Simple routes for improving polythiophene:fullerene-based organic solar cells

Abstract

Improved power conversion efficiency (PCE), by up to ∼27%, of organic solar cells based on poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS)/poly(3-hexylthiophene):[6,6]-phenyl-C 60-butyric acid methyl ester (P3HT:PCBM) were obtained via simple modifications, widely applicable, in the fabrication of the spin-coated PEDOT:PSS layer. These included (i) further diluting the original PEDOT:PSS solution with deionized water, (ii) mixing the original PEDOT:PSS solution with ethylene glycol (EG), and (iii) spin coating EG over a PEDOT:PSS layer fabricated using the original solution. The optimal dilutions, spin coating rates, and durations were determined. Approach (iii) resulted in the best cell with a PCE of 4.7% as compared to 3.7% for the untreated PEDOT:PSS. To evaluate the origin of the improvements we monitored the PEDOT:PSS conductivity, external quantum efficiency of the devices, and their I–V curves that indicated an increase of ∼16% in the short-circuit current I SC. Other characteristics included the PEDOT:PSS layer thickness, its transmittance, P3HT:PCBM absorption spectra, its morphology, and surface chemical composition. The results indicate that in addition to the enhanced PEDOT:PSS conductivity (following some of the treatments) that improves charge extraction, enhanced PEDOT:PSS transmission and especially, enhanced P3HT:PCBM absorption contribute to improved solar cell performance, the latter by increasing I SC. While the various treatments in the optimized devices had a minor effect on the PEDOT:PSS thickness, its morphology, and consequently that of the active layer, were affected. The surface roughness of the active layer increased significantly and, importantly, in devices with PEDOT:PSS/EG/P3HT:PCBM, PCBM aggregates were observed near the cathode. Such aggregates may also result in increased absorption and improved charge extraction.