Loss Mechanisms in Solution-Processed Small Molecule Bulk Heterojunction Solar Cells
Amidst looming concerns over increasing carbon emissions and global climate change, solar cells made from solution processed small molecules have garnered considerable attention because of their potential to serve as an economically viable, low-carbon source of electricity. However, as with the other classes of organic materials, organic solar cells made from solution processed small molecules are not yet efficient enough to be commercially viable. The aim of this dissertation is to understand the loss mechanisms that limit the power conversion efficiency of these organic solar cells and to suggest strategies for improvement.
Using a combination of electrical characterization techniques, it was found that two of the primary loss mechanisms in solar cells made from solution processed small molecules include field dependent generation and the recombination of free charge carriers. While field dependent generation is a significant loss mechanism in some cases, it was shown that it can also be completely overcome by careful control of the film morphology. The reduction of field dependent generation was found to be correlated with progressively purer and more ordered domains within the small molecule film.
In contrast to field dependent generation, it was found that in all small molecule solar cells, there is some degree of free carrier recombination particularly at low fields close to open circuit. The nature of this recombination was found to be primarily bimolecular – meaning a free hole recombining with a free electron (as opposed to a trap mediated process). While there is some variation in the rate coefficient of bimolecular recombination between systems, it was shown empirically that the charge carrier mobility is typically the most important determinant of the degree of voltage dependent recombination losses. For a 100 nm solar cell, both holes and electron mobilities should be at least 10-4 cm2/Vs in order to efficiently extract charge carriers before they recombine. In most material systems, it was found that the hole transporting molecule was the limiting factor in the charge transport of blend films and the hole mobility measured in neat films sets the upper limit for blend films. Further investigation revealed that increased order along the π-π stacking direction in donor molecules is correlated with lower activation energy for hole transport however even if donor crystallization is achieved the transport in blend films may still be limited by the number of conductive pathways