Fabrication and characterization of water-based organic solar cells

Abstract

The current benchmark for organic photovoltaics (OPVs) using poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) involve toxic solvents such as chlorobenzene, which are harmful to both humans and the environment. Using water as the solvent instead is much better since it is cheaper and “greener” than chlorobenzene. However, currently, record performance is only ~0.1% efficiency for water based OPV, as compared to 5% for P3HT/PCBM devices. To improve power conversion efficiency of these water based devices, it is imperative that detailed studies on the photophysics are carried out to gain a clear understanding of the physical properties and charge generation and recombination dynamics of these materials.

The causes for low efficiency may be attributed to the low mobility of electrons in the water based fullerene derivative. The morphology of the water based polymer/fullerene blend are not influenced by annealing, as opposed to P3HT/PCBM which aggregates and forms well-connected networks. We hypothesize that the –OH groups on the water soluble C60(OH)24/26 molecule do not interact well with the hydrophobic thiophene chain of the polymers. Ultrafast spectroscopy shows that charges undergo nongeminate recombination much faster in water based solar cells than in P3HT/PCBM, and less excitons are dissociated by the acceptor in water based cells compared to P3HT/PCBM, thus causing less charges to be available for extraction.