Studies On Morphology, Performance And Stability Of Polymer Photovoltaic Devices

This work is focused on factors that influence the performance of the bulk-heterojunction polymer solar cells. The main contents of dissertation include three parts. Firstly, temperature-dependent morphology alteration of the P3HT:PCBM blend solar cells during annealing processes were investigated. Secondly, in order to obtain a lower HOMO energy level donor material, the copolymer material FTQ was synthesized by using the fluorine atom to substitute the hydrogen atom in the TQ, which effectively improved the open circuit voltage of the FTQ:PCBM devices. Finally, the optimised PBDTTT-C:PCBM devices with high photovoltaic performance were fabricated. Compared to the conventional stucture devices, the inverted structure devices have better illumination and thermal stability, where the ZnO and MoO3as buffer layers.1The effect of thermal treatment on the morphology of P3HT:PCBM blend film and the performance of device based on P3HT:PCBM was studied.The influence of the thermal treatment processing on PCBM aggregations in P3HT:PCBM blend film were discussed. The results showed the size of PC61BM and PC71BM aggregations gradually enhance with the increasing temperature of thermal treatment, however the shape of two aggregations are significantly different, where the PC61BM aggregation with dendritic pattern, and the PC71BM aggregation with stellate pattern.The impact of Al electrode on the formation of PCBM aggregations was discussed. A1layer can limit the formation of microns PCBM aggregations in the active layer when A1electrode vapor-deposited before thermal annealing, which benefits to the dissociation of excitons and the transport of carriers.The influence of the thermal treatment on the P3HT crystalline state and the optical absorption of the film were discussed. With the temperature thermal treatment increased, the absorption and crystallinity of the P3HT blend film are enhance. Meanwhile the absorption peak shifts to long wavelength slightly.The impact of the annealing on the performance of P3HT:PCBM devices were studied. When the annealing temperature rised from130to170°C, the device performance has also been improved. The max efficiency of the was achieved, when the devices were annealed at170°C for30minutes. 2The fluorine-substituted TQ material, FTQ, were characterized, and the photovoltaic performance of the devices based on TQ and FTQ were studied and compared.The FTQ with optical band gap of1.70eV, the HOMO level of-5.51eV and the hole mobility up to5.7×10-3cmV-S-1. Compared to TQ, FTQ, fluorine-substituted by TQ, showed lower HOMO level and higher relative dielectric constant, which decides that the devices based on FTQ have higher Voc.The changes in the absorption of FTQ:PC71BM blend film during thermal treated at110°C was investigated. After thermal annealing, the obvious redshift of absorption peak of FTQ happens and the absorption increases with the increasing time.The device performance was optimized. The efficiency of5.3%was obtained for FTQ:PC71BM device with a mass ratio of1:1in its active layer, processing with a short time of1minute thermal annealing at110°C.3To study the key factor to affect the device performance, two kinds of device based on PBDTTT-C:PC71BM were fabricated, where the PEDOT:PSS and the ZnO were used as modified layers in conventional and inverted structure devices, respectively. By optimizing the parameters, the high efficiency device was obtained. The stability of the two kinds of devices in the long-term storage, illumination and thermal conditions were investigated.The effect of solvents and additives on the performance of the device were investigated. While DCB as the solvent to add3%DIO, the efficiency up to6.7%was achieved.In a nitrogen atmosphere, the aging of the devices with two structures were investigated in different conditions:the long-term storage, the illumination of solar simulator, and high temperature. The results show that conventional-type device decay faster than the inverted-type device. Due to using stable inorganic compound such as the ZnO and the MoO3as modified layer, the efficiency of the inverted-type device decays more slowly.ZnO, as a cathode modified layer, not only has good electron transport mobility, but also improves the lifetime and stability of the devices. The inverted-type device is an ideal practical solar cell candidate.