The Investigation Of Charge Carrier Recombination On Organic Semiconductors

Recently, the study on organic semiconductors has grown rapidly with remarkable achievements in organic solar cells, organic light emitting devices, organic field-effect transistors, organic memory and sensors. The charge transport has a significant effect on the performance of organic semiconductors device. Furthermore the charge carrier recombination is the main loss of organic semiconductor devices. However, the charge carrier lifetime will significantly increase if the charge carrier recombination can be effectively reduced. Herein, my master’s thesis focuses on charge carrier recombination of the organic semiconductors devices, based on the organic solar cell with the active layer of P3HT:PC61BM, and the photoelectric device with the organic layer of NPB.1. Trap-limited bimolecular recombination in poly(3-hexylthiophene)(P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester(PC61BM) blend films has been investigated by using photo-induced charge extraction by linearly increasing voltage(photo-CELIV) method. The bimolecular recombination rate is strongly dependent on the photoexcitation density, the PC61 BM composition and the thermal annealing process, but it slightly depends on the thickness of the blend film. The results show that the trap-limited bimolecular recombination is strongly affected by the distribution of the density of trap state(trap DOS). The higher trap-limited bimolecular recombination rate means the trap DOS centered at lower energy which is beneficial to charge carriers transportation and collection, due to the lower activation energy and faster release rate. And the recombination rate will increase when the transport of electrons and holes becomes more balanced. Our results will give a deeper understanding about the relationship between the trap-limited bimolecular recombination and the density of trap state in the polymer/fullerene system.2. Propose a device with the sandwich structure of glass/ITO/Si O2/N,N0-di-[(1-naphthyl)-N,N0-diphenyl]-(1,10-biphenyl)-4,40-diamine(NPB)/Al, and study the charge carrier lifetime of this device with CELIV method. The hole carrier lifetimes longer than 1 hour can be achieved. We attribute this four order of magnitude increase in carrier lifetime to a hill shaped band structure which forces photogenerated electrons and holes to spatially separate shortly after generation, the reduction in spatial overlap between the electron and hole population reduces the recombination rate. To understand and demonstrate how changes to the device structure and materials can be used to tune the charge carrier lifetime. To this end, we replace the spin coated Si O2 insulator used previously, with a vapor deposited layer of Al2O3 and consequently observe a charge carrier life times of over 2 hours. We also investigate if the NPB molecule it’s self played a special role in the previously reported long life times, by replacing it with a range of molecules of varying form factors but similar HOMO/LUMO energies. Finally, we perform life time measurements as a function of applied bias and light intensity to further elucidate and validate the mechanisms responsible for the ultra long life times. These results demonstrate that it is possible for organic semiconductors to have long charge carrier lifetimes, and opens up the possibility for new classes of ultra low light level photodetectors and memory elements.