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Controlling the Morphologies on Solar Cells Based on Cross-linked Semiconducting Polymers at the Nanoscale

Farinhas, J. ; Ferreira, Q. ; Charas, A. ; Morgado, J.

Controlling the Morphologies on Solar Cells Based on Cross-linked Semiconducting Polymers at the Nanoscale , Proc Cluster Workshop in Materials and Nanotechnology, Lisboa, Portugal, Vol. n/a, pp. 65 - 65, December, 2013.

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Abstract
Organic photovoltaic cells (OPVs), combining semiconducting polymers, as electron-donor materials, and fullerenes, as electron-accepting materials, are of high interest, both from the scientific and technological point of view. They combine low cost solution processability, fine-tuning of the optical and electrical properties upon modifications of the chemical structure of material and mechanical flexibility. However, the performance of OPVs is strongly depended of the organic active layer’s morphology: the excitons formed upon photon absorption have to reach the electron-donor/electron-acceptor interface for an efficient separation into charges (electrons and holes) before decaying and the generated charges have to reach the proper electrodes before they recombine. These constraints led to a proposed ‘ideal’ morphology for these cells consisting on an interdigitated-like structure of the donor-acceptor materials. To implement this approach, research has been focused on the patterning of the organic materials to form columns with ca. 20 nm in diameter.
In this communication, we report on a method to prepare insoluble cross-linked columnar films of a polyfluorene, upon phase separation in blends with polystyrene, used as assisting polymer [1]. The electron-donor nanostructured columnar films of these polymers were then coated with a soluble fullerene (PCBM), as the electron-acceptor, to form the active layer of OPVs [2]. The performance of these nanostructured solar cells is correlated with the column-like diameter of the patterned polymer, with the cells with the thinner columns showing the highest power conversion efficiencies (PCEs). Yet, the obtained PCEs still fall below that of the similar OPVs prepared upon blending of the two components. We will discuss on the origin of the PCE variation with column diameter and present the ongoing work to improve these OPVs performance.