Wide range thickness effect of hole-collecting buffer layers for polymer:fullerene solar cells

Here we report the wide range thickness effect of hole-collecting buffer layers (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)) on the performance of polymer:fullerene solar cells with blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The thickness of the PEDOT:PSS layers was controlled from 3 nm to 625 nm, followed by characterizations such as optical transmittance, electrical resistances in the in-plane and out-of-plane directions, work functions, contact angles, and device performances. Results showed that the optical transmittance was gradually decreased with the PEDOT:PSS thickness but a maximum value was measured for other properties in the thickness range of 10-30 nm. The device performance was noticeably improved with only 3 nm-thick PEDOT:PSS layer, while it was almost similar in the thickness range of 30-225 nm in the presence of gradual decrease in the surface roughness. The similar device performance between 30 nm and 225 nm has been assigned to the compensation effect between the reduced electrical resistance (increased conductivity) and the decreased optical transmittance as the thickness of the PEDOT:PSS layer increased.