(750c) 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.