Heterojunction Design And Interface Modification In Polymer Solar Cells

Organic solar cells based on conjugated polymers have been of great interest due to the prospect of low-cost, solution-based processing, and fabrication on flexible substrates, which offer a significant advantage over silicon technology. Among numerous photoactive donor/acceptor composites, the blend of poly(3-hexylthiophene)(P3HT) with fullerene derivatives [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) has been intensively investigated in recent years for organic solar cells. However, the efficiency is insufficient to meet the requirement of practice application. Much effort has been made toward improving the power conversion efficiency of organic solar cells.Liquid crystalline primitives has a strong self-assembled function, and able to induce the entire polymer molecules orderly arranged in certain conditions, thereby regulating the morphology and the heterojunction interface of the active layer. In polymer solar cells, the surface morphology of the active layer and the interface between the electrode and the active layer is crucial to improve the efficiency of polymer solar cell. Optimize the contact interface and regulate the morphology of the upper layer of the active layer through a certain modification material is a effective method to increase the device performanceIn my dissertation, a facile approach to develop a novel soluble liquid-crystalline fullerene derivative, N-methyl-2-[(4-(40-cyano) biphenyloxy) hexyloxyphenyl)]-3,4-fulleropyrrolidine, C60-bp-CN is reported. Compared to C60, the modified C60-bp-CN shows broadened light harvesting and lower LUMO level (-4.2eV), ascribing to the strong intermolecular interaction induced by self-assembled cyanobiphenyl mesogens, especially after thermal annealing from liquid crystalline states. When blended with poly (3-hexylthiophene)(P3HT), the absorption of the blend film is red-shifted to700nm, and the intensity is also remarkably increased. The results indicate that the modified C60-bp-CN is a promising acceptor for polymer solar cells (PSCs) based on BHJ active layers. Therefore, solar cell devices based on an ITO/PEDOT:PSS/P3HT:C6o-bp-CN/LiF/Al configuration are fabricated. Clearly, the performances of all the devices are dramatically enhanced relative to corresponding species based on P3HT/C60active layers. Among all the thermal treatments the mesophase annealing achieves the best performance with a short-circuit current density (Jsc) of5.5mA/cm2, an open-circuit voltage (Voc) of0.52V, a calculated fill factor (FF) of0.23, and a power conversion efficiency (PCE) of0.65%, which is three times higher than that of the untreated one.In addition, efficient and stable bulk heteroj unction polymer solar cells based on a P3HT:PC61BM active layer have been fabricated by inserting a convenient, low cost, solution-processed, PEDOT/PSS anode buffer layer doped with solution-processed MoO3. The MoO3consist of small "islands" in the blend film extends the interfacial contact area between the P3HT:PC61BM and PEDOT/PSS to facilitate hole collection, and meanwhile blocking electron transport at the anode. Hence the improved Jsc and FF dramatically increase the power conversion efficiency (PCE) from2.98%to3.94%. After anode buffer layer doped with the MoO3, the device stability also has a significant improvement.Finally, an facile approach for improving device efficiency of poly (3-hexylthiophene)(P3HT)/[6,6]-phenyl C61butyric acid methyl ester (PC61BM) bulk heteroj unction solar cells is presented. This method is used by simply pre-casting a tiny thin P3HT layer with high crystallinity between PEDOT:PSS and photoactive P3HT:PC61BM layers. The high crystalline thin P3HT layers are casted from three different solvents such as dichlorobenzene (o-DCB), tetrahydrofuran (THF) and dichloromethane (DCM). It is demonstrated that THF used for thin P3HT layer preparation is a suitable solvent for yielding a high crystalline film, which is unreadily washed away during the solution processing of the active layer. The results indicate that the morphology of P3HT:PC61BM active layers is strongly depended on the formation of P3HT buffer layer. A great morphology difference of P3HT:PC61BM is caused from crystallinity of P3HT buffer layers prepared by solvents. The thin P3HT layer with high crystallinity can improve the crystalline degree of P3HT in the active layer, subsequently inducing the whole active layer to form a well self-assembled pathway for efficient charge transfer and transportation to their respective electrodes. Therefore, a greatly enhanced short-circuit current density of the device is resulted. After optimization of thickness of P3HT buffer layer, an improvement of the power conversion efficiency is obtained from2.98%to5.14%, which is the highest power conversion efficiency of P3HT:PC61BM-based mono-junction solar cell reported to date.