Study of Electron Transport Layer in Organic Photovoltaic Cell on Flexible Substrate

Abstract

In this stduy, we investigated the effect of electron transport layer (ETL) on performances of inverted organic photovoltaic cells (OPVs) fabricated on flexible polyethylene naphthalate (PEN) substrates using poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thio-hene)-2-carboxylate-2-6-diyl)] and [6,6]-phenyl C₇₁-butyric acid methyl ester blend film as a photoactive layer. The photoconversion efficiency (PCE) of 5.92% was observed when using the sol-gel derived ZnO ETL, and was lower than that of reference device on a glass substrate (9.51%). In addition, a light soaking effect (LSE) and a relatively flat ZnO film surface were observed only the case of PEN substrate. This indicates that large number of defects in ZnO were generated due to the slow polycondensation speed of the sol-gel precursor solution when the PEN substrate was used and the carrier transport is inefficient. By using the nano-particle ZnO and the poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) as ETLs, the short circuit current density and fill factor were improved due to the reduced LSE compared when the sol-gel derived ZnO film was used. This result indicates that one reason of inferior OPV characteristics with the sol-gel derived ZnO ETL comes from the inhibition of electron transport/extraction due to the incomplete ZnO structure. As a result, we realized the highest PCE of 8.71% on the flexible PEN substrate.