Enhanced Vertical Concentration Gradient in Rubbed P3HT:PCBM Graded Bilayer Solar Cells.
Graded bilayer solar cells have proven to be at least as efficient as the bulk heterojunctions when it comes to the Poly(3-hexylthiophene) (P3HT) - [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) donor-acceptor system. However, control of the vertical concentration gradient using simple techniques has never been reported. We demonstrate that rubbing the P3HT layer prior to PCBM deposition induces major morphological changes in the active layer. Using the newly introduced energy-dispersive X-ray spectroscopy element mapping technique, we found that rubbing P3HT induces the formation of an ideal vertical donor-acceptor concentration gradient. Furthermore, the P3HT crystallites undergo a molecular reorientation from edge-on to face-on configuration inducing a better charge transport in the vertical direction. The combination of these two major morphological changes leads to the fabrication of high-performance solar cells that exhibit, to date, the record efficiencies for spin-coated graded bilayers solar cells.
†Japan Advanced Institute of Science and Technology (JAIST), School of Materials Science, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan.
Varun
V
Vohra
‡Istituto di Cristallografia del CNR - c/o Sincrotrone Elettra, Strada Statale 14-Km 163,5 Area Science Park, 34012 Basovizza, Trieste, Italy.
Gianmichele
G
Arrighetti
‡Istituto di Cristallografia del CNR - c/o Sincrotrone Elettra, Strada Statale 14-Km 163,5 Area Science Park, 34012 Basovizza, Trieste, Italy.
Luisa
L
Barba
†Japan Advanced Institute of Science and Technology (JAIST), School of Materials Science, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan.
Koichi
K
Higashimine
§Istituto per lo Studio delle Macromolecole (ISMAC) Via Bassini, 15, Milano, 20133, Italy.
William
W
Porzio
†Japan Advanced Institute of Science and Technology (JAIST), School of Materials Science, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan.
Hideyuki
H
Murata
P3HT; conjugated polymer; molecular orientation; organic photovoltaics; rubbing