Large Molecular Weight Polymer Solar Cells with Strong Chain Alignment Created by Nanoimprint Lithography

In this work, strong chain alignment in large molecular weight polymer solar cells is for the first time demonstrated by nanoimprint lithography (NIL). The polymer crystallizations in nonimprinted thin films and imprinted nanogratings with different molecular weight poly(3-hexylthiophene-2,5-diyl) (P3HT) are compared. We first observe that the chain alignment is favored by medium molecular weight (Mn = 25 kDa) P3HT for nonimprinted thin films. However, NIL allows large molecular weight P3HT (>40 kDa) to organize more strongly, which has been desired for efficient charge transport but is difficult to achieve through any other technique. Consequently P3HT/[6,6]-penyl-C61-butyric-acid-methyl-ester (PCBM) solar cells with large molecular weight P3HT nanogratings show a high power conversion efficiency of 4.4%, which is among the best reported P3HT/PCBM photovoltaics devices.