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

Abstract

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.