Publication date: 27th June 2014
State-of-the-art colloidal quantum dot (CQD) solar cells have been shown to achieve power conversion efficiencies (PCEs) approaching 9%. However, deposition of the CQD absorber layer is tedious requiring a layer-by-layer (LbL) buildup and is a major obstacle to industrial scale-up. Understanding why the LbL process is needed to achieve state-of-the-art devices is crucial to improving manufacturability and to improving solar cells further. In this study we have sought to evaluate the solvent-QD interactions in terms of physical and chemical changes to the CQD absorbing layer in order to identify an alternative and more facile ligand exchange route. We have demonstrated that an extended exposure of CQD films to methanol (MeOH) is harmful for the overall performance. This explains why the standard CQD solar cells made with MeOH require tedious LbL buildup involving nearly a dozen steps.
Understanding of solvent interactions with QD films has led us to develop more efficient processing conditions based on use of acetonitrile (ACN) which allow us to achieve state-of-the-art performance in only three steps. This work could shed light on ways to achieve a single step deposition and solid-state ligand exchange.