Inverted Organic Solar Cells Based on PTB7:PC71BM with PFN Electron Transport Layer on ITO-Free Flexible PEN Substrate

Polyfluorene: poly(9,9,-bis(3'-(N,N-dimethyl)-propyl-2,7-fluorene)alt- 2,7-(9,9-dioctylfluorene)) (PFN) has been recently reported as a good electron transport layer for organic bulk-heterojunction (BHJ) solar cells by Chang et al. [1]. They fabricated BHJ solar cells based on [4,8-biz[Poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl]] (PTB7)  and phenyl-C₇₁-butyric-acid-methyl-ester (PC₇₁BM) on indium tin oxide (ITO) glass substrates with PFN as an electron transport layer, and it showed the efficiency as high as 9.2%.

In this paper, we report on the improved long term reliability of similar solar cells with an ITO-free flexibleinverted solar cell structures of PEN/PEDOT:PSS/PFN/(PTB7:PC₇₁BM)/MoO₃/Au using, and found these devices showed much improved long-term reliability of the devices stored in air without any surface passivation layer. The J-C characteristics showed a slight reduction in the short cuircuit current density after storage in air for 4 days. The use of the PFN layer also significantly enhanced not only the open-circuit voltages and the power conversion efficiencies of the solar cells presumably due to the high electron transport efficiency and the better contact between the PEDOT:PSS cathode and the PTB7:PC₇₁BM active layer. Although the resulted conversion efficiency was only 1.8% at this moment, and further process improvement will be continued.Furthmore we tried efficiency of the device change in the PFN thickness by spin-coating time.

[1] Yi-Ming Chang, and Chi-Yi Leu. Journal of Materials Chemistry A 1.21 (2013): 6446-6451.