Correlation of diffusion and performance in sequentially processed P3HT/PCBM heterojunction films by time-resolved neutron reflectometry
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Date
2013-01-01
Journal Title
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Publisher
Royal Society of Chemistry
Abstract
Control over the structure of donor/acceptor blends is essential for the development of solution processable organic solar cells (OSCs). We have used time-resolved neutron reflectometry (NR) and in situ annealing to investigate the nanoscale structure and interdiffusion of sequentially spin-coated thin films of poly(3-n-hexylthiophene-2,5-diyl) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and correlated the evolving structure with the device performance. While the as-prepared film shows a clear two-layer structure it is evident that (19 wt%) PCBM has percolated throughout the lower P3HT layer. Upon heating, analysis of time-resolved NR data shows that the diffusion process is dependent on the annealing temperature. At temperatures up to 110 [degree]C, the two-layer structure is retained and only a small amount of PCBM diffuses from the interface into the lower layer, increasing the total PCBM content throughout the P3HT layer to 26 wt%. Significantly, this small change in acceptor content leads to a profound increase in device performance; with the power conversion efficiency (PCE) of the OSCs increasing from 0.47% (unannealed, 19 wt% PCBM) to 3.23% (annealed, 26 wt% PCBM) with the latter showing a similar efficiency to devices prepared from a blend containing 50 wt% PCBM. Further annealing at 120 and 130 [degree]C sees rapid interdiffusion between the two layers, along with an overall expansion in the thickness of the bilayer film. Despite the complete intermixing of the PCBM and P3HT to form a structure resembling a bulk heterojunction, essentially no improvement in device performance was observed for annealing at temperatures above 110 [degree]C. © 2013, Royal Society of Chemistry
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Keywords
Equipment, Reflectivity, Efficiency, Solubility, Interfaces, Stability
Citation
Lee, K. H., Zhang, Y., Burn, P. L., Gentle, I. R., James, M., Nelson, A., & Meredith, P. (2013). Correlation of diffusion and performance in sequentially processed P3HT/PCBM heterojunction films by time-resolved neutron reflectometry. Journal of Materials Chemistry C, 1 (14), 2593-2598. doi:10.1039/C3TC00063J