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

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;),the PEDOT:PSS/HPSA bilayer film has a conductivity of 1020 S cm;,which is improved by about 1400 times of the pristine PEDOT:PSS film(0.7 S cm;).The sheet resistance of the PEDOT:PSS/HPSA bilayer film was 98Ωsq;,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″-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(PC;BM)(PCDTBT:PC;BM)active layer exhibited a power conversion efficiency（PCE）of 5.52%,which is comparable to that of the traditional ITO-based devices.