Morphology control in co-evaporated bulk heterojunction solar cells

• Published in: Solar energy materials and solar cells 2013-10, Vol.117, p.22-28
• Main Authors: Kovacik, Peter, Assender, Hazel E., Watt, Andrew A.R.
• Format: Article
• Language: English
• Subjects: Annealing; Applied sciences; Blends; Buckminsterfullerene; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Energy; Exact sciences and technology; Fullerenes; Heterojunctions; Morphological development; Morphology; Natural energy; Organic photovoltaics; Photoelectric conversion; Photovoltaic cells; Photovoltaic conversion; Polymer–fullerene blend; Polythiophene; Solar cells; Solar cells. Photoelectrochemical cells; Solar energy; Vacuum thermal evaporation
• ISSN: 0927-0248; 1879-3398
• DOI: 10.1016/j.solmat.2013.05.021

Bulk heterojunction solar cells made by vacuum co-evaporation of polythiophene (PTh) and fullerene (C60) are reported and the blend morphology control through donor–acceptor composition and post-situ annealing demonstrated. Co-deposited heterojunctions are shown to generate about 60% higher photocurrents than their thickness-optimized PTh/C60 planar heterojunction counterparts. Furthermore, by annealing the devices post-situ the power conversion efficiency is improved by as much as 80%. UV–vis, XRD and AFM analysis shows that the enhanced photoresponse is a result of favorable morphological development of PTh upon annealing. Annealing-induced improvement of the donor–acceptor network is demonstrated across a wide range of compositions (20–80% PTh). This illustrates that good morphology control is possible in co-evaporated polymer–fullerene blends through multiple parameters and differentiates this donor–acceptor combination from the standard small-molecule systems. [Display omitted] •Vacuum deposition of polymer/fullerene bulk heterojunction solar cells is reported.•Their performance is optimized through composition as well as post-situ thermal annealing.•Annealing enhances efficiency over a wide range of D:A blend compositions (1:4–4:1).•Improved blend morphology results from favorable phase separation of the polymer.•Morphology control in the polymer blend is superior to the equivalent oligomer blends.