Impedance spectroscopy on ITO-free polymer solar cells

Indium tin oxide (ITO) is the most common transparent anode for polymer solar cells (PSCs). The limited flexibility of ITO, the increasing cost of indium and the patterning of ITO with a lot of chemicals, make this material not desirable for the realization of flexible, low-cost and efficient PSCs. Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has been investigated as innovative polymeric material for replacing ITO due to its high transparency, long-term stability and solution process ability. In this work we realized ITO-free polymer solar cells having as active layer a poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b´]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b]thiopene)-2,6-diyl] / [6,6]-phenyl C₇₁ butyric acid methyl ester (PBDTTT-C/[70]PCBM) blend film. The anode was realized using a highly conductive (HC) PEDOT:PSS layer. We studied the performances of this device by means of IV-dark, IV-light, external quantum efficiency and impedance spectroscopy measurements. The performance of the ITO-free device was compared with the corresponding one built using a standard ITO anode.