Quinacridone-Based Molecular Donors for Solution Processed Bulk-Heterojunction Organic Solar Cells

Abstract

New soluble quinacridone-based molecules have been developed as electron donor materials for solution-processed organic solar cells. By functionalizing the pristine pigment core of quinacridone with solubilizing alkyl chains and light absorbing/charge transporting thiophene units, i.e., bithiophene (BT) and thienylbenzo[c][1,2,5]thiadiazolethienyl (BTD), we prepared a series of multifunctional quinacridone-based molecules. These molecular donors show intense absorption in the visible spectral region, and the absorption range and intensity are well-tuned by the interaction between the quinacridone core and the incorporated thiophene units. The thin film absorption edge extends with the expansion of molecular conjugation, i.e., 552 nm for N,N'-di(2-ethylhexyl)quinacridone (QA), 592 nm for 2,9-Bis(5'-hexy1-2,2'-bithiophene)-N,N'-di(2-ethylhexyl)quinacridone (QA-BT), and 637 nm for 4-(5hexylthiophen-2-y1)-7-(thiophen-2-Abenzo[c][1,2,5]chiadiazole (QA-BTD). The change of molecular structure also influences the electrochemical properties. Observed from cyclic voltammetry measurements, the oxidation and reduction potentials (vs ferrocene) are 0.7 and 1.83 V for QA, 0.54 and 1.76 V for QA-BT, and 0.45 and 1.68 V for QA-BTD. Uniform thin films can be generated from both single component molecular solutions and blend solutions of these molecules with [6,61-phenyl C70-butyric acid methyl ester (PC70BM). The blend films exhibit space-charge limited current (SCLC) hole mobilities on the order of 1 x 10(-4) cm(2) V-1 s(-1) Bulk heterojunction (BHJ) solar cells using these soluble molecules as donors and PC70BM as the acceptor were fabricated. Power conversion efficiencies (PCEs) of up to 2.22% under AM 1.5 G simulated 1 sun solar illumination have been achieved and external quantum efficiencies (EQEs) reach as high as similar to 45%.