Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
DOI: https://doi.org/10.29363/nanoge.hopv.2018.106
Publication date: 21st February 2018
A strategy to broaden the absorption spectra of organic solar cells is to blend multiple donor or acceptor materials into one bulk heterojunction (BHJ) resulting in D1/D2/A or D/A1/A2 devices. However, the introduction of a third material complicates the morphology formation and may degrade the charge transport.1 A method that allows the use of conventional high-performance BHJs is by processing a D2/A layer on top of a D1/A blend. This cannot be realized by spin coating as the underlying layers will dissolve. In this work BHJs are processed by spontaneous spreading method which enables the fabrication of interesting device architectures. Here, a droplet of the D2/A blend spreads on a water surface due to surface tension gradients and upon spreading the solvent evaporates. The resulting layer is transferred to a glass/ITO/ZnO/D1/A substrate resulting in a double-layer ternary device. Solar cell characterization evidenced that D2/A double layer devices processed by spontaneous spreading (SS) are as efficient as spin coated (SC) layers (PMax,SC = 4.9% and PMax,SS = 5.1% at similar active layer thickness).
In the double-layer ternary device, the complementary absorption of the two active layers results in inhomogeneous charge generation and therefore built-in field. When applying a high intensity bias source of a specific wavelength, its selective absorption in one of the two layers can modify the built-in field and collection of the charges. Drift-diffusion calculation showed that bias illumination can result in an EQE enhancement up to 23%, partly corresponding to the measured results. However, measured EQEs > 100% cannot be explained by the drift-diffusion simulations and the origin of this effect is not fully understood. It is likely related to hampered electron transport over the interface as J–V characteristics showed wavelength dependent charge transport problems. At present, this inhibits steep efficiency improvement of the ternary device (PMAX = 5.9%) compared to the respective BHJ cells (PMAX, D1/A = 3.5% and PMAX, D2/A = 5.2%). Ongoing research is focused on the cause for the limiting charge transport properties and the development of new device architectures processed by spontaneous spreading.
1. N. Gasparini et al. Nature Energy, 2016, 1, 16118
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