Towards room temperature processed and completely flexible organic photovoltaics

This thesis presents the work towards identifying and developing materials and processes necessary to fabricate inexpensive, flexible and room temperature processed organic photovoltaic devices (OPVs). A method for rapidly fabricating and characterizing OPVs utilizing gallium-indium eutectic (liquid at room temperature) to replace thermally evaporated aluminum and a simple halogen light as a replacement for a solar simulator is described. Poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunctions were utilized for absorbing light and generating charge carriers. P3HT:PCBM OPVs using traditional transparent and conducting materials for carrier collection such as indium tin oxide (ITO) coated glass and fabrication processes such as thermal annealing were optimized. Thermal annealing is an integral step in developing the morphology necessary to create high efficiency OPVs, but is detrimental to many flexible (plastic) substrates. In order to overcome this limitation, a technique utilizing room temperature solvent vapor annealing for improving OPV efficiency was developed and the mechanisms leading to such improvement are elucidated through in situ and ex situ characterization. The analyses revealed that solvent vapor annealing at room temperature leads to comparable changes in morphology and charge transport as thermal annealing.;ITO is expensive and substantially loses conductivity when flexed. As an alternative, single walled carbon nanotube (SWNT) thin film as a transparent and conducting material was investigated. Using a solution of dispersed SWNTs, networks with densities just above the metallic percolation threshold were deposited with conductivity and transmission properties comparable to ITO. Additionally, they are flexible without a loss in conductivity with bending cycles. OPVs with SWNT thin films as the hole collecting electrodes showed better performance than ITO reference cells. The mechanism for the better performance was attributed to the three dimensional interface between the P3HT:PCBM and the SWNTs which allows efficient capture of holes. The presence of birefringence in SWNT thin films and its influence on OPV properties as a function of angle of illumination is also described. The results presented in this thesis should lead to a better understanding of the P3HT:PCBM system and demonstrate promising solutions towards the realization of flexible, room temperature processed and inexpensive OPVs.