Interfacial Modification Of Transparent Cathodes And Their Applications In Organic Opto-electronical Conversion Devices

Indium tin oxide（ITO） was widely used as transparent window electrodes in organic optoelectronic devices. ITO can be used as the anode in traditional devices due to its work function of about 4.7 e V, so active metals with low work function have to be used as the cathode, which is obviously not good for the stability and efficiency of devices. To improve the stability of devices, researchers fabricated devices with inverted structure. However, ITO cannot match well with the active layer in the inverted structure, and interface modifications of ITO are done to reduce its function. After adjusting the work function of ITO transparent cathode, the energy level of the electron transport layer may not match well with that of the active layer. It is important to modify the metal oxide of titanium dioxide（TiO₂）, which is used as the cathode buffer layer and the electron transport layer. However, there are different defects in TiO₂ prepared by a sol-gel method, which directly affects charge dissociation, charge transport and charge recombination at polymer/TiO₂ hybrid interfaces and in the TiO₂ film. It is also a research highlight at present, and it is not enough for adjusting energy levels of TiO₂ modified by biomaterials to resolve these problems, so it is important to do more modifications on the surface of TiO₂ to investigate this problem. A series of interface modifications were done in this work.Natural biomaterials such as amino acids and peptides are used to modify ITO, which produces significant reduction of the work function of ITO. The work function of ITO with the modification can match well with lowest unoccupied molecular orbital（LUMO） electronic energy level of most organic materials, so ITO can be widely used as the cathode of devices. In the application of organic optoelectronic devices, with the peptide modified ITO as the cathode, power conversion efficiencies（PCEs） of the organic solar cell devices significantly increased from 2.12% to 8.13%. Compared with the traditional forward devices with active metals as the cathode, the inverted devices with the peptide modified ITO as the cathode exhibited significantly superior device stability in air. In devices with a similar structure, the surface of ITO modified by self-assembly amino acids also show a very low work function, and high performance organic photodetectors can be achieved. Organic photodetectors（OPDs） based on the active layer of the blend of PBDTT-DPP and PC71 BM exhibited the highest projected detectivity of 5.38×1013 jones at the wavelength of 775 nm and projected detectivity higher th an 2.61×1013 jones at a wide range from visible to near infrared zone（350 to 820 nm）. The devices not only have a high detectivity, but also have a fast response speed with a rise time of 3.9 μs. This work provides a green, environment friendly and simple process method to prepare the transparent cathode with low work function and fabricate high performance organic solar cells and organic photodetectors.Amino acids were used to modify metal oxides of TiO₂, and high performance OPDs were achieved. This kind of OPDs have a wide response from ultraviolet to near-infrared light zone（350 to 900 nm）, a highest projected detectivity of 2.69×1013 jones at the wavelength of 775 nm and-0.1 V bias, a fast response speed, a wide linear dynamic range（LDR） and a long time stability. The performance of devices with TiO₂ modified by two group of amino acids was investigated. One group has different isoelectric point, and the other group has similar isoelectric point but different molecular polarity. The results show that both of two group of amino acids chemically react with TiO₂. When isoelectric point is different, devices with neutral amino acid show the best performance. When devices with similar isoelectric point but different molecular polarity, devices with nonpolar amino acids show better performance than that of devices with polar amino acids. This work provides a green and environmental protection way to modify the electron transport layer, and it provides the basis to choose amino acids in the application of electronic devices.For the problem of charge dissociation, charge transport and charge recombination at polymer/TiO₂ hybrid interfaces, we used TiO₂ nanofibers to modify the TiO₂ sol-gel film. Due to closer contact with the active layer by embedding TiO₂ nanofibers into the active layer and their own superior electron transport performance, the ability of charge dissociation and charge transport at the interfaces can be enhanced, so high performance OPDs were acquired. We also investigated the effect of the TiO₂ sol-gel film modified by different aligned TiO₂ nanofibers on the performance of devices, and the research results showed that the devices with one-ways aligned metal oxide nanofibers（AMONs） exhibited the highest projected detectivity and the fastest response speed. This work makes nanofibers prepared by a simple and controllable method applied in high performance OPDs. For charge dissociation, charge transport and charge recombination in TiO₂ sol-gel film, fullerenes(C₆₀) were embedded into TiO₂ film as the electron transport layer to fabricated hybrid solar cells and hybrid photodetectors, and performances of devices were significantly improved. Excitons in P3 HT can transfer into defects in TiO₂ through the f?ster resonance energy transfer（FRET）, and these excitons were trapped in defects and recombined. When C₆₀ were embedded into TiO₂, electrons of excitons in defects can transfer to C₆₀ and the cathode, which improves the performance of devices. This work states that inner modifications in the interface layer, not only at the interfaces, but also in the interface layer.