High-efficiency inverted polymer solar cells controlled by the thickness of polyethylenimine ethoxylated (PEIE) interfacial layers

In this work, we investigate the effect of the thickness of the polyethylenimine ethoxylated (PEIE) interface layer on the performance of two types of polymer solar cells based on inverted poly(3-hexylthiophene) (P3HT): phenyl C-61-butryric acid methyl ester (PCBM) and thieno[3,4-b]thiophene/benzodithiophene (PTB7):[6,6]-phenyl C-71-butyric acid methyl ester (PC71BM). Maximum power conversion efficiencies of 4.18% and 7.40% were achieved at a 5.02 nm thick PEIE interface layer, for the above-mentioned solar cell types, respectively. The optimized PEIE layer provides a strong enough dipole for the best charge collection while maintaining charge tunneling ability. Optical transmittance and atomic force microscopy measurements indicate that all PEIE films have the same high transmittance and smooth surface morphology, ruling out the influence of the PEIE layer on these two parameters. The measured external quantum efficiencies for the devices with thick PEIE layers are quite similar to those of the optimized devices, indicating the poor charge collection ability of thick PEIE layers. The relatively low performance of devices with a PEIE layer of thickness less than 5 nm is the result of a weak dipole and partial coverage of the PEIE layer on ITO.