Improved efficiency and stability of inverted polymer solar cells with a solution-processed BPhen interlayer and polystyrene beads

TitleImproved efficiency and stability of inverted polymer solar cells with a solution-processed BPhen interlayer and polystyrene beads
Publication TypeJournal Article
Year of Publication2013
AuthorsXiao T, Fungura F, Cai M, Anderegg JW, Shinar J, Shinar R
Journal TitleOrganic Electronics
Date Published10
Type of ArticleArticle
ISBN Number1566-1199
Accession NumberWOS:000323933600026
KeywordsBPhen, CsCl or CsI, degradation, Hole-blocking layer, interlayer, Inverted organic solar cells, layers, lifetimes, MECHANISMS, performance, photovoltaic devices, Polystyrene beads

We demonstrate improved power conversion efficiency (PCE) and strongly enhanced stability of inverted organic solar cells (OSCs) with Cs halides by solution casting BPhen (4,7-di(phenyl)-1,10-phenanthroline) on the halide layer and similar to 100 nm polystyrene beads (PSB) on the blank side of the OSC's substrate. The PCE of ITO/CsCl/P3HT:PCBM/MoO3/Al (where P3HT is poly 3-hexylthiophene and PCBM is [6,6]-phenyl-C-60-butyric acid methyl ester) improves by up to 46%, from 2.5% to similar to 3.7%, by adding a solution-processed BPhen layer between the CsCl and the active layer. For such cells with CsI (PCE similar to 3.3-3.4%) the increase was only 6-9%, to 3.5-3.7%. The PCE of cells devoid of the halides but with BPhen was similar to 3.3%. The cells were optimized by varying the BPhen concentration in a chlorobenzene solution. The results are consistent with reduced charge recombination at the ITO interface in the presence of the hole blocking BPhen interlayer. The use of hole blocking BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), as a substitute for BPhen, also showed an enhancement (though lower due to its lower electron mobility), verifying the effect of these materials as hole blocking interlayers. Interestingly, the stability of such non-encapsulated devices with CsCl/BPhen or CsI/BPhen improved significantly. For example, the PCE of unencapsulated cells with CsCl/BPhen kept in the dark under ambient conditions dropped by less than 2% after more than 3 weeks; the PCE of similar cells devoid of the BPhen layer dropped by similar to 60% during the same period. The PCE of the cell with CsCl/BPhen dropped by similar to 16% after 2 months. High humidity, as expected, resulted in faster deterioration in cell performance. The PCE, however, was restored to within similar to 10% of the original value for 2 week old cells by solution-application of a PSB layer on the blank side of the cell's glass substrate. These beads direct and scatter the light to enhance absorption in the active layer. The results demonstrate that a simple approach such as casting a film of similar to 100 nm diameter PSB from an aqueous suspension on the blank side of the OSC substrate can improve long-term performance, and that spin coating BPhen is a low-cost and easy approach to reduce charge recombination at the cathode in inverted structures for increased PCE and stability. (C) 2013 Elsevier B. V. All rights reserved.

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