Ambient Air Processing Causes Light Soaking Effects in Inverted Organic Solar Cells Employing Conjugated Polyelectrolyte Electron Transfer Layer

Inverted polymer:fullerene bulk heterojunction solar cells employing a conjugated polyelectrolyte electron transfer layer display light soaking effects as the oxygen adsorbed on indium tin oxide (ITO) during an ambient air device processing induces interface charge trap states in the conjugated polyelectrolyte layer and reduces its interface dipole. The light soaking populates the trap states with photoexcited electrons and reinstates the electric dipole, leading to a recovery of efficient charge extraction and normal illuminated currentvoltage characteristics consequently. The identified effect of adsorbed oxygen not only enables a remedy of the light soaking issue of the inverted solar cells via hydrogen plasma treatment of ITO but also suggests the importance of properly handling adsorbed oxygen species on ITO for achieving high performance organic devices based on ITO substrates in general.