Inkjet Printing Quasi-Miscible Droplets for Pseudo-Planar Organic Heterojunctions

The control of droplet mixing is of great interest for fundamental science (e.g. biomolecular analysis, molecular diffusion, analytes separation) and emerging technologies in lab-on-chip devices (e.g. multiplexed biochips, micrototal analysis systems, Point-of-care) [1]. In this scenario, printing methodologies are a scalable and versatile approach for droplets production, resulting in bespoke life-inspired platforms applicable to a vast range of disciplines [2], e.g. materials sciences, sensors, flexible electronics, and biotechnologies. Based on the rapid development of printing techniques for the fabrication of photovoltaic devices [3], this work shows an innovative platform involving inkjet printing to fabricate pseudo-planar heterojunction (PHJ) organic solar cells (OSC) on ITO/PET [4]. The process is based on the sequential deposition of two quasi-miscible inks containing poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM), the model donor/acceptor couple for OSCs. The optimization of printing process consists in finding a suitable ink formulation (chlorobenzene based for P3HT and chlorobenzene:dichlorometane in the ratio 1:1 for PCBM), droplet velocity (about 7-8 m/s), droplet spacing ȝP, ultimately leading to satellites-free spherical picolitre-scale droplets resulting in a continuous film on the ITO/PET support. The chemical analysis of the PCBM-on-P3HT printed film is realized by atomic force microscopy, X-ray photoelectron spectroscopy and fluorescence spectroscopy. The morphology is characterized by a continuous and low roughness surface at the center of a printed droplet (average roughness is about 1.3 nm), whereas nanometric aggregates at the droplet border are observed, as a result of Marangoni flows mainly involving PCBM molecules. Such instabilities bringing molecular recirculation to the droplet border agree with models developed for bisolvent droplets evaporation [5]. XPS depth profile under mild sputtering (1 kV) permits to analyze the two printed layers, demonstrating the lack of the separation of the P3HT and PCBM layers due the occurrence of complete mixing. The almost complete quenching of the P3HT fluorescence emission band at about 655 nm due to the PCBM layer printed on top of P3HT confirms the effective formation of an interface between the two films. The characterization of the solar cell device allows demonstrating the goodness of the proposed approach, since the extracted parameters are quite similar (in particular the power conversion efficiency) to those realized in a previous reference study which employs spin coating deposition of the same materials in a glove box under inert nitrogen atmosphere [6]. In conclusion, this study is a first step towards the analysis of functional interfaces for the realization of a new class of inkjet printed PHJs and, in general, printed mixing interfaces.