Transparent carbon nanotube sheets as 3-D charge collectors in organic solar cells

Strong transparent multiwall carbon nanotube (t-CNT) sheets [M. Zhang, S. Fang, A.A. Zakhidov, S.B. Lee, A.E. Aliev, C.D. Williams, K.R. Atkinson, R.H. Baughman, Science 309 (2005) 1215] can be used as effective anodes in organic photovoltaic cells (OPV) [A.D. Pasquier, H.E. Unalan, A. Kanwal, S. Miller, M. Chhiwalla, Appl. Phys. Lett. 87 (2005) 203511]. We prove here that t-CNT plays an essential role as three-dimensional (3-D) hole-collecting network with extended interface connectivity to photoactive layer of bulk heterojunction OPV. Although, the sheet resistance of as-prepared t-CNT is higher than that of ITO, we demonstrate that combining 3-D t-CNT network with planar ITO leads to twice increased photocurrent. So even in unoptimized OPV, a short circuit current density of 11 mA/cm2 is obtained, which is a significant increase, compared to only ∼5.5 mA/cm2 of photocurrent in sole ITO device, and in sole t-CNT case, increasing the overall efficiency to ∼2%. This enhancement is due to a combined effect of an enhanced hole collection by 3-D t-CNT and improved transport via planar ITO part. Flexible OPVs on plastic substrates also show enhanced performance with hybrid t-CNT–ITO anodes.