Improved efficiency and stability of inverted polymer solar cells with a solution-processed BPhen interlayer and 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 ∼100nm 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-C60-butyric acid methyl ester) improves by up to 46%, from 2.5% to ∼3.7%, by adding a solution-processed BPhen layer between the CsCl and the active layer. For such cells with CsI (PCE ∼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 ∼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 3weeks; the PCE of similar cells devoid of the BPhen layer dropped by ∼60% during the same period. The PCE of the cell with CsCl/BPhen dropped by ∼16% after 2months. High humidity, as expected, resulted in faster deterioration in cell performance. The PCE, however, was restored to within ∼10% of the original value for 2week 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 ∼100nm 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.