Sweeping out electrons with carbon nanotubes to achieve high efficiency solar cells

Luping Yu, Dmitri Talapin and collaborators from the University
of Chicago MRSEC are working to develop highly efficient bulk heterojunction
organic solar cells.  They have recently
created composite polymer solar cells from a combination of the semiconducting
polymer, PTB7, PC₇₁BM, and doped multiwall carbon nanotubes
(MCNT--these MCNT are doped with N (nitrogen), 
an n-type dopant or B
(boron),  a p-type dopant), illustrated at left.  The major effect
observed, was the significant enhancement of the current density.
Photoluminescence spectra suggest that the MCNT resides mainly in the the PC₇₁BM
phase; the luminescence quenching efficiency is unaffected by its
addition.  The optimal power conversion
efficiency (PCE) is obtained by using 1.5% of N-MCNT, selectively sweeping out
electrons from the PC₇₁BM domain. 
The resulting organic solar cells have the following characteristics:
photocurrent density, J_SC=20.0 mA/cm²; open circuit
voltage, V_OC=0.70V; fill factor, FF=66.5% and record PCE=9.4%.  These results point to an exciting direction
of nanoscale assemblies for energy conversion. [1]