Dual barrier to charge transport in copper coordination complexes as evidence of ionic movement George Harvey Morritt 1 , Marina Freitag 2 1 School of Maths, Stats and Physics,Hershel Building, Newcastle University, NE1 7RU Newcastle upon Tyne, UK, 2 School of Natural and Environmental Science, Bedson Building, Newcastle University, NE1 7RU Newcastle upon Tyne, UK Coordination complexes are a versatile class of charge transport materials with great potential for use in dye- sensitized solar cells [1, 2], particularly for indoor photovoltaics where tunable materials are especially important due to ever changing light sources [3] . Unlike traditional semiconductors, coordination complexes can be easily modified at a molecular level to adjust their properties [4] . They also outperform commonly used organic hole transport materials, such as Spiro-OMeTAD, in terms of cost and environmental impact. Understanding the charge transport mechanics of coordination complexes, the aim of this project, is a crucial aspect for the development of efficient and high-performance solar technologies. The primary method employed in this study is conductivity activation energy (E a ) measurements. Unlike the resistance of a material the E a gives us a more microscopic picture of the resistance to the flow of charge. By conducting these measurements, we can investigate how doping and modifications to the molecular structures impact charge transport, and identify and address factors that limit device efficiency [5] . Doping charge transport materials with sats like LiTFSI is commonplace to increase device efficiency, but its impact on charge transport mechanics and device performance is not well understood. Copper coordination complexes modified from Cu(tmby) 2 have been synthesized and studied. Figure 1 shows an E a measurement preformed on doped and undoped samples of Cu(tmby) 2 . The doped sample displays a bending effect in contrast to the expected linear Arrhenius plot of the undoped sample, indicating two charge transport mechanics. The degree of bending depends on ion concentration and weight, strongly indicating ionic movement is the cause. To the best of the authors knowledge, this is the first time E a measurements have been used to probe ionic movement in charge transport layers. Galvanostatic measurements confirm the presence of mobile ion species.
Figure 1: Ea measurements of undoed Cu(tmby) 2 (left) and samples doped with 0.5M LiTFSI (right).In the coming weeks, we will continue meaurements on coordination complexes at different doping concentrations, use established methods such as solid state impedance to further confirm mobile ions are present, correlate structural modifications with changes to the E a by measuring the modified versions of Cu(tmby) 2 and finally examine the effect of ionic movement on device performance. References 1. Energy Environ. Sci. 8, 2634-2637 (2015) DOI: 10.1039/C5EE01204J 2. J. Am. Chem. Soc., 138, 45, 15087–15096 (2016), DOI: 10.1021/jacs.6b10721
3. Chem. Sci., 11, 2895-2906 (2020) DOI: 10.1039/C9SC06145B 4. Chem. Phys. Rev. 3, 011306 (2022) DOI: 10.1063/5.0075283 5. ACS Appl. Energy Mater., 6, 4 (2023) DOI: 10.1021/acsaem.2c02999 6. Adv. Mater., 26, 38, 6629-6634 (2014) DOI: 10.1002/adma.201402415
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