Building Polymer Solar Cells with Top-Down Lithography

Solar cells based on π-conjugated polymers show promise as sources of clean energy: Manufacturing costs are low, and devices can be fabricated in light-weight, flexible, and durable plastic modules. Currently, the active layer in polymer solar cells is based on the bulk heterojunction (BHJ) design. BHJs are prepared by arresting the phase separation of a polymer/fullerene blend to produce a nanoscale, interpenetrating network. BHJs are cheap to fabricate and offer a large interfacial area for charge generation. However, such non-equilibrium structures are very difficult to control and reproduce, posing a significant challenge for fundamental structure-property investigations.  In this work, we demonstrate a new approach to build polymer solar cells with controlled morphologies based on electron-beam patterning of π-conjugated polymers. Heterojunction design is easily varied with a two-step fabrication process: First, a thin film of poly(3-hexylthiophene) (P3HT) is exposed to a periodic pattern of electron-beam radiation, which cross-links P3HT into stable structures at moderate radiation doses (ca. 300 μC/cm² at 30 keV).[1]  Nanodots and nanolines down to 40 nm in size are reliably printed with a 30 keV scanning electron microscope. Second, [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) is spun-cast on top of the patterned polymer to complete the heterojunction. As a proof-of-concept demonstration, solar cells with bilayer P3HT/PCBM heterojunctions were fabricated with electron-beam patterning.  Significantly, irradiated P3HT films retain their optoelectronic properties, and these bilayer devices yield power-conversion efficiencies in the range of 0.5-1%. Preliminary data for nanostructured devices will also be presented in this talk.

[1] S. Holdcroft, Adv. Mater. 2001, 13, 1753-1765.