Interface Engineering
We have pioneered a new form of nanoimprint lithography (NIL) that is suitable for a broad range of high temperature polymers; we call this process NIM (nanoimprint by melt) and the basic process flow is shown to the right. NIM generally involves the formation of nano-posts or nanoindentations in a reusable master stamp that is then used to emboss a freshly deposited polymer or other soft film. This is a process recently developed for the creation of grating-like features on optical fibers and waveguides and is now being scaled to dimensions relevant for organic solar cell (OSC) device processing.
This is an entirely new direction in the creation of OSCs, and promises a measure of control over interdigitation and vectoral charge transport that is currently missing in established bulk heterojunction (BHJ) technologies. NIM will enable us to create structures over a wide size range, and with separation of features in a controlled manner, allowing us to carefully study and separate the roles of exciton dissociation and charge transport in enhancing OSC efficiency.
The technique has been successfully demonstrated on a broad range of commodity polymers such as polymethylmethacrylate and polycarbonate, specialty electronic polymers such as P3HT and P3DT, and sol-gel materials. Below we show an AFM image of a poly(hexyl-thiophene) polymer (P3HT) that has been patterned by NIL and an SEM of a phthalocyanine dendrimer that has recently been patterned by NIM.
The features in the P3HT are on the order of 250nm in diameter, while those in the phthalocyanine dendrimer are as small as 25nm, thereby approaching dimensions of the same order as the exciton diffusion length in typical OSCs.
Molds used for NIM are usually prepared by e-beam lithography on silicon or nickel, and can be used several hundred times for printing without deterioration in quality of the imprinted nanostructure. Recent advances in NIM enable the formation of large area (1 inch x 1 inch) molds. Molds containing up to 600 different 1mm2 nanostructured areas are envisioned to allow for individual areas, having unique embossed architectures.
It should be possible to use such studies to estimate both the exciton diffusion length in embossed P3HT films, and the maximum length over which charges can be harvested before series resistance, due to finite charge mobilities, becomes evident in enhanced RS in the OSC device.
This will allow for rapid determination of luminescence quenching efficiencies in donor/acceptor composites films, as a function of the length and separation distance between donor posts, thereby providing exciton diffusion lengths as a function of processing conditions, in one chip.
One of our key goals is to reduce the size of embossed features to enable imprinting of polymer pillars with widths smaller than the minimum LD required for that material, and to systematically explore the internal photon conversion efficiency of OSCs created from P3HT, with vacuum deposited C60 overlayers, as a function of pillar width and depth.
