Organic Based Solar Cells with Morphology Control

Abstract

The field of organic solar cells has in the last years gone through an impressive development with efficiencies reported up to 12 %. For organic solar cells to take the leap from primarily being a laboratory scale technology to being utilized as renewable energy source, several issues need to be addressed. Among these are a more direct transfer of new materials tested on a laboratory scale to large scale production than offered by spincoating, a method offering direct control of the morphology in the active layer, and a more environmental friendly processing, where the vast use of organic solvents offers a great challenge. In this thesis the development of inks with a pre-arranged morphology was attempted by two methods. First by grafting of silicon nanoparticles with an organic phenylene vinylene oligomer, the resulting particles were analyzed by 1H-NMR, absorption spectroscopy, Atomic Force Microscopy and as solar cells in a blend with PCBM. It was concluded that these particles did not show a potential large enough for continuous work due to a high material loss and low efficiency when applied in solar cells. The second method to achieve was preparation of pre-arranged morphology organic nanoparticles consisting of a blend of donor and acceptor in an aqueous dispersion, thereby addressing two of the issues remaining in the field of organic solar cells. This approach was used on six different polymers, which all had the ability to prepare aqueous nanoparticle inks. The morphology of the nanoparticles was investigated both internally and externally, both were attempted to be controlled by variation in preparation solvent and particle sizes. The inks were slot-die coated on both the R2R coater and mini roll coater but only after a number of inks modifications and adjustments of the coating parameters. Solar cell prepared by large scale R2R-coating with an active layer consisting of these nanoparticles had an efficiency of up to 0.55 %. In order to shorten the distance from laboratory scale testing of solar cells to R2R production a mini roll coater was invented. This roll coater uses film deposition techniques which have been downscaled from the R2R coater i.e. slot-die coating and flexographic printing. Thereby allowing the device optimizations to be transferred almost directly from small to large scale. This is in contrast to devices prepared by spincoating. Another advantage with the laboratory roll coater is that, it enables the preparation of 250-300 solar cells in an hour from as little as 15-20 mg of polymer. This means that a number of parameters can be optimized with very little material compared to the amount necessary for optimization directly on the R2R equipment. The laboratory roll coater was used in the search of the high efficient organic solar cells by optimizing new electrodes and preparing a number of solar cells from different polymers with various optimization parameters such as thickness of the active layer, ratio between donor and acceptor, and coating temperature. Further small molecule inks were slot-die coated on this coater with a variation in ink formulations enabling the coating of films of a higher quality and coating of ink employing molecules with cross-linkable side chains to avoid dissolution of the active layer when coating subsequent layers. The mini roll coater was also used to develop a procedure for preparation of tandem devices where all layers were deposited by wet-processing on an ITO-free flexible substrate. The challenging part was the preparation of an intermediate layer, which was both easy to coat and had a good solvent resistance. Thus by pure intermediate optimizations the efficiency increased from 0.067 % to 1.7 %. Main aspects of this thesis worth of further investigation includes pre-arranged morphology by aqueous inks, the use of the mini roll coater to investigate and optimize new polymer, and the preparation of tandem devices with high efficiency.