Donor Design,Synthesis,Properties And Morphology Studies In Small Molecule Solar Cells Based On Porphyrin

The imminent shortage, the soaring prices and the accompanied environmental problems of fossil fuels have drawn researchers’ attention to develop clean and renewable energy throughout the world. Organic solar cells(OSCs) is considered as one of the most promising solutions because of its low cost, light-weight, and high mechanical flexibility.In most recent years, small molecular photo active materials have been progressed quite aggressively both in donors and acceptors. Molecular materials could be readily synthesized and purified with better and controllable quality. Meanwhile, molecular materials show defenite molecule weight and good structure order that could be used to manipulate active layer morphology and performances. Most recently, small molecule donor materials solar cells have achieved power conversion efficiency(PCE) over than 10%.On the one hand, in order to enhance the performance of OSCs, chemists in photovoltaics have been trying to extend the material’s absorption to long wavelength over 800 nm(namely very low energy band gap Eg) to absorb more light and device physists have been trying to optimize the device structure and look for better additives, interface, post treatments, etc. On the other hand, from research to manufacturing, the structural parameters for small molecule based active layer blends need to be thoroughly optimized and understood to obtaine the best performances. The intermolecular interactions between the donors and acceptors can be quite complicated, which would affect their self-assembly and thus morphologies and electronic properties.The studies in this thesis mainly include two parts. The first one is the design and synthesis of different series of small molecule donor materials using Sonogashira reaction, some of which ranks one of the best small molecules and takes irrelaceble roles in the OPVs. The second one is the morphology and mechanism studies of the active layers using different treatments and their photovoltaic properties, which demonstrated much important physical nature and many importanct structure-properties relationships. The details are as follows:In chapter 2, a series of small molecule donors based on porphyrins DPPEZnP-TRs were developed with the aim at getting very low energy band-gaps but high open circuit voltages(Voc) for OSCs. We designed and synthesized DPPEZnP-TEH molecule, in which a porphyrin ring was linked to two diketopyrrolopyrrole units by ethynylene bridges. The resulted material exhibited a very low energy band gap of 1.37 eV and a broad light absorption to 907 nm. An open circuit voltage of 0.78 V was obtained in bulk heterojunction(BHJ) organic solar cells based on DPPEZnP-TEH, showing a low energy loss of only 0.59 eV, which is the first report that small molecule solar cells show energy losses below 0.6eV. The optimized solar cells show remarkable external quantum efficiency, short circuit current, and power conversion efficiency values up to 65%, 16.76 mA/cm2, and 8.08%, respectively, which are the best values for BHJ solar cells with very low energy losses. Also, the possible reasons for the low energy loss were explored via eliminating method, which would have a great influence on the studies of enery loss of OPVs in the future.In chapter 3, OPV devices based on DPPEZnP-TRs were optimized via different and systematical methods. A remarkable current density of ~20 mA/cm2 is generated for the devices based on DPPEZnP-TBO, which is the highest in small molecule-based organic photovoltaics and the overall efficiency reached 9.06%, making these materials among the best deep absorbing light-harvesting materials. More importantly, the physical properties and the morphology of these unique materials have also been extensively explored. For the first time, we found that an additive can promote the intimate mixing of large chi binary mixtures. The physics behind is the kinetic interaction of pyridine molecules with both porphyrin molecules and PCBM. This mixing pushes the blend to a extreme of the non-equilibrium morphology, which then can be further manipulated to reach an ideal state, as we have shown in the manuscript of using thermal annealing and DIO additive. This is also the first time we demonstrated in real space and reciprocal space that multi-length scale morphology existed in small molecule OPV blends. We have tried energy filter TEM to resolve the poor resolution resulting from the similar structure in polymer OPV blends. With the help of high-powered TITAN HAADF imaging, we are surprised to see that Z-contrast imaging can provide large amount of morphological information in complicated non-equilibrium blends.In chapter 4, we systematically studied the solvent vapor annealing(SVA) on small molecule BHJ solar cell blends with PCBM and a low band gap DPPZnPor-TBO. In experiments, we see that molecular interaction and material crystallization are quite complicated factors that dictate the morphology and performances of BHJ thin films. When different thin film fabrication processes are used, the same solvent vapor annealing procedure can be of quite different physical nature in regulating their morphology. Also, the time scale of SVA is also a critical parameter that needs to be carefully optimized.In chapter 5, different small molecules were developed and synthesized using Sonogashira reactions which can avoid the usage of toxic stannyl intermediates and potentially dangerous lithiation reactions. Among the OSCs based on these materials, some show very large Vocs and some exhibit very low energy losses comparable to perovskite solar cells.