The role of alkane dithiols in controlling polymer crystallization in small band gap polymer:Fullerene solar cells

Abstract

The effect of the addition of 1,8‐octanedithiol (ODT) during processing on the microstructure of blend films of poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b′]dithiophene)‐alt‐4,7(2,1,3‐benzothiadiazole)] (PCPDTBT) and [6,6]‐phenyl‐C71 butyric acid methyl ester ([70]PCBM) is studied. Grazing incidence X‐ray diffraction and absorption spectroscopy show that the crystalline order of PCPDTBT increases when ODT is introduced in the solution phase either to neat polymer systems or to blends with [70]PCBM. The increased crystalline order is accompanied by less dispersive hole transport in the polymer, and leads to a more efficient formation of a percolating fullerene network within the blend. This contributes to an increase in photocurrent generation. However, the bimolecular recombination rate as determined from photovoltage transients increases upon addition of ODT, limiting the power conversion efficiency to values well below those expected from the energy levels of PCPDTBT. We propose some explanations for this increase in bimolecular recombination, based also on variable angle spectroscopic ellipsometry measurements. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

On the morphology of polymer‐based photovoltaics

“…Subsequently, it was noticed that PCPDTBT crystallized under the influence of the additive. 94,95 GIXD showed the ordered packing of PCPDTBT chains and the chain packing assumed an edge-on orientation, similar to that seen in thin films of P3HT [ Fig. 15(e)].…”

Electron Collection as a Limit to Polymer:PCBM Solar Cell Efficiency: Effect of Blend Microstructure on Carrier Mobility and Device Performance in PTB7:PCBM

“…[ 18 ] It was proposed that the increase in electron mobility on annealing is due to the aggregation of P3HT, driving phase separation and the formation of a connected and more ordered PCBM phase. [ 4,19,20,[35][36][37][38][39][40] Those studies concluded that electron collection limits the performance of the PCPDTBT:PCBM blend, but that the performance of the blend could be enhanced through use of ODT or DIO solvent additives, or by substituting the polymer with a silicon-bridged analogue. [ 34 ] The adverse effect on electron mobility of blending polymer and fullerene has also been demonstrated for PCPDTBT:PCBM in a number of studies.…”

Significantly Reduced Bimolecular Recombination in a Novel Silole‐Based Polymer: Fullerene Blend

“…However, a more recent paper by Agostinelli et al reported a β of 4 × 10 − 12 cm 3 s − 1 for the C-PCPDTBT:PC 70 BM blend. [ 14 ] Furthermore, Peet et al [ 7 ] have shown that the fi ll factor of Si-PCPDTBT:PCBM blend devices drops rapidly with increasing active layer thickness (from > 60% at 100 nm to < 45% at 200 nm). This result is inconsistent with Si-PCPDTBT being a nonLangevin system.…”