High-efficiency ITO-free polymer solar cells using highly conductive PEDOT:PSS/surfactant bilayer transparent anodes

By spin-coating a surfactant layer, glycerol monostearate (GMS) atop poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) film, a PEDOT:PSS/surfactant bilayer film was prepared facilely for the first time and applied as the transparent anode for high-efficiency ITO-free bulk heterojunction polymer solar cell (BHJ-PSC) devices. A significant improvement of the conductivity of PEDOT:PSS films (from ∼1 S cm−1 to more than 1000 S cm−1) was achieved by GMS modification and the highest conductivity reaches 1019 S cm−1 for Clevios PH 1000 under an optimized spin-coating speed of GMS layer. The Clevios PH 1000/GMS bilayer film exhibits a sheet resistance of 98 Ω sq−1 and a transparency of around 80% in the visible range, which are comparable to those of ITO, fulfilling its function as the transparent anode. The conductivity improvement by GMS modification is proposed to result from the GMS-induced segregation of PSS chains and the conformational change of the conductive PEDOT chains within PEDOT:PSS. While the highly hydrophobic –(CH2)16CH3 groups of GMS preferentially interact with the hydrophobic PEDOT of PEDOT:PSS, the highly hydrophilic –COOCH2–CHOH–CH2OH groups preferentially interact with the hydrophilic PSS chains with the hydroxyl groups playing an important role on the consequent phase separation between PEDOT and PSS chains. Using Clevios PH 1000/GMS bilayer films as the transparent anodes replacing ITO, high-efficiency ITO-free BHJ-PSC devices based on poly[N-9′′-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT) blended with [6,6]-phenyl C71-butyric acid methyl ester (PC71BM) (PCDTBT:PC71BM) and thieno[3,4-b]-thiophene/benzodithiophene (PTB7):PC71BM systems exhibit power conversion efficiencies (PCE) of 5.90% and 7.06%, respectively, which are comparable to the corresponding devices based on the traditional ITO anode.