Organic materials are widely viewed to have significant potential as next-generation materials for high-tech applications. Potential uses include low-cost solar cells, light-emitting diodes, and displays. As a specialty chemical supplier in the space of organic electronics, Dow Electronic Materials has a continued interest in understanding how materials purity impacts customer device performance.
Dow scientists, in work with the University of Minnesota, have addressed this and related questions to provide Dow with an improved understanding of how pure active materials must be in order to meet industry specifications. In recent work published in Applied Physics Letters, researchers at the University of Minnesota have demonstrated a clear correlation between materials purity and the efficiency of energy (exciton) transport in organic semiconductors. Excitons play a fundamental role in the operation of all organic optoelectronic devices, spanning light-emitting diodes, photodetectors and solar cells. This work involved collaboration with Dow for active materials synthesis and materials purity analysis.
In their article titled “Exciton diffusion in organic photovoltaic cells,” University of Minnesota researchers S. Matthew Menke and Russell J. Holmes explained:
Exciton generation, migration, and dissociation are key processes that play a central role in the design and operation of many organic optoelectronic devices. In organic photovoltaic cells, charge generation often occurs only at an interface, forcing the exciton to migrate from the point of photogeneration in order to be dissociated into its constituent charge carriers. Consequently, the design and performance of these devices is strongly impacted by the typically short distance over which excitons are able to move. The ability to engineer materials or device architectures with favorable exciton transport depends strongly on improving our understanding of the governing energy transfer mechanisms and rates.
The design and performance of organic photovoltaic cells is dictated in part by the magnitude of the exciton diffusion length (LD). Although this parameter is quite important, there have been few investigations connecting LD and materials purity. In an article titled “Role of impurities in determining the exciton diffusion length in organic semiconductors,” originally published in Applied Physics Letters 108, 163301 (2016), Dow and the University of Minnesota showcased their research on LD for the organic small molecule N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (α-NPD) as native impurities were systematically removed from the material. This work demonstrated that removal of impurities acts to decrease the nonradiative exciton decay rate and increase the radiative decay rate, leading to enhancements in both the exciton diffusivity and lifetime.
For α-NPD, LD was increased from (3.9 ± 0.5) nm to (5.3 ± 0.9) nm as the thin film high performance liquid chromatography (HPLC) purity increased from 97.1% to 99.0%. Experimentally measured values of LD agree with predicted values calculated using the separately measured changes in exciton decay rates and a diffusion model based on Förster energy transfer.
In summary, the results of this research highlight the role of impurities in determining LD, while also providing insight into the degree of materials purification necessary to achieve optimized exciton transport. This collaborative work has also demonstrated that native molecular impurities present in organic semiconductors can impede exciton diffusion by measuring LD as native molecular impurities are systematically removed from films of α-NPD.
 Energy Environ. Sci., 2014,7, 499-512, DOI: 10.1039/C3EE42444H, 2013