Bulk heterojunction solar cells based on preformed polythiophene nanowiresvia solubility-induced crystallization

Abstract

Here, we report the preparation of well-controlled nanoscale morphologies in photoactive thin films. The fabrication of bulk heterojunction structures in blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) employed two steps to achieve first the in situ formation of self-organized P3HT nanowires using a marginal solvent, and second, phase separation via mild thermal annealing. Morphological changes in the active layers that had been spin-cast from a marginal solvent, with varying annealing temperatures, were systematically studied and compared to the morphologies of films spin-cast from a good solvent. The interpenetrating nanowire structure yielded power conversion efficiencies as high as 4.07% due to the enhanced charge transport. Hole and electron mobilities increased substantially to 1.6 × 10⁻³ cm² V⁻¹ s⁻¹ and 1.4 × 10⁻³ cm² V⁻¹ s⁻¹, respectively, due to the two step process of P3HT crystallization by nanowire formation and subsequent phase separation. Photovoltaic performances improved with increasing film thickness up to 300 nm as a result of the interpenetrating donor/acceptor network structure.