Conductivity enhancement of PEDOT:PSS film via sulfonic acid modification: application as transparent electrode for ITO-free polymer solar cells

Abstract

3-Hydroxy-1-propanesulfonic acid (HPSA) was applied as a modification layer on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film via spin-coating, resulting in a massive boost of the conductivity of PEDOT:PSS film, and thus the as-formed PEDOT:PSS/HPSA bilayer film was successfully used as a transparent electrode for ITO-free polymer solar cells (PSCs). Under the optimized concentration of HPSA (0.2 mol L−1), the PEDOT:PSS/HPSA bilayer film has a conductivity of 1020 S cm−1, which is improved by about 1400 times of the pristine PEDOT:PSS film (0.7 S cm−1). The sheet resistance of the PEDOT:PSS/HPSA bilayer film was 98 Ω sq−1, and its transparency in the visible range was over 80%. Both parameters are comparable to those of ITO, enabling its suitability as the transparent electrode. According to atomic force microscopy (AFM), UV-Vis and Raman spectroscopic measurements, the conductivity enhancement was resulted from the removal of PSS moiety by methanol solvent and HPSA-induced segregation of insulating PSS chains along with the conformation transition of the conductive PEDOT chains within PEDOT:PSS. Upon applying PEDOT:PSS/HPSA bilayer film as the transparent electrode substituting ITO, the ITO-free polymer solar cells (PSCs) based on poly[ N -9 ² -hepta-decanyl-2,7-carbazole-alt-5,5-(4 ¢ ,7 ¢ -di-2-thienyl-2 ¢ ,1 ¢ ,3 ¢ -benzothiadiazole)]:[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) active layer exhibited a power conversion efficiency (PCE) of 5.52%, which is comparable to that of the traditional ITO-based devices.