexible substrates, making them a promising alternative to the ITO electrode. However, the transparency of metal thin film electrodes still needs to be improved. Most of previous work about metal thin film electrodes has been mainly focused on the effect of optical interference and the variation in the species of the metal. It should be noted that making the metal layer as thin as possible while maintaining its good electrical properties is of vital importance to improve thetransparency of metal thin film electrodes. However, few studies have been done in the view of how to reduce the metal percolation threshold. That is what we concern about in this dissertation. The optimized electrodes have been successfully applied to the fabrication of OSCs with different structures and active layers. All the devices achieve a good performance, showing the huge potential of the electrode. The main research results and contributions of this thesis are listed as follows:(1) The formation mechanism of Ag film is systematically studied. The best device performance of OSCs is generally achieved close to the percolation threshold of Ag, which indicates the best trade-off between the conductivity and transparency for the Ag thin film electrode. In our experiments, the percolation threshold of Ag on glass is 11 nm. Normal organic solar cells(OSCs) using Ag thin films with different thicknesses as the transparent anode are investigated. The poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester(PC61BM) films are employed as the active layer. The PCE of 2.57% is obtained for the device with the 11 nm thick Ag layer under 1 sun AM 1.5G simulated illumination. As discussed above, it is the optimal value among the devices based on pure Ag anodes, comparable to that of ITO-based reference OSCs(PCE of 2.85%).(2) A method of improving the transparency of the Ag thin film electrode by decreasing the percolation threshold of Ag with a Mo O3 interlayer has been proposed. It is observed that the introduction of the Mo O3 interlayer can effectively improve the wetting of Ag on the substrate and reduce the percolation threshold of Ag from 11 nm to 9 nm. Thus a similar sheet resistance and higher transmittance are achieved for the optimal Mo O3(2 nm)/Ag(9 nm) anode, compared with Ag(11 nm) anode. And the PCE of OSCs based on this Mo O3(2 nm)/Ag(9 nm) anode has been further enhanced to 2.71%, which has achieved the international advanced level.(3) The optimal Mo O3(2 nm)/Ag(9 nm) anode has been applied to ITO-free semitransparent OSCs with P3HT:PC61BM as the active layer. Different thickness combinations of Ca and Ag are employed as the cathode. It is observed from our results that OSCs with Ca(15 nm)/Ag(15 nm) cathode have the optimal transparency. Meanwhile, the PCE of 1.79% and 0.67% is obtained for corresponding devices when illuminated from the anode and cathode side respectively, which has reached the international level.(4) Mo O3(2 nm)/Ag(9 nm) anode has also been extended to flexible substrates. Based on this anode, ITO-free OSCs with normal structure are fabricated on flexible poly(ethylene terephthalate)(PET) substrates. P3HT:PC61BM is used as the active layer. A PCE of 2.50% is achieved for such flexible OSCs, comparable to that of the same devices on the rigid glass substrates(PCE of 2.71%). Meanwhile, our ITO-free OSCs based on PET substrates show a good mechanical flexibility. A 10% degradation in PCE is observed after 500 inner bending cycles with a bending radius of 1.5 cm while a 5% decrease in PCE is observed after 500 outer bending cycles.(5) In order to improve the device stability, inverted OSCs are successfully fabricated based on Mo O3/Ag electrodes with P3HT:PC61BM as the active layer. An aqueous processed Zn O electron extraction layer is used as the cathode buffer layer and Mo O3 is employed as the anode buffer layer here. A PCE of 2.76% is achieved for such inverted ITO-free OSCs, comparable to that of ITO-based reference OSCs(PCE of 2.99%). Moreover, inverted ITO-free OSCs based on Mo O3/Ag electrodes show a good stability. The PCE of such OSCs still remains 70% of the original efficiency after 729 h.(6) The blend of poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’] dithiophene-2,6- diyl] [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]](PTB7) and [6,6]-phenyl-C71-butyric acid methyl ester(PC71BM) with better absorption has been introduced to improve the device performance. A good PCE of 3.61% is achieved for OSCs with normal structure based on Mo O3/Ag electrodes, superior to that of the similar devices with P3HT:PC61BM as the active layer(PCE of 2.71%). Moreover, a PCE of 5.55% is achieved for inverted ITO-free OSCs with PTB7:PC71BM as the active layer, comparable to that of ITO-based reference OSCs(PCE of 6.11%). Meanwhile, inverted ITO-free OSCs with PTB7:PC71BM as the active layer also show an excellent stability. The PCE of the devices still remains above 85% of the original efficiency after 729 h, which is slightly superior to ITO-based reference OSCs where a 16% degradation in PCE is observed after 729 h. The above performance has achieved the reported international advanced level.