The Morphology Control And The Performance Of Bulk Heterojunction Solar Cells And Photodetectors Based On Porphyrin Small Molecules

Bulk heterojunction(BHJ) organic solar cells(OSCs) have attracted tremendous attention in both academic institutions and industrial companies for their potential advantages of light weight, low cost and large scale fabrication on flexible substrates. To achieve high performance OSC device, optimized film morphology with suitable nanophase separation between the donor(D) and acceptor(A) components is required since it determines the properties of D/A interfacial layers and interpenetrating network for efficient exciton dissociation and charge transportation, respectively. In addition, the distribution of the components of donor and acceptor along vertical direction in active layer also plays a key role in charge transportation and final collection by the electrodes. In this dissertation,with dedicated device engineering, especially using additive to improve the active layer morphologies, we fabricated high performance bulk heterojunction organic solar cells and photodetectors based on small molecular porphyrin donor materials, which are expected to create new efficient strategies to develop high efficient organic optoelectronic devices.Firstly, in this study, linear bidentate ligand 4,4′-bipyridyl(Bipy) was introduced to bind a porphyrin zinc(Por-1) by the coordination interaction. The optical properties, crystalline properties and the surface morphologies of the pure small molecule films and donor/acceptor blend films under different ratio of Bipy additive were well studied, and hole only devices were also fabricated to investigate the charge transportation properties within the active layer. The results show that in the film the porphyrin exists as the(Por-1)2·Bipy complex when the balanced ratio of Bipy to porphyrin was introduced, and the Bipy-induced porphyrin complex starts to dissociate to form Por-1·Bipy when the ratio of Bipy was further increased. The formation of the(Por-1)2·Bipy complex help to increase the short-range ordering and the electronic coupling between the two porphyrin π-systems, the absorption intensity, degree of crystallinity and hole mobility of the films were all significantly enhanced. When PC71 BM was used as the electron donor, the PCE was enhanced to 2.83%, which was an increase of 53% compared to the solar cell without Bipy. The research is expected to create a new efficient strategy to develop high efficiency organic solar cells based on porphyrin materials.Secondly, conjugated porphyrins Por-2 and Por-3 are easy to self-aggregate due to the big π-system and the planar conformation, when blended with PCBM, the serious aggregation of the porphyrin has adverse impact for the exciton dissociation and charge transportation. Pyridine with the lone electron pair on the nitrogen atom could coordinate with the Zn atom in the porphyrin core. In this work, we used pyridine to improve the morphology of the bend films by disrupting the intermolecular π-stacking of the porphyrin also utilizing this coordination interaction. The PCEs of the BHJ OSCs are increased to 4.78% and 4.15% from 3.71% and 1.61% by incorporating 3.0% and 1.0% pyridine into the host solvent of Por-2:PC61BM and Por-3:PC71BM, respectively. The enhanced PCEs are attributed to that small quantity of pyridine additive could lead to a better interpenetrating network by preventing the active layers from forming large-scale phase separation, a more balanced charge transport by increasing the hole mobility and decreasing the electron mobility, and slightly enhance the crystallinity of the molecule in the blend films.Thirdly, we successfully demonstrated high performance conventional and inverted structure solar cells based on a small molecule Por-4 and PC61 BM with DIO additive. During conventional device fabrication, 0.4% DIO is used to improve the crystallinity of Por-4 in blend films, and a good morphology with high and balanced charge mobility is obtained, the PCE is increased from 5.83% to 7.23% compared with the device without DIO additive. In contrast, higher amount of DIO additive is needed in the inverted device to achieve optimal PCE. The results show that higher degree of Por-4 crystallinity in BHJ film and better vertical phase separation with donor enriched at the top of the active layer are responsible for the significant increase in the efficiency of the inverted cell. In addition, slow drying process of the residual DIO under ambient pressure also shows significant benefits toward increasing the crystallinity of Por-4 and its hole mobility in active layer, better-sized phase separation was also obtained. Inverted structure OSC with a very high JSC of 19.25 m A cm-2 and a PCE of 7.78% have been demonstrated, which is among the highest reported for the inverted devices and expected to create a new efficient strategy for the fabrication of inverted solution processed small molecule solar cells with additives.Fourthly, solution processed organic near-infrared photo-detectors based on a porphyrin small molecule(Por-3) as the donor material were fabricated. The reversed dark current was significantly reduced with enhanced EQE in the range from 600 to 960 nm when the thickness of the active layer was increased. Operating at room temperature and at bias of 0 V, the optimized photodetectors show a broad spectral response from 380 to 960 nm, a high EQE around 20% in NIR region, a low dark current density of 3.44 n A/cm2, a high photo-to-dark current ratio of about 105, and detectivities over 1012 Jones from 380 to 930 nm. These high performance parameters are among the best reported small molecule photodetectors.