Brush-Painted Solar Cells from Pre-Crystallized Components in a Nonhalogenated Solvent System Prepared by a Simple Stirring Technique

Extensive efforts have been employed to improve the power conversion efficiency of organic solar cells. One of the most successful approaches is the morphological control of the solar cells' active layer, the bulk heterojunction of the donor and acceptor. However, many morphological control techniques have faced challenges transferring from the lab to industrial scale due to lack of process scalability, stringent environmental requirements, and high temperature requirements during thermal annealing. In this report, we develop a novel strategy to manipulate the organic solar cells' morphology using a simple stirring technique. By stirring of poly(3-hexylthiophene) (P3HT) and mixtures of P3HT and phenyl-C61-butyric acid methyl ester (PCBM) solutions in a nonhalogenated and less toxic solvent system, more perfect P3HT crystals can be made. Networks of P3HT crystal fibrils, percolated P3HT crystal domains, and PCBM phase separated domains are critical factors for optimal solar cell performance. We demonstrate a simple fabricating technique utilizing brush painting to easily deposit the precrystallized components of the devices' active layer. As a result, the painted solar cells (without thermal annealing) achieve similar performance to a control group prepared via the commonly used standard process of spin-coating from a mixture of P3HT and PCBM dissolved in dichlorobenzene, followed by thermal annealing at an elevated temperature. Moreover, this report reveals an increase in power conversion efficiency of 60% to 90% from the painted devices made from precrystallized components (after stir solutions) in comparisons to the devices made from the pristine components (before stir solutions). We have demonstrated an easy thin-film processing technique that achieves high degrees of morphological control, showing promise not only for applications in other semiconducting polymers, but also demonstrating a technique that is scalable for mass production.