Crystallinity and Morphology Effects on a Solvent-Processed Solar Cell Using a Triarylamine-Substituted Squaraine

Abstract

2,4-Bis­[4′-(<i><i>N,N</i></i>-di­(4″-hydroxyphenyl)­amino)-2′,6′-dihydroxyphenyl]­squaraine (Sq-TAA-OH, optical bandgap 1.4 eV, HOMO level −5.3 eV by ultraviolet photoelectron spectroscopy) is used as an active layer material in solution processed, bulk-heterojunction organic photovoltaic cells with configuration ITO/PEDOT:PSS/Sq-TAA-OH:PC₇₁BM/LiF/Al. Power conversion efficiencies (PCEs) up to 4.8% are obtained by a well-reproducible procedure using a mixture of good and poor Sq-TAA-OH solubilizing organic solvents, with diiodooctane (DIO) additive to make a bulk heterojunction layer, followed by thermal annealing, to give optimized <i>V</i>_OC = 0.84–0.86 V, <i>J</i>_SC = 10 mA cm^–2, and FF = 0.53. X-ray diffraction and scattering studies of pristine, pure Sq-TAA-OH solution-cast films show <i>d</i>-spacing features similar to single-crystal packing and spacing. The DIO additive in a good solvent/poor solvent mixture apparently broadens the size distribution of Sq-TAA-OH crystallites in pristine films, but thermal annealing provides a narrower size distribution. Direct X-ray diffraction and scattering morphological studies of “as-fabricated” active layers show improved Sq-TAA-OH/PC₇₁BM phase separation and formation of crystallites, ∼48 nm in size, under conditions that give the best PCE